Effect of high pressure on the electrical resistivity of glassy oxygenated selenium

Abstract

The application of pressure is one of the well-known methods for investigating the properties of glassy semiconductors ([1] and references cited therein). It is well established that small quantities of impurities can change the electrical and optical properties of some chalcogenide glasses [2]. In the case of amorphous selenium, the d.c. conductivity increases by six to seven orders of magnitude when small amounts (10 to 200 p.15.m.) of oxygen are added. Simultaneously the thermal activation energy of conductivity is lowered from 1.2 to 1.0 eV [3, 4]. Twaddell et al. [4] found the corresponding changes of the infrared transmission spectra in the range 4 to 25/~m and concluded that the conductivity alteration depended not only on the total concentration of impurity but also on the definite impurity bonding configuration. On the other hand, many other investigators found the conductivity of pure bulk glassy selenium to lie in the range 10-~0 to 10-13~-I cm -1 [5-12]. In this letter, a systematic study of the pressure and temperature dependence of the electrical resistivity of glassy oxygenated selenium is reported. The sample undergoes a continuous pressure-induced semiconductor-to-metal transition. The value of the thermal activation energy obtained for the electronic conduction for the samples agrees very well with the Motrs model [13]. The sample remains amorphous even up to 14GPa pressure, whereas pure glassy selenium crystallizes at 13 GPa pressure with a discontinuous transition to metallic state. Bulk glassy selenium (99.9999% purity) was obtained from Nuclear Fuel Complex, Hyderabad. The oxygenated glass Se + 400 p.p.m. O was prepared (without any special purifying) by incorporating oxygen in the form of SeO2. The mixture of SeO2 and selenium was heated in cleaned and evacuated (10-6 torr) quartz ampoules under argon atmosphere. The molten alloy (at 800 K) was agitated thoroughly in a rotary furnace with a speed of 10 r.p.m, for 2 days. Then the homogeneous glass was obtained by quenching the melt in an ice-water mixture. The glassy nature was confirmed by X-ray diffractometry. High pressure measurements were carried out in a Bridgman anvil apparatus up to 14GPa pressure under quasihydrostatic pressure environment. The details of high pressure apparatus and its calibration were published elsewhere [14, 15]. The electrical resistivity measurements at atmospheric pressure were made in a four-probe configuration using the van der Pauw technique. In calculating the electrical resis