Electronic properties of amorphous siliconand its interface with crystalline silicon

B Bader Theory for charge analysis 77 Bibliography 89 Introduction The wide eeort of the past decade to understand physical properties of the semiconductor heterojunctions has provided many fundamental and applica-tive results 1, 2]. The experience acquired on simple systems like isovalent common anion heterojunctions, e.g. GaAs/AlAs, has been successfully extended to heterovalent systems and also, in part, to metal-semiconductor contacts 92]. Also junctions between materials with diierent crystalline structure have been partially studied but, to our knowledge, the properties between crystalline and amorphous semiconductors is an almost unexplored eld. Stimulated by recent measurements made on c-Si(100)=a-Si 1?x C x : H by the experimental group of Evangelisti in Rome 3, 4, 5], we have faced in this thesis the interesting problem of a junction between amorphous (a-Si) and crystalline silicon (c-Si), which is nowadays a subject of interest. The modern silicon solar cells 6], for instance, are realized with junctions of diierent slabs of amorphous material with diierent doping and/or growing amorphous-crystalline junctions that are employed also in a variety of other devices 7]. In this thesis we aim to contribute to a better understanding of the physics of c-Si/a-Si interfaces, in particular on the band alignment between these two \phases", that the measurements have found quite insensitive to the geometrical microscopic features of the interface. To this aim a preliminary study of the amorphous state itself (we refer to it as \bulk" 2 Introduction amorphous silicon, as opposed to the amorphous slab in the interface), is required and we have performed an accurate analysis of the structural and overall of the electronic properties. We have also studied the hydrogenated amorphous silicon (a-Si:H), which can be deposited from SiH 4 gas in a more controlled structure than the pure a-Si 7]. Moreover it is more relevant for technological applications and it is the material actually interfaced with c-Si. The strategic importance of silicon in the recent development of applied science can not be discussed. We use electronic devices based on silicon 8] in most of the objects of every day life: clock, radio, TV, computers, and only a small percentage of the electronic devices is based on other materials. Probably the most serious limits for crystalline silicon are related to its indirect energy gap that forbids its application for realizing eecient optoelectronic devices. The crystalline momentum conservation in optical inter-band transition , in fact, implies that, in silicon, most …