Note: Descriptions are shown in the official language in which they were submitted.
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Backgrou_d of the Invention
This invention relates to a method of producing
semiconductor photodiodes having an indium antimonide epitax-
ial layer of one type conductivity onto an indium antimonide
~ substrate of another type conductivity, and more particularly
,~ to such a method wherein the antimony in the epitaxial layer
is partially replaced by another element.
Epitaxial growth is a process of growing solid
material from a suitable environment onto a substrate. The
growth is epitaxial when the material grown forms an extension
of the crystal structure of the substrate. By the addition
of gases comprising donor or acceptor type impurities, the
epitaxially deposited layer may be of n-type or p-type
conductivity as desired.
It is commonly known that diffusion processes are
used to produce the p-type region of pn-InSb photovoltaic
detectors. Uniformly n-type doped indium antimonide crystals
I are enclosed together with p-type dopants such as cadmium or
z~nc in a sealed quartz ampoule. The diffusion will take
place at 450-50boC. After removal of the diffused substrate,
the material is further processed into single or multi-
element semiconductor photodiodes by utilizing standard
planar or mesa technologies.
These methods require costly and complicated etching
and optimization processes which greatly aggravate the
establishment
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of reproducible, thin p-layer regions and fail completely
when extremely thin, uniform thicknesses are required. There-
fore, the resulting photoelectric devices suffer in their
performance characteristics. Still another method being
utilized is the growth of an epitaxial layer of indium anti-
monide having the opposite conductance type on an indium
antimonide monocrystal. This method suffers greatly from the
nonstoichiometric transport properties of both species, the
indium and antimony, which will result in an epitaxial layer
of very low quality.
It was, therefore, necessary to find a method of pro-
ducing indium antimonide single and multi-element detectors
according to the epitaxial growth method, which does not have
the deficiency of the conventional epitaxial methods.
The main demands placed upon the epitaxial process
are eestoration of the stoichiometry of the epitaxial layer,
to assure equivalent and a sufficient partial pressure of all
gas species involved and to apply standard device processing
technologies to the production of photoelectric indium anti-
monide diodes. These requirements can be fulfilled byexecuting the epitaxial process in an apparatus having the
capability of producing epitaxial layers of the desired
composition.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to
eliminate the above and other deficiencies of the prior art.
These and other objects are attained in the invention
which encompasses a method and devices produced thereby, and
wherein an indium antimonide epitaxial layer of p-type
conductivity is epitaxially grown onto an indium antimonide
substrate of n-type conductivity. Photodiodes and other
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devices are then fabricated therefrom. The epitaxial layer
is InAsxSbl x,wherein x is from 0.01 to 0.50, preferably
within the range of 0.01 to 0.05, and most preferably 0.05.
A feature of the invention is a method wherein there
is partial substitution of antimony with arsenic in the
indium antimonide epitaxial layer.
; A further feature of the invention is the use of an
; epitaxial layer of InAsxSbl x~ wherein x is from 0.01 to
0.50, preferably 0.01 to 0.05, and most preferably 0.05.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 depicts an illustrative apparatus to practice
the invention wherein vapor phase epitaxial depositing is
accomplished.
FIG. 2 depicts an illustrative apparatus to practice
the invention wherein liquid phase epitaxial depositing is
accomplished.
FIG. 3 depicts an illustrative substrate having an -
epitaxial layer grown thereon, with the epitaxial layer
having partial substitution of arsenic or phosphorus for
antimony in the indium antimonide.
FIGS. 4A-4F depict the step by step production of a
mesa configuration photoconductive device employing the ;
inventive method; and
FIGS. 5A-5G depict step by step production of a
planar configuration photoconductive device employing the
inventive method.
DETAILED DESCRIPTION OF PREFERRED 2MBODIMENTS
; FIG. 1 depicts a typical epitaxial gas or vapor phase
reactor apparatus for growing epitaxial layers on a substr-
ate, comprising a furnace chamber 1, having a temperature and
heating control system not shown, a removable cap 2, gas
~ 4
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inlet 4 and gas exhaust 3. Through inlet 4 may be
supplied different gases for the deposition of or growth
of a suitable layer. Within the chamber l may be placed
crystal dish 9 on which a substrate lO, such as of indium
antimonide may be placed. Also in chamber l there may be
placed another dish ll on which may be placed solid indium
12, and another dish 13, on which may be placed, for
example, a dopant 14 such as zinc or cadium for p-type
doping and tin or tellurium for n-type doping. From
sources 5, 6, 7 and 8, there is supplied
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for example, reagent gases H2, HCl, AsH3, or PH3, and SbH3,
respectively, with suitable flow control with known type of
metering.
