CA2234517A1 - Blue-green lase diode - Google Patents

Abstract

A II-VI compound semiconductor laser diode (10) is formed from overlaying layers of material including an n-type single crystal semiconductor substrate (12), adjacent n-type and p-type guiding lasers (14) and (16) or II-VI semiconductor forming a pn junction, a quantum well active layer (18) of II-VI semiconductor between the guiding layers (14) and (16), first electrode (32) opposite the substrate (12) from the n-type guiding layer (14), and a second electrode (30) opposite the p-type guiding layer (16) from the quantum well layer (18). Electrode layer (30) is characterized by a Fermi energy. A p-type ohmic contact layer (26) is doped, with shallow acceptors having a shallow acceptor energy, to a net acceptor concentration or at least 1 x 10 17 cm-3, and includes sufficient deep energy states between the shallow acceptor energy and the electrode layer Fermi energy to enable cascade tunneling by charge carriers.

Description

~v 92~21 170 PCI'/US9~/0 ~, PIN IN THIS AM~NDED
TEXT TRl~N~-l~T~lsl BLUE-GREEN LASER DIODE

BA~RO~ND OF T~ ~NY~NTTON
5Semiconductor laser diod~s are generally known and disclosed, for example, in Chapter 12 of Sze, Phys~cs of Semiconductor Devices, 2nd ed. pp. 681-742 (1981). To date, most commercially available laser diodes are fabricated from Group III-V compol~n~ semiconductor6 and 10 their ~lloys such as G~As and AlG~As. These devices emit light in the infrared and red portion~ of the spectrum, eg., at wavelengths between 630 and 1550 nm. Laser diodes of these types are used in a wide range of appllcations such as communications, recording, sensing 15 and imaging systems.
Nonetheless, there are many applications for which the wavelength of light generated by infrared and red laser diodes is not suit~ble. Commercially viable laser diodes which emit radiation at shorter wavelengths, 20 for example in the green and blue portions of the spectrum (ie., at wavelengths between 590 and 430 nm) would have widespread application. Shorter wavelength laser diodes would also increase the performance and capabilities of mAny system6 which currently use infrared 25 and red laser diodes.
Wide band gap II-VI semiconductors and alloys, and in particular ZnSe, have for many years been called promising materials for the fabrication of blue and green light emitting devices. In the 1960's, laser action was 30 demonstrated in several II-VI ~emiconductors using electron-beam pumping technigues. Colak et al., Electron BeAm Pumped II-VI L~sers, J. Crystal Growth 72, 504 (1985) includes a review of this work. There have also been more recent demonstration~ of photopumped and 35 electron-beam pumped lasing action from epitaxial II-VI
semiconductor materials. See eg., Potts et al., Electron Beam Pumped Lastng In ZnSe Grown ~y Molecular-Beam WC '~21170 PCTJUS92/03~8 Epltaxy, Appl Phys Lett , 50, 7 (1987) and ~ing et ~l , L~ser Action In ~he ~lue-Gre~n From optlc~lly ~ump~d (8n,Cd)S~/ZnSe S~ngle Qu~ntum Well StructurQs, Appl Phy6 Lett 57, p 2756 (l990) As re--arch on wide band 5 gap II-YI ~emiconductor dQvices ~6~ d, ~-v-ral k-y technological difficulties w~re identifi~d These difficulties included l) the inability to ~L Gduce lo~-re~istivity p-type ZnSe and related alloy~; 2) the inability to form d~vic--quality ohmic contacts to p-type lO ZnSe and related alloy~, and 3) the lack of a 6uitable lattice-match~d heterostructur~ material ~ystem Modern epitaxial growth tec~n~gues such a6 molecul~r beam ~pitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) are now used to fabricat~ device 15 quality undoped ~nd n-type ZnSe layers, typically on GaAs substrates The growth of low re~iRtivity p-type ZnSe u~ing Li and N (NH3) a~ Aorants has also been reported For ~ome time it appear~d that the upper limit of obtAinable net acceptor conc~ntratlon~ (N~-ND) wa~ ~bout 20 l017cm-3 Recently, however, signiflcantly gr~ater net acceptor concentration~ h~ve bQ~n achi-vQd ln ZnSQ N
grown by MBE using nitrogen ~r~ radicals proA~c~ by an rf pla~ma 60urcQ S~e ~g , Park Qt al , P-type ZnSe By N~trogen Atom Be~m Doping Our~ng Mol~cular B~m ~p~tax~l 25 Growth, Appl Phy6 Lett 57, 2127 (l990) Th~ largQct net acceptor concontration in ZnSe achieved through the use of the~e techniquQ~ is 2XlOI~C~ 3 . U~ing th~e technologies, rudimentary blue light em~tting diode~ have been reported by 6everal laboratorie- 8ee ~g , the Park 30 et al Appl Phy~ L~tt article rQferred to imm-diat~ly above Of the wid~ band gap II-VI s~miconductor sy~tems that are reasonably w~ll developed, iQ ., ZnS~Te, CdZnSe, ZnSSe and CdZnS, only CdZnS-ZnS~ offers a 3g lattice-matched ~ystQm Unfortunat-ly, thi~ ~y~tem offer~ only a very ~mall band gap difference (about O OS
eV), which i~ far too ~mall for the carrier confinem~nt ~1 ~2/21170 PCl'rUS92/03 needed for simple double hetero~tructure laser diodes.
Therefore, to achieve A band gap difference greater than 0.2 eV, it would be n~co~ry to use a 6trained-layer system (eg., ZnSe-Cd~Znl,Se with x > 0.2). To prevent 5 misfit di~loc~tion~ which degr~de the luminescence efficiency, the thickness of the ctrained layer should be kept less than the critical thickne~. However, a ~imple double hetero6tructure la~er made accordingly would have an active layer thickness ~o thin (due to the large lo mismatch required for sufficient b~nd gap difference) that the optical mode would be very poorly confined.
Thus, the confinement factor (overlap between the optical mode and the light generating region) would be small, and substrate losses would be high, causing prohibitively 15 high threshold current~. Therefore, si~ple double heterostructure laser diodes are not practical in these wide band gap II-VI materiAl~.
~ or the~e rea~ons, tbere have ~een no known demonstrations of laser diodes fabricated from II-VI
20 compound 6emiconductors. Commercially ~i~ble laser diodes of thi~ type would be extremely desirable and have widespread application.
Because of the wide range of important applications for these devices, considerable amounts of 25 research and development have been devoted to these materials. Many ma~or obQtacles to the ~.Gduction of commercially viable II-VI devices have been identified as a result of this work. In fact, despite all this reseArch, rudimentary blue light emitting diodes (LEDs) 30 fabricated from an epitaxial II-VI semiconductor (ZnSe) were only first reported in 1988. See eg., Ya~uda et al., Appl . Phys . Lett . 52, 57 ( 1988). There are no known reports of laser diodes fabricated from these materials.
A significant problem was the inability to p-35 type dope ZnSe or other appropriate II-VI ~miconductor material~ to sufficient net acceptor concentrations.
Improvements have recently been made in this area. See W~ 2/21170 PCT/US92/037 eg., P~rk et al., P-~ype 8nSe By Nl trogen Atom ~m Doping Dur~ng Nolecul~r Be~D ~pltaxi~l Growth, Appl.
Phys. Lett. vol. 57, p. 2127 (1990).
Another recQnt advance in II-VI t~chnology 5 lnvolves growlng ~pit~xial ~ at low t-mperatur-~uslng molecul~r b~am ~pitaxy and ~ th~roal-cracklng source for the Group VI elc~ent. 8ee eg., Ch-ng et al., Low Temper~ture Grolrth Of Znse By ~ol-cul~r B-~ ~p~taxy Us~ng Cr~ckod Selenlum, Appl. Phys. Lett., vol. 56, p.
10 848 (lg90).
The abllity to make low r--l-tanc- ohmlc contacts to both the p- and n-type II-VI e-~iconductor a18O presented probl~m~. Good ohmic contact~ are nece~s~ry for commercially vlable ~eg., low oper~ting 15 voltage and low heat generation) II-VI device~.
Conventional tech~ques for fabricating ohmic metal-~emiconductor contacts utillze a metal ~y~tem (often ther~ally alloyed) to produce a 8m~11 barrier to carrier in~ection, and/or to dope the semiconductor 20 contact layer with ~hallow (energy level) i~purities as heavily a~ po~ible at the ~urface of the layer. DUQ to the ~mall barrier height and the high doping lovel in the ~emiconductor layer, the potential barriers are ~o thin that tunneling of carriers through the barriQrs become~
25 very significant. Mo~t all commerci~lly viable cemiconductor devi¢-~ and integr~ted circuit~ ~mploy thls approach for current in~sction.
It was commonly assumed that this tech~jque (eg., doping and Au evaporation) would al~o b- cultable 30 for producing ohmic contacts to p-type ZnSe and other II-VI ~emiconductors. In f~ct, now that low resistance p-type ZnSe can be reproducibly grown, it has been determined th~t convention~l t-chnique~ c~nnot be relied upon to produce acceptable ohmic contacts. The ~table 35 low-barrier metal system and very high doplng levels are, a~ of yet, not avail~ble for these semiconductor~. One exception to these probl~ ZnT-, which can b- ~asily ~ ~ .r~ _ CA 02234~17 1998-0~-28 doped p-type. It ls also possible to make ohmlc contacts for thls semlconductor using conventlonal techniques. Nonetheless, it 1~
evident that there is a need for improved ohmic contact technology for other p-type II-VI wlde band gap semlconductors.
~RIEF DESCRIPTION OF THL DRA~INGS
Fig. 1 is a cross sectlonal view (not to scale) illustrating the structure of a II-VI semlconductor laser diode in accordance wlth the present lnventlon.
Flg. 2 is a schematic lllustratlon of a molecular beam epitaxy system used to fabricate the laser diode shown ln Fig. 1.
Fig. 3 ls a graph of the I-V characterlstlc of sample Au ohmic contacts on p-type ZnSe and slmllar to that lncorporated into the laser dlode shown ln Fig. 1.
Flg. 4 ls an energy band dlagram of an ohmlc contact to p-type ZnSe whlch is slmllar to that lncorporated lnto the la~er dlode shown ln Fig. 1.
Flg. 5 is a graph of the mea~ured optical power output from the laser diode shown in Fig. 1 as a functlon of applled current.
Flg. 61 is a graph of the measured intensity of light output from the laser diode shown in Fig. 1 as a function of wavelength, the upper trace representlng spontaneous (non-lasing) light and the lower trace representing stlmulated ~laslng) light.
Flg. 62 is a detalled lllustratlon of the central wavelength portlon of the graph of the stimulated llght output in Fig. 61.
Fig. 7 is a cross sectional vlew illustrating the structure of an alternatlve rib waveguide embodlnent of the laser shown in Fig. 1.
Fig. 8 is a graph of the low-temperature photoluminescence (PL) spectrum of the p-type ohmlc contact layer sample similar to that incorporated lnto the laser diode shown ln Flg. 1.
Fig. 9 18 a dlagram of a molecular beam epltaxy chamber for doplng semlconductors in accordance wlth the present lnvention.

