Method of making a crystalline multilayer structure at two pressures the second one lower than first

A process for fabricating a multilayer crystalline structure of nitrides of metals from group III of periodic table including GaN, AlN and InN is provided. The process includes the steps of heating a group III metal (26) to a temperature T1 under an equilibrium nitrogen pressure while maintaining group III metal nitride stability to form a first crystal layer of the group III metal nitride. Thereafter the method includes the step of forming a second crystal layer (28) of the group III metal nitride by decreasing the nitrogen pressure such that the second crystal layer grows on the first layer with a growth rate slower than the growth rate of the first layer at a temperature T2 not greater than temperature T1. The second layer (28) grows on at least a portion of the first layer at a predetermined thickness under the new nitrogen pressure.

FIELD OF THE INVENTION 
This invention relates to a process of manufacturing crystalline structure 
and more specifically to crystalline multilayer structures based on 
nitrides of group III metals, and manufacturing method thereof. 
BACKGROUND OF THE INVENTION 
Gallium nitride "GaNt" Aluminum nitride "AlN" and Indium nitride "INN" are 
known as semiconductor compounds of large direct energy gaps. As such they 
are important electronic materials. 
AlN, in the form of ceramic substrate, is applied in high power electronic 
applications, because of its high heat conductivity, thermal expansion 
co-efficient close to that of silicon, and good stability at high 
temperatures. 
It has long been known that among the nitrides of group III metals, GaN has 
potentially the best useful properties as a semiconductor device. 
Specifically, GaN has semiconducting properties for temperatures up to 
600.degree. C. as compared to silicon semiconductor with temperature 
stability of up to 120.degree. C. The temperature stability and large 
energy gap of GaN can provide many new high temperature applications for 
electronic products. 
A second important characteristic is that a GaN p-n Junction light emitting 
diode ("LED") emits visible blue light with a wavelength of approximately 
450 nm. GaN has a high efficiency of radiative recombination, and low 
dislocation mobility. The other semiconductors which are known to emit 
light in that band are silicon carbide (SIC) and generally A.sup.II 
B.sup.VI semiconductors such as ZnSe and CdF.sub.2. However, because it is 
an indirect bandgap material, the luminous efficiency of SiC is only about 
0.04 lumen/watt. The A.sup.II B.sup.VI are known to have high defect 
mobilities and dislocation densities, which reduce their useful life and 
the power level at which they can operate. In contrast, it is anticipated 
that LED's made from GaN would have a luminous efficiency of about 0.6 
lumen/watt, and remain extremely stable over time. 
Thus-GaN and other group III metal nitrides are viable candidates for 
applications in short wavelength optoelectronics, blue laser systems, full 
color display systems and high temperature electronics. 
Despite their many advantages, nitrides of group III metals including GaN 
have not been used extensively because of the many difficulties involved 
in growing such nitrides in bulk crystals. Their thermodynamic properties 
preclude the standard techniques for the growth of bulk single crystals, 
appropriate for commercial use. For instance, the high melting temperature 
and high N.sub.2 pressure at melting, of GaN is in the range where the 
compound is unstable and readily dissociates. Due to the high melting 
temperature, the substrate crystals of GaN cannot be obtained by typical 
crystal growing methods like Czochralski or Bridgman growth from the 
stoichiometric melts. 
Because of the difficulties to produce substances of pure crystalline 
nitrides of group III metals, the prior art methods use substrates made of 
materials other than group III nitrides, to develop crystalline nitrides. 
For example, the nitrides of group III metals like gallium nitride, 
aluminum nitride, indium nitride or their alloys are deposited on 
crystalline substrates of different chemical compositions like sapphire or 
silicon carbide, by Molecular Beam Epitaxy ("MBE") or Metal Organic 
Chemical Vapor Deposition ("MOCVD"). 
Specifically atoms of group III metals like gallium and atoms of nitrogen 
are deposited on a single crystalline substrate by causing them to collide 
with the substrate. In such known procedures gallium atoms are provided by 
vaporizing liquid gallium at 1800.degree. C. Nitrogen atoms ere generated 
from a flow of molecular nitrogen exposed to plasma causing its molecules 
to dissociate. It is also possible to apply accelerated positive ions by 
using an electric field for the acceleration to dissociate the nitrogen 
molecules. 
