Buried reverse bias junction configurations in semiconductor structures employing photo induced evaporation enhancement during in situ epitaxial growth and device structures utilizing the same

In situ removal of selected or patterned portions of semiconductor layers is accomplished by induced evaporation enhancement to form reversed bias current confinement structures in semiconductor devices, such as heterostructure lasers and array lasers.

BACKGROUND OF THE INVENTION 
This invention relates generally to semiconductor laser structures and more 
particularly to laser structures having buried back biased current 
confinement means formed in situ that function as optical and current 
confinement mechanisms for such laser structures. 
The employment of buried current blocking layers or current confinement 
regions to channel current through a selected active region of 
semiconductor laser devices, such as heterostructure quantum well lasers 
and array lasers, are well known in the art. However, there is no report 
or disclosure known by us that attempts or contemplates the fabrication of 
such current confinement means in situ during growth without at least the 
requirement of an additional step of, for example, photolithography or 
chemical etching or masking, to provide such current confinement 
structures. What is desired is a process, particularly as implemented in 
MBE or MOCVD, wherein layer patterning can be achieved in situ without 
growth interruption by some off-line or nongrowth procedure or process. 
There are two examples known to us where patterning may be achieved by 
quasi-in situ thermal processing wherein thermal etching is employed to 
selectively remove GaAs. In one example, a n-GaAs layer over a p-AlGaAs 
layer is first, selectively chemically etched in a particular region 
followed by thermal etching to remove the remaining thin GaAs left from 
chemical etching before proceeding with regrowth of the p-AlGaAs layer. 
This forms a buried reverse biased current confinement mechanism in a 
double heterostructure laser. H. Tanaka et al, "Single-Longitudinal-Mode 
Self Aligned AlGa(As) Double-Heterostructure Lasers Fabricated by 
Molecular Beam Epitaxy", Japanese Journal of Applied Physics, Vol. 24, pp. 
L89-L90, 1985. In the other example, a GaAs/AlGaAs heterostructure 
partially masked by a metallic film is thermally etched in an anisotropic 
manner illustrating submicron capabilities for device fabrication. A. C. 
Warren et al, "Masked, Anisotropic Thermal Etching and Regrowth for In 
Situ Patterning of Compound Semiconductors", Applied Physics Letters, Vol. 
51(22), pp. 1818-1820, November 30, 1987. In both of these examples, 
AlGaAs masking layers are recognized as an etch stop to provide for the 
desired geometric configuration in thermally etched GaAs, although it is 
also known that, given the proper desorption parameters, AlGaAs may also 
be thermally etched at higher temperatures with different attending 
ambient conditions visa vis GaAs. 
However, none of these techniques employ in situ photo induced evaporation 
as a technique in a film deposition system to incrementally reduce, on a 
minute scale, film thickness in patterned or selective locations at the 
growth surface either during or after film growth, producing smooth 
sculptured surface morphology which is a principal objective of this 
invention. 
It is another object of this invention to bring about in situ removal or 
desorption of selected surface regions or layers of compound 
semiconductors employing induced evaporation enhancement in metalorganic 
chemical vapor deposition (MOCVD) epitaxy and to apply this method in the 
fabrication of optical waveguides and buried heterojunction lasers and 
laser arrays with in situ fabricated buried back biased junctions for 
internal current confinement. 
