Fast response ZnO:Al/CuO nanowire/ZnO:Al heterostructure light sensors fabricated by dielectrophoresis

Abstract

Metal oxide nanowire (NW) photoconductors tend to exhibit high photoconductive gains and long recovery times mainly due to surface effects. In this work, p-type CuO NWs are synthesized by direct oxidation of copper and deposited on n-type ZnO:Al electrodes by dielectrophoresis. The heterostructure is electro-optically characterized showing recovery times in the 10 ls range, mainly limited by the resistance-capacitance product of the equivalent circuit, without signs of persistent effects. The fast response is attributed to short transit times across space charge regions built between CuO and ZnO:Al materials and fast carrier recombination at neutral regions. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4811128] Metal oxides (MOs) are attractive compounds for industrial purposes since high-quality crystal nanowires (NWs) can be obtained by cost-effective methods that do not require expensive vacuum systems or environmentally unfriendly chemical reactants. The high surface-to-volume ratio of such nanostructures leads to the enhancement of the device sensitivity prompting their use as building blocks in low-cost optical, biological, and gas sensors. [1][2][3][4] Regarding light detectors, NWs may help to reduce the operation voltages of high-gain sensors through the proximity of the surface and the subsequent enhancement of the local electric field. In addition, their small dimensions open possibilities for the integration of NWs from different materials or with different structures such as core-shell, p-n junctions, or quantum well superlattices. 5,6 However, the development of NW-based devices has to face issues such as the lack of performance reproducibility or the need for advanced methods to handle and manipulate nanostructures on a high demand basis. Dielectrophoresis (DEP) has long been used in biochemistry to separate biomolecules, but nowadays, it has also been proved to be a costeffective technique to align NWs between metal electrodes. 7,8 The technique is highly scalable and makes possible the precise placement of multiple NWs on Si integrated circuits at room temperature. Thus, ZnO NW devices have been fabricated using this method on Au electrodes. 9 Due to their high conductance and mechanical stability, Al-doped ZnO (AZO) is an alternative material to metals for the preparation of DEP electrodes. Moreover, its visible transparency also enables the alignment of NWs on desired locations of transparent substrates aiming at the development of sensors in transparent electronics.Concerning the performance of NW devices, the understanding of the surface effects is crucial since many of the electrical and optical characteristics are strongly influenced by surface electronic states. On many MO-NW light sensors, these surface effects lead to obtain photoresponse values well beyond those achieved in mesoscopic devices, at the expense of showing long recovery times. 10,11 Unfortunately, these slow transients hinder their practical application in many fields.Recently, p-type cupric oxide (CuO) NWs ha...

Fast, effective manipulation of nanowires for electronic devices

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Continuous-flow system and monitoring tools for the dielectrophoretic integration of nanowires in light sensor arrays

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