Forming disordered layer by controlled diffusion in heterojunction III-V semiconductor

A method is disclosed for converting a multilayer semiconductor structure, that includes active semiconductor regions interposed between semiconductor barrier layers, into a disordered alloy by introduction of a specified disordering element into the multilayer structure. Devices made using the method are also disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to improvements in semiconductive devices, and more 
particularly to a method for producing integrated light emitting 
semiconductor devices. The United States Government has rights in this 
invention as a result of financial support under NSF grants. 
Light emitting semiconductors are well known in the prior art. One of the 
more widely used light emitting devices is a heterojunction light emitter 
fabricated, for example, using a gallium arsenide/aluminum gallium 
arsenide material system. In such devices, a pair of relatively wide band 
gap layers (aluminum gallium arsenide) of opposite conductivity type are 
sandwiched around an active region (gallium arsenide). The interfaces 
between the active region and the wide band gap layers form a pair of 
heterojunctions. These heterojunctions effectively provide both carrier 
and optical confinement. The devices are generally used as light emitting 
diodes or lasers and may be energized using an electrical current or by 
optical pumping. 
An improved light emitting device is described in my U.S. Pat. No. 
4,439,782, assigned to the same assignee as this application. Therein is 
described a light emitting device wherein the active region comprises one 
or more layers of gallium arsenide separated by aluminum arsenide barrier 
layers. The aluminum arsenide binary layers replace previously employed 
aluminum gallium arsenide ternary barrier layers for the reason that the 
latter ternary layers have been found to be inherently disordered and to 
exhibit alloy clustering in the regions adjacent to the gallium 
arsenide/aluminum gallium arsenide interface. That clustering leads to the 
device requiring larger threshold currents and exhibiting lower 
efficiencies. The disclosure and teachings of the aforementioned patent 
application are incorporated herein by reference. 
Light emitting devices such as those described above are generally, 
although not necessarily, grown by metal-organic chemical vapor deposition 
("MO-CVD"), which is described, for example, in a publication entitled 
"Chemical Vapor Deposition for New Material Applications", appearing in 
the June, 1978, issue of "Electronic Packaging and Production". Such 
devices are also grown by molecular beam epitaxy, liquid phase epitaxy, or 
other suitable deposition techniques. The MO-CVD and MBE processes are 
generally the preferred ones. 
In the aforementioned processes, the light emitting devices are produced in 
wafer form, which wafer is then cleaved or cut to produce individual 
light-emitting diodes or lasers. This is in contrast to the well-known 
integrated circuit technology wherein large numbers of active devices are 
constructed and interconnected on a single chip. Such integration, 
heretofore, has been unavailable, on a practical basis, for the 
above-mentioned light emitting-semiconductor devices. Attempts to 
integrate light emitting devices have generally been rather 
crude--involving the actual physical emplacement of light-emitting 
structures in etched-out substrates. Such a structure is shown in U.S. 
Pat. No. 4,165,474 to D. J. Myers. 
It is clear that an economic method of integrating heterojunction light 
emitting devices into larger scale integrated circuits would be an 
important contribution to the expansion of optical data processing and 
data communications. 
Accordingly, it is among the objects of this invention to provide a method 
which enables the integration of III-V compound heterojunction devices 
into an overall integrated structure. 
It is a further object of this invention to provide a method for 
constructing integrated optoelectronic and field effect devices which 
method is not unduly complex, fits with present semiconductor processing 
technology, and allows fabrication of complementary N and P types of 
devices. 
SUMMARY OF THE INVENTION 
In accordance with the above objects, III-V semiconductive structure are 
disordered and shifted up in energy gap, while maintaining the crystalline 
structure, by introduction of a disordering element. In U.S. Pat. No. 
4,378,255 of N. Holonyak and W. Laidig, it is disclosed that such 
disordering can be achieved by diffusion of zinc atoms. For example, all 
or selected portions of a multilayer of either gallium arsenide/aluminum 
arsenide or gallium arsenide/ aluminum gallium arsenide can be converted 
into single crystal aluminum gallium arsenide having a higher energy gap 
than that of the original structure by the process of a zinc diffusion (as 
disclosed in the above referenced patent) or by introduction of silicon or 
krypton or zinc, such as by ion implantation thereof. Other active devices 
can then be constructed in the higher energy gap material using 
established semiconductor processing steps. 
