Glasses and amorphous materials
Amorphous silicon for solar cells and thin-film transistors is made by plasma CVD from silane as a-Si:H containing about 10% hydrogen. What is the main reason?
- AHydrogen acts as an interstitial donor and makes the film n-type even without phosphorus doping
- BHydrogen promotes crystallization of Si and turns the film into high-mobility polycrystalline silicon, improving its properties
- CHydrogen terminates Si dangling bonds and so reduces defect levels in the band gap
Show answer and explanation
Answer: C. Hydrogen terminates Si dangling bonds and so reduces defect levels in the band gap
Amorphous silicon without hydrogen contains as many as about 10¹⁹–10²⁰ cm⁻³ dangling bonds without a partner because of network disorder, and the levels they create near the middle of the band gap trap electrons and holes. As a result, adding dopants barely moves the Fermi level, and carriers generated by light recombine quickly. When hydrogen terminates these with Si-H bonds, the defect density falls to about 10¹⁵–10¹⁶ cm⁻³, making n-type and p-type doping with phosphorus or boron possible. Hydrogen does not act as a donor, and under ordinary deposition conditions the film stays amorphous (silane strongly diluted with hydrogen can also give microcrystalline films, but that serves a different purpose). The Staebler–Wronski effect, in which prolonged illumination increases defects, is also related to this hydrogen.