Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to -the growth of epitaxial
films comprising Groups III-V semiconduc-tor compounds. More
particularly, the present invention relates to a method for
the deposition of epitaxial layers of gallium arsenide and
gallium aluminum arsenide by means of an organometallic
chemical vapor deposition process.
Gallium arsenide and gallium aluminum arsenide are
well-known compound semiconductor materials in the Group
III-V system which have been widely used commercially in
numerous applications. Prior to applicants' entry into -the
field, it was common practice -to prepare these materlals by
the well-known organometallic chemical vapor deposition
process in which a hydride is employed as the source of the
Group V element. Thus, in the preparation of gallium
arsenide or gallium aluminum arsenide, arsenic trihydride
(AsH3) was employed as the arsenic source material, either
alone or in combination with other hydride and
organometallic compounds.
Although this procedure had been used by workers in
the art for several years with a modicum of success, the
total exploitation of -the process was inhibited by -the high
risk of exposure to harmful Ievels of toxicity of -the
arsenic trihydride which is commonly known as arsine.
Furthermore, -the use of hydrides was plagued by a
contamina-tion problem created by -the presence of both oxygen
and water vapGr therein. Applicants focused -their atten-tion
upon obviating these known prior art limi-tations and, much
to their surprise, discovered a method which completely
eliminated the need for hydrides as the source of the Group
V element.
This end was successfully attained in a process in
which the arsenic source material, in elemental or compound
form, is maintained in a first conduit separa-te and apart
from a second condui-t which contains vapors of either an
organometallic gallium compound or an organometallic gallium
aluminum compound. The arsenic source material is -then
vaporized and upon confluence of vapors from the firs-t and
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second condui-ts, -the arsenic vapors are diluted with
hydrogen, so precluding condensation of the arsenic as it
travels to the next stage of the reaction. It is this
unique processing sequence which applicants believe to be
the situs of invention. In other words, applicants made a
surprising departure from the prior art practice of using a
hydride of arsenic as an arsenic source. They eliminated
the need for the hydrlde by using elemental arsenic or
arsenic in compound form as the arsenic source, a concept
not previously contemplated by workers in the ar-t since it
was widely recognized that vapors of the Group V elemen-ts,
such as arsenic, could not be transported in a compatible
fashion with vapors of the Group III organometallic
compounds which decompose at vaporization temperatures of
arsenic. Thus, applicants departure from the prior art
practice is even more surprising. In essence, they
discovered that vapors of arsenic could, indeed, be
transported compatibly with vapors of a Group III
organometallic compound by diluting the arsenic vapors with
hydrogen as the arsenic vapors join the Group III
organometallic compound vapors.
The prime advantage of the described process
resides in the elimination of the use of the highly toxic
arsine gas. However, ancillary benefits include the
elimination of parasitic reactions between the arsenic
source and the organometallic compounds which lead to
contamination of the desired epitaxial film.
Our invention will be more readily understood by
reference to the accompanying drawing the single Figure of
which is a fron-t elevational view, in cross-section, of an
apparatus suitable for use in the practice of the present
invention.
With reference now more particularly to the
drawing, there is shown a conventional cold wall reactor 11
having disposed therein a quar-tz block 12, a susceptor 13
which serves as a support for a substrate wafer 31, and a
quartz support member 30 for susceptor 13. Reactor 11 is
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adapted with an inlet conduit 14, an exhaus-t condui-t 15 and
a cap member 16 which serves as a seal at the exit end of
reactor 11. Rf coil 17 is provided around the midsection of
reactor 11 for the purpose of heating the susceptor during
the reaction. A heating tape 18 is also provided around the
inlet end of the reactor for the purpose of obviating the
likelihood of the group V element condensing during the
course of the reaction but it does not heat this region
sufficiently to cause parasitic reactions between the group
III organometallic compound and the vapors of -the group V
element. The -tempera-ture in this region is within the range
of 200-300C. Also included as par-t of -the apparatus used
in the practice of the invention is a vaporizing furnace 19
for vaporization of the group V source material, furnace 19
having disposed -therein boat 20 for containing the source
material, boat 20 being coupled with quartz tube 21 having a
soft iron bar 22 sealed therein. A hook means 23 connected
to tube 21 permits the magnetic movement of the boat during
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the processing sequence. Eurnace 19 also includes inlet
conduit 24, a cap 25 to seal the inlet end and e~it
conduit 26 which is connected to inlet conduit 1~ of cold
wall reactor 11. ~leating of the source material exitlng
Erom furnace 19 is effected hy means of a heating tape 27
wrapped around exit conduit 26. Also shown connected to
conduits 26 and 14 is conduit 2~ for introduction of
organometallic compounds and hydrogen to reactor 11.
A brief description of the procedure followed in
the practice of our invention will now be given.
Initially, a substrate member is selected upon which
deposition of the desired epitaxial films will be
effected. Substrates suitable for this purpose may be
conductive or insulating in nature, ultimate selection
being dependent upon the type of device contemplated.
Typical materials found suitable for this purpose are
gallium arsenide, indium phosphide and the like. These
substrates may conveniently be obtained from commercial
sources.
Prior to insertion of the substrate wafer 31
upon the surface of susceptor 13, the substrate is
subjected to a conventional degreasing and etching
sequence, thereby assuring the presence of a clean surface
for deposition. A typical cleansing sequence would
involve degreasing with trichloroethylene, acetone and
methyl alcohol followed by etching with a 3:1:1 mixture of
sulfuric acid, hydrogen peroxide and water. Etching is
continued for a time period sufficient to remove
approximately 2 microns from the surface of the substrate.