In accordance with a further improvement hereof, disordering is implemented 
by diffusion of a disordering element which serves as a donor (n dopant), 
this process being complementary with the above-described 
acceptor-diffusion disordering process (e.g. zinc p dopant), thereby 
facilitating, inter alia, fabrication of complementary devices in an 
integrated circuit. Silicon, germanium or tin impurities, which are 
amphoteric but serve here as donors, are diffused into the structure e.g. 
superlattice or single quantum well) at temperatures which permit 
selective disordering, preferably in the range between about 700.degree. 
C. and 850.degree. C. It has been shown that silicon and germanium can be 
successfully diffused, using an encapsulant, into gallium arsenide using 
rapid thermal processing at temperatures 850.degree. to 1050.degree. C. 
[See M. E. Greiner and J. F. Gibbons, Appl. Phys. Lett. 44, 750 (1984).] 
In the present invention, where the diffused element is used for selective 
disordering, lower temperature, preferably in the range 700.degree. C. to 
850.degree. C., is used so that uncontrolled thermal disordering, beyond 
the region into which the disordering element is diffused, will not 
substantially occur. 
Further features and advantages of the invention will become more readily 
apparent from the following detailed description, when taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, there is shown a semiconductor heterostructure 
device of the type described in U.S. Pat. No. 4,439,782. The entire device 
is constructed on a gallium arsenide semi-insulating or conducting 
substrate 10. A pair of outer buffer or contact layers 12 and 14 encompass 
a pair of injecting/collecting (i.e., injecting or collecting) regions 16 
and 18, which are preferably, although not necessarily, of opposite 
conductivity type. A superlattice (many-layer) structure 20 is encompassed 
between regions 16 and 18 which structure is shown in a blown-up view to 
the right of the device. Superlattice 20 comprises a plurality of 
interleaved lower gap active regions 22 sandwiched between higher gap 
barrier layers 24. 
The injecting/collecting confining regions 16 and 18 are of a relatively 
wide band gap semiconductor material and active layers 22 are of a 
relatively narrow band gap binary semiconductor material. Barrier layers 
24 are of a binary semiconductor material that is lattice-matched to the 
active layer material 22. While not the most preferred embodiment, barrier 
layers 24 can also be a ternary semiconductor material which is 
lattice-matched to the binary active material 22. 
In superlattice 20, each active layer 22 is a quantumwell having a 
thickness in the range of about 20 to 500 Angstroms, with the preferred 
thickness range being 20 to 200 Angstroms. Each barrier region 24 should 
have a thickness of at least about 10 Angstroms and preferably be in the 
range of between about 10 and 200 Angstroms. The number of active layers 
22 is essentially subject to choice, but generally is in the range of 4 to 
100 layers with the number of barrier regions 24 being one more in number. 
However, as noted herein, the invention also has applicability to 
disordering of a single quantum well active region that is adjacent a 
barrier region. 
An embodiment of the structure of FIG. 1 is as follows: 
layer 12: 1 .mu.m GaAs: Se (n .about.1.times.10 .sup.18 cm.sup.-3) 
layer 16: 0.5-2.0 .mu.mAl.sub.0.4 Ga.sub.0.6 As: Se 
(n.about.5.times.10.sup.17 cm.sup.-3) 
layers 24: (thickness=L.sub.B) AlAs (doped or undoped) 
layers 22: (thickness=L.sub.z) GaAs (doped or undoped) 
layer 18: 0.5-2.0 .mu.m Al.sub.0.4 Ga.sub.0.6 As: 
Zn(p.about.2.times.10.sup.17 cm.sup.-3) 
layer 14: 1 .mu.m GaAs: Zn (p.about.2.times.lO.sup.18 cm.sup.-3) 
The diffusion of zinc atoms into superlattice 20 can cause the superlattice 
to become compositionally disordered Al.sub.x Ga.sub.1-x As, with its 
energy gap (in one specific case) changed from about E.sub.g =1.61 eV (for 
the gallium arsenide active layer 22) to about E.sub.g =2.08 eV. (From 
dull red to yellow). 
