Stimulated Emission in Zn(Se,S) Single Crystals

Abstract

Two different types of optically pumped stimulated emission were observed at 1.8 K in the high quality Zn(Se,S) crystals grown by low temperature physical vapour transport. One type, occurring at moderate excitation levels, is ascribed to the recombination of excitons localized because of chemical disorder in the two-anion mixed crystal. The other type, occurring at high excitation levels, is related to the inelastic exciton-exciton scattering. The high quality Zn(Se,S) crystals were grown by low temperature physical vapour transport technique. The photoluminescence was investigated at 1.8 K. The samples were immersed in pumped liquid helium. Two different ways of optical excitation were used: a continuous beam from a 7 mW He-Cd laser (3.8 eV) or pulses (5 ns, 10 Hz) from a frequency tripled Nd:YAG laser (3.49 eV).He-Cd laser was used to obtain the photoluminescence spectra for the low excitation levels. Opposite to the rich luminescence spectra, which we obtained for ZnSe and for the twocation mixed crystals (Zn,Mg)Se, the near-edge spectra of the twoanion crystals were usually limited to one or two lines in the vicinity of the free exciton (FE) energy.As the energy gap and the free exciton energy depend on the composition of a ternary crystal, we measured the reflectivity spectrum in order to determine the position of the free exciton structure. Then, the photoluminescence lines were reliably located with respect to the middle of the FE structure in reflection.A ZnSe 0.88 S 0.12 crystal was chosen to detailed studies. The low-excitation photoluminescence spectra of the samples from this crystal contained mainly one strong line in the near band-edge region. A second line, 5-6 meV below FE, was more or less pronounced in various samples. The second line can be easily identified as belonging to the donor-bound exciton (see for example [1]). The concentration (871)

Technology and Characterization of Single-Crystalline Substrates Made of ZnSe-Based Wide-Gap II-VI Semiconductor Compounds

“…The line at 2783.1 meV (known as I deep 1 ) belongs to the exciton bound to the neutral acceptor consisting of a zinc vacancy and a lithium interstitial [23] (see also [24] and references therein). The broader and smaller peak centred at about 2780 meV is related to the inelastic scattering of two free excitons [25] (and references therein). Both the line at 2783.1 meV and the peak at 2780 meV are accompanied by phonon replicas.…”