CA1126374A - Strip buried heterostructure laser - Google Patents

Abstract

Abstract of the Disclosure A double heterostructure laser having a pair of opposite-conductivity-type, wide bandgap cladding layers separated by a narrower bandgap active region is characterized in that the active region includes a low-loss wavesuide layer and contiguous therewith a narrower bandgap active layer in the form of a narrow strip which extends along the longitudinal (resonator) axis of the laser.
Suitable lateral current confinement means, such as reversed biased p-n junctions, are provided to constrain pumping current to flow in a narrow channel through the active layer. Lasers of this type exhibit relatively high pulsed power outputs (e.g., 400 mW), linear L-I
characteristics, stable fundamental transverse mode and single longitudinal mode oscillation. In another embodiment the surfaces of the waveguide layer adjacent the active layer are provided with distributed feedback gratings. Also described are techniques for shaping the active layer without the introduction of debilitating defects therein, as well as procedures for LPE growth on Al-containing Group III-V compound layers which are exposed to processing in the ambient.

SCAN, R.A. 31-3

Description

6i379~

Strip buried heterostructure laser Back~round of the Invention . ., ,_ . . _ . _ This invention relates to semiconductor junction lasers.
The stripe-geometry contact for junction lasers was proposed by R. A. Furnanage et al. (U.S. patent No.
3,363,195, granted January 9, 1968) more than a decade ago and has been incorporated, in one form or another, in various heterostructure laser configurations in use and under study today. These lasers, which range from 10 the simple double heterostructure (DH) (I. Hayashi, U.S.
patent 3,758,875, granted September 11, 1973) to more com-plicated buried heterostructure (BH) [T. Tsukada, Journal of Applied Physics, Vol. 45, p. 4899 (1974)], each have one or more advantageous operating characteristics.
The DH laser has the longest lifetime of all semi-conductor lasers, exceeding 105 hours to date, and is characteri~ed by low thresholds and fundamental trans-verse mode operation. On the other hand, it has a wide beam divergence, a nonlinearity (known as a "kink") in 20 its light-current (L-I) characteristic, and incomplete lateral current confinement.
The Tsukada BH laser, which includes a GaAs active region completely surrounded by Alo 3Ga0 7As, has effective transverse mode stabilization, but the 25 refractive index change along the junction plane is so large that stable fundamental mode lasing is possible ~q.

~.26374 only for active layer widths of ~ l~m, resulting in low output power (e.g., 1 mW) and large beam divergence in both transverse directions. In BH lasers with wider active layers, higher order modes are easily excited near threshold.
Summary of the Invention According to one aspect of the invention there is provided a method of epitaxially growing a Group III-V
compound second layer from the liquid phase on an Al-containing Group III-V compound first layer comprising the steps of: after growth of said Al-containing first layer, forming a non-Al-containing Group III-V compound epitaxial protective layer about several hundred Angstroms thick on a major surface thereof before exposing said first layer to an ambient which would otherwise oxidize said major surface, exposing said layers to said ambient, said protective layer preventing oxidation of said growth surface, bringing a molten solution of said Group III-V
compound of said second layer into contact with said protective layer so as to dissolve said protective layer into said solution and epitaxially grow said second layer directly on said major surface of said Al-containing first layer.
Other aspects of this invention are claimed in our patent application Serial No. 315,705 filed on October 31, 1978 of which the present application is a division, and in a second division thereof.
In accordance with one preferred embodiment of our invention a strip buried heterostructure ~SBH) laser com-prises a pair of opposite-conductivity-type, wide bandgap, semiconductor cladding layers separated by a narrower bandgap, semiconductor active region characterized ~1.2~i37~

in that the active re~ion includes a low-loss waveguide layer anà contiguous therewith a narrower band~ap active layer in the form of a narrow strip which extends along the longitudinal (resonator) axis of the laser. Preferably, S the bandgap difference between the waveguide and active layers is sufficiently large to confine to the active layer minority carriers injected therein when the cladding layers are forward biased, yet small enough to allow a significant portion of the stimulated radiation generated in the active 10 layer to be coupled into the waveguide layer, thereby reducing the optical power density at the mirror facets.
In addition, it is also preferable that the band~ap difference between the waveguide layer and the contiguous opposite-conductivity-type cladding layer be large enough 15 to prevent significant leakage current between the two layers under normal operating conditions of the laser.
~ leans are also preferably provided for constraining pum~ing current to flow in a narrow channel through the strip-shaped active layer. In one embodiment, 20 the constraining means includes a pair of laterally spaced reverse-biased p-n junctions-near the top surface of the laser. however, other constraining means, such as proton bombardment, are also suitable.
An illustrative embodiment of our SBH laser was 25 fabricated from the GaAs-AlGaAs materials syste,n and exhibited, over a wide operating range, high power output, a linear L-I characteristic for all currents up to catastrophic failure, stable fundamental transverse and single longitudinal mode oscillation and reduced beam 30 divergence, along with adequate lasin~ thresholds and external quantum efficiencies.
In other embodi,nents of our invention, the strip active layer is partially embedded in the low-loss ~aveguide layer instead of being formed on top of a major 35 s~rface of the waveguide layer. In either case, however, the portions of the major surface of the waveguide layer adjacent the active layer can be provided with distributed feedback gratings.

