CA2132986C - Semiconductor optical devices and techniques - Google Patents
Semiconductor optical devices and techniques Download PDFInfo
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- CA2132986C CA2132986C CA002132986A CA2132986A CA2132986C CA 2132986 C CA2132986 C CA 2132986C CA 002132986 A CA002132986 A CA 002132986A CA 2132986 A CA2132986 A CA 2132986A CA 2132986 C CA2132986 C CA 2132986C
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- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/02104—Forming layers
- H01L21/02107—Forming insulating materials on a substrate
- H01L21/02109—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates
- H01L21/02112—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer
- H01L21/02172—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides
- H01L21/02175—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides characterised by the metal
- H01L21/02178—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides characterised by the metal the material containing aluminium, e.g. Al2O3
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- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/02104—Forming layers
- H01L21/02107—Forming insulating materials on a substrate
- H01L21/02225—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer
- H01L21/02227—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process
- H01L21/0223—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process formation by oxidation, e.g. oxidation of the substrate
- H01L21/02233—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process formation by oxidation, e.g. oxidation of the substrate of the semiconductor substrate or a semiconductor layer
- H01L21/02241—III-V semiconductor
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- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/02104—Forming layers
- H01L21/02107—Forming insulating materials on a substrate
- H01L21/02225—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer
- H01L21/02227—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process
- H01L21/02255—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process formation by thermal treatment
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
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- H—ELECTRICITY
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/1021—Coupled cavities
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- H01S5/00—Semiconductor lasers
- H01S5/02—Structural details or components not essential to laser action
- H01S5/026—Monolithically integrated components, e.g. waveguides, monitoring photo-detectors, drivers
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/1003—Waveguide having a modified shape along the axis, e.g. branched, curved, tapered, voids
- H01S5/101—Curved waveguide
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/1028—Coupling to elements in the cavity, e.g. coupling to waveguides adjacent the active region, e.g. forward coupled [DFC] structures
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
- H01S5/1071—Ring-lasers
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- H01S5/00—Semiconductor lasers
- H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
- H01S5/4025—Array arrangements, e.g. constituted by discrete laser diodes or laser bar
- H01S5/4031—Edge-emitting structures
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- H—ELECTRICITY
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- H01S5/00—Semiconductor lasers
- H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
- H01S5/4025—Array arrangements, e.g. constituted by discrete laser diodes or laser bar
- H01S5/4031—Edge-emitting structures
- H01S5/4056—Edge-emitting structures emitting light in more than one direction
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- H01S5/00—Semiconductor lasers
- H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
- H01S5/4025—Array arrangements, e.g. constituted by discrete laser diodes or laser bar
- H01S5/4031—Edge-emitting structures
- H01S5/4068—Edge-emitting structures with lateral coupling by axially offset or by merging waveguides, e.g. Y-couplers
Abstract
The disclosure is directed to improved techniques and devices employing an aluminum-bearing III-V
semi-conductor material and a native oxide of aluminum that is formed in the semiconductor material. In a form of the disclosure, two linear arrays of end-coupled minicavities, defined by a native oxide of an aluminum-bearing III-V
semiconductor material, are arranged side by side to obtain a two dimensional array, with resultant lateral coupling between the linear arrays. The two dimensional array exhibits mode switching and multiple switching in the light power (L) versus current (I) characteristic (L-I) with increasing current. In another form of the disclosure, a stripe laser (1210) is transversely coupled (or side-coupled) with a linear array of end-coupled minicavities (1221-1275). Bistability and switching are demonstrated in the light versus current (L-I) characteristic of a native-oxide-defined structure of this type.
semi-conductor material and a native oxide of aluminum that is formed in the semiconductor material. In a form of the disclosure, two linear arrays of end-coupled minicavities, defined by a native oxide of an aluminum-bearing III-V
semiconductor material, are arranged side by side to obtain a two dimensional array, with resultant lateral coupling between the linear arrays. The two dimensional array exhibits mode switching and multiple switching in the light power (L) versus current (I) characteristic (L-I) with increasing current. In another form of the disclosure, a stripe laser (1210) is transversely coupled (or side-coupled) with a linear array of end-coupled minicavities (1221-1275). Bistability and switching are demonstrated in the light versus current (L-I) characteristic of a native-oxide-defined structure of this type.
Description
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SEMICONDUCTOR OPTICAL, DEVICES AND TECIiNI~UES
FIELD OF THE INVENTION
This invention relates to semiconductor devices and, more particularly; to techniques which employ a grown native oxide of aluma.num to obtain improvements in III-V semiconductor lasers dnd wa~rec~uides~ and alto relates to semiconductor lasers wh~:ch exhibit imprQVed properties, including improved single mode operation, optical switching and bistability.
The p~°es~nt invention was made, in part, with U.S.
Government support, and the U.S. Government has certain fights in this invention.
gp,CKGROUND OF THE INVENTION
Semiconductor lasers in the shape of a ring, or a partial virago have been known in the art-for a number of years.
Reference can be made, for example, to J. Caxran et al., IEEE
~. quantum E~eC~r~~. ~E-~ ~ (..~~~0.).% A.S aH. ~~a~~et. al . , APpl'. P~~s. I,.ett. 36; 801 (1980) and PSansonetti ~t al., ~.lec~tron Lett. ~3, 485 (,1988) . These tlrpes of devices have carious applications and proposed ~~pla.cations. For example, 3t hays been proposed that a semiconductor.ria~g lassr; in which light circulates in both clockwise and qe~unt~r-dlockwise diredtioins,,could be used asa very small and inexpensive ,, gyx~eascop~. Briefly, certain'motion of the gyroscope would have a different effect 'on the clockwise' and counter-Gloc~c~aise fight components, and the effect can be deferred -~o determine 'the motion or orientation of the de~rice.' R~.ng lasers,, or ;'circular resonators" hsve also been proposed far applications such as filtering and multiple~cing in so-ca~.Led opto-electronic or integrated optical circuits. L'rac~inns of a ring, such as a halt-ring or a quarter-ring, with cleaved ~;:~: . ~ ,., :rx ;-.5..c ,~~. , f m.f ;a r p . .r"~ mr:.. ~ : .~i-,. 5 .;~4., '. 1 . . n.4...
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W~ 93/2~5~1 ~'(..'fIlJS93/02844'~~..r ~I3~~~~b' facets, have been used for various applications in optical communications.
In a ring laser the curved light path makes optical confinement more difficult. Because of the greater incident angles the light subtends with respect to the confining walls (particularly for a small radius of curva,t'ure), the difference -. .,.
in indices of refraction must be relativ~L"y large to ensure internal reflection of sufficient light~in the ring laser "waveguide". It i~ among the objects of the present invention to overcome difficulties in the prior art of producing a laser in the shape of w ring or having a curved light path, for example part of a ring or a non-circular arc.
Another application where control of index of refraction is important is in coupled-strige laser diode arrays. These arrays offer the possibility of obtaining high output powers with decreased beam divergence anti single-longitudinal mode operation: Index-guided arrays, compared to their gain-guided counterparts, haws ad~rantages of increased mode stability and coherence, and decreased beam astigmatism. Several methods have been employed to fabricate index-guided arrays, including: channel etching,'epitaxial regrowth or overgrowth, and imp~rfty induced layer disordering ('°IILD°') [see, for example, D. G: Deppe e~ al:, Appl. Phys.'Lett. 50, 632 (1987);
L. J. Guido, Appl. Phys. Lett. 50, 757 (1987) and J. S.' Major, Jr. et al., Apple Phys~ Lett. ~5, 271 (1989)]. Many of these techniques require relatively sophisticated processing andlor provide limi ed control of the index-step between em~ateas. More precise'adjustment of the index-step would permit conbrol of the optical field between emitters and, thus~~ control:.'of the doupling between stripes. This coupling dramatically affects the far-field radiation patterns, determ3:ning the supermode(s) in which the array will , oscillate:
Opta-electronic circuits (in which devices in a , semiconductor chip, have interacting optical and electronic elements) are utilized in conjunction with fiber optics communications systems and are expected to ultimately have . :~.-. ~ ._ >,., f., , ;.
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:~ ~3iz~s~~ ~ 1 ~ ~ ~ ~ 6 . ~~--riu~~~i42 widespread application for other systems. In such circuits, circular or other curved optical signal paths are needed, particularly for the design and fabrication of relatively complex circuits. Tt is among the further objects of the present invention to pr~vide an efficient semiconductor optical waveguide for use in opto-electronic semiconductor circuits.
The high gain required for oscillation in semiconductor lasers results in a large optical bandwidth in which laser operation is possible. This large bandwidth generally results in multiple-longitudinal-made operation. For many applications, single-longitudinal-mode operation is required.
Consequently, sophisticated structures such as the distributed feedbac3c (DFB) laser [see D. R. Scifres, R. D. Burnham, and W.
Streifer; Appl. Phys. Lett. 25, 203 (1974)] and the cleaved-coupled-cavity (C3) laser (see W: T. Tsang, Lightwave Commun~:cations Technology, Part B, Semiconductor Injection Lasers, I, edited by W: T'. Tsang, in Semiconductors and Semimetals, Vol. 22, edited by'R. K: Willardson and A. C. Beer (Academ~.c, Orlando, x.985). Chap. 5, PP~ 2~7-373 have been developed to encore single-mode'operation. Tha DFB laser employs a fine-kale periodic corrugation of relatively small index s$;eps to interact witlh the electromagnetic wave. The C3 laser relies on several 'large-scale nonp~rioelic monolith~.c cavities for feedbacl~ and mode selection.
Qp~ical switching and bistability ire important for applicata.ons such as optical memories, optical signal processing, end optical logic elements. A variety of semiconductor 7.aser'devices have exhibited swi~.ching and bista~ility.: indluding: lasers with saturable absorbers [see, ~' M. I. Nathan, J: C. Marinate, a. F. Rutz, A. E. Michel, and G.
J. Lasherp 3. Appl. Phys. 36, 473 (2965); C. Harder, K. Y.
Lau, and A. Yariv, TEEE J. Quantum Electron. QE-18, 1351 (1882); N> Yamada and J: S.' Harris, Jr.P APP1~ Phys. Lett. 60, 2453 (1~92)~, ordinary tancism coupled-Cavity lasers [see N. K.
Dutta, G. P: Agrawal, and M. W. Focht, Appl. Phys. Lett. 44, 30 (1984)] and,vertical-cavity surface-emitting lasers [see D.
r(.
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W~ 93/20581 PC.'f1~S93/02i344xy' ~13~~~6 G, ~eppe, ~C. Lei, T. J. Rogers, and B. G. Streetman, Appl.
Phys. Lett. 58, 2616 (1991)]. It is also among the,objects of the present invention to provide a semiconductor laser that exhibits relatively large amplitude switching and bistability .
in its light versus current characteristics.
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SEMICONDUCTOR OPTICAL, DEVICES AND TECIiNI~UES
FIELD OF THE INVENTION
This invention relates to semiconductor devices and, more particularly; to techniques which employ a grown native oxide of aluma.num to obtain improvements in III-V semiconductor lasers dnd wa~rec~uides~ and alto relates to semiconductor lasers wh~:ch exhibit imprQVed properties, including improved single mode operation, optical switching and bistability.
The p~°es~nt invention was made, in part, with U.S.
Government support, and the U.S. Government has certain fights in this invention.
gp,CKGROUND OF THE INVENTION
Semiconductor lasers in the shape of a ring, or a partial virago have been known in the art-for a number of years.
Reference can be made, for example, to J. Caxran et al., IEEE
~. quantum E~eC~r~~. ~E-~ ~ (..~~~0.).% A.S aH. ~~a~~et. al . , APpl'. P~~s. I,.ett. 36; 801 (1980) and PSansonetti ~t al., ~.lec~tron Lett. ~3, 485 (,1988) . These tlrpes of devices have carious applications and proposed ~~pla.cations. For example, 3t hays been proposed that a semiconductor.ria~g lassr; in which light circulates in both clockwise and qe~unt~r-dlockwise diredtioins,,could be used asa very small and inexpensive ,, gyx~eascop~. Briefly, certain'motion of the gyroscope would have a different effect 'on the clockwise' and counter-Gloc~c~aise fight components, and the effect can be deferred -~o determine 'the motion or orientation of the de~rice.' R~.ng lasers,, or ;'circular resonators" hsve also been proposed far applications such as filtering and multiple~cing in so-ca~.Led opto-electronic or integrated optical circuits. L'rac~inns of a ring, such as a halt-ring or a quarter-ring, with cleaved ~;:~: . ~ ,., :rx ;-.5..c ,~~. , f m.f ;a r p . .r"~ mr:.. ~ : .~i-,. 5 .;~4., '. 1 . . n.4...
'. ./ J. ,.: . ~ 1 <..
'.k. /Eh 4! S
J x! n ~, ..
J A..:..: :.t:...
3. . .:;
?. tY. ,7 .. 1k.
i "~4 . s.. l , w' y ..'r': .,., .0,.4.' Y . J
I .~r.~.,. ,....y.. .. .s ;: y ri:~' ~ r. , . v. e. i.:.. f.....~r . '1 . eW 4 ,'!. s. ,. r ~~.n.:.~ 4. . v r'TI. . 2 . . ~c1 , f. ~~., , . /~N ",.: k i . 1 .:. . .), .
x .~ ~-. . h .~.XS~
~,~;~ V ...'.1., .. S . . ~F
... L.~....,..., 1 p p. .. 6, r l, ... n ~Ys .. ,:~ . . r a .,-a :Y...~.. ~..~.:,..i . ..,.:7. .. I
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.z ....:,f:.~.v a r., . . .L .. ,.'<, ,";,.. v ,..
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a~:, <. , a J ~ . . ,. .. .., ~t - . ,. , . . . .. ., ... ~. . . . , .~ ,. ., . . . . , ... . .. . , t..........n....... .e. . .......f....:a..,... .,~..........vtr.., <. ~.
~.,.."~.m .......<.... 1., . .. ,. ...~. . r.s..... (ta.,r...5r x,n.s,...ae....u ....f. . ~.~S41 a. ...... ....n........ .... .....
W~ 93/2~5~1 ~'(..'fIlJS93/02844'~~..r ~I3~~~~b' facets, have been used for various applications in optical communications.
In a ring laser the curved light path makes optical confinement more difficult. Because of the greater incident angles the light subtends with respect to the confining walls (particularly for a small radius of curva,t'ure), the difference -. .,.
in indices of refraction must be relativ~L"y large to ensure internal reflection of sufficient light~in the ring laser "waveguide". It i~ among the objects of the present invention to overcome difficulties in the prior art of producing a laser in the shape of w ring or having a curved light path, for example part of a ring or a non-circular arc.
Another application where control of index of refraction is important is in coupled-strige laser diode arrays. These arrays offer the possibility of obtaining high output powers with decreased beam divergence anti single-longitudinal mode operation: Index-guided arrays, compared to their gain-guided counterparts, haws ad~rantages of increased mode stability and coherence, and decreased beam astigmatism. Several methods have been employed to fabricate index-guided arrays, including: channel etching,'epitaxial regrowth or overgrowth, and imp~rfty induced layer disordering ('°IILD°') [see, for example, D. G: Deppe e~ al:, Appl. Phys.'Lett. 50, 632 (1987);
L. J. Guido, Appl. Phys. Lett. 50, 757 (1987) and J. S.' Major, Jr. et al., Apple Phys~ Lett. ~5, 271 (1989)]. Many of these techniques require relatively sophisticated processing andlor provide limi ed control of the index-step between em~ateas. More precise'adjustment of the index-step would permit conbrol of the optical field between emitters and, thus~~ control:.'of the doupling between stripes. This coupling dramatically affects the far-field radiation patterns, determ3:ning the supermode(s) in which the array will , oscillate:
Opta-electronic circuits (in which devices in a , semiconductor chip, have interacting optical and electronic elements) are utilized in conjunction with fiber optics communications systems and are expected to ultimately have . :~.-. ~ ._ >,., f., , ;.
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:~ ~3iz~s~~ ~ 1 ~ ~ ~ ~ 6 . ~~--riu~~~i42 widespread application for other systems. In such circuits, circular or other curved optical signal paths are needed, particularly for the design and fabrication of relatively complex circuits. Tt is among the further objects of the present invention to pr~vide an efficient semiconductor optical waveguide for use in opto-electronic semiconductor circuits.
The high gain required for oscillation in semiconductor lasers results in a large optical bandwidth in which laser operation is possible. This large bandwidth generally results in multiple-longitudinal-made operation. For many applications, single-longitudinal-mode operation is required.
Consequently, sophisticated structures such as the distributed feedbac3c (DFB) laser [see D. R. Scifres, R. D. Burnham, and W.
Streifer; Appl. Phys. Lett. 25, 203 (1974)] and the cleaved-coupled-cavity (C3) laser (see W: T. Tsang, Lightwave Commun~:cations Technology, Part B, Semiconductor Injection Lasers, I, edited by W: T'. Tsang, in Semiconductors and Semimetals, Vol. 22, edited by'R. K: Willardson and A. C. Beer (Academ~.c, Orlando, x.985). Chap. 5, PP~ 2~7-373 have been developed to encore single-mode'operation. Tha DFB laser employs a fine-kale periodic corrugation of relatively small index s$;eps to interact witlh the electromagnetic wave. The C3 laser relies on several 'large-scale nonp~rioelic monolith~.c cavities for feedbacl~ and mode selection.
