(The) effect of hydrogen on growth, structure and stability in a-Si:H

Abstract

The doped microcrystalline silicon films have been deposited by the radio frequency(13.56MHz) glow discharge decomposition of $SiH_2F_2+B_2H_6+H_2$ and $PH_3+SiH_2F_2$ mixing gases with varying substrate temperature (100$^\circ$C $\sim$ 310$^\circ$C) and rf power density (160mW/$cm^2\sim$530mW/$cm^2$). With increasing substrate temperature, growth rate decreases up to 200$^\circ$C and then increases slowly at low rf power level. The growth rate not changes with substrate temperature at high rf power level. It was found that the chemical equilibrium between the deposition and etching of Si on the growing surface formed microcrystalline Si. The microcrystalline Si was confirmed from X-ray diffraction and Raman scattering. A new method of the fabrication, layer by layer deposition technique, with hydrogen radical annealing in between have been studied to accomplish the high stability in a-Si:H films with preparation parameters of substrate temperature and hydrogen plasma exposure (HPE) time. With the increase of HPE time, the dark conductivity decrease up to 15 sec and then increases rapidly, resulting in that maximum photosensitivity $3\times10^6$ can be reached by about 15 sec of HPE time. Above 50 sec of HPE time the microcrystalline Si is formed, confirmed from the dark conductivity. It was found from the experiments that the deposision rate per cycle od the film decreased monotonously in proportion to the hydrogen plasma exposure time, indicating that the deposited film is etched slightly during the hydrogen plasma exposure. As results, a-Si:H films produced by layer by layer deposition technique having photosensitivity of $>10^6$, conductivity activation energy of 0.9 eV, and hydrogen content of $< 10$ at.\% were obtained. The Staebler-Wronski effect of this films is greatly reduced compared with single-layered a-Si:H film deposited at the same temperature.