Surface preparation of ZnSe substrates for MBE growth of II–VI light emitters

Abstract

Abstract This paper describes substrate surface preparation techniques used in the development II–VI light emitting diode and laser diode structures on high-quality, bulk ZnSe substrates supplied by Eagle-Picher Industries. The use of ZnSe substrates eliminates many of the problems associated with lattice mismatch in heteroepitaxy of II–VI light emitters on GaAs substrates. However, defects still form during nucleation of an epitaxial layer on ZnSe substrates because of surface roughness, contamination, and defects. We have employed a variety of wet chemical etches, vacuum anneals, plasma treatments, and characterization techniques such as RHEED, Auger electron spectroscopy, and SEM studies to improve the ZnSe substrate surface prior to MBE film growth. A combination of hydrogen plasma exposure and annealing was found to be the most effective way to remove contaminants from ZnSe substrates but less than optimum homoepitaxial quality showed that the surface preparation is more complex than simply cleaning the polished surface. Since polishing can leave residual damage in the form of near-surface defects, the top layer of these substrates was removed by reactive ion etching with BCl 3 . Parameters were chosen such that this etch was homogeneous and smoothed the ZnSe surface. Etch pit density measurements revealed that the polish-induced damage to ZnSe extended up to about 5 μm deep. A dramatic improvement in the characteristics of blue/green light emitting devices was observed for devices grown on ZnSe substrates from which this damaged layer had been removed. This surface preparation procedure has led to the brightest and longest lasting II–VI green LEDs made in the world today.