Method of relaxing a stressed film by melting an interface layer

The present invention relates to the technological field of manufacturing semiconductor materials for optoelectronic and microelectronic components, and it relates specifically to the method of making stacks of metamorphic layers of materials having lattice mismatches of several percent between one another or relative to the substrate.

FIELD OF THE INVENTION 
The present invention relates to the field of technologies for 
manufacturing semiconductor materials in thin layers for optoelectronic 
and microelectronic components, and it relates specifically to a method of 
making stacks of metamorphic layers of material having lattice mismatches 
of several percent between one another or relative to the substrate. 
BACKGROUND OF THE INVENTION 
The possibility of making stacks of thin layers of semiconductor materials 
that are of different natures and thus of different properties, lies 
behind numerous optoelectronic and microelectronic components, some of 
which are already being produced on an industrial scale. In most cases, 
such structures are built up from materials possessing lattice parameters 
that are equal or very similar (relative difference a few parts per 
thousand). This restriction is associated with a physical limitation that 
is often described as being the critical thickness for a material E of 
parameter a.sub.e grown epitaxially on a substrate material S of parameter 
a.sub.s, and which depends on the relative lattice mismatch (a.sub.e 
-a.sub.s)/a.sub.s between the two materials, and on implementation 
conditions. 
At less than the critical thickness, the material E grows in two 
dimensional and stressed manner: its lattice parameter is subjected 
elastically to tetragonal deformation so as to be equal to that of the 
substrate in the plane of the layers. The layer of material E is said to 
be pseudomorphic. Pseudomorphic materials are in widespread use at 
present. 
Above the critical thickness, either dislocation generation is observed in 
the interface plane, or the two-dimensional layer is observed to transform 
into islands of material. Both phenomena enable material E to relax. 
In both cases, continued growth of material E will give rise to 
dislocations. These dislocations will pass through the thickness of the 
material E to find a free surface at which to terminate. The material E is 
thus degraded. When the material E has returned to its natural lattice 
parameter, the layer is said to be metamorphic. In order to obtain 
metamorphic layers of good quality, two kinds of approach can be found in 
the prior art. 
A first approach consists in using intermediate buffer layers which serve 
to accumulate defects and to preserve the upper layers. 
Such buffers may, for example, be stressed super-lattices (T. Won, S. 
Agarwala and H. Morkoc, Appl. Phys. Lett. 53 (1988), 2311) or graded 
composition layers (G. H. Olsen, M. S. Abrahams, C. J. Buiocchi and T. J. 
Zamerowski, J. Appl. Phys. 46 (1975), 1643, and J. C. Harmand, T. Matsuno 
and K. Inoue, Jap. J. Appl. Phys. 28 (1989), L1101), or indeed layers 
grown epitaxially at very low temperature (T. Ueda, S. Onozawa, M. Akiyama 
and M. Sakuta, J. Cryst. Growth 93 (1988), 517). They are generally more 
than one micron thick. 
One case that has been studied extensively in the prior art is that of 
growing GaAs epitaxially on Si. 
The use of thick buffers presents the drawback of buffer volume, given that 
the purpose of the buffer is to collect structural defects and not to have 
an active role in the operation of the component that is to be made 
subsequently. This "dead" volume greatly limits the possibilities offered 
by associating materials having lattice parameters that are very different 
since it does not enable two materials to be close enough together on 
either side of the buffer. 
A more recent approach consists in using epitaxial adhesive between the two 
materials. 
Each of the layers to be juxtaposed is then grown epitaxially on a 
substrate with a matching lattice. Their surfaces are put into contact 
with pressure being applied, and it is ensured that atomic bonds are 
formed by heating under a controlled atmosphere of hydrogen or nitrogen. 
