Stress Reduction in AlGaAs-AlGaAsP Multilayer Structures

The invention relates to the reduction of stress between contiguous layers of different semiconductor materials and, more particularly, to double heterostructure junction lasers with reduced current density thresholds for lasing.

In the summer of 1970 I. Hayashi and M. B. Panish reported successful c.w. operation at room temperature of a semiconductor p-n junction laser now known as the double heterostructure (DH) laser (Applied Physics Letters, Vol. 17, No. 3, pages 109-111, Aug. 1, 1970). This achievement was the result of intensive efforts both in physics, which led to the design of the DH laser, and in chemistry, which led to improved liquid phase epitaxy (LPE) techniques for realizing the DH laser.

Briefly, the DH laser comprises a narrow bandgap active region, which may be either n-type, p-type or contain a p-n junction in which case it has both conductivity types. The active region is sandwiched between relatively wider bandgap, opposite-conductivity-type layers which form two heterojunctions, one at each interface with the active region. These heterojunctions, as is now wellknown, serve to confine injected carriers as well as stimulated radiation to the active region. Consequently, it was early recognized by Hayashi and Panish that the heterojunctions should have as few defects as possible because such defects could act as nonradiative recombination centers which would tend to reduce efficiency and increase lasing thresholds. They, therefore, fabricated their DH lasers by LPE from the GaAsAlAs semiconductor system; i.e., early forms of the DH lasers comprised an n-GaAs substrate on which were grown the following layers:

Because GaAs and AlAs are nearly lattice matched at the growth temperature (about 800.degree.C), GaAs and AlGaAs were even better lattice matched, so that particularly good heterojunctions were formed during LPE fabrication.

When operated c.w. at room temperature, however, these early forms of the DH laser typically had relatively short lifetimes ranging from only a few minutes to tens of hours. Consequently, systematic studies of DH lasers were undertaken in order to identify degradation mechanisms and to develop solutions to the short lifetime problem.

Since the reduction of process-induced stress (e.g., stress due to bonding of contacts) has been shown by R. L. Hartman and A. R. Hartman to increase the lifetime of DH lasers (see Applied Physics Letters, Vol. 23, p. 147 (1973)), we believe that the reduction of growth-induced stress may further extend the lifetime. More particularly, we have recognized that although GaAs and AlAs are nearly lattice matched at the LPE growth temperature, their lattice mismatch is magnified at room temperature (where the lasers are intended to operate) because of the different thermal expansion coefficients of the two materials. Therefore, at room temperature there is considerable stress in the grown epitaxial layers.

We propose that the process be reversed; i.e., that a slight lattice mismatch be intentionally introduced at the growth temperature so that the difference in thermal expansion, before a deleterious factor, can be taken advantage of to produce a better lattice match at room temperature where DH lasers are most often operated. In accordance with one embodiment of our invention, therefore, the average stress at room temperature between contiguous layers of Al.sub.x Ga.sub.1.sub.-x As and Al.sub.y Ga.sub.1.sub.-y As (0 .ltoreq. x,y .ltoreq. 1, y > x) can be reduced by the addition of small, controlled amounts of phosphorus during the growth of the latter layer so that the quaternary Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z is grown instead of the ternary Al.sub.y Ga.sub.1.sub.-y As. We have found that the amount of phosphorus added must be properly chosen: too little added may reduce stress by only a minimal amount whereas too much added may in fact increase stress and may generate dislocations. In particular, in order to reduce the average stress in the quaternary layer to less than about 2 .times. 10.sup.8 dynes/cm.sup.2 the amount of phosphorus added should satisfy the condition: ##EQU2##

Evidence of the beneficial effect of adding phosphorus was found in experiments which demonstrated that DH lasers comprising AlGaAsP-GaAs-AlGaAsP layers grown on a GaAs substrate, in accordance with our invention, generally exhibited lasing thresholds about 25 percent lower than conventional AlGaAs DH lasers containing no phosphorus. Moreover, many DH lasers containing phosphorus as aforesaid have been successfully operated c.w. at room temperature for more than 5000 hours, and several for more than 10,000 hours. Although in the latter case we have not as yet been able to correlate completely the addition of phosphorus to longer lifetimes, it is believed that the reduction of stress of this form should enhance laser lifetime.

At times we will use for convenience the abbreviations AlGaAs and AlGaAsP to represent, respectively, the ternary compound, aluminum gallium arsenide and the quaternary compound, aluminum gallium arsenide phosphide.

Our invention, together with its various features and advantages, can be more easily understood from the following more detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 shows contiguous layers of GaAs and Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z fabricated in accordance with one aspect of our invention;

FIG. 2 is a schematic drawing of a DH laser in accordance with one embodiment of our invention;

FIG. 3 is a graph showing how the average stress between layers of GaAs and AlGaAsP is related to the amount of phosphorus and aluminum in the latter layer; and

FIG. 4 is a graph showing the variation with temperature of the lattice parameters of AlAs (curve I), GaAs (curve II) and AlGaAsP (curve III, which is estimated for a quaternary composition lattice matched to GaAs at room temperature).

We have established that liquid phase epitaxy (LPE) layers of Al.sub.y Ga.sub.1.sub.-y As grown on GaAs substrates are compressively stressed at room temperature to levels of 10.sup.8 to 10.sup.9 dynes/cm.sup.2 depending on y, the fraction of aluminum in the grown layer. The stress, which is elastic in nature (i.e., no misfit or growth-induced dislocations are generated), can be accounted for by the differences in the thermal expansion coefficients between layer and substrate. These grown-in stresses are believed to be potentially deleterious to the performance and lifetimes of double heterostructure lasers. We have demonstrated, however, that LPE layers of Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z grown on GaAs substrates reduce or eliminate heterojunction stresses, provided the amount of phosphorus added is properly chosen. The essence of our invention is to use a quaternary compound to deliberately mismatch the epi-layer at the growth temperature so that on cooling the different thermal expansion coefficients of the substrate and layer will enable the lattice parameters to be substantially matched at room temperature. In contrast, at room temperature a ternary Al.sub.y Ga.sub.1.sub.-y As layer has a larger lattice parameter than the GaAs substrate, a situation which, as previously mentioned, induces considerable compressive stress in the grown layer.

The fraction z of phosphorus in the quaternary layer Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z was estimated from the change in stress in the grown layer, compared to that in Al.sub.y Ga.sub.1.sub.-y As with the same fraction y of aluminum. This stress reduction was converted to an equivalent strain reduction since the strain .epsilon. = .DELTA.a/a = .sigma.(2.nu./E), where .nu. and E are Poisson's ratio and Young's modulus, respectively, a is the lattice parameter of the binary substrate, and .DELTA.a is the lattice parameter difference between the quaternary layer and binary substrate. Using (i) the values of E and .nu. for GaAs published by Brantley in Journal of Applied Physics, Vol. 44, p. 534 (1973), (ii) a substrate lattice parameter of 5.6537 Angstrom, and (iii) the stress data determined as described below as a function of the phosphorus composition in the growth solution, we calculated a lattice parameter shift of about 0.0012 Angstroms per 10.sup..sup.-5 atom fraction of phosphorus in the growth solution. Since the room temperature lattice parameter difference between GaP and Ga.sub.0.66 Al.sub.0.34 As is about 0.21 Angstroms, we assumed Vegard's law was operative and estimated a phosphorus concentration ratio between the solid and liquid of about 300 for growth at 790.degree.C.

The stress in the quaternary layer, the substratelayer lattice parameter difference, and defect structure of the quaternary layer were analyzed using a modified Lang X-ray topography camera in three different modes of operation. First, the substrate lattice curvature due to the presence of the heteroepitaxial quaternary layer was determined by plotting the change in the position of the Bragg angle, .DELTA..theta..sub.B, as a function of sample position. The plots were obtained automatically and were called ABAC traces (Automatic Bragg Angle Control) and the substrate radius was converted to layer stress in a manner well known in the art and as described by G. A. Rozgonyi in Review of Scientific Instruments, Vol. 44, p. 1053, (August 1973). A {220} transmission X-ray topograph was then obtained. Since the LPE layer did not cover the entire substrate, defects unique to the layer were readily identified. Once the transmission X-ray topograph plate was exposed, the crystal was rotated and a (400) reflection rocking curve was obtained by operating the ABAC apparatus so that it plotted the intensity, I, as a function of Bragg angle. The I vs. .theta..sub.B traces contained characteristic K.sub..alpha..sub.1 -K.sub..alpha..sub.2 peaks from both the layer and substrate. The measured peak shift (K.sub..alpha.PS) was used to estimate the room temperature lattice parameter differences between layer and substrate.

As shown in FIG. 1, Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z quaternary layers were grown at 790.degree.C on (100) oriented GaAs substrates in an apparatus of the type described by M. B. Panish in Journal of Applied Physics, Vol. 44, p. 2659 (June 1973) at a cooling rate of 0.1.degree.C/minute for times ranging from 5 to 60 minutes. Illustratively, the growth liquid contained 0.0020 atom fraction Al, 0.985 atom fraction Ga and 0.013 atom fraction of As, with the phosphorus concentration varied in the 10.sup..sup.-6 to 10.sup..sup.-5 atom fraction range. The starting liquid concentration required to yield the quaternary compositions of interest were estimated from a thermodynamic treatment of the quanternary in which the liquid was taken to be a simple solution and the solid a strictly regular solution.

For the above composition, the value of y (aluminum fraction) was determined from the peak energy of the photoluminescence to be approximately 0.34 by comparison with the data given in FIG. 1 of an article by M. B. Panish in Journal of Applied Physics, Vol. 44, p. 2667 (June 1973).

As a result of the foregoing experiments, we measured the stress in quaternary layers of Al.sub.0.34 Ga.sub.0.66 As.sub.1.sub.-z P.sub.z for various values of z ranging from 0 to 0.04 approximately, and plotted the data as graph I of FIG. 3. Graph I shows that the room temperature stress is, within experimental error, linearly dependent on the amount of phosphorus incorporated in the quaternary layer. An important aspect of graph I is the fact that as z increases the stress first decreases from a maximum compressive stress of about 8 .times. 10.sup.8 dynes/cm.sup.2 at z = 0 (i.e., an Al.sub.0.34 Ga.sub.0.66 As layer on a GaAs substrate) to substantially zero stress at z = 0.013. The addition of more phosphorus from this point on causes the stress to increase and to switch from compressive to tensile. In fact, the addition of too much phosphorus (i.e., z .ltorsim. 0.03) produces a tensile stress which exceeds that of a ternary Al.sub.0.34 Ga.sub.0.66 As layer (z = 0).

In a similar fashion, if the quaternary layer contains less aluminum than y = 0.34, then graph I shifts to the left in FIG. 3, e.g., graph III for y = 0.17. On the other hand, if the quaternary layer contains more aluminum than y = 0.34, then graph I shifts to the right in FIG. 3, e.g., graph II for y = 0.51. In the latter two cases, therefore, the point of substantially zero average stress also shifts, being approximately z = 0.006 for y = 0.17 and being approximately z = 0.019 for y = 0.51. In general, substantially zero average stress occurs in the quaternary layer on a GaAs substrate when the amount of phosphorus z is related to the amount of aluminum y as follows:

However, as a practical matter for many device applications (e.g., DH lasers) it will be sufficient if the average stress is reduced below 2 .times. 10.sup.8 dynes/cm.sup.2, which occurs whenever the following inequality is satisfied approximately:

Even more generally, however, if the substrate is not purely GaAs but contains some aluminum (i.e., an Al.sub.x Ga.sub.1.sub.-x As substrate or underlying layer), then substantially zero average stress occurs at ##EQU3## and to reduce the average stress to less than about 2 .times. 10.sup.8 dynes/cm.sup.2, the following inequality should be satisfied approximately: ##EQU4## where the quantity (y - x) is the absolute value of the difference between y and x. Note that inequalities (1) and (5) are identical.

The origin of equation (4) can be understood from the following brief discussion. It is known in the prior art, and confirmed by our experiments on GaAlAs-GaAs layers grown by LPE, that the room temperature lattice mismatch between AlAs and GaAs is about 0.008 Angstroms (see FIG. 4). We have also determined that in quaternary Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z layers grown on GaAs substrates that the lattice parameter change is approximately 0.002 Angstroms for each percent of phosphorus added to the Group V component of the compound. Therefore, generalizing for the amount of phosphorus required to reduce to substantially zero the average stress in a quaternary layer of Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z grown on a ternary layer of Al.sub.x Ga.sub.1.sub.-x As, we can write ##EQU5## which corresponds to equation (4).

It should be emphasized that in the foregoing procedure, because the substrate-layer lattice parameters are intentionally mismatched at the growth temperature, the probability for formation of interfacial misfit dislocations is increased. Those samples which exhibit stress relief by misfit dislocation generation will necessarily bend the substrate to a lesser degree and give a lower value of layer stress. It was, therefore, important that the total layer misfit strain energy at the growth temperature (which is proportional to both the layer thickness and the lattice mismatch at that temperature) be kept below a threshold value which is defined by FIG. 6 of the concurrently filed application of Petroff and Rozgonyi, Case 2-7, supra. In particular, compressively stressed layers in the vicinity of the minimum room temperature stress region in FIG. 3 (i.e., z satisfying inequality (5)) were grown without misfit dislocations provided that the thickness of the quaternary was maintained less than a value in the range of approximately 1.mu.m to 2.mu.m when z was in the range of about 0.03 to 0.01. Of course, fabrication and/or device considerations limit how thin the layer can be. Within this thickness range the actual thickness used depended on the value of z, i.e., larger values of z required thinner layers. As is well known in the art, however, growth without the creation of interfacial misfit dislocations does not eliminate substrate defects which extend into the quaternary layer. The manner in which a unidirectional sheet of infinitely long misfit dislocations is intentionally created at the substrate-layer interface in order to prevent substrate defects from extending into the quaternary layer is described by Petroff and Rozgonyi in the concurrently filed application; Case 2-7, supra.

Application of the foregoing stress reduction procedures to the LPE growth of double heterostructure lasers has resulted in significant and reproducible reductions in the threshold current density for lasing. As shown in FIG. 2, a DH laser in accordance with an illustrative embodiment of our invention comprises an n-GaAs substrate 10 on which were grown by LPE the following layers in the order recited: an n-Al.sub.y Ga.sub.1.sub.-y As.sub.1.sub.-z P.sub.z layer 12; an n-type, p-type or compensated GaAs layer 14; a p-Al.sub.q GA.sub.1.sub.-q As.sub.1.sub.-p P.sub.p layer 16 and a p-GaAs layer 18. In general, of course, layer 14 may also include A1 or A1 and P and the conductivity types of the layers may be reversed if a p-type substrate is used. As is well known in the art, however, the fraction of A1 added to layer 14 should be less than about 0.4, the point at which AlGaAs switches from a direct gap to an indirect gap semiconductor.

Layers 12, 14, and 16 comprise a double heterostructure and the interfaces 13 and 15 therebetween form a pair of heterojunctions which serve to confine both light and injected carriers to the active region (layer 14) when the device is forward biased. Typically the heterostructure forms a symmetric waveguide in which case y = q and z = p. Layer 18 is utilized to make good electrical contact to the device and usually includes a thin (0.2 .mu.m) high conductivity surface layer (not shown) formed by diffusing Zn therein. Electrical contacts 20 and 22 are deposited, or otherwise formed, on layer 18 and substrate 10, respectively. A resonator is formed by mirrors 24 and 26 which are typically cleavage faces of the GaAs crystal. One or more heat sinks (not shown) may be thermally coupled to substrate 10 or layer 18 by means well known in the art.

