The application of mismatched combinations of heteroepitaxial semiconductors has been quite limited due to the presence of high threading dislocation densities. In recent years, great progress has been made toward solving this problem using compliant substrates and lateral epitaxial overgrowth. We have proposed another approach which we call patterned heteroepitaxial processing (PHP), and which involves post-growth patterning and thermal annealing. In this paper we describe the successful application of the PHP technique to the ZnSe/GaAs (001) material system.
Epitaxial layers of ZnSe on GaAs (001) were grown to thicknesses of 2000 - 6000 Å by photoassisted metalorganic vapor phase epitaxy (MOVPE). Following growth, layers were patterned by photolithography and then annealed at elevated temperature under flowing hydrogen. Threading dislocation densities were determined using a bromine/methanol etch followed by microscopic evaluation of the resulting etch pit densities.
We found that as-grown layers contained more than 107 cm-2 threading dislocations. The complete removal of threading dislocations was accomplished by patterning to 70 μm by 70 μm square regions followed by thermal annealing for 30 minutes at temperatures greater than 500°C. Neither post-growth annealing alone nor post-growth patterning alone had a significant effect. By studying the annealing temperature dependence, we have determined that the dislocation removal by PHP is thermally activated. It appears that the maximum dimension for patterned regions in PHP is determined by the annealing temperature rather than an effective range for image forces.
These results show that PHP can be used to completely remove threading dislocations from lattice-relaxed heteroepitaxial layers. In principle this approach should be generally applicable to mismatched heteroepitaxial materials.