SUMMARY OF THE INVENTION 
According to this invention, in situ removal or thinning of portions or all 
of selected regions of deposited films are brought about by a technique 
employing an irradiation energy source directed to a spot or region of 
exposure on the growth surface of a substrate or support in a deposition 
system, e.g., MBE or MOCVD system. This technique, termed "induced 
evaporation enhancement", is taught in U.S. Pat. No. 4,962,057. In 
particular, the invention herein is directed to the employment of this 
technique in fabricating in situ buried current confinement and index 
waveguide mechanisms in heterostructure lasers and array lasers wherein 
induced evaporation enhancement purely removes or prevents the continued 
growth of the epitaxially deposited materials in selected regions at the 
growth surface without the need or introduction of masking or chemical 
etching processes. The application of this technique, as particularly 
described here, provides the opportunity to produce in situ current 
confinement configurations which effectively channel current to designated 
regions of a semiconductor device by means of patterned desorption of 
layer induced evaporation enhancement. For typical semiconductor laser 
structures, a reverse bias junction configuration is created employing a 
doped quantum well layer which is heavily doped using a spike doping 
technique wherein both before and after the growth of the quantum well 
layer, certain sources of elemental constituents, e.g., Group III sources, 
are paused to allow an accumulation of a submonolayer coverage of the 
growth surface by the dopant. In the case of a quantum well of n-GaAs, the 
dopant source may be, for example Se, which is supplied with a 1% 
arsine/hydrogen mixture with the Ga source placed on hold to produce a Se 
spike before and/or after the growth of the n-GaAs quantum well. In this 
manner, a highly effective but ultra thin buried reverse bias 
configuration can be produced by a single quantum well layer no thicker 
than 5 to 10 nm thick. The use of such a thin layer of GaAs has the 
advantage of being selectively patterned by removal of layer portions 
within a relatively short period of time, e.g., within several hundreds of 
seconds. 
Thus, while buried reverse bias junctions per se are known in the art, the 
novelty herein is (1) the in situ formation of such a junction without the 
use of nongrowth type processes and (2) the use of an ultra thin quantum 
well layer to form the junction and, more particularly, in combination 
with sheet doping to provide an effective ultra thin reverse bias 
junction. 
Other objects and attainments together with a fuller understanding of the 
invention will become apparent and appreciated by referring to the 
following description and claims taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In describing the devices of this invention, reference is generally made to 
individual discrete structures although it is generally the case that a 
plurality of such devices would be fabricated on a wafer substrate in a 
MOCVD reactor wherein the processing laser beam would be scanned and/or 
modulated to selected locations to perform the desired induced evaporation 
enhancement in patterned fashion across the wafer surface. Also, many of 
the structures disclosed contain a separate confinement cavity for the 
active region for purposes of illustration, which is not a specific 
requirement in the exercise of this invention. 
Reference is now made to FIG. 1 wherein there is shown a laser 10 having a 
single emitter and fabricated using MOCVD processing as described in 
incorporated U.S. Pat. No. 4,962,057. Laser 10, for example, may comprise 
a substrate 12 upon which are deposited the following layers or regions: a 
cladding layer 14 of n-Ga.sub.1-x Al.sub.x As; an active region 16 being 
undoped, or p-type doped or n-type doped and can comprise a relatively 
thin conventional double heterostructure (DH) active layer or a single 
quantum well of either GaAs or Ga.sub.1-y Al.sub.y As where x&gt;y or a 
multiple quantum well structure of alternating well layers of GaAs or 
Ga.sub.1-y Al.sub.y As and corresponding barrier layers of either AlAs or 
Ga.sub.1-y Al.sub.y' As, where x, y'&gt;y or a separate single or multiple 
quantum well structure in a separate confinement cavity; a cladding layer 
18 of p-Ga.sub.1-z Al.sub.z As where x, z, y'&gt;y; a n-GaAs layer 19 
followed by another cladding layer 20 of p-Ga.sub.1-z Al.sub.z As, which 
is an extension of layer 18 and, finally cap layer 22 of p+GaAs. 
Fabrication of these layers is continuous through the growth of layer 19 at 
which time epitaxial growth is interrupted and, as taught in U.S. Pat. No. 
