Method for improvement of optical quality and reduction of background doping in gainSB/INAS superlattices

Post-growth annealing of GaInSb/InAs superlattices at about 400.degree. to 650.degree. C. in an antimony flux followed by cooling results in enhanced optical properties as determined by photoluminescence and in reduced background doping levels as determined by Hall measurements. Accordingly, the annealing procedure represents an advantage over previous fabrication techniques for Ga.sub.1-x In.sub.x Sb/InAs superlattices.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to optical and electronic devices, 
and, more particularly, to processes for improving the optical properties 
of GaInSb/InAs materials employed in infrared lasers and detectors. 
2. Description of Related Art 
Ga.sub.1-x In.sub.x Sb/InAs superlattices are candidate materials for 
future generation infrared detectors and lasers from mid- to very 
long-wavelengths (3 to 5 .mu.m and beyond). The optimum substrate 
temperature (as determined by surface morphology and x-ray diffraction) 
for fabrication of these superlattices by molecular beam epitaxy is below 
400.degree. C.; see, e g., D. H. Chow et al, "Growth of InAs/Ga.sub.1-x 
In.sub.x Sb infrared superlattices," Journal of Crystal Growth, Vol. 111, 
pp. 683-687 (1991); I. H. Campbell et al, "Far-infrared photoresponse of 
the InAs/GaInSb superlattice", Applied Physics Letters, Vol. 59 (7), pp. 
846-848 (1991); and J. L. Davis, "Optimum growth temperature determination 
for GaInSb/InAs strained layer superlattice", Journal of Vacuum Science 
and Technology B, Vol. 11, No. 3, 861-863 (May/June 1993). 
The Chow et al publication outlines a molecular beam epitaxy (MBE) growth 
procedure which yields Ga.sub.1-x In.sub.x Sb/InAs superlattices with good 
surface morphology, but poor photoluminescence efficiency and high 
background doping levels. Photoluminescence efficiency (as it relates to 
minority carrier lifetime) and low background doping are both key material 
properties for the realization of high performance optical and electronic 
devices such as infrared detectors and lasers. 
Thus, there is a need to provide GaInSb/InAs superlattice materials having 
improved photoluminescence efficiency and low background doping. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, post-growth annealing of the 
superlattices in an antimony flux results in enhanced optical properties 
as determined by photoluminescence and in reduced background doping levels 
as determined by Hall measurements. 
In particular, the method of the invention for treating a Ga.sub.1-x 
In.sub.x Sb/InAs superlattice formed on a III-V substrate comprises: 
(a) heating the substrate to a temperature within the range of about 
400.degree. to 650.degree. C.; 
(b) providing a flux of antimony species onto a surface of the Ga.sub.1-x 
In.sub.x Sb/InAs superlattice; 
(c) maintaining the substrate at the temperature and the flux of antimony 
atoms onto the superlattice surface for a period of time sufficient to 
reduce the density of as-grown point defects; and 
(d) cooling the substrate to room temperature. 
The annealing procedure of the present invention results in improved 
optical quality and in reduction of background doping. Accordingly, the 
annealing procedure represents an advantage over previous fabrication 
techniques for Ga.sub.1-x In.sub.x Sb/InAs superlattices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is directed to a procedure which improves the optical 
quality and reduces background doping in Ga.sub.1-x In.sub.x Sb/InAs 
superlattices grown on substrates. These results are of significance for 
the attainment of high performance devices such as infrared detectors and 
lasers based on Ga.sub.1-x In.sub.x Sb/InAs superlattices. In the compound 
semiconductor, x may range from 0 to 1, thereby providing superlattice 
compositions ranging from GaSb/InAs to InSb/InAs. Typically, the layers 
comprising the superlattice range from about 3 to 100 .ANG. in thickness, 
with at least two periods. 
FIG. 1 is a schematic layer sequence diagram which illustrates 
schematically the structure obtained by prior art processes used to 
fabricate Ga.sub.1-x In.sub.x Sb/InAs superlattices 10, comprising 
alternating layers of InAs 10a and Ga.sub.1-x In.sub.x Sb 10b. This 
structure has been previously published in the prior art by several 
different authors. 