Qp~ical switching and bistability ire important for applicata.ons such as optical memories, optical signal processing, end optical logic elements. A variety of semiconductor 7.aser'devices have exhibited swi~.ching and bista~ility.: indluding: lasers with saturable absorbers [see, ~' M. I. Nathan, J: C. Marinate, a. F. Rutz, A. E. Michel, and G.
J. Lasherp 3. Appl. Phys. 36, 473 (2965); C. Harder, K. Y.
Lau, and A. Yariv, TEEE J. Quantum Electron. QE-18, 1351 (1882); N> Yamada and J: S.' Harris, Jr.P APP1~ Phys. Lett. 60, 2453 (1~92)~, ordinary tancism coupled-Cavity lasers [see N. K.
Dutta, G. P: Agrawal, and M. W. Focht, Appl. Phys. Lett. 44, 30 (1984)] and,vertical-cavity surface-emitting lasers [see D.
r(.
~'.,.. y y v ..( .r f..'~.~., ::2-yyt-.:~(:~ pa 4 a .y!y ~.:~i7 , ..~t.v.s ":(a ~:: « t v r. .. . , .. , o...
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W~ 93/20581 PC.'f1~S93/02i344xy' ~13~~~6 G, ~eppe, ~C. Lei, T. J. Rogers, and B. G. Streetman, Appl.
Phys. Lett. 58, 2616 (1991)]. It is also among the,objects of the present invention to provide a semiconductor laser that exhibits relatively large amplitude switching and bistability .
in its light versus current characteristics.
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9 93/20581 PCffL1S93/02~4 SUMMARY OF THE INVENTION
An aspect of the present invention is directed to improved techniques and devices employing, inter alia, an aluminum-bearing III-V semiconductor material and a native oxide of aluminum that is formed in the semiconductor material.
There has been previously disclosed a technique of forming a high quality, stable, and compact native oxide layer from an aluminum-bearing Group III-V semiconductor material.
[See Dallesasse et al., Appl. Phys. Lett. 57 (26), 2844-6, 24 December 1990; Dallesasse et al., Appl. Phys. Lett. 58 (4), 394-396, 28 January 1991; Dallesasse et al., Appl. Phys. Lett 58 (8), 834-836, 25 February 1991; and Sugg et al., Appl.
Phys: Lett 58 (11), 1199-12p1, 18 March 1991.] The technique comprises exposing an aluminum-bearing Group III-V
semiconductor material to a water-containing environment and a temperature of at Least about 375°C to convert at Least a portion of the aluminum-bearing Group III-V semiconductor material to'a native oxide. The thickness of said native oxide formed thereby is subs antially the same as or Less than the thickness of that portion of said aluminum-bearing III-V
semiconductor material converted int~ the native oxide. The native ox~.de Dyer thus grown is denser and more stable °than oxide iaye~~ formed from previous methods, meaning, for ' example, that they do not degrade under conditions of normal use aa~d atmospheric ex~osur~e. Further, the native oxide was demonstrated to exhibit improved operating and performance characteristics, f~r exa.mpTe with regard to metalli~ation adherence and dielectric properties. The native oxides were described as being useful. in lasers, transistors, capacitors, waveguides and'in other electrical and opto-electrical devices: Anhydrous oxides of alumin~zm were noted to exhibit a relatively low index of refraction (less than about 2.0) and index of refraction can be used to distinguish the anhydrous oxide forms'from the higher index hydrated oxide forms that are generally unsx~itable for semiconductor applications due to r~r,y~
~~ 9mzoss~ Pcrms9~ioz~a b::.~
~~~z~gs s properties such as expansion and instability.
A form of the invention is directed to a method of making a semiconductor laser having a light path that is at least partially curved, and comprises the following steps: forming -a layered semiconductor structure comprising an active region between first and second semiconductor confining layers, the .
first and second semiconductor confining layers being of opposite conductivity types, and said first semiconductor confining layer being any aluminum-bearing III-V semiconductor material; applying a mask pattern over said first semiconductor confining layer, the pattern including a stripe that is at least gart~.ally curved; exposing unmasked portions of the first semiconductor confining layer to a water-containing environment and a temperature of at least 375 degrees C for a time sufficient to form a thick native oxide of aluma.num iz~ said first semiconductor confining layer; and coupling respective electrodes with said first and second semiconductor confining layers. Generally, the active region includes at least a waveguide layer and a quantum well layer, and the respective electrodes are coupled to the semiconductor confining layers through further respective semiccanductor layers. The aluminum-bearing materialmay comprise, for example, AlxGal~XAs, where x 'is at least 0.3. Generally, a higher aluminum fraction, for example x = 4:7 or greater-will be used to facilitate the thick oxide growth rate, which also depends'an temperature.' A temperature of at least about 450 degrees C is generally preferred. For'a ring laser, the time of exposure may be selected to have said native oxide extend through at least most o~ the thickness of said first confining layer, a,nd possibly; through the entire; thickness of said,, first confining layer. Another form, of the a.nvention comprises a sem3.conductor passive optical waveguide, having a light path which is at least partially; curved, tl~a~ employs a thick na~:ive oxide of aluminum. , In a further form of the invention; two linear arrays of end-coupled cavities (called minicavities) of a QWH
semiconductor laser are defined by a native oxide of an _ ~~.3~~~b ',~~ 19312n~~' PCT/US93/02&~4 aluminum-bearing III-v semiconductor material and are arranged side by side to obtain a two dimensional array, with resultant lateral coupling between the linear arrays. The two dimensional array exhibits mode switching and multiple switching in the light power (L) versus current ,(I) characteristic (L-I) with increasing current.
In another form of the invention, a stripe laser is transversely coupled (or side-coupled) with a linear array of end-coupled minicavities. Bistability and switching are demonstrated in the Light versus current (L-I) characteristic of a native-oxide-defined structure of this type. The device, witY~'internally coupled elements and the current partitioned among the dlemants, exhibits a large hysteresis in the L-I
curve, with switching from the stimulated to the spontaneous regime occurring over substantial power (light) and current ranges. The liner array of "minilasers°' and its resonance modulates arid switches the stripe laser operation.
In acdordance with a further definition of the invention, a semiconductor laser device includes first and second adjacent laser units formed on the same semiconductor substrate, daCh of the units including a laser cavity. The laser cavity of the first unit has a substantially different longitudinal mode selection characteristic than the laser cavity. of said second unit. [As used herein, substantially different longi udinal mode s~c ion charactera:stics means that the first unit has a cavity mode ~g~cin~ that is at least 1t~
percent greater that the cavity mode spacing of the second uwit, andlor a primary emission wavelength that is at least 50 1~ greater than the:primary emission wavelength of the second unit.,) Meana are provided for applying energizing signals to the first end second units to obtain laser emission from the unitsand lateral.coupling betweem the cavities of the units.
In an.embodim~nt of the invent3oa~ there is disclosed a semiconduc or laser device: that includes a semiconductor active region dispersed between first and second semiconductor confining layers. An electrode array hay electrode elements coupled with the first confining layer. [As used herein, the WO 93l2~5~1 PCTh1JS93/028n4 ~, term "electrode elements'° is intended to include electrical contact regions (e. g. highly doped semiconductor regions) that contact an underlying semiconductor structure.] At least one opposing electrode is coupled with the.second confining layer. .
The electrode elements of the array are~~spaced apart and form a two-dimensional array that incluc~ea"a plurality of electrode .
. ,. .
elements along a line and at leas~t~~one further electrode element laterally spaced from the electrode element of said line. Means are provided for applying electrical signals between the electrode elements and the at least one further electrode element and the opposing electrode to effect light emission in the active regions defined under the plurality electrode elements and at least one active region defined under the at least one further electrode element, and to effect lateral coupling of the emissions.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.
~ ~ 3 ~ ~ 8 ,~ ~~riu~~3io~s4a g~~zosg~
HRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a scanning electron microscope image of a stained cross-section of a device in accordance with an embodiment of the invention.
Fig. 2 is a graph of cw light output (both ring ends) versus current for a device in accordance with an embodiment of the invention, and shows, in an inset, a surface photograph of the device.
Fig. 3 is a graph of pulsed light output (both ring ends) versus current for a device in accordance with an embodiment of the invention and shows, in the inset, single mode operation.
F~.g. 4 shows near field image plots for the Fig. 3 device.
Fig. 5 is a graph of pulsed light output versus current for another device in accordance with the invention, the inset slowing the device geometrlr.
Fig. 6 is a simplified cross-sectional representation of a semiconductor laser diode device in accordance with an embodiment of the inventioaa.
~,i~. 7 is a simplified cross-sectional diagram of another semiconductor laser' device in accordance with a embodiment of the invention.
Fig. 8 is a simplified ~r~s~°sectional representation of ~ s;emiconductor optical: wav~eguide in accordance with an embod3:ment of the ixwention. ._ Fig. 9 i.ll:us~rates the surface configuration of a ring laser novice.
,Fig. 10 illustrates the surface configuration of a ~u~rter-ring laser or waveguide.
Figs I1 and 12 illustrate the surface configuration of ring lasers or waveguides with different branch coupling arrangements.
Figs 13 and 14 illustrate the surface configuration of mufti.-stripe lasers or waveguides with ring coupling.
~'ig. 15 illust~cates the surface configuration of a series '..1F'.,~a '.'.7,.
.., ...,,,. .. ... ~. . ... ... , ..a,.:.. .... ...,.
._....., . "... .. ,... .. Y .... ..~._.~ , . ". ,., , ;.,r« ..... ,.... , ".a ,. . . ..... ...... . . , .., . . ., .
CVO 93!20581 . P~CflU593102844 , ' ~~~.~~9~~
la of coupled half-ring lasers or waveguides.
Fig. 16 illustrates the surface configuration of a multi-stripe laser or waveguide with half-ring coupling.
Fig. 17 illustrates the surface configuration of a curved .
laser or waveguide in which the light path becomes laterally -'~,:~
offset. , Fig. 18 illustrates the surface,-'configuration of a laser or waveguide which couples light in 'a single branch with four curved branches.
Fig. 19 shows continuous (cw) 300 K light output (single facet) versus current characteristic (L-T) of a native-oxide-defined two-dimensional (2-D) coupled-cavity AlXGa1_XAs-GaAs QWH
laser array (uncoated facets, - 304 pm total cavity length).
The threshald is ~5 mA~ and the power peaks at - 12.5 mW (115 mA). The inset shows a surface photomicrograph of the unmetalli~ed 2-D twin lineax array. The rectangular minicavities are 4 Nm wide, 19 Nm long, and separated end-to-end by - 3 um. The two ccaupled linear arrays are separated by ~ um:
Fig. 24 sows longitudinal mode spectra (cw, 300 K) of the diode of F'ig. I9 at (a) 115, (b) 154, and (c) 164 mA
(points shown on the L-I o~ Figa 5). The single mode behavior at (a) 8280 ~'(1I5 mA) shif s to 8313 ~ at (b) 150 mA. At (c) 164 mA single mode operation has switched off and the a res~nances of the 19 arm long minicavities aye evident and marked with arrows. The mode spacing i~s ~~ 50 ~,, which agrees with the 19 um minicavity length.
Fag. 2~ shows he li.ght'output versus current characteristic (L-I-, cw, 300 K) of a diode with the same ge~me~try as that shown in Fig. 19. The diode turns on and off twice as the current is increased. The dashed Sine shows that the emission intensity in the valley region is in the range of spontaneous emassion. The inset shows single mode behavior (840 ~) persists to at least 415 mA (- BT~n), and is marked with a solid dot on the L-I curve.
Fig. 22 shows the near field (NF) emission patterns and longitudinal mode spectra o~ the diode of Fig. 21 near the r , : 93/20581 ~ 1 ~ ~ ~ ~ ~j ~'CfItJS93102844 diamond-shaped point at ~ 70 mA on the L-I characteristic. At (a) 40 mA (spontaneous regime) the near field (NF) shows two intensity peaks of the twin linear array, with the width of 9.2 pm in accord with the geometry shown in Fig. 19. At (b) 71 mA the NF is twin lobed, with the device ogerating single mode (8260 ~) but with also strong satellite longitudinal modes. At (c) 72 mA, the. NF emission from the right stripe disappears abruptly, with also an abrupt disappearance of the satellite longitudinal modes:
F~.g: 23 shows the continuous 300 K light output (single facet, uncoated) versus current characteristic (L-T) of a native-oxide-defined AlXGaI_xAs-GaAs single laser stripe side-coupled to a linear array of end-coupled minilasers. The laser threshold is 32 mA, with abrupt switching from the stimulated (ON} to spontaneous (OFF) regime occurring at 168 mA. The device exhibits bistability, switching back shargly from the spontaneaus (OFF} to stimulated (ON) regime at 123 mA: The diode geometry (prior to metallization) is shown in the inset ahd consists of a single ~ 6 ~m-wide laser stripe side-coupled (°- 5 pm away) to a linear army of end-coupled min3~lasers (6 ~m-wide, 19 p~m long and 22 pm centers).
Fig. 24 shows the continuous 300 K light output (single facet, uncoatedj versus current'characteristic (L-I) of a device of the form of Fig. 23 (inset). The laser exhabi~ts a :threshold of 27 mA, with switching and bistability occurring "in the range 96-100'mA. Throughout the entire operating range, the device output consists essentially of a ~ 5.5 um 'Gau~sian near-field (NF} pattern fr~m the single continuous stripe of Fig. 23. The NF:pattern is hown dust before swific,hing at ( a ) 99 .C inset ) . After, switching at ( b ) .100 .,,,. ,.
mA, essentiall~,r no output is observed; on a higher sensitivity scale (b'}, however, the same NF pattern is revealed.
Fig. ~5 shows longitudinal mode spectra (cw, 300 K) of the diode 4~ Fig. 24 corresponding to single mode stimulated emission (ON) at (a} and switched JFF to spontaneous emission at (b). Single mode laser=operation ~.s observed from thgeshold (~ 27 mA) to (a) ~9 ~~ with output at large '~~3~Zg~6 W~ 93/2051 _ PCf/~J~93/02~84'";.~;
amplitude from only the continuous stripe of the diode (left stripe in Fig. 23 inset). In the spontaneous emission OFF
regime (b) the lower energy group of modes corresponds to the laser stripe and the higher energy group of modes to the linear array (see inset of Fig. 23).
Fig. 26 is diagram of a portion of~'the top surface of the device described in conjunction with lEi~ures 23-25.
Fig. 27 is a cross-sectional diagram (not to scale), as taken through a section of the Fig. 26 device defined by arrows 13-13.
Fig. 28 is a cross--sectional diagram (not to scale), as taken through a section of the Fig. 26 device defined by arrows 14-14.
Fig. 29 is a cross-sectional diagram (not to scale), as taken through a section of the Fig. 26 device defined by, arrows 15-15.
Fig: 30 illustrates a tyro-dimensional array that can be operated using two, three, or four terminals.
Fig; 31 illustrates a twa-dimensional array with terminal dantrol in both dimensions.
Fl.gures 32-35 show plan views of ring lasers including a~inicavities in curved configurations in accordance with embodiments of theinvention.
I'igur~s 36-39 show plan views of adjacent rang and w straight line lasers, ~aith'transverse caupling between laser cavities, and including configurations where the ring, the straight line, or laoth, are divided ir~t~ minicavities.
Fig. ~0 is a cross-sectional diagram (not to scale) of a vertical cavity laser device w~:th transverse coupling between adjaqentt laser cavitie having different made selection, characteristics .
Fig. ~1 illus rates a two-dimensianal gray of vertical cavity laser units of the type illustrated in Fig. 40.
~~ ~:~ ~ 9128581 ~ 1 ~ ~ ~ ~ ~ P~.°f/US93/02844 DETAILED DESCRIPTION
In an example hereof, a quantum well heterostructure is grown by metal-organic chemical vapor deposition ["MOCVD" -see for examgle, R.D. Dupuis et al., Proceedings of The International Symposium on GaAs and Related Compounds, pp. 1-9, Institute of Physics, London, 1979, and M.J. Ludowise, J.
Appl. Phys., 58, R31, 1985] on an n-type GaAs substrate.