. 31-3 i37~
Another aspect of our invention is a method for defining the geometry of the strip active layer, or other àcvice active region, without the introduction of debilitating defects therein during shaping operations. A
thin eyita~ial protective is grown on the active layer before it is mas~ed and shaped (e.g., by etching and/or anodization), the protective layer is selectively etched away in mas~ openings to expose the active layer, and thin ~ortions of the active layer are removed te.g., by anodization) to de~ine the desired geometry. In the fabrication of our SBH laser, these procedures are followed by growing over the shaped structure a cladding layer having a composition essentially identical to that of the protective layer so that the latter is incorporated into 1~ the former.
One additional aspect of our invention entails a procedure for epitaxially growing a Group III-V compound second layer (e.g., the AlGaAs cladding layer) from the licluid phase on an ~l-containing Group III-V compound first 20 la~er (e.~., the ~lGaAs wave9uide layer). ~fter growth of the ~irst layer, a non~Al-containing Group III-V compound epitaxial protective layer (e.g., GaAs) about several hundred angstroms thick is for.ned on a major surface of the first layer before exposing the first layer to an ambient 25 Which would otherwise o~idize it. Processing, such as etching and/or anodization, can be used to form the protective layer from a much thic~er layer (e.g., that from which the active strip of an SBH laser is formed), or in sonle applications the thin layer may be grown directly (e.g., by deposition of the first layer and the protective layer by molecular beam epitaxy). In either event, state of the art techllology has demonstrated that LPE yields better quality layers for optical devices than ~IBE, but LPE
growth on ambient-exposed Al-containing layers is difficult 33because Al tends to oxidize so readily. The protective layer enables the use of LPE because the molten solution used to grow the second layer dissolves the protective layer so that the second layer grows directly on the first .~hN, R. A. 31-3 ~ 2637~
layer.
~rief Description of the Drawing Our invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the accompanying drawing, in which the figùres are not drawn to scale for clarity of illustration.
FIG. 1 is a schematic isometric view of an SBH
laser in accordance with one embodiment of our invention in -which the strip active layer is formed on top of a majorsurface o~ the waveguide layer;
FIG. 2 is an end view of an SBH laser in accordance with another embodiment of our invention in which the strip active layer is partially embedded in the waveguide layer; and FIG. 3 is a schematic isometric view of an SBH
laser in accordance witn yet another embodiment of our invention in which distributed feedback gratings flank the strip active layer.
Detailed Description _ _ . ....
SBII Laser StLucture llith reference now to FIG. 1, there is shown an SB~ laser 10 formed on a substrate 11 and comprising first and second opposite-conductivity-type, wide bandgap, semiconductor cladding layers 12 and 14 separated by a narrower bandgap, semiconductor active region 16 characterized in that active region 16 includes a low-loss waveguide layer 16.1 and contiguous therewith a narrower bandgap active layer 16.2 in the form of a.narrow strip which extends along the longitudinal (resonator) axis 28 of the laser.. Tne narrow strip may be formed on top of a major surface of waveguide layer 16.1 as in FIG. 1 or, as si~own by layer 16.2' of FIG. 2, may be partially embedded in waveguide layer 16.1'. In the latter case, the major surface of the waveguide layer is essentially coplanar with a major surface of the active layer.
tleans 18 is provided for constraining pumping current to flow in a narrow channel throu~h the active ~!-.AtJ 1?. A. ~
-~.2637~