The superlattices 10 are grown either on nearly lattice-matched III-V 
substrates 12, such as GaSb or InAs substrates, or on thick stress-relaxed 
GaSb buffer layers on GaAs or InP substrates (not shown). The 
superlattices may also be grown on silicon substrates. In any event, 
unusually low InAs growth rates are used (approximately 0.2 monolayers per 
second) to minimize cross-incorporation of arsenic in the antimonide 
layers and to maintain a reasonable Ga.sub.1-x In.sub.x Sb growth rate. 
Any of the known prior art techniques for growing the superlattices 10 may 
be employed in the practice of the present invention. Such prior art 
techniques include molecular beam epitaxy (MBE), metal organic MBE 
(MOMBE), chemical vapor deposition (CVD), and chemical beam epitaxy (CBE) 
and variants thereof. 
It has been determined empirically that the superlattices 10 must be 
deposited at low substrate temperatures (&lt;400.degree. C.) to obtain good 
surface morphology and structural quality. However, growth at these low 
substrate temperatures tends to result in defects (such as antisite 
defects, interstitials, and vacancies) incorporated into the structures, 
leading to high background doping levels and strong nonradiative 
recombination channels. As these properties are deleterious to detector 
and laser performance, it is desirable to reduce the density of as-grown 
point defects. The procedure of the present invention accomplishes this 
goal. 
The procedure disclosed here is as follows: (a) the Ga.sub.1-x In.sub.x 
Sb/InAs superlattice 10 is deposited using a prior art process such as 
that described above; (b) the substrate temperature is raised to an 
annealing temperature in a vacuum environment while maintaining an 
incident Sb-flux on the superlattice surface 10'; (c) the annealing 
temperature and incident Sb-flux are maintained for a period of time 
sufficient to reduce the density of as-grown point defects; and (d) the 
substrate is cooled in the incident Sb-flux and then to room temperature. 
The cooling to room temperature may be performed in the ultrahigh vacuum 
environment or ex situ. 
The annealing temperature employed in the practice of the present invention 
ranges from about 400.degree. to 650.degree. C. The lower limit is 
constrained by considerations related to time: a temperature of less than 
about 400.degree. C. would require too long a time of annealing to be 
practical. The upper limit is constrained by the loss of integrity of the 
superlattice 10 as the layers begin mutual interdiffusion. Preferably, the 
annealing temperature is about 500.degree. C. 
The antimony flux is intended to suppress loss of Sb from the superlattice 
by vaporization; such loss would generate defects in the superlattice 
structure. The amount of the flux is based on the known vapor pressure of 
Sb at the annealing temperature, and is typically at least about twice the 
calculated loss of Sb. The flux of antimony species may comprise monomers, 
dimers, and tetramers, and mixtures thereof. 
The time that the sample is maintained at the annealing temperature depends 
on the annealing temperature. Lower annealing temperatures require more 
annealing time, while higher annealing temperatures permit shorter 
annealing time. Consistent with these considerations, the annealing time 
may range from about 10 seconds to 24 hours. At an annealing temperature 
of 500.degree. C., an annealing time of about 30 minutes is appropriate. 
Neither the rate of heating to the annealing temperature nor the rate of 
cooling appears to be very critical. However, heating and cooling rates on 
the order of 0.2 to 1.degree. C./sec have been found to be sufficient. 
The Sb-flux does not need to be maintained below about 350.degree. C. since 
the loss of Sb at that temperature is minimal. Accordingly, the Sb-flux 
may be turned off when the substrate reaches that temperature during 
cooling. Indeed, if the substrate is too cool, metallic Sb will be formed 
on the surface, which is undesirable. 
Once the Sb-flux is turned off, the cooling may proceed under the vacuum 
conditions employed during the annealing. The vacuum is typically in the 
range of 10.sup.-8 to 10.sup.-10 Torr, which is the range employed during 
MBE deposition of the superlattice. However, use of a vacuum during 
annealing does not appear to be critical, and the annealing may 
alternatively be done ex situ. 