In order to accomplish the zinc diffusion only in desired areas, a silicon 
nitride mask 28 is laid down on the surface of layer 12 using well-known 
photolithographic processes. The exposed portions of contact region 12 are 
etched away, exposing the upper surface of confining layer 16. The 
semiconductor structure along with ZnAs.sub.2 is then placed in a quartz 
ampoule and the combination is placed in a diffusion oven. Zinc is 
introduced by diffusion in the crystal in the temperature range of 
500.degree.-600.degree. C., a temperature well below the normal cross 
diffusion temperature of the superlattice components. The diffusion time 
is, of course, dependent upon the device structure, but diffusion times 
ranging from 10 to 60 minutes are appropriate. 
The zinc atoms diffuse into the exposed regions and cause active regions 22 
and barrier regions 24 in superlattice 20 to become compositionally 
disordered alloy Al.sub.x Ga.sub.1-x As. In other words, the various thin 
superlattice layers are combined in such a manner as to lose their 
individual identities, while maintaining the crystalline structure. If 
barrier regions 24 are AIAs and active regions 22 are GaAs, 
x.about.L.sub.B /(L.sub.B +L.sub.z). If the barrier regions 24 are 
Al.sub.y Ga.sub.1-y As, then x.about.yL.sub.B /(L.sub.B +L.sub.z). In this 
instance, y represents the fraction of barrier layer 24 that can be 
considered as AlAs. 
Ordinarily, aluminum/gallium interdiffusion in the temperature range 
500.degree.-600.degree. C. is negligible. It has been shown, however, that 
when zinc is diffused, even at such a low temperature, into AlAs/GaAs 
superlattices, the zinc enhances the aluminum/gallium interdiffusion. 
Thus, at a low temperature, and in any pattern desired, the GaAs/AlAs or 
GaAs/Al.sub.x Ga.sub.1-x As superlattice can be fully disordered and, 
depending upon the GaAs layer's thickness L.sub.z and the L.sub.z /L.sub.B 
ratio, can be increased in energy gap or even shifted, from direct gap to 
indirect gap. 
In a form of the invention, it has been found that introduction of silicon 
or krypton, such as by ion implantation, can also cause the superlattice 
to become compositionally disordered, while maintaining the crystalline 
structure. In one example, the AlAs-GaAs superlattice, grown by 
metalorganic vapor deposition, had alternating undoped layers of GaAs (86 
Angstroms) and AlAs (80 Angstroms), with 126 layers altogether for a total 
thickness of 1 .mu.m. Silicon ions were implanted into the superlattice 
structures at room temperature at 375 keV at an angle of 7.degree. with 
respect to the substrate. The ion dose was 10.sup.14 cm.sup.-2. Some 
samples were subsequently annealed at 675.degree. C. for 4 hours in an 
arsenicrich atmosphere. Compositional disordering was found to result, 
with the energy gap, in an annealed case, for the disordered region being 
about Eg=1.99 eV, and therefore substantially higher than was exhibited by 
the ordered superlattice, which was about Eg=1.57 eV. Annealing, 
preferably in a temperature range of about 500.degree. C. to 700.degree. 
C., and below the temperature at which the superlattice was originally 
grown, appears to be an important aspect of the process in these examples. 
For an implanted dose of 10.sup.14 cm.sup.-2 it appears that, before 
anneal, compositional disordering has not occurred and considerable 
crystal damage is present. After anneal at temperatures less than the 
original growth temperature, the damage in the implanted region is removed 
and compositional disordering is extensive, though not complete. 
In other examples, zinc and krypton ions were implanted under similar 
conditions, and resulted in observable compositional disordering after 
annealing, although the results for the same dosages exhibited less 
disordering than was the case with silicon implantation. 
FIG. 2 shows the structure of FIG. 1 after introduction of the disordering 
element, and in FIG. 3 the silicon nitride layer 28 has been removed and 
replaced by metallization layer 36. A similar layer of metallization has 
been applied to the underside of substrate 10 (substrate 10 being 
conductive in this instance) enabling a light emitting structure to be 
completed. A plan view of the structure is shown in FIG. 4. 
When a potential is applied via contact 36 to the heterojunction laser (or 
if there are no contacts,the device is optically pumped), a red light is 
emitted by the GaAs active layers 22 along the long dimension as shown by 
arrows 33. Since the Al.sub.x Ga.sub.1-x As regions 30 and 32, into which 
the disordering element was introduced, are of a higher energy gap (orange 
or yellow) than the GaAs regions 22, the red light is able to pass 
therethrough without hindrance. Wafer edges 35 act as Fabry-Perot 
reflectors, creating a cavity of nonabsorbing Al.sub.x Ga.sub.1-x As for 
the laser. Obviously, the structure of the cavity for the heterojunction 
laser can be designed as desired for optimum performance characteristics. 
