2.10 Pulsed photovoltaic effect in thin film MIS devices

Abstract

A MIS structure, illuminated by pulsed light, can generate a pulsed photocurrent. We have observed this effect in various kinds of MIS structures (Si 3N 4/Si; Polymer/Si). We have shown that the detected photovoltage pulse across a high impedance load is directly related to surface potential Φ s ( q Φ s KT ; band bending at insulator/semiconductor interface). Indeed, the photovoltage versus external bias V curves are found in good agreement with the Φ s (V) curves obtained by capacitance-voltage measurements. For instance, photovoltage is found to reverse polarity, like Φ s , at flatband voltage (V = V FB). This pulsed photovoltaic effect is then studied on thin film sandwiches: Metal/Polymer/CdSe/Metal. The CdSe film, evaporated on to a chromium contact, is coated by a polyparaxylene film (2000 A-rf plasma polymerized) and a semi-transparent counter-electrode. A pulse of light creates a 0.2 V voltage pulse across a 1 M Ω load resistor. Furthermore, after a +5 V short time bias, photo response vanishes, and reappears by reversing the bias. Hysteresis investigations of the capacitance-voltage curves is used to explain this memory effect. A short time bias is sufficient to fill or empty polymer traps near the interface. The respective semiconductor surface is then accumulated (low/ Φ s ) or deplated (high/ Φ s ). Photoresponse is in good agreement with these results. Pulsed photovoltaic effect may be used to investigate MIS structures, and also to make up a new thin film memory device.