CA1215160A - Stripe-geometry solid-state laser with light guidance by transverse refractive-index gradient - Google Patents
Stripe-geometry solid-state laser with light guidance by transverse refractive-index gradientInfo
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- CA1215160A CA1215160A CA000429594A CA429594A CA1215160A CA 1215160 A CA1215160 A CA 1215160A CA 000429594 A CA000429594 A CA 000429594A CA 429594 A CA429594 A CA 429594A CA 1215160 A CA1215160 A CA 1215160A
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- 238000002513 implantation Methods 0.000 claims description 2
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- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 2
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 2
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 2
- 239000003574 free electron Substances 0.000 description 2
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- 230000003287 optical effect Effects 0.000 description 2
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- 229910001218 Gallium arsenide Inorganic materials 0.000 description 1
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Classifications
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- 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
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/24—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a grooved structure, e.g. V-grooved, crescent active layer in groove, VSIS laser
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- 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
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- 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
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/2054—Methods of obtaining the confinement
- H01S5/2059—Methods of obtaining the confinement by means of particular conductivity zones, e.g. obtained by particle bombardment or diffusion
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- 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
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
- H01S5/223—Buried stripe structure
- H01S5/2237—Buried stripe structure with a non-planar active layer
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Geometry (AREA)
- Optics & Photonics (AREA)
- Computer Hardware Design (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Semiconductor Lasers (AREA)
- Liquid Deposition Of Substances Of Which Semiconductor Devices Are Composed (AREA)
Abstract
A LASER-TYPE LIGHT-EMITTING SEMICONDUCTOR
DEVICE WITH REFRACTIVE INDEX GRADIENT GUIDANCE, AND A METHOD OF FABRICATION OF SAID DEVICE
Abstract of the Disclosure A stripe-geometry solid-state laser with light guidance by the refractive index as applicable to systems for data transmission through optical fibers comprises an active layer buried in an etched V-groove of the substrate.
The active layer forms a heterojunction with two confine-ment layers and has a cross-section in the shape of a meniscus with zero thickness at the edges, thus producing a transverse refractive-index gradient. The layers are obtained by localized epitaxy within the groove under conditions of zero supersaturation.
DEVICE WITH REFRACTIVE INDEX GRADIENT GUIDANCE, AND A METHOD OF FABRICATION OF SAID DEVICE
Abstract of the Disclosure A stripe-geometry solid-state laser with light guidance by the refractive index as applicable to systems for data transmission through optical fibers comprises an active layer buried in an etched V-groove of the substrate.
The active layer forms a heterojunction with two confine-ment layers and has a cross-section in the shape of a meniscus with zero thickness at the edges, thus producing a transverse refractive-index gradient. The layers are obtained by localized epitaxy within the groove under conditions of zero supersaturation.
Description
This invention relates to a semiconductor laser device of the stripe geometry type providing light guidance by refractive index control, the device being primarily applicable to optical-fiber telecommunication systems. The invention is also concerned with the method of fabrication of said laser by liquid-phase epitaxy.
The structure which has been studied improves the per-formances of the laser while also permitting easier fabrication and industrial production.
Semiconductor lasers employed as radiation sources in optical telecommunications through glass or silica fibers are of the stripe-geometry type in the majority of instances. In other words, a laser structure of this type has an active region in the form of a so-called stripe in which the electron-hole pairs of electric charges and the light radiation are confined.
These structures can be grouped in two classes according to the principal physical effect which results in guidance of the light radiation along the principal axis of the stripe, namely the gain in stimulated emission or the effective index of refraction.
In lasers of the "gain guidance" type, the real portion of the refractive index is of minimum value at the center, which in fact produces an anti-guidance effect.
On the other hand, in "index guidance" lasers, the real portion of the refractive index is in fact of maximum I
The structure which has been studied improves the per-formances of the laser while also permitting easier fabrication and industrial production.
Semiconductor lasers employed as radiation sources in optical telecommunications through glass or silica fibers are of the stripe-geometry type in the majority of instances. In other words, a laser structure of this type has an active region in the form of a so-called stripe in which the electron-hole pairs of electric charges and the light radiation are confined.
These structures can be grouped in two classes according to the principal physical effect which results in guidance of the light radiation along the principal axis of the stripe, namely the gain in stimulated emission or the effective index of refraction.
In lasers of the "gain guidance" type, the real portion of the refractive index is of minimum value at the center, which in fact produces an anti-guidance effect.
On the other hand, in "index guidance" lasers, the real portion of the refractive index is in fact of maximum I
2--value at the center ox the stripe (that is, in the trays-verse direction), which ensures light guidance but usually involves a fairly complex fabrication process. After formation of the active layer by epitaxy, it is indeed necessary to localize the injection region by means similar to those employed for gain guidance lasers, which therefore complicates fabrication operations and reduces the definition of the structure since a greater number of operations and locating steps are involved in the lo fabrication process.
The laser in accordance with the invention circumvents these disadvantages. In this device, provision is made for a buried active layer in which the injection region is focalized during the epitaxial growth operation within a groove cut in the substrate and is limited solely to the width of the groove. When viewed in cross-section, the active layer has the shape of a crescent, the thickness of which is zero at the edges which are in contact with the groove walls. This structure, in which the injection region is limited in the transverse directions obtained by means of a single operation of liquid-phase epoxy with zero supersaturation during which the epitaxial growth process it rapid on the concave surfaces and zero on the convex surfaces.