CA 02234~17 1998-0~-28 Flg. lOAl ls a graph of the PL lntensity vs. energy for a particular sample.
Flg. l0A2 shows a portlon of the graph of Flg. 10 drawn to an enlarged scale.
Flg. lOBl is a graph of the PL intensity vs. energy for a further sa~ple.
Flg. lOB2 shows a portion of the graph of Fig. 10 drawn to an enlarged scale.
Flg. ll~A) i8 a graph of 1/C2 versus bias voltage for a semlconductor sample de~crlbed ln the Detalled Descrlptlon Of The Preferred Embodiments.
Flg. ll(B) is a graph of a net acceptor density versus depletion wldth for a ~emiconductor descrlbed in the Detalled De~cription Of The Preferred Embodlments.
Fig. 12(A) is a cross sectlonal vlew (not to scale) of a light emitting diode described in the Detailed Description Of The Preferred Embodlments.
Flg. 12(B) 18 a graph of the EL lntenslty versus wavelength relatlonshlp of the llght emltting diode shown ln Flg.
12(A) at 77K.
Fig. 13 18 a graph of the EL lnten~ity versus wavelength relatlonshlp for the llght emltting diode shown ln Fig. 12~A) at room temperature.
Flg. 14 i8 a cross sectional view (not to scale) of a second llght emitting diode described in the Detailed Description Of The Preferred Embodiments.
DETAILED DESCRIPTION OF THE PREFERRED EHBODIHENTS
The structure of a laser diode 10 ln accordance wlth the present invention is illustrated generally in Figure 1. Laser dlode 10 is a wlde band gap II-VI device fabricated from heteroepitaxial layers of ZnSxSel x~ ZnSe, and CdyZnl ySe grown by molecular beam epitaxy (MBE) on a GaAs substrate. Prototypes of this device have exhibited la~er action, emitting coherent blue-green light near 490 nm from a CdyZnl ySe quantum well structure under pulsed current in~ection at 77 K.

CA 022345l7 l998-05-28 Laser diode 10 i8 fabricated on a GaAs sub~trate 12, and lncludes lower (flr~t) and upper (~econd) ZnSe llght-guiding layers 14 and 16, 6a ~ _92/211~0 PCT/US92/0 respectiv~ly, ~teparated by a Cd~Zn~Se quantum well active layer 18. The surfAces of light-guiding layers 14 and 16 opposite active layer 18 are bounded by lower and upper ZnS~Se~ cladding layers 20 ~nd 22, respectively. A lower 5 ZnSQ ohmic contact layer 24 i~ positioned on the curface of lower ~ ng layer 20 opposite light-guiding layer 14, while an upper ZnSe ohmic contact layer 26 is positioned on the surface of upper cl~ ng layer 22 opposite light-guiding layer 16. A GaAs buffer layer 28 10 $eparate~ ~ubstrate 12 from lower ZnSe contact layer 24 to assure high crystalline quality of the contact and su~sequently deposited layers. A polyimide insulating layer 34 coYer~ the surface of upper ohmic contact layer 26 opposite upper cladding layer 22. Electr$cal contact 15 to the ohmic contact layer 26 iB made by Au electrode 30 which is formed in a window stripe in insulating layer 34. A t~in Ti layer 31 and subsequently a final Au layer 33 are applied over polyimide layer 34 and exposed portions of Au electrode 30 to facilitate lead bonding.
20 Electrical contact to the lower sidQ of la~er diode 10 is made by an In electrode 32 on the surface of substrate 12 opposite the lower ohmic contact layer 24.
Layers 24, 20 and 14 are all doped n-type with Cl (ie., are of a first conductivity type) in prototypes 25 of laser diode 10. Layers 16, 22 and 26 are all doped p-type with N (ie., are of a second conductivity type).
Active layer 18 is an undoped quantum well layer of CdO2ZnO~Se semiconductor deposited to a thickness of 0.01 ~m. Light-guiding layers 14 and 16 are both 0.5~m thick.
30 Lower light-guiding layer 14 i8 doped to a net donor concentration of lxlO~7cm-3, while upper light-guiding layer 16 is doped to a net acceptor concentration of 2xlOI~cm-3.
Claddlng layers 20 and 22 are layer~ of ZnSOO7SeO93 sem~conductor depositQd to thickne-ses of 2.5~m ~nd 1.5~m, respectively. The net donor concentration of the lower cladding layer is lxlO~3cm-3. The net acceptor concentration of the upper cladding layer is 2xlO~7cm~.

