Surface cleaning and etching of CdZnTe and CdTe in H-2/Ar, CH4/H-2/Ar, and CH4/H-2/N-2/Ar electron cyclotron resonance plasmas

This paper compares H-2/Ar, CH4/H-2/Ar, and CH4/H-2/N-2/Ar plasma etch processes for CdZnTe and CdTe substrates in view of their potential to provide high-quality substrate surfaces for subsequent HgCdTe epitaxy. An electron cyclotron resonance source was used as plasma generator, and ellipsometry, angle-resolved x-ray photoelectron spectroscopy and low energy electron diffraction were applied to characterize roughness, composition, and order of the resulting substrate surfaces. It was found that CdZnTe is much more susceptible to evolving surface roughness under H-2/Ar plasma exposure than CdTe. The severe roughening observed at 100 degrees C sample temperature was found to be correlated with a build-up of ZnTe at the surface, which suggests that the roughness formation may result from a preferential etching of the CdTe component. This surface degradation could be reduced by the addition of CH4 to the process gases, However, only a further addition of nitrogen gas balanced and substantially improved the plasma process so that atomically clean, very smooth, and stoichiometrically composed CdZnTe surfaces of long-range order were eventually obtained.