Advantageous results have been obtained with this technique, with mismatch 
defects remaining confined close to the adhesive interface without 
spreading through the volume of the materials (G. Patriarche, F. Jeannes, 
F. Glas and J. L. Oudar, Proceedings of the 9th Int. Conf. on Microscopy 
of Semiconducting Materials, Oxford, 1995). This technique is presently 
being developed for making vertical cavity semiconductor lasers. Active 
layers that match InP are applied, for example, on mirrors that have been 
grown epitaxially on GaAs (D. I. Babic, K. Streubel, R. P. Mirin, N. M. 
Margalit, J. E. Bowers, E. L. Hu, D. E. Mars, L. Yang, K. Carey, Phys. 
Tech. Lett. (1995)). GaAs has been applied to Si (Y. H Yo, R. Bhat, D. M. 
Hwang, C. Chua and C. H. Lin, Appl. Phys. Lett. 62 (1993), 1038) or indeed 
InP has been applied to Si (K. Mori, K. Tobutome, K. Nishi and S. Sugou, 
Electron. Lett. 56 (1990), 737) using this technique. 
In the context of epitaxial adhesion, work has so far been done on samples 
of very small surface area, of the order of 1 cm.sup.2. There is no 
guarantee that such a method will continue to be effective for larger 
areas. Also, that technique requires two separate epitaxial procedures on 
respective substrates, followed by said adhesion, and selective removal of 
one of the substrates. If it is desired to use such adhesion on a 
plurality of layers, the method becomes cumbersome. Finally, the presence 
of impurities such as O, C, and Si, at high concentration is probable at 
the adhesion interface, since, prior to adhesion, both surfaces are 
covered in a native oxide. 
OBJECTS AND SUMMARY OF THE INVENTION 
The present invention seeks to provide a method of making stacks of 
metamorphic layers of semiconductor materials of III-V, II-VI, or IV type, 
having lattice mismatches of several percent between one another or 
relative to the substrate. The method of the invention makes it possible, 
advantageously, to mitigate the limitations of the techniques known in the 
prior art, such as the use of thick buffers or of epitaxial adhesion, by 
implementing metamorphic stacks made in a single operation, independently 
of the size of the sample, and without degrading the state of the 
material, and in particular without having dislocations passing through 
its volume. The method of the invention enables structural defects to be 
confined in an interface zone that is extremely thin, thus enabling said 
materials to be close to one another. 
The method of the invention implements a material E having a melting point 
T.sub.F.sup.e to be grown epitaxially on a substrate material S having a 
melting point T.sub.F.sup.s via an interface layer, itself constituted by 
a material I. The melting point T.sub.F of the material I is low enough to 
be compatible with avoiding destruction of the materials E and S. In 
particular, the substances InSb, GaSb, CdTe, and Ge having respective 
melting temperatures of 530.degree. C., 712.degree. C., 800.degree. C., 
and 937.degree. C. can be used as interface layer materials. 
The sequence of growth steps in the method of the invention is as follows: 
a) pseudomorphic deposition of material I of lattice parameter a.sub.i on 
the substrate material S of lattice parameter a.sub.s, at a temperature T 
lower than T.sub.F. 
To obtain a pseudomorphic deposit, the thickness of the material I must be 
less than the critical thickness which is a function of the mismatch 
(a.sub.i -a.sub.s)/a.sub.s and of the temperature T at which the deposit 
is made. 
The interface zone constituted by material I is made using epitaxy 
equipment, e.g. equipment making use of molecular beam epitaxy (MBE), and 
it contains a very low concentration of impurities. It is also as thin as 
possible, but it must provide at least one plane essentially constituted 
by atomic bonds specific to the material I constituting it. It comprises 
less than ten monoatomic layers, typically two or three such layers. 
T can be lower than the usual growth temperature in order to enable the 
critical thickness to be greater, should that be necessary. 
b) Pseudomorphic deposition of material E of lattice parameter a.sub.e on 
the material I constituting the interface layer, at a temperature T less 
than T.sub.F. 
The condition for obtaining a pseudomorphic layer is that the thickness of 
E must be less than the critical thickness which is a function of (a.sub.e 
-a.sub.s)/a.sub.s and that the total thickness of both deposits is less 
than the critical thickness which is a function of the mismatches (a.sup.i 
-a.sub.s)/a.sub.s and (a.sub.e -a.sub.s)/a.sub.s that may be cumulative or 
compensate, depending on their signs. 