Another prior art method for developing GaN crystal is known as metal 
organic chemical vapor deposition. Accordingly, the gallium nitride is 
deposited on a sapphire substrate, by simultaneously applying two chemical 
reactions: first, decomposing ammonia End second decomposing a 
metalorganic compound, like trimethylgallium, which is a suitable carrier 
of gallium. Gallium obtained from the decomposition of the metalorganic 
compound and the nitrogen derived from ammonia, are deposited on the 
surface of a sapphire substrate and as a result create a two layer 
structure. Using a similar method, aluminum nitride deposited on a 
sapphire substrate has been produced by using trimethylaluminum as a 
source of aluminum. 
Another method for producing gallium nitride crystal is disclosed in the 
Polish Patent No. 127099. The patent discloses a procedure for 
crystallization of gallium nitride from a gas phase by sublimation and 
condensation process under high nitrogen pressure. specifically, according 
to the disclosed method gallium nitride powder sublimates at 
temperatures-exceeding 1000.degree. C., at nitrogen pressure higher than 
1000 bar. Thereafter, gallium nitride condensation occurs on a sapphire 
substrate. The temperature difference between the starting material and 
the substrate would not exceed 500.degree. C. 
The procedures disclosed in prior art are therefore mainly limited to the 
growing of GaN crystal or other group III metal nitride crystals, on a 
different substrate. Such growth procedures are known as heteroepitaxy 
production. The gallium nitride structures obtained by such known 
heteroepitaxy procedures are of low crystalline quality. Their half width 
at half maximum of the X-ray double crystal reflection curve, known as the 
rocking curve, is not lower than 200 arcsec, which is not satisfactory for 
many applications. 
One main reason for the poor quality crystals of the prior art is the 
difference between the lattice constants of the substrates and the 
deposited layers, which causes a strain field in the structure. The large 
lattice mismatch, which is 14% for sapphire and 3.4% for SiC substrate, 
leads to the creation of dislocations, cracking of the layers, island 
growth and the formation of incoherent boundaries between crystalline 
grains. 
Another difficulty with GaN is its failure to maintain a chemical balance 
or stoichiometry. Gallium nitride is not stoichiometric because of the 
high propensity for nitrogen atoms to leave gallium nitride crystals. 
Therefore, stoichiometric nitrides free of nitrogen vacancies are 
difficult to obtain. It is commonly believed that the high concentration 
of nitrogen vacancies is the source of numerous native donor states which 
are responsible for high free electron concentration observed in group 
III-nitride semiconductors. 
Hence there is a need for multilayer high quality group III metal nitride 
crystals and consequently n and p type semiconductors derived from such 
crystals in order to benefit from their potentially important properties. 
OBJECTS AND SUMMARY OF THE INVENTION 
One object of the present invention is to fabricate a crystalline 
multilayer gallium nitride structure. 
Another object of the invention is to fabricate multilayer crystals based 
on nitrides of group III metals or their alloys. 
Yet a further object of the invention is to obtain gallium nitride crystals 
with satisfactory growth and quality which can be used in optoelectronics 
and high temperature electronics. 
A further object of the invention Is to deposit different layers of gallium 
nitride upon a gallium nitride substrate. 
Another object of the invention is to produce P type gallium nitride layer 
to ultimately produce GaN p-n junctions. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows. 
According to the present invention, the foregoing and other objects and 
advantages are attained by a method for fabricating a group III metal 
nitride crystal by homoepitaxial growth. For example in order to achieve a 
GaN crystal growth, a first layer is grown by melting gallium at a 
temperature T1 in the range of 400.degree.-2000.degree. C. and exposing 
the gallium solution to high nitrogen pressure. Instead of nitrogen a 
mixture of gases containing nitrogen may also be used to obtain a first 
crystalline layer during a period of about 1 hour. Then the pressure of 
nitrogen or nitrogen mixture is decreased and a second layer grows at 
temperature T2 not higher than T1 until the second layer of a desired 
thickness is obtained. The decrease in pressure is such that the growth 
rate of the second layer is significantly slower than the growth rate of 
the first layer. Furthermore, the thickness of the second layer is much 
less than the thickness of the first layer. Remarkably, a decrease of 
pressure of about 200 bars or more is usually sufficient to allow the 
growth of a second layer with better crystalline quality than the first 
layer. The second layer has better surface flatness, and lower 
concentration of N-vacancies than the first layer. Thus the resulting 
crystalline structure is of such quality that allows the attainment of 
highly desired industrial applications mentioned above. Typically the 
width of x-ray rocking curve of the second layer is about 20 arcsec and 
the difference between the width of rocking curves for first and second 
layers is about 10 arcsec. The x-ray rocking curve indicates an 
improvement by a factor of 10, over prior art crystalline structures. 
