This invention relates generally to semiconductor devices and more particularly to semiconductor devices having a superlattice region formed on a base substrate.
In thin film heteroepitaxy, the problem of lattice mismatch between the overgrowth and the substrate often poses a major limitation as to the choice of an optimum heteroepitaxial system. This mismatch causes a high density of misfit dislocations to be formed. Where the base substrate is of such a composition that it is impossible to match the lattice parameter of the superlattice with that of the substrate an additional substrate layer with a lattice parameter equal to the average parameter of the superlattice has heretofore been grown by gradually changing the composition from that of the base substrate composition to a composition which matches the average lattice parameters of the multilayer. Such a concept has been shown and described in U.S. Pat. No. 4,088,515 entitled, "Method of Making Semiconductor Superlattices Free of Misfit Dislocations" which issued to A. E. Blakeslee, et al. on May 9, 1978.
Because a relatively large lattice mismatch exists between InAs and GaAs and between GaSb and GaAs, the growth of either InAs-GaSb or In.sub.1-x Ga.sub.x As-GaSb.sub.1-y As.sub.y superlattices onto a GaAs substrate presents a problem for which the continuous grading layer solution taught in the Blakeslee patent does not result in overcoming the mismatch problem. Undesirable carrier concentrations for undoped superlattices in the order of 1.times.10.sup.17 -1.times.10.sup.18 cm..sup.-3 for n-type InAs-GaSb superlattices with mobilities of less than 300 cm..sup.2 /V-sec. and concentrations of 1.times.10.sup.17 -1.times.10.sup.18 cm..sup.-3 for p-type In.sub.0.5 Ga.sub.0.5 As-GaSb.sub.0.5 As.sub.0.5 superlattices have been observed.
It is an object of the present invention, therefore, to provide an improvement in overcoming the lattice mismatch between semiconductor compositions.
It is a further object of the present invention to eliminate or at least to minimize misfit dislocations between a substrate and the superlattice structure grown thereon.
It is yet another object of the present invention to overcome the problem of lattice mismatch between the composition of a superlattice grown on a GaAs substrate.
And still a further object of the present invention is to overcome the mismatch problem of films of InAs and GaSb grown on GaAs substrates.