Absorption and photoluminescence studies of lightly alloyed CdTe(S) and CdS(Te)
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Absorption and photoluminescence studies of lightly alloyed CdTe(S) and CdS(Te). Diana Shvydka, A.D. Compaan, and K.J. Price The University of Toledo, Dept. of Physics and Astronomy, Toledo, OH 43606, USA ABSTRACT CdTe-rich and CdS-rich alloy films were deposited at temperatures less than 300C using an rf sputtering technique. Transmission-reflection and photoluminescence measurements were conducted to study the near-band edge properties of this ternary system for films of different compositions at room temperature and at 10K. Results were compared to data on defect states introduced in CdTe single crystal by annealing at several different overpressure conditions, including CdS and CdCl2 overpressures. A below-band-gap photoluminescence excitation technique was used to study systematically the effect of different steps in the production of the complete solar cell on band gap states of CdSxTe1-x alloys as well as pure binary constituents. INTRODUCTION Formation of a CdSxTe1-x mixed crystal layer at the CdS/CdTe interface of thin-film solar cells based on CdTe is an unavoidable result of post-growth CdCl2 heat treatment which is crucial to cell performance. Due to the large miscibility gap which prevents formation of alloys with concentrations in range of x between about 0.06 and 0.97 at 415 C, it is very important to study the optical properties of light alloys in the miscibility range. The presence of defect states in CdTe as well as CdS is important for electrical properties of solar cells, since these defects can control the minority-carrier lifetime. EXPERIMENTAL For this work a set of thin (~2 µm) film samples was prepared by rf sputtering from both comercial and cold-pressed targets made from premixed powders of CdS and CdTe. Details of film preparation are given in [1]. Concentrations of the films were checked by WDS measurements and corresponded to target concentrations within experimental error. Double-side polished CdTe single crystals were obtained commercially. All CdCl2 heat treatments were done at 387C for 30 min in a flow of dry air. Absorption measurements were performed at 10 K using a tungsten halogen white light source and photomultiplier coupled with triple-grating spectrometer. Kr (752 nm), Ar (458 nm) and tunable diode (770-790 nm) lasers were used for photoluminescence (PL) studies at room temperature and 10 K. PL spectra were recorded with the CCD detector. CdTe-RICH ALLOYS Absorption and PL data on polycrystalline thin films Figure 1 shows absorption coefficient data for pure CdTe (x = 0) and the CdSxTe1-x alloy of concentration x = 0.04 measured at 10 K. In both cases, the film quality improves substantially with CdCl2 treatment, so that the absorption edge sharpens and the free exciton peak can clearly H6.2.1
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