Gas-phase process for the production of an epitaxial layer of indum phosphide

A process and an apparatus for epitaxy in a gaseous phase, producing thin and homogeneous layers of monocrystalline indium phosphide. The process comprises two steps. In the first step, the phosphine is decomposed in a pyrolysis chamber which extends through a kiln in accordance with the reaction: ##EQU1## Thereafter, in a second step, the phosphorus is reacted with triethylindium in an atmosphere of hydrogen and nitrogen: ##EQU2## The residual gases are drawn off by a vacuum pump.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The invention relates to a process for obtaining, in a gaseous phase, 
epitaxial layers of monocrystalline indium phosphide for the purpose of 
producing semiconductor devices. 
Description of the Prior Art 
It is known that this type of process, in particular in a reactor, termed a 
"horizontal" reactor, operating at low pressure (76 Torrs), permits the 
obtainment of epitaxial layers of monocrystalline semiconductors having 
excellent qualities for very small thickness, of the order of 2000 
Angstroms, and of relatively large areas up to several square centimeters. 
The suitably doped monocrystalline indium phosphide is particularly 
recommended for producing diodes of the Gunn type of higher power and 
higher efficiency than those obtained with gallium arsenide. Likewise, 
there are advantages in using indium phosphide for very high frequency and 
high gain field effect transistors and for electro-optical devices such as 
laser diodes and photo-diodes. 
However, the obtainment of indium phosphide by epitaxy in a gaseous phase 
encounters a serious difficulty when it is attempted in particular to 
operate in a manner similar to the epitaxy of gallium arsenide. Indeed, if 
there is employed a combination reaction of an organo-metallic compound 
(triethyl indium in the present case) with a hydrogenated compound 
(phosphine in the present case) according to the diagram: 
EQU In (C.sub.2 H.sub.5).sub.3 +PH.sub.3 .fwdarw.In P+C.sub.2 H.sub.6 ( 1) 
Indeed, the following parasitic reaction occurs: 
EQU In (C.sub.2 H.sub.5).sub.3 +PH.sub.3.fwdarw.H.sub.3 P In (C.sub.2 
H.sub.5).sub.3 ( 2) 
Indeed, as the reaction (1) is slower than the reaction (2) and the 
addition compound resulting from the reaction (2) is particularly stable, 
the reaction (1) practically does not occur. The formation of this 
particularly stable compound is explained by the fact that the indium has 
a vacancy of electrons whereas the phosphorus has a nonbinding doublet 
which promotes the formation of an addition compound at the cost of the 
formation of In P. 
The invention aims at overcoming these difficulties. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a process for producing in a 
gaseous phase an epitaxial layer of indium phosphide, comprising the 
following steps: 
(a) formation of phosphorus vapour by pyrolysis of phosphine in a gaseous 
phase at a temperature of 700.degree. C. to 1000.degree. C., according to 
the diagram 
EQU 4 PH.sub.3 .fwdarw.P.sub.4 +6H.sub.2 
)b0 reaction of the phosphorus vapour, in an atmosphere containing nitrogen 
and hydrogen with triethyl indium, in the presence of a substrate of 
monocrystalline In P, according to the diagram: 
EQU 3/2 H.sub.2 +1/4P.sub.4 +In (C.sub.2 H.sub.5).sub.3 .fwdarw.In P+3C.sub.2 
H.sub.6 
at a temperature of 350.degree. to 700.degree. C. 
According to another feature of the invention, the aforementioned steps 
occur at a pressure which is low relative to atmospheric pressure, namely 
between 10 Torrs and atmospheric pressure, and preferably at 76 Torrs or 
in the range of 70 to 80 Torrs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A better understanding of the invention will be had, and other features 
will appear, from the ensuing description with reference to the single 
FIGURE which shows diagrammatically an apparatus for carrying out the 
process according to the invention. 
The successive reactions occur in a gaseous phase, the gases passing in 
succession through the component parts of a reactor 10 (single feature). 
These parts are the following: 
a tubular pyrolysis chamber 11 of glass of the "quartz" type inserted in a 
kiln 14; 
an epitaxy chamber 12 comprising a cylinder, a flared portion closed by a 
removable door 121 and an outlet 122 leading to means 13 for pumping the 
gases. 
The kiln 14 is capable of causing a temperature of 700.degree. C. to 
1000.degree. C. to prevail in the tube 11 which extends through the kiln, 
which causes the decomposition of the phosphine. The optimum temperature 
is about 750.degree. C. 
