Reduction of CdZnTe substrate defects and relation to epitaxial HgCdTe quality

Abstract

We have conducted annealing experiments on CdZnTe wafers to restore stoichiometry, eliminate or reduce second-phase (Cd or Te) inclusions, and investigate effects on the quality of epitaxial HgCdTe grown on the thermally treated substrates. Two categories of second phase features were revealed in these materials. Category 1 has a star-like shape with sixfold symmetry (as seen by infrared transmission microscopy) and a central core consisting of cadmium. These stars were observed only in the more stoichiometric materials (having good infrared transmission characteristics). Category 2 consists of triangular, hexagonal, and irregular shaped tellurium inclusions which are present in the off-stoichiometry materials (which exhibit strong IR absorption). Substrates were annealed at temperatures ranging from 500 to 700°C for one to seven days, in vapor derived from elemental Cd or Cd_1-xZn_x alloy (x = 0.005). These anneals were able to eliminate the excess IR absorption and decrease the apparent sizes of both categories of second-phase features. It was found that pinhole-like morphological defects on the surface of a HgCdTe layer grown by liquid phase epitaxy can be caused by Cd and Te inclusions located within the CdZnTe substrate near the interface. Additionally, measurement and spatial mapping of copper concentration by sputter initiated resonance ionization spectroscopy showed 10 to 100 times higher Cu concentration in the inclusions than in the surrounding matrix areas.