Electrical activity of boron and phosphorus in hydrogenated amorphous Silicon

Using realistic models of hydrogenated amorphous silicon and density functional methods, we explore doping and transport with the most popular impurities: boron and phosphorous. We discuss conventional analogies of doping based upon shallow acceptors and donors in a crystalline matrix and highlight the limitations of such an approach. We show that B enters the network always with considerable strain, whereas P is much more "substitutional" in a tetrahedral site. We show that H is attracted to strained centers, especially for B, which increases the likelihood of H passivation effects on B impurities. We elucidate doping and nondoping static configurations in doped a-Si-H, and the role of H passivation as a partial explanation for the well-known low doping efficiency the material exhibits. We show that thermal fluctuations (that induce both network motion and H hopping) can also significantly impact conduction. We draw comparisons to experimental work.