Hydrogen has long been recognized as having an important role in improving the electrical and optical properties of amorphous semiconductors. The period of the middle and late nineteen eighties saw a tremendous amount of work performed on the properties of hydrogen in crystalline semiconductors such as Si, GaAs and related materials. It was soon established that atomic hydrogen passivates the electrical activity of both shallow acceptor and donor dopants in virtually all semiconductors. Since this discovery was preceded by the recognition of hydrogen passivation of deep-level impurities in these same materials, hydrogen is seen to act as an all-purpose impurity or defect passivant. Deep-level passivation is usually more thermally stable than shallow-level passivation, and is usually thought to have more practical application. The microscopic nature of shallow dopant-hydrogen complexes has been the subject of extensive experimental and theoretical investigations, and in most cases a detailed understanding has been developed. Since the microscopic nature of many deep levels themselves is not clear, the mechanisms for deep-level passivation are at a much cruder stage.
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