After a GaAs buffer layer, an Alo,BGa~.2As lower confining layer is grown to a thickness of about 1 pm. [The confining layers are also sometimes called cladding layers.) The active region ~f the quantum well heterostructure is then grown, and includes symmetrical Alp.zSGao.~5As waveguide layers, undo~ed and of thickness about 750 ~l eachr on either side of a GaAs quantum well of thickness about 100 A. An upper confining (or cladd~:ng) layer of p-type AloaaGdo.aAs a.s grown to a thickness of about 0e6 ~m~ and a heavily doped p-type GaAs contact layer is grown thereon, the contact layerhaving a thickness of about X00 d~. In this example, fabrication of a laser begins with the patterning of about 1000 l~ of Si3N4 into rings [ 25-dam wide anraulu~250 hum inside diameter (IDj, 300 um outside diameter (OD)], The Si~~l4 rings serve as a mask for the chemical etching ( HZS04: HZOZ : H20, l s 8 : 80 ) of the contact layer, thus leaV.in.g the Alx~al_xAs upper confiniing layer exposed i.nsi~e and outside of the masked rings: The sample is then placed in an open tube furnace, supplied with HzO vapor and NZ, at 450°C for 35 minutes. Th~.s process xesults an the coxwersion of the upper c~nfining layer (aahere exposed) ~o a native oxide having an index of refraction of aDaout 1.6. In this case, at the ring edges the.;,oxide extends downward through the entire upper confining layer as shown in Fig. 1 by the scanning electron microscope (SEM) image of a stained crow section. The oxide is deeper ~t the ring edge than beyond (~o the right in F'ig.
1):~ This effect may be a result of changes in H20 adsorption, O/H diffusion, or s ress induced by the presence of the masking stripe: ~ The oxide ~arofile is fairly isotropic, however, extending laterally essentially to the same extent as r r. ~~
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~~~986 it does in depth. Transmission electron microscope (TEM) , images of similarly oxidized crystals indicate that some oxidation (about 200 A) of the underlying Alo.z3Gao.~~As waveguide region occurs. Thus, the low-index native oxide extends into -the waveguide layer, creating large latexal index steps for sidewise optical confinement and waveguiding. Calculations .
based on propagation in a 4-layer slab waveguide (see G.E.
Smith, IEEE J. Quantum Electron., QE-4, 288 (1968)] for this deep oxide edge indicate an effective lateral index step greater than 0.05. For this example, structures with the native oxide located about 1000 A vertically away from the waveguide result in an insufficient index step for ring oscillation. However, as discussed further below, effective lasers can be made with lateral native oxide that extends only partially through the confining layer.
After the indicated oxidation, the Si~N4 masking rings are selectively removed in'a CFA plasma, resulting in a self-aligned geometr~r. The-'sample is then Zn-diffused (540°C, 20 min) to improve the contacts and metallized with Ti-Au for the p-hype contact and Ge-Ni-Au for the n-type contact. In an example hereof the rings are then cleaved ~:n half (or on a chord through the annulus) and the remaining three sides of the crystal are'saw cut (SC) to prevent resonance across the edges. The sur~ace'of a typical half-ring laser diode hereof after metallization, cleaving, and sa~ring is shown in the inset of Fi:g. 2:
The 300 K cw bight versus currant (~-~) curve of a typical half-ring laser diode hereof is shown in Fig. 2. The threshold current is 105 mA (~° 890 A~cm~). The curve is linear above threshold with a total external differential .. ' I I, quantum efficiency (~ of 49g) and a total output power (both ends of the'half ring) exceeding 40:mW. The pulsed threshold (2 us pulse' width, -0.5~' duty circle) of this diode is 78 mA.
The pulsed'(2 Ns, a.5~ duty cycle) L--I characteristic of a moderate guality half-ring laser diode, fabricated as above, is shown in Fig. 3: The diode threshold current is - 103 mA.
Longitudinal, mode spectra show well defined modes, with .r 9312058 ~ I ~ ~ ~ ~ ~ PC1'/US93/02844 single-mode operation occurring at 150 mA {Fig. 3, inset).
The mode spacing (~~l) is - 1.7 A, corresponding to a cavity length of ~ 560 Nm. This is longer than the half-circumference (- 470 ~um} and may be due to some misalignment of the cleave (creating a longer cavity) or a longer effective path length caused by the curved resonator, Operation around the curved resonator is confirmed by imaging the output of each end of the half-ring laser separately into a monochromator. The longitudinal mode spectrum is confirmed tc be identical from each end, indicating that laser emission indeed occurs from the circular cav~:ty (data not shown}. Further evidence of oscillation arQUnd the circular cavity was provided by sawing a half-ring device, which is originally observed to lase, in two along the vertical arrows "SC'° and "25 um" in the inset of Fig. 2.; This was found to destroy the resonator and the laser operation.
If the device were originally lasing linearly from the front cleave to the opposite saw cut (SC}, the device should continue to exhibit laser operation, which is not the case.
Thus, there is strong confirmation that laser operation occurs around-the ring. However, half-ring laser diodes that were cleaved in two {not saw cut) to form quarter-ring diodes continue to lace, with stimulated emission being observed from both perpendicular clewed.facets.
The near field (NF) intensity profiles of the laser diode of this example are collected with a f/U.95 25 mm focal length lens. A low magnification view {Si MOS camera) with the diode operating at 1~0 Ma (pu~.sed;) shows distinct emission from the two ends of the half-ring laser (Fig. 4{a)). The 267 pm ,.:,,, center- o-center separation agrees well with the device geometry. The corresponding intensity profiles (CCD array image} are shown in Fig. 4(b). Both g~aks exhibit asymmetry, with the intensity,drogping off faster towards the outside diameter (OD} of the annulus. This asymmetry is more evident in the higher magnification view of he right-hand end (Fig.
4(c)); Such asymmetric intensity profiled agree well with those calculated for a circular waveguide (see E. I~arcatilli, L.~1 i~V~ 931205$1 P~'/US93/0284~ ~': ;,:
~~~~°~~ ~ 6 Bell Syst. Tech. J. 48, 2103, 1969}.
Polarization-resolved L-I characteristics indicate that the half-ring diodes lass in the TM mode., This behavior is opposite to that observed in conventional GaAs QWH laser diodes and in native-oxide defined linear resonator QWH laser diodes, which lass in the TE mode. The radiation losses in the native-oxide circular resonator for the TE modes are greater than for the TM modes, indicating application for mode ,;..
filtering.
Fig. 5 shows the L-I characteristic of a native oxide ring laser diode fabricated in similar manner but on a lower ( vertical ) confinement AlxGa1_X~rs-GaAs QWH laser crystal ( x p.6~,confining layers). A cleave through the ring annulus permits laser light to leak out (inset of Fig. 5), with oscillation still maintained around the ring.
Fig. ~ is a simplified diagram of a laser device 6a0 made ~xsing,the forgoing technique. The device, on Gars substrate and buffer layers 610 and 615, includes an active region 630 between AlXGal_xAs confining layers 640 and 650, of opposite conductivity types. The active reg~:on includes the quantum well 633 between undaped AlXGa1_XAs waveguide layers 635 and 637: The diagxam also shows the curv~d top contact stripe 660, the underlying GaAs cap layer 670, and bottom electrode 605: As noted a.n the f~regoing description the native oa~ide of this exa~tple, 680', extends through the entire upper confining layer G50 and slightly into the upper waveguide layer 637.
Inthe description in conjunction with Fig s 1-6, the native oxide of aluminum extends thr~ugh the entire upper confiding layer of the laser diode and even, to a small ~' extent, znto the waveguide region. Applicant has discovered :that effective optical confinement; 'taile~red to obtain desired operating conditions, can be achieved with a thick (generally, about 3000 ~ or more) native oxide that does not necessarily extend through the entire confining layer. Generally, a native oxide that extends through at least one-third of the canfin.ing Iayer is preferred. Fig. 7 shows an~embodiment of . . ] 93/2051 ~ ~ ~ ~ ~ $ ~ ~crrus9~roz~aa the invention having a linear stripe 760 and wherein the thick oxide 780 is controlled (e.g. by controlling the time of exposure andlor temperature in producing the native oxide) to extend about half way through the upper confining layer. In this example, the aluminum fraction (x) of the AlXGa1_XAs confining layers 7~O and 750 is relatively low, for example about 0.4, which results in lower vertical (i.e., in the direction transverse the layers) confinement of the laser beam. (Layers with like reference numerals to those of Fig. 6 represent similar structure.] As described further hereinbelow, less vertical confinement permits greater expansion of the beam into the confining layers and, accordingly, a larger effective lateral refractive index step encountered by the beam as a result of the native oxide in the Confining layer.
Reference can be made to the following publications which relate, inter alia, to control of the optical (field and gain profile by adjusting the thickness of native oxide outside the active strige and to control of oxide thickness to determine the degree of optical eonfinemen~ts F:A. Kish, S:J. Carac~i, 1~. Holonyak, Jr., J.M.
Dallesasse, K.C. Hs~.eh, M.J~ Raes, S.C. Smith, & R.D. Burnham, °'Planar Native-Ox~.~le Inde%-Guided AlXGa~_XAs-GaAs Quantum Well Heteros~ructure Lasers", Appl. Phys. Lett. 59, 1755, September 30, 1991;
F:A. Kish, S.J. Caracci, N: Holonyak, Jr., and S.A.
Maranowski, J.Nt. Dallesasse, R.D: Burnham, and S.C. Smith.
t,~~sible Spectrum Idati~re°-O~ide Coupl~cl-Stripe Ino.~(Al~Ga~-x)o.~P_ In0.5~a0.5P Quantum Well , Heterostructure Laser Arrays'" , Appl .
Phys.~ Lett. 59.2883, November 25r 1991; , FA. Kish', S.J. Caraeci, ~T~ Holonyak, Jr., P. Gavrilovic, K. Meehan, ~ ~.~~ William, °"Coupled-Strfipe In-Phase Operation Of Planar Native-Oxide Index°~ua.ded AlYGa~_yAs-GaAs-InXGa~_XAs Quantum-Well Hfetex'ostructure Laser Arrays", Appl. Phys. Lett.
71e J~IriuaY'~r 6, 1992;
F.A. K sh, S:J« Garacci, S:A. Maranowski, N. Holonyak, ~'r,, K.C. Hsieh, C:P. Kuo, R.M: Fletche.r, T.D. Osentowski, &
PG'f/tJS93/02~:. ~>
Wo 9~~2~''3 2 9 ~ ~
M.G. Craford, "Flanar Native-Oxide Buried-Mesa AlxGa1lxAs-Ino.S(AlyGal_~)o.sP-Ino.a(AlZGa1_Z)o.sP Visible-Spectrum Laser Diodes", J. Appl. Phys. ?1, 2521, March 15, 1992.
Fig. 8 illustrates a passive curved waveguide in accordance with a form of the invention. The waveguide, which can be coupled (directly, or evanescently) with a suitable light source [not shown], includes, for example, GaAs substrate and buffer layers 810 and 815, and an AlxGa1_xAs (x =
0:8; for example) waveguide layer 820. The GaAs cap layer 870, native oxide confining regions 880 (which extend about half way through the alumanum-bearing material in this case), a~ad the contact stripe 860, can be formed using the previously described techniques.
Figs. 9-18 illustrate configurations of lasers or waveguides (cross-sections of which may be, for example, ,,of the-types shown in Eigs. 6, 7 and/or 8) that can be advantageously implemented utilizing the principles of the invention. In these Figures, the white regions represent .either the laser stripe configuration, which has thereunder, aster axia, a waveguide reg~.on with he index of refraction confinement in accordance with the Present invention or, in the case of a waveguide, the index-confined waveguide region in accordance with the princ~.ples hereof; Figo 9 illustrates a ring configuration, wi h light energy travelling in bobh directions. Fag. 1,O illustrates'a quarter ring, with light energy again travelling in both directions. This configuration, in an active or a passive device, can be utilized to obtain a ninety degree change raf direction of the light path. Figs 11 and 12 illustrate ring laser or waveguide confi~gu~ations with tangentially coupled branches.
In Figs 1~ and 14, mul i-stripe lasers are shown as being coupled by'ring lasers, such as for phi a locking. The stripe spacing can: be substantial. Fig. 16 shows a similar arrangement, but with half-ring lasers, and Fig. 15 shows a series of coupled half-rings. Thelocking or tuning provide by these configurations'can result in enhanced longitudinal and/or transverse mode-operation. Fig. 17 shows curved 3 93/20581 ~, ~, J ~i ~ ~ ~ PC.'f/USg3/02$44 sections in an "S-bend" arrangement for providing an active or passive lateral offset of the optical beam path. Fig. 18 illustrates the surface configuration of a laser or waveguide which couples light in a single branch with four curved branches.
In a further form of the invention, a quantum well heterostructure is grown by metal-organic chemical vapor deposition [°'MOCVD"] on an n-type GaAs substrate. After n-type buffer layers of GaA~ ( ~0.5~um) and an Alo,z3Gao_~7As ( - l~cm) payer, an AIo.~Gao.SAs lower confining layer is grown to a thickness'of - l:Sum: The active region of the quantum well heterostructure is,then grown, and includes a -- 2100 waveguide region of undoped Alo.z3Gao.~7As with ~ 100 ~ undoged ~aAs quantum well (QW) grown inside the waveguide region - 700 ~, fram the lower confining layer. An upper confining layer of p-type Alo:eGao.2As 1,s grown to ~ thickness of about 3500 ~, a:~d a heavily doped p-type GaAs contact layer is grown thereon, the contact payer having a thickness of about 800 ~.
The position of QW is displaced from the center of the wraveguide for more effective overlap of the high-gain region with the optical mode, which is displaced towards the substrate due to the asymmetric confining layers. This asymmetry is purposely introduced to minimize the effects of thensurface of the laser crystal (located - 3500 ~ from°the 'waveguide) ~y shift~.ng the optical field toward the substrate.
The shallow upper confining'layer is desirable in order to m~:n~:mize current spreading, allow finer pattern definition, and'improved heat dissipation with the crystal mounted p side ''d~wn" and thus the. active region closer to the heat sink.
The thin upper confina.ng layer structure combined with ~.he p-type metallizatiorimay also serve to reflect light emitted toward the surface back into the crystal for improved device properties. [~ laser diode, fabricated using the described type of QW heterostruc~ure, and comprising a linear array of small rectangular 3.nternal coupled cavi ies delineated by oxidation of the high-gap AIxGaI~XAs upper, confining layer, . is described in NEl--rein, F.A. Kish, N. Holonyak, Jr., A.R.
~ f.r f-. 7 71F7 L
,rwF r~...:n : a0.". 4 s»-irrvT.i7=-Y f ' o i .°. aio r a . rt t f.
x,., < r- s .a .e, d .. , a . :. G- . . ..
. . . , r x.. . > ... ,. . . ° a.. « .. I : 'a _... _ ___.._ . ......,.. .... . .....,..... ~n,.. ....~x , . . . .. .... . , . ,. . a r. ,.-i ,.. . . ... . ... , ,~ ... ., w27 s. . x. m.. ,4., .. ,.w ., . ... a _ . , .<.. ., n_ , a.. s.. .:"~. , . . ,... 2~.e~.w w~ ~3i2oss~ . Pc-rms~3~o~a~a <r Sugg, M.J. Ries, S.C. Smith, J.M. Dallesasse, and R.D.
iBurnham, "Native-Oxide Coupled-Cavity AlxGa~_XAs-GaAs Quantum Well Heterostructure Laser Diodes°', Appl. Phys. Lett. 59, 2838, November 25, 1991.]
A laser diode array in accordance with an embodiment hereof is fabricated by patterning - 1Ø00 ~ of Si3N4 into repeated (masked) rectangular cav~.tie's (- 19 um long, - 4 Nm width, ~ 3 arm end-to-end spacing), which axe arranged lengthwise in two parallel stripes with - 1 Nm separation.
The exposed GaAs cap is then removed by chemical etching (HZS04:H202:H20, 1:8:80) and the crystal is placed in an open-tube furnade (supplied with a N2 carrier gas bubbled through HzO at 95°C) at 425°C for 20 min: As above.,,,, this process results in he transformation of 1300 A of the Alo.eGao.zAs upper confining layer to native oxide outside of the repeated cavities. The S13N4 is then removed in a CF4 plasma. The inset-of Fi.g: 19 shows a photomicrograph of the surface of the crystal after these processing steps. In order to increase the doping ~:n the rectangular GaAs contact areas, the crystal is sealed xn a.n'evacuated quartz ampoule and is shallow Zn-diffused (ZnAs2;source, 540°C for 20 min), The crystal is then lapped and Qolished (on'the substra~.e side) to a thickness of 100 ~t~n, and is metalli:zed with Ti°Au across the oxide and the g-type GaAs "contacts" and with Ge~Ni-Au on the n-type substrate side. The. sample is then cleaved into 250-500 um wide Fabry-Perot resonators, diced, and indiv~.dual dies are mounted'p-side downon Ih°coated Cu'heat sinks for continuous ,(cw) operation.
The unusual switching behavior of the resultant 2-D
stra.pe lasers i~s evident from the L-I characteristic shown in, Fiq: 19, which, after reaching ~ peak ire power of - 12.5 mW at (a) 115mA, decreases over 50$ in power from (a) 115 to (b) 150 _ mA, and simultaneously shifts its single mode operation (Fig.
20) to longer wavelength. At (c) 164 anA (Fig: 20) the single , mode operation of (a) and (b) has switcfied off, and in the braad spectrum of (c) 164 mA the resonances of the 19 Nm long minicavities are evident and marked with straws. In the broad .~. . , .!