WC~ ~t~2117o PCI/US92/0378L

Ohmic contact l~yers 24 and 26 ar- both depocited to a thickness of O l~m in these prototyp- d~vices The lower contact layer i6 doped n-type to a net donor concentration of lxlO"cm~ The upper contact layer i8 5 doped p-type to a net acceptor ~-o,~ tr~tion of lxlO~cm3 Other param-ter~ and materi~l~ can also be u~ed in the fabrication of laser diode~ 10 in accordance with the pre~ent invention For example, the thickne~se~ of layers 24, 20, 14, 16, 22 and 26 can be vari~d as needed lo for given applicat$ons Typical thicknQs6 rangQs for contact, cladding and light-guiding layors ar- O 03 to 1 0 ~m, 0 5 to 5 0 ~m, and 0 1 to 1 0 ~m, re~pectivQly In general, the thickne~-s of light-guiding layers 14 and 16 ~hould be chosen to minimize the width of the 15 optical mode If the layer~ 14 and 16 are too thin, the evanescent tails will extend far into cladding layers 20 and 22 Cladding layers 20 and 22 must be thick enough to make absorption of th- optical ~ode in ~ub~trate 12 and electrod~ 32 negligible The compo~ition of the 20 CdsZn~sSe (which det-rmines the la~er w~velength) with x of approximately 0 2 was ~elected to provid~ rge enough band gap difference (~ of approximately 0 2 eV) to facilitate effective carrier confinement Larger x will provide deeper quantum wQlls, but would require a 25 thinner layer due to increa~sd lattice mismatch, thereby decreasing the efficiency of the collection of carrierC
into the well The composltion of the ~nSy~ with y of approximatQly 0 07 was ~ cted to provid~ ~ufficient 30 difference in refractive index from the index of the ZnSe guiding layer~ to form a low-los~ wav-gulde Thl~
composition also provides excellent morphology since it is nearly lattlce matched to the GaA~ ~ub~trat- at the growth temperature of 300 C
Other n-type dopants which may b- u~ed include Ga, Al, In, I, F, and Br Oxygen or Li acceptors can also be used for the p-type dopants Other Group V p-~_g2/21170 PCT/US92/03 type dopants which might be u~d include ar~enic and phosphorous. Greater donor and acceptor concentrations can also be used, although they should not be 80 high as to cause exce~ive free-carrier absorption.
The prototype~ of l~er diode 10 are fabricated on Si-doped n~-type GaA~ ~ub~trat- 12 having a (100) cry~tal orientation. Sub~trat~s 12 of this type are commercially available from a number of manufacturers including Sumitomo Electric Indùstries, Ltd. GaAs buffer lO layer 28 is deposited to a thickness of 1 ~m in this embodiment, and doped n+ with Si to a net donor concentration of lxlOI~cm~3. Other appropriate 6ubstrates (eg., ZnSe, GaInAs or Ge) and buffer layers such as AlGaAs, AlAs, GaInP, AlInP, AlInAs or GaInAs can also be 15 used. The thickness of buffer layer 28 can al60 be varied while providing an appropriate high-quality surface for growing the II-VI semiconductors. If an appropriate high-guality ~ub~trate and appropriate surface preparation iB u~ed, buffer layer 28 may not be 20 needed.
The lattice con6tants of the ZnSSe cladding layers 20 and 22 and the ad~acent ZnSe layers 24, 14 and 16, 26, respectively, are mismatched by about 0.3%.
Preliminary transmi6sion electron microscopy (TEM) 25 studies indicate that the ZnSe of light-guiding l~yers 14 and 16 is at least partially relaxed by dislocations formed at the interf~ces of the light-guiding layers and the adjacent ZnSSe cla~ng layers 20 and 22, respectively. These preliminary ~tudies also indicate 30 that the thickness of the CdZnSe quantum well active layer 18 is less than the critical thickne~R for this material system. Quantum well active layer 18 is therefore pseudomorphic, minimizing dislocations in the light-emitting region of laser diode 10. The ~aximum 35 pseudomorphic thicknQs~es for ~trained epit~xial layers ~uch as 18 depends on the composition and can be c~lculated from formulae describQd in MatthQws et al., WC /21170 PCT/US92/0378~

