Scanning photoinduced impedance microscopy using amorphous silicon photodiode structures.

Scanning photoinduced impedance microscopy (SPIM) is an impedance imaging technique, which is based on photocurrent measurements at electrolyte-insulator-semiconductor (EIS) and metal-insulator-semiconductor (MIS) field-effect structures. The material to be investigated has to be deposited on top of the insulator (E/I or M/I interface). The lateral resolution of SPIM is limited by the lateral diffusion of minority charge carriers. Therefore, it would be advantageous if semiconductors with a short diffusion length of charge carriers such as amorphous silicon could be employed. However, field-effect capacitors fabricated using amorphous silicon suffered from a large number of interface states, high leakage currents through the insulator, and small photocurrents. In this work, field-effect capacitors were replaced by amorphous hydrogenated silicon photodiode structures (a-Si:H p-i-n/SiO2 or n-i-p/SiO2) as this was expected to result in higher photocurrents and eliminate the necessity of a high-quality insulator. The photodiode structures were shown to be suitable for SPIM measurements. The resolution of photocurrent measurements was found to depend strongly on the frequency of the modulated light and the doping concentration of the amorphous silicon layer closest to the insulator. An equivalent circuit model was developed to simulate this behavior.