GROWTH-PROCESS STUDIES BY REFLECTANCE ANISOTROPY SPECTROSCOPY ON MOVPE ZNSE

The growth of ZnSe on GaAs by metal organic vapour phase epitaxy (MOVP E) has been studied using reflectance anisotropy spectroscopy (RAS). T he RA spectra of ZnSe an significantly different for growth on initial ly Se- or Zn-exposed GaAs surfaces. The Se-terminated GaAs (001) RA sp ectrum has Se-dimer-related features at 3.3 and 5.1 eV, and the large, high energy peak dominates during ZnSe growth on this surface. Transm ission electron microscopy (TEM) analysis has been used to show that t hese large RA signals arise from anisotropic surface corrugation of th e growing ZnSe epilayer. Under initially Zn-stabilised growth conditio ns, the ZnSe epilayer RA spectrum is largely featureless, showing only a weak peak at 4.7 eV and a dip at 5.1 eV. The corresponding surface anisotropy is greatly reduced in comparison with growth from the initi ally Se-terminated surface. These observations reveal RAS to be an imp ortant technique for ensuring the desired initial GaAs surface since t he grown ZnSe surface morphology is critically dependent on the pre-gr owth substrate surface treatment. However, as the characteristic ZnSe RA spectra are relatively insensitive to changes in substrate temperat ure and VI-II ratio, RAS is of more limited use as an in-growth surfac e probe for MOVPE-grown ZnSe.