In more exact terms, the invention consists of a light-emitting device of the laser type which emits light trough a stripe-geometry active layer within which light it guided by a refractive-index gradient. The device comprises a substrate having an etched V-groove in which the active layer of the laser is buried between two confinement layers with which said active layer forms two heterojunctions. Said device further comprises a contact layer deposited on the external confinement layer and two elect rical-contact metallizations deposited on the two principal faces of the substrate. The distinctive feature of the laser lies in the fact that, on the one hand, the first confinement layer, the active layer and the second confinement layer are limited to the interior of the etched V-groove and that, on the other hand, the active layer is limited by two heterojunctions of concave shape, the concave side of which is directed towards the external confinement layer. The active layer has zero thickness at the edges which are in contact with the groove ; the difference in thickness between the central portion and the edges of the active layer thus produces a refractive index gradient for guiding the emitted light.
Other features of the invention will be more apparent upon consideration of the following description and accompanying drawings, wherein :
- Fig. 1 is a simplified view of a stripe-geometry semiconductor laser ;
- Fig. 2 is a sectional view of a "gain guidance"
laser in accordance with the prior art ;
- Fig. 3 is a sectional view of an "index guidance" laser having a buried heterostructure in accordance with the prior art ;
- Fig. 4 is a sectional view of an etched-substrate index guidance laser in accordance with the prior art ;
- Fig. 5 is a sectional view of the index guidance laser in a first alternative embodiment of the invention ;
- Fig. 6 illustrates another alternative embodiment in accordance with the invention.
Before entering into a description of the invention, a preliminary summary of the different known solutions and difficulties involved will serve to gain a clearer understanding of the object of the invention and its advantages.
Fig. 1 is a simplified view of a stripe-geometry solid state laser which, as stated earlier, is the most commonly employed in conjunction with optical fixers. Various methods are known for fabricating a structure of this type including an active region in the form of a stripe in which the radiation and toe electron-hole pairs are confined.
A laser of this type comprises a substrate 1 which carries an active layer 2 limited by two confinement layers 3 and 4 with which said active layer forms two heterojunctions. Light emission is produced solely by a stripe 5 formed within the active layer 2 in order to ensure concentration and guiding o-E the beam.
The thickness d of the active region 5 is usually within the range of 0.05 to 1 em and is determined by heterojunctions. The length 1 is within the range of 100 to 500 em and is equal to the distance between the cleaved faces which limit the crystal at the ends of the stripe 5. The large number of known stripe-geometry structures are related to the mode of determination of the width w of the active region 5 which may be within the range of 1 to 10 em in the case of low-power lasers for telecommunications.
lo These structures can be grouped in two classes according to the principal physical effect which produces radiation guidance in the transverse direction Ox of the stripe whereas guidance in the direction Ox is ensured by the heterojunctions.
Gain guidance constitutes a first class of structure illustrated in Fig. 2.
The same references have been retained in Fig. 2 in order to facilitate a comparison with Fig. 1.
Transverse guidance is ensured in this case by the profile of the stimulated emission gain which is in turn determined by the density profile of injected I
carriers which must exhibit a maximum at the center of the active region. A number of means for the achievement of this result have already been described in the literature. All these methods are based on deposition of uniform epitaxial layers on a flat substrate. Means are then employed for preventing the flow of current out side the active region. For example, in the laser of Fig. 2 which comprises an active layer 2 formed on a substrate and limited by two confiIlement layers 3 and 4, lo the emission gain profile is determined by two insulating compartments 6 and 7 formed in -the top confinement layer 4.
A metallization layer 8 covers the entire external face of the laser on the side remote from the top face of the substrate 1 but, by reason of the insulating compartments 6 and 7, the density profile of electric charge carriers is at a maximum at the center of the active region. This variation in density is designated schematically in Fig. 2 by arrows which are closely spaced in proximity to that portion ox the metallization layer 8 which is in contact with the confinement layer 4 and consequently in proximity to the central region of the active layer 2 but are less closely spaced in the external regions of said active layer.
The present Applicant has already fabricated structures of this type by implantation of protons to a depth of approximately l micron in order -to provide local insulation of the compartments 6 and 7.
These structures suffer from two drawbacks. On the one hand, the effective width of the stripe cannot be reduced to less than a few microns by reason of the lateral S diffusion of the free carriers injected into the active region. Furthermore, since the density profile of free carriers has a maximum value at the center, the real portion of the refractive index of the material is con-sequently at a minimum value, thus producing an anti-guidance effect and finally an increase in losses. Theist effects set a lower limit of about 40 PA on the threshold current value of lasers at an emission wave-length of 0.85 em and an upper limit of over 100 ma at 1~3 em.
Beneath the structure of Fig. 2, there is shown A curve representing the real portion of the refractive index _ as a function of the abscissa x in the active layer 2. This curve clearly shows that the refractive index n is at a minimum (in its real portion) at the center of the stripe which is delimited by the density profile.
The second light guidance class utilizes the refractive index and is illustrated in Figs. 3 and 4.
