Characterization of hydrogenated amorphous silicon by capacitance-voltage and surface photovoltage measurements using liquid Schottky barriers

Abstract

Experimental studies have been made on the electrical properties of hydrogenated amorphous silicon (a-Si:H) using liquid Schottky barriers. We have found that the quasi-static capacitance-voltage (C-V) method can be applied to the a-Si:H/quinone-hydroquinone (Q-HQ) liquid Schottky junction. This method enables us to determine the net density of positive space charge due to ionized traps and impurities in a-Si:H (Ne), the built-in potential (Vb), and the width of the surface space-charge layer (W), of this liquid junction. The barrier height of an undoped a-Si:H/Q-HQ junction has been estimated to be more than 1 eV from the value of Vb thus obtained. By C-V and surface photovoltage (SPV) measurements on the same samples, we have studied the changes in the properties of a-Si:H with doping and with prolonged illumination. It has been found that phosphorous (P) doping drastically decreases the hole diffusion length measured by the SPV method and increases the value of Ne. Slight boron (B) doping increases the ambipolar diffusion length L1 and the field assisted carrier collection length L2, both of which have been determined by SPV. These results explain the observed enhancement of the photovoltaic properties of a-Si:H p-i-n solar cells with the slight B doping to the i layers. The values of L1 and L2 have a distinct correlation with the photo-voltaic properties in the slightly B-doped samples, which has confirmed the effectiveness of the SPV method in characterizing a-Si:H as a photovoltaic material. We have found that the increase in Ne and the decrease in L1 occur simultaneously in undoped a-Si:H with prolonged illumination. On the other hand, the increase in Ne does not always accompany the decrease in L1 in the photoinduced changes in P-doped or slightly B-doped a-Si:H, which suggests the difference in the mechanism of the changes between undoped and doped samples.