The cylindrical part of the epitaxy chamber 12 has its axis in the 
extension of the pyrolysis tube 11 which promotes the rapid circulation of 
the gases. This part is surrounded, over a length slightly greater than 
that of the epitaxy zone proper, by a high-frequency induction winding 
100, for example having a frequency of 50 kHz. The electric field thus 
created produces by the effect of eddy currents a substantial heating in a 
dissipator, termed susceptor, 101 formed by a metal plate. The latter is 
carried by a support 102 and inclined in such manner as to decrease the 
angle of incidence of the gases impinging on the substrate placed on the 
suspector. The dimensions of the metal plate are sufficient to enable 
substrates of large area to be placed on the susceptor. The current in the 
winding 100 is set in such manner as to obtain a temperature of 
350.degree. C. to 700.degree. C. in the susceptor 101. The optimum 
temperature of the substrate to be epitaxied is of the order of 
500.degree. C. 
The pumping means comprise a primary vacuum type pump preceded by a 
molecular sieve trap. The capacity of the pump is of the order of 100 
m.sup.3 /hour. 
Connected to the reactor 10 are two gas supply lines namely a supply line 
21 at the entrance of the pyrolysis chamber 11, and consequently opposite 
to the epitaxy chamber 12, and another supply line 22 at the entrance of 
the chamber 12. The supply line 21 receives the phosphine (PH.sub.3) 
through a flow meter 212 provided with a valve 211. 
Converging onto the supply line 22 are: the hydrogen supply line 23, the 
nitrogen supply line 24 and a supply line 25 for a mixture of hydrogen and 
triethyl indium. 
The nitrogen passes through a flow meter 34 provided with a valve 33. 
The hydrogen is supplied on one hand through a flow meter 32 provided with 
a valve 31 connected to the supply line 23, and on the other hand, through 
a flow meter 36 provided with a valve 35 connected to a supply line 26 
immersed to the bottom of a tank containing triethyl indium at the 
temperature of 20.degree. C. from which the supply line 25 extends. 
By way of example, the flows are the following in respect of an epitaxy 
chamber whose cylindrical part has a diameter of 5 cm: 
PH.sub.3 : 0.1 liter/minute; 
H.sub.2 (principal): 5 liters/minute; 
H.sub.2 bubbling in In(C.sub.2 H.sub.5).sub.3 : 1 liter/minute; 
N.sub.2 : 3 liters/minute. 
In general, the flow of P H.sub.3 should be 1/10 of the flow of the 
principal hydrogen, the flow of triethyl indium being regulated by a 
saturation of an auxiliary current of hydrogen which should be 1/5 of the 
principal hydrogen flow, and the flow of nitrogen should be 3/5 of the 
principal hydrogen flow. 
If one were content to introduce into the reactor pure hydrogen, phosphine 
and triethyl indium one could not avoid the pollution of the epitaxy by 
the product of the reaction (2), that is to say, triethyl indium on the 
phosphine which escapes from the pyrolysis, since the latter strictly does 
not reach an efficiency of 100%. 
A successful attempt has been made to slow down this troublesome reaction 
by mixing nitrogen with the hydrogen so as to decrease the partial 
pressures of the other gases and increase their speed of passage on the 
substrate to be epitaxied. 
For a flow of nitrogen at 30 to 70% of the total flow of nitrogen and 
hydrogen (the partial pressure of PH.sub.3 and In (C.sub.2 H.sub.5).sub.3 
being negligible), there is observed a practically complete absence of 
pollution of the epitaxy. The nitrogen may be replaced by any other inert 
gas. 
The results are particularly good when the mean pressure of the gases in 
the reactor is of the order of 76 Torrs. At this total pressure, the 
partial pressure of the phosphorus vapour is sufficient to guarantee a 
good yield of InP, without reaching values for which the gases circulate 
too slowly. At pressures close to atmospheric pressure, there is indeed 
produced nucleations in the gaseous phase which results in the formation 
of smoke and pollution of the epitaxial layers. 
If the process according to the invention is compared with more 
conventional epitaxy methods in a liquid phase (using indium and 
phosphorous) or in a gaseous phase (using indium and P Cl.sub.5), the 
following advantages are observed: 
As concerns the quality of the layer, the transition between the substrate 
and the epitaxial layer is more sudden and the same is true if a plurality 
of epitaxial layers are produced with different dopings. 
As concerns the dimensions of the surface areas obtained, the process 
according to the invention lends itself well to the obtainment of large 
areas. Indeed, epitaxy in a liquid phase is limited to very small areas in 
crucibles employed with kilns of usual dimensions and these areas could 
not be increased without the use of very large and expensive kilns. As 
concerns epitaxy in a vapour phase by the halide method, it is produced 
only in a well-defined zone where the temperature gradient has a definite 
value, which results in a limitation of the area of the epitaxied layers. 
In contrast to this, in the process according to the invention, the 
epitaxy can occur throughout the length and throughout the width of the 
tubular part of the epitaxy chamber, the conditions of pressure and flow 
of the gases being achieved simultaneously throughout this length.