In these structures, transverse guidance results from a profile of the real portion of the effective refractive index of the guide which is so designed as to provide a maximum at the center of the active region which is distinctly higher than the minimum mentioned earlier.
The effective refractive index of a guide can be con-ridered as a suitably weighted average of the refractive indices of the different materials used locally in said guide and therefore depends both on the materials and on the structure. The index guidance structures described up to the present time can be placed in two classes.
The first class illustrated in Fig. 3 relates to buried heterostructure lasers.
The active zone 9 of a laser of this type has a rectangular cross-section and is inserted in a p-n junction of materials having a forbidden band of greater width and consequently having a lower refractive index.
The transverse profile of the effective index of the guide is therefore a jump profile. The method of construction comprises a first uniform deposition of epitaxial layers 10, and 11 on a flat substrate 1. A photo etching operation then localizes the active region 9 in the form of a mesa A second epitaxial deposit then covers the sides of said mesa with two compartments 12 and 13 : the light-emitting s-tripe 9 is therefore buried and delimited geometrically by the -two confinement layers 10 and 11 and by the -two compartments 12 and 13. The conductivity types are shown in Fig. 3 by way of indication and could be reversed A metallization layer 8 deposited on the free face of the laser serves to make an electrical contact.
The curve of refractive index n which has been added beneath the structure of Fig. 3 shows that its real portion has a maximum value.
The second type of index-gradient guidance structure is of the etched substrate type and is illustrated in Fig. 4, On a substrate 1 in which a groove has been etched, a first layer 14 is grown by epitaxy. Said first layer has a depression directly above the groove, in which epitaxial growth is more rapid than in the flat portions of the substrate During epitaxial deposition of a layer 15 which is the active layer, a meniscus is formed within the depression of the first layer 14, thus producing a maximum thickness in the central region of the active layer 15. The result thereby achieved is that the profile of the effective index also exhibits a maximum as shown by the index curve beneath the structure. An index-gradient profile is thus obtained A second confinement layer 16 and a metallization layer 8 complete the structure.
It is then necessary to localize the current injection region by a method of the type employed for gain-guidance lasers by means of insulating compartments implanted at the surface since the active layer occupies the entire width of the laser device.
The threshold currents obtained in the case of index-guidance lasers are much lower than those of gain-guidance lasers namely 10 ma instead of 40 ma This current could be of even lower value if it were possible to eliminate the leakage current passing through the junctions located on each side of the active region.
It is therefore particularly advantageous to ensure that the active layer is limited in the transverse direction in order to prevent any lateral leakage current which only causes a reduction in laser efficiency while requiring a higher threshold current. However, it is also desirable to ensure that the active region is buried since a structure of this type has a long lifetime and low thermal resistance.
The laser structure in accordance with the invention and as shown in Fig. 5 satisfies these require-mints.
Said structure is formed on a p-type substrate of In doped by zinc with a free-hole concentration in the vicinity of 10 8 cm . The indium phosphide In is one ox the substrates in most common use at the present time for solid state lasers but this specific choice does not limit the scope of the invention which is generally applicable to all types of materials both for the substrate and for the layers of the light-emitting structure as well as to conductivity types other than those herein described.
Said substrate 1 has a V-shaped groove 17, the angular distance of which is approximately 5 microns. It is solely within this groove that four epitaxial layers are grown successively.
The first layer 18 is a p-type confinement layer of In doped by zinc with a free-hole concentration within the range of 1017 to 1018 cm 3 and a thickness of 2 microns at the center. By reason of the fact that the epitaxial growth is more rapid within the concave portions, the layer 18 has a tendency to fill the bottom of the V-shaped groove 17 and the top face of said layer has the shape of a concave meniscus at the end of the epitaxy " 15 operation.
On said first layer 18 is grown a second layer 19 constituting the active layer of unhoped Go In As Pi the composition of which is chosen so as to ensure that this solid solution has a lattice parameter which is identical with that of the substrate and that the emission wavelength is 1.30 em. It is known that these conditions are all met when x = 0.28 and y = 0.60 The conditions of epitaxy are so adjusted that the active layer has a crescent shape with a thickness at the center within the range of 0.1 to 0.5 em and zero thickness at the edges. This shape is obtained quite naturally since epitaxial growth taxes place on a surface which is already concave, namely that of the firs-t layer 18, which is con-disavow to greater growth at the center of the concave portion.
The laser receives a second heterojunction formed by the second confinement layer 20 of n-type In doped by tin with a free-electron concentration which is higher than the free-hole concentration of the layer 18 but is lets than 10 8 cm 3. Said second confinement layer also has the shape of a meniscus since its epitaxial growth started on the active layer 19, this layer itself being concave on its top surface.
Finally, a contact layer 21 of n-type In doped by tin with a free-electron concentration greater than em is deposited on top of the second confinement layer 20. The conditions of epitaxial growth of this layer are so adjusted that its surface is located at the same level as the substrate and is substantially flat.
I Protons are implanted on each side of the groove to a depth of approximately 1 em in order to make the material insulating in accordance with well-known practice. The implanted insulating compartments 22 and 23 have the effect of isolating the substrate 1 from the top metallic contact outside the groove. In fact, metallization layers 24 and 25 are deposited on the top and bottom faces of the structure in order to make electrical contacts.
