The effect of the electron irradiation on the electrical properties of thin polycrystalline CdSe and CdS layers
Introduction
The AII–BVI semiconductor compounds, particularly CdS, CdSe, CdTe, etc. are of great interest because they are potential candidates in many practical applications like solar cells, optical detectors, dosimeters of ionized radiation, field-effect transistors and optoelectronic devices. For example the heterojunctions, based on CdS or CdSe thin layers [1], [2], [3], are very promising structures for solar cells because of suitable band gap, optical absorption and good stability of the used materials. The development of low-cost solar cells depends on the exploitation of thin films and thus CdS, CdSe or CdTe thin films obtained under various experimental conditions, requires comprehensive electrical characterization. In spite of the considerable amount of work, which has been done on the structural and electrical properties of these kind of films [4], [5], [6], [7], [8], [9], [10], only a few studies on the influence of the ionizing radiations on their electrical properties have been made [11], [15]. That is why in the present paper the effect of electron irradiation on the dark conductivity of polycrystalline CdS and CdSe thin films was investigated.
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Experimental procedures
The sandwich structures like Au/CdS/Au and Au/CdSe/Au, respectively, were prepared by the following procedure. A gold film having the shape of a strip of 8 mm length, 2 mm width and 300 nm thickness was initially deposited on the substrate which was a glass plate maintained at 220°C during the deposition of the semiconducting thin layer. A 25 μm layer of CdS and 24 μm layer of CdSe, respectively, were then deposited on the gold electrode. The powders were vacuum sublimated at a pressure of 10−5 Torr
Structure
Fig. 1 shows the experimental XRD pattern of a CdSe sample. It indicates a mixture of hexagonal-close-packed (h.c.p.) and face-centered-cubic (f.c.c.) phases, which is usual for the systems that present a normal h.c.p. and/or an f.c.c. structure. In the case of CdSe, the h.c.p. structure is the normal one in the system, while the f.c.c. structure can be described simply as the result of the segregation of the stacking-faults developed in the films during the growth process (superstructure
Conclusions
Using measurements of I–V characteristics for different temperatures in the range (150–400 K), before and after irradiation, the effect of high-energy electron irradiation on the electrical properties of CdSe and CdS polycrystalline thin films was investigated.
In the range of low-applied voltages in both non-irradiated and irradiated CdSe and CdS layers the ohmic conduction is observed. The temperature dependence study of the conductivity showed that in the high temperature range the band
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