Synthesis of aluminum nitride

Solid aluminum nitride, deposited for example, in epitaxial layers, is prepared by reacting aluminum and selenium to form aluminum monoselenide, and transporting the aluminum monoselenide in an inert carrier gas to a heated deposition zone where it is contacted with and reacts with nitrogen to give the aluminum nitride, the carrier gas flushing away elemental selenium also produced in the deposition zone.

The present invention is concerned with a process for the production of 
aluminum nitride, with its adaptation to the production of epitaxial 
crystalline layers suitable for surface acoustic wave (SAW) devices, 
single crystals of aluminum nitride and to such crystalline layers and 
crystals. 
Aluminum nitride is a refractory material. It does not sublime appreciably 
at normal pressures until the temperature is in the range 2000.degree. to 
2450.degree. C. and it also has useful electrical properties notably as a 
thin film insulator and as a piezoelectric material of high 
surface-acoustic-wave (SAW) velocity. 
Several processes are known for the production of aluminum nitride of which 
the most important for the production of material for SAW devices are the 
halide process and the organo-metallic process. 
In the prior art process ammonia and aluminum trichloride react in the gas 
phase in a multi-stage reaction to produce overall solid aluminum nitride 
and gaseous hydrogen chloride. This process has the disadvantage that 
aluminum trichloride is very hygroscopic and difficult to handle and the 
resulting aluminum nitride films tend to be subject to hydrolysis. 
In the prior art organo-metallic process the ammonia is reacted in the 
gaseous phase again in stages with aluminum trimethyl to yield overall 
aluminum nitride solid and gaseous methane. The disadvantage of this 
process is that as simple organo-metallic compounds tend to be unstable 
the final reaction stage tends to be homogeneous resulting in the 
deposition of aluminum nitride powder on the growing layer of aluminum 
nitride. Aluminum trimethyl is also a dangerous material to handle and 
difficult to purify by distillation. 
Despite these problems the organo-metallic process has tended to give the 
most useful results heretofore. 
It is an object of the present invention to provide a process for the 
production of aluminum nitride employing more readily purified starting 
materials and more thermally stable reaction intermediates than the prior 
art processes. 
In accordance with the present invention a process for the production of 
aluminum nitride includes the steps of causing aluminum and selenium to 
react to form aluminum monoselenide, transporting the gaseous aluminum 
monoselenide in an inert carrier gas to a heated deposition zone within 
which it is contacted and reacts with nitrogen gas to form solid aluminum 
nitride and gaseous elemental selenium, the latter being flushed away by 
the inert carrier gas. 
It is believed that as a first step aluminum and selenium react to form the 
expected aluminum tri-selenide Al.sub.2 Se.sub.3 and that further reaction 
takes place in the two stages set forth below: 
EQU Al.sub.2 Se.sub.3 (liq)+4Al(liq).fwdarw.3Al.sub.2 Se(gas) (1) 
EQU Al.sub.2 Se(gas)+N.sub.2 (gas).fwdarw.2AlN(solid)+Se(gas) (2) 
As the gaseous aluminum monoselenide is evolved it is entrapped in the 
inert carrier gas and carried to the heated deposition zone in which the 
reaction set forth in equation (2) takes place. It is to be understood 
that the foregoing explanation is not to be construed as limiting in any 
way upon the scope of the invention. 
Although the reaction will take place with a wide range of relative molar 
proportions of the two reactants aluminum and selenium, it has been found 
that the preferable proportions are two molar proportions of aluminum to 
one molar proportion of selenium. 
For the purposes of the present invention the term "inert gas" means a 
gaseous material which does not react with any of the starting materials, 
products, apparatus construction materials of the process disclosed in the 
specification, and advantageously an inert gas is one of the so called 
noble gases of group VIII of the periodic table, the preferred inert gas 
being helium. 
The relative flow rates of the inert gas and the nitrogen must be carefully 
adjusted to ensure heterogeneous decomposition of the aluminum selenide on 
the substrate for the most efficient operation of the process. In general 
the optimum relative flow rates must be determined by trial and error 
because the exact dimensions of the reaction tube affect the flow patterns 
of the reacting gases and thus the relative flow rates required for 
optimum operation. If the flow rate of the inert gas is too low relative 
to the flow rate of nitrogen there is excessive back diffusion of nitrogen 
to the aluminum selenide source causing a passivation reaction leading to 
slowing of the rate of production of aluminum selenide, which in turn 
results in very low deposition rates of aluminum nitride. If the inert gas 
flow rate is too high the aluminum selenide tends to be carried too fast 
through the heated deposition zone which result again in slow deposition 
rates of aluminum nitride. 
The process of the present invention may be used to produce bulk single 
crystals of aluminum nitride or the aluminum nitride may be deposited as 
epitaxial crystalline layers upon appropriate substrates. For example, 
single crystal aluminum oxide or silicon carbide may be placed in the 
aluminum nitride deposition zone in order to obtain films of aluminum 
nitride upon the substrate. 
It has been found convenient to combine the aluminum/selenium reaction zone 
and the heated deposition zone into a single reaction tube and the 
temperature of the heated reaction zone must be controlled so as to avoid 
deleterious reactions or effects. It must be above about 1500.degree. C. 
because of the need to prevent condensation of aluminum tri-selenide which 
may be formed by the disproportionation of aluminum monoselenide. The 
upper limit is set by the need to avoid chemical reaction between the 
substrate upon which the aluminum nitride is being deposited and one or 
more of the chemical species present before, during or after the 
production of aluminum nitride. For example if aluminum nitride is being 
deposited on alumina the temperature must be kept below about 1600.degree. 
C. in order to prevent the formation of an oxynitride spinel phase. 
In accordance with an aspect of the present invention a process for the 
production of an epitaxial film of aluminum nitride upon an alumina 
substrate includes placing a slice of single crystal aluminum oxide having 
a desired crystallographic surface orientation in the heated aluminum 
nitride deposition zone of a process as hereinbefore defined. Preferably 
the crystallographic orientation of the single crystal slice of aluminum 
oxide is 1100 (hereinafter designated M-plane), and in accordance with a 
preferred aspect of the present invention an epitaxial film of aluminum 
nitride is deposited by the process hereinbefore defined upon a slice of 
single crystal aluminum oxide oriented so that the crystal face which is 
presented has the crystallographic orientation 1100.