The laser structure in accordance with the invention has an appearance which is remarkable for the shape of the groove 17 and the limitation of the active and confinement layers within said groove.
In fact, the groove 17 which is etched in the substrate 1 originally has a V-shape with flat faces prior to commencement of the epitaxy operations, with the result that a sharp edge is formed a-t the point of junction of the groove with the top face of the substrate.
However, as stated earlier, the epitaxial growth process develops more rapidly in the concave regions than in the regions which are convex or which have a sharp convex edge. In consequence, during the different epitaxy operations which are necessary in order to form the layers of the laser, said layers are preferentially formed within the groove, are limited to said groove and buried therein. At the same time, the sharp edge formed between the groove faces and the top face of the substrate is slightly dissolved at each operation in the epitaxial bath. When fabrication of the laser has been completed, the V-groove 17 consequently has a convex rounded surface 26 which forms a junction with the top plane of the substrate and on which epitaxial growth does not take place.
The fact that the layers of the laser are limited to the interior of the groove 17 is due in particular to the convex surface 26 which joins the groove to the top plane of the substrate.
During epitaxy operations the materials con-stituting the epitaxial layers may possibly be deposited on the flat regions of the substrate. The methods of epitaxial growth are adjusted as indicated hereinafter in order to ensure that the layers deposited on the flat regions are of appreciably smaller thickness than within the groove and are in any case separated from the layers deposited within the groove by the convex surface 26 of the substrate.
A first alternative embodiment of the laser structure described in the foregoing is illustrated in Fig. 6. Isolation between the substrate 1 and the top metallization layer 24 is obtained by means of a thin layer 27 of insulating material such as alumina which no-places the implanted compartments 22 and 23. Said insulating layer 27 is provided with a window directly above the groove in order to make an electrical contact between the top metallization layer 24 and the contact layer 21.
Another possible alternative consists in reversing the conductivity types ox the materials so that the substrate 1 and the first confinement layer 18 is I
n-type whilst the second conrinemenet layer 20 and the contact layer 21 are p-type.
Similarly, the materials mentioned in the fore-going do not imply any limitation of the invention. The choice of such materials corresponds to criteria such as the desired emission wavelength. Thus the materials of the GaxAll system deposited on a Gays substrate will be chosen so as to emit within the spectral region of 0.7 - 0.9 em. The essential condition to be satisfied lies in the fact that the forbidden bandwidth of the active layer 19 is smaller by at least 0.2 eve than the bandwidth of the confinement layers 18 and 20 so as to constitute a double heterojunction structure.
Finally, the contact layer 21 can have a compost-lion which is different from that of the confinement Lowry in order to minimize the contact resistance. Thus in the case of materials of the Galas system, said contact layer will advantageously be of Gays whereas, in the case of the materials of the GaInAsP system, said layer can be of Go inn assay, which will be preferable if the substrate is of n-type In.
As stated in the foregoing, the method of fabrication of the laser in accordance with the invention is based on localized epitaxy within the groove. However, in order to ensure that the layers are effectively localized within the groove, the conditions of epitaxy are very precise and correspond to what is known as zero supersaturation. Two epitaxial growth techniques are employed, namely liquid-phase epitaxy and vapor-phase epitaxy. In all cases, the fabrication of lasers is clearly a collective operation on substrate wafers on which a plurality of devices are assembled together.
In a first technique, the substrate is provided only with the grooves which have been previously photo-etched. Said substrate is placed in a shutter-type liquid-epitaxy crucible of known design in which it can be brought successively into position beneath a number of wells containing liquid solutions, the compositions of which are chosen as a function of the composition of the epi-taxial layers to be deposited. The epita~ial deposit lion process is different from the usual techniques. In fact, the composition of each liquid solution is adjusted so as to ensure that its temperature of equilibrium T
with a flat substrate corresponds to within + 0.5C to the constant temperature To which is chosen for the epitaxial growth process and depends on the materials considered (550 to 650C in the case of GaInAsP/InP, 750 to 850C in the case of GaAlAs/GaAs). The substrate is then introduced successively beneath each liquid solution. Growth is zero on the flat portions of the substrate at equilibrium but is effective within the grooves in which the concave portion is directed upwards or in other words towards the epitaxy wells, thus determining the profiles of epitaxial layers described with reference to Figs. 5 and 6. The growth process ends with the layer 21 when the surface reaches the level of the substrate.
Since supersaturation is defined by IT = To To' this growth technique is referred-to as "zero superstar-lion" growth.
In a variant which corresponds to fabrication of the laser insulated with alumina, the substrate come proses an alumina layer which is open above the grooves as shown in Fig. 6. The epitaxy technique is similar to that described earlier but, in this case, the presence of the alumina allows the use of a less narrow tolerance in regard to supersaturation.
Vapor-phase epitaxy techniques are also employed, in particular those which are close to thermodynamic equilibrium and which make it possible in accordance with known practice, by adjusting the composition of the vapor phase, to proceed from etch-cleaning of the substrate to epitaxial growth and consequently to adjust the super-saturation.
It is therefore as a result of zero superstar-lion that the special growth of limited layers takes place within a groove for the fabrication of a laser having a buried active layer in accordance with the invention as will be defined in the appended claims.