As during the first deposition, T may be lower than the usual growth 
temperature so that the critical thickness can be greater, should that be 
necessary. 
The thickness of the second layer is of the order of a few monoatomic 
layers, typically fewer than ten. 
The sequence of steps a) and b) may advantageously be repeated, typically 
one to ten times, so as to obtain a periodic stack of layers I and E 
enabling the material S to be encapsulated better thus preventing it from 
being degraded during annealing. 
c) The interface layer is annealed at a temperature which is high enough to 
force relaxation of the layer of material E by generating dislocations 
that are located in I. 
The bonds within the interface layer become weaker. 
The materials E and S are thus decoupled, and the stress applied to the 
film E by the substrate S disappears. The film E relaxes. The lattice 
mismatch is accommodated by atoms being displaced in the interface layer. 
The temperature at which this step is performed can be equal to T.sub.F, 
in which case the interface layer melts. 
Rapid annealing is preferred (less than 1 minute) in order to avoid the 
stack being brought into equilibrium which would run the risk of causing 
atoms to be inter-changed between the various layers, and thus giving rise 
to morphological changes. 
d) The interface layer is cooled. 
While it is resolidifying, the material I of the interface layer conserves 
all of the structural defects that enables it to accommodate the lattice 
parameter difference between the materials E and S. 
e) The material E is grown epitaxially. 
The material E or any other material having the same lattice parameter 
a.sub.e can then be grown epitaxially to arbitrary thickness, greater than 
the critical thickness, without creating new defects. 
This sequence of steps can be repeated an indefinite number of times in 
order to obtain the desired stack of material layers, with the material E 
of a first sequence of steps acting as the material S in a second sequence 
of steps, etc. 
In a preferred implementation of the invention, it is possible to select a 
combination of materials S, I, and E such that: 
EQU a.sub.s &lt;a.sub.i 1) 
and 
EQU a.sub.e &lt;a.sub.i 2) 
which generally leads to the melting temperature T.sub.F of the interface 
layer material I being less than that of the two materials E and S. 
The interface material I can be selected from InSb, GaSb, CdTe, and Ge. 
In particular, it is possible to select a combination of materials S, I, E 
such that: 
EQU a.sub.s &lt;a.sub.e &lt;a.sub.i 3) 
or preferably such that 
EQU a.sub.e &lt;a.sub.s &lt;a.sub.i 4) 
in order to facilitate pseudomorphic deposition of the two materials I and 
E in ultrathin layers on the substrate material S. 
On the basis of condition 4), it is possible, for example, to select the 
following combinations {S, I, E}={GaSb, InSb, InP}, {GaSb, InSb, GaAs}, 
{InAs, InSb, GaAs}, {InP, GaSb, AlAs}, and {Si, Ge, III-AsN}. 
The present invention also provides any material obtained by the 
above-described method and used in making optoelectronic or 
microelectronic devices, e.g. a vertical microcavity laser. 
DESCRIPTION OF A SPECIFIC EXAMPLE 
The following example illustrates the invention without restricting its 
scope.

EXAMPLE 
The stack S/I/E was made using the following materials InP/GaSb/AlAs. In 
that case: 
The layer of interface material GaSb was deposited at a growth rate of 0.1 
to 1 monolayers per second and at a temperature of 300.degree. C. to 
400.degree. C. The thickness of the deposit was 3 monolayers, i.e. about 1 
nm. 
The AlAs material to be grown epitaxially was deposited at a growth rate of 
1 monolayer per second and at a temperature in the range 300.degree. C. to 
400.degree. C. The thickness of AlAs deposit was about 5 nm. 
Melting step: the temperature was raised for a few seconds to 712.degree. 
C. (it could have been a little less). Cooling also took place in a few 
seconds. 
Growth of the AlAs material was continued at 680.degree. C.