1. Field of the Invention
This invention relates to structures including layers of materials which are not lattice matched but which can be combined in layered structures wherein the final layer is fully relaxed and exhibits sufficiently low defect densities to be useful for device applications, either as a device-bearing layer or as a substrate for a device-bearing layer. More particularly, this invention relates to such structures and to processes for making them, an especially interesting example being a fully relaxed structure including a silicon substrate, and a Si:Ge layer of graded composition formed on the silicon substrate, and having a final composition including up to 100% Ge where the surface region of the SiGe (or Ge) layer can be used for the formation of devices therein or as a substrate layer for additional epitaxy.
2. Description of the Prior Art
In materials science, and particularly in the field of microelectronics, it is often the situation that layered structures are formed. If there is substantial lattice matching between the substrate and the layer to be formed thereon, various thicknesses of layers can be formed which will be fully relaxed and exhibit low defect densities. In this context, low defect densities are defined as those densities which make the layer suitable for use in devices, i.e., suitable for use either as a substrate for a device-containing layer formed thereon, or suitable for the formation of devices therein.
As an example of an epitaxial structure of the type described in the previous paragraph, it is well known to deposit epitaxial layers of either n or p type silicon on an underlying silicon substrate, where the lattice matching is such that varying thicknesses of epitaxial layers can be formed. In contrast with this situation, however, is a situation in which the substrate and the layer to be deposited thereon do not have close lattice matches. In this latter situation, epitaxy can be attempted, but there will be a critical thickness at which the strain that occurs due to the lack of lattice match will cause the nucleation of defects in the growing epitaxial layer. These defects will quickly propagate and multiply so that the resulting epitaxial film contains very high defect levels. If the thickness of the epitaxial layer is less than the critical thickness at which defect nucleation occurs, there can be a very low defect density in the epitaxial layer. However, when the maximum layer thickness must be less than the critical thickness a severe limitation can result. For example, there may be limitations on the types of device which can be fabricated, as well as limitations on the electrical properties that can be obtained.
The critical thickness of an epitaxial layer formed on a substrate to which it is not lattice-matched depends upon the degree of mismatch between the epitaxial layer and the substrate. As the degree of mismatch increases, the critical thickness of the epitaxial layer becomes smaller. For example, Ge has about a 4% mismatch with Si. This means that, when Ge is to be epitaxially formed on Si, only about five or six atomic layers of Ge can be formed before dislocation defects occur. On the other hand, if the epitaxial layer material and the substrate material have only a very small mismatch, the epitaxial layer can be made very thick prior to the formation of dislocation defects therein.
In the formation of structures including non lattice-matched layers, it is well known that misfit dislocations will occur and that threading parts of such dislocations can propagate toward the edges of the growing epitaxial film, as the thickness of the growing film increases. However, there is a limit, termed the "glide distance", over which the threads will move toward the edges of the growing epitaxial film. At temperatures less than 600.degree. C., gliding is slow and the glide distance is exceptionally small, mostly due to the repulsive interaction between intersecting dislocations. This results in numerous threading dislocations being pinned in the final epilayer.
In the prior art it is known that misfit dislocations can be controlled somewhat to reduce their appearance in epitaxial layers that are not lattice matched to the substrate. One such technique is the use of strained super lattice layers that are utilized to bend threading dislocations to the edge of the wafer. This is known as "dislocation filtering", and is described by Matthews et al in J. Crystal Growth, 29, 273 (1975). This technique has been used in GaAs based systems to provide some reduction in the density of threading dislocations, but does not work in SiGe systems. A publication by B. W. Dodson, J. Electrical Materials, 19, 503 (1990) reports the result of this technique to provide a 10.sup.4 reduction in the density of threading dislocations in GaAs based systems. Another article by R. Hull et al, appearing in J. Appl. Phys. 65, 4723 (1989) describes the use of dislocation filtering in SiGe alloy systems. As noted in this article, this filtering technique does not work to provide defect density reduction in SiGe alloy systems.
It is also known in the art to use intermediate layers between two layers which are not lattice matched, in order to provide structures having reduced defect densities. It is further known to form structures in which a layer having a graded composition is utilized between two layers which are not lattice matched. However, in both of these approaches it has not been possible to produce fully relaxed structures wherein the epitaxial layer has sufficiently low defect densities to be suitable for any useful purpose.
