Doping mechanism ina-Si:H

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Abstract
A bonding mechanism is developed to account for the unique features of substitutional doping of hydrogenated amorphous silicon (a-Si:H) by trivalent elements. Following Street we argue that doping is produced by dopant–valence-alternation pairs (DVAP’s) consisting of an ionized substitutional dopant and an oppositely charged Si dangling bond. We argue that DVAP’s are stabler than simple substitutional sites because of their diamagnetism, having no unpaired bonding or antibonding electrons. DVAP’s are an excited configuration of the electrically inactive ground state of the dopant in the random network, the threefold-coordinated site, which obeys the 8-N rule and we note that dopants are accommodated substitutionally in crystalline Si because both the DVAP and the threefold site are topologically forbidden. We also show that the DVAP has a negative U, accounting naturally for the saturation of doping efficiency and the strong increase in dangling-bond density found experimentally. We relate the diamagnetism of DVAP’s to the general defect bonding in amorphous semiconductors of groups IV–VI. Modulation doping using superlattices is noted to produce doped a-Si with lower defect density.