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°rs.~ ~ .. .. . .. . .... ....,. .., .. " . ,.. " , ... _ ,..... .., ._ _ ..,..
~" ~i X312~581 , ~ ~ ~ ~ ~ P~'f/US93J02844 spectral region of weak stimulated emission, the minicavities tend to store photons, making the mode amplitudes (marked with arrows) smaller (c of Fig. 20). Note that the mode structure near the peak of the spectrum in (c) is sufficiently complicated that it is not evident that at (b) 150 mA the single mode laser operation has shifted fully, from (a) 115mA, to a minicavity resonance (e.g., bhw - 6 meV vs. DE ° 9 meV
fr~m resonance to resonance). It is evident from (a) to (b) to (c) in Figs. l9 and 20, however, that single mode operation is turning-off and mufti-mode operation, and weaker stimulated emission, is turning-on as the current is increased.
The unusual switching behavior of these 2-D array QWH
stripe laser diodes is much more evident in Fig. 21. The L-I
characteristic shows that, with increasing current, the laser turns on and off twice. As shown by the inset, which corresponds to the peak Uf the L-I characteristic (> 12 mW, 415 mA, anarked); single mode operation still occurs. In the valley region between 220 and 300 mA, broad-spectrum multi-moda operation similar to that of Fig. 20(c) occurs (data not shown). As the dashed line of Fig. 21 shows, the emission intensity an this region:-is at or somewhat above spontaneous em~.ssic~n. Most of the 2-D array lasers examined behaved as Shown 3.n Fig. 21.
The data of Fig. 22 show in some detail the behaviar of the di~de of Fig. 21 near he diamond°shaped point located at mA on the L-I characteristic. For comparison, at (a) 40 mA in the spontaneous regime ttie near field (NF) exhibits two intensity peaky expected of a twin linear array, with the spacing of ~.2 um agreding with the 2-D array width shown in he inset c~f' Fang. 19: At (b) 71 .mA the near field still, exhibits twin a.ntensity peaks; and the spectx'um a single main mode corresponding to the left NF peak and significant satellite longitudinal modes corresponding to the right NF
peak. A small current change of 1 mA (71 ~ 72 mA) produces abrupt switching: The satellite longitudinal modes (Fig. 22c) vana.sh abruptly, and simu~.taneously the right NF emission.
peak: It is clear that the strong coupling of one side of the »~~,,.
-,..: -,.. , "<.::;: , .; ".; .; ,.. :; :~: ,. . . . .. ; .... _,...
r"i h v ~Yt7 93120581 PCT/US93/02844 :, F
~~~2g~6 zz diode interferes, constructively or destructively, with the other side Uf the diode. Also it is evident that the manner in which the current has been partitioned among many identical coupled rectangular minicavities insures that the resonant operation between the cavities is favored.
The data of Figures 19-zz demonstrates a laser diode having two parallel linear arrays c~f.'small coupled rectangular cavities delineated by oxidation of the high-gap AlXGaX_lAs-GaAs QWH. The two dimensional laser array exhibits mode switching and switching in the L-I characteristic with increasing current. Depending on the bias position (current) on the L-I
curve, the laser operates in a sinc3le longitudinal modes in or near the spontaneous regime. For example, the resonances of the minicavities are evident in the spontaneous spectra in spite of small heating effects and carrier-induced changes in dielectric properties. As above, optimization of the geometry, sire, and number of the minicavities, and their coupling, may result in improved behavior of these devices.
In another form of the invention, described beginning with Fig. z3, the QW hetero~tructure crystal is substantially the same as the one described above in conjunction with the previous device. In'the present embodiment; the laser diode array 3s fabricated by first depositing 1000 ~ of Si3Na on the crystal surface, which is then patterned into end-to-end reputed (masked) rectangular cavities (minicavitzes, 6 pm wide and 19 p~m long on 2zpm centers) arzanged lengthwise.
Next, 6 ~m p~~otQresist (PR) Stripes ire patterned - 5 ~m away from the linear array of mina.c~vities: The patterned PR and Si3Na then serve as a mask for the chemical etching (HaSOy~~2Oa:H20, 1:8:80) of the GaAs cap layer, leaving the ~iigh-gage AlXGa1_XAs exposed outside of the patterned regions , The PR is then removed and the sample is placed immediately in an epen°tub~ furnace (~z5°C) supplied with H20 vapor in an N2 carrier'gas for z0 anin. Again, this process results in the conversion of the exposed ~~.gh-gap A1XG~1_XAs to a low-index ( n 1.6) insulating nativeoxide looted ~ 1000 A above the QWH
waveguide region. The patterned 5i3N4 and unetched GaAs ~n~r 93f20581 - ~ ~ ~ ~ ~ ~ ~ P~lffllS93/02844 regions are unaffected by this treatment. The patterned Si3N4 is then removed in a CF4 plasma. The inset of Fig. 23 shows the surface of the device after these processing steps. Next the sample is ~n-diffused (540°C, 20 min) to increase the doping in the contact regions (labeled "GaAs" in Fig. 23)..
The crystal is then lapped and polished to a thickness of --125 um and, again, metallized over the entire top surface with Ti-Au for p-type contacts and with Ge-Ni-Au for n-type contacts. Finally the crystal is cleaved, diced, and individual dies mounted on In-coated copper heat sinks for continuous (cw) operation.
The large amplitude switching praperties of the single-stripe laser coupled to the active linear array are shown by the 300 K continuous (cw) L-I curve of Fig. 23. The laser threshold current is 32 mA, and laser operation persists;up to a current of 168 mA. At this po~.nt the diode switches abruptly from stimulated emissi~n, ON (19.6 mW/facet, undoated), to the spontaneous regime, OFF (0.4 mW/facet, uncoated). This behavior corresponds to a large ONaOFF pawer ~rati~ c~f 4g: These ara inherently nanlin~ar devices, and exhibit bistable operation with a large hysteresis. As the current is decreased (returned) to 123 mA, the diode switches back from the spontaneous regime, OFF, to the simulated segue; ON. For further current ~.ncreas~ beyond 168 mA,°after the dev3;ce has switched OFF with increasing current, only a slight increase in the spontaneous output is observed until fai3ure at 18? mA. 'We mentir~n that, although hysteresis occurs in the LaI characteristics, no hy~t~resi is observed 3n the current versus voltage (i-V) ch~racteri~stics of these, devices: ;;
The L,-T characteristic (cw 300 K) of another diode exhibiting similar switching behavior is sh~wn in Fig. 24.
The laser threshold current is 2? mA, with the device exhi;bit'a,ng essentially a single ~ 5.5 pm~wide Gaussian near-f3eld pattern (data not shown). This intensify pattern corresponds to laser operation of the ~ 6 um wide uniform stripe (inset of Fig. 23); which is expected to reach WO 93/205S1 PC'f/U593/02R44 i~ ~~ -a ~~ ~ s~ :~
threshold before the segmented linear array. From gain-loss considerations, the linear array with its repeated unpumped absorbing sections should have a higher laser threshold.
Throughout the entire operating., range, a single-stripe near--field gattern persists, i.e.; only very weak output is observed from the linear array~.portion of the device. The near-field pattern (300 K, cw.,bperation) at (a) 99 mA just before the switching from ON to OFF, i.e., before switching fram single-mode stimulated emission to spontaneous emission, is shown in the inset of Fig. 24. Similar to operation just abave threshold, only a 5.5 Nm Gaussian near-field is observed at significant amplitude. After the diode switches OFF at (b) 100 mA, no pattern is observed on the same sensitivity scale of the CCD detector. However, at higher sensitivity (11.3x), emissi.an from the same aperture (- 5.~
Nm) ~:s observed (b'). Thin near-field pastern also is observed as the laser is switched back from OFF to ON. These data indicate unambiguously that only the uniform laser stripe prouides much of the optical'output of the system. The side-coupled linear array serves mainlx to effect the interferences and swit~hing~ ON°OFF, and does not contribute primarily to the optical output;
Further understanding of the operation of these diodes is obtained by'examining the output spectra. Somewhat above the threshold at 30 mAr the diode of Fig. 24 operates in a single longitudinal mode (~ 8353 ~. data not shown). This behavior eantinues to the peak of the L-I curve'of Fig. 24 (63 mA), where the single mode operation "hops" to longer wavelength g367 1~, data not shown). :Throughout the entire stimulated emission operating regime (,30 -~ 99 mA), the output occurs in a well developed single longitudinal made. For example, at 63 mA the 'laser exhibits aside-mode suppression of 29 dH. The :mode hopping, and corresponding structure in the L-I curve (Fig. 24), is attributed. to the interaction (interference) of the single laser str~.pe with side-coupled active linear array and its resonances and stop hands.
Longitudinalmode spectra in the higher-current switching "'i> 93/2~158a PCT/US93l02844 regime of the device of Fig. 24 are shown in Fig. 25.
Immediately before switching from ON (stimulated emission) to OFF (spontaneous regime), (a) in Fig. 24, the laser operates in a single longitudinal mode at ~, w 8415 A, which is shown as (a) in Fig. 25. When the diode switches OFF to the spontaneous regime, (b) of Fig:. 24, the longitudinal mode spectra appear as shown in (b) of Fig. 25. At this point, the output consists of the spontaneous emission of the single stripe laser (group of lower energy modes) and the linear array (group of higher energy modes). The coupling of the linear array to the stripe laser leads to interference. The resonances of the minicav~ities of the linear array are apparent (clearer in the laboratory data) at higher energy in the spectrum of Fig. 25(b) and are marked with arrows. The spacing of these resonances (~~1 - 50 .~) corresponds to the -19 um minicavity length shown in the inset of Fig. 23. It is noted that the output in the OFF spontaneous regime (Fig.
25(b)) differs s~.~nificaratly from that observed in the spontaneous regime below laser threshold (< 27 ~1), where only the longitudinal mode output of the single laser stripe (group 'of lower energy modes) is observed (data not shown).
Tg~~ described switchihg and storage are fundamentally different from previously ~~Qo~ted switching laser devices.
The 0~-O~'F switching behavior occurs in this embodiment sn a single unbroken or uninterru~ated laser stripe. The switching behavior is owing to the influence (via sidewise coupling) of an active 1'inear army. The switching and bistability is effected by the periodic structure ~f the,linear array (see ~i.G. Wiriful, J,H. Marbuxvger, and'. Carmir~s Appl. Phys. Lett.
35,; 3,79 ( 1979) °,, Jr ~e and M: Cada, IEEE J. Quantum Electron.
QE°27, 1182 (1991)) and the obvious inhomogeneous carrier distribution, aa~d i.nhomogeneous operation, resulting from the nativ~--oxid~ patterning of the gray.
Thus, this embodiment sets forth a new form of optical switching element in which a conventional single-stripe laser is side-coupled to a linear array of coupled minilasers. The resulting many-eleanent twin-stripe laser is easily realized via native-oxide device processing. The planar devices exhibit large hysteresis in the L-I curve, with large amplitude switching from the peak of the stimulated emission regime (ON) to the spontaneous regime (OFF). Changes in the coupling, e.g., the spacing between the laser stripe and linear array and between the array elements, a.nd in the geometry of the structure should improve the switching l;~ehavio:r of these lasers. Independent control of the current (carrier population) in t:he single laser stripe in the linear a~°ray, e.g., via a third terminal electrode, should allow control o:F the switching behavior, and other variations are possible.
Fig. 26 shows a part of the surface of the device described in conjunction with Fi~~ures 23-25, and is used as a reference to show the cross-sections used for the illustrations of Figures 27-29. In Figure 26 the stripe is labelled 1210 and the minicavities, or porticans thereof, are labelled 1221-1225. The cross-section 13-13 is taken through the stripe 1210 and an adjacent minicavity 12'..4. The illustrated layers, which were previously de:acribed, include the bottom contact metallization 1250 (it bein<~ understcaod throughout: that references to "bottom"
or "top" are ?°or ease c:~f description, as the device may be mounted and used in an~,r desired orientation), followed, in ascending ordE~r, by the:':ntype GaAs substrate layer 1255, the n-type GaAs buff=er layer 1.258, the n-type A1o.23Gao.,.,As buffer layer 1260, the n-t~rpe Alo.SGa.a.SAs lower confining layer 1262, and active active region 1:?70 that includes a GaAs quantum well layer 1271 between waveguide layers 1273 a.nd 1275 of undoped Alo.z3Gao..,~As. r~.bove the active region is the upper confining layer 1278 of p-type: Alc,_BGao,;,As. The layer thicknesses may be, for example, as previously indicated above for the experimental device. The p--type GaAs contact 1281 and the p-type GaAs contact 1283 respectively define the contact positions of the stripe 1210 and the minicavity 1226: of Fig. 26. The native oxide is shown at 1291, 1292, and 1293, a.nd, in this example, has a thickness of about 1300 ~ The oxide a:Lso extends somewhat under the GaAs contact regions. The top (p-side) metallization is labeled 1240.
The diagram of F:~.g. 28 illustrates the cross-section defined by arrows 14-14 of Fig. 26. In this view, only the contact region 1281 of t:he stripe 1210 is visible. The oxide (1294) extends continuously to the :right of the stripe.
Fig. 29 shows the cross-section defined by arrows 15-15 of Fig. 26. This view is longitudinally through the minicavities, with two minicavities being shown between three oxide regions 1296, 1297, 1298. The :longitudinal dimension of the contact 1283 is seen in this view.
In the illustrated embodiments, operation may be "two terminal", such as by applying the electrical potential between the bottom electrode and the top common metallization. The device can alternatively be made for operation as a three terminal or multiple terminal devic::e. For example, Fig. 30 illustrates a device having a stripe 1610 with mini cavities 1620 on both sides, each line having a common metallization (represented by the joining lanes between minicavities) and its own terminal, so that the device can be operated with four independent terminals, with three terminals (~:or example, t:he terminals of only two adjacent line, and the bottom terminal, or with the two outside lines in common) or twc:a terminals, with all three lines in common. [In this diagram, and in other diagrams hereof where a plan view of the minicavity and/or stripe configuration is shown, the underlying structure is of the type previously described, the fabrication m<~sking pat::t~erns being consistent with the structures illustrated.) Fig. 31 ~.llustrates a two-dimensional array o.f adjacent liner of mini cavities, with individual terminals coupled with the mini cavities. It will be understood that a terminal can be coupled with any desired combination or group of cavities or m_Lnicavitie:~s.
The previous embodiments illustrate straight line minicavity and stripe c.~.onfigurations, but it will be understood that the principles of the invention also apply to minicavities and stripes arranged in curved line r.."' 1~J0 93/20581 PCflUS93/02844 <:' configurations and arrays. Figures 32-39 illustrate some representative embodiments (with bottom electrode and various possible top electrodes not shown). In Fig. 32 there is shown a ring laser which is divided into curved minicavities 1815, to obtain the types of effects described in N. El-Zein, F.A.
Dish, N. Holonyak, Jr., A.R. Sugg, M.J. Ries, S.C. Smith, J.I~.
Dallesasse, and R.D. Hurnham, "Native-Oxide Coupled-Cavity AlXGa1_xAs-GaAs Quantum Wel l Heterostructure Laser Diodes °' , Appl. Phys. Lett. 59, 2838, November 25, 1991, in the context of a ring laser. [It will be understood throughout that any of the curved configuratibns need not be precisely circular, that any desired portions of rings or curves can be used, and that the rings or portions thereof can be cleaved at any desired position to obtain one or more outputs.] Fig. 33 illustrates two concentric ring lasers, each divided into minicavities 1915, so that lateral coupling can be achieved, as first described in conjunction with Figures 19-22 above for the case of straight line arrays. I~ Fig. 34, one of the concentric rings 2010 is continuous, and the other is divided into minicavities 2015, to ob~air~ a curved version of the embodiment described in conjunction with Figures 23-29. Fig.
35 i.I:lu~trates a ~a.reular configuration with sector-shaped minicavities 21.5 separated by xa$ial "spokes" of native oxide:
Fic3. 36 shows s rung laser 2210 laterally coupled with a stripe laser 2220, In Fig. 37 the ring is divided into mihacavl,ties 2315, end in Fig. 38, the stripe is divided into minicavities 2415. In F3.g.-39, both the ring and the stripe are divided into minicavit3es (2515 and 2525, respectively).
It w~.ll b~ understood that'the previous indicated variations with regard to numbers and types of striges, array elements, and/or terminal connections are applicable to embodiments with curved lines or minicavities.
