Process for producing epitaxial semiconductor material layers on monocrystalline substrates via liquid phase shift epitaxy

A process for producing epitaxial semiconductor material layers on monocrystalline substrates via liquid phase shift epitaxy wherein, in order to avoid bead-growth on a substrate, at least the lower region of an epitaxy solution chamber is clad with a substrate material so as to displace the location of the bead growth away from the actual substrate and toward the region of the cladding. This process is useful for producing GaAs-(Ga, Al) As and (Ga, In) (As, P) mixed crystal layers for luminescent diodes and laser diodes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The invention relates to epitaxial layers of semiconductor materials and 
somewhat more particularly to a method and apparatus for producing 
expitaxial layers via liquid phase shift epitaxy. 
2. Prior Art: 
Processes for producing epitaxial layers of semiconductor material, such as 
composed of A.sub.III B.sub.V -compounds, on monocrystalline substrates 
via liquid phase shift epitaxy are known wherein a surface of a substrate 
is contacted with a solution of such compounds and out of which, as a 
result of oversaturation, an epitaxial layer is deposited on the substrate 
surface and the solution is then removed from the substrate and the 
overlying epitaxial layer. 
For example, German Offenlegungsschrift No. 26 41 347 describes a liquid 
phase shift epitaxy process of this type and a device for practicing such 
process. 
Normally, the liquid phase shift epitaxy process involves a strong growth 
at the edges of the epitaxial layer, which is generally referred to as 
"bead-growth". However, when a plurality of layer sequences are produced 
in one operation, certain disadvantages occur because of such bead-growth. 
One disadvantage is that residues of a removed solution remain suspended 
on the beads and during the next application of a solution, which can be 
different from the first removed solution, are drawn into such subsequent 
solution and contaminate it. Another disadvantage is that portions of the 
beads break-off when the solution chamber is passed over the newly 
deposited epitaxial layer and thus mechanically damage such epitaxial 
layer. 
These disadvantages make it nearly impossible to produce quality 
semiconductor components, such as for laser diodes. A further disadvantage 
resulting from the presence of beads is that instances when the substrate 
and overlying epitaxial layer are further processed via planar techniques, 
a lack of definition occurs during focusing in the photographic process. 
A plurality of methods for avoiding bead-growth at the edges of a deposited 
epitaxial layer or substrate are known. For example, German 
Offenlegungsschrift 24 04 017 describes a process wherein, prior to the 
epitaxial process, the edge zones of the substrate are covered with a 
masking layer composed of a heat-resistant material (silicon dioxide or 
silicon nitride). 
Western Electric Technical Digest, No. 46, page 77 describes another means 
of avoiding bead-growth whereby the chambers containing the melt solution 
are additionally heated by heating coils positioned thereabout in order to 
minimize and/or eliminate heat discharge through the graphite chamber 
walls. 
The Journal Cryst. Growth, Vol. 29, (1975), pages 62-64 describes yet 
another means of avoiding bead-growth whereby a so-called step-cooling 
process is used during the epitaxial deposition. 
Finally, the earlier mentioned German Offenlegungsschrift No. 26 41 347 
suggests that bead formation at the substrate edges can be avoided by 
modifying the spatial orientation of the substrate edges, based on the 
fact that the speed of crystal growth at edges is dependent upon the 
spatial orientation of such edges. In this process, substrates are 
selected which are split and sawn in such a manner that the boundary edges 
of the substrate surfaces are not edges at which rapid bead-growth 
typically occurs. 
SUMMARY OF THE INVENTION 
The invention provides an improved method and apparatus for suppressing 
bead-growth at the edges of an epitaxial layer or a substrate during 
liquid phase shift epitaxy. 
In accordance with the principles of the invention, at least the lower 
regions of chambers containing the solution from which an epitaxial layer 
is to be deposited, are clad with a layer of a substrate material so that 
the location of the bead-growth is moved away from the actual substrate 
deposition surface and toward such cladding. In preferred embodiments of 
the invention, the side walls of the solution-receiving chamber in a 
liquid phase shift epitaxy device are clad with the same semiconductor 
material as forming the actual substrate. This principle effectively 
enlarges the substrate and displaces the location of beadgrowth toward the 
semiconductor material cladding on the side walls of the chamber. The 
invention is based on the recognition that bead-growth always occurs at 
the three-phase boundary defined between a solid semiconductor material, a 
melt solution and an external phase. 
The principles of the invention are particularly useful in producing 
epitaxial gallium arsenide layers, gallium aluminium arsenide layers 
and/or (Ga, In) (As, P) mixed crystal layers for luminescent diodes and 
laser diodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For the purposes of promoting and understanding the principles of the 
invention, reference will now be made to an exemplary embodiment of the 
invention, which relates to production of epitaxial gallium arsenide 
layers with the aid of the device illustrated in the drawings. However, it 
will be understood that other epitaxial semiconductor material layers can 
also be produced from the invention, for example from A.sub.III B.sub.V 
-semiconductor compounds. 
An operational liquid phase shift epitaxial device is comprised of a 
housing 1, for example composed of graphite and is provided with a 
plurality of solution-receiving chambers 2 within the housing 1. A carrier 
plate 5, for example composed of graphite, is positioned in a displaceable 
manner along the bottom of each chamber 2. Each plate 5 is provided with a 
recess 5a for receiving a substrate 6 therein for movement with each 
respective plate 5. In the embodiment here under discussion, substrate 6 
is composed of a monocrystalline gallium arsenide wafer. A cladding 3 is 
provided along at least the lower regions of side walls of each chamber 2 
so as to at least contact the carrier plate 5 and preferably the outer 
edge regions of substrate 6 and extend upwardly therefrom along such side 
walls. In the embodiment here under discussion, cladding 3 is composed of 
gallium arsenide. An operative device of this type includes means for 
periodically moving the carrier plate and substrate away from the interior 
of chamber 2 as well as means for maintaining a solution of a 
semiconductor material in each chamber 2. A saturated or slightly 
over-saturated solution 4 composed of, for example molten gallium as a 
solvent and an amount of molten arsenic equal to a few atomic % is 
provided within each chamber 2. The exact composition of solution 4 is 
determined in accordance with solubility diagrams and growth temperatures, 
based on relevant parameters of gallium arsenide and corresponding 
dopants. During operation, the graphite carrier plate 5 is periodically 
displaced within body 1 relative to chamber 2 and the substrate 6 
positioned within recess 5a in plate 5 can also be displaced with the 
graphite plate. When the carrier plate 5 is displaced relative to the 
chamber 2, the variously doped gallium arsenide solution can consecutively 
contact the upper surface of substrate 6 and deposit corresponding 
epitaxial layers on such surface. An advantage of this type of operation 
is that between individual growth layers, the last-grown epitaxial layer 
is not exposed to atmosphere and thus cannot become oxidized. 
The presence of the semiconductive material cladding 3, i.e., gallium 
arsenide, on the side walls of chamber 2 ensures that the strong edge 
growth, which occurs in known processes, is displaced away from substrate 
6 and toward the cladding 3 on regions 7 thereof. In this manner, the 
so-produced epitaxial layers are extremely level, even in instances of 
multi-layer growth and with hetero-structures. 
As is apparent from the foregoing specification, the present invention is 
susceptible of being embodied with various alterations and modifications 
which may differ particularly from those that have been described in the 
preceding specification and description. For this reason, it is to be 
fully understood that all of the foregoing is intended to be merely 
illustrative and is not to be construed or interpreted as being 
restrictive or otherwise limiting of the present invention, excepting as 
it is set forth and defined in the hereto-appended claims.