4,962,057, supra, the TMG source to the MOCVD chamber is turned off, the 
substrate temperature is increased to about 825.degree. C. and a radiation 
beam is focused to an elongated region 19' of layer 19 and scanned in the 
X direction perpendicular the plane of FIG. 1 for a period of time 
sufficient to thermally evaporate region 19' to the interface 21 at layer 
18 at a rate of 1 .ANG./second. The temperature at region 19' during the 
evaporation process in the As/H attending environment may be about 
1000.degree. C. to 1030.degree. C. After the removal of GaAs region 19', 
epitaxial growth is continued with the deposition of layers 20 and 22. The 
p-Ga.sub.1-z Al.sub.z As region now formed in region 19' through n-GaAs 
film 19 forms a current channel while the remaining regions of film 19 
form reverse biased p-n junctions 15 to provide for current confinement 
through channel 19' to active region 17 of laser 10. Appropriate 
metallization may then be performed at the outer surfaces of layer 22 and 
substrate 12 as is known in the art. The processing techniques in U.S. 
Pat. No. 4,962,057 permit the full in situ processing of a buried 3-D 
current confinement configuration in semiconductor devices without removal 
of the structure from the MOCVD reactor or movement or masking of the 
structure in the reactor. 
FIGS. 2 and 3 disclose laser structures including a reverse bias junction 
in the cladding region of the laser for purposes of current confinement 
and a buried active region, both of which are formed in situ during 
epitaxial growth employing induced evaporation enhancement. In FIG. 2, 
reverse bias junction laser 30 comprises substrate 32 of n-GaAs upon which 
is epitaxially deposited an outer confinement layer 34 of n-Al.sub.x 
Ga.sub.1-x As (e.g., 0.66 .mu.m thick and x=0.8), inner confinement 
waveguide layer 36 of n-Al.sub.y Ga.sub.1-y As where x&gt;y (e.g., 0.06 .mu.m 
thick and y=0.4), active region 38 of undoped, or p-type doped or n-type 
doped layer or layers and may comprise a relatively thin conventional 
double heterostructure (DH) active layer or a single quantum well of 
either GaAs or Al.sub.z Ga.sub.1-z As where z is very small and x&gt;y&gt;z 
(e.g., in either case 13.6 nm thick and z=0.01), or a multiple quantum 
well structure of alternating well layers of GaAs or Al.sub.z Ga.sub.1-z 
As and corresponding barrier layers of either AlAs or Al.sub.z Ga.sub.1-z' 
As, where z&gt;y&gt;z'&gt;z. 
Upon completion of the growth of active region 38, however, epitaxial 
growth is temporally discontinued, the metalorganic sources are vented, a 
1% arsine/hydrogen mixture is introduced into the MOCVD chamber and a 
laser beam or combination laser beam is focussed on the surface of active 
region 38 in areas indicated by dotted lines 37 and in accordance with the 
teachings of U.S. Pat. No. 4,962,057, these areas of the active region are 
desorbed down to the interface with inner confinement layer 36 resulting 
in active region strip region 39. To be noted is that the Al.sub.y 
Ga.sub.1-y As layer 36 functions as a desorption stop to the optical 
patterned desorption because AlGaAs is much more difficult to desorb 
according to this process as compared to the desorption of GaAs. Epitaxial 
growth is then continued with the growth of inner cladding layer 40 of 
p-Al.sub.y Ga.sub.1-y As (e.g., 0.06 .mu.m thick and y=0.8) and first 
outer confinement layer 42 of p-Al.sub.A Ga.sub.1-A As (e.g., 0.06 .mu.m 
thick and A=0.8) where x.gtoreq.or .ltoreq.A.gtoreq.y&gt;z'&gt;z. 