For instance, a larger cavity will provide longer photon transit times, 
less cavity end loss, a higher Q and a resultant lower threshold laser. 
A plurality of light emitting devices such as those above described have 
been grown on a single substrate and then subsequently isolated by 
selective introduction of a disordering element, as described, to create 
individual devices in a monolithic environment. When a zinc diffusion or 
implantation is used, it creates a p region which is substantially 
semiconductive in its own right. By subsequently bombarding the exposed p 
regions with a suitable source of protons, those regions can be 
sufficiently damaged while still remaining single crystalline so as to 
create high resistivity isolating barriers between the active devices. 
Such bombardment does not effect the red light transmissivity of the 
bombarded regions. 
Superlattices having active regions 22 (i.e., GaAs) as thick as 500 
Angstroms (L.sub.z) can be compositionally disordered as taught herein. 
Preferably, however, the thickness of active region 22 should be 
approximately 200 Angstroms or less for optimum results and, in any event, 
be sufficiently thin to exhibit quantum size effects. 
Referring now to FIG. 5, there is shown a sectional view of a Schottky 
barrier field effect transistor constructed employing the heterostructure 
configuration of FIG. 1 and isolated from other portions of the circuitry 
by the disordering process described above. In this instance, regions 12 
and 16 have been etched away to open a channel which exposes superlattice 
layer 20. Metallization contact 50 has been deposited and is used as the 
gate electrode. N type metallizations 52 and 54 are alloyed into layers 12 
and 16 and contact superlattice layer 20. These provide the source and 
drain contacts for the device. The device of FIG. 5 is illustrated to show 
the versatility of the selective disordering technique hereof in that a 
plurality of devices can be integrated into a single monolithic chip and 
then isolated by the higher gap disordered regions--which are later 
converted to higher resistivity, if necessary, by proton bombardment. 
Obviously, additional active devices can be constructed in the disordered 
regions, if such are desired. 
The method of constructing the Schottky barrier device shown in FIG. 5 is 
conventional in that layer 20 can be high mobility modulation doped, i.e., 
a donor grown into the barriers but none in the adjacent regions (i.e., 
GaAs). Layers 12 and 16 are selectively etched away after the upper 
surface of the device is suitably masked. The last step involves the 
alloying of junction contact 52 and 54, again after suitable masking. 
Referring now to FIG. 6, there is shown a plan view of an integrated 
structure constructed in accordance with the invention. In this instance, 
however, contact layer 12 and confining layer 16 over the superlattice 
layer have been removed to show an integrated laser/waveguide structure. 
Laser active regions 60 and 62 are constructed identically to that shown 
in FIGS. 2-3, except that each terminates in a pair of superlattice 
waveguides 64 and 66. A metal contact 68 (similar to that shown in FIG. 5) 
overlays waveguide 66 and is reverse (or even forward) biased to provide a 
Schottky barrier junction between itself and underlying superlattice 66. 
The individual devices have been isolated by selective disordering as 
described above. 
Laser 60 is biased in such a mode as to generate light; however, laser 62 
is biased sufficiently below threshold that it can be optically pumped by 
in-phase radiation traveling along superlattice waveguides 64 and 66. Due 
to fact that the lower gap material (red) exhibits a higher index of 
refraction than the yellow material, the emitted red light tends to stay 
within the lower gap material making up waveguides 64 and 66 (so long as 
there are no abrupt changes of direction of the waveguide materials). By 
properly energizing contact 68, a retarding electro-optic effect can be 
achieved which will alter the phase of the signal on waveguide 66 so as to 
create an out-of-phase signal at the juncture feeding laser 62. Under 
these conditions, laser 62 is inhibited from lasing. If contact 68 is not 
energized, in phase optical pumping enables laser 62 to lase, thereby 
providing an electro-optic logic device. 
