Semiconductor-based gas sensors have been used for a wide range of applications over the last few decades. In this thesis, sensing properties of pure ZnO and Ga doped ZnO are investigated. There are three types of tested gas species, H2, O2 and NO, and three test temperatures, 300oC, 400oC and 500oC. After measurements of response to exposure to H2 and O2, it is concluded that Ga doped ZnO and ZnO are both n-type metal oxides. In measurements of NO, two test conditions were considered, the case with background O2 (10%) in the gas flow and the case without background O2. NO can be oxidized to NO2 or reduced to N2 and O2. The resistance of Ga doped ZnO and ZnO sensors always decreases for all exposures to NO except for the case in which the Ga doped ZnO sensor was exposed to NO in a background of O2 at 500 oC. In this special case, the resistance of Ga doped ZnO actually increases during exposure to low concentrations of NO (< 30 ppm). It is not clear whether the change in response direction is due to an n-p transition or different reactions between gas molecules and Ga doped ZnO. Work function measurements were therefore conducted to understand more about the electron transfer during gas exposure. The work function measurements suggest that there are probably several stages of interactions between gas molecules and Ga doped ZnO during each gas pulse exposure.

abstract: Transparent conductive oxides (TCOs) are used as electrodes for a number of optoelectronic devices including solar cells. Because of its superior transparent and conductive properties, indium (In) tin (Sn) oxide (ITO) has long been at the forefront for TCO research activities and high-volume product applications. However, given the limited supply of In and potential toxicity of Sn-based compounds, attention has shifted to alternative TCOs like ZnO doped with group-III elements such as Ga and Al. Employing a variety of deposition techniques, many research groups are striving to achieve resistivities below 1E-4 ohm-cm with transmittance approaching the theoretical limit over a wide spectral range. In this work, Ga-doped ZnO is deposited using pulsed laser deposition (PLD). Material properties of the films are characterized using a number of techniques. For deposition in oxygen at pressures >1 mTorr, post-deposition annealing in forming gas (FG) is required to improve conductivity. At these higher oxygen pressures, thermodynamic analysis coupled with a study using the Hall effect measurements and photoluminescence spectroscopy suggest that conductivity is limited by oxygen-related acceptor-like defects in the grains that compensate donors, effectively reducing the net carrier concentration and creating scattering centers that reduce electron mobility. Oxygen is also responsible for further suppression of conductivity by forming insulative metal oxide regions at the grain edges and oxygen-related electron traps at the grain boundaries. The hydrogen component in the FG is thought to passivate the intra-grain acceptor-like defects and improve carrier transport across these grain boundaries. Given this deleterious effect of oxygen on conductivity, depositions are performed in pure argon (Ar), i.e., the only oxygen species in the growth ambient are those ejected directly from the PLD solid source target. Ga-doped ZnO deposited in Ar at 200 °C and 10 mTorr have resistivities of 1.8E-4 ohm-cm without the need for post deposition annealing. Average transmittance of the Ga-doped films is 93% over the visible and near infrared (IR) spectral regions, but free carrier absorption is a limiting factor further into the IR. After annealing in FG at 500 °C, a 300 nm Ar film has a Haacke figure of merit of 6.61E-2 sq. ohm. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2011