D~cts In Ep~t~x~al Mult~l~yer~, J. Cry~tal Growth, vol 27, p 118 ~1974) The inclusion of quantum well layer 18, which could al~o be ~ F~ Aomorphlc layer of other semiconductor material such a~ ZnSeTe, facilitates the 5 low threshold curr-nt operatlon Or la~er diode 10 when po~itloned within the thicker, low-lo~ II-VI waveguide The waveguide can be made with higher r-fractlve index light-guiding layers 14 and 16 ~nd lower refractive index cladding layers 20 and 22 which can have a relatively 10 small dif~erence in their band gaps and need not be exactly lattice m~tched The composition o~ the light-guiding layers may be graded to minimize dislocations and/or to form a graded index waveguide Figure 2 is an illustration of a molecular beam 15 epitaxy (MBE) ~ystem 50 u~ed to f~bric~te the laser diode 10 described above MBE system 50 includes two MBE
chambers 52 and 54 interconn-ctet by ultrahlgh vacuum (UHV) pipeline 56 Each chamber 52 and 54 includes a high energy electron gun 58, a pho~phoruQ ~creen 60, a 20 substrate heater 90 and a flux monitor 6Z MBE chamber6 ~uch a~ 52 and 54 are g~nerally known and commercially available A Perkin-Elmer Model 430 MBE system was used to produce the prototype la~er diodes 10 MBE chamber 52 i~ used to grow the GaAs buffer 25 layer 28 on sub~trate 12 and include~ a Ga erfu~ion cell 64 and an A~ cracking cell 66 A 8i efru~ion cell 6B iB
al60 provided ~B a ~ource o~ n-type dopant~ Substrate 12 i~ cleaned and prepar~d u~lng con~ntional or otherwi~e known tec~n~ues, and mount-d to a Molybdenum 30 sample block (not shown in Fig 2) by In solder before being positioned within chamber S2 By way of example, substrate preparation t-c~n~ques de6cribed in th- Cheng et al. articl~ Mol~cul~r-B-~m Eplt~xy Growth o~ 8nSe Us~ng A Cr~c~c~d Sel en ~ um Source, J . Vac . Sc~ . Technol , 35 B8, 181 (1990) were used to produc- the prototype laser diode 10 The Si doped buffer l~yer 28 can be grown on substrate 12 by operating MBE chamber 52 in a ~2/21l70 PCT/US92/03 conventional manner, such ag that described in ~echnology and Physics of Molecular Be~m ~p~t~xy, ed. E.H.c. Parker, Plenum Pres~, 1985. The rQsulting ~uffer layer 28 ha~ an As-rich ~urface which exhibited a c(4x4) reconstruction 5 as observed by reflection high energy electron diffraction (Kh~ ). The sample block bearing the GaAs substrate 12 and buffer layer 28 is then tran~fered to MBE chamber 54 through UHV pipeline 56 for further proces~ing.
Device layer_ 24, 20, 14, 18, 16, 22, and 26 are all grown on the buffer layer 28 ~nd GaAs substrate 12 within MBE chamber 54. To this end, chamber 54 includes a Zn effusion cell 70, cracked-Se effuqion cell 72, ZnS effusion cell 74 (as a source of S), Cd effusion 15 cell 76 and a standard Se (ie., primarily Se6) effusion cell 79. A~ shown, cracked-Se effusion cell 72 includes a bulk evaporator 84 and high temperature cracking zone 82, and provides a 60urce of cracked Se (including Se2 and other Se molecules with les~ than 6 atoms). The bulk 20 evaporator 84 and high temperature cracking zone 82 used to produce the prototype laser diodes lo are of a custom design, the details and capabilities of which are described in the Cheng et al. J. Vac. Sci. Technol.
article referenced above. Cl effusion cell 78 which 25 utilizes ZnCl2 source material provideR the Cl n-type dopant. The p-type dopant i8 provided by N free-radical source 80. Free-radical source 80 i8 connected to a source 86 of ultra-pure N2through leak-valve 88. The free-radical source 80 used in tbe fabrication of la6er 30 diodes 10 is commercially available from Oxford Applied Research Ltd. of Oxfordshire, England (Model No. MPD21).
This ~ource ha~ a length of 390 mm. The beam exit plate at the end of the 60urce is made of pyrolytic boron nitride (PBN) and haq nine 0.2 mm diameter holes through 35 it. This source is mounted on a ~t~n~ard port for an effusion cell through a lo" extension tube. N2 source 86 u~ed to fabricate la~er diode~ 10 i~ of re~earch purity CA 02234~17 1998-0~-28 grade produced by Matheson Gas Products. The pressure at the inlet of the leak-valve of source 86 is 5 psi.
MBE chamber 54 is operated in the manner described in the Cheng et al. article "Growth Of p- and n- Type ZnSe by Molecular Beam Epitaxy", J. Crystal Growth 95, 512 (1989) using the Se6 source 79 as the source of Se to grow the n-type contact, cladding and light-guiding layers 24, 20 and 14, respectively, of the prototype laser diode 10. Quantum well active layer 18 is grown in a manner described in the Samarth et al. article, "Molecular Beam Epitaxy of CdSe and the Derivative Alloys Znl xCdxSe and Cd1 xMnxSe", J. Electronic Materials, vol. 19. No. 6, p. 543 (1990).
MVE chamber 54 is operated in a manner described in the Parker et al. article entitled "p-type ZnSe by Nitrogen Atom Beam Doping During Molecular Beam Epitaxial Growth", published in Appl. Phys. Lett. 57 (20), 12 November 1990, using the Se6 source 79 to grow the p-type light-guiding layer 16 and cladding layer 22. Relevant portions of the above-referenced article are summarized immediately below.
An atomic dopant beam (either nitrogen or oxygen), produced by a free-radical source, is used to dope ZnSe during molecular beam epitaxy which produces p-type ZnSe epitaxial thin films. When electromagnetic power at the frequency of 13.52 MHz is coupled to an RF plasma discharge chamber of the free-radical source, atomic dopant species are generated inside the chamber of the free-radical source from a gaseous source of ultra-high purity. A diffuser plate having 18 holes of about 0.3 mm diameter each was used to separate the free-radical source and the molecular beam epitaxy chamber. The amount of the atomic dopant species generated is controlled by the level of the RF power coupled to, and the pressure in the RF plasma discharge chamber. The atomic dopant species, which effuse into the molecular W~92/21170 PCT/US92/037 beam epitaxy ch~mber through openings in the diffuser plate, are u~ed as the dopants during the molecular beam epitaxy growth of ZnSe.
In one embodiment, ZnSe thin layers are grown 5 on a well-polished GaAs surface with the surfacQ normal vector es~entially along the t0011 crystal orientation.
There are many ~uppliers of either the GaAs substrate, available from, for example, Sumitomo Electric Industries, Ltd., 1-1 Koyakita l-Chome, Itami, Hyogo, 664 10 Japan, or the GaAs epitaxial layer, available from Splre Corporation, Patriots Park, ~edford, M~ chusett~, 01730, for this purpose. Before loading into the molecular beam epitaxy sy6tem for the ZnSe growth, the GaAs substrates are degrea~ed in trichloroethane, 15 acetone, and isopropanol, rinsed in deionized water and blown dry by high purity nitrogen gas. The degreased substrates are chemically etched in a solution consisting of six parts of sulfuric acid, one part of hydrogen peroxide and one part of deionized water for several 20 minutes (about two to five minutes). The substrate is rinsed in deionized water and blown dry by high purity nitrogen gas. The degreased and chemically-etched GaAs substrates are t~en attached to a Mo samplQ block u~ing molten In of high purity a~ solder. The ~ub~trate 25 assembly is immediately lo~ded into the molecular beam epitaxy system. The GaAs substrates are heated in the ultra-high vacuum growth chamber to about 610~C for a~out one to five minutes to desorb the native oxides and expose the underlying cry6talline strUcture on which the 30 ZnSe with the ~ame crystal 6tructure iB to be grown. The typical growth conditions for ZnSe by molecular beam epitaxy are a Zn to Se beam eguivalent pre~sure ratio of 1:2 (in the range of about 1:4 to 2:1) and a growth temperature of 275~C (in the range of about 250~C to 35 400~C). Typical layer thickne~e~ and growth rate~ are 2 ~m and 0.5 ~m/h (in the range of about 0.4 ~m/h to 2.0 ~m/h) respectively. The atomic dopants generated by W~2/2ll70 PCT/US92/037~-the free-radical ~ource ar~ incorporated into t~ ZnSe by opening the mechanical 6hutter whicb block~ the line of sight path betw~en t~e free-radical source and the heated substrates The ma~or focus in recent year~ r~g~rding research on the wide-bandgap II~-VIA com~ou ~Qmiconductor, ZnS~ (~ ~2 67-Y at room tu~perature), has b~en on producing low re~i~tivity p-type ~aterial Tbe pr~sent invention utilize- a m~thod and apparatu~ for thQ
10 ln-situ production of ~pitaxial structur-~ comprising ZnSe pn ~unction~ Thi~ i~ u~-ful in th- fabrication of efficient light-emitting deYices, ~uch a~ light--mitting diode~ and diode lasers which operate in the blue region of the vi~ible ~pQctrum Either nitrogen or oxygen are an excellent p-type dopant element in ZnSe In addition to providing large net acceptor densitie~ (greater than about 5xlO~Icm3 and low compQn~ation ~ND/NA le~ than about 0 8)), nitrogen and oxyg-n ar~ ~tabl~ ln ZnSe at t-mp-ratures up 20 to 375~C
Large concentrations of net nitrogen acceptor impuritie~ are incorporated into ZnS~/GaA~ ~pitaxiA1 layer6 which involves nitrogQn atom beam doping during molecular beam epitaxial growth Net accQptor d~n~ities 25 as largQ as 4 9xlO~7cm3 hav~ b-en m~asured in th~ re~ultant p-type ZnSe material Fig 9 ~hows a mol ~ r b~am ~pltaxy cy~t~m 110 MolQcular beam ~pitaxy ~y~tem 110 includes a molecular b~am epitaxy chamber 112 which ~nclo~-s a 30 6ubstrate 114 NolQcular b~am epitaxy chamb-r 112 ~ncludes an electron gun 116, a rho~horus scr-en 118 and a flux monitor 120 Effusion cell~ 122, 124, 126, and 128 are carried in molecular beam epitaxy chamber 112 Effueion cells 122, 124, 126, and 128 may compri~e, for 35 example, effusion cells for Zn, Se, and ZnCl2 Molecular beam epitaxy ~y~tem 10 also includes a free-radical source 130 Free-radical source 130 may comprise a W~J ~2/21170 PCI'/US92/03;....

