Surface passivation of IV-VI semiconductors with As.sub.2 S.sub.3

The process of coating epitaxial films of lead chalcogenide materials with s.sub.2 S.sub.3 to insulate the films from the effects of oxygen upon exposure to air.

BACKGROUND OF THE INVENTION 
The present invention relates to the process for passivating and 
stabilizing the surface of lead chalcogenides, and to the devices prepared 
thereby. 
As used here, it is convenient to describe the surface of a semiconductor 
material that has been rendered inert to ambient gases as a passivate 
surface. It is well known that adsorption of oxygen onto the surfaces of 
lead chalcogenide crystals causes the formation of a strong p-type surface 
layer. For those not familiar with recent advances in this art, the 
following brief bibliography is offered: 
Surface Interaction of H and O.sub.2 On Thin Epitaxic Films, by G. F. 
McLane and J. N. Zemel, Thin Solid Films, Vol. 7, pg. 229 (1971); 
Photoconductivity In Lead Selenide, Experimental, by J. N. Humphrey and W. 
W. Scanlon, Physical Review, 105, 469 (1957); 
Surface Transport Phenomena In PbSe Epitaxial Films, by M. H. Brodsky and 
J. N. Zemel, Physical Review, 155, 780 (1967); 
The Effect Of Oxygen On Epitaxial PbTe, PbSe And PbS Films, by R. F. 
Egerton and C. Juhasz, Thin Solid Films, 4, 239 (1969). Although extensive 
work has been done with passivation of silicon aand germanium, (e.g., 
Oxidation Of Semiconductive Surfaces For Controlled Diffusion, U.S. Pat. 
No. 2,802,760, August, 1957 or Method Of Fabricating An Ensulated Gate 
Field-Effect Device, U.S. Pat. No. 4,010,290, March, 1977) little is known 
about passivation of the lead chalcogenides. 
SUMMARY OF THE INVENTION 
A process for preparing and, a lead chalcogenide device having a stabilized 
surface conductivity upon exposure to ambient gases. The surface layer of 
a semiconductor, created by exposure of the semiconductor to a source of 
gas (e.g., oxygen, hydrogen) such as air or moisture, is removed by vacuum 
annealing, and the semiconductor coated with arsenic-trisulfide, As.sub.2 
S.sub.3. Alternately, as-grown semiconductors may be cooled and coated 
with arsenic-trisulfide prior to exposure to air. 
Accordingly, it is an object of the present invention to provide a process 
for stabilizing the electrical properties of lead chalcogenide 
semiconductors. 
It is another object to provide a process for stabilizing the surface 
conductivity of lead chalcogenide semiconductor devices. 
It is another object to provide a process for stabilizing the surface 
conductivity of lead chalcogenide semiconductor devices. 
It is still another object to provide a lead chalcogenide device having a 
stabilized surface conductivity upon exposure to ambient gases such as 
ionic oxygen or hydrogen. 
It is yet another object to provide a stable, protective coating for lead 
chalcogenide semiconductor devices. 
It is still yet an object to provide a stable coating for insulating lead 
chalcogenide semiconductor devices from the effects of ambient gases.

DETAILED DESCRIPTION 
Epitaxial films of PbS.sub.x Se.sub.1-x (x = 0, 0.50, 0.80, 0.85, 1) and 
Pb.sub.1-y Sn.sub.y Se (y = 0.07) were deposited on freshly cleaved (111) 
BaF.sub.2 using the apparatus shown in FIG. 1. The apparatus is a 
conventional glass belljar 56 system of the type disclosed in the 
copending patent application, Equilibrium Growth Technique For Preparing 
PbS.sub.x Se.sub.1-x Epilayers, filed on 27 May, 1977 and assigned Ser. 
No. 801,431, with a nitrogen cold trap (not shown) and an oil diffusion 
pump 54. Deposition pressures and substrate temperatures were on the order 
of 10.sup.-6 Torr (.about.1.3 .multidot. 10.sup.-4 Pa) at gauge 52 and 
350.degree. to 400.degree. C., respectively. The main furnance 20 was 
maintained at 600.degree. C. Growth rates were in the range of two to four 
microns per hour. The source 2 to substrate 12 distance was four 
centimeters, and the main furnance 20 was two centimeters in diameter. 
