Structural quality ofHg1−xCdxTe: Equilibrium point defects

The Phillips–Van Vechten dielectric theory for estimating enthalpies of defect formation and defect concentrations is applied to CdTe and narrow-gap ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Cd}}_{\mathrm{x}}$Te (${\mathrm{Hg}}_{0.8}$ ${\mathrm{Cd}}_{0.2}$Te). It is found that favorable ionization levels in CdTe lead to large charged native defect densities (∼${10}^{18}$/${\mathrm{cm}}^3$; ∼4×${10}^{17}$/${\mathrm{cm}}^3$ net at 1365 K), which can condense into extended defects, such as dislocations and Te inclusions at lower temperatures. Intentional incorporation of small amounts of impurities or use of substitutional alloys, such as ${\mathrm{Cd}}_{0.96}$ ${\mathrm{Zn}}_{0.04}$Te, has often been suggested as a means of improving the quality of CdTe crystals, and addition of Zn or Mn has also been suggested as a means of reducing cation-vacancy concentrations in Hg-rich ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Cd}}_{\mathrm{x}}$Te. It is shown that equilibrium concentrations of native point defects are not reduced substantially by adding small amounts of other elements either to CdTe or to Hg-rich ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Cd}}_{\mathrm{x}}$Te, although macroscopic and nonequilibrium phenomena, such as formation of dislocations, may be significantly affected. Increasing metallicity upon addition of Hg to the alloy and the volatility of elemental Hg lead to preferential loss of Hg from ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Cd}}_{\mathrm{x}}$Te, which must be balanced, in equilibrium, by a larger proportion of Hg in the external phase. The observed strong p-type character of ${\mathrm{Hg}}_{0.8}$ ${\mathrm{Cd}}_{0.2}$Te, in contrast to the strongly compensated character of CdTe at high temperature, results both from this and from highly favorable cation-vacancy ionization levels.