source of any group VA or oxygen free-radicals. For example, free-radical source 130 may provide a ~ource of nitrogen free-radicals, in which free-radical source 130 is supplisd with ultra-pure Nl fro~ ~n ultra-pure N2 5 ~ource 132 through a valve 133. Fr-e-radical ~ource 130 is available from Oxford Applied Rer-Arch Ltd.
(Oxfordshire, UK). Free-radical source 130 might comprise other type~ of sources which produce free-radicals. For example, an electron cyclotron resonance (ECR) free-radical 60urce may be u~ed (available from, for example, Wavemat, Inc., 44780 Helm Street, Plymouth, Michigan). A microwave cracker coupled into the gas source through a microwave tube may be used to produce free-radical~. A DC plasma di~charge chamber may also be 15 used. Furthermore, any appropriate thermal cracker or disassociation cell (available from, for example, EPI, 261 East Fifth Street, St. Paul, Minnesota 55101) may be used.
ZnSe layers were grown on GaAs sub~trates in a 20 molecular beam epitaxy sy6tem of the type described herein. The6e layers were grown at a substrate temperature of 275~C with a Zn to Se beam equivalent pressure ratio of 1:2 (typical layer thickne66es and growth rates were 2 ~m and 0.5 ~mth, rQspectively).
25 P-type doping of the ZnSe layers wa8 achieved by a free-radical source which was incorporated in the molecular beam epitaxy system, rather than a conventional effusion source. The free-radical 60urce provided a flux of atomic nitrogen (together with a much larger flux of non-30 di6sociated N2) created in a RF plasma di6charge chamber A RF frequency of 13.5 MHz was used to generate nitrogen atoms from a ga6eous source of ultra-pure N2. The atomic nitrogen flux level was controlled by suitably adjusting the intensity of the RF plagma discharge.
The nitrogen actively incorporated into the ZnSe was much greater using the free-radical atomic beam than that of molecular nitrogen, a~ evidenced by CA 02234~17 1998-0~-28 comparing lOK photoluminescence (PL) spectra recorded from ZnSe layers grown with a flux of N2 only and with a flux of N
+ N2. As shown in Figs. lOA1 and A2, the lOK PL spectrum recorded from a ZnSe layer grown using a flux of N2 only, (in this case an equilibrium background pressure of N2 in the molecular beam epitaxy chamber of 5x10-7 Torr was maintained) appears to be identical to that recorded from undoped ZnSe heteroepitaxial layers (see R.M Park, C.M. Rouleau, M.B.
Troffer, T. Koyama, and T. Yodo, J. Mater. Res., 5, 475 (1990)). The dominant peaks in the excitonic regime are the split free-exciton (Ex) and donor-bound-exciton (I2) transitions, the splitting being due to the thermal expansion coefficient mismatch between ZnSe and GaAs which renders the ZnSe layers under inplane biaxial tension (see K. Shahzad, D.J. Olego, D.A. Cammack, Phys. Rev. B 39, 13016 (1989)).
Consequently, at such low background N2 partial pressures, molecular nitrogen is completely unreactive at the ZnSe surface. The situation changes dramatically, however when a plasma discharge is created in the free-radical source, as shown in the lOK spectrum of Figs. lOB1 and B2. Again the background N2 partial pressure in the molecular beam epitaxy chamber during growth was 5x10-7 Torr with power applied to the RF plasma discharge. The excitonic regime is dominated by split acceptor-bound-exciton (IN1) transitions due to the incorporation of nitrogen acceptor impurities (see P.J. Dean, W. Stutius, G.F. Neumark, B.J. Fitzpatrick, and R.N. Bhargava, Phys. Rev. B 27, 2419 (1983)). In addition, the complete PL
spectrum is dominated by donor-to-acceptor (D-A) transitions (QNO-represents the no phonon transition, with several LO
phonon replicas of QNO also indicated) as opposed to excitonic transitions. Thus, the rate of substitutional incorporation of atomic nitrogen is much greater than that of molecular nitrogen at the growing ZnSe surface. The sample from which the PL spectrum shown in Figs. lOB1 and B2 was obtained was found to have a net acceptor concentration of lxlO17cm~3.

W~92/21170 PCT/US92/03 Net acceptor concentrations, NA-ND, in the nitrogen doped ZnSe/GaAs layers were determined using capacitance-voltage (C-V) profiling. Since the ZnSe epitaxial layers were ~ .. on semi-insul~ting GaAs, 5 planar profiling between two Schottky contacts on the ZnSe surface was carried out. T~e surface contact pattern consisted of a ~eries of 762 ~m diameter Cr/Au dots physically isolated from a large Cr/Au ~lL~unding electrode. The separation between the inner (dot) lo electrodes and the outer electrode was 25 ~m, a sm~ll separation being necessary ln order to maintain a low series resistance. The contact pattern was created by thermally evaporating 75 ~ of Cr followed by lOOo A of Au and performing photolithographic and lift-off processes.
15 In all of these measurements the outer electrode was held at ground potential and bias was applied to the inner Schottky contact.
With this sign convention the ma~ority carrier type is given by the sign of the slope of the l/C2 versus 20 V plot; a pOfiitiVe slope would indicat~ th~ material to be p-type. The net acceptor (NA-ND) concentration is proportional to the slope of l/C2 versus V. The l/C2 versus V plot and the NA-ND ver~us depletion width profile obtained from a heavily-doped ZnSe layer are illustrated 25 in Figs. ll(a) and ll(b), respectively. As 6hown in Figs. ll(a) and ll(b), the material is p-type with a net acceptor concentration around 3.4xlOI7cm-3. As shown in Fig. ll(b), the doping profile is rather flat from zero bias (0.068 ~m) out to where rever~e bias breakdown 30 occurs (1.126 ~m). Breakdown o~ L~d at 3.8 V which is consistent with avalanche breakdown in ZnSe material doped at this level, ie, 3.4xlO~cm~3 p-type.
Further evidence of the p-type nature of the nitrogen doped ZnSe material was obtained through the 35 fabrication of blue light-emitting diodes based on epitaxially grown ZnSe:N/ZnSe:Cl pn homo~unctions. The n-type ZnSe layers in the~ pn ~unctions were grown using w092/2l170 PCT/US92/03782 Cl as th~ dop~nt QlemQnt, t~Q ~ource Or the Cl atom~
being a ZnClt effusion cell inco~v~ated in the molecular beam epitaxy ~ystem ~ number of ZnSe ~amples grown u~ng molecular S beam epltaxy were tested The r~ult6 were a~ follow~

1 Undoped Zn8e Zn to Se b-a~ eguivalent pressure ratio 1 2 lo Growth Temperature 275 C
Results Low t-mp-rature photolumine~cence ~pe_L~um indlc~ted sample wa~ not p-typs C-Y
mea~urem-nt indicat-d ~mple was in~ulating 2 Doped ZnSe u~ing N~ with no RF power to free-radical source Zn to Se beam eguivalent pressure ratio 1 2 Growth Temperature 275 C
RF power 0 watt~
Backyr~ul~d pre~ure 5x10-7 Torr Result~ Low temperature photoluminescence spectrum indic~ted ~ample wa~ not p-type C-Y
mea~urements indicated ~mple was in~ul~ting.

3 Doped ZnSe using N2 wlth RF pow~r to free-radical ~ource Zn to S- b-am equivalent pre~ure ratio 1 2 Growth temperature 275 C
RF power 320 watt~
Background pre~sure 5x107 Torr Re~ult~ Low tempQrature ~._92/21170 PCT/US92/03 _ ~ -- 19 --photoluminocr~nc~ spectrum, current-voltage mea~urement ~nd capacitance-voltage mea~urement lndicated that sample was p-type. ND/N~SO.8 (high doping efficiency) and NA-ND=3 . 4xlO~7cm~3.

4. Doped ZnSe using ~2 with RF power to free-radical source:
Zn to Se beam equivalQnt pressure ratio: 1:2 Growth temperature: 275~C
RF power: 3 20 watts ~ackground pre~sure: 5x10-7 Torr Results: Low tempersture photoluminescence ~pectrum, current-voltagemeasurement, and cap~citance-voltage measurement indicated that sample wa6 p-type and N~-2 0 ND-3 . OX10~6Cm-3 . .

