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BACKGROUND OF THE INVENTION
This application relates to a method for preparing semiconductor materials
and devices, and more particularly to a method of preparing defect free
layers of semiconductor materials for use as a substrate or a buffer layer
in a semiconductor device.
The growth of high quality, defect free semiconductor materials for use as
a substrate or a buffer layer in a semiconductor device is a prerequisite
for the subsequent growth of high quality semiconductor layers in the
device.
The preparation of defect free semiconductor materials is easiest when a
substrate of the same material is used. However, often times this is not
possible and defects are created during the preparation of the
semiconductor at the interface between the semiconductor and the
substrate, as a result of the lattice and thermal mismatch between the
semiconductor and the substrate materials. Buffer layers and other
multi-layer schemes are often used to form a transition between the
substrate and the semiconductor layer, but these have proven insufficient
in many cases.
For example, in the case of III-Nitride semiconductor materials, many of
the defects propagate nearly along the growth direction, that is
perpendicular to the substrate/layer interface, without stopping as the
film is grown thicker. It is therefore necessary to effectively stop the
propagation of these defects.
SUMMARY OF THE INVENTION
An object then of the subject invention is the growth of a high quality
layer of semiconductor material.
A further object is the growth of defect-free III-Nitride materials.
These and other objects are attained by the subject invention which
comprises the method of preparing defect-free semiconductor material
layers by growing the semiconductor material buffer layer under optimum
conditions on the substrate; the buffer layer was grown thin (10
.ANG.-10000 .ANG.); the buffer layer is next masked with a dielectric film
which is then etched to open periodic seed windows having a width of
0.1-50 .mu.m and periods of 0.2-500 .mu.m. The length of the seed window
can be as long as needed and may reach the edges of the substrate wafer.
Another layer of a III-V or II-VI material is then grown in the
longitudinal direction from the seed window, followed by lateral growth of
the same material to form an epitaxial film and a structure which provides
a defect free surface for further epitaxial layers. The substrate may also
be removed by an etching solution to yield a defect free semiconductor
material layer for use as a substrate.
DETAILED DESCRIPTION OF THE INVENTION
The reactor and associated gas-distribution scheme used herein are
substantially as described in U.S. Pat. No. 5,384,151. The system
comprises a cooled quartz reaction tube (diameter 5 cm in the substrate
area) pumped by a high-capacity roughing pump (120hr.sup.-1) to a vacuum
between 10 and 300 Torr. The substrate was mounted on a pyrolytically
coated graphite susceptor that was heated by rf induction at 1 MHz. The
pressure inside the reactor was measured by a mechanical gauge and the
temperature by an infrared pyrometer. A molecular sieve was used to impede
oil back-diffusion at the input of the pump. The working pressure was
adjusted by varying the flow rate of the pump by using a control gate
valve. The gas panel was classical, using 1/4-inch stainless steel tubes
and Swagelock fittings. Flow rates were controlled by mass flowmeters.
The reactor was purged with a hydrogen flow of 4 liters min.sup.-1, and the
working pressure of 78 Torr was established by opening the gate valve that
separated the pump and the reactor. The evacuation line that was used at
atmospheric pressure was automatically closed by the opening of the gate
valve. The gas flow rates were measured under standard conditions, i.e., 1
atm and 20.degree. C., even when the reactor was at subatmospheric
pressure. The pressure in the gas panel was regulated by the needle valve
placed between the gas panel and the reactor. The needle valve was
adjusted to maintain a constant pressure of 1 atm on the gas panel,
thereby ensuring reproducibility of flow-rate measurements.
The gas sources used in this study for the growth of GaN, AlGaN, and GaInN
by LP-MOCVD are listed below.