The laser in accordance with the invention circumvents these disadvantages. In this device, provision is made for a buried active layer in which the injection region is focalized during the epitaxial growth operation within a groove cut in the substrate and is limited solely to the width of the groove. When viewed in cross-section, the active layer has the shape of a crescent, the thickness of which is zero at the edges which are in contact with the groove walls. This structure, in which the injection region is limited in the transverse directions obtained by means of a single operation of liquid-phase epoxy with zero supersaturation during which the epitaxial growth process it rapid on the concave surfaces and zero on the convex surfaces.
In more exact terms, the invention consists of a light-emitting device of the laser type which emits light trough a stripe-geometry active layer within which light it guided by a refractive-index gradient. The device comprises a substrate having an etched V-groove in which the active layer of the laser is buried between two confinement layers with which said active layer forms two heterojunctions. Said device further comprises a contact layer deposited on the external confinement layer and two elect rical-contact metallizations deposited on the two principal faces of the substrate. The distinctive feature of the laser lies in the fact that, on the one hand, the first confinement layer, the active layer and the second confinement layer are limited to the interior of the etched V-groove and that, on the other hand, the active layer is limited by two heterojunctions of concave shape, the concave side of which is directed towards the external confinement layer. The active layer has zero thickness at the edges which are in contact with the groove ; the difference in thickness between the central portion and the edges of the active layer thus produces a refractive index gradient for guiding the emitted light.
Other features of the invention will be more apparent upon consideration of the following description and accompanying drawings, wherein :
- Fig. 1 is a simplified view of a stripe-geometry semiconductor laser ;
- Fig. 2 is a sectional view of a "gain guidance"
laser in accordance with the prior art ;
- Fig. 3 is a sectional view of an "index guidance" laser having a buried heterostructure in accordance with the prior art ;
- Fig. 4 is a sectional view of an etched-substrate index guidance laser in accordance with the prior art ;
- Fig. 5 is a sectional view of the index guidance laser in a first alternative embodiment of the invention ;
- Fig. 6 illustrates another alternative embodiment in accordance with the invention.
Before entering into a description of the invention, a preliminary summary of the different known solutions and difficulties involved will serve to gain a clearer understanding of the object of the invention and its advantages.
Fig. 1 is a simplified view of a stripe-geometry solid state laser which, as stated earlier, is the most commonly employed in conjunction with optical fixers. Various methods are known for fabricating a structure of this type including an active region in the form of a stripe in which the radiation and toe electron-hole pairs are confined.
A laser of this type comprises a substrate 1 which carries an active layer 2 limited by two confinement layers 3 and 4 with which said active layer forms two heterojunctions. Light emission is produced solely by a stripe 5 formed within the active layer 2 in order to ensure concentration and guiding o-E the beam.
The thickness d of the active region 5 is usually within the range of 0.05 to 1 em and is determined by heterojunctions. The length 1 is within the range of 100 to 500 em and is equal to the distance between the cleaved faces which limit the crystal at the ends of the stripe 5. The large number of known stripe-geometry structures are related to the mode of determination of the width w of the active region 5 which may be within the range of 1 to 10 em in the case of low-power lasers for telecommunications.
lo These structures can be grouped in two classes according to the principal physical effect which produces radiation guidance in the transverse direction Ox of the stripe whereas guidance in the direction Ox is ensured by the heterojunctions.
Gain guidance constitutes a first class of structure illustrated in Fig. 2.
The same references have been retained in Fig. 2 in order to facilitate a comparison with Fig. 1.
Transverse guidance is ensured in this case by the profile of the stimulated emission gain which is in turn determined by the density profile of injected I
carriers which must exhibit a maximum at the center of the active region. A number of means for the achievement of this result have already been described in the literature. All these methods are based on deposition of uniform epitaxial layers on a flat substrate. Means are then employed for preventing the flow of current out side the active region. For example, in the laser of Fig. 2 which comprises an active layer 2 formed on a substrate and limited by two confiIlement layers 3 and 4, lo the emission gain profile is determined by two insulating compartments 6 and 7 formed in -the top confinement layer 4.
A metallization layer 8 covers the entire external face of the laser on the side remote from the top face of the substrate 1 but, by reason of the insulating compartments 6 and 7, the density profile of electric charge carriers is at a maximum at the center of the active region. This variation in density is designated schematically in Fig. 2 by arrows which are closely spaced in proximity to that portion ox the metallization layer 8 which is in contact with the confinement layer 4 and consequently in proximity to the central region of the active layer 2 but are less closely spaced in the external regions of said active layer.
The present Applicant has already fabricated structures of this type by implantation of protons to a depth of approximately l micron in order -to provide local insulation of the compartments 6 and 7.
These structures suffer from two drawbacks. On the one hand, the effective width of the stripe cannot be reduced to less than a few microns by reason of the lateral S diffusion of the free carriers injected into the active region. Furthermore, since the density profile of free carriers has a maximum value at the center, the real portion of the refractive index of the material is con-sequently at a minimum value, thus producing an anti-guidance effect and finally an increase in losses. Theist effects set a lower limit of about 40 PA on the threshold current value of lasers at an emission wave-length of 0.85 em and an upper limit of over 100 ma at 1~3 em.
