Investigations of ZnSe based laser structures on ZnSe substrates by highresolution x-ray diffraction

Abstract

The structural quality of homoepitaxially grown ZnSe based layers depends sensitively on the ZnSe substrate preparation prior to the growth. In this paper the consequences of an in situ hydrogen plasma cleaning of ZnSe substrates to the crystalline quality of II-VI based laser structures are discussed. The crystalline properties of the resulting laser structures are compared with those of homoepitaxially grown structures on non-hydrogen cleaned substrates as well as with the characteristics of heteroepitaxially grown lasers on GaAs. The samples were grown by molecular beam epitaxy and characterized by high-resolution x-ray diffraction. Remarkable differences in the strain state of the homoepitaxially grown samples are observed. While the structures grown on hydrogen cleaned ZnSe substrates are pseudomorphic, layers of comparable thickness and lattice mismatch on non-hydrogen cleaned substrates show clear strain relaxation. Triple axis (004) rocking curves reveal distinct differences between the three sample types investigated. According to these measurements, the in situ hydrogen plasma cleaning of ZnSe substrates results in a drastic improvement of crystalline perfection of the homoepitaxial laser structures. However, even the defect density of the optimized homoepitaxial samples exceeds that of heteroepitaxial reference lasers by two orders of magnitude as estimated from the diffuse scattered intensity.