According to another aspect of the invention, once the first layer of GaN 
is formed, its position is changed. Meanwhile, the pressure of nitrogen is 
decreased. The first layer is then subjected to thermal or chemical 
treatment at temperatures higher than 300.degree. C. and, finally, its 
surface is covered by atoms of gallium metals present in the atmosphere or 
present in the flow of nitrogen gas. The atoms of gallium metals can 
emanate from vapors, beam of atoms, metal compounds containing gallium or 
metalorganic compounds containing gallium metal. Consequently a second 
layer of GaN crystal deposits on a previous layer of GaN crystal at a 
significantly slower growth rate than the growth rate of the previous 
layer. 
Once the two layer structure is obtained according to the present invention 
the next layers may be deposited by known methods in the art like chemical 
vapor deposition, molecular beam epitaxy or plasma phase epitaxy. 
Other objects and advantages of the present invention will become readily 
apparent to those skilled in the art from the following detailed 
description, wherein only the preferred embodiments have been shown and 
described.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates a high pressure chamber 10 used to fabricate group III 
metal nitride crystals of the present invention. A boron nitride crucible 
12 for holding group III metals or their alloys is placed in a three zone 
furnace 14, designed for work at high gas pressures of up to 20 kbar. The 
furnace 14 with the crucible 12 is placed in the high pressure chamber 10. 
The furnace 14, consists of three temperature zones 16, 18 and 20 supplied 
by electric currents of different values. The desired pressure is provided 
by adjusting the input pressure to chamber 10 by connecting a gas 
compressor (not shown) to the chamber through a high pressure inlet 22 and 
a vacuum outlet 24. 
According to the invention, a multi-layer group III nitride crystal is made 
in chamber 10. Instead of pure group III metals discussed above, group III 
metal alloys can also be used to attain a crystalline growth. Group III 
metal alloys are any combination of group III metals that result in a 
crystalline growth. Since III-N compounds are fully miscible, where III is 
a group III metal and N is nitrogen, many combinations of such group III 
metals can be used to grow crystalline layers according to the present 
invention. 
A sample 26 of a metal from group III of the periodic table or group III 
metal alloy as defined above is placed in crucible 12. Thereafter the 
crucible is placed in the three zone furnace 14. The furnace with the 
crucible is then placed in the high pressure chamber 10. The crucible is 
placed near zone 16 with temperature Td, and zone 18 with higher 
temperature T.sub.e such that the furnace causes a temperature gradient in 
the metal sample. The chamber is filled with nitrogen gas or a gas mixture 
containing a certain percentage of nitrogen. The metal sample is thus 
exposed to a pressure of nitrogen or partial nitrogen pressure. 
Temperatures T.sub.d and T.sub.e are both above the metal's melting point 
and the nitrogen pressure is such that the metal sample remains in the 
form of a liquid solution. 
During the growth of the first crystalline layer, the pressure of nitrogen 
is high enough to maintain GaN stability for the entire metal sample 
solution which is exposed to heat zones 16 and 18. The first crystal layer 
is grown for a period of about 5 hours. The growth period is discretionary 
and depends on the desired thickness and mechanical strength of the 
crystalline layer. Typically a first layer with a thickness of few 
millimeters is appropriate for many applications. 