Fig. 40 illustrates, in cross-section, a form of the invention 4ahich couples cavities with d~.fferent longitudinal mode characteristics, in the form of a vertical cavity laser array. vertical cavity lasers are well known in the art (see, ~~~~~'~ 93/20S81 ~ ~ ~ ~ ~ ~ ~ P~.°T/US93~a2~4 for example, H. Soda et a1. Japan J. Appl. Fhys. 18, 59 {1979) and K. Iga et al., Electron Lett. 23, 134 (1987), and include, as in the lefthand unit of Fig. 40, a bottom contact 2610 on a substrate {e.g. GaAs 2605), an n-type superlattice 2620, an active region 2630 that includes a quantum well layer 2632 between waveguide layers 2634 and 2636, a p-type superlattice 2640, and one or more contacts 2650. Various materials may be used. As one example, the superlattices may comprise a number of alternating layers of AlAs and GaAs [ or AlxGa~_XAs and AlYGaI_ yAs, x~y, gr combinations of AlXGa1_xAs and conductive dielectric stacks ( a . g . Ti02/Si02, ZnSe/CaF2 ) ] , and the act~.ve region may comprise . Aln.lGao,9As { or GaAs ) waveguides layers with a GaAs { or Ino,lGao.9As ) quantum well layer with total thickness of typically - 250 ~. The contacts at the surface may comprise for examgle Au or Ag with a standard (e.g. Ge-Au) backside (substrate) side contact. A two-dimensional vertical davity coupled array of such devices is described for example in I~.G. Deppe, J.P. Van der Ziel, Nasesh Chand, G.J. Zydzik, and S,N.G. Chu, Appl. Phys. Lett. 56, 2089 (1990). Briefly, in operation, the multiple reflections from the superlattice interfaces provide a relatively short effective cavity length (typically ~ Sum) from the limited thickness device, and the cavities are coupled wanescen~ly:
In accordance with a feature of a form of the inven-~ion, adjacent vertical cavity laser aanits are pr~v~.ded with active reg.ion~ of d~.fferent thickn~sses, as illustrated in Fig. 40, where the active region 2630' is substantially thicker than he active region thickness of its neighboring unit. In the illustration, the quantum well layer is c~ntinuous through adjacent active regions, although this is not necessary., The t.v name ~s'true of the superlattice layers above the active region 2630'. Variation o~-the upper ~r lower superlattice thidkness laterally varies the effectide lateral mirror areflecti.vity. Such variations may also be employed with a uniform thickness active region to achieve local variations in the cavity structure. In addition to the Standard evanescent coupling, other schemes may be employed to'couple such devices W~ 93/20581 ~'Cf/US93/02844 'i~.,.~~;
such as varying the mirror (e.g. superlattice) angles to directly reflect some of the light from one cavity into adjacent cavities.
There are various other techniques that can be utilized to obtain adjacent vertical cavities having substantially different effective vertical cavity lengths, so that lateral coupling thereof can be advantageously exploited to obtain properties such as switching, bistability, andlor tuning. [As used herein, vertical cavities having substantially different effective cavity lengths means that the cavities have substantially different longitudinal mode characteristics, as previously defined.] For example, the active regions of adjacent units may comprise different materials. Another alternative is to provide adjacent units with superlattices of different thicknesses, or superlattices of different configuration: An exam~lQ of the latter would be to provide one unit having superlattices of alternating 100 ~ GaAs and Al~s layers and the other unit with superlattices having alternating 200 ~1 Gags layers and 100 A AlAs layers, which results an different effective cavity lengths.
Fag. 41 illustrates a checkerboard-type array of such;
units, with the cross-hatched units representing the units having tie thicker active regions. The array can be operated as a two terminal device, with the top contacts coupled in common, and potential applied between the top and bottom c~ntact, or can be driven as a three termznal or multiple terminal device with wparate connections to contacts.
Various other shapes and configurations in one-dimension or wo~-dimensions can be utilized. 'In one example of fabricating an arrays the growth may; be terminated at the active region (2630'). A two-dimensional pattern (e.g,. a checkerboard) is then masked using standard photolithography techniques and the sample is subjected to chemical etching to remove a portion of he active region (2636) in the unmasked area. This process results in a lateral variation in the active layer thickness.
The photoresist is then removed and the upper p-type supperlattice is grown on the patterned active region, such as ,, ». . ,...-. .. .; . .. -,:. , ;; ., ,...:. ..::.., >..:... : . . ..
P~'/US93/02844 by MOCVD or MBE (molecular beam epitaxy). A circular (dot) metallization can then be applied on the upper p-type supperlattice for contact and reflectivity purposes.
The invention has been described with reference to particular preferred embodiments, but variations within the spirit and scope of the invention will occur to those skilled in the art. For example, while the aluminum-bearing III-V
semiconductor material aluminum gallium arsenide has been described in embodiments hereof, it will be understood that the devices and technique hereof can employ other aluminum-bearing IIr-V semiconductor materials, such as indium aluminum gallium phosphide, indium aluminum gallium arsenide, or aluminum gallium phosphide. [Reference can be made to F. ICish et al:, J. of Appl: Phys. 71, T5 March, 1992.] Also, it will be understood that the indicated confining layers can be;
multiple layers, one or more of which comprises the aluminum-bear~.ng III-V semiconductor material. It will further be unders~odd that devices can integrate the aluminum-bearing III-V semiconductor material (from which the native oxide is formed)ywitka other nQn-III--V semiconductor materials. It will also b~ understood that laterally coupled cavities as described herein can be utilized for tuning as well as indicated functions such as switching. The lateral coupling described herein is particularly facilitated by using the native oxidE formed in an aluminum-bearing III°V semiconductor material to separate laterally coupled cavities. In some of the-configurations hereof, 5:ess gre~erred cavity definition can alternatively be implemented by,tedhniques such as 'muZ:'~iple regrowths/overgrowths, etch and xegrowth/overgrowth:, r~,d~geiFormation, ridge formation and overgrowth, impurity induced layer disordering, and proton implantation.
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9 93/20581 PCffL1S93/02~4 SUMMARY OF THE INVENTION
An aspect of the present invention is directed to improved techniques and devices employing, inter alia, an aluminum-bearing III-V semiconductor material and a native oxide of aluminum that is formed in the semiconductor material.
There has been previously disclosed a technique of forming a high quality, stable, and compact native oxide layer from an aluminum-bearing Group III-V semiconductor material.
[See Dallesasse et al., Appl. Phys. Lett. 57 (26), 2844-6, 24 December 1990; Dallesasse et al., Appl. Phys. Lett. 58 (4), 394-396, 28 January 1991; Dallesasse et al., Appl. Phys. Lett 58 (8), 834-836, 25 February 1991; and Sugg et al., Appl.
Phys: Lett 58 (11), 1199-12p1, 18 March 1991.] The technique comprises exposing an aluminum-bearing Group III-V
semiconductor material to a water-containing environment and a temperature of at Least about 375°C to convert at Least a portion of the aluminum-bearing Group III-V semiconductor material to'a native oxide. The thickness of said native oxide formed thereby is subs antially the same as or Less than the thickness of that portion of said aluminum-bearing III-V
semiconductor material converted int~ the native oxide. The native ox~.de Dyer thus grown is denser and more stable °than oxide iaye~~ formed from previous methods, meaning, for ' example, that they do not degrade under conditions of normal use aa~d atmospheric ex~osur~e. Further, the native oxide was demonstrated to exhibit improved operating and performance characteristics, f~r exa.mpTe with regard to metalli~ation adherence and dielectric properties. The native oxides were described as being useful. in lasers, transistors, capacitors, waveguides and'in other electrical and opto-electrical devices: Anhydrous oxides of alumin~zm were noted to exhibit a relatively low index of refraction (less than about 2.0) and index of refraction can be used to distinguish the anhydrous oxide forms'from the higher index hydrated oxide forms that are generally unsx~itable for semiconductor applications due to r~r,y~
~~ 9mzoss~ Pcrms9~ioz~a b::.~
~~~z~gs s properties such as expansion and instability.
A form of the invention is directed to a method of making a semiconductor laser having a light path that is at least partially curved, and comprises the following steps: forming -a layered semiconductor structure comprising an active region between first and second semiconductor confining layers, the .
first and second semiconductor confining layers being of opposite conductivity types, and said first semiconductor confining layer being any aluminum-bearing III-V semiconductor material; applying a mask pattern over said first semiconductor confining layer, the pattern including a stripe that is at least gart~.ally curved; exposing unmasked portions of the first semiconductor confining layer to a water-containing environment and a temperature of at least 375 degrees C for a time sufficient to form a thick native oxide of aluma.num iz~ said first semiconductor confining layer; and coupling respective electrodes with said first and second semiconductor confining layers. Generally, the active region includes at least a waveguide layer and a quantum well layer, and the respective electrodes are coupled to the semiconductor confining layers through further respective semiccanductor layers. The aluminum-bearing materialmay comprise, for example, AlxGal~XAs, where x 'is at least 0.3. Generally, a higher aluminum fraction, for example x = 4:7 or greater-will be used to facilitate the thick oxide growth rate, which also depends'an temperature.' A temperature of at least about 450 degrees C is generally preferred. For'a ring laser, the time of exposure may be selected to have said native oxide extend through at least most o~ the thickness of said first confining layer, a,nd possibly; through the entire; thickness of said,, first confining layer. Another form, of the a.nvention comprises a sem3.conductor passive optical waveguide, having a light path which is at least partially; curved, tl~a~ employs a thick na~:ive oxide of aluminum. , In a further form of the invention; two linear arrays of end-coupled cavities (called minicavities) of a QWH
semiconductor laser are defined by a native oxide of an _ ~~.3~~~b ',~~ 19312n~~' PCT/US93/02&~4 aluminum-bearing III-v semiconductor material and are arranged side by side to obtain a two dimensional array, with resultant lateral coupling between the linear arrays. The two dimensional array exhibits mode switching and multiple switching in the light power (L) versus current ,(I) characteristic (L-I) with increasing current.
In another form of the invention, a stripe laser is transversely coupled (or side-coupled) with a linear array of end-coupled minicavities. Bistability and switching are demonstrated in the Light versus current (L-I) characteristic of a native-oxide-defined structure of this type. The device, witY~'internally coupled elements and the current partitioned among the dlemants, exhibits a large hysteresis in the L-I
curve, with switching from the stimulated to the spontaneous regime occurring over substantial power (light) and current ranges. The liner array of "minilasers°' and its resonance modulates arid switches the stripe laser operation.
In acdordance with a further definition of the invention, a semiconductor laser device includes first and second adjacent laser units formed on the same semiconductor substrate, daCh of the units including a laser cavity. The laser cavity of the first unit has a substantially different longitudinal mode selection characteristic than the laser cavity. of said second unit. [As used herein, substantially different longi udinal mode s~c ion charactera:stics means that the first unit has a cavity mode ~g~cin~ that is at least 1t~
percent greater that the cavity mode spacing of the second uwit, andlor a primary emission wavelength that is at least 50 1~ greater than the:primary emission wavelength of the second unit.,) Meana are provided for applying energizing signals to the first end second units to obtain laser emission from the unitsand lateral.coupling betweem the cavities of the units.
In an.embodim~nt of the invent3oa~ there is disclosed a semiconduc or laser device: that includes a semiconductor active region dispersed between first and second semiconductor confining layers. An electrode array hay electrode elements coupled with the first confining layer. [As used herein, the WO 93l2~5~1 PCTh1JS93/028n4 ~, term "electrode elements'° is intended to include electrical contact regions (e. g. highly doped semiconductor regions) that contact an underlying semiconductor structure.] At least one opposing electrode is coupled with the.second confining layer. .
The electrode elements of the array are~~spaced apart and form a two-dimensional array that incluc~ea"a plurality of electrode .
. ,. .
elements along a line and at leas~t~~one further electrode element laterally spaced from the electrode element of said line. Means are provided for applying electrical signals between the electrode elements and the at least one further electrode element and the opposing electrode to effect light emission in the active regions defined under the plurality electrode elements and at least one active region defined under the at least one further electrode element, and to effect lateral coupling of the emissions.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.
~ ~ 3 ~ ~ 8 ,~ ~~riu~~3io~s4a g~~zosg~
HRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a scanning electron microscope image of a stained cross-section of a device in accordance with an embodiment of the invention.
Fig. 2 is a graph of cw light output (both ring ends) versus current for a device in accordance with an embodiment of the invention, and shows, in an inset, a surface photograph of the device.
Fig. 3 is a graph of pulsed light output (both ring ends) versus current for a device in accordance with an embodiment of the invention and shows, in the inset, single mode operation.
F~.g. 4 shows near field image plots for the Fig. 3 device.
Fig. 5 is a graph of pulsed light output versus current for another device in accordance with the invention, the inset slowing the device geometrlr.
Fig. 6 is a simplified cross-sectional representation of a semiconductor laser diode device in accordance with an embodiment of the inventioaa.
~,i~. 7 is a simplified cross-sectional diagram of another semiconductor laser' device in accordance with a embodiment of the invention.
Fig. 8 is a simplified ~r~s~°sectional representation of ~ s;emiconductor optical: wav~eguide in accordance with an embod3:ment of the ixwention. ._ Fig. 9 i.ll:us~rates the surface configuration of a ring laser novice.
,Fig. 10 illustrates the surface configuration of a ~u~rter-ring laser or waveguide.
Figs I1 and 12 illustrate the surface configuration of ring lasers or waveguides with different branch coupling arrangements.
Figs 13 and 14 illustrate the surface configuration of mufti.-stripe lasers or waveguides with ring coupling.
~'ig. 15 illust~cates the surface configuration of a series '..1F'.,~a '.'.7,.
.., ...,,,. .. ... ~. . ... ... , ..a,.:.. .... ...,.
._....., . "... .. ,... .. Y .... ..~._.~ , . ". ,., , ;.,r« ..... ,.... , ".a ,. . . ..... ...... . . , .., . . ., .
CVO 93!20581 . P~CflU593102844 , ' ~~~.~~9~~
la of coupled half-ring lasers or waveguides.
Fig. 16 illustrates the surface configuration of a multi-stripe laser or waveguide with half-ring coupling.
Fig. 17 illustrates the surface configuration of a curved .
laser or waveguide in which the light path becomes laterally -'~,:~
offset. , Fig. 18 illustrates the surface,-'configuration of a laser or waveguide which couples light in 'a single branch with four curved branches.
Fig. 19 shows continuous (cw) 300 K light output (single facet) versus current characteristic (L-T) of a native-oxide-defined two-dimensional (2-D) coupled-cavity AlXGa1_XAs-GaAs QWH
laser array (uncoated facets, - 304 pm total cavity length).
The threshald is ~5 mA~ and the power peaks at - 12.5 mW (115 mA). The inset shows a surface photomicrograph of the unmetalli~ed 2-D twin lineax array. The rectangular minicavities are 4 Nm wide, 19 Nm long, and separated end-to-end by - 3 um. The two ccaupled linear arrays are separated by ~ um:
Fig. 24 sows longitudinal mode spectra (cw, 300 K) of the diode of F'ig. I9 at (a) 115, (b) 154, and (c) 164 mA
(points shown on the L-I o~ Figa 5). The single mode behavior at (a) 8280 ~'(1I5 mA) shif s to 8313 ~ at (b) 150 mA. At (c) 164 mA single mode operation has switched off and the a res~nances of the 19 arm long minicavities aye evident and marked with arrows. The mode spacing i~s ~~ 50 ~,, which agrees with the 19 um minicavity length.
Fag. 2~ shows he li.ght'output versus current characteristic (L-I-, cw, 300 K) of a diode with the same ge~me~try as that shown in Fig. 19. The diode turns on and off twice as the current is increased. The dashed Sine shows that the emission intensity in the valley region is in the range of spontaneous emassion. The inset shows single mode behavior (840 ~) persists to at least 415 mA (- BT~n), and is marked with a solid dot on the L-I curve.
Fig. 22 shows the near field (NF) emission patterns and longitudinal mode spectra o~ the diode of Fig. 21 near the r , : 93/20581 ~ 1 ~ ~ ~ ~ ~j ~'CfItJS93102844 diamond-shaped point at ~ 70 mA on the L-I characteristic. At (a) 40 mA (spontaneous regime) the near field (NF) shows two intensity peaks of the twin linear array, with the width of 9.2 pm in accord with the geometry shown in Fig. 19. At (b) 71 mA the NF is twin lobed, with the device ogerating single mode (8260 ~) but with also strong satellite longitudinal modes. At (c) 72 mA, the. NF emission from the right stripe disappears abruptly, with also an abrupt disappearance of the satellite longitudinal modes:
F~.g: 23 shows the continuous 300 K light output (single facet, uncoated) versus current characteristic (L-T) of a native-oxide-defined AlXGaI_xAs-GaAs single laser stripe side-coupled to a linear array of end-coupled minilasers. The laser threshold is 32 mA, with abrupt switching from the stimulated (ON} to spontaneous (OFF) regime occurring at 168 mA. The device exhibits bistability, switching back shargly from the spontaneaus (OFF} to stimulated (ON) regime at 123 mA: The diode geometry (prior to metallization) is shown in the inset ahd consists of a single ~ 6 ~m-wide laser stripe side-coupled (°- 5 pm away) to a linear army of end-coupled min3~lasers (6 ~m-wide, 19 p~m long and 22 pm centers).
Fig. 24 shows the continuous 300 K light output (single facet, uncoatedj versus current'characteristic (L-I) of a device of the form of Fig. 23 (inset). The laser exhabi~ts a :threshold of 27 mA, with switching and bistability occurring "in the range 96-100'mA. Throughout the entire operating range, the device output consists essentially of a ~ 5.5 um 'Gau~sian near-field (NF} pattern fr~m the single continuous stripe of Fig. 23. The NF:pattern is hown dust before swific,hing at ( a ) 99 .C inset ) . After, switching at ( b ) .100 .,,,. ,.
mA, essentiall~,r no output is observed; on a higher sensitivity scale (b'}, however, the same NF pattern is revealed.
