Use of Electron Cyclotron Resonance Plasmas to Prepare CdZnT6 (211 )B Substrates for HgCdTe Molecular Beam Epitaxy

摘要

Without any additional preparation, Cd_1-yZn_yTe (211)B (y ~ 3.5/100) wafers were cleaned by exposure to an electron cyclotron resonance (ECR) Ar/H_2 plasma and used as substrates for HgCdTe molecular beam epitaxy. Anger electron spectra were taken from as-received wafers, conventionally prepared wafers (bromine: methanol etching, followed by heating to 330-340 deg. C), and wafers prepared under a variety of ECR process conditions. Surfaces of as-received wafers contained ~1.5 monolayers of contaminants (oxygen, carbon, and chlorine). Conventionally prepared wafers had ~ 1/4 monolayer of carbon contamination, as well as excess tellurium and/or excess zinc depending on the heating process used. Auger spectra from plasma-treated CdZnTe wafers showed surfaces free from contamination, with the expected stoichiometry. Stoichiometry and surface cleanliness were insensitive to the duration of plasma exposure (2--20 s) and to changes in radio frequency input power (20--100 W). Reflection high energy electron diffraction patterns were streaked indicating microscopically smooth and ordered surfaces. The smoothness of plasma-etched CdZnTe wafers was further confirmed ex situ using interferometric microscopy. Surface roughness values of ~0.4 nm were measured. Characteristics of HgCdTe epilayers depos- ited on wafers prepared with plasma and conventional etching were found to be comparable. For these epilayers, etch pit densities on the order of 10~5 cm~-2 have been achieved. ECR Ar/H_2 plasma cleaning is now utilized at Night Vision and Electronic Sensors Directorate as the baseline CdZnTe surface preparation technique.