High‐Density Arrays of Germanium Nanowire Photoresistors

Abstract

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, E g , of approximately 0.66 eV (temperature T = 300 K) [1] and has been widely used for the fabrication of photodetectors, [2][3][4][5] radiation detectors, [6][7][8] charged particle and photon tracking devices, [9] far-infrared photoresistors, [10] and numerous other devices. [11] 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. [12][13][14] 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) [15,16] or mesoporous materials [17,18] 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, [19,20] 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 [21][22][23] and metal/ semiconductor core/shell [24] 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. [25][26][27][28] However, photoconductivity measurements on ordered semiconducting nanowire arrayshave 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. [29] To our knowledge, this study is the first analysis of photoconductivity in ordered semicond...

Role of Defects and Growth Directions in the Formation of Periodically Twinned and Kinked Unseeded Germanium Nanowires

“…The kinked area possesses longitudinal faults, however, the short dimension of this region indicates that the favourable growth direction is the dominant factor in determining the morphology of this NW. The SAED pattern (Figure 4 (c) inset), taken from the kinked NW, which corresponds to the [1][2][3][4][5][6][7][8][9][10][11] zone axis, shows additional reflections at 1/3{(-224)}-type positions, which are related to the stacking fault content in this kinked NW. As previously noted by Gibson et al 39 for crystalline Si, a 20 complete diamond cubic unit cell having an AaBbCc stacking sequence, produces diffracted amplitude from each pair of layers that are phase shifted by 120°, resulting in exactly zero intensity at the 1/3 (-224)-type position.…”

Synthesis of Germanium/Multi-Walled Carbon Nanotube Core-Sheath Structures via Chemical Vapor Deposition

“…Although there is a tendency for amorphous deposits to cover the AAO template, the coating can be mechanically removed by polishing to expose MWCNT-encapsulated Ge nanowire arrays which protrude out of the AAO matrix. Past work suggests that it should be possible to exploit these arrays to fabricate gas sensors and optoelectronic devices (Baughman et al, 2002;Agarwal & Lieber, 2006;Polyakov et al, 2006;Erts et al, 2006).…”

“…Furthermore, Ge has much higher electron and hole mobility than Si (Sze, 1981), which is especially required when electronic devices are scaled down to the sub-100 nm regime. Several growth methods have been developed for the synthesis of Ge nanowires, including laser ablation (Morales & Lieber, 1998;Zhang et al, 2000), thermal evaporation (Gu et al, 2001;Nguyen et al, 2005;Sun et al, 2006;Das et al, 2007;Sutter et al, 2008), supercriticalfluid synthesis (Ryan et al, 2003;Polyakov et al, 2006;Ziegler et al, 2004;Erts et al, 2006), liquid-state synthesis (Heath & LeGoues, 1993;Song et al, 2009), molecular beam epitaxy (Omi & Ogino, 1997), and chemical vapor deposition (CVD) (Kodambaka et al, 2007;Ryan et al, 2003). CVD has been the most widely employed of these synthesis methods, with the aim of synthesizing Ge nanowires in a controllable way via the selection of suitable Ge ordered mesoporous aluminosilicate thin films (Ryan et al, 2003;Ziegler et al, 2004) and AAO membranes Erts et al, 2006) as templates to grow Ge nanowire arrays by the degradation of diphenylgermane (C 12 H 12 Ge) in supercritical CO 2 .…”

Photoconductive properties of Bi2S3 nanowires

“…For most of these studies laser irradiation, with energies between 70-100 W•cm -2 12 , or halogen and tungsten filaments were employed for sample illumination 18,19 . Although heating of the samples due to the high intensity illumination employed was not reported in these studies, Polyakov et al have previously shown that high-powered illumination of Ge nanowires can have a detrimental effect on their photoconductivity by causing exponential thermogenerated current increase inside them 8 . In our study, low-power illumination of 1-4 W•cm -2 was used to study the optoelectronic response of Bi2S3 nanowires as a function of illuminating light photon energy, minimizing any effects due to sample heating and illustrating high optical sensitivity of the nanowires.…”