The compound ZnAs 2 is a II-V semiconductor which crystallizes in a monoclinic structure, with the space group P21/C [1]. The progress which has been achieved in the technology for gas phase synthesis of perfect zinc diarsenide crystals, especially the method of directional melt crystallization, has made it possible to carry out detailed studies of various physical properties of this material [2-8]. The optical properties of zinc diarsenide crystals were studied in [3-8]. The direct nature of the band-gap has been established, the intrinsic absorption edge has been used to determine the gap of ZnAs 2 to be 0.92 eV at 300 K [3, 4]. The cathodoluminescence of ZnAs 2 crystals grown from the gas phase was studied in [5]. At low temperatures a number of luminescence bands have been observed in the optical spectrum, due to exciton-impurity complexes. In the present work we have used a high resolution spectral apparatus [9] to study the low temperature photoluminescence of ZnAs 2 crystals grown by directional crystallization from the melt. The luminescence properties of ZnAs 2 produced in this manner have not been studied previously. ZnAs 2 crystals with carrier densities in the range 1015 to 4.1015 cm -3 at 300 K were grown by the vertical Bridgeman technique. Before the luminescence was measured the samples were mechanically polished and chemically etched in a 2:3:1 mixture of HNO3:HCI:H20 to reduce the surface recombination rate for the nonequilibrium charge carriers. In different cases, the luminescence was observed from the natural crystal facets or from fresh cleavage surfaces. The luminescence spectra were recorded for temperatures from 4.2 to 300 K. The nonequilibrium carriers were excited by light from a high pressure xenon lamp, model DKSSh-1000, through a water filter 10 cm thick and Karl-Zeiss CZC-22 and VG-17 filters or by use of a Karl-Zeiss ILA-120 argon laser, with a cw multiline power of 4 W. The spectra were analyzed using an MDR-23 monochromator with a 600 line/mm diffraction grating, providing a dispersion of 2.6 nm/mm. A copper-compensated germanium photomultiplier cooled with liquid nitrogen was used to detect the luminescence. In Fig. 1 we show survey spectra of the photoluminescence for zinc diarsenide crystals, recorded at the temperatures 300, 78, and 4.2 K with spectral resolutions of 2, 1, and 0.5 meV, respectively. The room temperature luminescence exhibits a band with a maximum at 0.955 eV (Fig. la). The band is clearly asymmetric, with a steeper low-energy wing, and its halfwidth is - 38 meV ( - 1.5 kT at 300 K). If we suppose that this band is due to radiative recombination of nonequilibrium carriers via energy states in the allowed bands, then the band gap of ZnAs 2 at 300 K, determined from the maximum of the emission, would be -0.955 eV, which is significantly larger than the band gap (-0.92 eV) obtained from IR absorption spectra [3, 4]. However, analysis of the temperature dependence of the shape and energy displacement of the 0.955 eV band as the temperature is lowered indicates that it is due to optical transition via the energy states in the gap. The luminescence of samples immersed in liquid nitrogen exhibits a number of bands with maxima at 1.042, 1.033, 1.011, and 0.965 eV, or 1.033, 1.027, and 0.965 eV, depending on the structural perfection of the zinc arsenide crystal. The band at 0.955 eV (Fig. la) shifts by 10 meV as the temperature is lowered from 300 to 78 K, and its shape becomes more symmetric. We note that the shape of the luminescence spectra detected at 78 K apparently is determined by the presence of some local detects in the crystals, and by their relative concentrations. As a rule, the spectra fall in two classes (Fig. lb), observed for different crystals. The energy position of the most intense band and the appearance of the 1.011 eV maximum depend on which luminescence centers have the dominant concentration in the crystal. The contributions of different sorts of defects to the spectral shape detected at 78 K is more readily visible when the spectra are observed at 4.2 K. In Fig. lc we show the luminescence spectrum of a ZnAs 2 crystal immersed in liquid helium. A large number of lines are observed in the
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May 1, 2018
A A Pruchkina + 6