Referring to FIG. 7, there is shown a semiconductor heterostructure device 
as shown in FIG. 1, but with the N-type and P-type layers being of 
opposite conductivity type; i.e., with layers 712 and 716 being P+ and P 
type layers, respectively, and with layers 710, 714 and 718 being N, N+ 
and N type layers, respectively. The superlattice 720 includes active 
layers and barrier regions, as in FIG. 1. The materials, thicknesses and 
doping levels may be, for example, substantially the same as those 
indicated for corresponding layers of the FIG. 1 structure, but with the 
layer dopants being reversed; i.e., Se for layers 714 and 718 and Zn for 
layers 712 and 716. In this case, the diffusion of silicon, germanium or 
tin atoms into the superlattice 720 causes the superlattice to be 
compositionally disordered. As in the FIG. 1 example, a silicon nitride 
mask can be employed in a photolithographic process such that the exposed 
portions of the contact region 712 are etched away, exposing the upper 
surface of confining layer 716. A layer of silicon germanium or tin to be 
diffused is then evaporated on the structure and an encapsulating layer 
such as silicon dioxide is deposited over the silicon. Diffusion of the 
disordering element into the layer structure is then implemented at a 
temperature, preferably in the range 700.degree. to 850.degree. C. As 
noted above, diffusion time is dependent upon the device structure, a 
diffusion time in the range of 1 to 24 or more hours generally being 
appropriate. The energy gap changes in the superlattice will be similar to 
the case for zinc diffusion, as described above. 
FIG. 8 shows the structure of FIG. 7 after the selective disordering has 
been implemented and the encapsulating layer has been etched away. The 
regions 730 and 732, into which the silicon, germanium or tin has been 
diffused, are rendered N-type in this case. As in FIG. 3, metallization 
layers 736 and 737 can be applied (or optical pumping can be used) to 
obtain a light emitting device, as previously described. Again, a 
plurality of devices can be grown in a single substrate and isolated, 
and/or complementary devices can be made using both donor and acceptor 
diffusion processes as described herein. Further, it will be understood 
that field effect transistors, such as shown in FIGS. 5 and 6 can be 
constructed using the described donor and/or acceptor disordering and, 
again, complementary devices can be made using both processes. Also, 
active regions with a single quantum well and barrier layer interface can 
be disordered using the described processes. 
An experiment will now be described wherein an aluminum gallium arsenide - 
gallium arsenide superlattice is selectively disordered using silicon 
diffusion. Initially a 1 .mu.m layer of Al.sub.x Ga.sub.1-x As 
(x.about.0.6) is grown on a substrate followed by a .about.0.4 .mu.m GaAs 
layer and then the superlattice. The layers are grown by metalorganic 
vapor deposition. The superlattice consisted of 40 periods of GaAs quantum 
wells of thickness L.sub.z .apprxeq.280 Angstroms coupled by A1.sub.x 
Ga.sub.1-x As (x.about.0.6) barriers of thickness L.sub.B .apprxeq.320 
Angstroms. A bright-field transmission electron micrograph (TEM) of a 
section of the superlattice is shown on the right side of FIG. 10. (The 
diffused region on the left is described later.) 
Prior to Si diffusion into the superlattice, Si.sub.3 N.sub.4 is deposited 
(for diffusion masking) on the wafer, and for convenience a stripe pattern 
(15 .mu.m stripes on a 25 .mu.m period, FIG. 11) is developed on 
photoresist deposited on the Si.sub.3 N.sub.4. The Si.sub.3 N.sub.4 is 
then plasma etched (CF.sub.4) leaving 10 .mu.m bare stripes on the wafer. 
Next the photoresist is removed, and the wafer is cleaned in HCl just 
before .about.100.ANG. of Si is electron-beam evaporated onto the wafer at 
7.times.lO.sup.-7 Torr. Immediately after the evaporation is completed 
.about.0.5 .mu.m of SiO.sub.2 is deposited onto the wafer. The 
superlattice, with a small piece of As, is then sealed in a quartz ampoule 
facedown on a "slab" of Si to insure a uniform temperature across the 
wafer (with also an overpressure of As). After the diffusion, which is 
accomplished in one example at 850.degree. C. for 10 hr, most of the 
SiO.sub.2 is removed with NH.sub.4 F:HF (7:1, 3.5 min). The remaining 
SiO.sub.2, Si, and Si.sub.3 N.sub.4 are removed in a CF.sub.4 plasma. The 
sample is then cleaved, with one part employed for TEM specimens and the 
other samples for photopumping. 
