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±0.02 Å) is very close to that observed in the alloy (2.73 Å), but is substantially different from the Mn—Te bond length in pure (NiAs-type) MnTe (2.92 Å). (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 ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$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 ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Te show a vanishing optical bowing, a Mn $d_{\uparrow }$ band at $E_v$-2.5 eV and explains the observed optical transitions. (vii) The fact that ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$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.