The Role of Charged Defects In Current Transport Through Hydrogenated Amorphous Silicon Alloys
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ABSTRACT Current transport in metal-semiconductor-metal structures based on amorphous silicon alloys has been studied in relation to the density of dangling bond state defects. The density of defects was changed by varying alloy composition or by current stressing. We show that the change of current-voltage characteristics and activation energy with defect density and the onset of Poole-Frenkel conduction with composition require charged defects. It is found that there are more charged defects in amorphous silicon nitride (a-Sil.xNx:H) than in amorphous silicon carbide (a-Sil.xCx:H). In addition, an excess of negatively charged dangling bond defects compared to positively charged dangling bond defects is observed in a-Sii-xNx:H films. This is attributed to the presence of N4' act as the donor states in silicon nitride. We find that the density of charged dangling bond defects can be higher than 1019 cm 3 . INTRODUCTION Before the concept of charged defects in hydrogenated amorphous silicon (a-Si:H) thin films first proposed by Alder [1], it was assumed that the neutral Do dangling bonds were solely responsible for the light induced degradation or Staebler-Wronski (SW) effect [2] in the material. Charged defects have since been of large scientific interest, both experimentally and theoretically, with the aim of understanding the degradation mechanism. It has been shown by combining the results of electron spin resonance (ESR), light-induced ESR (LESR) and constant photocurrent method (CPM) that, in addition to neutral defects, there are a large amount of charged defect states in a-Si:H and its alloys [3]. The defect pool model [4,5] suggested that there are about four times more charged defects than neutral defects in intrinsic a-Si:H. Fig. 1 shows the distribution of different charged silicon dangling bond states in which a band of positively charged D' defect states is located above midgap, and negatively charged D defect states located below the midgap. This model has been used to explain the large differences in the optoelectronic properties of aSi:H thin films [6,7]. However, the effects of charged defects on electronic properties in amorphous silicon alloys have not been studied in detail. Most of the studies have been limited to a narrow range of amorphous silicon alloys with the N and C content less than 12 at.% and the corresponding band gaps of below 2.0 eV [3,8]. It has been reported that the current-voltage characteristics in metal-semiconductor-metal (MSMI) structures based on amorphous silicon alloys can be changed significantly by current stressing [9,10]. This phenomenon is attributed to the formation of metastable silicon dangling bond defects by the energy released during electron-hole recombination in a manner similar to the SW effect. The defects created by current stressing can be annealed out and re-introduced. A useful feature of this method of introducing defects is that a wide range of defect concentrations can be introduced without altering the strength of the band tails. In this article we
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