Note: Descriptions are shown in the official language in which they were submitted.
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-1-
This application is related to Canadian patent
application Serial No. 541,724-4 filed July 9, 1987,
(Shambhu K. Shastry) entitled "Method of Epitaxially
Growing Gallium Arsenide on Silicon" and assigned to the
assignee of the present application.
This invention relates to semiconductor materials.
More particularly, it is concerned with methods of
epitaxially growing a semiconductor material on a
substrate of the same or a different material.
In the fabrication of compound semiconductor
devices and integrated circuits which include compound
semiconductor devices, it is necessary to epitaxially grow
layers of single crystal compound semiconductor materials
directly on substrates of insulating and semiconducting
materials. Metalorganic vapor phase epitaxy (MOVPE)
techniques have been employed for this purpose.
The density of dislocations in the single crystal
structure of III-V compound semiconductor materials is
high compared to silicon. The dislocations are due to
thermally induced stress while the bulk crystal is cooling
from its growth temperature. These dislocations are
present in wafers or substrates produced from the bulk
crystal, and propagate in material epitaxially grown on
the substrates. It has been even more difficult to obtain
satisfactory device grade layers of III-V compound
semiconductor materials grown on substrates of different
materials, for example, silicon and sapphire. Although
some techniques such as introducing strained layer
superlattice structures have been employed to reduce the
dislocation density in epitaxially grown III-V compound
1301(}3S
87-3-255 CN -2-
materials, additional complications are inherent with
these structures.
Accordingly, the present invention provides a method
of epitaxially growing a compound semiconductor material
on a substrate comprising: providing a substrate having
exposed surface areas; introducing sodium ions onto said
surface areas; and metalorganic vapor phase epitaxially
growing single crystal compound semiconductor material on
said surface area of the substrate.
The presence of the sodium ions during the epitaxial
growth process greatly improves the reproducibility of the
process in obtaining low dislocation density, single
domain compound semiconductor layers directly on
insulating and semiconducting substrates.
One embodiment of the invention will now be
described, by way of example, with reference to the
accompanying drawings in which:
FIG. 1 is a profile of the distribution of As, Si, and Na
ions in a sample of MOVPE-grown GaAs on a substrate
of silicon in accordance with the present invention;
and
FIG. 2 are photomicrographs illustrating the surface
morphology of (a) a sample of MOVPE-grown GaAs on a
sodium-treated substrate of silicon in accordance
with the method of the present invention, and (b) a
sample of MOVPE-grown GaAs on a substrate of silicon
not treated with sodium.
For a better understanding of the present invention,
together with other and further objects, advantages, and
capabilities thereof, reference is made to the following
disclosure and appended claims in connection with the
above-described drawings.
The present invention is concerned with the epitaxial
growth of semiconductor materials on substrates of
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essentially single crystal semiconducting or insulating
materials. More specifically, the method is directed to
the MOVPE growth of compound semiconductor materials on
substrates of compound semiconductor materials, silicon,
and A12O3. The compound semiconductor materials of
concern includes III-V compound semiconductor materials
such as GaAs, InP, InAs, InGaAs, GaAlAs, and InGaAsP, and
also combinations thereof which form III-V heterojunction
materials such as GaAlAs/GaAs and InGaAsP/InP. The method
may also be employed for the homo-epitaxial and hetero-
epitaxial growth of II-VI compound semiconductor
materials. Various conductivity type imparting materials
may be introduced into compound semiconductor materials to
establish the desired conductivity characteristics of
regions thereof. Typical conductivity type imparting
materials include silicon, sulphur, tellurium, selenium,
beryllium, zinc, cadmium, and magnesium.
The substrate and epitaxially grown material may be
the same, for example gallium arsenide on gallium
arsenide, or may be different, for example gallium
arsenide on silicon or A12O3 (sapphire). In order for the
epitaxially grown material to be of device quality the
substrate is essentially single crystal material.
Substrates of A12O3 (sapphire or alumina) may have a large
number of discontinuities in their single crystal
structure relative to single crystal silicon and still be
considered as essentially single crystal for the purpose
of serving as a substrate for the growth of hetero-
epitaxial layers of compound semiconductor materials.
In practicing the method of the invention for the
homo-epitaxial or hetero-epitaxial growth of compound
semiconductor materials, a trace amount of sodium (Na)
ions is introduced onto the surface of the substrate which
is to become the interface between the substrate and the
epitaxially grown compound semiconductor material. The
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87~3-255 CN -4-
sodium may be present in the amount of one or two
monolayers. The sodium-treated substrate is processed in
accordance with generally known MOVPE techniques, and
preferably the two-step MOVPE process as disclosed in the
aforementioned application of Shambhu K. Shastry may be
employed.
