26 May 2017

In the recent past ZnO has emerged as a promising alternative to Si and GaN in devices like light-emitting diodes (LEDs), photodetectors, and optically pumped lasers for the UV region(1-3). ZnO has several special properties such as direct wide bandgap (~3.37eV)(4), radiation resistance, high adsorption capacity, high exciton energy (~60meV)(4), high mechanical and thermal stabilities, and transparency in the visible range of the electromagnetic radiation(4-6). In recent times, one-dimensional (1-D) nanostructures of ZnO have attracted considerable attention of researchers, because of its unique properties (such as controllable shape and size)(7-10). A variety of 1-D nanostructures of ZnO, such as nanostructures(7), nanowires (NWs)(8), nanorods (NRs)(9), nanoparticles(10), spirals(11), nanoneedle(12), and nanocombs(13) can be grown by different synthesis techniques(7-13).

Among these 1-D nanostructures, NRs and NWs are the most popular and commonly used structures of ZnO, for different applications. The ZnO-NRs can be grown by a variety of techniques like sol-gel method(14), atomic layer deposition (ALD)(15), thermal evaporation(16), electrodeposition(17), spray pyrolysis(18), hydrothermal(9), and chemical vapor deposition(19). Most of these growth techniques are complex and require high growth temperatures (600-1000°C)(12, 16). The hydrothermal method has attracted considerable interest because of its simplicity and low-temperature processing(9, 20-23). Different nanostructures of ZnO such as nanoflowers(21), nano-crystals(22), and nanopencils(23) could be grown by hydrothermal techniques. In the past decade, a lot of work has been done on ZnO-NR-based devices like optically pumped lasers(24), field effect transistors(25), and biological and chemical sensors etc(26).

Among these, ZnO-NR-based UV detectors and optical switches have been the focus of wide studies(27, 28). In recent times, many groups have reported the UV detection properties of ZnO thin films and ZnO nanostructures-based devices(28-30). Li et al. reported Au/ZnO NR array-based UV photodetectors (UV-PDs) with good sensitivity (contrast ratio ~ 4.7)(30). They have grown ZnO-NR arrays on F-doped SnO2 (FTO) substrates by hydrothermal synthesis. Humayun et al. reported a ZnO nanostructure decorated microgap electrodes UV sensor. They have compared the UV sensing properties of Au/Ti/ZnO thin film and Au/Ti/ZnO NR array deposited in selective areas of the microgap electrodes spacing(31). They concluded that the fabricated devices could be used for low power miniaturized devices having rapid response and reproducibility(31).

Witkowski et al. have reported UV detector properties of ZnO-NRs grown on quartz substrates by the hydrothermal method. They have fabricated ohmic contacts of Ti/Au on ZnO-NRs and their detector showed a sensitivity of 20 mW/m2 upon UV illumination(32). Zhou et al. reported Pt/ZnO-NR and Pt/modified ZnO-NR based Schottky UV detectors. They have used different seed layers and metal oxide (MgZnO, MgO, and Al-doped ZnO) modifying layer materials. They reported that the ZnO-NRs UV-PD, which was grown on MgZnO seed layer and without oxide material-coating, demonstrated bigger responsivity and a larger detectivity than PDs with a ZnO seed layer(33).

Liu et al. have reported UV detectors based on the vertically aligned ZnO micro/nanowires on graphene, which showed high responsivity of 1.62 A W?1 per volt(34). Nie et al. have reported monolayer graphene (MLG) film/ZnO NR Schottky UV detectors with quick response of millisecond rise time/fall times(35). Dang et al. have reported ZnO nanostructure/graphene (Gr) based UV detectors with high responsivity (RI ~ 3 × 105 A W?1)(36).
Although there have been several works that reported on the UV-sensing properties using ZnO NRs, there are only a few reports on the UV-sensing of ZnO-NRs (grown by the hydrothermal-method) at a low-voltage.

The main focus of this work is to examine the UV-sensing characteristics of Ag/ZnO-NRs Schottky devices at forward applied bias over the range 0 V to 1 V.
The results show that these devices could be useful for cost-effective and low-voltage UV detection applications.