Beneath the structure of Fig. 2, there is shown A curve representing the real portion of the refractive index _ as a function of the abscissa x in the active layer 2. This curve clearly shows that the refractive index n is at a minimum (in its real portion) at the center of the stripe which is delimited by the density profile.
The second light guidance class utilizes the refractive index and is illustrated in Figs. 3 and 4.
In these structures, transverse guidance results from a profile of the real portion of the effective refractive index of the guide which is so designed as to provide a maximum at the center of the active region which is distinctly higher than the minimum mentioned earlier.
The effective refractive index of a guide can be con-ridered as a suitably weighted average of the refractive indices of the different materials used locally in said guide and therefore depends both on the materials and on the structure. The index guidance structures described up to the present time can be placed in two classes.
The first class illustrated in Fig. 3 relates to buried heterostructure lasers.
The active zone 9 of a laser of this type has a rectangular cross-section and is inserted in a p-n junction of materials having a forbidden band of greater width and consequently having a lower refractive index.
The transverse profile of the effective index of the guide is therefore a jump profile. The method of construction comprises a first uniform deposition of epitaxial layers 10, and 11 on a flat substrate 1. A photo etching operation then localizes the active region 9 in the form of a mesa A second epitaxial deposit then covers the sides of said mesa with two compartments 12 and 13 : the light-emitting s-tripe 9 is therefore buried and delimited geometrically by the -two confinement layers 10 and 11 and by the -two compartments 12 and 13. The conductivity types are shown in Fig. 3 by way of indication and could be reversed A metallization layer 8 deposited on the free face of the laser serves to make an electrical contact.
The curve of refractive index n which has been added beneath the structure of Fig. 3 shows that its real portion has a maximum value.
The second type of index-gradient guidance structure is of the etched substrate type and is illustrated in Fig. 4, On a substrate 1 in which a groove has been etched, a first layer 14 is grown by epitaxy. Said first layer has a depression directly above the groove, in which epitaxial growth is more rapid than in the flat portions of the substrate During epitaxial deposition of a layer 15 which is the active layer, a meniscus is formed within the depression of the first layer 14, thus producing a maximum thickness in the central region of the active layer 15. The result thereby achieved is that the profile of the effective index also exhibits a maximum as shown by the index curve beneath the structure. An index-gradient profile is thus obtained A second confinement layer 16 and a metallization layer 8 complete the structure.
It is then necessary to localize the current injection region by a method of the type employed for gain-guidance lasers by means of insulating compartments implanted at the surface since the active layer occupies the entire width of the laser device.
The threshold currents obtained in the case of index-guidance lasers are much lower than those of gain-guidance lasers namely 10 ma instead of 40 ma This current could be of even lower value if it were possible to eliminate the leakage current passing through the junctions located on each side of the active region.
It is therefore particularly advantageous to ensure that the active layer is limited in the transverse direction in order to prevent any lateral leakage current which only causes a reduction in laser efficiency while requiring a higher threshold current. However, it is also desirable to ensure that the active region is buried since a structure of this type has a long lifetime and low thermal resistance.
The laser structure in accordance with the invention and as shown in Fig. 5 satisfies these require-mints.
Said structure is formed on a p-type substrate of In doped by zinc with a free-hole concentration in the vicinity of 10 8 cm . The indium phosphide In is one ox the substrates in most common use at the present time for solid state lasers but this specific choice does not limit the scope of the invention which is generally applicable to all types of materials both for the substrate and for the layers of the light-emitting structure as well as to conductivity types other than those herein described.
Said substrate 1 has a V-shaped groove 17, the angular distance of which is approximately 5 microns. It is solely within this groove that four epitaxial layers are grown successively.
The first layer 18 is a p-type confinement layer of In doped by zinc with a free-hole concentration within the range of 1017 to 1018 cm 3 and a thickness of 2 microns at the center. By reason of the fact that the epitaxial growth is more rapid within the concave portions, the layer 18 has a tendency to fill the bottom of the V-shaped groove 17 and the top face of said layer has the shape of a concave meniscus at the end of the epitaxy " 15 operation.
On said first layer 18 is grown a second layer 19 constituting the active layer of unhoped Go In As Pi the composition of which is chosen so as to ensure that this solid solution has a lattice parameter which is identical with that of the substrate and that the emission wavelength is 1.30 em. It is known that these conditions are all met when x = 0.28 and y = 0.60 The conditions of epitaxy are so adjusted that the active layer has a crescent shape with a thickness at the center within the range of 0.1 to 0.5 em and zero thickness at the edges. This shape is obtained quite naturally since epitaxial growth taxes place on a surface which is already concave, namely that of the firs-t layer 18, which is con-disavow to greater growth at the center of the concave portion.
The laser receives a second heterojunction formed by the second confinement layer 20 of n-type In doped by tin with a free-electron concentration which is higher than the free-hole concentration of the layer 18 but is lets than 10 8 cm 3. Said second confinement layer also has the shape of a meniscus since its epitaxial growth started on the active layer 19, this layer itself being concave on its top surface.
Finally, a contact layer 21 of n-type In doped by tin with a free-electron concentration greater than em is deposited on top of the second confinement layer 20. The conditions of epitaxial growth of this layer are so adjusted that its surface is located at the same level as the substrate and is substantially flat.