layer 16.2 ~or 16.2'~ when cladding layers 12 and 14 are forward biased above the lasing threshold. Forward bias voltage is applied by means of suitable ohmic contacts 20 and 22 formed on substrate 11 and means 18, respectively.
In the embodiment shown, constraining means 18 com-prises a pair of laterally spaced p-n junctions 18.1 and 18.2 which are reverse biased when cladding layers 12 and 14 are forward biased. The junctions are formed by de-positing on cladding layer 14 a layer 18.3 of the same 10 conductivity type and then forming a bifurcated, wider bandgap, opposite-conductivity-type layer 18.4 on layer 18.3. This fabrication technique is described more fully in our U.S. Patent Serial No. 4,169,997 issued on October 2, 1979. The junctions 18.1 and 18.2 are thus 15 separated by a window which exposes a strip of layer 18.3. That strip is contacted by the central portion 22.1 of ohmic contact 22 so that pumping current flows transversely through the layers in a narrow channel from contact portion 22.1 in the window to active layer 16.2.
20 Current spreading can be further reduced by incorporating an additional pair of spaced, reverse biased p-n junctions at the substrate interface by the techniques described in our aforementioned copending application or by using other prior art schemes referenced in that application.
In addition, making the bandgap of cladding layer 14 sufficiently greater than that of active layer 16.2 pre-vents any substantial amount of pumping current from bypassing the active layer 16.2 by flowing directly between the cladding and waveguide layers; i.e., the 30 turn-on voltage of p-n heterojunctions 16.3 between the waveguide and cladding layers in larger (e.g., 1.6V) than the turn-on voltage of p-n heterojunction 16.4 between the waveguide and active layers (e.g., 1.4V).
The pumping current causes the injection of minority 35 carriers into active layer 16.2 where they undergo radiative recombination to generate stimulated radia-tion. A significant portion of the optical field of this radiation preferably penetrates into the waveguide.

