Nondestructive analysis of Hg1−xCdxTe (x=0.00, 0.20, 0.29, and 1.00) by spectroscopic ellipsometry. II. Substrate, oxide, and interface properties

Abstract

Properties of Hg1−x CdxTe substrates, their electrochemically grown anodic oxides, and their interfaces are studied under as-grown conditions using the nondestructive analytic capabilities of spectroscopic ellipsometry to avoid ambiguities arising from destructive analytic techniques. Pseudodielectric functions that are the most accurate representations of the intrinsic bulk responses that we are able to achieve are given for each of the materials studied. The dielectric functions of the oxides are calculated from (tan ψ, cos Δ) data in a three-phase model. These oxides are uniform with sharp interfaces at both substrate and ambient. Residual Te layers of thicknesses of the order of 6%–8% of the initial oxide thicknesses are found on the Hg-containing substrates after their native oxides are stripped with HCl. Four-phase model calculations show that these Te layers are not present (±2 Å) in the as-grown condition for the Hg-containing materials and therefore cannot be invoked to explain leakage in integrated-circuit devices. However, a small residual amorphous (a-Te) layer ∼1.5 Å thick between the semiconductor and electrochemically grown anodic oxide is seen for CdTe. The optical data show the oxide on CdTe to be predominantly the tellurite CdTeO3. The composition of the oxide on HgTe cannot be identified uniquely, but it probably contains a substantial fraction of TeO2. A residual absorption tail below the fundamental edge on all oxides is consistent with the presence of ∼3%–4% a-Te in the oxides. The oxides on the alloys can be represented as physical mixtures of those of the endpoints, suggesting an oxidation mechanism and a structure consisting of TeO2 and some Hg tellurites captured in a relatively insoluble matrix of CdTeO3. After correcting for residual Te overlayers, all substrate dielectric functions for a given material are found to be identical to within 1% regardless of sample history, placing upper limits of less than 2% for cationic or anionic outdiffusion in these materials. A similar consistency of broadening parameters implies that the actual limits may be much lower. The properties of the Hg1−x CdxTe compounds studied in this work vary smoothly with x and show no chemical or physical anomalies.