I Protons are implanted on each side of the groove to a depth of approximately 1 em in order to make the material insulating in accordance with well-known practice. The implanted insulating compartments 22 and 23 have the effect of isolating the substrate 1 from the top metallic contact outside the groove. In fact, metallization layers 24 and 25 are deposited on the top and bottom faces of the structure in order to make electrical contacts.
The laser structure in accordance with the invention has an appearance which is remarkable for the shape of the groove 17 and the limitation of the active and confinement layers within said groove.
In fact, the groove 17 which is etched in the substrate 1 originally has a V-shape with flat faces prior to commencement of the epitaxy operations, with the result that a sharp edge is formed a-t the point of junction of the groove with the top face of the substrate.
However, as stated earlier, the epitaxial growth process develops more rapidly in the concave regions than in the regions which are convex or which have a sharp convex edge. In consequence, during the different epitaxy operations which are necessary in order to form the layers of the laser, said layers are preferentially formed within the groove, are limited to said groove and buried therein. At the same time, the sharp edge formed between the groove faces and the top face of the substrate is slightly dissolved at each operation in the epitaxial bath. When fabrication of the laser has been completed, the V-groove 17 consequently has a convex rounded surface 26 which forms a junction with the top plane of the substrate and on which epitaxial growth does not take place.
The fact that the layers of the laser are limited to the interior of the groove 17 is due in particular to the convex surface 26 which joins the groove to the top plane of the substrate.
During epitaxy operations the materials con-stituting the epitaxial layers may possibly be deposited on the flat regions of the substrate. The methods of epitaxial growth are adjusted as indicated hereinafter in order to ensure that the layers deposited on the flat regions are of appreciably smaller thickness than within the groove and are in any case separated from the layers deposited within the groove by the convex surface 26 of the substrate.
A first alternative embodiment of the laser structure described in the foregoing is illustrated in Fig. 6. Isolation between the substrate 1 and the top metallization layer 24 is obtained by means of a thin layer 27 of insulating material such as alumina which no-places the implanted compartments 22 and 23. Said insulating layer 27 is provided with a window directly above the groove in order to make an electrical contact between the top metallization layer 24 and the contact layer 21.
Another possible alternative consists in reversing the conductivity types ox the materials so that the substrate 1 and the first confinement layer 18 is I
n-type whilst the second conrinemenet layer 20 and the contact layer 21 are p-type.
Similarly, the materials mentioned in the fore-going do not imply any limitation of the invention. The choice of such materials corresponds to criteria such as the desired emission wavelength. Thus the materials of the GaxAll system deposited on a Gays substrate will be chosen so as to emit within the spectral region of 0.7 - 0.9 em. The essential condition to be satisfied lies in the fact that the forbidden bandwidth of the active layer 19 is smaller by at least 0.2 eve than the bandwidth of the confinement layers 18 and 20 so as to constitute a double heterojunction structure.
Finally, the contact layer 21 can have a compost-lion which is different from that of the confinement Lowry in order to minimize the contact resistance. Thus in the case of materials of the Galas system, said contact layer will advantageously be of Gays whereas, in the case of the materials of the GaInAsP system, said layer can be of Go inn assay, which will be preferable if the substrate is of n-type In.
As stated in the foregoing, the method of fabrication of the laser in accordance with the invention is based on localized epitaxy within the groove. However, in order to ensure that the layers are effectively localized within the groove, the conditions of epitaxy are very precise and correspond to what is known as zero supersaturation. Two epitaxial growth techniques are employed, namely liquid-phase epitaxy and vapor-phase epitaxy. In all cases, the fabrication of lasers is clearly a collective operation on substrate wafers on which a plurality of devices are assembled together.
In a first technique, the substrate is provided only with the grooves which have been previously photo-etched. Said substrate is placed in a shutter-type liquid-epitaxy crucible of known design in which it can be brought successively into position beneath a number of wells containing liquid solutions, the compositions of which are chosen as a function of the composition of the epi-taxial layers to be deposited. The epita~ial deposit lion process is different from the usual techniques. In fact, the composition of each liquid solution is adjusted so as to ensure that its temperature of equilibrium T
with a flat substrate corresponds to within + 0.5C to the constant temperature To which is chosen for the epitaxial growth process and depends on the materials considered (550 to 650C in the case of GaInAsP/InP, 750 to 850C in the case of GaAlAs/GaAs). The substrate is then introduced successively beneath each liquid solution. Growth is zero on the flat portions of the substrate at equilibrium but is effective within the grooves in which the concave portion is directed upwards or in other words towards the epitaxy wells, thus determining the profiles of epitaxial layers described with reference to Figs. 5 and 6. The growth process ends with the layer 21 when the surface reaches the level of the substrate.
Since supersaturation is defined by IT = To To' this growth technique is referred-to as "zero superstar-lion" growth.
In a variant which corresponds to fabrication of the laser insulated with alumina, the substrate come proses an alumina layer which is open above the grooves as shown in Fig. 6. The epitaxy technique is similar to that described earlier but, in this case, the presence of the alumina allows the use of a less narrow tolerance in regard to supersaturation.
