Conducting properties of nearly depleted ZnO nanowire UV sensors fabricated by dielectrophoresis

Abstract

ZnO nanowires (NWs) with different radii (rNW) have been aligned between pre-patterned electrodes using dielectrophoresis (DEP) for the fabrication of high gain UV sensors. The DEP conditions (voltage amplitude and frequency) and electrode material, geometry and size were optimized to enhance the efficiency during the DEP process. To understand the alignment mechanism of the ZnO NWs, the dielectrophoretic force (FDEP) was analyzed as a function of the DEP conditions and NW dimensions. These studies showed that the DEP alignment process tends to trap NWs with a smaller radius. The effects of NW size on device performance were analyzed by means of I-V measurements in darkness and under illumination (200 nm < λ < 600 nm). In darkness, the NW resistance increases as rNW decreases due to the reduction of the conduction volume, until saturation is reached for rNW < 65 nm. On the other hand, the NW spectral photoresponse shows high values around 10(8) A W(-1) (measured at 5 V and λ < 370 nm) and follows a linear trend as a function of the NW cross section. In addition, the cut-off wavelength depends on rNW, presenting a clear blue-shift for NWs with a lower radius (rNW < 50 nm). Transient photoresponse studies show that NWs with lower radii have longer rise times and shorter decay times mainly due to surface trapping effects. Regardless of NW size, passivation of the surface using a dielectric capping layer of SiO2 reduces the dynamic range of the photoresponse due to a strong increase of the dark current.

Effects of Hydroxylation and Silanization on the Surface Properties of ZnO Nanowires

“…33 Quantum theory predicts that the increase of the carrier population in the NW increases the transmission probability through a potential barrier. This result agrees with a previous studies on NWs of different diameters, 25 where it is found that the increase of the conductive volume provokes, it is observed that the NW silanization improves the linear characteristic of the contact formed between the NW and the Al electrode mainly due to the increase of the total free charge in the NW, which can benefit the output parameters of ion-gated field-effect transistors.…”

“…Similar results have been obtained in a previous work, where current levels of devices with the same geometry were demonstrated to be directly related by rNW of the ZnO NW. 25 Therefore, that higher current levels measured in the silanized device (or even higher 18 untreated ZnO NW based devices) are suggested to be a direct consequence of the SCR narrowing, which means an increase of the conductive volume.…”

Photodetector fabrication by dielectrophoretic assembly of GaAs nanowires grown by a two-steps method

“…Under illumination conditions, electron-hole pairs are created by photon absorption, increasing the carrier density in the NW body; band-bending is then reduced and also the depletion width near the NW surface, increasing the NW effective section for conduction. This effect, also found in other semiconductor NW technologies, explains the observed reduction of the device series resistance under illumination and the corresponding increase of the forward current [21]. Figure 11.…”

“…In this regard, contactless NW manipulation methods using electromagnetic fields -like dielectrophoresis (DEP)-are usually much softer than mechanical methods such as contact transfer printing or tape peeling, often resulting less destructive. Different assembly methods have been developed to manipulate NWs, including fluidic alignment, roll contact printing [15,16], Langmuir-Blodgett (LB) film deposition technique, drop-casting [17,18], nano-manipulation [19,20], and dielectrophoresis [21][22][23][24][25][26]. Fluidic alignment, roll contact printing transfer, and LB techniques, all offer the ability to align NWs in parallel but do not allow for precise NW placement in functional systems.…”

“…However, the mechanical interaction between the AFM cantilever and the NW can harm the NW structure during the manipulation process; in addition, this technique is limited to the manipulation of a reduced number of NWs at the same time. The assembly of NWs through dielectrophoresis has attracted attention of researchers due to the ability of this technique to precisely position NWs at specific places on a substrate [21][22][23][24][25][26], making possible the fabrication of NW based applications including field effect transistors (FET) [27,28], biosensors [29], and other functional electronic devices [30]. Moreover, this approach allows to assemble NWs directly on flexible substrates and at large-areas which is promising for the development of flexible light-emitting diode (LED) displays [31], and transparent flexible nano-electronics [32].…”