Hot exciton luminescence in ZnTe/MnTe quantum wells

It has been established experimentally in a number of wide-gap II-IV semiconductors, notably ZnTe, that the recombination spectrum of photon energies at and above the bandgap often consists of well-defined peaks separated by LO phonon energies. The physical issues subject to debate have been (1) the distinction between hot luminescence (HL) and resonance Raman scattering (RRS), and (2) whether the energetically relaxing electron-hole pair maintains an excitonlike character throughout the entire secondary emission process. It is shown how the availability of a ZnTe-based quantum well with advanced experimental methods gives direct insight into this problem. Experiments were carried out on a ZnTe/MnTe single-quantum-well structure with very thin MnTe barrier layers (22 angstrom). Tunneling of electron-hole pairs out of the ZnTe quantum well strongly reduces the normal thermalized PL (photoluminescence) emission, so the entire secondary emission from the quantum well is dominated by hot luminescence. The emission, ranging from the laser excitation to the region of the n = 1 excitonic bandgap, shows how the fifth-order LO-phonon-related emission is strongly resonating at this gap. Its linewidth is considerably broadened compared with the lower orders, and a rapid loss of polarization memory with increasing order is observed. To investigate the question of exciton versus hot-electron and hole character in the secondary emission, the spectra were measured in magnetic fields up to 23 T. The key point is that, apart from small spectral shifts, the main features of the spectrum are preserved, a direct indication of the exciton nature of the process of recombination.