Amorphous silicon
Amorphous silicon (a-Si) is a non-crystalline form of the pure semiconductor silicon . The term amorphous (= disordered) refers to the lack of long-range order of the a-Si. It is an amorphous semiconductor .
Structure, electronic and optical properties
Amorphous silicon is a modification of the element; H. it consists of the same atoms, which are geometrically composed differently. In the short-range order , it is comparable to crystalline silicon (c-Si). The bond lengths and angles deviate more and more from the periodic structure of the c-Si in the long-range order, so that from the fourth bond length there is no longer any correlation in the distance and orientation of the Si atoms. This leads to many unsaturated bonds ( dangling bonds ) of the silicon. The unpaired electrons of these bonds represent localized states in the band gap, which result in the Fermi level being fixed in the center of the band gap. A doping this material does not change the conductor properties of pure amorphous silicon no semiconductor devices can thus be manufactured. Deposition of silicon on a cold, non-crystalline substrate with the incorporation of hydrogen results in these defects being saturated by hydrogen atoms . This creates hydrogenated amorphous silicon (a-Si: H). They are usually manufactured using plasma-assisted chemical vapor deposition (PECVD) at temperatures below 200 ° C. This greatly reduces the density of states of the defects and makes doping of amorphous silicon possible in the first place.
Amorphous silicon has a high absorption capacity of electromagnetic waves in the optical and near-infrared spectral range and can therefore be used in solar cells with particularly small layer thicknesses. The usual layer thicknesses are about a factor of 100 smaller than with crystalline silicon. This compensates for the low efficiency of around 6 to 8% due to the defects and makes a-Si economically interesting for applications in the photovoltaic industry.
Web links
- Crystalline and amorphous silicon in solar technology
- Amorphous silicon for solar cells ( Memento from July 19, 2011 in the Internet Archive ) (PDF file; 326 kB)
literature
- Charles Kittel : Introduction to Solid State Physics . 11th edition. Oldenbourg Wissenschaftsverlag, Munich 1996, ISBN 3-486-23596-6 (Original title: Introduction to Solid State Physics . Translated by Monika Ziegler).
- Peter Y. Yu, Manuel Cardona : Fundamentals of Semiconductors . Physics and Materials Properties. Springer, Berlin, Heidelberg 1996, ISBN 3-540-61461-3 .