High-Performance Enhancement-mode ZnO Nanowire Field-Effect Transistors with Organic Nanodielectrics: Effects of Ozone Treatments

Abstract

Nanowire transistors are of significant interest for future electronic and optoelectronic applications, including flexible electronics and displays. While relatively high mobilities have been achieved in nanowire transistors using various semiconductors, the transistors reported to date typically suffer from relatively poor on/off ratios, due to the relatively thick gate dielectrics and the use of metal (Schottky) source/drain contacts to moderate band-gap materials, which leads to ambipolar effects. ZnO nanowire field-effect transistors (ZnO NW-FETs) are of particular interest for future display devices because of the potential transparency due to the wide bandgap (3.37 eV), as well as the inherent flexibility of nanowires. Initial studies of ZnO NW-FETs showed mobilities significantly lower than bulk values.' As fabricated, the ZnO-NW FETs in our prior work exhibited good transistor characteristics, although the measured subthreshold slopes and on/off ratios were not suitable for large-scale integration.2 In this study, we report significant improvements in device performance metrics following annealing and ozone treatment ofZnO NW-FETs. The single-wire device structure (Fig. 1) uses a heavily doped ntype Si substrate as a common back-gate and aluminum (Al) source/drain contacts. The crystalline nanowires had a diameter of approximately 120 nm. The gate dielectric consists of three layer-by-layer self-assembled organic trilayers (15 nm).3 This self-assembled superlattice (SAS) film is compatible with lithographic processes, and exhibits excellent insulating properties (Fig. 2) with a large specific capacitance (180 nF/cm2) and a low leakage current density (1 X10-6 A/cm2 up to 2V). The device characteristics following annealing and ozone treatments are illustrated for a representative device in Figs. 3 (a) and 3 (b). First, annealing in air (130°C, 15 min) was performed to reduce fixed positive charges in the SAS,3 resulting in an improved subthreshold slope (230 mV/dec). The on-current degraded upon 4 annealing, consistent with reported annealing-induced effects on ZnO nanowires. Compared to asfabricated devices, subsequent ozone treatments resulted in complete on-current recovery, a positive threshold voltage (Vth) shift from -0.4 V to 0.2 V, a subthreshold slope reduction from 400 mV/dec to 130 mV/dec and a large on-off current ratio (-10o7). The resulting enhancement-mode devices operate at subIV with an on-current of 4 ,uA at 0.9V and a transconductance of 1.4 ,uS (Fig. 3). The effective mobility, extracted from the measured transconductance and a cylindrical capacitance model, is as high as -1200 cm2/V-sec. Since I-V based mobility extraction does not permit independent determination of carrier concentration and mobility, an extended analysis is being developed to explain the relatively large value compared to bulk ZnO mobility ( 200 cm2/V-sec). Nominally undoped ZnO nanowires are lightly doped n-type, so Al is expected to have a good workfunction line-up, as shown in Fig. 4 (b). Ozone treatments change the ZnO nanowires from partially depleted by surface oxygen (Fig. 4 (c)) to fully depleted (Fig. 4 (d)) and reduce interfacial impurities, enabling enhancement-mode operation.