High-Density Arrays of Germanium Nanowire Photoresistors

Here we present for the first time a study of the photoresistive properties and dynamics of ordered, high-density arrays of germanium nanowire photoresistors. Germanium is a wellknown semiconducting material with an indirect bandgap, Eg, of approximately 0.66 eV (temperature T = 300 K) and has been widely used for the fabrication of photodetectors, radiation detectors, charged particle and photon tracking devices, far-infrared photoresistors, and numerous other devices. During the last few years there has also been increasing interest in the use of nanostructures (quantum dots and wires) of both germanium and silicon as materials for potential applications in sensors, nanophotonics, and nanoelectronics. However, in order to successfully integrate onedimensional semiconductors into useful devices, ordered architectures of aligned nanowires are required. Using templates such as anodized aluminium oxide (AAO) or mesoporous materials as hosts for nanowires offers a viable method for forming high-density arrays of ordered, crystalline nanowires. Significantly, AAO membranes with ordered and highly oriented pore structures have recently been synthesized on silicon substrates, which is very promising for the integration of such materials into current complementary metal oxide semiconductor technologies. At University College Cork we have developed supercritical-fluid-inclusion phase methods for forming semiconductor and metal/ semiconductor core/shell nanowires and nanotubes within the pores of mesoporous matrices and AAO membranes. Supercritical-fluid-inclusion methods are ideal solvents for forming high-density arrays of nanowires within AAO templates as they do not suffer from the inherent problem of pore blocking associated with other methods, such as electrodeposition and incipient wetness techniques. The electrical conductivity and photoluminescence properties of semiconductor nanowire arrays have been investigated by several research groups. However, photoconductivity measurements on ordered semiconducting nanowire arrays have not yet been performed. An investigation into the photoconductivity of ordered arrays of nanowires is important in order to fully understand their potential in future photodetection devices, for example, as photoresistors or photodiodes. In this paper, we report the photoconductive properties of germanium nanowire photoresistors with mean diameters of 50 and 100 nm, incorporated within the pores of AAO membranes. A comparative study of the photoresistive properties of germanium nanowire photoresistor arrays with different optically transparent electrodes, namely ultrathin gold films and tin-doped indium tin oxide (ITO) layers, is described in this paper. ITO is a well known n-type semiconductor widely used in the fabrication of transparent electrodes in various optoelectronic devices. To our knowledge, this study is the first analysis of photoconductivity in ordered semiconducting nanowire arrays. Germanium nanowires, with mean diameters of 50 and 100 nm were synthesized in the pores of AAO membranes. The electrical and structural properties of these Ge nanowire arrays has previously been reported (see Supporting Information). Figure 1a shows a current-distribution map of the encapsulated nanowires, with a mean diameter of 100 nm, obtained using conductive atomic force microscopy (C-AFM). Practically all of the Ge nanowires incorporated within the AAO membranes demonstrated similar electrical conductivC O M M U N IC A TI O N S