- ~26~74 layer 16.1 so as to reduce the optical power density at the mirror facets and thereby increase the threshold for catastrophic damage. To this cnd the bandgap (or refractive inaex) difference between active layer 16.2 and 5 waveguide layer 16.1 should be small enough to permit such penetration, yet large ellough to confine injected minority carriers to the active region and thercby maintain relatively high electronic 9ain. This laser configuration, we have found, exhibits relatively hi~h pulsed power 10 outputs (in the hundreds of milliwatts range) and, suprisingly, a linear L-I characteristic-free of kin~s - at all power levels up to the catastrophic damage threshold.
In addition, this SBH laser exhibited stable fundamental tranverse and single longitudinal mode operation.
In order to reduce the number of nonradiative recombination defect centers at the heterojunction interfaces between the various layers of our SBH laser, it is preferred that essentially lattice matched materials be utilized. Fewer defect centers in general means lower 20 lasiny thresholds and longer lifetimes. In the Group III-V
compound system these materials include, for example, GaAs-AlGaAs, GaAs-AlGaAsP, GaAsSb-AlGaAsSb and InP-InGaAsP.
Cf these, GaAs-AlGaAs has the advantage that it is substantially lattice matched over all solid solutions of 25 GaAs and AlAs. Using the latter system, the SBH laser of FIG. 1 would typically comprise an n-GaAs substrate on which the following layers would be epitaxially grown: an n-AlxGal_xAs cladding layer 12 (0 < x ~ 1); an n-AlyGal~yAs wave~uide layer 16.1 (0 < y < 1; y < x); an n-, p- or 30 compensated active layer 16.2 of AlzGal~zAs [0 ~ z ~ 0.4;
z < y; and (y-z) adapted to confine injected carriers to the active layer while at the same time permitting the optical field to penetrate from the active layer into the waveguide layer]; a p AlqGal~qAs cladding layer 14 [0 < q < 1; q > z and y; and (q~y) > (y-z) to prevent significant pumping current from flo~ing across heteroj~nctions 16.3]; a p-GaAs stop-etch and contacting layer 18.3, and an n-AlrGal_rAs bifurcated layer 18.4 ~A'~. P~. A. ~1-3 i37~
~0 < r < 1). Of course, it is obvious that the conauctivity types of the various layers can be reversed.
For efficient operation at room temperature the SBH laser is mounted on a heat sink (not shown) by means 5 well ~nown in the art, and for continuous wave operation at room temperature, the thickness of the active layer 16.2 should be less than 1.0 ~m and preferably about 0.15-0.20 ~m.
An alternative embodiment of our SBH laser 10 incorporates a distributed feedback ~DFB) grating which provides frequency selectivity and in integrated optics applications obviates the need for a discrete resonator formed by cleaved mirror facets. As shown in FIG. 3, the D~B grating comprises a plurality of parallel grooves 30 15 which are forl~ed on the same major surface of waveguide layer 16.1 as active layer 16.2, i.e., on heterojunctions 16.3. But, the grooves are formed on opposite sides of the active layer 16.2 and extend perpendicular to the resonator a~is 28 (i.e., perpendicular to the elongated dimension of 20 strip active layer 16.2). ~s is well known in the art, to provide feedback the periodicity of the grating should preferably be equal to an odd integral number of half wavelengths of the laser radiation as measured in the semiconductor. This grating would typically be formed, for 25 e.~arnple, by ion milling or chemically etching waveguide layer 16.1 after depositing and suitably masking active layer 16.2. Note that the interior ends of the grating grooves should preferably be as close to the sides of the active layer as possible to allow the optical field in the 30 active layer 16.2 to penetrate into the grating.
Illustratively, the grating enàs should be within 1-2 ~m of tne active region. ~lthough not depicted, the DFB
configuration of FIG. 3 could also be incorporated into the ~in~odiment of FIG. 2 by forming the grating, as before, on 35 tne heterojunctions 16.3' on opposite sides of active layer 16.2'. In this case, the interior ends of the grating 3rooves can ~e made right next to the sides of the embedded active strip 16.2' by fabricating the grating first, r~ , R. A. 31-3 ~lZ63~4 uniormly everywhere, then etching the channel for the embedded strip 16.2'.
In prior art buried heterostructure ~BH) lasers, effective transverse mode stabilization has been achieved 5 by introducing a built-in refractive index change along the junction plane; for example, by embedding an active GaAs core completely in Alo 3GaO 7As cladding. ~lowever, the index change along the junction plane is so large that stable fundamental mode lasing is possible only for active 10 layer widths of < 1 ~m, resulting in low output power and large beam divergence in the two transverse directions.
Yet, in lasers with wider active layers, higher order modes are easily excited near threshold~
In our SB~ laser, the introduction of the 15 waveguide layer converts the core in a BH laser to a strip-loaded waveguide having the thin active layer as the strip and the thicker low-loss waveguide as the supporting layer. This structure significantly reduces the effective refractive index change alony the junction plane. ~lence, 20 Eundamental transverse mode along the junction plane can be easily obtained with much wider strip widths. ~s a result, the output power is increased and the beam divergence is reduced, while mode stabilization is maintained.
Furthermore, better device fabrication and performance 25 control can be achieved.
In the direction perpendicular to the junction plane, the introduction of the waveguide layer greatly increases the cavity thickness (e.g., from about 0.2 ~m to about 1.6 ~m) while still providing enough potential 30 barrier to confine the injected carriers in the active strip. This thickening of the optical cavity does not affect the threshold current but increases the output power before catastrophic mirror failure and reduces the beam divergence. Since the active strip is much thinner than 35 the ~aveguide layer, the fundamental transverse mode (perpendicular to the junction plane) acq~ires more gain than higher order modes. This provides mode ~iscrimination against higher order modes even though they have slightly L~A~I, R. A. 31-3 ~26374 higher mirror reflectivity. Finally, the waveyuide layer is not expected to decrease the quanturn e~ficiency of the laser because it is essentially lossless at the lasiny wavelength. Therefore, low current threshold, stable fundamental transverse mode operation with linear liyht-current characteristic and narrow beam divergence in both transverse àirections up to substantially high injection current levels, and high output ~ower should be obtainable with our SB~ lasers. Indeed these properties have been 10 observed as discussed in the example which follows.
Example .
The following describes the fabrication of an SB~
laser from the GaAs-AlGaAs materials system. Dimensions, materials, conductivity types and carrier concentrations 15 are intended to be illustrative only and should not be construed as limitations on the scope of the invention.
~ sing a two-cycle liquid phase epitaxy (LPE) tcchnique, with suitable masking, etching and anodization steps between the two cycles, we fabricated SBH lasers of 2n the type depicted in FIG. 1 colnprisiny: an (001) n-GaAs substrate 11 doped with Si to about 101~ cm 3 and about 100 ~m thic~; an n-A10 3GaO 7~s cladding layer 12 doped with Sn to about 2xl ol 7cm~3 and about 1.4 ~m thick; an n-~ lGaO 9As waveguide layer 16.1 doped with Sn to about 25 2xl~l7cm~3; a p-GaAs active layer 16.2 doped with Ge to about 3~1017cm~3 and about 0.2 ~m thick and of various widths ~ 2.5, 3.5, 5, 7.5 or 10 ym; a p-Alo 3GaO 7As cladding layer 14 doped with Ge to about 3xlO17cm~3 and about 2.5 ~m thick; a p-Ga~s contacting and stop-etch 30 layer 18.3 aoped with Ge to about 5xlO17cm~3 and about 0.5 ~m tilick; and an n-A10 45Gau 55As layer 18.4 doped with Sn to about 1017cm~3 and about 1 ~m thick. The layer 18.4 had various window openinqs of comparable size to the underlying active strips 16.2 and in substantial 3~ registration therewith. The substrate contact 2U comprised a ~u-Sn alloy whereas the top contact 22 comprised a Au-Zn alloy.
Ihe fabrication of these SBH lasers proceeded as '~, R. A. 31-3 .. .

