Understanding the oxidation and reduction process in transparent conducting oxides

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 133-141).Transparent conductors play important roles in many optoelectronic devices such as LEDs, thin film solar cells, and smart windows through their ability to efficiently transport both photons and electrons. Simultaneous requirements of a wide band gap, high free carrier concentration, and high electron mobility limits the selection of available transparent conductor materials. Further improvements in the optical and electrical properties, along with improvements in processing tolerance, are highly desirable for this material class. One key limitation of current transparent conducting oxides is their response to oxidation, which can cause severe decreases to the conductivity of the material through ionic compensation. Materials with slow oxygen kinetics or resistance to the formation of compensating ionic defects could lead to more flexible operating and processing conditions for applications requiring transparent conductors.The properties of transparent conducting oxides, Al-doped ZnO and La-doped BaSnO₃, were examined through a variety of methods with a focus on the impact of processing on the free carrier concentration, electron transport, and optical properties. Al-doped ZnO was examined as a well-known alternative to indium tin oxide (ITO) that has been shown to be limited by relatively narrow processing conditions and large variances in reported properties. BaSnO₃ is a comparatively new material in the field of transparent conductors, attractive mainly due to its exceptionally high electron mobility for an oxide. Little is currently known about the nature of defects and processing on the optical and electrical properties of this material, but this information will be important to understand before implementing this material in practical devices.For these materials, I examined the roles of oxygen stoichiometry and point defect formation in impacting properties and stability under both processing conditions and harsh operating conditions and explored the limitations and opportunities provided by these transparent conducting oxide systems. Al-doped ZnO thin films were produced by pulsed laser deposition under a variety of oxygen conditions demonstrating the strong dependence of free electron concentration and mobility on the oxidation state of the material. The free carrier absorption in the infrared photon range was measured and modeled and found to agree well with theory assuming ionized impurity scattering as the limiting electron scattering mechanism. These effects were understood through the framework of the formation of compensating zinc vacancies under oxidizing conditions, leading to decreases in the free electron concentration.Atom probe tomography was applied to Al-doped ZnO thin films deposited on Si substrates, demonstrating an effective accumulation of Al near the ZnO/Si interface, but with no detected precipitation or agglomeration in the x-y plane of the film, even for heavily doped films. This was surprising due to the high concentration of Al-dopant in the material, exceeding the thermodynamic solubility limit of bulk ZnO. An accumulation of Al-dopant was observed at the ZnO/Si interface under multiple conditions, with the oxygen atmosphere during deposition and nature of the Si substrate affecting the degree of accumulation. Because transparent conductors are typically used to transfer charge through interfaces, understanding the nature and implications of this observed accumulation effect could be essential to understanding device performance.La-doped and undoped BaSnO₃ thin films and bulk samples were tested for their electrical conductivity in-situ under various temperatures and oxygen partial pressures. In the undoped case, a p-type to n-type transition was observed at lower temperatures with decreasing oxygen partial pressure, with the behavior correlated to the formation and annihilation of oxygen and cation vacancies. Under donor-doping, a measurable, but weak n-type dependence of conductivity was demonstrated, pointing to a surprisingly weak role played by cation vacancy charge compensation over the measured temperature ranges. Compared to other similar oxide systems, compensation by cation vacancies would normally be expected to be strong under oxidizing conditions.This is a key advantage for La-doped BaSnO₃ as a high temperature oxygen stable material compared to other competing materials that are more susceptible to conductivity degradation due to ionic compensation of the donor dopant under oxidizing conditions. This was directly demonstrated in the testing of the conductivity response of La-doped BaSnO₃ thin films that maintained high conductivity under a large range of oxygen and temperature conditions. Oxygen diffusion in the material was estimated from conductivity relaxation and further explored with oxygen tracer diffusion studies. These studies revealed an activation energy of 2 eV for the oxygen diffusion process, as well as a depth dependent diffusivity leading to depressed oxygen diffusivities near the surface. Study of epitaxial and polycrystalline thin films of La-doped BaSnO₃ revealed a difference in the rate of oxidation response of the conductivity.Epitaxial thin films exhibited a weak power law dependence on temperature while polycrystalline thin films under oxidizing conditions exhibited an activation energy of 0.36 eV. This effect was attributed to the formation of narrow space charge regions at the grain boundaries under oxidizing conditions. Simultaneous measurements of the infrared transmission and electrical conductivity of thin films were performed as a means of correlating infrared transmission with conductivity at high temperatures under various controlled atmospheres. These two measurements were found to be strongly correlated and were demonstrated to be connected to the formation and annihilation of free carriers in the thin films. A novel measurement technique was explored in which the conductance response was measured across a substrate during pulsed laser deposition of Al-doped ZnO.The measured conductance profile as a function of time was correlated to the expected growth regimes typical of an island growth mode, and the thickness dependence of resistivity was directly observed. Additional information about the growth conditions was obtained through conductance relaxation after single pulses, performed under different growth chamber atmospheres.by Michael J. Campion.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineerin