Method of Making a Double Heterostructure Laser

FIG. 1 is a perspective view of a semiconductor laser of the invention.

FIGS. 2-4 are cross-sectional views of a body at different steps in the device fabrication.

In FIG. 1 a semiconductor laser incorporating the present invention is designated as 10. The laser 10 comprises a body 12 of single crystal semiconductor material having spaced, parallel end facets 14 which are partially reflecting of light, with at least one of the end facets 14 being partially transparent so that light may be emitted therefrom. The body 12 also includes spaced, parallel side surfaces 16 which extend between and are perpendicular to the end facets 14.

The semiconductor body 12 includes a substrate 18 having spaced, substantially parallel major surfaces 20 and 22 which extend between and are perpendicular to the end facets 14 and the side surfaces 16 of the semiconductor body 12. In the first major surface 20 there is a channelled region 24 which extends between the end facets 14. The channelled region 24 comprises channels 26 which extend a distance into the substrate 18 from the first major surface 20 and mesas 28 between the channels 26. The top surfaces of the mesas 28 are typically below the level of the surrounding major surface 20. The channelled region 24 in effect forms a trough in the major surface 20. A buffer layer 30 overlies the first major surface 20, the channels 26 and the mesas 28 and has a surface 32 which is substantially planar over a center portion 34 of the channelled region 24. A first cladding layer 36 overlies the buffer layer 30 and has a substantially planar surface 38 over the center portion 34 of the channelled region 24. A cavity region 39 overlies the first cladding layer and is composed of a guide layer 40 and an active layer 44. The guide layer 40 overlies a surface of the first cladding layer and has a concave surface portion 42 over the center portion 34 of the channelled region 24. The active layer 44 overlies the guide layer and has a thick portion 46 over the concave surface portion 42 of the guide layer 40. A second cladding layer 48 overlies the active layer 44 and a capping layer 50 overlies the second cladding layer 48. An electrically insulating layer 52 overlies the capping layer 50 and has an opening 54 extending therethrough over the center portion 34 of the channelled region 24. A first electrical contact 56 overlies the electrically insulating layer 52 and the capping layer 50 and the region of the opening 54 in the electrically insulating layer 52. A substrate electrical contact 58 overlies the second major surface 22 of the substrate 18.

The substrate 18, the buffer layer 30 and the first cladding layer 36 are of one conductivity type and the second cladding layer 48 and the capping layer 50 are of the opposite conductivity type. In the cavity region 39 the positions of the guide layer 40 and the active layer 44 are interchangeable. The guide layer 40 is preferably positioned between the first cladding layer 36 and the active layer 44 and, in this case, has the same conductivity type as the first cladding layer 36. In some applications the guide layer 40 may be positioned between the active layer 44 and the second cladding layer 48 in which case it has the same conductivity type as the second cladding layer 48. The active layer 44 may be of either conductivity type but is typically only lightly electrically conducting. It is to be understood that the conductivity types of each of these layers may be reversed so long as the relationships of the conductivities of the various layers described above is maintained.

The compositions of particular layers of the laser 10 should be such that the refractive index of the active layer 44 is greater than the refractive index of the guide layer 40 which in turn is greater than the refractive indexes of the cladding layers 36 and 48. The laser may be formed of materials such as GaAs and AlGaAs which have the requisite refractive index differences. Alternatively, other combinations of group III-V alloys such as alloys containing indium, phosphorus, gallium and arsenic can be used.

The substrate 18 is preferably composed of N-type GaAs having a first major surface 20 parallel to the (100) crystallographic plane. The substrate may be misoriented from this orientation but preferably a {100} plane is used. The channels 26 are typically between about 1 and 4 .mu.m deep, having a width at the surface 20 between about 1 and 4 .mu.m. The channels are typically vee-shaped but may have another shape such as a flat bottom. The mesas typically have a hieght of between about 0 and 2 .mu.m above the bottom of the channels.

The buffer layer 30 is typically composed of N-type GaAs. This layer is typically between about 1 and 3 .mu.m over the substrate surface 20 and is typically between about 0.5 to 1.5 .mu.m over the tops of the mesas 28. The surface portion 32 of the buffer layer 30 is substantially planar over the center portion 34 of the channelled region 24 and is depressed below the remainder of the surface 20 forming a trough over the channels 26. The curvature at the corners of this trough provide the initial curved surfaces which will cause a tapering in the lateral direction in the thickness of the overlying layers.

The first cladding layer 36 is typically composed of N-type Al.sub.r Ga.sub.l-r As where r is between about 0.25 and 0.45 and preferably between about 0.30. This layer is preferably between about 1 and 2 .mu.m thick over the channels 26. Preferably, the planar surface 38 of the first cladding layer 36 in the central portion 34 of the channelled region 24 is not coplanar with the surface 20 of the substrate 18 and is at greater height above the mesas 28 than the surface 20. The guide layer 40 is typically composed of N-type Al.sub.x Ga.sub.l-x As where x is between about 0.15 and 0.30 and is preferably about 0.22. This layer is typically between about 0.5 and 1.5 .mu.m thick over the center portion 34 of the channels and has a concave surface portion 42 over the center portion 34 on which the active layer is deposited. The active layer 44 is typically composed of Al.sub.y Ga.sub.l-y As where y is between about 0.0 and 0.15 and prefer 0.07. This layer is typically between about 0.08 and 0.16 .mu.m thick, the thickest portion being over the center portion 34 of the channelled region 24 and tapering in decreasing thickness in the lateral direction. The second cladding layer 48 is typically composed of P-type Al.sub.z Ga.sub.l-z As where z is between about 0.25 and 0.45 and preferably about 0.30. This layer is typically between about 0.8 and 1.5 .mu.m thick. The capping layer 50 is typically composed of P-type GaAs and is used to facilitate ohmic electrical contact between the underlying semiconductor material and the overlying first electrical contact 56. This layer is typically between about 0.5 and 1.0 .mu.m thick.

The electrically insulating layer 52 is preferably composed of silicon dioxide which is deposited on the capping layer 50 by pyrolytic decomposition of a silicon-containing gas, such as silane, in oxygen or water vapor. The opening 54 is formed using standard photolithographic masking techniques and etching processes. The first electrical contact 56 is then deposited over the capping layer 50 where it is exposed to the opening 54 and is preferably composed of sequentially evaporated titanium, platinum, and gold. The substrate electrical contact 58 is deposited on the major surface 22 by sequential evaporation and sintering of tin and silver followed by a plated nickel layer and a layer of gold.

The emitting end facet 14 of the laser 10 may be coated with a layer of Al.sub.2 O.sub.3 or a similar material having a thickness of about one-half wave at the lasing wavelength as disclosed by Ladany et al. in U.S. Pat. No. 4,178,564. The opposed end facet 14 is coated with a reflector consisting of an electrical insulator such as SiO.sub.2 coated with a layer of gold as disclosed by Caplan et al. in U.S. Pat. No. 3,701,047. Alternatively, the mirror may be a multi-layer reflector such as that disclosed by Ettenberg in U.S. Pat. No. 4,092,659. All three of these patents are incorporated herein by reference.

The laser of the invention, may be fabricated using well-known photolithographic masking techniques and etching processes to form the channels 26 with the mesas 24 therebetween followed by standard liquid-phase epitaxy deposition of the layers such as those disclosed by Lockwood et al. in U.S. Pat. No. 3,753,801, incorporated herein by reference, and by Botez in U.S. Pat. No. 4,347,486 to deposit the layers over the substrate surface containing the channels. The etching processes to form the channels may include selective chemical etching of a surface having a particular crystallographic orientation or ion etching. These techniques are well known in the art.

Alternatively the layers may be formed by first forming a series of, preferably equally spaced, adjacent vee-shaped channels thereby forming a corrugated surface as shown in FIGS. 2 and 3. In FIG. 2 a GaAs substrate 300 has formed on a major surface 302 thereof, which is preferably the (100) crystallographic plane, a plurality of stripes 304 composed of an etch-resistant material such as SiO.sub.2. The stripes are formed using standard photolithographic masking techniques and etching processes and are preferably oriented along a (011) crystallographic direction on the (100) oriented surface. A preferential etch is then applied to the exposed surface of the substrate to form the vee-shaped channels 402 as shown in FIG. 3. Channels 402 having a vee shape are formed by undercutting the stripes 304 to the point where only a small portion of the original surface 302 remains to support the stripes 304.

The substrate having the corrugated surface is then inserted into a liquid-phase epitaxy apparatus such as that disclosed by Lockwood et al. and brought into contact with the solution from which the first layer is to be deposited on the channels 402 and the surface 302.

The sequence of events which occurs at the onset of the liquid-phase epitaxy deposition process depends critically on the characteristics of the solution and the shape of the substrate surface contacted by the solution. In the simplest case, a super-saturated solution containing the elements to be deposited and a solvent for the substrate material is brought into contact with a planar surface and uniform deposition onto the substrate occurs. If the solution is just saturated, nothing will happen. If the solution is under-saturated, meltback of the surface will occur until the solution becomes saturated.

However, if the surface is not planar but has a locally varying radius of curvature, then the degree of saturation of the contacting solution also locally varies. If the solution is just saturated for a planar surface, it will be super-saturated over concave portions of the surface, as viewed from the direction of the solution, and under-saturated over convex portions of the surface. Over the curved portion of the surface two effects can then occur: deposition occurs on the concave portions of the surface over which the solution is super-saturated and meltback of the substrate occurs on the convex portions of the surface over which the solution is under-saturated.

Applying these principles to the corrugated surface of the substrate 300, as shown in FIG. 4, the convex portions of the corrugations, that is, the tips of the projections which form the channels 402, undergo meltback forming the mesas 404 between the concave portions of the corrugations, now forming the channels 402. This meltback may continue until the mesas are eradicated thereby leaving a single wide channel. The meltback locally delays the growth over the newly formed mesas 404 and lowers the surfaces of the mesas below that of the surface 302 of the substrate. The growth proceeds, however, in the channels 402 and over the surface 302 so that the layer 406 is formed over the surface 302 and the surfaces of the channels 402 and the mesas 404. The layer 406 has a surface composed of three parts: a planar portion 408 over the channelled region, a planar portion 410 over the surface 302 which differ in their relative heights and are connected by the curved surface portions 412. The difference in height between the planar surface portions 408 and 410 together with the curved surfaces 412 provide a trough over the channels 402. This trough will cause the subsequently deposited layers to have locally concave surfaces over at least the central portion 34 of the channeled region 24, as shown in FIG. 1, which is the key to providing the proper lateral tapers for the overlying guide and active layers. We have found that a GaAs buffer layer or AlGaAs cladding layer can be deposited on the corrugated surface to provide this trough over the channelled region by carefully controlling the temperature of the solution, the degree of super-cooling used, and the cooling rate. In particular, we have found that an initial growth temperature, typically the temperature at which the solution is equilibrated, between 700.degree. C. and 800.degree. C., and preferably about 760.degree. C., should be used. The range of temperatures through which the solution and substrate are cooled to a lower temperature is between about 2.degree. and 10.degree. C., and preferably between about 4.degree. and 5.degree. C. The cooling rate is typically between about 0.5.degree. and 5.degree. C., and preferably about 1.degree. C. per minute. In this temperature range the meltback can be controlled and the rate of growth is significantly slower for the deposited layer than for the range of growth temperatures between 850.degree. C. and 950.degree. C. which are typically used.

The steps of the novel method of fabricating a double heterostructure laser is to form a plurality of adjacent corrugations on the surface of a substrate with the axis of the corrugations extending between what will become the end facets of the semiconductor body. Typically, a solution containing the elements to be deposited is equilibrated at a first temperature in contact with a source wafer as disclosed by Lockwood et al. in U.S. Pat. No. 3,741,825, incorporated herein by reference. Preferably, the solution and the substrate are then separately cooled through a range of temperatures to create a super-saturated growth condition for concave and planar portions of the surface and a slightly under-saturated growth condition for convex portions of the surface. The solution and the substrate are then contacted, typically by sliding a wafer into contact with the solution, thereby causing a partial meltback of the convex portions, or tips, of the corrugations forming mesas between the concave portions of the corrugations, i.e., the channels. Deposition of the first layer begins and continues until the planar surface is formed over the channels formed of the original corrugations and over the surrounding planar substrate thereby forming the trough. The remaining semiconductor layers of the laser are then deposited on this non-planar surface using standard liquid-phase epitaxy techniques.

This approach to the deposition of a layer by liquid-phase epitaxy in the fabrication of a semiconductor layer is also a more general method for locally delaying the growth of a layer on a surface of a body. The portion of the surface over which the growth is to be delayed is initially corrugated, preferably with the corrugations adjacent to one another. The corrugated portion of the surface may be a portion which is coplanar with the remainder of the surface. Alternatively the corrugated portion may be the surface of a mesa projecting outwards from the surface or it may be the surface of a channel of arbitrary shape extending into the body of semiconductor material. The key to the local delay of the deposition rate is the meltback of the convex portions of the corrugations at the onset of the deposition process. Thus the corrugated surface must initially be brought into contact with a solution comprising the elements to be deposited and an element which will meltback the material of the body having the corrugated surface. The condition of the solution should be such that there is a super-saturated growth condition for concave and planar portions of the surface and an under-saturated growth condition for convex portions of the surface. The degree of super-saturation will determine the rate of growth in the concave portions of the corrugations. The degree of under-saturation will determine the rate of meltback of the convex portions of the corrugations. The degree of saturation of the solution can be controlled by the initial temperature and the range of temperatures through which the solution is cooled.

In the operation of the laser of the invention under forward bias, electrical current flows through the first electrical contact 56 over the center portion 34 of the channelled region 24. Lasing action occurs over the central portion 34 of the channelled region 24 and propagation of the lasing light beam occurs in the tapered active and guide layers 44 and 40 respectively at a distance above the surface 20 such that the light beam does not interact with the substrate 18.