Group-III Sources Group-V Source
Al(CH.sub.3).sub.3 t-butylamine
Al(C.sub.2 H.sub.5).sub.3 NH.sub.3
In(CH.sub.3).sub.3
In(C.sub.2 H.sub.5).sub.3
(CH.sub.3).sub.2 In(C.sub.2 H.sub.5)
Ga(CH.sub.3).sub.3
Ga(C.sub.2 H.sub.3).sub.3
An accurately metered flow of purified H.sub.2 or N.sub.2 is passed through
the appropriate bubbler. To ensure that the source material remains in
vapor form, the saturated vapor that emerges from the bottle is
immediately diluted by a flow of hydrogen or N.sub.2. The mole fraction,
and thus the partial pressure, of the source species is lower in the
mixture and is prevented from condensing in the stainless steel pipe work.
The metal alkyl or hydride flow can be either injected into the reactor or
into the waste line by using two-way valves. In each case, the source flow
is first switched into the waste line to establish the flow rate and then
switched into the reactor. The total gas flow rate is about 8 liters
min.sup.-1 during growth. Stable flows are achieved by the use of mass
flow controllers.
The dielectric film etching solution used in the subject invention may be
any of those commonly used, such as a solution of sulfuric acid and
hydrogen peroxide. Alternatively, a solution of HF and ammonium fluoride
may be used.
The substrate can be GaAs, Si, Al.sub.2 O.sub.3, MgO, SiC, ZnO,
LiGaO.sub.2, LiAlO.sub.2, MgAl.sub.2 O.sub.4 or GaN. Preferably, Si is
used as the substrate. The epitaxial layer quality is sensitive to the
pretreatment of the substrate and the alloy composition. Pretreatment of
the substrates prior to the procedure of the subject invention was thus
found to be beneficial. One such pretreatment procedure is as follows:
1. Dipping in H.sub.2 SO.sub.4 for 3 minutes with ultrasonic agitation;
2. Rinsing in Deionized H.sub.2 O;
3. Rinsing in hot methanol;
4. Dipping in 3% Br in methanol at room temperature for 3 minutes
(ultrasonic bath);
5. Rinsing in hot methanol;
6. Dipping in H.sub.2 SO.sub.4 for 3 minutes;
7. Rinsing in deionized H.sub.2 O, and
8. Rinsing in hot methanol.
After this treatment, it is possible to preserve the substrate for one or
two weeks without repeating this treatment prior to growth.
The epitaxial layers of the subject invention may be grown by low-pressure
metalorganic chemical vapor deposition (LP-MOCVD), although MBE, LPE and
VPE procedures may be used with equal effect.
Growth takes place in MOCVD procedures by introducing metered amounts of
the group-III alkyls and the group-V hydrides into a quartz reaction tube
containing a substrate placed on an rf-heated susceptor surface. The hot
susceptor has a catalytic effect on the decomposition of the gaseous
products; the growth rate is proportional to the flow rate of the
group-III species, but is relatively independent of temperature between
500 and 600.degree. C. and of the partial pressure of group-V species as
well. The gas molecules diffuse across the boundary layer to the substrate
surface, where the metal alkyls and hydrides decompose to produce the
group-III and group-V elemental species. The elemental species move on the
hot surface until they find an available lattice site, where growth then
occurs.
For best results, all surfaces of the growth reaction chamber are coated
with a barrier coating capable of withstanding high temperatures and not
reacting with the reactants and dopants utilized therein at such high
temperatures. Preferably, a coating of AlN or of SiC is grown in situ in
the reaction chamber to cover all surfaces therein. There is thus formed a
stable layer that prevents oxygen and other impurities originating within
the reaction chamber from reacting with the semiconducting layer to be
grown.
High quality III-V layers may be grown in the method of the subject
invention by low pressure metalorganic chemical vapor deposition
(LP-MOCVD) although MBE, LPE and VPE may be used as stated above. The
layers are grown by an induction-heated horizontal cool wall reactor. The
sources set forth above are typically used as the sources for the element
desired. Pure and diluted ammonia gas (NH.sub.3) is generally used as the
N source although other substrates such as GaAs, Si, MgO, SiC, ZnO,
LiGaO.sub.2, LiAlO.sub.2, MgAl.sub.2 O.sub.4 or GaN can be used. A buffer
layer of AlN, GaN, InN, GaAs, GaP or any alloy of these may be
individually laid at thicknesses from 500 .ANG. to a few microns. The
optimum growth conditions for representative layers are listed in Table 1.
