Defect formation during deposition of undoped a-Si:H by rf glow discharge

Abstract

Dependence of the as-grown defect concentration in undoped a-Si:H deposited by rf glow discharge on the deposition parameters is investigated. It is found that the defect density behaves similarly to the concentration of ${\mathrm{SiH}}_{2}$ configuration in the films deposited at substrate temperatures below 300 \ifmmode^\circ\else\textdegree\fi{}C. The defect concentration varies proportionally to about the third to fourth power of the ${\mathrm{SiH}}_{2}$ concentration depending on the deposition conditions. The Urbach energy increases in samples deposited under the condition where ${\mathrm{SiH}}_{2}$ concentration increases. An additional energy is necessary to convert the weak bond in the valence-band tail into the defect. The observed characteristics are analyzed by a model in which this additional energy is produced by the surface reaction of ${\mathrm{SiH}}_{3}$ incorporating the ${\mathrm{SiH}}_{2}$ configuration into the network. These characteristics are compared with those of the Staebler-Wronski effect. The increase of weak bond density contributes to the increase of defect creation efficiency. The analyses give a quantitative explanation of the observed characteristics and estimation of some parameters involved in the conversion process. Similarities in the mechanisms are pointed out between the as-deposited defect formation and the Staebler-Wronski effect.