B. S. Myerson et al, Appl. Phys. Lett. 53, 2555 (1988) describes the formation of a series of Si:Ge alloys formed by ultra high vacuum chemical vapor deposition (UHVCVD) in which intermediate layers of pure Si are located between each Si:Ge alloy layer. These layers were formed over an initial silicon substrate, with the Ge content being increased during the formation of each Si:Ge layer until the top Si:Ge layer contained about 20% Ge. The alternating layers of pure Si were used as marker layers for later cross-sectional transmission electron microscopy (TEM). Both incommensurate (fully relaxed) structures and commensurate (strained) structures were formed. The defect density in the uppermost layer of the commensurate structures was less than about 10.sup.4 defects/cm.sup.2, a number consistent with what would be expected for strained structures. The defect density of the uppermost layer of the incommensurate structures was not measured, the sensitivity of the instrumentation not allowing measurement to less than about 10.sup.8 defects/cm.sup.2. It was noted in this article that the final structures had defects present in the substrate, but no mention was made of any threading mechanism for movement of dislocation defects to the edges of the wafer. In addition, no mention was made of any criticality with respect to the rate of compositional grading and strain relaxation, and the maximum Ge content in the Si:Ge layers did not exceed 20%.
It is apparent from the prior art that, until the present invention, no technique existed for the production of fully relaxed, non lattice-matched systems of arbitrary compositions having reduced defect densities, where the defect densities were reduced by substantial amounts. Further, the prior art does not teach any technique for reduction of defect densities in fully relaxed non lattice-matched systems where the technique can be applied to many different kinds of materials, indeed to a very arbitrary mix of non lattice-matched materials. Still further, the prior art does not illustrate a low temperature technique for the formation of reduced defect, incommensurate layers of diverse lattice parameters where the substrate and the final epitaxial layer can be elements, alloys or compounds, or wherein the non lattice-matched materials can be metallics, semi-conductors, insulators, superconductors, etc.
Accordingly, it is a primary object of the present invention to provide structures and processes for making these structures wherein layers of non lattice-matched materials can be formed having low defect densities in fully relaxed layered structures.
It is another object of the present invention to provide a low temperature process for forming incommensurate layered structures having sufficiently low defect densities to be useful for device applications.
It is another object of the present invention to provide incommensurate layered structures of non lattice-matched materials where the materials can be elements, alloys, or compounds.
It Is another object of the present invention to provide epitaxial layered structures which combine materials of differing lattice constant in a manner to provide low defect densities.
It is another object of the present invention to provide incommensurate layered structures including non lattice-matched epitaxial layers integrating Si and GaAs-based materials.
It is another object of the present invention to provide a structure including a crystalline substrate layer, an intermediate layer of varying composition, and a third layer formed on said intermediate layer and substantially lattice-matched to said intermediate layer, where the third layer has a low defect density and is not lattice-matched to said substrate.
It is a further object of this invention to provide a structure and a process for making the structure where the structure includes a first silicon layer, a compositionally graded Si:Ge layer, and an overlying GaAs-based layer substantially lattice matched to the surface of the intermediate layer, where the GaAs-based layer having a low defect density.
It is a still further object of the present invention to provide a structure and a process for making it where the structure includes a silicon layer and, a graded Si:Ge layer containing up to 100% Ge in the topmost portion of the Si:Ge layer, this topmost portion being suitable as a device layer or as a substrate layer for epitaxy thereon.
It is another object of the present invention to provide a technique for maximizing the glide distance of dislocation defects in incommensurate layered structures of non lattice-matched materials.
It is also an object of the present invention to provide a technique which modifies the nucleation of dislocation defects in a layered epitaxial structure of non lattice-matched materials, and which increases the mobility of threading segments in such a structure. It is an object of the invention to form an incommensurate layer using non-matched lattice systems, including Si and Go, III-V, and II-VI semiconductors.
It is a further object of the invention to provide a technique which allows the provision of one material on another where the lattice matching is such that normally only stressed commensurate films can be produced with low defect densities.
It is a further object of the invention to provide fully relaxed, incommensurate films where the defects in the top layer are significantly reduced or less than or equal to 10.sup.5 threading dislocations per cm.sup.2 over that which would be expected, i.e., 10.sup.12 per cm.sup.2.
It is a further object of the invention to provide incommensurate films having low defect densities in the final or terminal layer followed by epitaxial lattice-matched film thereover, for example, GaAs on Ge on Si.
It is a further object of the invention to provide a semiconductor laser medium having a direct bandgap due to zone folding of the bandgap.