Amorphous silicon

Amorphous silicon ( a-Si) is a non-crystalline allotropic form of the pure semiconductor silicon. The term amorphous ( = unordered) refers to the lack of long-range order of a-Si.

Construction

In the short-range order, it is of crystalline silicon (c- Si) is comparable. The bond lengths and angles soft while in the long-range order further and further from the periodic structure of c -Si, so that no correlation exists in distance and orientation of the Si atoms from the fourth bond length more. This leads to many dangling bonds ( dangling bonds Sheet ) of the silicon. These defects are technically saturated by hydrogen atoms. This results in hydrogenated amorphous silicon (a -Si: H). The preparation usually takes place by plasma enhanced chemical vapor deposition (PECVD) at temperatures below 200 ° C.

Amorphous silicon has a high absorption of electromagnetic waves in the optical and near-infrared spectral range, and can therefore be used in solar cells, with very low film thicknesses. The usual layer thicknesses are smaller than the crystalline silicon by a factor of 100. This compensates for the slight by the defects efficiency of about 6-8 % and makes a-Si economical for applications in the photovoltaic industry interesting.