In one embodiment a stripe contact geometry is defined by bombarding the spatially separate zones 28 with high energy protons along the length of device and approximately to the depth of heterojunction 13. As described in copending U.S. application Ser. no. '204,222 (D'Asaro 10-4-6-3) filed on Dec. 2, 1971 now U.S. Pat. No. 3,824,133 isued on July 16, 1974, such proton bombardment produces high resistivity in zones 28 which serves to confine the electrical current applied through contact 20 to the unbombarded zone 30 (about 13 .mu.m wide). In addition, this configuration tends to confine the laser radiation to oscillation in the fundamental transverse mode parallel to the plane of the heterojunctions. We have also constructed, however, broad-area-contact lasers in which no proton bombardment was performed and, therefore, the laser was pumped across its entire width and length.

For the case of a symmetric structure, numerous lasers of the type shown in FIG. 2 were fabricated by utilzing phosphorus compositions ranging from about 30 to 150 .mu.gm GaP per 4 gm Ga in the growth solutions of layers 12 and 16. The layers were grown by LPE at about 780.degree. C with a cooling rate of 0.1.degree. C/min. The corresponding fraction z of phosphorus, therefore, ranged from about 0.004 to 0.02 whereas the fraction (y = q ) of aluminum was about 0.24. For different devices the thickness d of the active region (layer 14) ranged from about 0.15 .mu.to 0.40.mu.m. In an illustrative example of the DH laser of FIG. 2, the substrate 10 comprised n-GaAs doped with Si to 3 .times. 10.sup.18 /cm.sup.3, layer 12 was 1.95 .mu.m thick and comprised n-Al.sub.0.24 Ga.sub.0.76 As.sub.1.sub.-z P.sub.z doped with Te to 2.5 .times. 10.sup. 17 /cm.sup.3 layer 14 comprised p-GaAs doped with Ge to 4 .times. 10.sup.17 /cm.sup.3, layer 16 was 0.79 .mu.m thick and comprised p-Al.sub.0.24 Ga.sub.0.76 As.sub.1.sub.-p P.sub.p (p = z) doped with Ge to 3.3 .times. 10.sup. 17 /cm.sup.3, and layer 18 comprised p-GaAs doped with Ge to 2 .times. 10.sup. 18 /cm.sup.3. In addition, the resonator length was about 380 .mu.m. Substituting y = 0.24 and x = 0, inequality (1) reduces to 0.007 <z < 0.012 approximately for stress reduction in these layers.

The lowest thresholds for such lasers were compared with the lowest thresholds of similar ternary DH lasers (no phosphorus in layers 12 and 16). On the average, the quaternary lasers (FIG. 2) exhibited thresholds which were about 25 percent lower than the thresholds of ternary lasers. For example, at d = 0.15 .mu.m the lowest threshold of stripe geometry quaternary lasers was about 1.4 kA/cm.sup.2 whereas that of ternary lasers as 1.8 kA/cm.sup.2. At d = 0.4 .mu.m the quaternary threshold was 3.0 kA/cm.sup.2 whereas the ternary was 3.7 kA/cm.sup. 2. Similarly, for broad-area-contact lasers with d = 0.15 .mu.m, the lowest threshold of the quarternary lasers was about 0.92 kA/cm.sup.2 whereas that of the ternary was 1.22 kA/cm.sup.2. At d = 0.3 .mu.m the corresponding comparison was 1.15 kA/cm.sup.2. Thus, it is apparent that the addition of appropriate amounts of phosphorus to layers 12 and 16 produced significant reductions in lasing thresholds.

It is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which can be devised to represent application of the principles of our invention. Numerous and varied other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. In particular, the structures and techniques described herein find application in numerous other devices such as single heterostrucure lasers and heterostructre LEDs of the type desccribed by I. Hayashi in application Ser. No. 307,219 (Case 8), now U.S. Pat. No. 3,801,928 issued on Apr. 2, 1974, and application Ser. No. 307,377 (Case 7) now U.S. Pat. No. 3,812,516 issued on May 21, 1974, concurrently filed with one another on Nov. 17, 1972. Moreover, use is also contemplated in non-optical devices where it is desired to reduce stress or misfit dislocations.