~ 11 ~
~6374 follows. During the first I.PE growth cycle, layers 12 and 16.1 as described above were depositcd on an n-Ga~s wafer (i.e., on the substrate 11) and then a p-Ga~s layer was, deposited having a thic~ness equal to that desired for the 5 active layer 16.2. A thin (about 0.2 ~m) p-Alo 3GaO 7As layer was then grown on the p-GaAs layer. ~lote, the last layer was deposited to protect the top interface of the active layer during subsequent processing steps and does not yet correspond to the much thicker cladding layer 14.
10 This intermediate structure was removed from the LPE
chamber and the toy surface of the thin Alo 3GaO 7As layer was anodized to form a native oxide masking layer thereon.
Standard photolithographic techniques were then used to form mask strips along the (110) direction in the oxide 15 layer and to expose the thin Alo 3GaO gAs layer between the strips. The exposed Alo 3GaO 07As was selectively etched in an iodine etchant (113 g KI, 6S g I2, 100 cc H2O) to expose the p-GaAs layer between the strips. Standard anodization (which forins a native oxide and consumes a 20 portion of the seniconductor) and stripping were then used to r~;nove nearly all of the p-Ga~s layer between the strips. It was important, ho;wever, to leave a thin (about 200 Angstco~ thick) layer of p-GaAs between the strips so as not to expose the underlying n-A10 lGaO gAs to the 25 atmosphere~ Such exposure makes subsequent LPE growth on Al-containing Group III-V compounds very difficult.
After removing the oxide strip masks and subsequent chemical cleaning, the structure on the wafer comprised layers 12 and 16.1 with strip mesas of p-GaAs 30 ~i.e., active layer 16.2) protected by the thin Alo 3GaO 7As layer. The spaces between mesas were protected with the thin (about 200 Angstro.~ thick) p-GaAs layer.
Next, the wafer was returned to the LP~ chamber 3snd p-Al~ 3GaO 7As layer 14 was grown thereon. During this growth step the thin p-Alo 3GaO 7As layers protecting the tops of the active layers were incorporated into layer 14, ¦ and th-? tbin p-GaAs layer between the strips was dissolved ' ~r.QN ~ A ~

37~
into the rnelt used to grow layer 14. Therefore, layer 14, for all practical purposes, grew directly on the portions of waveguide layer 16.1 between the strips as well as on the strips themselves.
The contactiny and stop-etch p-GaAs layer 18.3 was thetl grown followed by an n-A10 45Ga0 55As layer. The latter was Masked, using the same photolithographic mask used to define the strips, and then selectively etched, using the iodine etchant previously described, down to the 10 p-GaAs layer 18.3, thereby bifurcating the n-A10 45Ga0 55As layer as depicted by layer 18.4 of ~IG. 1. Individual SBH
laser diodes were then formed by conventional metallization, cleaving and heat-sinking procedures.
Light-current ~L-I) characteristics of our SBH
15 lasers without anti-reflection mirror coatings were made using standard measurement procedures. The measurements with pulsed injection (150 ns pulse width, 1000 pulses/sec) were made for active layer widths of about 5 ~m and 10 ~m and lengths of 380 ~m. The top channel (window in 20 layer 18.4) widths of the lasers with 10 llm and 5 llm wide active strips were typically about 15 ~m and 10 ~m, rcspectively. All lasers tested displayed excellent linearity in L-I characteristics. For lasers with 10 ~m strips, this linearity continued, without catastrophic 25 failure, to about 10 tirnes threshold current where a peak power output of 400 m~ per face was measured. One laser with a 5 ym strip was tested to the catastrophic failure limit. For that laser linearity continued up to about 15 times threshold at which a pea~ power output of 230 m~1 per 30 face was measured. At this power catastrophic failure occurred. Similarly, we measured the light-current characteristics of other SBH lasers with 5 ~m wide active layers pumped only to an output power of 100 ~W per face to avoid burnout. The uniformity and linearity of these 35 lasers was evident.
E'or SB~ lasers with 10 ~m and 5 ~m wide active layers, current thresholds were 150 mA-180 mA and 90 mA-150 mA, respectively, while the external quantum !3GA~J, R A. 31-3 cfficiencies were ~%-63~ and 25~-35~. The lower external quantum efficiency of the lasers with 5 ~m strips was due to: (1) the larger top channel-to-strip width ratio, about