An effective procedure for treating substrates prior
to the epitaxial growth process in order to provide the
desired amount of sodium ions is to introduce the sodium
by way of the otherwise conventional cleaning solution.
The sodium ions may be in the form of Na, NaCl, NaF, or
NaOH dissolved in the cleaning solution. Sodium ions are
present in the solution in the amount of approximately 1
percent by weight of the solution. More specifically, Na,
NaCl, or NaF is added to the 20% HCl solution which is
conventionally employed to clean gallium arsenide
substrates. Typically, silicon substrates are cleaned
with a 20% solution of HF. In accordance with the present
invention NaF is added to the dilute HF solution. For
cleaning sapphire (A12O3) substrates Na, NaOH, or NaF is
added to the de-ionized water typically used. It is
important that the sodium-based reagent employed be of
high purity, preferably 99.99~ or better. The metallic
impurities which are known to be exceptionally detrimental
to semiconductor devices such as iron, magnesium,
beryllium, etc. must be at a minimum, since these
impurities are electrically active in gallium arsenide.
Additionally, applying an anodic bias of 3 to 5 volts
to the substrate during this treatment enhances the
introduction of sodium ions onto the surface. The
electrochemical potential of sodium ions with respect to a
hydrogen electrode is about -2.718 volts, and thus the
natural tendency for the positive sodium ions would be to
become neutral. The anodic bias of greater than 2.718
volts decreases this natural tendency, thereby increasing
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87-3-255 CN -5-
the concentration of sodium ions on the surface of the
substrate.
After receiving the sodium treatment, a substrate is
placed in an introductory chamber of an MOVPE reactor
which is pumped down and flushed several times with argon.
The substrate is then transferred into the reaction
chamber and placed on a graphite susceptor. The graphite
susceptor is heated by RF energy applied to induction
heating coils encircling the quartz reactor tube.
More specifically, when a a homo-epitaxial layer of
gallium arsenide on gallium arsenide is grown in accor-
dance with the method of the invention, a single crystal
gallium arsenide substrate is treated by immersing in a
20~ HCl solution containing approximately 1% by weight
sodium in the form of Na, NaCl, or NaF. A potential of ~3
volts is applied to the substrate with respect to a
platinum electrode also immersed in the solution.
Treatment is carried on for 10-15 seconds, and then the
substrate is blow dried.
The gallium arsenide substrate is placed in the MOVPE
reactor and the pressure is reduced to 25-50 torr. The
temperature of the substrate is raised and when it reaches
300C, arsine (10~ AsH3 in hydrogen) is introduced at a
rate of 56 standard cubic centimeters per minute (sccm).
The temperature is raised to 600C, and triethylgallium
(2% TEG in hydrogen) is also introduced into the reactor
chamber at a rate of 125 sccm. The vapors containing the
constituent elements are carried into the reactive chamber
with a hydrogen carrier gas flow rate of 5 standard liters
per minute (slm). Under these conditions the gallium
arsenide grows at a rate of about 40 nanometers per minute
and layers of between 10 and 12 microns are grown in four
to five hours. Silicon-doped gallium arsenide layers are
grown by introducing silane ~0.2% SiH4 in hydrogen),
together with the arsine and triethylgallium, while
heating the substrate at a temperature of 650C.
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87-3-255 CN -6-
For hetero-epitaxially growing gallium arsenide on a
substrate of single crystal silicon, the substrate is
first anodically treated at +3 volts for 10-15 seconds
while immersed in a 20% HF solution containing approxi-
mately 1~ by weight of sodium ions provided by the
addition of NaF or NaCl to the solution.
The treated substrate is then placed in an MOVPE
reactor chamber and processed in accordance with the
teachings of the aforementioned application of Shambhu K.
Shastry. The pressure is reduced to between 25 torr and
50 torr, preferably about 40 torr, and the temperature of
the substrate is raised. When the temperature reaches
about 300C, arsine (10% AsH3 in hydrogen) is admitted to
the chamber at a rate of 56 sccm. When the temperature
becomes stabilized between 425C and 450C, preferably at
about 450C, triethylgallium (2% TEG in hydrogen) is
introduced into the reactor chamber at a flow rate of
8 sccm. The ratio of arsenic atoms to gallium atoms
admitted to the reactor chamber is about 300 to 1, and
desirably is not lower than about 200 to 1. The
conditions within the reactor chamber are such that the
growth rate of gallium arsenide on the silicon substrate
is at about 3 nanometers per minute. These conditions are
maintained from about 2 to 3 minutes to produce a seed
layer of about 5 to 10 nanometers thick. The substrate
temperature is raised to a temperature of between 575C
and 650C, preferably to about 600C (650C if the gallium
arsenide is being doped with silicon from silane). The
flow rate of arsine vapor is held the same, and the flow
rate of the triethylgallium-hydrogen mixture is raised to
125 sccm. Under these conditions a gallium arsenide
buffer layer is grown at a rate of about 40 nanometers per
minute. These conditions may be maintained as long as
desired to obtain a layer of gallium arsenide of the
desired thickness.
