Silicon metal-semiconductor–metal photodetector with zinc oxide transparent conducting electrodes

Abstract

Transparent conducting oxides thin layers, due to their optical and electrical properties, can be used as transparent electrodes in various optoelectronic devices. We present a metal-semiconductor–metal photodiode (MSM-PD) on silicon as optically active layer with zinc oxide (ZnO) thin layer as interdigitated Schottky transparent electrodes. The advantage of using a ZnO thin layer as Schottky electrodes consists in the improvement of the photoresponse by eliminating the shadowing of the active area by opaque metallic electrodes. ZnO thin layers were deposited on 10 Ω cm resistivity silicon epitaxial wafers by the vacuum thermal evaporation method. High purity metallic powders were mixed with an (Al+Sn)/Zn ratio of 0.03. In order to obtain transparent layers the metallic depositions were thermally treated at 450 °C for 2 h. The Al, Sn co-doped ZnO layers of 0.5–0.8 μm were investigated regarding structural, optical and electrical properties and surface morphology. The obtained thin layers have a high transparency (T>85%) over a large spectral range and the resistivity is quite low, ρ~10−4 Ω cm. The interdigitated Schottky contacts of ZnO were configurated onto the optically active Si layer providing an MSM-PD structure of 0.143 mm2 active area and finger spacing and finger width of 6 μm. The optoelectronic characteristics were measured and the Schottky barrier height of 0.62 eV was determined from the current–voltage characteristic. A responsivity of 0.2 A/W at 475 nm and a capacitance of 1.4 pF at 10 V bias were obtained for the MSM-PD structure with transparent conducting ZnO Schottky electrodes.