Steady-state photoconductivity in hidrogenated amorphous silicon and $Co_{0.2}Ga_2S_{3.2}$ single crystal

Abstract

In this thesis the steady-state photoconductivity have been studied for undoped, phosphorus-and borondoped hydrogenated amorphous silicon (a-Si:H) as functions of the temperature and the illumination intensity. The investigation is focused on the following three points: (1) Are the observed photoconductivity activation energy and power exponent in respective doping ratio or temperature range or illumination intensity range given by what recombination process? (2) What are the changes of steady-state photoconductivity properties by light-soaking? And are these changes led by what changes in recombination process? (3) What is the mechanism about the appearance and disappearance of thermal quenching and supralinearity in a-Si:H with the variation of dark Fermi level position? It is revealed that the general feature of steady-state photoconductivity in a-Si:H is controlled by the two types of recombination center located at or below midgap. One of these is a fast center (presumably the dangling bond) and the other is a sensitizing slow center in thermal contact with valence band and have small capture cross section for electron. When the dark Fermi level ($E_f$) is close to $E_A$ as in phosphorus-doped sample, the predominant recombination center is slow center, resulting in high photosensitivity, bimolecular-type recombination and the activation energy of photoconductivity ($E_{ph}$) to be $E_{ph} = E_c-E_A 0.20$ eV. In the mean-time, when $E_f$ is located near midgap, now predominant recombination center is fast center, resulting in low photosensitivity, monomolecular-type recombination and $E_{ph}$ to be nearly zero. And in this study we find that there are some correlation between $E_{ph}$ and $\gamma$, the power exponent in intensity dependence of photoconductivity such that the sample with a large $E_{ph}$ have a small $\gamma$, and vice versa. As $E_f$ shift toward midgap by introduction of metastable defects near midgap by light-soaking, the effectiveness o...