At this time a n-type doped QW layer 44 is grown on confinement layer 42 
comprising n-GaAs heavy doped, for example, with Si or Se at a 
concentration of 10.sup.19 /cm.sup.3 and is represented by the bold lines 
outlining this layer. This layer 44 may be about 7.5 mm thick. The growth 
of layer 44 is accomplished by first shutting off for a short period of 
time Group III sources, i.e., the Ga source, with continued flow of a 
mixture of As and H.sub.2 to form an impurity spike comprising a 
submonolayer or monolayer of an impurity species, such as Se, followed by 
the growth of n-GaAs via the return of the Ga source flow to form layer 44 
followed by the growth of a second or another submonolayer or monolayer of 
impurity species. The characterization of these two submonolayers are also 
referred to as impurity sheet doping, e.g., Se sheet doping, and are 
represented by the bold lines outlining reverse bias configuration. In 
this manner, a highly effective but ultra thin buried reverse bias 
configuration can be produced by a single quantum well layer no thicker 
than 5 to 10 nm thick. The use of such a thin layer of GaAs has the 
advantage of being selectively patterned by removal of layer portions 
within a relatively short period of time, e.g., within several hundreds of 
seconds. 
In connection with the above example, it should be understood that only one 
doping sheet possibly need be employed rather than two doping sheets as 
described. The single doping sheet would be placed on the surface of laser 
44 that is to form the reverse bias function with a contiguous layer 
relative to the forward applied injection current. In the case here, that 
would be the second mentioned submonolayer of impurity species. 
After the growth of layer 44, the growth is again temporarily discontinued, 
the metalorganic sources are vented, a 1% arsine/hydrogen mixture is 
introduced into the MOCVD chamber and a laser beam is focussed to the 
central region of layer 44 to provide a temperature gradient sufficient to 
induce the desorption of a strip in the n-GaAs down to interface 49 with 
outer confinement layer 42 forming channel 47 in layer 44. The Al.sub.A 
Ga.sub.1-A As layer 42 functions as a desorption stop to the optically 
patterned desorption at 47 because AlGaAs is more difficult to desorb 
compared to GaAs. As an example here, for a period of 300 seconds, GaAs is 
desorbed at channel 47 at a rate of about 0.03 mm/s with a temperature of 
approximately 1030.degree. C. from a 3-mm diameter focussed laser spot. 
After removal of channel 47 by induced evaporation enhancement forming 
reverse bias junction configuration 45, epitaxial growth is continued with 
the growth of second outer confinement layer 46 of p-Al.sub.B Ga.sub.1-B 
As (e.g., 0.45 .mu.m thick and B=0.8 where x.gtoreq.A.gtoreq.or 
.ltoreq.B&gt;y&gt;z'&gt;z, followed by the growth of cap layer 48 of p+-GaAs (e.g., 
Mg doped and 0.05 .mu.m thick). 
As an example, layers 42 and 46 may both be comprised of Al.sub.0.8 
Ga.sub.0.2 As. This structure provides for an index guide that has 
emphasis on lateral waveguiding properties. On the other hand, with B&gt;A, 
an antiguiding structure can be created in conjunction with reverse biased 
junction configuration 45, e.g., layer 42 may be Al.sub.0.8 Ga.sub.0.2 As 
and layer 46 may be Al.sub.0.4 Ga.sub.0.6 As. This antiguiding structure 
enables high power operation in a stable mode by spreading optical power 
across a large lateral dimension at the output facet of the laser 
structure. 
FIG. 6 illustrates actual data pertaining to the nature of operation of 
reverse bias junction configuration 45 as analyzed laterally across its 
width relative to forward voltage at 10 mA as a function of position 
laterally along the device. Layer 44 was a Se doped GaAs QW layer 
approximately 7.5 nm thick. Total laser power employed was 7.4 W and 
temperature of the substrate was 800.degree. C. It took approximately 300 
seconds to remove layer 44 to form channel 47 therethrough. It can be seen 
that the forward voltage is about 2 fold greater over the configuration 45 
as compared to channel 47. The series resistance under forward bias is 
also reduced to 8.OMEGA. in channel 47 compared to 11.5.OMEGA. outside 
channel 47. The spatial variation of the forward voltage and series 
resistance clearly indicates the region or channel 47 over which reverse 
biased layer 44 has been removed. The extent of the desorbed region 47 
approximately corresponds with the spot size of the laser radiation at the 
surface of layer 44. Near the center of the laser beam spot the forward 
voltage increases indicative possibly of effects due to over exposure with 
the laser beam. 