In operation of FIG. 1, the dish~9 having indium
antimonide slice 10, such as of n-type conductivity, thereon
is charged through the right end having cap 2 removed there-
from, and into the chamber 1. Also, the contalners 11 and 13
having loaded thereon solid indium and zinc dopant, respectively,
are also charged into the chamber 1, through the open right end.
Then cap 2 is used to close chamber 1. Chamber 1 is then purged
by supplying hydrogen gas from source 6 through inlet 4. The
chamber is then raised in temperature to a range of about 480C
to 520C. The temperature of the indium is raised to about 700C
and that of zinc 14 is raised to about 380C. As the tempera-
ture of the reaction chamber approaches a constant value, the
epitaxial growth atmosphere is established in chamber 1 by
supplying for example, antimony hydride as an antimony source
from source 5 and arsine as an arse~ic source from source 8,
and hydrogen chloride as a transport agent for both the indium
and zinc, from source 7. The gases are supplied through the
single common inlet 4, or through a plurality of individual
inlets. The hydrogen is continuously supplied, serving as a
carrier and reducing gas. Zinc is utilized to provide p-type
doping while the epitaxial layer is being grown. Of course,
cadmium can also be used to provide p-type doping. If n-type
doping is desired tin or tellurium may be used.
By suitably adjusting the molecular ratio of stibine
and arsine, the compositional ratio of epitaxial film can be
altered. The mixed crystal InAsxSbl x should have the ratio
wherein x is between 0.01 to 0.50, and preferably toward the
lower range of 0.01 to 0.05, and most preferably wherein
x = 0.05. The same ratios are applicable for use of phos-
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phorus in place of arsenic. In the gas phase operation, thiscan be done by suitable ad~ustment of the flow rates of the
reagent gases using conventional metering systems. For
example, using a 36 inches long, 2 inches inner diameter
reactor, flow rates as follows will produce InAsO 05Sbo 95 : -
H2 = 500 to l,OOO cc/min.; SbH3 = lO to 25 cc/min; AsH3 =
50 to lOO cc/min and HCl = 3 to 6 cc/min.
When a sufficient period of time has elapsed, such as
for example 30 to 60 minutes, for the growth of p-type indium
antimonide (using zinc for the dopant) epitaxial layer, to the
desired thickness, such as 0.4 microns, the reagent gases are
turned off. After sufficient time for purging of the atmos-
phere in the chamber l, such as about 30 minutes, the furnace
is turned off and left to cool down to room temperature. The
dish 9 is then remo~ved from the chamber l and the chip is then
ready for further fabrication, such as placement of ohmic
contacts, which may be of gold of 2,000 to 5,000 A! and
etching and shaping of the chips.
In FIG. 2, there is depicted a liquid phase epitax-
20 ial reactor comprising a furnace chamber 21, which is supported -
by support 22 to be tiltable for example up to 30 in either
direction from the horizontal, an inlet 25 for supplying
hydrogen gas, and an outlet 24 for exhaus~ing the atmosphere,
and a removable end cap 23 for sealing the chamber 21. Within
the reactor there may be placed a boat 26 having therein a
slice 29 for example of indium antimonide on a support 28,
and a mixture 27 of for example indium, antimony, and arsenic
in desired compositional proportions and a dopant such as zinc
for p-type conductivity. The slice 29 may already be of an
n-type material, as was the case in the FIG. l slice lO. The
arsenic may be replaced wi~h phosphorus. Initially, an indium
antimonide slice 29 is placed on one end of boat 26 in such a
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manner as to not be in contact with the metallurgical mix-
ture 27 which is placed at the opposite end of the boat 26.
The boat 26 containing the slice 29 on support 28 and mixture
27 may be charged through the right end of chamber 21, and
thereafter capped with removable cap 23.
