ABSTRACT
Existing models for radiative interband recombination, valid only for idealised band structures, are extended to the case of many-valley semiconductors with aspherical surfaces of constant energy. The theoretical lineshape calculations are based on the (combined) density of states of valence and conduction band in a magnetic field and on the occupation of the Landau levels at finite temperatures by free carriers due to doping as well as due to photo-excitation. The models explicitly take into account band gap renormalisation due to many-particle effects, carrier heating by the exciting laser radiation, possible violation of momentum conservation and onset of stimulated emission. The lineshape models are used for the interpretation of photoluminescence spectra of a variety of IV-VI semiconductors. In this paper we exemplary present the results obtained on Pb1_xEuxSe epitaxial films (Eu contents x approximately 4 %) with well-known energy band structures. Photoluminescence was excited by the radiation of a Q-switched Nd:YAG laser at different pump intensities around and well above the threshold for the onset of stimulated emission. Experiments were done at a temperature of 1.7 K in magnetic fields up to 7 T. Increasing excitation intensity shifts the spectra to lower energies, demonstrating that at any pump level a homogeneous nonequilibrium electron hole plasma is observed. Plasma densities up to 5x1017 cm-3 yielded a band gap reduction up to 7 meV. Keeping the excitation intensity constant, the band gap renormalisation is increased proportional to B1/3 by 4 meV in a magnetic field of 4 T. These numerical values, surprisingly high for materials with high dielectric constants and small effective masses, clearly demonstrate that many-particle effects in IV-VI compounds must not be neglected.