In FIG. 3, the reverse biased junction laser 30' is identical in structure 
to laser 30 except that reverse bias junction layer 44 is p-doped with 
junction configuration 45 in the lower outer cladding layer 34 so that 
epitaxial growth is interrupted after completion of epitaxial growth of 
layers 34 and 44. As in the case for FIG. 2, the laser beam is focussed to 
the central region of layer 44 to provide a temperature gradient 
sufficient to induce the desorption of a strip in the p-GaAs down to 
interface 51 with layer 34 forming a channel 47 in layer 44. Growth is 
then continued with the epitaxial growth of outer cladding layer 35 of 
n-Al.sub.w Ga.sub.1-w As where w&gt;y&gt;z and thence layers 36 and 38 with the 
previously described formation of buried heterostructure active region 39 
and thence the sequential growth of layers 39, 40, 42 and 48. 
Current confinement strip 52, defined by proton or ion implant regions 50 
is optional in this embodiment. 
In FIG. 4, there is disclosed a surface emitting laser 90 comprising a 
n-GaAs substrate 92 of n-GaAs upon which is epitaxially deposited a DBR 
comprising alternating layers of n-Al.sub.w Ga.sub.1-w As and n-Al.sub.u 
Ga.sub.1-u As where w&gt;u followed by a first reverse junction layer 96 of 
p-GaAs, which may be of quatum size and provided with sheet doping at its 
surfaces, e.g., Mg. DBR 94, as optically and current confined by reverse 
bias junction layers 96 and 99, functions as a mirror for optical feedback 
for the vertically disposed Fabry-Perot of the laser structure with 
surface emission at 114. 
After the growth of layer 96, epitaxial growth is temporarily discontinued, 
the metalorganic sources are vented, a 1% arsine/hydrogen mixture is 
introduced into the MOCVD chamber and a laser beam is focussed to the 
central region of layer 96 to provide a temperature gradient sufficient to 
induce the desorption of a circular aperture 97 or other shaped region in 
this n-GaAs layer down to interface 98 with DBR 94. Epitaxial growth is 
then continued with the growth of a second junction layer 99 of n-Al.sub.x 
Ga.sub.1-x As, forming the reverse bias junction with layer 96, followed 
by relatively thick active region 100 of undoped, or p-type doped or 
n-type doped layer or layers and may comprise a relatively thick 
conventional double heterostructure (DH) active layer or a multiple 
quantum well structure of alternating well layers of GaAs or Al.sub.z 
Ga.sub.1-z As and corresponding barrier layers of either AlAs or Al.sub.z' 
Ga.sub.1-z' As, where x, y&gt;z'&gt;z, cladding layer 102 of p-Al.sub.y 
Ga.sub.1-y As and n-type doped QW layer 104 which comprises n-GaAs heavy 
doped, for example, with Si or Se at a concentration of 10.sup.19 
/cm.sup.3, which layer may be about 7.5 nm thick. 
After the growth of layer 104, the growth is again temporarily 
discontinued, the metalorganic sources are vented, a 1% arsine/hydrogen 
mixture is introduced into the MOCVD chamber and the laser beam spot is 
focussed to the central region of layer 104 to provide a temperature 
gradient sufficient to induce the desorption of a circular opening in the 
n-GaAs down to interface 106 with layer 102 forming a circular aperture 
105 which is in appropriate alignment with aperture 97. 
After removal of region 105 by induced evaporation enhancement, epitaxial 
growth is continued with the growth of second outer confinement layer 108 
of p-Al.sub.B Ga.sub.1-B As where A.gtoreq.B&gt;x, y&gt;z'&gt;z, followed by the 
growth of cap layer 110 of p+-AlGaAs, for example, Al.sub.0.5 Ga.sub.0.95 
As. If cap layer 110 is QW size, p+-GaAs, an aperture below output 
aperture 114 must be formed in the layer to prevent radiation absorption 
by the GaAs. Zn diffusion is performed at 112 for good ohmic contact with 
metal contact 111. 
