Light and thermally induced metastabilities in doped amorphous silicon films

Abstract

In this thesis, various metastabilities occurred in compensated and delta-doped hydrogenated amorphous silicon(a-Si:H) films are observed, and I proposed a microscopic mechanisms for the generation excess carriers by rapid cooling or by light illumination. I examined the conductivity changes induced by thermal quenching in compensated a-Si:H films with various compensation ratios of boron and phosphorus as well as in delta-doped a-Si:H films. The magnitude of the excess conductivity upon rapid cooling increases as the Fermi-level approaches to the band edges. The relaxation of the excess conductivity follows the stretched exponential form. However, the activation for the relaxation time is found to be smaller then those for undoped and singly doped a-Si:H films. This facts indicate that the relaxation mechanism strongly governed by hydrogen diffusion. The effect of band tail carriers are also studied in metastable state annealing processes when the density of band tail carriers($n_BT$) are large. Previously, Street et al., showed that the band tail carriers affect metastable relaxation processes, we create large persistent photoconductivities by light illumination upon fully compensated sample at several temperatures. Keeping the illuminating temperature constant, slow decay of persistent photocurrents are measured according to time. Decay behavior fits well with numerical analysis introducing band tail carrier dependent hydrogen diffusion coefficient as $D_H=(Dn_BT^ε+D_min)(Vt)^{-α},ε dependence of $D_H$, can be determined through widely accepted values for $T_0$ and $D_min$. From this experimental results, band tail carriers participate not only in the creation of the metastable state but also in the annealing processes. Therefore, we must take into account of band tail carrier effect on metastable state creation or annealing processes when $n_BT$ is large.