2, as compared to about 1~5 for lasers with 10 ~m strips, 5 and (2) the fact that as the top channel width decreases, the amount of lateral current spreading in the p-GaAs and p-~lo 3GaO 7~s layers increases rapidly. By using more efficient lateral current confinement schemes, such as laterally spaced, reverse-biased junctions at the substrate 10 interface in addition to those of FIG. 1, we believe that much lower current thresholds can be obtained.
The far-field patterns, both along and perpendicular to the junction plane, at various current levels above threshold were also measured for a typical SBH
15 laser with a nominal 5 ~m wide active layer. These patterns were measured under pulsed operation up to 9 times threshold. In the current region examined, the lasers operated stably in the fundamental mode in both transverse directions ~ith no significant distortion of the field 20 l)atterns. In ~Jeneral, the beam divergences were about ~-10 degrees and 26-30 degrees in the directions parallel and perpendicular to the junction plane, respectively. For lasers with 10 ~m wide active layers, higher order modes along the junction plane were excited near threshold and 25 successively changed into even higher order modes as the current injection level was increased. We observed, however, no "kink" or other nonlinearity associated with mode transition. Lasers with 5 ~m wide active layers, under pulsed operation, exhibited sinyle longitudinal mode 30 oscillation at injection currents as high as twice threshold. In general, lasing occurred in several longitudinal modes at slightly above the threshold current Ith (< 1.05 Ith), but the lasing power quickly concentrated into a single longitudinal mode with a slight increase in 35 current. With increasing current, the longitudinal mode shifted to an adjacent shorter wavelength mode, staying predominantly a single mode over wide current intervals except during the orie~ mode transitions. Such current i37~
intervals shortened for high injcction current levels.
It is to be understood that the above-described arrangements are merely illustrative of the many specific emboaiments which can be devices to represent application 5 of the principles of our invention. Numerous and varied other arran~ements can be devices in accordance with these principles by those s~illed in the art without departing from the spirit and scope of the invention. In particular, in each of the embodiments of our SBH laser it is readily 10 possible to fabricate the strip active layer so that it is shorter than the resonator (i.e., the active layer terminates short of the mirror facets), thereby virtually eliminating surface recombination of the facets. Thus, the active layer would be entirely embedded in wider bandgap 15 material. Also note that with this modification to FIG. 3, the DFB gratings near the facets can be made to extend across the width of the laser.

Claims (6)

Claims:

1. A method of epitaxially growing a Group III-V compound second layer from the liquid phase on an Al-containing Group III-V compound first layer comprising the steps of:
after growth of said Al-containing first layer, forming a non-Al-containing Group III-V compound epitaxial protective layer about several hundred Angstroms thick on a major surface thereof before exposing said first layer to an ambient which would otherwise oxidize said major surface, exposing said layers to said ambient, said protective layer preventing oxidation of said growth surface, bringing a molten solution of said Group III-V
compound of said second layer into contact with said protective layer so as to dissolve said protective layer into said solution and epitaxially grow said second layer directly on said major surface of said Al-containing first layer.

2. The method of claim 1 wherein said protective layer comprises GaAs.

3. The method of claim 2 wherein said first and second layers comprises AlGaAs.

4. The method of claim 1 wherein said forming step in-cludes epitaxially growing a relatively thick protective layer, and thinning said layer until only several hundred Angstroms remain.

5. The method of claim 4 wherein said thinning step comprises anodizing said protective layer to form a native oxide which consumes a portion of said layer and removing said oxide layer so that only several hundred Angstroms of said protective layer remains.

6. The method of claim 1 wherein said first layer is grown by molecular beam epitaxy (MBE) and, without removing said first layer from the MBE growth chamber, said protective layer is formed by directly depositing by MBE several hundred Angstroms of said non-Al-containing Group III-V compound.