Electrical properties of ZnTe nanowires and ZnTe Schottky diodes

Abstract

In this Master thesis, a single ZnTe nanowire was characterized and used to develop a nano Schottky diode. ZnTe nanowires were deposited on a silicon substrate using Vapor Liquid Solid method and characterized. A MEMS device was fabricated to make electrical contact between a metal pad and a single nanowire and used to measure electrical properties of ZnTe nanowires. On the whole four devices were fabricated

nanowire under two gold contacts, nanowire on top of the two gold contacts with Pt pads, nanowire on top of four gold contacts with Pt pads and nanowire on top of four nickel contacts with Pt pads. The first two devices were used to investigate electrical properties of ZnTe nanowire. The I-V curves of the nanowires were obtained and some electrical parameters such as resistivity, carrier concentration and mobility were estimated from the corresponding curves. The effects of thermal annealing, Joule heating, and doping on the electrical conductivity of nanowires were investigated. After thermal annealing and Joule heating five orders of magnitude of improvement in the electrical conductivity was seen. P-doping of the as deposited ZnTe nanowires carried out by an immersion method in Cu(NO_3 )_2 solution, followed by a thermal treatment process for dopant activation, resulted in conductivity enhancement by five orders of magnitude. Each single ZnTe nanowires between two metal contacts can be modeled as two head-to-head Schottky diodes. Two other devices with four gold and nickel terminals were used to extract Schottky diode parameters. First a thermionic emission model was applied to understand charge transport mechanism in our nano Schottky structure. According to the calculations there is a large deviation in ideality factor of our Schottky diodes, implying thermionic filed emission is not dominant in the charge transport of ZnTe nanowire and a tunneling current can be used to model behavior of device. Schottky barrier height estimated in devices with gold and nickel contacts. The results shows in contrast to conventional large diodes, work function variation of metal contact does not change Schottky barrier height significantly. This weak relationship between work function and barrier height, demonstrates the strong pinning of surface Fermi level because of surface traps. A four-probe structure used to measure I-V characteristic of single nanowire without effect of contacts. Four-probe measurements show resistance of ZnTe nanowires is in range of Mega Ω while in two-probe approach this value is in the range of Giga Ω which is a evidence of low quality electric contact between metal and semiconductor. This weak electric contact can be attributed to surface contamination and also oxide layer around the ZnTe nanowire. Annealing of Schottky diode at 400 oC improved ideality factor because of reduction of surface traps and establishing better contact between semiconductor and metal.