Optimization of the reactive magnetron sputter deposition, and studies of hydrogen diffusion in hydrogenated amorphous silicon based materials.

摘要

The applications of hydrogenated amorphous silicon in photovoltaic devices have been hindered due to two key problems: the light-induced metastable defect creation and poor doping. To improve these properties, I have chosen the reactive magnetron sputtering method for the growth of a-Si:H and its doped alloys. The advantages of this technique are the independent control of hydrogen incorporation via the pressure of H{dollar}sb2{dollar} injected into the plasma, and the energetic nature of the deposition species. The effects of growth parameters on the electrical and microstructural properties have been studied, and the films appear to be optimized.; Earlier studies indicated that the mid-gap defect density of high quality a-Si:H increases from 10{dollar}sp{lcub}15{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar}, and saturates at 9 x 10{dollar}rmsp{lcub}16{rcub} cmsp{lcub}-3{rcub}{dollar} after intense light exposure. Using reactive magnetron sputtering, I have achieved a very stable high quality a-Si:H: the saturated defect density reaches the lowest value (2{dollar}-{dollar}3 x 10{dollar}rmsp{lcub}16{rcub} cmsp{lcub}-3{rcub}{dollar}) reported so far in the literature. This significant improvement of the stability could result in a high stabilized solar cell efficiency. For p{dollar}sp+{dollar} a-Si,C:H, films with much lower thermal activation energy (0.28-0.33 eV) have been obtained, compared with films grown by plasma enhanced chemical vapor deposition (0.4 eV). Microstructural investigation shows that the microvoid contents in these films are much less than that of high quality boron doped a-Si,C:H prepared by plasma enhanced chemical vapor deposition. I attribute the significant improvements of stability and doping efficiency of the sputtered amorphous silicon and its alloys to the better amorphous network.; Hydrogen diffusion in a-Si:H has been associated with the metastable defect creation and annealing. A systematic study of hydrogen diffusion kinetics in sputtered a-Si:H is carried out for the first time. The independent control of hydrogen incorporation allows me to study explicitly the effect of hydrogen content on hydrogen diffusion kinetics. I have found that H diffusion coefficient varies strongly with the hydrogen concentration in a-Si:H film. The H diffusion coefficient is thermally activated, with the activation energy decreases from 1.6 to 1.0 eV as the hydrogen concentration increases from 1.2-12 at.%. For film with higher hydrogen content ({dollar}sim{dollar}18 at.%), the H diffusion profile shows an exponential decay. This observation indicates that the diffusion process is deep trapping limited. Based on the experimental results, I have proposed a revised H density of states model to account for the strongly concentration dependent diffusion behavior. As the H chemical potential shifts closer to the transport level with the increase of hydrogen content, the diffusion activation energy decreases and the diffusion coefficient increases.