Metal organic chemical vapor deposition of indium oxide for ozone sensing

In this work, a novel type of ozone sensor based on the photoreduction and oxidation principles operating at room temperature was developed. First, MOCVD techniques were used to grow high-quality Indium oxide thin films and nanostructures, to be used as ozone sensing materials. On sapphire substrates, highly textured bcc-In2(O)3 films were grown using a low-temperature indium oxide film as the buffer layer. Furthermore, for the first time, meta-stable rh-In2(O)3 films were epitaxially grown on sapphire substrate by means of MOCVD, which usually can only be obtained under high-temperature and high-pressure conditions. In2(O)3 nanoparticles were also obtained at low substrate temperatures using TMIn and water vapour as the precursors, leading to a low-cost fabricating process. In addition, In2(O)3 thin films were successfully deposited on III-N (GaN, AlN, and InN) substrates. By using a thin h-InN film as a buffer layer, epitaxial single crystalline bcc-In2(O)3 (111) was grown. By comparing the electrical properties of different In2(O)3 thin films, it was found that the In2(O)3 nanostructures and In2(O)3 thin layers on AlN substrates are most suitable for the application of ozone sensing. To characterize ozone sensors, an automated ozone measuring station was established which allows gas sensing measurements under different conditions (in vacuum, in different gases, in humid atmospheres). The integrated ozone sensor structure and its electronic control unit were developed. Different operational modes of ozone sensors were studied, and it was found that sensor operation in pulse mode results in stable and fast sensing. The ideal operation temperature of ozone sensors based on the photoreduction and oxidation principle was determined to be room temperature. The required photon energy and light intensity were also determined for the reactivation of the active In2(O)3 layer, which enables the integration of In2(O)3 nanoparticle based ozone sensors with UV-LEDs, leading to compact, low-energy consumption, low-cost ozone sensors. The smallest integrated ozone sensor developed was 300 ?m * 300 ?m. The ozone sensitivity was determined to be greater than 10(5) in vacuum. Not only in vacuum but also in synthesized air, the sensor showed good ozone sensing results. The lowest detectable ozone concentration was found to be ~ 13 ppb. In addition, this type of ozone sensor showed a good long-term stability, and the cross-sensitivity against other oxidizing gases, such as NOx, CO(2), O(2) was very low. Tanking into account that the integrated LED operates at a current of 10 mA at 3 V, the total energy consumption including the electronic control unit was determined to be less than 50 mW. Furthermore, measurements under real conditions were carried out by an ozone sensor controlled by the developed electronic unit in the center of the Freiburg city, Germany. Ozone concentrations as low as 12 ppb were measured. Thus, compact, portable, low-cost, low-energy consumption, environmental ozone sensors were developed, which can operate at room temperature and are suitable for integration in plastic packages, in cellular phones and PDAs. To understand the mechanisms of photoreduction and oxidation effects, electrical and structural characterization of ozone sensing layer after various treatments were performed. It was found that the contamination on the nanoparticle surface was reduced after the first cycle of photoreduction and oxidation. The adsorbed oxygen species, which was analyzed to be O- compared to the widely accepted O3 - in the literature , plays a major role during ozone sensing, and the adsorption and desorption process takes place mainly at the nanoparticle surfaces. The adsorbed O-species form a dipole layer with the positive-charged vacancies, leading to an upward shift of the work function on nanoparticle surfaces. Furthermore, the photoreduction process occurs throughout the sensing layer at the same time, while gas diffusion dominates in the oxidation process. Due to the high oxygen deficiency on indium oxide nanoparticle surfaces, physical models were proposed.