Electrical behaviour of chemically modified amorphous Se studied by xerographic depletion discharge

The extent to which the photoelectronic properties of amorphous chalcogenides can be influenced by specific chemical dopants is a key issue in the study of this technologically important class of semiconductors. The elemental nature and ambipolarity of a-Se make it a particularly attractive candidate for study. The effect on thermal generation and deep trapping in a-Se by halogen and alkali doping has been investigated using xerographic, that is open circuit, techniques. The effect of dopants on magnetic properties has been measured simultaneously by electron spin resonance techniques. It has been demonstrated that in all cases thermal generation is controlled by mono-energetic diamagnetic gap states. In a-Se, thermal generation of free holes is accompanied by simultaneous formation of deeply trapped negative space charge, a process which is progressively enhanced by halogen doping. On the other hand, alkali-metal doping enhances free-electron generation, leaving a deeply trapped positive space charge behind. In this sense halogenated and undoped a-Se are p-type materials whereas alkali-metal doped a-Se becomes an n-type material. Application of a simple model leads to the prediction that the number of thermal generation centres increases from about 10 14 cm −3 in pure a-Se to about 10 18 cm −3 in the most heavily doped specimens. Thermal cycling experiments suggest a doping mechanism in which chemical species interact with structural defects.