The films of the subject invention were grown on semi-insulating substrates
such as Si using a horizontal, rf-heated, LP-MOCVD reactor. Triethyl
Gallium (TEG), was used as the precursor of Gallium; Ammonia, NH.sub.3,
was used as the precursor of N. Palladium-diffused hydrogen was used as a
carrier gas with a total flow rate of 1.5 l/min. Growth conditions, such
as temperature, pressure and V/III ratio, were varied to obtain good
quality films as shown for a few III-V compounds in Table I. II-VI
compounds may also be utilized in the method of the subject invention.
In one embodiment of the invention, a conventional SOI technique using the
ELO (Epitaxial Lateral Overgrowth) method "Lateral Epitaxial Overgrowth of
Silicon on SiO.sub.2, " by D. D. Rathman et al., JOURNAL OF
ELECTRO-CHEMICAL SOCIETY SOLID-STATE SCIENCE AND TECHNOLOGY, October,
1982, p. 2303) may be used. First a semiconductor layer of AlN, GaN, InN,
GaAs, GaP or any alloy of these is grown on top of a semiconductor
substrate such as GaAs, Si, Al.sub.2 O.sub.3, MgO, SiC, ZnO, LiGaO.sub.2,
LiAlO.sub.2, MgAl.sub.2 O.sub.4 or GaN. Then, a barrier film made of
dielectric such SiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, or of a
metal is then deposited on the previously grown semiconductor layer. The
barrier film is then covered with photoresist and a pattern is applied
through conventional photolithography to achieve exposed and covered areas
on the barrier film. The exposed areas of the barrier film are then etched
using an etching solution to thereby open seed windows. This is followed
by selective growth of GaN in the longitudinal direction from the seed
windows and, subsequently, lateral epitaxial growth, to form a low defect
epitaxial layer on the barrier layer.
Once a complete low defective GaN layer is formed, a semiconductor device
can be grown on it.
TABLE 1
Optimum growth conditions for
representative semiconductor materials.
GaInN GaN
Growth Pressure 76 76
Growth Temperature (.degree. C.) .about.800 1000
Total H.sub.2 Flow Rates 3 3
TMI (cc/min) 200 --
TEG (cc/min) 120 120
NH.sub.3 (cc/min) .about.3000 .about.3000
Al(CH.sub.3).sub.3 (cc/min) -- --
As(CH.sub.3).sub.3 (l/min.) -- --
Growth Rate (.ANG./min) 30 250
EXAMPLE
Very thin (500 .ANG.) AlN buffer layers were grown on (111) Si substrates
and used as template layers for the subsequent LEO procedure.
Single crystal undoped GaN films were then grown on the AlN buffer layers
on (111) Si substrates. By reducing the thickness of the GaN(down to 2000
.ANG.), it was possible to achieve crack free films.
The GaN layer is masked and etched using a HF solution buffered with
ammonium fluoride and distilled water, thereby opening a seed window.
Selective epitaxial growth of GaN in the longitudinal direction from the
seed windows follows. Lateral epitaxial growth follows to form an
epitaxial layer of GaN.
This structure may be used as a basis for growth of further epitaxial
layers of different compositions or the SiO.sub.2 substrate may be removed
by etching or other process and the defect-free GaN layer may then be used
as a substrate itself.
While the invention has been described with reference to a preferred
embodiment, it will be understood by those skilled in the art that various
changes may be made and equivalents may be substituted for elements
thereof without departing from the scope of the invention. In addition,
many modifications may be made to adapt a particular situation or material
to the teachings of the invention without departing from the essential
scope thereof. Therefore, it is intended that the invention not be limited
to the particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include all
embodiments and equivalents.
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