The effect of the electron irradiation on the electrical properties of thin polycrystalline CdS layers

The electrical properties of nonirradiated and electron irradiated structures, containing a polycrystalline thin layer of CdS, sandwiched between two gold electrodes, were investigated. The thin films of CdS, obtained through thermal-vacuum evaporation on the glass substrate at a temperature of 220 degrees C, were subjected to two sessions of irradiation with 7 MeV electrons to the fluences of 4 X 10(15) and 6 X 10(15) e/cm(2), respectively. In the case of nonirradiated structures, under low voltages, the Ohm's law is followed with a thermally activated electron concentration of n(0) congruent to 3 x 10(16) cm(-3), an electron mobility mu(0) congruent to 0.1 cm(2)/V s and a room temperature electrical conductivity sigma(0)congruent to 4 x 10(-4) Ohm(-1) cm(-1). In this range of voltage the electron irradiation induces a small increase in the activation energy of mobility, determining, of course, a small decreasing of the mobility. At high-applied voltage, there is a space-charge-limited conductivity controlled by a single trap level having the depth of E-c - E(t)congruent to 0.086 eV and the total trap concentration N(t)congruent to 8.9 x 10(15) cm(-3). In this range of voltage, the electron irradiation modifies the trap distribution. After the first session of irradiation a uniform trap distribution appears and after the second session, an exponential trap distribution was induced in the band gap of CdS layer. All the induced trap distributions are characterized and their effect on the charge transport mechanisms is discussed. (C) 1998 American Institute of Physics. [S0021-8979(98)02014-3]