OPTICAL PROPERTIES OF CdIn2S4 SINGLE CRYSTALS

-The optical absorption of CdIn2S4 single crystals has been measured at several temperatures in the spectral range from 0.4 to 0.8 p. The interband gap for the indirect allowed transition was found to have a temperature coefficient of 9.5 X 10J eV/deg between 100 K and 300 K and to have a value of 2.7 eV when extrapolated to 0 K. From a temperature dependence of the photocurrent corresponding to the knee on the photocurrent-temperature curve, the energy level of the recombination centers (sensitizing centers) was found to be about 0.7 eV above the valence band edge. It was found frommeasurement of thermally stimulated current that there were three kinds of temporary trap levels for the electrons at about 0.1, 0.2 and 0.5 eV below the conduction band. In a study of the current-voltage characteristics, low frequency (0.05 Hz-l0 Hz) oscillations of the photocurrent were observed. Two types of the oscillations were observed i. e., sinusoidal type and pulse type. The frequencies of the oscillations increases with an increase of the light intensity and also they do with an increase of the applied voltage. The oscillations are probably directly associated with exhaustion of the traps, recombination and thermal quenching. Introduction. Recently the ternary compound CdIn2S, is attracting increasing attention. One of the reasons for this interest is the fact that CdIn,S, is a photosensitive semiconductor and has a possibility of use as pl~otoconductor in the visible region of the spectrum. The optical absorption, reflection, the photoconductivity and the photoluminescence have been measured by several workers [l]-[9]. However, the results obtained by these authors were not always in agreement. We have succeeded in growing Iarge and homogeneous single crystals of Cdln2S4 and have measured several transport and optical properties [10]-[13]. The work presented here is the result of more detailed measurements of the optical properties of Cdln2S4 than have been made previously. The result of the study on the low frequency oscillations of the photocurrent in Cdln2S, will also be presented. (*) Department of Electrical Engineering, Faculty of Engineering, Tokyo University of Science, Shinjuku-ku, Tokyo, Japan. 1 . Experimental procedure. The samples were cut out of the single crystal grown from a melt with excess sulphur. Electrical resistivity of the samples in the dark is about 105 Q-cm. Samples for the optical measurements were thin slices with their surfaces polished by emery paper and diamond paste. A Nihon Bunko SS-50 grating monochrometer was employed. The absorption coefficient a and the sample thickness t are connected by the following equation neglecting multiple reflections, I = Io(l R)' exp( at) where I , and I are the incident and the transmitted light intensity, respectively and R is the reflectivity of the sample. A plot of the In ( I / I o ) vs t should give a straight line, the slope of which gives cr if R is constant throughout the measurement. The absorption coefficient was determined from such a plot of the relative transmittance of the sample having various thicknesses ranging from 0.03 mm to 0.2 mm. MeasuArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975330 C3-164 H. NAKANISHI, S. E N D 0 A N D T. JRIE rements below room temperature were made using a cryostat having quartz windows. The photocurrent was recorded as potential drop across the series resistance which was selected to be low enough to insure constant voltage across the sample. 2. Experimental results and analysis. The absorption coefficient X was measured at several temperatures in the spectral range from 0.4 to 0.8 p. For the shorter wave length than 0.4 p the apparatus was not sensitive enough to detect the weak transmitted light. In order to determine the energy gap for the direct transition, or2 is plotted as a function of the photon energy in figure 1. Extrapolating a linear por-