Thermal equilibration processes in hydrogenated amorphous silicon

Abstract

The fast cooling effects on undoped and P-doped a-Si:H, a-Ge:H and a-SiGe:H:F alloys have been studied. Films with high defect density such as a Ge;H and a-SiGe: H:F do not show a change in the dark conductivity by fast cooling. For undoped a-Si:H films, the dark conductivity is decreased and the dark conductivity activation energy is increased after fast cooling. And the subband-gap optical absorption coefficient increases. It is concluded, therefore, that the metastable dangling bonds increased upon fast cooling from above the equilibrium temperature ($T_E$) for undoped a-Si:H. Changes of the $T_E$ and defect density with annealing temperature have been studied for P-doped a-Si:H films. From the correlation between the defect density and the $T_E$, it is concluded that the hydrogen diffuses via the defects in doped a-Si:H. From the isothermal relaxation experiments on the excess dark and photo conductivity by fast cooling and light soaking at different temperatures, it is concluded that the metastable phenomena in a-Si:H have a common origin. The structural changes that underline the metastabilities are made possible by the diffusive motion of hydrogen. The metastable dangling bonds and active dopants increase upon fast cooling from above the $T_E$ or light soaking.