In the operation of the apparatus of FIG. 2, the
boat 26 which may be of quartz is first loaded with the metall-
urgical mixture 27 at one end and the support 28 having an
indium antimonide slice 29 at the other end. Then boat 26 is
placed in chamber 21. Then, cap 23 is closed. Then, hydrogen
gas is supplied, such as at a flow rate of 500 to 1,000 cc/min.
in a 2" cylindrical inner diameter chamber 20" long, through -
inlet 25 to purge the chamber. After sufficient purging, which
may be about 30 minutes, heat is applied to chamber 21 and -~
raised to a temperature of about 500C. This temperature will
produce a uniform melt of the mixture 27 without affecting the
characteristics of the indium antimonide monocrystal 29. As
the temperature of the chamber reaches equilibrium, which is
about 20 minutes, the chamber 21 is tilted in such a way that
the liquid melt 27 will roll over slice 29 in a complete manner,
suah as shown in FIG. 2. After a suitable time, such as five
minutes, has elapsed, the temperature of chamber 21 is sequen-
tially lowered in small increments, in order to shift the
.
thermodynamic equilibrium of the melt to solid state, thus
resulting in the growth of the epitaxial layer. The thickness
of the epitaxial layer is a function of the length of the
intervals and composition of the melt. After the desired
layer thickness has been attained, such as 0.4 micron, thé
chamber 21 is purged, the heat turned off and the chamber is
cooled to room temperature. The boat 26 is then removed from
chamber 21.
FIG. 3 shows a typical device wherein onto substrate
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31, for example of indium antimonide, is epitaxially grown,
an epitaxial layer 32, wherein the antimony has been partially
substituted with arsenic or phosphorus, in the ratio af
InAsxSbl x' wherein x is 0.01 to 0.50, preferably 0.01 to
0.05 and most preferably 0.05. The same holds for phosphorus.
The foregoing apparatuses of FIGS. 1 and 2 may be used to
produce such epitaxial growth with appropriate doping.
FIGS. 4A-4F show a photoconductive device using a
mesa type configuration. Either of the above apparatus may
be used to obtain this devlce together with ordinary photo-
lithographic and fabrication techniques. First a slice of
indium antimonide of n-type conductivity is used as a substrate
40. Onto this substrate 40 is epitaxially grown an indium
antimonide epitaxial layer 41 (see FIG. 4B) having the antim-
ony partially substituted with either arsenic or phosphorus
in amounts to satisfy that above compositional ratios, and
doped, for example, with p-type conductivity. Next, a photo-
resist 42 is placed on the epitaxial layer 41 and subsequently
stripped to provide a window, and in the window a gold ohmic
contact 43 is deposited or placed. The ohmic contact is about
2,000 to 5,000 A in thickness. The photoresist may be, for
example, "Shipley Photoresist AZ 1350" and the stripper may be
a "Shipley Stripper", both of which are readily available on the
commercial market. The results are shown in FIG. 4C. Then, the
photoresist 42 is stripped again, and another photoresist 44 is
placed over both the epitaxial layer 41 and the gold contact 43
as shown in FIG. 4D. Then, as depicted in FIG. 4E, the epitax-
ial layer 41 and the substrate 40 are both etched away, resulting
in a mesa type arrangement. The photoresist 44 is removed, and
as shown in FIG. 4F, leads are placed on the contact-43, ahd on
the substrate 40. The substrate is mounted on a metallic plate
45, such as by use of epoxy.
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In FIGS. 5A-5G, there are depicted a photodetecting
device produced in accordance with the method of this invention
using a planar configuration. In FIGS. 5A and 5B, an indium
antimonide substrate 50 of n-type conductivity has deposited
thereon silicon dioxide layer 53. The silicon dioxide precip-
itation on the crystai surface 50 is preferably effected through
pyrolysis of organosilicon compounds, such as silicon chloroform,
at a temperature of approximately 400-500C. The carrier con-
centration of the substrate 50, as in the substrate of FIGS.
4A-4F, may be in the order of 1015 to 1016 atoms/cm3. Then,
as shown in FIG. 5C, a photoresist 52 is placed on the silicon
dioxide layer 53 to form a window 54 as shown. Then, the
oxide layer 53 is etched away in that window area 54 to leave
as shown in FIG. 5D, the surface of substrate 50 exposed. The
etching may be done by using buffered hydrofluoric acid on the
exposed area, such as the window 54, to remove the oxide layer.
The photoresist is stripped away, and the oxide layer 53 is
left remaining. Then,the substrate 50 and oxide layer 53 are
placed in either the apparatus of FIGS. 1 or 2, and the epitax-
ial layer 51 having the antimony of the indium antimonidepartially replaced by arsenic or phosphorus in accordance with
the ratio as discussed above, is grown onto substrate 50. This
is shown in FIG. 5E. Then, in FIG. 5F, a photoresist layer 56
with a window 57 is placed on the oxide layer 53 and epitaxial
layer 51, as shown. Into the window 57, a gold ohmic contact
61 is deposited in contact with the epitaxial layer 51. Next,
the substrate is placed securely on a metallic plate 58, such
as by means of an epoxy. Then, as shown in FIG. 5G, the photo-
resist 56 is removed, and the electrode lead 59 is placed on
ohmic contact 61 and electrode lead 60 is placed on metallic
plate 58.