Thereafter the pressure of the nitrogen or partial nitrogen pressure in the 
mixture is decreased by about 200 bars or more. With the decrease in 
pressure, the portion of the sample exposed to the warmer zone 18 with 
temperature T.sub.g comes out of GaN stability range and liquid phase 
metal contacts directly with gaseous nitrogen, while the portion of the 
solution exposed to the cooler zone 16 with temperature T.sub.d remains in 
GaN stability range. The second crystal layer grows at temperature T.sub.d 
at a significantly slower growth rate than the first layer until the 
second layer of a desired thickness is obtained. The thickness of the 
second layer is less than the thickness of the first layer and is 
typically around 1/2 micron. Therefore, although the growth rate of the 
second layer is much less than the growth rate of the first layer, the 
growth period necessary to grow a second layer with a desired thickness is 
comparable with and in some instances less than the growth period of the 
first layer. 
According to another embodiment of the invention, as illustrated in FIG. 2, 
after the first crystalline layer is obtained, together with decreasing 
pressure of nitrogen, the first crystalline layer is moved to zone 20 with 
temperature T.sub.i, which is lower than both T.sub.d and T.sub.g. 
Thereafter the first crystalline layer is subjected to a chemical or 
thermal treatment. At lower pressure of nitrogen, the metal solution in 
crucible 12 turns into vapor phase and begins to evaporate towards zone 20 
and in combination with nitrogen flow causes the growth of a second layer 
28 in zone 20 over the first layer. In the alternative, the atoms of group 
III metals can be obtained from vapors, beam of atoms or compounds of 
these metals or from decomposition of metalorganic compound in atmosphere 
or flow of nitrogen or gases containing nitrogen. 
The temperatures T.sub.g and T.sub.d at which the metal is first heated are 
in the range of 400.degree.-2000.degree. C. at a specified pressure of 
nitrogen. The necessary pressure of nitrogen can be determined based on 
the pressure-temperature curve of the group III metal nitride. 
FIG. 3(a) illustrates the pressure-temperature curve of GaN. FIG. 3(b) 
illustrates the pressure-temperature curve of AlN, and FIG. 3(c) 
illustrates the pressure temperature curve of InN. The 
pressure-temperature curves illustrate the minimum required pressure of 
N.sub.2 at different temperatures, under which the compound remains within 
a stability range. As illustrated, the higher the temperature of the 
nitride compound, the higher the pressure required to maintain the 
stability condition. Therefore, the area to the left of the curves 
represents pressure and temperature conditions under which no metal 
nitride stability is achieved and the area to the right of the curves 
represents metal nitride stability conditions. 
Thus for GaN, the desired pressure of N.sub.2 at a specified temperature T 
is higher than the equilibrium pressure P.sub.N2eq (T), according to the 
equilibrium state as illustrated by the pressure-temperature curve of FIG. 
3(a). Furthermore, the desired pressure of N.sub.2 is preferably lower 
than three times the equilibrium pressure P.sub.N2eq (T). At higher 
pressures the quality of the obtained crystal begins to deteriorate. If 
the gas provided in the chamber is not pure nitrogen and only partially 
contains nitrogen, the minimum nitrogen content in the gas mixture is 
preferably about 20% or more. 
For AlN, the pressure of gas is in the range of 200 bar to 10 kbar. This 
pressure range prevents Al evaporation and gas phase reaction, as 
illustrated by the pressure-temperature curve of FIG. 3(b). In the event 
that the gas provided in the chamber only partially contains nitrogen, the 
minimum nitrogen content in the gas mixture is about 1% or more. 
When pure nitrogen is used to develop a multilayer AlN crystalline 
structure, the desired pressure decrease necessary for growing the second 
layer with a sufficiently slow growth rate to develop a high quality 
crystal layer is about 6.4 kbars. In the alternative the temperature 
change is adjusted to decrease the growth rate of the second layer with 
high quality characteristics. Therefore, during pure nitrogen growth of 
AlN crystal the first layer is grown at pressures of 6.5 kbar or more, and 
the second layer is grown at a low pressure of 0.1 kbar and less. 
Finally for InN, the desired pressure of N.sub.2 --similar to GaN--is 
higher than the equilibrium pressure P.sub.N2eq (T), according to the 
equilibrium state as illustrated by the desired pressure-temperature curve 
of FIG. 3(c). Furthermore, the pressure of N.sub.2 is preferably lower 
than three times the equilibrium pressure. At higher pressures the 
obtained crystal begins to deteriorate. 