In FIG. 10 the region at the left is impuritydisordered bulk-crystal 
Al.sub.x' Ga.sub.1-x' As (0 &lt;x'&lt;x). This region (and TEM image) is taken 
from the lower part of the superlattice near a Si.sub.3 N.sub.4 mask edge, 
and is just the depth (2.4 .mu.m) to which the Si impurity penetrates 
during the diffusion process. The TEM micrograph of FIG. 10 does not 
reveal any dislocations at the diffusion boundary with the undisturbed 
superlattice. Furthermore, at higher magnification (not shown) lattice 
fringe images can be resolved that extend undistorted from the as-grown 
superlattice into the disordered Al.sub.x' Ga.sub.1-x' As, which obviously 
is single crystal. It is noted that the 850.degree. C.-10 hr anneal cycle 
has not caused any noticeable change in the as-grown superlattice (right 
side of FIG. 1). This was confirmed by separate TEM images of the as-grown 
(non-annealed) superlattice. 
FIG. 11 shows in a different manner the effect of the selective 
(stripe-pattern) disordering. To obtain this figure white light has been 
transmitted through a sample after the substrate and buffer Al.sub.x 
Ga.sub.1-x As and GaAs layers have been removed from the superlattice. The 
red stripes are the regions into which Si has been selectively diffused 
and creates disordered Al.sub.x 'Ga.sub.1-x 'As of increased energy gap. 
Separately diffused samples without masking stripes have been examined in 
absorption and in photoluminescence. These measurements confirm that the 
"red" diffused region has, indeed, been shifted to higher gap 
(x'.gtorsim.0.32). Because of the combination of high gap and heavy doping 
of the Si-diffused ("red") region, photoluminescence signals are too weak 
to be recorded. The masked region, i.e., the "black" stripes of FIG. 11 or 
the corresponding superlattice region shown on the right in FIG. 10, 
operates as expected as a continuous (cw) 300 K photopumped laser. A 
portion of the stripe-pattern sample of FIG. 11 has been heat sunk under 
diamond and is shown in cw 300 K photopumped laser operation in FIG. 12. 
Curve (a) corresponds to laser threshold (4.times.lO.sup.3 W/cm.sup.2 or 
J.sub.eq .about.1.7.times.10.sup.3 A/cm.sup.2), and (b) at 
5.times.lO.sup.3 W/cm.sup.2 (J.sub.eq .about.2.1.times.10.sup.3 
A/cm.sup.2) single mode operation is well established. These data show 
that the masked portion of the superlattice is not damaged by the thermal 
annealing cycle, that the adjacent higher gap "red" bulk Al.sub.k 
'Ga.sub.1-k, As crystal (or the diffusion interface) does not draw the 
excess carriers into non-radiative recombination, and that the quality of 
the .about.80 layer interfaces is high. It can be noted that the 
photopumping excitation beam, a 5145 .ANG. Ar.sup.+ laser, is not focused 
to much smaller than a 35 .mu.m diameter spot and strikes not more than 10 
.mu.m of "as-grown" superlattice and photoexcites more of the red-gap 
impurity-disordered Al.sub.x 'Ga.sub.1-x 'As. It is clear that in this 
form of excitation "geometry" defects at the red-black diffusion boundary 
(FIG. 11), if present, would quench the laser operation. 
The 2.4 .mu.m disordering depth into the SL crystal (FIG. 10) indicates 
that, for thermal annealing at 850.degree.C. for 10 hr (or much longer 
than the rapid anneals described by Greiner and Gibbons (referenced above) 
the effective Si diffusion constant is D(SL).about.1.6.times.lO.sup.-12 
cm.sup.2 /s, which is .about.2.7x greater than D(GaAs) 
.about.6.times.lO.sup.-13 cm.sup.2 /s, described by Greiner and Gibbons. 
This generally agrees with earlier observations that a disordering 
impurity (Zn acceptor) diffused into an Al.sub.x Ga.sub.1-x As-GaAs SL has 
a significantly higher diffusion coefficient than in GaAs. Thus, to 
achieve practical diffusion depths of .ltorsim.1 .mu.m, the annealing 
times and temperatures of this example can be reduced. Applicant has 
observed that diffusion temperatures in the range 700.degree. to 
850.degree. C. are preferred.