Field-induced exchange effects in (Cd,Mn)Te and (Cd,Mn)Se from photoluminescence measurements.

Abstract

Photoluminescence measurements are made on the semimagnetic semiconductors ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Se for 0<x\ensuremath{\le}0.15. Large energy shifts (\ensuremath{\sim}50 meV) of the exciton recombination peaks and large optical polarizations (\ensuremath{\sim}100%) are produced by applying a magnetic field. This study examines changes in luminescence using magnetic fields up to B=15 T and at temperatures between T=1.5 and 60 K. When viewed along the direction of the field (Faraday geometry), the luminescence due to both exciton recombination and impurity-related transitions becomes circularly polarized. At T=2 K the polarization shows saturation at only 0.5 T.This low saturation field results from the large internal exchange field which produces large spin Zeeman splitting of the bands. We found that the polarization gives a quantitative measure of the exchange effects even when the luminescence features are broader than the splittings. In the Voigt geometry (luminescence observed perpendicular to the magnetic field), the linear polarization gives information on the orbital part of the wave functions of the recombining carriers. Differences in the polarization of the two exciton-related peaks in the (Cd,Mn)Te spectra indicate that one peak arises from an acceptor-bound exciton complex and the second to a simple electron-hole exciton. The latter is attributed to a trapped exciton. With increasing magnetic field, both the (Cd,Mn)Te and (Cd,Mn)Se spectra show splittings of the bound exciton related peaks.At low fields the low-energy feature dominates while at high fields the higher-energy feature is dominant. This is attributed to a field-induced instability of the binding of the exciton to the impurity, resulting from the exchange interaction and Pauli exclusion. At high magnetic fields and low temperature the energy of the exciton peak shows a step in the field-tuning curve. This step reflects a transition of the antiferromagnetically coupled nearest-neighbor manganese ion pair. A value of J/k=-8.4 K is determined for the exchange constant. Finally, the luminescence spectra indicate that samples grown by liquid-phase epitaxy and the traveling-solvent method have much higher purity than Bridgman-grown materials. The manganese concentrations in (Cd,Mn)Te are determined from the energies of the exciton luminescence peaks.