Total-energy and band-structure calculations for the semimagnetic Cd1-xMnxTe semiconductor alloy and its binary constituents.

Abstract

Spin-polarized, self-consistent local-spin density total-energy and band-structure calculations have been performed for CdTe, antiferromagnetic (AF) MnTe in its NiAs structure, ferromagnetic (F) ${\mathrm{CdMnTe}}_{2}$, and the hypothetical zinc-blende phase of MnTe in the F and AF spin arrangements. We find the following: (i) The alloy environment stabilizes a zinc-blende form of MnTe, hitherto unknown to exist in the phase diagram of pure MnTe. Its calculated Mn---Te bond length (2.70\ifmmode\pm\else\textpm\fi{}0.02 A\r{}) is very close to that observed in the alloy (2.73 A\r{}), but is substantially different from the Mn---Te bond length in pure (NiAs-type) MnTe (2.92 A\r{}). (ii) AF zinc-blende MnTe is more stable than F zinc-blende MnTe due to a reduced p-d repulsion in the upper valence states. (iii) F ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te is more stable than its zinc-blende constituents CdTe + F MnTe, hence, once formed, this ordered alloy will not disproportionate. (iv) Nevertheless, AF ${\mathrm{CdMnTe}}_{2}$ is more stable than its ferromagnetic counterpart, but it is unstable relative to its constituents CdTe + AF MnTe. Hence, if F ${\mathrm{CdMnTe}}_{2}$ converts into AF ${\mathrm{CdMnTe}}_{2}$, the latter will disproportionate into antiferromagnetic domains of MnTe. (v) The band structure of F zinc-blende MnTe and F ${\mathrm{CdMnTe}}_{2}$ predicts a novel type of negative (p-d) exchange splitting, whose origins are discussed in terms of a p-d repulsion mechanism. (vi) The calculated electronic states of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te show a vanishing optical bowing, a Mn ${d}_{\ensuremath{\uparrow}}$ band at ${E}_{v}$-2.5 eV and explains the observed optical transitions. (vii) The fact that ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te does exhibit localized multiplet transitions but NiAs-type MnTe does not, is explained in terms of the coexistence of covalency and low symmetry in the latter case.We discuss the electronic structures, local magnetic moments, exchange interaction coefficients, and the general features of the chemical bonds in the semimagnetic semiconductor.