Fig. ~5 shows longitudinal mode spectra (cw, 300 K) of the diode 4~ Fig. 24 corresponding to single mode stimulated emission (ON) at (a} and switched JFF to spontaneous emission at (b). Single mode laser=operation ~.s observed from thgeshold (~ 27 mA) to (a) ~9 ~~ with output at large '~~3~Zg~6 W~ 93/2051 _ PCf/~J~93/02~84'";.~;
amplitude from only the continuous stripe of the diode (left stripe in Fig. 23 inset). In the spontaneous emission OFF
regime (b) the lower energy group of modes corresponds to the laser stripe and the higher energy group of modes to the linear array (see inset of Fig. 23).
Fig. 26 is diagram of a portion of~'the top surface of the device described in conjunction with lEi~ures 23-25.
Fig. 27 is a cross-sectional diagram (not to scale), as taken through a section of the Fig. 26 device defined by arrows 13-13.
Fig. 28 is a cross--sectional diagram (not to scale), as taken through a section of the Fig. 26 device defined by arrows 14-14.
Fig. 29 is a cross-sectional diagram (not to scale), as taken through a section of the Fig. 26 device defined by, arrows 15-15.
Fig: 30 illustrates a tyro-dimensional array that can be operated using two, three, or four terminals.
Fig; 31 illustrates a twa-dimensional array with terminal dantrol in both dimensions.
Fl.gures 32-35 show plan views of ring lasers including a~inicavities in curved configurations in accordance with embodiments of theinvention.
I'igur~s 36-39 show plan views of adjacent rang and w straight line lasers, ~aith'transverse caupling between laser cavities, and including configurations where the ring, the straight line, or laoth, are divided ir~t~ minicavities.
Fig. ~0 is a cross-sectional diagram (not to scale) of a vertical cavity laser device w~:th transverse coupling between adjaqentt laser cavitie having different made selection, characteristics .
Fig. ~1 illus rates a two-dimensianal gray of vertical cavity laser units of the type illustrated in Fig. 40.
~~ ~:~ ~ 9128581 ~ 1 ~ ~ ~ ~ ~ P~.°f/US93/02844 DETAILED DESCRIPTION
In an example hereof, a quantum well heterostructure is grown by metal-organic chemical vapor deposition ["MOCVD" -see for examgle, R.D. Dupuis et al., Proceedings of The International Symposium on GaAs and Related Compounds, pp. 1-9, Institute of Physics, London, 1979, and M.J. Ludowise, J.
Appl. Phys., 58, R31, 1985] on an n-type GaAs substrate.
After a GaAs buffer layer, an Alo,BGa~.2As lower confining layer is grown to a thickness of about 1 pm. [The confining layers are also sometimes called cladding layers.) The active region ~f the quantum well heterostructure is then grown, and includes symmetrical Alp.zSGao.~5As waveguide layers, undo~ed and of thickness about 750 ~l eachr on either side of a GaAs quantum well of thickness about 100 A. An upper confining (or cladd~:ng) layer of p-type AloaaGdo.aAs a.s grown to a thickness of about 0e6 ~m~ and a heavily doped p-type GaAs contact layer is grown thereon, the contact layerhaving a thickness of about X00 d~. In this example, fabrication of a laser begins with the patterning of about 1000 l~ of Si3N4 into rings [ 25-dam wide anraulu~250 hum inside diameter (IDj, 300 um outside diameter (OD)], The Si~~l4 rings serve as a mask for the chemical etching ( HZS04: HZOZ : H20, l s 8 : 80 ) of the contact layer, thus leaV.in.g the Alx~al_xAs upper confiniing layer exposed i.nsi~e and outside of the masked rings: The sample is then placed in an open tube furnace, supplied with HzO vapor and NZ, at 450°C for 35 minutes. Th~.s process xesults an the coxwersion of the upper c~nfining layer (aahere exposed) ~o a native oxide having an index of refraction of aDaout 1.6. In this case, at the ring edges the.;,oxide extends downward through the entire upper confining layer as shown in Fig. 1 by the scanning electron microscope (SEM) image of a stained crow section. The oxide is deeper ~t the ring edge than beyond (~o the right in F'ig.
1):~ This effect may be a result of changes in H20 adsorption, O/H diffusion, or s ress induced by the presence of the masking stripe: ~ The oxide ~arofile is fairly isotropic, however, extending laterally essentially to the same extent as r r. ~~
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~~~986 it does in depth. Transmission electron microscope (TEM) , images of similarly oxidized crystals indicate that some oxidation (about 200 A) of the underlying Alo.z3Gao.~~As waveguide region occurs. Thus, the low-index native oxide extends into -the waveguide layer, creating large latexal index steps for sidewise optical confinement and waveguiding. Calculations .
based on propagation in a 4-layer slab waveguide (see G.E.
Smith, IEEE J. Quantum Electron., QE-4, 288 (1968)] for this deep oxide edge indicate an effective lateral index step greater than 0.05. For this example, structures with the native oxide located about 1000 A vertically away from the waveguide result in an insufficient index step for ring oscillation. However, as discussed further below, effective lasers can be made with lateral native oxide that extends only partially through the confining layer.
After the indicated oxidation, the Si~N4 masking rings are selectively removed in'a CFA plasma, resulting in a self-aligned geometr~r. The-'sample is then Zn-diffused (540°C, 20 min) to improve the contacts and metallized with Ti-Au for the p-hype contact and Ge-Ni-Au for the n-type contact. In an example hereof the rings are then cleaved ~:n half (or on a chord through the annulus) and the remaining three sides of the crystal are'saw cut (SC) to prevent resonance across the edges. The sur~ace'of a typical half-ring laser diode hereof after metallization, cleaving, and sa~ring is shown in the inset of Fi:g. 2:
The 300 K cw bight versus currant (~-~) curve of a typical half-ring laser diode hereof is shown in Fig. 2. The threshold current is 105 mA (~° 890 A~cm~). The curve is linear above threshold with a total external differential .. ' I I, quantum efficiency (~ of 49g) and a total output power (both ends of the'half ring) exceeding 40:mW. The pulsed threshold (2 us pulse' width, -0.5~' duty circle) of this diode is 78 mA.
The pulsed'(2 Ns, a.5~ duty cycle) L--I characteristic of a moderate guality half-ring laser diode, fabricated as above, is shown in Fig. 3: The diode threshold current is - 103 mA.
Longitudinal, mode spectra show well defined modes, with .r 9312058 ~ I ~ ~ ~ ~ ~ PC1'/US93/02844 single-mode operation occurring at 150 mA {Fig. 3, inset).
The mode spacing (~~l) is - 1.7 A, corresponding to a cavity length of ~ 560 Nm. This is longer than the half-circumference (- 470 ~um} and may be due to some misalignment of the cleave (creating a longer cavity) or a longer effective path length caused by the curved resonator, Operation around the curved resonator is confirmed by imaging the output of each end of the half-ring laser separately into a monochromator. The longitudinal mode spectrum is confirmed tc be identical from each end, indicating that laser emission indeed occurs from the circular cav~:ty (data not shown}. Further evidence of oscillation arQUnd the circular cavity was provided by sawing a half-ring device, which is originally observed to lase, in two along the vertical arrows "SC'° and "25 um" in the inset of Fig. 2.; This was found to destroy the resonator and the laser operation.
If the device were originally lasing linearly from the front cleave to the opposite saw cut (SC}, the device should continue to exhibit laser operation, which is not the case.
Thus, there is strong confirmation that laser operation occurs around-the ring. However, half-ring laser diodes that were cleaved in two {not saw cut) to form quarter-ring diodes continue to lace, with stimulated emission being observed from both perpendicular clewed.facets.
The near field (NF) intensity profiles of the laser diode of this example are collected with a f/U.95 25 mm focal length lens. A low magnification view {Si MOS camera) with the diode operating at 1~0 Ma (pu~.sed;) shows distinct emission from the two ends of the half-ring laser (Fig. 4{a)). The 267 pm ,.:,,, center- o-center separation agrees well with the device geometry. The corresponding intensity profiles (CCD array image} are shown in Fig. 4(b). Both g~aks exhibit asymmetry, with the intensity,drogping off faster towards the outside diameter (OD} of the annulus. This asymmetry is more evident in the higher magnification view of he right-hand end (Fig.
4(c)); Such asymmetric intensity profiled agree well with those calculated for a circular waveguide (see E. I~arcatilli, L.~1 i~V~ 931205$1 P~'/US93/0284~ ~': ;,:
~~~~°~~ ~ 6 Bell Syst. Tech. J. 48, 2103, 1969}.
Polarization-resolved L-I characteristics indicate that the half-ring diodes lass in the TM mode., This behavior is opposite to that observed in conventional GaAs QWH laser diodes and in native-oxide defined linear resonator QWH laser diodes, which lass in the TE mode. The radiation losses in the native-oxide circular resonator for the TE modes are greater than for the TM modes, indicating application for mode ,;..
filtering.
Fig. 5 shows the L-I characteristic of a native oxide ring laser diode fabricated in similar manner but on a lower ( vertical ) confinement AlxGa1_X~rs-GaAs QWH laser crystal ( x p.6~,confining layers). A cleave through the ring annulus permits laser light to leak out (inset of Fig. 5), with oscillation still maintained around the ring.
Fig. ~ is a simplified diagram of a laser device 6a0 made ~xsing,the forgoing technique. The device, on Gars substrate and buffer layers 610 and 615, includes an active region 630 between AlXGal_xAs confining layers 640 and 650, of opposite conductivity types. The active reg~:on includes the quantum well 633 between undaped AlXGa1_XAs waveguide layers 635 and 637: The diagxam also shows the curv~d top contact stripe 660, the underlying GaAs cap layer 670, and bottom electrode 605: As noted a.n the f~regoing description the native oa~ide of this exa~tple, 680', extends through the entire upper confining layer G50 and slightly into the upper waveguide layer 637.
Inthe description in conjunction with Fig s 1-6, the native oxide of aluminum extends thr~ugh the entire upper confiding layer of the laser diode and even, to a small ~' extent, znto the waveguide region. Applicant has discovered :that effective optical confinement; 'taile~red to obtain desired operating conditions, can be achieved with a thick (generally, about 3000 ~ or more) native oxide that does not necessarily extend through the entire confining layer. Generally, a native oxide that extends through at least one-third of the canfin.ing Iayer is preferred. Fig. 7 shows an~embodiment of . . ] 93/2051 ~ ~ ~ ~ ~ $ ~ ~crrus9~roz~aa the invention having a linear stripe 760 and wherein the thick oxide 780 is controlled (e.g. by controlling the time of exposure andlor temperature in producing the native oxide) to extend about half way through the upper confining layer. In this example, the aluminum fraction (x) of the AlXGa1_XAs confining layers 7~O and 750 is relatively low, for example about 0.4, which results in lower vertical (i.e., in the direction transverse the layers) confinement of the laser beam. (Layers with like reference numerals to those of Fig. 6 represent similar structure.] As described further hereinbelow, less vertical confinement permits greater expansion of the beam into the confining layers and, accordingly, a larger effective lateral refractive index step encountered by the beam as a result of the native oxide in the Confining layer.
Reference can be made to the following publications which relate, inter alia, to control of the optical (field and gain profile by adjusting the thickness of native oxide outside the active strige and to control of oxide thickness to determine the degree of optical eonfinemen~ts F:A. Kish, S:J. Carac~i, 1~. Holonyak, Jr., J.M.
Dallesasse, K.C. Hs~.eh, M.J~ Raes, S.C. Smith, & R.D. Burnham, °'Planar Native-Ox~.~le Inde%-Guided AlXGa~_XAs-GaAs Quantum Well Heteros~ructure Lasers", Appl. Phys. Lett. 59, 1755, September 30, 1991;
F:A. Kish, S.J. Caracci, N: Holonyak, Jr., and S.A.
Maranowski, J.Nt. Dallesasse, R.D: Burnham, and S.C. Smith.
t,~~sible Spectrum Idati~re°-O~ide Coupl~cl-Stripe Ino.~(Al~Ga~-x)o.~P_ In0.5~a0.5P Quantum Well , Heterostructure Laser Arrays'" , Appl .
Phys.~ Lett. 59.2883, November 25r 1991; , FA. Kish', S.J. Caraeci, ~T~ Holonyak, Jr., P. Gavrilovic, K. Meehan, ~ ~.~~ William, °"Coupled-Strfipe In-Phase Operation Of Planar Native-Oxide Index°~ua.ded AlYGa~_yAs-GaAs-InXGa~_XAs Quantum-Well Hfetex'ostructure Laser Arrays", Appl. Phys. Lett.
71e J~IriuaY'~r 6, 1992;
F.A. K sh, S:J« Garacci, S:A. Maranowski, N. Holonyak, ~'r,, K.C. Hsieh, C:P. Kuo, R.M: Fletche.r, T.D. Osentowski, &
PG'f/tJS93/02~:. ~>
Wo 9~~2~''3 2 9 ~ ~
M.G. Craford, "Flanar Native-Oxide Buried-Mesa AlxGa1lxAs-Ino.S(AlyGal_~)o.sP-Ino.a(AlZGa1_Z)o.sP Visible-Spectrum Laser Diodes", J. Appl. Phys. ?1, 2521, March 15, 1992.
Fig. 8 illustrates a passive curved waveguide in accordance with a form of the invention. The waveguide, which can be coupled (directly, or evanescently) with a suitable light source [not shown], includes, for example, GaAs substrate and buffer layers 810 and 815, and an AlxGa1_xAs (x =
0:8; for example) waveguide layer 820. The GaAs cap layer 870, native oxide confining regions 880 (which extend about half way through the alumanum-bearing material in this case), a~ad the contact stripe 860, can be formed using the previously described techniques.
Figs. 9-18 illustrate configurations of lasers or waveguides (cross-sections of which may be, for example, ,,of the-types shown in Eigs. 6, 7 and/or 8) that can be advantageously implemented utilizing the principles of the invention. In these Figures, the white regions represent .either the laser stripe configuration, which has thereunder, aster axia, a waveguide reg~.on with he index of refraction confinement in accordance with the Present invention or, in the case of a waveguide, the index-confined waveguide region in accordance with the princ~.ples hereof; Figo 9 illustrates a ring configuration, wi h light energy travelling in bobh directions. Fag. 1,O illustrates'a quarter ring, with light energy again travelling in both directions. This configuration, in an active or a passive device, can be utilized to obtain a ninety degree change raf direction of the light path. Figs 11 and 12 illustrate ring laser or waveguide confi~gu~ations with tangentially coupled branches.
In Figs 1~ and 14, mul i-stripe lasers are shown as being coupled by'ring lasers, such as for phi a locking. The stripe spacing can: be substantial. Fig. 16 shows a similar arrangement, but with half-ring lasers, and Fig. 15 shows a series of coupled half-rings. Thelocking or tuning provide by these configurations'can result in enhanced longitudinal and/or transverse mode-operation. Fig. 17 shows curved 3 93/20581 ~, ~, J ~i ~ ~ ~ PC.'f/USg3/02$44 sections in an "S-bend" arrangement for providing an active or passive lateral offset of the optical beam path. Fig. 18 illustrates the surface configuration of a laser or waveguide which couples light in a single branch with four curved branches.
In a further form of the invention, a quantum well heterostructure is grown by metal-organic chemical vapor deposition [°'MOCVD"] on an n-type GaAs substrate. After n-type buffer layers of GaA~ ( ~0.5~um) and an Alo,z3Gao_~7As ( - l~cm) payer, an AIo.~Gao.SAs lower confining layer is grown to a thickness'of - l:Sum: The active region of the quantum well heterostructure is,then grown, and includes a -- 2100 waveguide region of undoped Alo.z3Gao.~7As with ~ 100 ~ undoged ~aAs quantum well (QW) grown inside the waveguide region - 700 ~, fram the lower confining layer. An upper confining layer of p-type Alo:eGao.2As 1,s grown to ~ thickness of about 3500 ~, a:~d a heavily doped p-type GaAs contact layer is grown thereon, the contact payer having a thickness of about 800 ~.
The position of QW is displaced from the center of the wraveguide for more effective overlap of the high-gain region with the optical mode, which is displaced towards the substrate due to the asymmetric confining layers. This asymmetry is purposely introduced to minimize the effects of thensurface of the laser crystal (located - 3500 ~ from°the 'waveguide) ~y shift~.ng the optical field toward the substrate.
The shallow upper confining'layer is desirable in order to m~:n~:mize current spreading, allow finer pattern definition, and'improved heat dissipation with the crystal mounted p side ''d~wn" and thus the. active region closer to the heat sink.
