Semitransparent all-oxide p-NiO/n-ZnO nanowire ultraviolet photosensors
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All-oxide ultraviolet (UV) photosensors based on NiO/ZnO nanowire heterostructure were fabricated on corning glass substrates. The p-type NiO layers were directly deposited on the ZnO nanowire arrays grown on the AZO bottom electrode/glass for the formation of a p–n diode, followed by the growth of the ITO top electrode layer for the electrical interconnection of nanostructures. The fabricated device structure showed a transmittance value of about 60% in the visible region, resulting in semitransparent properties. The current–voltage (I–V) characteristics of the fabricated p–n heterostructure showed a typical rectifying behavior with a current rise at about 4 V and an I(forward)/I(reverse) ratio of about 11.3 at 8 V. In addition, the ITO/p-NiO/n-ZnO/AZO structure responded at a wave-length position of 370 nm in reverse bias, together with weak photoresponse in the visible region. An UV sensor based on the all-oxide ZnO nanowire absorber exhibited improved photoresponse compared to the device based on a ZnO thin film.
I. INTRODUCTION
Recently, transparent optoelectronic devices have attracted attention due to novel applications such as light-emitting diodes,1 solar cells, thin-film transistors,2,3 and ultraviolet (UV) sensors.4 UV sensors have been widely used in various commercial and military applications, including secure space-to-space communications, pollution monitoring, water sterilization, flame sensing, and early missile-plume detection.5 Among various materials suggested for the realization of transparent electronic devices, ZnO has been strongly considered to be a promising functional material because it has a direct band gap of 3.37 eV, a large exciton binding energy of 60 meV at room temperature, and a high transparency in the visible range with native n-type.6 In particular, well-aligned, onedimensional (1D) ZnO arrays are considered to induce high-performance devices due to the increased surface area and quantum effect.7 Excellent vertical alignment of ZnO nanowires on large area is very important for the fabrication of optoelectronic devices using nanostructures as an active layer, because it allows the use of semiconductor processing and metal interconnection on the top of all nanowires. However, the ZnO nanowires with intrinsically n-type characteristics require the development of p-type oxide materials to fabricate all-oxide p–n junctions with rectifying properties. The reports on optoelectronic devices consisting of all-oxide p–n junctions a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.242 J. Mater. Res., Vol. 28, No. 18, Sep 28, 2013
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have been rare because of the absence of p-type oxide materials with reliable characteristics. Up to now, most studies on the ZnO-nanowire-based optoelectronic devices have used p-Si and p-GaN layers as p-type semiconductors. Schottky,8 p–n junction, p–i–n junction, and metal– semiconductor–metal9 structures have been proposed for semiconductor UV sensors. In the cas
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