According to the present invention, the generation of nitrogen vacancies in 
the substrate is avoided due to the pressure growth technique disclosed 
herein. The concentration of free electrons in pressure grown crystals 
depends on growth temperature but also on the growth rate of the crystal. 
In the crystals growing slower, this concentration can be substantially 
reduced. 
According to another embodiment of the present invention, doping of the 
first and the second layer is achieved by the addition of small amounts, 
of around 10%, of other metals er non-metals to the metal sample 26, in 
order to introduce impurities in the growing crystalline layers. Such 
impurities include Zn, Mg, Cd, Si or P. An example of a resulting 
crystalline layer is a ternary system III-X-N, where III is a group III 
metal, X is an impurity and N is nitrogen, with a solidus which contains 
only one solid phase, that is, the nitride doped with the impurity X up to 
1at. %. Group III metal alloys for growing GaN crystalline structure may 
contain any combination of In, Al, Si, Mg, Zn, Ce, Bi, and P. 
Higher order crystalline structures containing more than one impurity can 
also be grown. The partial group III metal having about 10 at. % of 
dopants and its crystallization by methods described above results in a 
doped group III metal nitride crystal and partial compensation of free 
electrons. The resulting impurity content in the crystal is about 0.1 at. 
%. 
For obtaining p-type conductivity it is necessary to reduce N-vacanies 
content. This is achieved by either crystallization of the second layer 
from the vapor phase described above at high N.sub.z pressure, or by 
annealing an n-type crystal doped with acceptors like Mg or Zn, at 
temperatures higher than 1500.degree. C. at high N.sub.2 pressures. 
Three examples for growing a multilayer group III nitride crystal using the 
homoepitaxy growth of the present invention is herein described. It can be 
appreciated by those skilled in the art that the same examples are 
applicable to crystal growths of AlN, InN and their alloys. 
EXAMPLE 1 
During the operation of furnace 14, the nitrogen in the chamber is 
compressed under a pressure P.sub.1 of approximately 10 kbar. The system 
is then heated to reach the conditions for growth of GaN crystals from 
nitrogen solution in the liquid gallium, in a temperature gradient 
illustrated in FIG. 4. Accordingly FIG. 4 illustrates the temperature T as 
a function of position X in the sample of liquid gallium in crucible 12 
during the crystallization process of the first layer. Temperature T.sub.d 
of zone 16 is maintained at 1350.degree. C. and temperature T.sub.g of 
zone 18 is maintained at 1410.degree. C. Under pressure p.sub.1 the 
equilibrium temperature T.sub.r is greater than both temperatures T.sub.d 
and T.sub.e. FIG. 5 illustrates crucible 12 with 2 cm.sup.3 gallium sample 
26 shown in liquid form. FIG. 6 illustrates the concentration of nitrogen 
N, as a function of position X in the sample of liquid gallium during the 
crystallization process of the first layer. As illustrated, the 
concentration of nitrogen in the liquid gallium sample increases with the 
increasing temperature. 
As mentioned above, the process is carried out at conditions where GaN is 
stable in the entire temperature range. Therefore, the highest temperature 
of the sample, 1410.degree. C., does not exceed the equilibrium 
temperature (Tr) for coexistence of three phases GaN, liquid Ga and 
N.sub.2 gas, corresponding to the nitrogen pressure of 10 kbar. In these 
conditions the surface of the liquid gallium begins to be covered by a 
thin GaN crystalline layer. Due to the temperature gradient in the system, 
nitrogen dissolved in the warmer part of the crucible is transported, by 
diffusion and convection, to the cooler part where GaN crystals in the 
form of single crystalline hexagonal platelet grow from the supersaturated 
solution as a first substrate layer. In an 8 hour process the crystal 
reaches the dimensions of 0.5.times.2.times.2 mm. The next step according 
to the present invention is the homoepitaxial growth of a second 
crystalline layer at a growth rate slower than the growth rate of the 
first layer, in a lower supersaturation controlled by the change of 
pressure and temperature of the process. Thus, the pressure in the system 
is decreased by 1000 bar which changes the distribution of concentration 
of nitrogen in the liquid gallium 26 of FIG. 8, based on the curve 
illustrated in FIG. 9. The equilibrium temperature for pressure of 9000 
bar is between the temperatures of the warmer and the cooler parts of the 
crucible as illustrated by FIG. 7. Under this condition, as illustrated by 
FIG. 9, in the warmer part of the crucible, gallium nitride is not stable 
and the liquid phase gallium has a direct contact with gaseous phase 
nitrogen. 
