SURFACE PREPARATION OF ZNSE SUBSTRATES FOR MBE GROWTH OF II-VI LIGHT EMITTERS

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 Industr ies. The use of ZnSe substrates eliminates many of the problems associ ated 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, conta mination, and defects. We have employed a variety of wet chemical etch es, vacuum anneals, plasma treatments, and characterization techniques such as RHEED, Auger electron spectroscopy, and SEM studies to improv e the ZnSe substrate surface prior to MBE film growth. A combination o f hydrogen plasma exposure and annealing was found to be the most effe ctive way to remove contaminants from ZnSe substrates but less than op timum homoepitaxial quality showed that the surface preparation is mor e complex than simply cleaning the polished surface. Since polishing c an leave residual damage in the form of near-surface defects, the top layer of these substrates was removed by reactive ion etching with BC1 (3). Parameters were chosen such that this etch was homogeneous and sm oothed the ZnSe surface. Etch pit density measurements revealed that t he polish-induced damage to ZnSe extended up to about 5 mu m deep. A d ramatic improvement in the characteristics of blue,green light emittin g devices was observed for devices grown on ZnSe substrates from which this damaged layer had been removed. This surface preparation procedu re has led to the brightest and longest lasting II-VI green LEDs made in the world today.