The degreased, etched substrate 31 is then placed upon
susceptor 13 in cold wall reactor 11.
In accordance with an aspect of our invention, a
group V source material in solid, elemental or compound
form of high purity is selected. For -this purpose, it is
desirable to employ the material in a form of the highest
purity available, typically 99.9999+% purity. ~rsenic,
gallium arsenide polycrystals, indium arsenide
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polycrystals, indium phosphide polycrystals, and
phosphorous are the materials most commonly employed for
this purpose. The source material so selected is then
placed in boat 20 in vaporizing furnace 19. The weight of
the source material is not considered critical, the only
requirement being that sufficient material be present to
obtain the desired III-V compound. The source selected
may be obtained in solid form from known commercial
sourc~:i or it may be generated in situ by cracking halides
such as arsenic trichloride or phosphorous trichloride and
condensing the arsenic or phosphorous vapors prior to a
deposition run. In this latter case, the furnace 19 is
made movable and the movable boat eliminated.
In the operation of the process, the source
material in boat 20 is heated with hydrogen flowing into
the furnace via inlet 24. Heating is effected at a
temperature sufficient to vaporize the solid source, the
hydrogen serving as a means for transporting the vaporized
source to reactor 11. The amount of vapor which is
transported determines the carrier type and concentration
level, the morphology of the deposited layer and the
growth rate. These parameters must be determined
experimentally for each system, such being dependent upon
considerations relating to the intended use of the
deposited film. Heating tape 27 is heated to a
temperature equal to that of furnace 19, thereby assuring
that source vapors leaving furnace 19 will not condense in
the conduit leading to cold walled reactor 11.
Simultaneous with heating of the source
material, hydrogen is bubbled through a liquid
organometallic compound of a group III element at a
temperature a few degrees lower than room temperature or
lower depending on the vapor pressure of the group III
organometallic compound, so resulting in vapors of the
organometallic compound being transported via conduit 28
past the region 33 of confluence with the vapor from solid
source 20 and through conduit 14 into the growth region in
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reactor 11. Transport of the organome-tallic compound may
be regulated by controlling the rate at which hydrogen is
bubbled through the organometallic compound. The hydrogen
introduced through conduit 28 serves to dilute the group V
vapors in the region 33. I-t will be understood by those
skilled in the art that during the growth sequence it is
feasible to alter the source of the group III compound
and/or add suitable dopants to the system such as hydrogen
selenide, silane, diethylzinc and the like.
The substrate material 31 contained in the cold
wall reactor 11 is heated to a temperature sufficient to
permit growth of the desired compound semi-conductor. In
general this temperature may range from 475-750C with a
general preference being found for a range of 600-750C,
such range being dictated by considerations relating to
layer quality.
Epitaxial films prepared in accordance with the
described procedure may be used in a wide variety of
device applications which will be readily appreciated by
those skilled in the art. Typical of such devices are
field effect transistors, light emitting diodes, lasers,
etc.
An example of the practice of the present
invention is set forth below. It will be understood that
this example is solely for purposes of exposition and is
not to be construed as limiting.
This example describes the growth of a gallium
arsenide epitaxial film utilizing an apparatus of the type
shown in the Figure. The substrate employed was a semi-
insulating chromium doped gallium arsenide wafer oriented
six (6) degrees off the (100) crystalline surface toward
the (111)~ surface. The source material chosen was
99.9999% purity elemental arsenic, obtained Erom
commercial sources.
The arsenic source material was placed in the
boat of the vaporizing furnace and heated to a temperature
of approximately 450C. The susceptor in the cold wall
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reactor was then heated using radio frequency induction toa temperature of 650C, the susceptor being heated without
substantially heating the quartz reactor tube above the
wafer of gallium arsenide. High purity hydrogen was then
passed through the vaporizing furnace at a flow rate of 6
liters per minute, so resulting in a flow velocity of
about 25 cm./sec. at the leading edge of the susceptor.
The amount of arsenic transported to the deposition region
ranged from 0.025 grams/min to 0.125 grams/min
corresponding to an arsenic source temperature of ~25-
470C. ~Iydrogen was then bubbled through trimethyl
gallium and trimethyl aluminum and the resultant vapors
transported to the cold wall reactor. ~t the deposition
site, a trimethyl gallium partial pressure of lxlO 4 was
established to commence growth. Growth was initiated at a
rate of approximately 0.125 ,um/min. The resultant grown
- layer was p-type and had a carrier concentration of
approximately 2X10l5 cm~3.
Studies of the reaction described in that
foregoing example revealed that deposition was dominated
by the decomposition reaction of the As~ molecule produced
by heating solid arsenic. This was found to result in
deposition characteristics markedly different from those
observed with the commonly used arsine. This conclusion
is supported by the fact that a (100) facet was observed
on an epitaxial layer deposited on a 6 off (100) toward
(lll)A substrate, the layer being of p-type when using a
solid arsenic source. However, when arsine is used, no
(lO0) facet is observed and the resultant layers are under
similar growth conditions generally of n-type.
As indicated above, the prime advantage of the
described process resides in the elimination of the use of
the highly toxic arsine gas. However, ancillary benefits
include the elimination of parasitic reactions between the
group V source and the organometallic compounds, such
reactions being of particular concern with indium
containing organometallics.
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Lastly, it will be appreciated that optimigation
of the deposition parameters permits deposition of any of
the group III-V compounds or their alloys.