Approximately twenty grams of granulated source material 2 was placed in 
the upper quartz furnance 20. This was sufficient material to obtain 
fifteen to twenty epilayers 14 of constant composition. A coaxial, 
auxiliary furnance 30, maintained at nearly room temperature, was used to 
coevaporate a small amount of sulfur during growth of the PbS.sub.x 
Se.sub.1-x epilayers (0.5 .ltoreq. x .ltoreq. 1). This source was needed 
to obtain nearly stoichiometric p-type films. In the Pb.sub.1-y Sn.sub.y 
Se films (0 .ltoreq. y .ltoreq. 0.07) stoichiometry was controlled by the 
ingot composition from which source charge 2 was obtained, and from which 
the films were grown. A 0.5% metal-rich ingot yielded n-type films while a 
nearly stiochiometric ingot produced p-type films of low minority charge 
carrier density. A more complete description of the details of preparing 
lead chalcogenide epilayers is given in the earlier mentioned patent 
application, Ser. No. 801,431. 
After cooling, the films were exposed to the atmosphere and inserted into a 
second vacuum system in which gold electrical contacts were evaporated for 
the transport measurements. Conventional direct current electrical 
measurements were made at room temperature and 77.degree. K. using silver 
paint over gold pads to make electrical contacts. The samples were 
measured in air at 300.degree. K. and immersed in liquid nitrogen for the 
77.degree. K. measurement. Typical sample currents were of the order of 
0.5 to 1 mA. The magnetic field was in the range of 1 to 3kG. The Hall 
mobility and carrier concentration were calculated from .mu..sub.H = 
.sigma.R.sub.H and R.sub.H = -1/nq. 
Coating a lead chalcogenide film with As.sub.2 S.sub.3 stabilizes the 
film's electrical properties. Samples of several epilayers of lead 
chalcogenide alloys were annealed in a vacuum chamber of the type shown in 
FIG. 2 (i.e., a second stage of a multi-stage vacuum deposition apparatus 
having the furnance shown in FIG. 1 as a first stage), cooled to about 
room temperature, and coated with about 3,000 A of As.sub.2 S.sub.3. 
Annealing was performed by heating the sample epilayers to 150.degree. 
Celsius for thirty minutes at a gauge pressure 52 of 1 .multidot. 
10.sup.-6 Torr (.about.1.3 .multidot. 10.sup.-4 Pa) in order to remove 
ambient gases such as oxygen and hydrogen from the surface of the 
epilayers. After annealing, the epilayer is cooled to a temperature 
between 4.degree. Kelvin and 100.degree. C. Prior to coating, a portion of 
the 99.999% As.sub.2 S.sub.3 charge (i.e., a fine powder) was heated to 
remove moisture and oxides. The As.sub.2 S.sub.3 charge 3 was evaporated 
onto the cooled epilayer from a quartz ampoule 21 with a nichrome heater 
winding 22'. A rate monitor 60 with a quartz crystal face 62 was used to 
measure the amount of As.sub.2 S.sub.3 deposited. The deposition rate was 
approximately 1000 A per minute. The resistance of each sample was 
recorded during the vacuum anneal, during the coating procedure, and after 
exposure to air. 
The electrical properties of six of the sample films, measured before and 
after coating, are shown in Table 1. 
__________________________________________________________________________ 
As-grown Overcoated 
Sample Thickness (.mu.m) 
##STR1## 
.mu.H(cm.sup.2 V.sup.-1.sbsp.s.sup.-1) 
##STR2## 
.mu.H(cm.sup.2 V.sup.-1.sbsp.s.su 
p.-1) 
__________________________________________________________________________ 
PbS.sub.0.8 Se.sub.0.2 
14 0.10 +3.1 .times. 10.sup.18 
1800 
+5.3 .times. 10.sup.17 
1600 
PbS.sub.0.5 Se.sub.0.5 
12 0.18 +1.8 .times. 10.sup.18 
2100 
+8.3 .times. 10.sup.17 
1600 
PbSe 9 0.27 +1.7 .times. 10.sup.18 
4300 
+9.3 .times. 10.sup.17 
3500 
PbSe 19 0.50 +9.1 .times. 10.sup.17 
3300 
-4.1 .times. 10.sup.17 
2400 
Pb.sub.0.93 Sn.sub.0.07 Se 
73 0.29 +7.5 .times. 10.sup.17 
2300 
+2.5 .times. 10.sup.17 
4500 
Pb.sub.0.93 Sn.sub.0.07 Se 
122 0.68 +4.4 .times. 10.sup.17 
10,000 
+3.2 .times. 10.sup.17 
11,500 
__________________________________________________________________________ 
The surface charge carrier concentration was reduced significantly in all 
the samples coated. This change is greatest in the thinner samples, and 
occurred during the annealing procedure thus indicating that the change is 
due to desorption of oxygen and reduction of the excess surface charge. 