The solubility of the gas in Ga, in contrast to the solubility of GaN, is a 
decreasing function of temperature. The chemical potential of gas, at 
constant pressure, decreases with temperature due to rapidly decreasing 
density. Similarly for the same temperature as the pressure decreases the 
solubility of nitrogen in Ga also decreases. The change in temperature 
dependence of nitrogen concentration in the solution leads to the lowering 
of the supersaturation in the growth region of the solution. At the 
conditions of this example, the average growth rate of the layer is of 
order of 10.sup.-3 mm/h. The width of the rocking curve for the layer 
deposited on GaN crystal is typically 20-24 arcsec. The lowering of the 
supersaturation and the slower growth rate provides for the growth of a 
better quality crystal. 
Furthermore, during the growth of the second layer, the part of the gallium 
sample with temperature T.sub.g above the equilibrium temperature T.sub.r 
is not covered by the GaN surface crust. The second layer grown at these 
conditions has better qualities than the first substrate layer. It can be 
appreciated by those skilled in the art that the decrease of pressure in 
the second step should be such that the equilibrium temperature remains 
between the temperatures of zone 16 and 18 of the furnace. Otherwise, no 
stable region in the sample remains and the GaN crystal can readily 
decompose. 
EXAMPLE 2 
FIG. 2 illustrates the second embodiment of the invention. The process of 
growth of the first gallium nitride layer, which is the substrate crystal 
in the form of the hexagonal plate, is carried out as explained above in 
reference to FIG. 1, at a nitrogen pressure of approximately 10 kbar, in a 
temperature gradient provided by zones 16 and 18, during an 8 hour 
crystallization process, until GaN crystal with dimensions of 
0.5.times.2.times.2 mm is obtained. In the next step of the process, the 
crystal is displaced to temperature zone 20 in the furnace, where its 
temperature is approximately 1250.degree. C. Simultaneously, the pressure 
of nitrogen is decreased by 2000 bar. At lower pressure conditions, the 
substrate crystal is thermodynamically stable, whereas the liquid gallium 
evaporates easily. As illustrated in FIG. 3(a), the temperature 
1410.degree. C. at zone 18 is higher than the equilibrium temperature 
necessary for GaN stability at 8000 bar. Then, Ga vapors are transported 
by convection towards the substrate and deposited on it, reacting with 
nitrogen to form the second layer of GaN. Since the second layer is grown 
in N.sub.2 -rich side of the phase diagram, the resulting crystal has low 
concentration of N-vacancies. 
It can be appreciated by those skilled in the art that depending on 
temperature in the decrease of pressure in the second step should be such 
that the new decreased pressure be high enough to prevent decomposition of 
GaN substrate, yet be low enough to allow sufficient evaporation of the 
gallium liquid. Consequently, if the growth of the second layer is 
performed at low temperatures, for example, lower than 
900.degree.-1000.degree. C., the pressure can be decreased to even less 
than 1 bar, since at low temperatures GaN remains in a metastable state. 
It can also be appreciated by those skilled in the art that at lower 
temperatures mentioned above, it is also possible to grow the second 
crystal layer by Molecular beam epitaxy or chemical vapor deposition or 
plasma phase epitaxy methods. 
EXAMPLE 3 
Using growth techniques discussed above a Ga.sub.0.98 In.sub.0.02 N was 
grown from the solution containing 90 at. % Ga and 10 at. % In, at N.sub.2 
pressure of 10 kbar in a temperature range of 1200.degree. C. to 
1300.degree. C. 
Once the two layer structure is fabricated according to the present 
invention, it is possible to add more layers by CVD or MBE processes. This 
enables growth of multilayer structures such as superlattices and 
heterostructures. 
Consequently, the present invention teaches a method to fabricate 
multi-layer crystals of group III metal nitrides, while avoiding the 
disadvantages of prior art fabrication methods. The homoepitaxy growth of 
the present invention provides a good quality crystal with many potential 
applications in optoelectronics and high temperature electronics.