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The gallium arsenide layers thus grown are of single
domain and have very low dislocation densities, Fig. 1
illustrates the depth profile of Na, Si, and As in a
1 micron thick GaAs epitaxial layer grown on a (100)
silicon substrate as determined by secondary ion mass
spectrometry techniques (SIMS). The sample was produced
in accordance with the two-step MOVPE method as previously
described. As may be noted from Fig. 1, a high Na peak is
observed at the GaAs-Si interface. Fig. 2 is a
photomicrograph showing the surface morphology of samples
of gallium arsenide grown on silicon and subjected to a
preliminary sodium treatment in accordance with the
invention (a) and without the preliminary sodium treatment
(b). The sodium-treated sample produced a mirror-like,
smooth, epitaxial gallium arsenide layer without any
detectable antiphase domains. The layer grown without the
sodium treatment was hazy to the naked eye and consisted
of antiphase domains.
Typically, in their simplest form the antiphase
boundaries in III-V compound semiconductor materials
exist at two adjacent planes of Group III or Group V
atoms. That is, they are electrically charged planes of
imperfections. When epitaxially growing gallium arsenide
on a silicon substrate, after the formation of the initial
monolayer of GaAs, the crystal steps on the silicon
surface effectively cause the formation of gallium
arsenide with differing polarity over the adjacent steps.
As a result antiphase domain boundaries form over the
edges of the steps. It is theorized that in treating the
substrate in accordance with the present invention the
presence of sodium ions at or near the steps probably
reduces polarity reversal situations and thus reduces the
density of antiphase domains. It is also probable that as
sodium is incorporated into the growing epitaxial gallium
arsenide layer, sodium anions move and bond to arsenic
cations at the antiphase boundaries. Relative to sodium,
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87-3-255 CN -8-
~allium in gallium arsenide is anionic, and thus sodium
cations would move and bond to gallium anions. This
action would completely block the movement of the
antiphase boundaries, since one of the rows of the
Group III or Group V atoms is neutralized by sodium ions
in this manner. Since sodium ions are known to be quite
mobile in amorphous media, and in the initial state during
nucleation, gallium arsenide tends to be amorphous due to
the randomness in the nucleation process, sodium ions
could be mobile during this stage of the process.
Dislocations are line defects in the crystal
structure and hence form a subset of antiphase boundaries.
It is probable that the sodium attachment process reduces
or eliminates dislocations in the same manner as it does
the antiphase boundaries. In the initial stages of
gallium arsenide epitaxial growth on silicon or sapphire,
dislocations are formed first, and lateral accumulation of
dislocations then essentially seeds the antiphase
boundary. Thus, if the formation of dislocations in the
initial first or second atomic layers is suppressed,
antiphase boundaries can be considered to be absent
altogether. It is probable that the presence of sodium
ions on the substrate surface accomplishes this purpose.
It is also possible that both the As-Na and Ga-Na bonds
are more ionic than the As-Ga bonds in gallium arsenide.
That is, the sodium ions do not need to place themselves
rigidly in certain angular and spatial positions. This
ionic character of the bonds thus would help relax the
lattice mismatch between the substrate material and the
epitaxial material thereby reducing the dislocation
density at the interface.
Although certain specific combinations of substrates
and epitaxially grown layers have been discussed
hereinabove, the method of the invention can be extended
to other hetero-epitaxial structures. The basic necessity
is to overcome the problem of lattice mismatch and
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%7-3-255 CN -9-
interface charge imbalance problems during the formation
of the first or second monolayer of the epitaxially grown
material. By introducing sodium ions onto the substrate
surface in accordance with the present invention these
problems are removed permitting the epitaxial growth of
compound semiconductor materials with the improved results
as discussed hereinabove.
The electrical quality of epitaxial gallium arsenide
layers grown in accordance with the present invention do
not degrade. Electron mobility of about 8,000cm2/V-s at
300 K, 210,000cm /V-s at 77 K, and peak mobility of
309,000cm /V-s at 42 K have been measured in
homo-epitaxial gallium arsenide layers grown at a tempera-
ture of 650C. In addition, characterization of these
layers using 4.2 K photoluminescense indicates high
optical quality of the epitaxial material.
While there has been shown and described what are
considered preferred embodiments of the present invention,
it will be ohvious to those skilled in the art that
various changes and modifications may be made therein
without departing from the invention as defined by the
appended claims.