Turning now to FIG. 1 the reaction is carried out in a silica tube 10 
having aluminum end caps 11 and 12 and provided with a RF coil 13 by which 
energy is provided to a susceptor 14 by means of which the necessary heat 
is supplied to the reactants. The term "susceptor" is a well-known term of 
art for a material which is heated by generation of eddy currents therein 
when RF radiation from a nearby source falls on it, graphite being a 
commonly used susceptor material. The original solid reactants 15 are 
contained in a carbon boat 16. One end 12 of the silica tube 10 is 
enclosed within a laminar air flow clean station (not shown) so that the 
substrates 17 can be subjected to final cleaning in clean room conditions 
as they pass from the substrate supply 18 to the apparatus. 
A helium supply 19, controlled and purified by conventional means (not 
shown), passes through one aluminum end cap 11, and a nitrogen supply 20, 
also controlled and purified by conventional means (not shown), passes 
through the other aluminum end cap 12, through which exhaust gases are 
withdrawn to waste 21 and collected by conventional means (not shown). 
The process is controlled by monitoring the temperature within the silica 
tube 10 by means of a thermocouple (not shown) and the temperature profile 
along the silica tube can be altered by changing the RF coil in accordance 
with known practice. 
For the best results the starting materials should be of the highest purity 
available. For example, the nitrogen used is generally of electronic 
grade. 
In order to operate the process, aluminum and selenium powders of high 
purity are weighed out in the proportion of two molar parts of aluminum to 
one of selenium, thoroughly mixed mechanically and pressed into pellets in 
a PTFE (polytetrafluoroethylene) lined die. The pellets are placed in the 
carbon boat 16 which is then put in place. The substrates, which are 
single crystal slices of aluminum oxide oriented so that the surface 
displays the 1100 crystallographic orientation produced by standard 
methods, are cleaned in hydrogen peroxide/hydrochloric acid mixtures and 
placed in the apparatus. 
In a typical example of the process a total gas (that is helium plus 
nitrogen) flow rate is about 2 liters per minute of which up to a half, 
that is 1 liter per minute is nitrogen, and the reactor temperature is 
1550.degree. C. Once the reactor reaches the operation temperature the 
reaction is allowed to proceed for a period of 6 hours and deposits a 
layer of epitaxial aluminum nitride of one to three microns thickness. At 
the end of this time the reactor is cooled at a rate of 10.degree. C. per 
minute to prevent damage to the apparatus caused by thermal shock. 
The epitaxial films of aluminum nitride upon aluminum oxide are 
sufficiently smooth to permit the deposition thereon of metal films by 
standard methods of photolithography. 
The epitaxial film of aluminum nitride can have a pattern of conducting 
films deposited on it by conventional photolithographic means to produce a 
SAW device such as the simple uniform interdigital transducer illustrated 
in FIG. 2, which shows an epitaxial film of aluminum nitride 30 carried 
upon a substrate of single crystal aluminum oxide 31, the aluminum oxide 
surface being in the crystallographic orientation 1100. The film of 
aluminum nitride 30 carries a pattern of two interdigital electrodes 32. A 
signal source 33 is connected across one end of the electrodes 32 and a 
signal detector 34 across the other. 
When an alternating voltage is applied to the electrodes 32 by the signal 
source 33 a surface acoustic wave is generated and travels along the 
device in the direction of the arrow 35 and is detected by the signal 
detector 34. The characteristics of the surface wave are determined by the 
size and spacing of the interdigital electrodes 32 and by the frequency of 
the applied signal. By suitable adjustment of these parameters this simple 
device can act as a filter. 
It will of course be realised that the SAW device of FIG. 2 is purely 
illustrative and that many devices well known within the SAW art can be 
produced.