Field effect in amorphous Se-Te-As-Ge semiconductors

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Abstract
The field-effect experiment has been used to study the electronic transport processes in thin amorphous films of Se-Te-As-Ge prepared by flash evaporation of the bulk ingot powder (nominal composition Se12Te49As29Ge10). The field-effect measurements were carried out at room temperature and 343 K as a function of a.c. (≤ 1 kHz) and d.c. fields applied between the gate and the source electrodes. The magnitude of the induced surface charge was kept to less than 4 × 1011 cm−2. All the films investigated were found to be p-type. The d.c. modulated source-drain current was found to decay monotonically with time and no standing field-effect modulation could be observed. The a.c. field-effect data have been analysed by considering a trap-limited drift mobility mechanism due to charged D− and D+ centres (Street and Mott 1975) in the accumulation and depletion regimes respectively. A set of hole (D−) traps 0.25-0.32 eV above the valence-band mobility edge and of approximately 3 × 1019 cm−3 in density have been identified. Furthermore, a set of electron (D+) traps roughly 0.34 eV below the conduction-band mobility edge have also been detected in one of the thin films. A new model has been proposed to explain the observed monotonic decay in conductivity modulation : by considering the potential distribution in the semiconductor in the presence of spatially fixed deep donor-acceptor centres, the final expected modulation has been shown to be very much less than present theories would account for.