Vapor-phase epitaxy techniques are also employed, in particular those which are close to thermodynamic equilibrium and which make it possible in accordance with known practice, by adjusting the composition of the vapor phase, to proceed from etch-cleaning of the substrate to epitaxial growth and consequently to adjust the super-saturation.
It is therefore as a result of zero superstar-lion that the special growth of limited layers takes place within a groove for the fabrication of a laser having a buried active layer in accordance with the invention as will be defined in the appended claims.
Claims (7)
1. A light-emitting semiconductor device of the laser type for light emission through a stripe-geometry active layer within which the light is guided by a refractive-index gradient, the device being constituted by a substrate having an etched V-groove in which the active layer of the laser is buried between two confinement layers with which said actice layer forms two heterojunctions, said device being further constituted by a contact layer deposited on the external confinement layer and two electrical-contact metallization layers deposited on the two principal faces of the substrate, wherein, on the one hand, the first confinement layer, the active layer and the second confinement layer are limited to the interior of the etched V-groove and wherein, on the other hand, the active layer is limited by two heterojunctions of concave shape, the concave side of which is directed towards the external confinement layer, the active layer being such as to have zero thickness at the edges which are in contact with the groove, the difference in thickness between the central portion and the edges of said active layer being such as to produce a refractive index gradient for guiding the emitted light.
2. A semiconductor device according to claim 1, wherein injection of current into the laser is limited to the groove region by two compartments which are made insulating by implantation of protons and isolate the top metallization layer from the substrate.
3. A semiconductor device according to claim 1, wherein the injection of current into the laser is limited to the groove region by an alumina layer which has an open portion directly above the groove which isolates the top metallization layer from the substrate.
4. A semiconductor device according to any one of claims 1 to 3, wherein the substrate and the first confine-ment layer are of n-type InP, the active layer is of GaxIn1-xAsyP1-y with x = 0.28 and y = 0.60, the second confinement layer is of p-type InP and the contact layer is of p-type Ga0.47In0.53As.
5. A semiconductor device according to any one of claims 1 to 3, wherein the substrate and the first confine-ment layer are of p-type InP, the active layer is of GaxIn1-xAsyP1-y with x = 0.28 and y = 0.60, the second confinement layer and the contact layer are of n-type InP.
6. A semiconductor device according to claim 1, wherein the V-groove has two surfaces which are joined to the top face of the substrate and are of convex shape, the convex side of said surfaces being directed towards the groove.
7. A method of fabrication of a laser-type semi-conductor device according to any one of claims 1 to 3, wherein the active layer, the confinement layers and the contact layer are obtained by epitaxial growth localized within the groove under conditions of zero supersaturation .DELTA.T = Te - Tc, where Te is the temperature of equilibrium of the epitaxial medium with a flat substrate and Tc is the epitaxial growth temperature, the composition of the epitaxial medium for growth of a layer being chosen so that Te = Tc.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR8209735 | 1982-06-04 | ||
| FR8209735A FR2528234A1 (en) | 1982-06-04 | 1982-06-04 | INDEX GRADIENT GUIDED LASER TYPE SEMICONDUCTOR DEVICE AND METHOD OF MAKING SAME |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1215160A true CA1215160A (en) | 1986-12-09 |
Family
ID=9274630
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000429594A Expired CA1215160A (en) | 1982-06-04 | 1983-06-02 | Stripe-geometry solid-state laser with light guidance by transverse refractive-index gradient |
Country Status (6)
| Country | Link |
|---|---|
| EP (1) | EP0096613B1 (en) |
| JP (1) | JPS58219772A (en) |
| KR (1) | KR840005276A (en) |
| CA (1) | CA1215160A (en) |
| DE (1) | DE3364511D1 (en) |
| FR (1) | FR2528234A1 (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2563051B1 (en) * | 1984-04-13 | 1987-08-07 | Telecommunications Sa | LASER DIODE WITH ELECTRICAL CONTAINMENT BY REVERSE JUNCTION |
| US4634928A (en) * | 1985-04-19 | 1987-01-06 | Trw Inc. | Superluminescent light-emitting diode and related method |
| DE3728566A1 (en) * | 1987-08-27 | 1989-03-09 | Telefunken Electronic Gmbh | Optoelectronic semiconductor component |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1570479A (en) * | 1978-02-14 | 1980-07-02 | Standard Telephones Cables Ltd | Heterostructure laser |
-
1982
- 1982-06-04 FR FR8209735A patent/FR2528234A1/en active Granted
-
1983
- 1983-05-24 EP EP83401029A patent/EP0096613B1/en not_active Expired
- 1983-05-24 DE DE8383401029T patent/DE3364511D1/en not_active Expired
- 1983-06-01 KR KR1019830002439A patent/KR840005276A/en not_active Application Discontinuation
- 1983-06-02 CA CA000429594A patent/CA1215160A/en not_active Expired
- 1983-06-03 JP JP58098178A patent/JPS58219772A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| KR840005276A (en) | 1984-11-05 |
| FR2528234B1 (en) | 1985-05-17 |
| DE3364511D1 (en) | 1986-08-21 |
| EP0096613B1 (en) | 1986-07-16 |
| EP0096613A1 (en) | 1983-12-21 |
| FR2528234A1 (en) | 1983-12-09 |
| JPS58219772A (en) | 1983-12-21 |
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