The Optical Properties of Noncrystalline Silicon and Si1−xHx Films

Abstract

The optical properties of noncrystalline (NC) silicon and Si1−xHx films are analyzed in terms of single average excitation energy, Eo(Si), and normalized macroscopic' oscillator strength, f(Si), of the Si network; the latter is an indicator of bond orbital delocalization and short range order (SRO) in the NC Si network. In general, the Si network can be viewed as a mixture of four‐fold coordinated local configurations the extremes of which resemble the diamond type (d‐Si) and the high density b.c.c. (b‐Si) polymorphs. Higher deposition temperatures (e.g. chemically deposited “CVD” NC Si films) favor the increased SRO of the b‐Si configuration as evidenced by values of f(Si) which fall close to that characterizing crystalline Si. The NC network resembling d‐Si in vacuum‐deposited or sputtered NC films, on the other hand, yields more localized bonds with reduced SRO and f(Si) values. The effect of hydrogen in Si1−xHx is more complex than simple saturation of dangling bonds. The Si‐H bonds modify the NC Si network so that the Eo(Si) value increases with hydrogen content. Also, the Si network probably develops more SRO as a result of hydrogen incorporation since the f(Si) values are higher (i.e., more crystalline‐like) for Si1−xHx than for vacuum‐deposited or sputtered NC Si films.

Departure of Hydrogen from a-Si:H

“…After having analysed the hydrogen evolution based on a variation of spectroscopic parameters with temperature, we must say that our study is evidently macroscopic and found upon a calculated model which might draw criticism concerning the hypothesis about coordination number and valency of anions [21].…”