The resistance of the films did not change either during the coating 
procedure or when the films were subsequently exposed to air. This 
indicates that the As.sub.2 S.sub.3 coating does not produce over about 1 
.multidot. 10.sup.13 ionized surface states, but forms a stable, 
protective layer. No degradation was observed in any of the coated films, 
even after temperature cycling from room temperature to 77.degree. Kelvin 
numerous times, an indication that the As.sub.2 S.sub.3 layer either is 
flexible and obtains a good mechanical bond to lead chalcogenide, or that 
there is a near identity between the coefficients of thermal expansion of 
As.sub.2 S.sub.3 and lead chalcogenide films. 
A discussion of other details of these experimental procedures, and of the 
principles upon which they are based appears in Surface Charge Transport 
In PbS.sub.x Se.sub.1-x And Pb.sub.1-y Sn.sub.y Se Epitaxial Films, 
written by the inventors hereof, and published in the Journal Of Vacuum 
Science Technology, Volume 13, No. 4, July/August, 1976. 
Lead chalcogenide epilayers passivated according to the present invention 
may be further processed to prepare any of the typical semiconductor 
devices such as the photovoltaic cell shown in FIG. 3. By 
photolithographic techniques well known to those skilled in the arts, 
windows may be etched through the arsenic trisulfide insulating layer 16. 
Regions of epilayer 14 underlying the exposed areas may be converted to 
regions of opposite type conductivity either by vacuum diffusion or ion 
implantation techniques. Ohmic and non-ohmic electrical contacts may be 
attached to selected of the exposed areas. In the alternate, a complete 
semiconductor device may be prepared, annealed in a vacuum, cooled, and 
then coated with an insulating layer of arsenic trifulfide. 
FIG. 4A is a graph of the current-voltage characteristic of a typical prior 
art photodiode after exposure to air. FIG. 4B is a graph of the same 
characteristic of a photodiode passivated according to the present 
invention. In both of the photodiodes represented by FIGS. 4A and 4B, an 
indium dot was diffused into an epitaxial layer 14 of lead sulfide, to 
form a shallow planar junction. A comparison of the two FIGS. shows that 
both of the diodes exhibit normal conduction when forward biased. The 
prior art diode however, behaves as a very leaky diode when reverse 
biased, while the As.sub.2 S.sub.3 passivated diode has a leakage current 
that is an order of magnitude less. 
It is apparent from the details of the preceeding description that a 
coating of arsenic trisulfide insulates a lead chalcogenide film from not 
only oxygen, but any gas in atomic form (i.e., ionized) that would alter 
the electric properties of the films. Such gases include hydrogen, 
fluorine, chlorine, bromine, iodine, sulfur, selenium, and tellurium. 
While the samples presented in the description were identified as 
epitaxial thin films of PbS.sub.x Se.sub.1-x and Pb.sub.1-y Sn.sub.y Se, a 
coating of arsenic trisulfide may be applied to passivate any lead 
chalcogenide device, whether monolithic or multilayer, whether 
monocrystalline or polycrystalline, whether a thin-film or a bulk device, 
or whether a binary such as PbS, PbSe, PbTe, a ternary alloy such as a 
lead-tin or lead-cadmium chalcogenide e.g.; PbSnS, PbSnSe, PbSnTe, PbCdS, 
PbCdSe, PbCdTe, PbSSe or PbSeTe, or a quarternary alloy such as PbSnSSe, 
PbSnSTe, PbCdSSe, PbCdSTe or PbCdSeTe. A coating of arsenic trisulfide 
does not produce over 1 .multidot. 10.sup.13 ionized surface states but 
forms a stable, flexible, insulating layer without thermal expansion 
mismatch over a temperature range extending from four degrees Kelvin. 
Additionally, the arsenic trisulfide layer improves the junction 
characteristics of a lead chalcogenide device by removing the leakage 
current. It should be noted that as arsenic trisulfide is transparent over 
a region extending from the visible to the far infrared spectrum, it has 
particular application to photoconductive and photovoltaic devices. 
Accordingly, it is apparent that the thickness of the insulating coating, 
earlier described as 3000 A, is not crucial, and may be tailored to also 
serve as a quarter wave anti-reflective coating.