The influence of annealing in liquid zinc on the photoluminescence spectrum of single crystals of ZnSe

Abstract

In most cases, single crystals of zinc selenide grown without the deliberate introduction of impurities either do not exhibit luminescence at all or show extremely weak luminescence. In order to increase the brightness of the luminescence, ZnSe crystals are annealed in liquid zinc [1]. It is known that this is accompanied by the extraction of Cu, Fe, and other heavy metal impurities from the material of the semiconductor [2]. In addition, the formation of zinc vacancies leads to an increase in the intensity of serfactivated luminescence [1, 3]. At the present time it is possible to find contradictory information on the localization of the serfactivated luminescence bands of zinc selenide. Some authors consider that the serf-activated luminescence band of ZnSe lies in the range 640-650 am [4, 5]. Other workers [3, 6, 7] assign the band with ~max = 600610 am to the serf-activated luminescence. It may be noted that the emission of ZnSe due to copper impurity is also localized in the orange-red range of the spectrum [3, 8], so that the problem of differentiating the self-activated and impurity luminescence in this range of the spectrum remains. We studied single crystals of zinc selenide grown from the melt. In the original state, the specimens did not exhibit appreciable luminescence in the visible range of the spectrum, and showed only an infrared band with a maximum at ~ = 950 nm (curve i in Fig. 1). The crystals were annealed in quartz ampuls evacuated to a pressure of 10 -5 mm Hg and containing a certain quantity of highly pure zinc. When the appropriate temperature was reached, the zinc melted, and the crystals were immersed in the resulting molten metal. The crystals were heated at a melt temperature of 900~ The duration of the treatment could be varied from 1 h to several tens of hours. The measurement of the luminescence spectra of the crystals subjected to annealing in molten zinc made it possible to divide them into two groups, The specimens of the first group (after annealing for a period of 48 h) showed three luminescence bands at a temperature of 103~ with maxima localized in the region of 550, 640, and 950 am (curve 2 in Fig. 1). The specimens of the second group, obtained after annealing for a period of 72 h at the same temperature, were characterized by a single luminescence band localized in the region of 610 am. Subsequently, we paid particular attention to the characteristics of the luminescence band in the orange (~max = 610 nm) and red (Xmax = 640 urn) regions of the spectrum. It was found that the excitation spectra of these bands are different. The radiation with ~max = 610 nm is excited only by light from the region of the characteristic absorption of ZnSe, whereas the band with Xmax = 640 nm is also observed with excitation of the crystal with light from the region of the impurity absorption. It was also shown that the temperature dependence of the intensity of the luminescence in each of these bands, plotted on coordinates In [(I0/D -1] vs l/T, is characterized by two activation energies (Fig. 2).