Subsequent layers (requiring substrate temperatures below 500.degree. C.) 
may be deposited prior to step (d), if desired. For example, a thin GaSb 
or InAs layer (not shown), approximately 50 to 1,000 .ANG. in thickness, 
may be grown on top of the superlattice layer 10 as a cap layer. Such a 
cap layer is well-known for its use in preventing charge separation at the 
exposed surface 10' of the superlattice layer 10. 
The annealing procedure of the invention serves to eliminate certain point 
defects by increasing bulk diffusion in the as-grown superlattice. For 
example, Group-III-on-Sb-site defects may be eliminated by exchanging the 
misplaced Group III atom with an Sb-atom from the incident flux. 
FIG. 2 displays photoluminescence data obtained from a superlattice grown 
with the herein-disclosed procedure, the data having been measured at a 
temperature of 5K. The nominal growth parameters for the superlattice were 
13 monolayers of InAs 10a and 8 monolayers of Ga.sub.0.75 In.sub.0.25 Sb 
10b, repeated 40 times for a total film thickness of approximately 0.25 
.mu.m. X-ray diffraction revealed a superlattice period close to that 
expected from the nominal parameters. A strong photoluminescence peak was 
observed near 10 .mu.m (120 meV), consistent with band edge emission from 
the structure (the energy gap of the structure was independently 
determined by spectral photoconductivity). Samples grown without the use 
of the disclosed procedure displayed no photoluminescence beyond 8 .mu.m 
(155 meV). 
The presence of the peak near 10 .mu.m permits its use as a practical 
diagnostic tool in determining the effectiveness of annealing. Its absence 
indicates that the particular annealing conditions were not effective. 
This provides a relatively simple evaluation procedure without undue 
experimentation. 
FIG. 3 displays Hall effect data taken from one superlattice grown with the 
disclosed procedure of the present invention (Curve 20) and another 
superlattice grown without the disclosed procedure (Curve 22). Both 
superlattice structures were grown using the prior art MBE process. The 
background carrier concentration observed in the sample grown with the 
annealing procedure of the present invention is more than a factor of two 
smaller, dropping to a value of 4.times.10.sup.15 cm.sup.-3 at 10K. 
The data disclosed in FIGS. 2 and 3 depict the advantages of annealing in 
accordance with the invention over the prior art unannealed samples. In 
particular, the reduction in carrier concentration illustrated in FIG. 3 
will result in longer depletion lengths in photovoltaic detectors, 
directly improving their performance. Further, as illustrated by the 
photoluminescence of FIG. 2, the reduced carrier density significantly 
increases carrier lifetimes, both by raising fundamental Auger lifetimes 
and by suppressing Shockley-Reed-Hall nonradiative recombination 
processes. Increasing lifetime directly improves the performance of a 
photoconductive or photovoltaic detector (for either class of detector 
operated in a non-background-limited mode, detectivity 
D*.varies..tau..sup.1/2, except in the case of photovoltaic detectors for 
which the diffusion length is less than the thickness of the active layer, 
in which case D*.varies..tau..sup.1/4, where T is the carrier lifetime) 
The present invention applies to the general area of infrared imaging 
systems, such as passive forward-looking infrared (FLIR) systems, 
electro-optical missile seekers, and imaging cameras. Potential commercial 
applications include environmental monitoring and industrial process 
control systems based on infrared imaging. For all of these applications, 
GaInSb/InAs superlattice infrared detectors and lasers offer an 
alternative to present state-of-the-art HgCdTe detectors with significant 
system advantages if their performance nears theoretical predictions. The 
present invention is an important step toward such performance. Laser 
systems based on mid-wave IR lasers may be used in hydrocarbon monitoring, 
such as pollution emissions from vehicles and refineries. 
Thus, there has been disclosed an annealing procedure which improves the 
optical quality and reduces background doping in Ga.sub.1-x In.sub.x 
Sb/InAs superlattices. It will be readily apparent to those skilled in 
this art that various changes and modifications of an obvious nature may 
be made, and all such changes and modifications are considered to fall 
within the scope of the present invention, as defined by the appended 
claims.