The diodes formed using planar or mesa techniqués,
can then be used, for example, in infrared detectors of various
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108~83S
types, either as single devices or together in groups of two
or more in a multi-array arrangement.
Actual examples of the method are as follows.
EXAMPLE l.
Using a vapor phase epitaxial deposition reactor,
such as shown in FIG. l, an epitaxial layer of indium anti-
monide having the antimony partially replaced with arsenic
was grown onto an indium antimonide substrate. The substrate
was n-type conductivity and the epitaxial layer was p-type.
10 The cylindrical reactor of FIG. 1, was about 36 inches long, `
with an inside diameter of about 2 inches and thickness of
about 4 inches. The reagent gases used with the following
source pressures, were as follows: From source 6, H2 at 20.0
p.s.i., with a variance of +3 or 4 p.s.i.; from source 5,
SbH3 at 14.3 p.s.i., with a variance of a range of 10 to 15
p.s.i.; from source 8, ASH3 at 14.3 p.s.i. with a variable
range of 10 to 15 p.s.i., and from source 7, HCl at 14.3 p.s.i.
with a variable range of 10 to 15 p.s.i. Into a crystal dish
9, was placed a slice 10 of indium antimonide having n-type
conductivity; the slice was about 3 cm2, and had a thickness
of about 8 mils. Also, placed in the container ll was about
5 grams of solid indium 12 of high purity, such as over
99.9999 percent. In container 13 was placed a pellet 14
of about 2 milligrams of zinc for providing p-type doping
to the epitaxial layer. The chamber 1 was then closed with
cap 2, and flushed for about 30 minutes with a stream of hydro-
gen. The chamber 1 was raised in temperature of about 480C.
The flow rates of hydrogen was 500 cc/min. The flow rates of
AsH3 was 80 cc/min; the flow rate of SbH3 was 20 cc/min; and
the flow rate of HCl W3S 5 cc/min. The time of exposure of
the reagent gases to the substrate at the heated temperature
was about 30 minutes. This produced a thickness of epitaxial
layer of about 0.4 microns. The layer was of InAsO 05Sbo 95
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The chamber was then cooled to room temperature and the
c ~ substrate lO having the grown epitaxial layer was removed.
Then, as shown in FIGS. 4 and 5, devices were fabricated
therefrom.
EXAMæLE 2.
Example l was repeated except PH3 at a pressure of
14.3 p.s.i. and a variable range of lO to 15 p.s.i. at the
source 5, and a flow rate of 60 cc/min. was used. Similar
results as in Example l were obtained.
EXAMPLE 3.
A photodetecting device was produced using the
apparatus of FIG. 2, namely, a liquid phase epitaxial growth
apparatus. The cylindrical chamber 21 was about 18 inches
long, 2 inches inner diameter, and 4 inches thick. Hydrogen
at a pressure of 20 p.s.i. with a variable range of ~3 or 4
p.s.i. at the source 25 was supplied to purge the chamber at
a flow rate of 500 cc/min. The metallurgical mixture 27 was
comprised of 4.5 grams of indium, 4.5. grams of antimony, and
1.0 grams of arsenic, and 1 milligram of zinc pellets. The
substrate was indium antimonide slice 29 of 3 cm2 and 8 mils
thickness and n-type conductivity. After purging with hydro-
gen gas from source 25, the furnace 21 was heated to about
500C. The mixture was disposed on the right side of boat 26.
After about 20 minutes, the mixture melted. After about 5
minutes the melt was mixed, and chamber 21 was tilted to cause
the mixture melt 27 to completely cover slice 29. After about
lO minutes, an epitaxial layer was grown on the substrate 29.
The chamber 21 was purged, the temperature cooled to room
temperature, and the boat 26 was removed. The slice 29 having
an epitaxial layer was removed. The epitaxial layer was about
0.4 microns, and contained InAsO 05Sbo 95. The diode was then
made into devices, such as by planar or mesa techniques.
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~ EXAMPLE 4.
Example 3 was repeated except that phosphorus of
1.0 grams was used in place of arsenic. The results were
similar.
In the foregoing examples, also, the p-type dopant
could have been cadmium, and the n-type dopant tellurium or
tin.
The foregoing description is illustrative of the
principles of this invention. Numerous variations and
modifications thereof would be apparent to the worker skilled
in the art. All such variations and modifications are to be
considered to be within the spirit and scope of this invention.
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