The thin upper confina.ng layer structure combined with ~.he p-type metallizatiorimay also serve to reflect light emitted toward the surface back into the crystal for improved device properties. [~ laser diode, fabricated using the described type of QW heterostruc~ure, and comprising a linear array of small rectangular 3.nternal coupled cavi ies delineated by oxidation of the high-gap AIxGaI~XAs upper, confining layer, . is described in NEl--rein, F.A. Kish, N. Holonyak, Jr., A.R.
~ f.r f-. 7 71F7 L
,rwF r~...:n : a0.". 4 s»-irrvT.i7=-Y f ' o i .°. aio r a . rt t f.
x,., < r- s .a .e, d .. , a . :. G- . . ..
. . . , r x.. . > ... ,. . . ° a.. « .. I : 'a _... _ ___.._ . ......,.. .... . .....,..... ~n,.. ....~x , . . . .. .... . , . ,. . a r. ,.-i ,.. . . ... . ... , ,~ ... ., w27 s. . x. m.. ,4., .. ,.w ., . ... a _ . , .<.. ., n_ , a.. s.. .:"~. , . . ,... 2~.e~.w w~ ~3i2oss~ . Pc-rms~3~o~a~a <r Sugg, M.J. Ries, S.C. Smith, J.M. Dallesasse, and R.D.
iBurnham, "Native-Oxide Coupled-Cavity AlxGa~_XAs-GaAs Quantum Well Heterostructure Laser Diodes°', Appl. Phys. Lett. 59, 2838, November 25, 1991.]
A laser diode array in accordance with an embodiment hereof is fabricated by patterning - 1Ø00 ~ of Si3N4 into repeated (masked) rectangular cav~.tie's (- 19 um long, - 4 Nm width, ~ 3 arm end-to-end spacing), which axe arranged lengthwise in two parallel stripes with - 1 Nm separation.
The exposed GaAs cap is then removed by chemical etching (HZS04:H202:H20, 1:8:80) and the crystal is placed in an open-tube furnade (supplied with a N2 carrier gas bubbled through HzO at 95°C) at 425°C for 20 min: As above.,,,, this process results in he transformation of 1300 A of the Alo.eGao.zAs upper confining layer to native oxide outside of the repeated cavities. The S13N4 is then removed in a CF4 plasma. The inset-of Fi.g: 19 shows a photomicrograph of the surface of the crystal after these processing steps. In order to increase the doping ~:n the rectangular GaAs contact areas, the crystal is sealed xn a.n'evacuated quartz ampoule and is shallow Zn-diffused (ZnAs2;source, 540°C for 20 min), The crystal is then lapped and Qolished (on'the substra~.e side) to a thickness of 100 ~t~n, and is metalli:zed with Ti°Au across the oxide and the g-type GaAs "contacts" and with Ge~Ni-Au on the n-type substrate side. The. sample is then cleaved into 250-500 um wide Fabry-Perot resonators, diced, and indiv~.dual dies are mounted'p-side downon Ih°coated Cu'heat sinks for continuous ,(cw) operation.
The unusual switching behavior of the resultant 2-D
stra.pe lasers i~s evident from the L-I characteristic shown in, Fiq: 19, which, after reaching ~ peak ire power of - 12.5 mW at (a) 115mA, decreases over 50$ in power from (a) 115 to (b) 150 _ mA, and simultaneously shifts its single mode operation (Fig.
20) to longer wavelength. At (c) 164 anA (Fig: 20) the single , mode operation of (a) and (b) has switcfied off, and in the braad spectrum of (c) 164 mA the resonances of the 19 Nm long minicavities are evident and marked with straws. In the broad .~. . , .!
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°rs.~ ~ .. .. . .. . .... ....,. .., .. " . ,.. " , ... _ ,..... .., ._ _ ..,..
~" ~i X312~581 , ~ ~ ~ ~ ~ P~'f/US93J02844 spectral region of weak stimulated emission, the minicavities tend to store photons, making the mode amplitudes (marked with arrows) smaller (c of Fig. 20). Note that the mode structure near the peak of the spectrum in (c) is sufficiently complicated that it is not evident that at (b) 150 mA the single mode laser operation has shifted fully, from (a) 115mA, to a minicavity resonance (e.g., bhw - 6 meV vs. DE ° 9 meV
fr~m resonance to resonance). It is evident from (a) to (b) to (c) in Figs. l9 and 20, however, that single mode operation is turning-off and mufti-mode operation, and weaker stimulated emission, is turning-on as the current is increased.
The unusual switching behavior of these 2-D array QWH
stripe laser diodes is much more evident in Fig. 21. The L-I
characteristic shows that, with increasing current, the laser turns on and off twice. As shown by the inset, which corresponds to the peak Uf the L-I characteristic (> 12 mW, 415 mA, anarked); single mode operation still occurs. In the valley region between 220 and 300 mA, broad-spectrum multi-moda operation similar to that of Fig. 20(c) occurs (data not shown). As the dashed line of Fig. 21 shows, the emission intensity an this region:-is at or somewhat above spontaneous em~.ssic~n. Most of the 2-D array lasers examined behaved as Shown 3.n Fig. 21.
The data of Fig. 22 show in some detail the behaviar of the di~de of Fig. 21 near he diamond°shaped point located at mA on the L-I characteristic. For comparison, at (a) 40 mA in the spontaneous regime ttie near field (NF) exhibits two intensity peaky expected of a twin linear array, with the spacing of ~.2 um agreding with the 2-D array width shown in he inset c~f' Fang. 19: At (b) 71 .mA the near field still, exhibits twin a.ntensity peaks; and the spectx'um a single main mode corresponding to the left NF peak and significant satellite longitudinal modes corresponding to the right NF
peak. A small current change of 1 mA (71 ~ 72 mA) produces abrupt switching: The satellite longitudinal modes (Fig. 22c) vana.sh abruptly, and simu~.taneously the right NF emission.
peak: It is clear that the strong coupling of one side of the »~~,,.
-,..: -,.. , "<.::;: , .; ".; .; ,.. :; :~: ,. . . . .. ; .... _,...
r"i h v ~Yt7 93120581 PCT/US93/02844 :, F
~~~2g~6 zz diode interferes, constructively or destructively, with the other side Uf the diode. Also it is evident that the manner in which the current has been partitioned among many identical coupled rectangular minicavities insures that the resonant operation between the cavities is favored.
The data of Figures 19-zz demonstrates a laser diode having two parallel linear arrays c~f.'small coupled rectangular cavities delineated by oxidation of the high-gap AlXGaX_lAs-GaAs QWH. The two dimensional laser array exhibits mode switching and switching in the L-I characteristic with increasing current. Depending on the bias position (current) on the L-I
curve, the laser operates in a sinc3le longitudinal modes in or near the spontaneous regime. For example, the resonances of the minicavities are evident in the spontaneous spectra in spite of small heating effects and carrier-induced changes in dielectric properties. As above, optimization of the geometry, sire, and number of the minicavities, and their coupling, may result in improved behavior of these devices.
In another form of the invention, described beginning with Fig. z3, the QW hetero~tructure crystal is substantially the same as the one described above in conjunction with the previous device. In'the present embodiment; the laser diode array 3s fabricated by first depositing 1000 ~ of Si3Na on the crystal surface, which is then patterned into end-to-end reputed (masked) rectangular cavities (minicavitzes, 6 pm wide and 19 p~m long on 2zpm centers) arzanged lengthwise.
Next, 6 ~m p~~otQresist (PR) Stripes ire patterned - 5 ~m away from the linear array of mina.c~vities: The patterned PR and Si3Na then serve as a mask for the chemical etching (HaSOy~~2Oa:H20, 1:8:80) of the GaAs cap layer, leaving the ~iigh-gage AlXGa1_XAs exposed outside of the patterned regions , The PR is then removed and the sample is placed immediately in an epen°tub~ furnace (~z5°C) supplied with H20 vapor in an N2 carrier'gas for z0 anin. Again, this process results in the conversion of the exposed ~~.gh-gap A1XG~1_XAs to a low-index ( n 1.6) insulating nativeoxide looted ~ 1000 A above the QWH
waveguide region. The patterned 5i3N4 and unetched GaAs ~n~r 93f20581 - ~ ~ ~ ~ ~ ~ ~ P~lffllS93/02844 regions are unaffected by this treatment. The patterned Si3N4 is then removed in a CF4 plasma. The inset of Fig. 23 shows the surface of the device after these processing steps. Next the sample is ~n-diffused (540°C, 20 min) to increase the doping in the contact regions (labeled "GaAs" in Fig. 23)..
The crystal is then lapped and polished to a thickness of --125 um and, again, metallized over the entire top surface with Ti-Au for p-type contacts and with Ge-Ni-Au for n-type contacts. Finally the crystal is cleaved, diced, and individual dies mounted on In-coated copper heat sinks for continuous (cw) operation.
The large amplitude switching praperties of the single-stripe laser coupled to the active linear array are shown by the 300 K continuous (cw) L-I curve of Fig. 23. The laser threshold current is 32 mA, and laser operation persists;up to a current of 168 mA. At this po~.nt the diode switches abruptly from stimulated emissi~n, ON (19.6 mW/facet, undoated), to the spontaneous regime, OFF (0.4 mW/facet, uncoated). This behavior corresponds to a large ONaOFF pawer ~rati~ c~f 4g: These ara inherently nanlin~ar devices, and exhibit bistable operation with a large hysteresis. As the current is decreased (returned) to 123 mA, the diode switches back from the spontaneous regime, OFF, to the simulated segue; ON. For further current ~.ncreas~ beyond 168 mA,°after the dev3;ce has switched OFF with increasing current, only a slight increase in the spontaneous output is observed until fai3ure at 18? mA. 'We mentir~n that, although hysteresis occurs in the LaI characteristics, no hy~t~resi is observed 3n the current versus voltage (i-V) ch~racteri~stics of these, devices: ;;
The L,-T characteristic (cw 300 K) of another diode exhibiting similar switching behavior is sh~wn in Fig. 24.
The laser threshold current is 2? mA, with the device exhi;bit'a,ng essentially a single ~ 5.5 pm~wide Gaussian near-f3eld pattern (data not shown). This intensify pattern corresponds to laser operation of the ~ 6 um wide uniform stripe (inset of Fig. 23); which is expected to reach WO 93/205S1 PC'f/U593/02R44 i~ ~~ -a ~~ ~ s~ :~
threshold before the segmented linear array. From gain-loss considerations, the linear array with its repeated unpumped absorbing sections should have a higher laser threshold.
Throughout the entire operating., range, a single-stripe near--field gattern persists, i.e.; only very weak output is observed from the linear array~.portion of the device. The near-field pattern (300 K, cw.,bperation) at (a) 99 mA just before the switching from ON to OFF, i.e., before switching fram single-mode stimulated emission to spontaneous emission, is shown in the inset of Fig. 24. Similar to operation just abave threshold, only a 5.5 Nm Gaussian near-field is observed at significant amplitude. After the diode switches OFF at (b) 100 mA, no pattern is observed on the same sensitivity scale of the CCD detector. However, at higher sensitivity (11.3x), emissi.an from the same aperture (- 5.~
Nm) ~:s observed (b'). Thin near-field pastern also is observed as the laser is switched back from OFF to ON. These data indicate unambiguously that only the uniform laser stripe prouides much of the optical'output of the system. The side-coupled linear array serves mainlx to effect the interferences and swit~hing~ ON°OFF, and does not contribute primarily to the optical output;
Further understanding of the operation of these diodes is obtained by'examining the output spectra. Somewhat above the threshold at 30 mAr the diode of Fig. 24 operates in a single longitudinal mode (~ 8353 ~. data not shown). This behavior eantinues to the peak of the L-I curve'of Fig. 24 (63 mA), where the single mode operation "hops" to longer wavelength g367 1~, data not shown). :Throughout the entire stimulated emission operating regime (,30 -~ 99 mA), the output occurs in a well developed single longitudinal made. For example, at 63 mA the 'laser exhibits aside-mode suppression of 29 dH. The :mode hopping, and corresponding structure in the L-I curve (Fig. 24), is attributed. to the interaction (interference) of the single laser str~.pe with side-coupled active linear array and its resonances and stop hands.
Longitudinalmode spectra in the higher-current switching "'i> 93/2~158a PCT/US93l02844 regime of the device of Fig. 24 are shown in Fig. 25.
Immediately before switching from ON (stimulated emission) to OFF (spontaneous regime), (a) in Fig. 24, the laser operates in a single longitudinal mode at ~, w 8415 A, which is shown as (a) in Fig. 25. When the diode switches OFF to the spontaneous regime, (b) of Fig:. 24, the longitudinal mode spectra appear as shown in (b) of Fig. 25. At this point, the output consists of the spontaneous emission of the single stripe laser (group of lower energy modes) and the linear array (group of higher energy modes). The coupling of the linear array to the stripe laser leads to interference. The resonances of the minicav~ities of the linear array are apparent (clearer in the laboratory data) at higher energy in the spectrum of Fig. 25(b) and are marked with arrows. The spacing of these resonances (~~1 - 50 .~) corresponds to the -19 um minicavity length shown in the inset of Fig. 23. It is noted that the output in the OFF spontaneous regime (Fig.
25(b)) differs s~.~nificaratly from that observed in the spontaneous regime below laser threshold (< 27 ~1), where only the longitudinal mode output of the single laser stripe (group 'of lower energy modes) is observed (data not shown).
Tg~~ described switchihg and storage are fundamentally different from previously ~~Qo~ted switching laser devices.
The 0~-O~'F switching behavior occurs in this embodiment sn a single unbroken or uninterru~ated laser stripe. The switching behavior is owing to the influence (via sidewise coupling) of an active 1'inear army. The switching and bistability is effected by the periodic structure ~f the,linear array (see ~i.G. Wiriful, J,H. Marbuxvger, and'. Carmir~s Appl. Phys. Lett.
35,; 3,79 ( 1979) °,, Jr ~e and M: Cada, IEEE J. Quantum Electron.
QE°27, 1182 (1991)) and the obvious inhomogeneous carrier distribution, aa~d i.nhomogeneous operation, resulting from the nativ~--oxid~ patterning of the gray.
Thus, this embodiment sets forth a new form of optical switching element in which a conventional single-stripe laser is side-coupled to a linear array of coupled minilasers. The resulting many-eleanent twin-stripe laser is easily realized via native-oxide device processing. The planar devices exhibit large hysteresis in the L-I curve, with large amplitude switching from the peak of the stimulated emission regime (ON) to the spontaneous regime (OFF). Changes in the coupling, e.g., the spacing between the laser stripe and linear array and between the array elements, a.nd in the geometry of the structure should improve the switching l;~ehavio:r of these lasers. Independent control of the current (carrier population) in t:he single laser stripe in the linear a~°ray, e.g., via a third terminal electrode, should allow control o:F the switching behavior, and other variations are possible.
Fig. 26 shows a part of the surface of the device described in conjunction with Fi~~ures 23-25, and is used as a reference to show the cross-sections used for the illustrations of Figures 27-29. In Figure 26 the stripe is labelled 1210 and the minicavities, or porticans thereof, are labelled 1221-1225. The cross-section 13-13 is taken through the stripe 1210 and an adjacent minicavity 12'..4. The illustrated layers, which were previously de:acribed, include the bottom contact metallization 1250 (it bein<~ understcaod throughout: that references to "bottom"
or "top" are ?°or ease c:~f description, as the device may be mounted and used in an~,r desired orientation), followed, in ascending ordE~r, by the:':ntype GaAs substrate layer 1255, the n-type GaAs buff=er layer 1.258, the n-type A1o.23Gao.,.,As buffer layer 1260, the n-t~rpe Alo.SGa.a.SAs lower confining layer 1262, and active active region 1:?70 that includes a GaAs quantum well layer 1271 between waveguide layers 1273 a.nd 1275 of undoped Alo.z3Gao..,~As. r~.bove the active region is the upper confining layer 1278 of p-type: Alc,_BGao,;,As. The layer thicknesses may be, for example, as previously indicated above for the experimental device. The p--type GaAs contact 1281 and the p-type GaAs contact 1283 respectively define the contact positions of the stripe 1210 and the minicavity 1226: of Fig. 26. The native oxide is shown at 1291, 1292, and 1293, a.nd, in this example, has a thickness of about 1300 ~ The oxide a:Lso extends somewhat under the GaAs contact regions. The top (p-side) metallization is labeled 1240.
The diagram of F:~.g. 28 illustrates the cross-section defined by arrows 14-14 of Fig. 26. In this view, only the contact region 1281 of t:he stripe 1210 is visible. The oxide (1294) extends continuously to the :right of the stripe.
Fig. 29 shows the cross-section defined by arrows 15-15 of Fig. 26. This view is longitudinally through the minicavities, with two minicavities being shown between three oxide regions 1296, 1297, 1298. The :longitudinal dimension of the contact 1283 is seen in this view.