Fig. 12(a) ~hows a light emitting diode 134.
Light emltting diode 134 includes a p-type GaAs ~ub~trate 136. P-type GaA~ substrate 136 forms the base for 25 molecular beam epitaxial growth. A p-type ZnSe nitrogen doped layer 13 8 is depo6ited upon p-type GaAs substrate 136. P-type ZnSe layer 138 i~ deposited in accordance with the present invention u~lng a nitrogen free-radical source. An n-type ZnSe chlorine doped layer 140 is 30 deposited upon p-type ZnSe layer 138. An n+ ZnSe cap layer 142 is deposited upon n-type ZnSe layer 140. The deposition of layers 138, 140, and 142 i5 through molecular beam epitaxial growth. Ohmic contact~ 144 and 146 form electrical contacts to n~ ZnSe cap layer 14Z and 35 p-type GaAs substrate 136, respectivQly. I n o n e embodiment, p-type ZnSe layer 138 has a thickness of 2~m and has a net acceptor concentration o~ 1xlol~cm-3. N-type W('~2/21170 CA 02234517 1998-05-28 PCT/US92/03 ZnSe l~yer 140 has a thickness of O 5 ~m ~nd a net donor concentr~tion of lxlO1~cm3 The n~ ZnSe cap layer 142 has a thickness of 500 A and a net donor concentration of SxlO~cm3 s Fig 12(a) shows th- p-typ- ZnSe layer iB grown fir~t on a p'-typ~ GaA~ ~ubstrate Thi~ type of "buried p-type layer" structure avoids the ~erious problems associated wlth ohmic contact formation to p-type ZnSe (See M A Ha~se, H Ch~ng, J M DePuydt, and J E Potts, lO J Appl Phys , 67, 448 (l990)) However, a di~advantage with this device design iB that a large hole b~rrier exlsts At the p~-GaAs/p-ZnSe h-tero-interface (~ee L
Kassel, H Ab~d, J W G~rland, P M Raccah, J E Potts, M A Haase, and H Cheng, Appl Phy6 Lett , 56 42 (1990)) In tbis type of de~ice, hole in~ection across the p+-GaA~/p-ZnSe hetero-interface i~ only realized ~t ~val~nche breakdown Con~-quently, large turn-on voltages are required to observe electroluminescence associated with the ZnSe pn homo~unction Light-emitting diod~ rabrication wa6 accomplished u~ing conventional photolithographic techniques with devicQ i~olation being achieved by wet chemical etching to form 400 ~m diameter me~as The top electrode metalization was ring shap-d and w~ p~tterned 25 by vacuum evaporation and lift-orf Ultrasonic gold ball bonding was used to make cont~ct to the devices for electroluminescence characterization A typical electrolumine-c-nc- ~pectrum r-corded at 77K for light emitting diode 134 ~hown in Fig 12(a), 30 is illustrated in Fig 12(b) Th- device operating voltage ~nd current were 13 5 V ~nd 40 mA, r~spectlvely, for the ~pectrum ~hown in Fig 12(a) A~ c~n be ~een from Fig 12(b), the visible electrolumine~cence is dominated by blue emis~ion, the spQctrum comprising 35 number of resolved line~ principally at 447 7 nm, 459 6 nm and 464 7 nm The two highest energy peaks in the spectrum corre~pond clo~ely in energy to the 60s57-4s70D

W~92~21170 PCT/VS92/037 electroluminescence peaks ob~erved at 77~ from blue light-emitting diodes fabricated using a nitrogen-ion implantation and annealing procQdure as reported by Akimoto et al (See K. Akimoto, T. Hiyajima, and Y. Mori, 5 Jpn. J. Appl. Phys., 28, L528 (1989)). Infrared emission at 844 nm was al~o recorded from the~e devices (simultaneously with the blue emis~ion) which appear~ to be the result of electron injection into the p+-type GaA~
material under avalanche breakdown conditions at the lo hetero-junction (not shown in Fig. 12(b)).
An electroluminescence spectrum recorded at room temperature from the devlce 6tructure illustrated in Fig. 12(a) (visible region only) is ~hown in Fig. 13. As can be seen from the figure, dominant emission in the 15 blue region of the visible spectrum is observed, peaking in intensity at a wavelength of 465 nm. For the particuIar spectrum ~hown in Fig. 13, the voltage applied and current drawn were 22 V and 20 mA, respectively.
Fig. 14 shows a light emitting diode 148.
20 Light emitting diode 148 is a p on n device which operates similar to light emitting diode 134 of ~ig.
12(a). Light emitting diode 148 includes an n~ GaAs substrate 150, an n-type ZnSe layer 152 and p-type ZnSe layer 154. Contacts 156 and 158 make electrical contact 25 with p-type ZnSe layer 154 and n+ GaAs ~ubstrate 150.
The p-type ZnSe layer 154 i~ deposited using molecular beam epitaxy and a group VA free-radical source described above. In one embodiment, diode 148 shown in Fig. 14 n-type ZnSe layer 152 has a net donor concentration of 30 about lxlO~cm-~ and a thicknes~ of about 2.0 ~m and p-type ZnSe layer 154 has a net acceptor concentration of about lxlO~7cm~3 and a thickness of 0.5 ~m.
Using the method and apparatus de~cribed above, n-type IIB-VIA semiconductor film may alco be produced.
35 The resultant IIB-VIA semiconductor film may be used in pn junction devices such as light emitting diodes and light detectors as well as diode lasers and transistors.

W ~ ~/21170 PC~rJUS9210378 The free-radical source i~ in-~Gd~ced into a molso~llar bQam epitaxy growth chamber to provldo a dopant to a IIB-VIA ~e~iconductor during mol~cul~r bQam opltaxial growth.
The free-radical source ~ay be nitrogen, phosphorus, 5 arsenic, and anti~ony. Oxyg-n ~ay al~o be used as a ~uitable free-radical ~ource. The method and appar~tus may be used for N-doping and O-doping of ZnSe. P-type tern~ry II~-YIA semiconductor~ including Zn~Cd~Se, ZnSe~
~Te~, ZnSe~S" ZnSI~Te~, and Zn~Cd~S.
lo Referring again to Figure 1 and th- pr-sQnt invention, lower ZnSSe cla~in~ layer 20 iB doped n-type using the ZnCl2 ~ource. Other a~pect~ of the tsr~n~ques used to grow cladding layers 20 and 22 are de~cribed in the M~t~umur~ et al. article, Opt~mum Compos~t~on In MB~-15 ZnS~Se~JZnS~ For ~gh QU~l~ty ~teroep~t~l Growth, J.
Crys. Growth, vol. 99, p. 446 (1990).
A low resistivity p-type Zn8e ohmic contact l~yer 26 has been achieved by growing the contact layer at low temperature within MBE chamber 54 utilizing the 20 cracked Se source 72 (ie., cracking zone 82 and evaporator 84), while at tho came time doping the se~lconductor material of the contact l~yer p-type in accordance with the method d~cribed above. The low temperature growth technique usQd to produce the contact 25 layer 26 of the prototype laser diode 10 i~ deficribed generally in th~ ChQng et al. article Low Temper~ure Growth O~ ZnS~ By Nolecul~r Be~m Ep~t~xy Us~ng Cr~cked Selen~um, Appl. Phy~. Lett. (Feb. 1990). The ~emiconductor body with layer~ 28, 24, 20, 14, 18, 16 and 30 22 on ~ubfitrato 12 i~ heated to a temperatur- le~ than 250~ C but high enough to promote cry~talllne growth of the ZnSe doped with the N p-type dopant~ to a net acceptor concentration of at lea~t lx1017cm~. A net acceptor roncQntration of lxlOI~c~3 wa~ achieved in the 35 ohmic contact layer 26 of prototype la~er diod-c 10, wh~n grown at a substrate temperatur- of about 150- C.
How~ver, it is anticipated that ohmic contact layer~ 26 J2ttll70 PCT~US92/03 with acceptable characteristics can be achieved at other growth temperatures down to at least 130~ C. Other operating parameters of MBE chamber 54 used to produce the ohmic contact layer 26 of the prototype la~er diodes 5 10 are as follows:

Zn beam equivalent pressure: 1.0x107 Torr*
Se cracking zone temperature: 600~ C*
Se bulk ev~porator temperature: 250~ C*
Growth rate: 0.3-0.6 ~m/hr Surface rQconstruction: Zn-stabilized Nitrogen pressure in chamber: >3.5x107 Torr*
rf power: 150-250 ~
* parameters dependant upon specific MBE system configuration Figure 3 is the current-voltage characteristic of a sample with two coplanar Au metal electrodes on a p-type ZnSe contact layer produced for test purposes in a 20 manner substantially similar to that described above.
The ohmic nature of this contact is indicated by the substantially linear nature of the curve over the -6 to +6 volt range.
The mechani~ms believed to enable the ohmic 25 nature of contact layer 26 can be described with reference to Figure 4 which is an energy band diagram of the Au - p-type ZnSe contact layer interface. In addition to the expected shallow impurities 100 utilized by conventional ohmic contacts, additional electronic 30 energy states 102 are formed in the contact layer. These additional energy states 102 are relatively deep (within the forbidden gap) with respect to the valence band maximum, campared to the depth of the shallow impurity level 100. Energy states 102 are in effect intermediate 35 energy states located at an energy less than the Au Fermi level and greater than the ~hallow impurity level 100.
Since the probability of charge carriers tunneling W~ /2ll70 PCT/US92~0378 between two given energy states incr~ YponQntially with decreasing distance between the two ~tates, additional energy ~tates 102 greatly increase the tunneling probability by prov~d~ng a t-mporary re~idence 5 for the carrier~ and facilitate ~ulti-~tep or cascade tunneling. The optimum condition is illu-trated in Figure 4 where Ep is the ~ermi energy and E~ i~ the acceptor energy. A diagramatic depiction of an electron making a multi-step tunneling transfer between the ZnSe lo and Au layers through the additional energy state~ 102 i6 also ~hown in Figure 4. Even better cont_cts are attainable with electronic state~ ~t more than one energy level, ~uch that tunneling can occur from ~tate to ~tate across the barrier.
It i~ anticipated that the introduction of additionsl energy 6tates 102 can be achieved by a number of methods. Doping during growth, diffusion, ion implantation or other known t~c~niques can be used to incorporate impurities whlch produce deep l-vel~. One 20 import_nt type of deep level impurity is the i50-electronic trap. ~y way of example, Te iB thought to form a hole trap in ZnSe. The _dditlonal energy ~tates 102 can also be achieved by introducing proper native crystal defects ~uch a~, but not limited to, 25 di~locations, vacancies, interstitials or co~plexes into contact layer 26. Thi~ can be dona during the daposition of the contact layer by choo~ng tha molecular 8pecie~ of the precursor~, and/or by other appropriate growth condition6. Native defect~ can also be generated by 30 post ~wth treatment~ such a~ ~rradiation by electron beams, ion b-ams, radical b-ams or el-ctromagnetic radiation. However, thasa te~hn~ques mu~t be implemented without detrimentally degrading the conductivity of the ZnSe or other ~emiconductor material u~ed for the contact 35 layer.
It therQfore appQ_rs that the u~eful p-type contact ~ayer 26 ha~ a number of propertie~. The net W~ ~2/21170 PCTJUS92/03 acceptor density NA-ND is large, preferrably at least lxlO~cm3. This serves to reduce the width of the barrier through which the charge carriers must tunnel. The p-type dopant concentration (nitrogen in laser diode lO) 5 must also be larqe, preferrably at least lxlO~9cm-3. In addition to forming the ~hallow acceptor levels, the nitrogen impurities also appear to participate in the formation of the deep energy states. At a minimum, the amount of nitrogen required is that which will provide 10 adequate concentrations of both types of levels. The growth conditions must also be appropriate to form the defects at the energy levels de~cribed above. The low temperature growth technique described above has been shown to produce these material properties (contact 15 resistances less than 0.4 ohm-cm2 have been achieved).
The low-temperature photoluminescence (PL) spectrum from a good ohmic contact layer such as 26 is shown in Figure 8. The observed characteristics include:
l) the very weak near band edgQ PL; 2) the appearance of 20 the defect band at 2.3 eV (18,500 cm-l); and 3) the presence of a band (presumably a~sociated with donor-acceptor-pair recombination) at about 2.5 eV (20,400 cm~
The band edge PL is expected to be weak for materials which have significant concentrations of deep levels 25 since the deep levels provide long wavelength and nonradiative channels which compete with the near band edge proce~ses. The emission band at approximately 2.3 eY is as~ociated with a transition from the conduction band to a deep (acceptor) level about 0.5 eV above the 30 valence ~and maximum. Thi~ is near the energy position that iB believed to be the most effective for cascade tunneling. The emission band at 2.5 eY i8 believed to be related to transitions from donor to acceptor states. No or minimal donor states would be preferrable, eliminating 35 this transition, or shifting its occurance to ~lightly higher energies.