Reference is now made to FIG. 5 wherein the surface emitter laser 150 has a 
structure similar to the laser structure shown in FIG. 4 except that the 
order of epitaxial growth is reversed relative to the substrate so that 
the DBR is the last structure epitaxially grown rather than the first. 
With the DBR on top, current injection can be achieved via an impurity 
diffusion. 
Laser 150 comprises substrate 152 of n-GaAs upon which is grown an outer 
cladding layer 154 of n-Al.sub.x Ga.sub.1-x As followed by reverse biased 
junction layer 156 of heavily doped p-GaAs. After the growth of layer 156, 
epitaxial growth is temporarily discontinued, the metalorganic sources are 
vented, a 1% arsine/hydrogen mixture is introduced into the MOCVD chamber 
and a laser beam is focussed to the central region of layer 156 to provide 
a temperature gradient sufficient to induce the desorption of a circular 
spot in this p-GaAs layer down to its interface with layer 154 forming a 
circular aperture 157 in layer 156 for current channeling. 
Epitaxial growth is continued with the growth of inner cladding layer 158 
comprising n-Al.sub.x Ga.sub.1-x As followed by the growth of thick active 
region 160 of undoped, or p-type doped or n-type doped layer or layers and 
may comprise a relatively thick conventional double heterostructure (DH) 
active layer or a multiple quantum well structure of alternating well 
layers of GaAs or Al.sub.z Ga.sub.1-z As and corresponding barrier layers 
of either AlAs or Al.sub.z' Ga.sub.1-z' As, where z&gt;y&gt;z'&gt;z, inner cladding 
layer 162 of p-Al.sub.y Ga.sub.1-y As and reverse biased junction layer 
164 of n-GaAs. Both reverse bias junction layers 156 and 164 may be 
quantum well layers. 
After the growth of reverse biased junction layer 164, the growth is again 
temporarily discontinued, the metalorganic sources are vented, a 1% 
arsine/hydrogen mixture is introduced into the MOCVD chamber and the laser 
beam spot is focussed to the central region of layer 164 to provide a 
temperature gradient sufficient to induce the desorption of a circular 
opening or any other desired shape in the n-GaAs layer down to interface 
with layer 162 forming a circular aperture 165 in layer 164 which is 
approximately concentric with aperture 157. 
After removal of region 165 by induced evaporation enhancement, epitaxial 
growth is continued with the growth of outer confinement layer 166 of 
p-Al.sub.y Ga.sub.1-y As followed by the epitaxial deposition of DBR 168 
comprising alternating layers of undoped Al.sub.w Ga.sub.1-w As and 
Al.sub.u Ga.sub.1-u As where w&gt;u. The growth of DBR 168 is followed by a 
Zn diffusion into DBR 168 forming p-type regions 170 for current injection 
into layer 166. The Zn concentration level may be made sufficiently high 
during this diffusion step to provide current confinement outside the 
central portion of DBR 168 so that the undoped DBR 168 contains no free 
carriers and optical confinement to the central portion of DBR 168. In any 
case, the remaining portion of DBR 168 functions as a reflector for 
optical feedback for the propagating radiation. 
Surface emitter 150 is completed by an annular aperture 176 etched through 
the highly absorbent GaAs substrate 152 to provide for surface emission 
and the deposit of metal contacts 172 and 174. 
While the invention has been described in conjunction with a few specific 
embodiments, it is evident to those skilled in the art that many 
alternatives, modifications and variations will be apparent in light of 
the foregoing description. Accordingly, the invention is intended to 
embrace all such alternatives, modifications and variations as fall within 
the spirit and scope of the appended claims.