On-chip fabrication of ZnO-nanowire gas sensor with high gas sensitivity

Abstract

ZnO-nanowire gas sensors were fabricated by a selective growth of nanowires on patterned Au catalysts thus forming nanowire air bridges or ‘nanobridges’ between two Pt pillar electrodes. The gas sensing properties of nanobridge gas sensors were demonstrated using a diluted NO₂. The response, as a function of temperature, was highest at 225 °C and was linearly increased with the concentration of NO₂ in the range of 0.5–3 ppm and then showed a sign of saturation. Our sensor showed higher response compared with different types of sensors including ZnO nanocrystals, Sn- and In-doped ZnO thin film, or ZnO nanowires. The enhanced response was attributed to the additional modulation of the sensor resistance due to potential barrier at nanowire/nanowire junctions as well as the surface depletion region of each nanowire. Also nanobridge structure enabled fast recovery behavior because desorbed gas molecules can be easily swept away from the surface of ZnO nanowire without re-adsorption.

Introduction

Ever since Seiyama et al. [1] discovered in 1962 that the electrical conductivity of ZnO could be dramatically changed by the presence of reactive gases in the air, there have been tremendous reports on the applications of semiconducting-metal oxides as gas sensors due to their small dimensions, low cost, and high compatibility with microelectronic processing. It was known that unintentionally doped ZnO always displayed n-type behavior and the responsible donors were usually identified as the O vacancy V_O, the Zn interstitial Zn_I, or defect complex [2], [3], [4]. Thus, when a ZnO gas sensor is exposed to oxidizing ambient for example, oxygen molecules are adsorbed on the surface of ZnO and then extract electrons from the surface, resulting in the increase of the depletion layer.

Semiconducting-metal-oxide gas sensors have been usually fabricated in the form of thin films, in which ceramic powders are screen printed on prefabricated electrodes, followed by firing at high temperature [5], [6], [7]. In thin-film gas sensors, only a small fraction of the species adsorbed near the grain boundaries is active in modifying the electrical transport properties. This causes the low sensitivity because of the limited surface-to-volume ratio, which is difficult to overcome.

Recently, one-dimensional (1D) semiconductor nanostructures including nanowires, nanotubes, and nanobelts have attracted considerable interest for their potential as the building blocks for fabricating various nanodevices. Due to the high surface-to-volume ratios and high crystallinity of these 1D nanostructures, their major application were first made on the chemical/gas sensors [8], [9], [10], [11]. Nanowire gas sensors have been fabricated using a variety of metal-oxide semiconductors (e.g., SnO₂, ZnO, WO₃, In₂O₃, V₂O₅, TiO₂, etc.) as gas-sensing materials [12], [13], [14], [15], [16], [17]. Recently, there are also many reports on the enhancement of gas response by catalyzing or doping of metal-oxide semiconductor nanowires with noble-metal nanoparticles such as Pd, Ru, or Au [18], [19], [20]. On the other hand, from the viewpoint of device structure, nanowire-based gas sensors were fabricated either by pick-and-place process of a single nanowire followed by making electrical contacts to individual nanowires using expensive and time consuming fabrication techniques such as electron-beam lithography [21] or by a series of processes involving synthesis, sonication, and dispersal of ensemble nanowires on another substrate with prefabricated electrodes [22]. Due to these complicated fabrication processes, their practical application of nanowire gas sensors is very limited.

Here we report on the fabrication of ZnO-nanowire gas sensor, whose fabrication process does not require any expensive and complicated fabrication techniques such as e-beam lithography. The gas-sensing capability of ZnO-nanowire gas sensor was demonstrated for NO₂ gas and the responsible gas-sensing mechanism was suggested.