~'() ~2/21 1 7U CA 0 2 2 3 4 517 19 9 8 - 0 5 - 2 8 PCl tUS9t~0378 I

In g~ner~ nd other than the d$frer-nce~
de~cr$bQd below, conventional procs-~e- (ie , tho~e us~d for Si and III-V semlconductor devices) arQ used to complete the fabrication of prototype laser diode 10 5 Following the deposition of contact layer 26, the as yet incomplete la~er diode 10 i~ removed from MBE chamber 54 Electrode 30 includes Au which is ~acuum avaporated onto contact layer 26 and patterned into a 6tripe (typically ~bout 20 ~m wide) u~ing conventional photollthography and lo llft-off An in~ulating layer 34 of i~ then ~pplied ov-r electrode 30 and th~ expo~ed ~urface of contact layer 26 ~or an insulator that can be applied at low temperatures, polyimide photoresi~t i~ preferred Prob$m$de 408 from Ciba-Geigy Corp wa~ used to produce laser diode 10 A
15 stripe (about 20 ~m wide) of the polyimide layer 34 directly above electrode 30 i~ removed by UV exposure through ~ photomask ~nd d~v-lopment u~ing the manufacturer's recommended pl~ ~sing recipe, ~xc ~L for the post-developmQnt cure To cure the dQveloped 20 polyimide, the devic~ wa~ flood exposed to 1 J/C~2 of W
light from a mask aligner, and baked at 125~ C on a hot plate in ~ir for 3 minute~ Ti-Au layer 31 iB th-n evaporated on the expo6ed ~urface of the Au electrode 30 and polyimide layer 34 to facilitate lead-bonding The 25 In used for MBE substrate bonding ~180 ~er~ed a8 electrode 32 on substrate 12 Opposite end~ of the device were cleaved along (110) plane~ to form facQt mirrors Cavity length of the prototype devices 10 i~
about 1000 ~m La-er dioae~ 10 werQ then bond-d p-side 3Q up to cer~mic sampl- hold-r~ wlth ~ilvQr-fllled ~poxy Improved performanc~ of th-~e l~er deviceR can be gained by providing bett~r lateral confinement of the optical mode This can be achieved by forming an index-guided la~er 10' such as that ~hown in Figure 7 Index-35 guided laser 10' i5 6imilar to la~er lO ~nd can befabricated with the 6ame II-VI ~emiconductor layers Portions of l~er 10' which corre~pond to tho~e of l~ser lO are indicated with identical but primed (ie., "X"') reference numer~ls. In the embodiment shown, laser lO' includes a waveguide or rib 35 in the cladding layer 22' and contact layer 26'. Rlb 35 can be formed to a width 5 of about 5 ~m by ion beam etching with a Xe or Ar ion beam or by wet-chemical etching. Conventional photoresist can be uRed as a ma~k for this process.
other known and conventional techniques can al~o be used to provide lateral waveguiding. These techniques include lO using ~ubstrates in which grooves h~ve been etched (ie., channelled-subRtrate la~er~), or etching a rib and re-growing a top cladding layer (ie., a "buried heterostructure" laser). Im~rovements in the threshold current or the differential quantum efficiency may be 15 achieved by dielectric coatings of the facets to adjust the reflectivities.
Initial tests of the prototype laser diodes lO were conducted at 77 X by pulsing the devices, typically with 500 n~ec pul~es and a 500 ~sec period.
20 Current measurements were made with a box-car averager, while a large Si photodetector wa6 u~ed to collect and monitor the output light intensity from one end facet of the device. The measured light output as a function of current (ie., L-I) characteristics from one of the 25 devices is illustrated in Figure 5. The threshold current is 74 mA, whlch corrQ-ponds to a threshold current density of 320 A/cm2. Differential quantum efficiencies in excQss of 20% per facet have been measured, as have pulsed output powers of over lO0 mW per 30 facet. The coherent light is ~trongly TE polarized and a "speckle pattern" is clearly vi~ible. The output laser beam has an elliptical far-field pattern, with a divergence of roughly 40~x4~ for the central lobe. Side lobes are visible, indicating higher order transverse 35 modes.
The mea~ured L-I characterictics, ~uch as that shown in Figure 5, do indicate some dependence on pulse CA 02234~17 1998-0~-28 length. At high current densities, the gain in the single quantum well prototype devices tends to saturate. At the same time, the index of refraction is reduced due to the injection of excess carriers, which tends to make the region under the stripe of electrode 30 anti-guiding. Thermal effects become important at these current densities as thermal gradients and the temperature dependence of the index provide lateral optical confinement. It is expected that these characteristics will be alleviated by index-guided versions such as laser diode 10'.
Figures 61 and 62 show a graph of the optical spectra that are characteristic of the prototype laser diodes 10 at 77 K. The spectra illustrated in Figures 61 and 62 were acquired using a SPEX 1403 double monochromator. At currents below threshold, the spontaneous emission occurs at 490 mn and has a FWHM of about 3nm. Above threshold, the 1060 ~m long device operates in many longitudinal modes centered at 489.6 ~m, and which are separated by 0.03 nm.
Laser operation has been observed in the prototype laser diodes 10 for short periods of time at temperatures as high as 200 K. At room temperature the devices emit at 502 nm, but do not lase.
The operating voltage of the prototype laser diodes 10 at the threshold current is approximately 15 V. This characteristic indicates that there is still room for improvement in the ohmic contact between electrode 30 and contact layer 26 and/or improvement in the conductivity of p-type layers 16, 22, and 26. Reducing this series resistance and improving the heat-sinking of the device (ie., by solder-bonding the p-type side down) are expected to facilitate CW
operation at higher temperature.
It is expected that the inventive concepts disclosed herein and used to fabricate the prototype laser diode 10 are equally well suited to the fabrication ~ ~2t21170 PCT/US92/03 -- 2g --of laser diodes from a wide variety of other compound II-VI 5emiconductor alloy6, eBpecially from other ZnSe alloys. For example, improved operatlng character~sics will be achieved by using lattice matched materials such 5 ag Cd~Zn~S (with x of a~uximately 0.61) and ZnSe to form the waveguide. The quantum well in 6uch a device may include CdZnSe. ThiA ~emiconductor 6ystem will not suffer from misfit dislocation~ which c~n decrease efficiency and the useful li~etime of the devices. Also, lo a multiple quantum WQll active layer made of a ~trained-layer superlattice could replace the 6ingle pseudomorphic quantum well layer 18.
Ohmic contact layer 26 might also be improved by using thin layer~ of smaller band gap II-VI alloys 15 such as ZnSel~Tes, C~Znl~Se and Hg~Znl,Se. Group YI sources other than Se2 can al~o be used to produce ohmic contacts in accordance with the present invention. Other Group VI
specieR Xm wher- m<6, ~ w-ll a- other ~ource~ of the~Q
speciQs, should be suitable substitutQ~. Other metals (eg., Pt) or other electric~lly conductive materials having a large work function (eg., >5eV) and ~uitable for a stable semiconductor interface can also be used as electrodes. In conclu~ion, although the present invention has been described with reference to preferred 25 embodiments, change~ can b- mads in form and detail without departing from the ~pirit and scope of the invention.

Claims (19)

1. A method for producing an ohmic contact to a p-type II-VI semiconductor body, including: placing a p-type II-VI
semiconductor body in a molecular beam epitaxy chamber;
injecting at least one group II source into the chamber;
injecting at least one group VI source Xm into the chamber, where m<6; injecting at least one free-radical p-type dopant into the chamber; heating the semiconductor body to a temperature less than 250 degrees C but high enough to promote crystalline growth of a II-VI semiconductor layer doped with the free-radical p-type dopants to a net acceptor concentration of at least 1x10 17cm-3; and growing a crystalline II-VI ohmic contact layer doped with the free-radical p-type dopants to a net acceptor concentration of at least 1x10 17cm-3, on the semiconductor body.

2. The method of claim 1 wherein heating the semiconductor body includes heating the semiconductor body to a temperature high enough to promote crystalline growth of a II-VI semiconductor layer doped with free-radical p-type dopants to a net acceptor concentration of at least 1X10 18cm-3.

3. The method of claim 2 wherein heating the semiconductor body includes heating the semiconductor body to a temperature between about 130 and 200 degrees C.

4. The method of any one of claims 1 to 3 wherein:
injecting a group VI source includes injecting Se m where m<6;
injecting a group II source includes injecting Zn; and injecting free-radical p-type dopants includes injecting group V free-radicals.

5. The method of any one of claims 1 to 3 wherein injecting a group VI source includes injecting thermally cracked Se.

6. The method of any one of claims 1 to 3 wherein injecting group V free-radical p-type dopants includes injecting free-radicals from the group consisting of nitrogen, arsenic and phosphorous.

7. The method of any one of claims 1 to 3 wherein injecting free-radical p-type dopants includes injecting nitrogen free-radicals.

8. The method of any one of claims 1 to 3 and further including depositing a layer of conductive material on the ohmic contact layer.

9. The method of claim 8 wherein depositing a layer of conductive material includes depositing a layer of conductive material having a work function greater than about 5eV on the ohmic contact layer.

10. The method of claim 8 wherein depositing a layer of conductive material includes depositing a layer of metal on the ohmic contact layer.

11. A method of fabricating a laser diode including a layer of p-type II-VI semiconductor, including producing an ohmic contact to the layer of p-type II-VI semiconductor in accordance with the method of any one of claims 12 to 14.

12. A Group II-VI semiconductor device including: a p-type Group II-VI semiconductor device layer; a p-type Group II-VI crystalline semiconductor contact layer on a first side of the device layer; a conductive electrode layer characterized by a Fermi energy, on the contact layer; and the contact layer is doped, with shallow acceptors having a shallow acceptor energy, to a net acceptor concentration of at least 1x10 17cm-3, and includes sufficient deep states with energy between the shallow acceptor energy and the electrode layer Fermi energy to enable sufficient current flow for device operation with voltages less than about 2 volts across the contact layer.

13. The p-type ohmic contact of claim 12 wherein the contact layer is doped with shallow acceptors to achieve a net acceptor concentration of at least 1x10 18cm-3.

14. The p-type ohmic contact of claim 12 wherein the contact layer is doped with nitrogen shallow acceptors.

15. The p-type ohmic contact of claim 12 wherein the contact layer is doped with nitrogen to a concentration of 1X10 19cm-3.

16. The p-type ohmic contact in any one of claims 12 to 15 wherein the semiconductor device layer includes a ZnSe semiconductor layer.

17. The p-type ohmic contact of any one of claims 12 to 15 wherein the electrode layer includes a layer of material having a work function of at least 5eV.

18. A laser diode for emitting a coherent light beam in the blue and/or green portions of the spectrum, including a layer of p-type II-VI semiconductor, and an ohmic contact in accordance with any one of claims 23 to 26 to the layer of p-type II-VI semiconductor.

19. An electronic system including a laser diode in accordance with claim 18.