In the illustrated embodiments, operation may be "two terminal", such as by applying the electrical potential between the bottom electrode and the top common metallization. The device can alternatively be made for operation as a three terminal or multiple terminal devic::e. For example, Fig. 30 illustrates a device having a stripe 1610 with mini cavities 1620 on both sides, each line having a common metallization (represented by the joining lanes between minicavities) and its own terminal, so that the device can be operated with four independent terminals, with three terminals (~:or example, t:he terminals of only two adjacent line, and the bottom terminal, or with the two outside lines in common) or twc:a terminals, with all three lines in common. [In this diagram, and in other diagrams hereof where a plan view of the minicavity and/or stripe configuration is shown, the underlying structure is of the type previously described, the fabrication m<~sking pat::t~erns being consistent with the structures illustrated.) Fig. 31 ~.llustrates a two-dimensional array o.f adjacent liner of mini cavities, with individual terminals coupled with the mini cavities. It will be understood that a terminal can be coupled with any desired combination or group of cavities or m_Lnicavitie:~s.
The previous embodiments illustrate straight line minicavity and stripe c.~.onfigurations, but it will be understood that the principles of the invention also apply to minicavities and stripes arranged in curved line r.."' 1~J0 93/20581 PCflUS93/02844 <:' configurations and arrays. Figures 32-39 illustrate some representative embodiments (with bottom electrode and various possible top electrodes not shown). In Fig. 32 there is shown a ring laser which is divided into curved minicavities 1815, to obtain the types of effects described in N. El-Zein, F.A.
Dish, N. Holonyak, Jr., A.R. Sugg, M.J. Ries, S.C. Smith, J.I~.
Dallesasse, and R.D. Hurnham, "Native-Oxide Coupled-Cavity AlXGa1_xAs-GaAs Quantum Wel l Heterostructure Laser Diodes °' , Appl. Phys. Lett. 59, 2838, November 25, 1991, in the context of a ring laser. [It will be understood throughout that any of the curved configuratibns need not be precisely circular, that any desired portions of rings or curves can be used, and that the rings or portions thereof can be cleaved at any desired position to obtain one or more outputs.] Fig. 33 illustrates two concentric ring lasers, each divided into minicavities 1915, so that lateral coupling can be achieved, as first described in conjunction with Figures 19-22 above for the case of straight line arrays. I~ Fig. 34, one of the concentric rings 2010 is continuous, and the other is divided into minicavities 2015, to ob~air~ a curved version of the embodiment described in conjunction with Figures 23-29. Fig.
35 i.I:lu~trates a ~a.reular configuration with sector-shaped minicavities 21.5 separated by xa$ial "spokes" of native oxide:
Fic3. 36 shows s rung laser 2210 laterally coupled with a stripe laser 2220, In Fig. 37 the ring is divided into mihacavl,ties 2315, end in Fig. 38, the stripe is divided into minicavities 2415. In F3.g.-39, both the ring and the stripe are divided into minicavit3es (2515 and 2525, respectively).
It w~.ll b~ understood that'the previous indicated variations with regard to numbers and types of striges, array elements, and/or terminal connections are applicable to embodiments with curved lines or minicavities.
Fig. 40 illustrates, in cross-section, a form of the invention 4ahich couples cavities with d~.fferent longitudinal mode characteristics, in the form of a vertical cavity laser array. vertical cavity lasers are well known in the art (see, ~~~~~'~ 93/20S81 ~ ~ ~ ~ ~ ~ ~ P~.°T/US93~a2~4 for example, H. Soda et a1. Japan J. Appl. Fhys. 18, 59 {1979) and K. Iga et al., Electron Lett. 23, 134 (1987), and include, as in the lefthand unit of Fig. 40, a bottom contact 2610 on a substrate {e.g. GaAs 2605), an n-type superlattice 2620, an active region 2630 that includes a quantum well layer 2632 between waveguide layers 2634 and 2636, a p-type superlattice 2640, and one or more contacts 2650. Various materials may be used. As one example, the superlattices may comprise a number of alternating layers of AlAs and GaAs [ or AlxGa~_XAs and AlYGaI_ yAs, x~y, gr combinations of AlXGa1_xAs and conductive dielectric stacks ( a . g . Ti02/Si02, ZnSe/CaF2 ) ] , and the act~.ve region may comprise . Aln.lGao,9As { or GaAs ) waveguides layers with a GaAs { or Ino,lGao.9As ) quantum well layer with total thickness of typically - 250 ~. The contacts at the surface may comprise for examgle Au or Ag with a standard (e.g. Ge-Au) backside (substrate) side contact. A two-dimensional vertical davity coupled array of such devices is described for example in I~.G. Deppe, J.P. Van der Ziel, Nasesh Chand, G.J. Zydzik, and S,N.G. Chu, Appl. Phys. Lett. 56, 2089 (1990). Briefly, in operation, the multiple reflections from the superlattice interfaces provide a relatively short effective cavity length (typically ~ Sum) from the limited thickness device, and the cavities are coupled wanescen~ly:
In accordance with a feature of a form of the inven-~ion, adjacent vertical cavity laser aanits are pr~v~.ded with active reg.ion~ of d~.fferent thickn~sses, as illustrated in Fig. 40, where the active region 2630' is substantially thicker than he active region thickness of its neighboring unit. In the illustration, the quantum well layer is c~ntinuous through adjacent active regions, although this is not necessary., The t.v name ~s'true of the superlattice layers above the active region 2630'. Variation o~-the upper ~r lower superlattice thidkness laterally varies the effectide lateral mirror areflecti.vity. Such variations may also be employed with a uniform thickness active region to achieve local variations in the cavity structure. In addition to the Standard evanescent coupling, other schemes may be employed to'couple such devices W~ 93/20581 ~'Cf/US93/02844 'i~.,.~~;
such as varying the mirror (e.g. superlattice) angles to directly reflect some of the light from one cavity into adjacent cavities.
There are various other techniques that can be utilized to obtain adjacent vertical cavities having substantially different effective vertical cavity lengths, so that lateral coupling thereof can be advantageously exploited to obtain properties such as switching, bistability, andlor tuning. [As used herein, vertical cavities having substantially different effective cavity lengths means that the cavities have substantially different longitudinal mode characteristics, as previously defined.] For example, the active regions of adjacent units may comprise different materials. Another alternative is to provide adjacent units with superlattices of different thicknesses, or superlattices of different configuration: An exam~lQ of the latter would be to provide one unit having superlattices of alternating 100 ~ GaAs and Al~s layers and the other unit with superlattices having alternating 200 ~1 Gags layers and 100 A AlAs layers, which results an different effective cavity lengths.
Fag. 41 illustrates a checkerboard-type array of such;
units, with the cross-hatched units representing the units having tie thicker active regions. The array can be operated as a two terminal device, with the top contacts coupled in common, and potential applied between the top and bottom c~ntact, or can be driven as a three termznal or multiple terminal device with wparate connections to contacts.
Various other shapes and configurations in one-dimension or wo~-dimensions can be utilized. 'In one example of fabricating an arrays the growth may; be terminated at the active region (2630'). A two-dimensional pattern (e.g,. a checkerboard) is then masked using standard photolithography techniques and the sample is subjected to chemical etching to remove a portion of he active region (2636) in the unmasked area. This process results in a lateral variation in the active layer thickness.
The photoresist is then removed and the upper p-type supperlattice is grown on the patterned active region, such as ,, ». . ,...-. .. .; . .. -,:. , ;; ., ,...:. ..::.., >..:... : . . ..
P~'/US93/02844 by MOCVD or MBE (molecular beam epitaxy). A circular (dot) metallization can then be applied on the upper p-type supperlattice for contact and reflectivity purposes.
The invention has been described with reference to particular preferred embodiments, but variations within the spirit and scope of the invention will occur to those skilled in the art. For example, while the aluminum-bearing III-V
semiconductor material aluminum gallium arsenide has been described in embodiments hereof, it will be understood that the devices and technique hereof can employ other aluminum-bearing IIr-V semiconductor materials, such as indium aluminum gallium phosphide, indium aluminum gallium arsenide, or aluminum gallium phosphide. [Reference can be made to F. ICish et al:, J. of Appl: Phys. 71, T5 March, 1992.] Also, it will be understood that the indicated confining layers can be;
multiple layers, one or more of which comprises the aluminum-bear~.ng III-V semiconductor material. It will further be unders~odd that devices can integrate the aluminum-bearing III-V semiconductor material (from which the native oxide is formed)ywitka other nQn-III--V semiconductor materials. It will also b~ understood that laterally coupled cavities as described herein can be utilized for tuning as well as indicated functions such as switching. The lateral coupling described herein is particularly facilitated by using the native oxidE formed in an aluminum-bearing III°V semiconductor material to separate laterally coupled cavities. In some of the-configurations hereof, 5:ess gre~erred cavity definition can alternatively be implemented by,tedhniques such as 'muZ:'~iple regrowths/overgrowths, etch and xegrowth/overgrowth:, r~,d~geiFormation, ridge formation and overgrowth, impurity induced layer disordering, and proton implantation.
Claims (15)
1. A method of making a semiconductor laser having a light path that is at least partially curved, comprising the steps of:
forming a layered semiconductor structure comprising an active region between first and second semiconductor confining layers, said first and second semiconductor confining layers being of opposite conductivity types, and said first semiconductor confining layer being an aluminumbearing III-V
semiconductor material;
applying a mask pattern over said first semiconductor confining layer, said pattern including a stripe that is at least partially curved;
exposing unmasked portions of said first semiconductor confining layer to a water-containing environment and a temperature of at least 375 degrees C for a time sufficient to form a thick native oxide of aluminum in said first semiconductor confining layer, said native oxide of aluminum having a thickness of at least 3000 Angstroms; and coupling respective electrodes with said first and second semiconductor confining layers.
forming a layered semiconductor structure comprising an active region between first and second semiconductor confining layers, said first and second semiconductor confining layers being of opposite conductivity types, and said first semiconductor confining layer being an aluminumbearing III-V
semiconductor material;
applying a mask pattern over said first semiconductor confining layer, said pattern including a stripe that is at least partially curved;
exposing unmasked portions of said first semiconductor confining layer to a water-containing environment and a temperature of at least 375 degrees C for a time sufficient to form a thick native oxide of aluminum in said first semiconductor confining layer, said native oxide of aluminum having a thickness of at least 3000 Angstroms; and coupling respective electrodes with said first and second semiconductor confining layers.
2. The method as defined by claim 1, wherein said active region includes at least a waveguide layer and a quantum well layer, and wherein said respective electrodes are coupled to said semiconductor confining layers through further respective semiconductor layers.
3. The method as defined by claim 1, wherein said curved portion is in the shape of an annular arc.
4. The method as defined by claim 1, wherein said curved portion is an annular ring.
5. The method as defined by claim 3 or 4, wherein said time is sufficient to have said native oxide extend through at least most of the thickness of said first confining layer.
6. The method as defined by claim 5, wherein said native oxide extends through the entire thickness of said first confining layer.
7. The method as defined by claim 1, wherein said aluminum-bearing material comprises Al x Ga1-x As, where x is at least 0.3.
8. The method as defined by claim 1, wherein said water-containing environment comprises water vapor and an inert gas, and wherein said temperature is at least 450 degrees C.
9. A semiconductor ring laser device; comprising:
a semiconductor active region disposed between first and second semiconductor confining layers, said first and second semiconductor confining layers being of opposite conductivity type, and said first semiconductor confining layer being an aluminum-bearing III-V semiconductor material;
first and second electrode means respectively coupled with said first and second confining layers, said first electrode means including a conductive annular ring; and lateral confining regions of a native oxide of aluminum formed in said first confining layer around both peripheries of: said annular ring, said native oxide extending through at least most of the thickness of said first confining layer.
a semiconductor active region disposed between first and second semiconductor confining layers, said first and second semiconductor confining layers being of opposite conductivity type, and said first semiconductor confining layer being an aluminum-bearing III-V semiconductor material;
first and second electrode means respectively coupled with said first and second confining layers, said first electrode means including a conductive annular ring; and lateral confining regions of a native oxide of aluminum formed in said first confining layer around both peripheries of: said annular ring, said native oxide extending through at least most of the thickness of said first confining layer.
10. The device as defined by claim 9, wherein said native oxide extends through the entire thickness of said first confining layer.
11. The device as defined by claim 10, wherein said aluminum-bearing material comprises Al x Ga1-x As, where x is at least 0.3.
12. This device as defined by claim 9, wherein said active region includes at least a waveguide layer and a quantum well layer, and wherein said respective electrode means are coupled to said semiconductor confining layers through further respective semiconductor layers.
13. A semiconductor optical waveguide, comprising:
a semiconductor substrate;
a generally planar semiconductor waveguide layer disposed on said substrate, said waveguide layer being an aluminum-bearing III-V semiconductor material;
an elongated. optical path, which is at least partially curved, in said waveguide layer formed by said aluminum-bearing III-V semiconductor material between optically confining boundaries of native oxide of aluminum formed within the planar surface of said aluminum-bearing semiconductor material, said native oxide having a thickness of at least 3000 Angstroms and extending through at least one third the thickness of said aluminum-bearing III-V semiconductor material.
a semiconductor substrate;
a generally planar semiconductor waveguide layer disposed on said substrate, said waveguide layer being an aluminum-bearing III-V semiconductor material;
an elongated. optical path, which is at least partially curved, in said waveguide layer formed by said aluminum-bearing III-V semiconductor material between optically confining boundaries of native oxide of aluminum formed within the planar surface of said aluminum-bearing semiconductor material, said native oxide having a thickness of at least 3000 Angstroms and extending through at least one third the thickness of said aluminum-bearing III-V semiconductor material.
14. The waveguide as defined by claim 13, wherein said native oxide extends through most of the thickness of-said aluminum-bearing III-V semiconductor material.
15. The waveguide as defined by claim 13 or 14, wherein said aluminum-bearing material comprises Al x Ga1-x As, where x is at least 0.3.
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US860,454 | 1992-03-30 | ||
| US07/860,454 US5327448A (en) | 1992-03-30 | 1992-03-30 | Semiconductor devices and techniques for controlled optical confinement |
| US07/927,822 US5353295A (en) | 1992-08-10 | 1992-08-10 | Semiconductor laser device with coupled cavities |
| US927,822 | 1992-08-10 | ||
| PCT/US1993/002844 WO1993020581A1 (en) | 1992-03-30 | 1993-03-26 | Semiconductor optical devices and techniques |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2132986A1 CA2132986A1 (en) | 1993-10-14 |
| CA2132986C true CA2132986C (en) | 2004-06-22 |
Family
ID=27127572
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002132986A Expired - Lifetime CA2132986C (en) | 1992-03-30 | 1993-03-26 | Semiconductor optical devices and techniques |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0634052A4 (en) |
| JP (2) | JP3726240B2 (en) |
| AU (1) | AU3936893A (en) |
| CA (1) | CA2132986C (en) |
| WO (1) | WO1993020581A1 (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5400354A (en) * | 1994-02-08 | 1995-03-21 | Ludowise; Michael | Laminated upper cladding structure for a light-emitting device |
| US5550081A (en) * | 1994-04-08 | 1996-08-27 | Board Of Trustees Of The University Of Illinois | Method of fabricating a semiconductor device by oxidizing aluminum-bearing 1H-V semiconductor in water vapor environment |
| US6117699A (en) * | 1998-04-10 | 2000-09-12 | Hewlett-Packard Company | Monolithic multiple wavelength VCSEL array |
| US7561770B2 (en) * | 2007-07-30 | 2009-07-14 | Hewlett-Packard Development Company, L.P. | Microresonator systems and methods of fabricating the same |
| GB2478602B (en) * | 2010-03-12 | 2014-09-03 | Toshiba Res Europ Ltd | A semiconductor device and method of manufacturing a semiconductor device |
| US11658453B2 (en) * | 2018-01-29 | 2023-05-23 | Ronald LaComb | Concentric cylindrical circumferential laser |
| FR3078834B1 (en) * | 2018-03-08 | 2020-03-27 | Commissariat A L'energie Atomique Et Aux Energies Alternatives | LIGHT EMITTING DEVICE COMPRISING AT LEAST ONE VCSEL AND A DIFFUSION LENS |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4291327A (en) * | 1978-08-28 | 1981-09-22 | Bell Telephone Laboratories, Incorporated | MOS Devices |
-
1993
- 1993-03-26 CA CA002132986A patent/CA2132986C/en not_active Expired - Lifetime
- 1993-03-26 WO PCT/US1993/002844 patent/WO1993020581A1/en not_active Application Discontinuation
- 1993-03-26 JP JP51757193A patent/JP3726240B2/en not_active Expired - Lifetime
- 1993-03-26 EP EP93908605A patent/EP0634052A4/en not_active Withdrawn
- 1993-03-26 AU AU39368/93A patent/AU3936893A/en not_active Abandoned
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2002
- 2002-11-28 JP JP2002345786A patent/JP2003224330A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| CA2132986A1 (en) | 1993-10-14 |
| WO1993020581A1 (en) | 1993-10-14 |
| EP0634052A4 (en) | 1997-08-27 |
| EP0634052A1 (en) | 1995-01-18 |
| JPH07505503A (en) | 1995-06-15 |
| JP2003224330A (en) | 2003-08-08 |
| AU3936893A (en) | 1993-11-08 |
| JP3726240B2 (en) | 2005-12-14 |
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