Dependence of energy gap on x and T in Zn1−xMnxSe: The role of exchange interaction

Abstract

Photoluminescence and reflectivity measurements have been carried out on ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Se solid solutions in the complete range of crystal compositions, 0≤x≤0.55. The features of the photoluminescence and reflectivity data near the band edge enabled us to determine values of the fundamental band gap as a function of Mn molar fraction x and temperature (8 K≤T≤300 K). The energy gap in crystals with compositions in the vicinity of the zinc-blende–wurtzite structural transition (0.2≲x≲0.3) exhibited a scatter of values, probably associated with the presence of various polytypes (as evidenced by transmission-electron-microscopy measurements). The dependence of $E_g$ on x is anomalous for 0≤x≤0.2, showing a minimum in the $E_g$ vs x curve. A simple model (in terms of second-order perturbation theory in s-d and p-d interactions) relates this minimum to a maximum observed in the magnetic susceptibility as a function of x. For samples with x≳0.35, the $E_g$ dependence on T shows an onset of an additional blue shift as the temperature is lowered below ∼150 K. This onset, though not reproduced by our simple calculation, appears to be also related to the presence of s-d and p-d interactions.