Doctor of Philosophy
Stavola, Michael J.
Other advisers/committee members
Fowler, W B.; DeLeo, Gary; Biaggio, Ivan; Ferguson, Gregory S.
Transparent conducting oxides (TCOs) are wide-band-gap, metal-oxide semiconductors that combine high electrical conductivity with transparency to visible light. These properties have made TCOs attractive candidates for many optoelectronics applications. Although the origin of the conductivity of TCOs has traditionally been attributed to native defects such as oxygen vacancies and cation interstitials, theoretical calculations show that these defects are deep-level defects and therefore cannot be responsible for the n-type conductivity. Recent reports have shown that hydrogen, often incorporated unintentionally into the sample during crystal growth, acts as a shallow donor and is likely a source of the unexpected conductivity. Understanding the physical and chemical properties of hydrogen-related defects in these materials is vital for controlling the conductivity, and for the continued development of transparent electronic devices.Fourier Transform Infrared (FTIR) spectroscopy is a powerful tool for studying the vibrational properties of hydrogen-related defects in semiconductors, and can reveal information about the atomic composition, microscopic structure, and thermal stability of a defect. Measurement of the absorption in the IR spectral region due to free carriers provides a contact-free method for probing the electrical conductivity of a material. This dissertation presents three different studies of defects in TCOs.Experimental methods were developed to quantitatively measure the transmission spectra of materials in the near-infrared and mid-infrared spectral regions at different temperatures using an FTIR spectrometer. This technique was used to study the free carrier absorption and the carrier scattering mechanisms in n-type, hydrothermally grown, ZnO single crystals.Previous IR studies have assigned the 3306 cm-1 O-H vibrational line observed in as-grown and hydrogenated In2O3 single crystals to the interstitial hydrogen shallow donor (Hi+). Polarized IR absorption measurements of In2O3 single crystals under  and  uniaxial stresses reveal that the Hi+ defect has  monoclinic symmetry, which is consistent with that predicted by theory. A stress-induced alignment has been observed in samples under  stress. The reorientation process giving this alignment corresponds to a diffusion jump. The combination of diffusivities determined at low temperature from uniaxial stress experiments and high temperatures from bulk diffusion experiments (Ying Qin) reveal the diffusivity of Hi+ across 10 decades. Monoclinic gallium oxide (b-Ga2O3) is a TCO that has received renewed interest due to its large band gap of 4.9 eV, which makes it transparent at ultra-violet wavelengths. Hydrogenated b-Ga2O3 samples show a strong O-H vibrational line at 3437 cm-1 comprising two equivalent hydrogens. The line has a high thermal stability (~ 900 °C) and exhibits interesting polarization properties. The combination of experimental data and first principles calculations using CRYSTAL06 suggest that the 3437 cm-1 O-H line originates from a gallium vacancy complexed with two hydrogens.
Weiser, Philip Michael, "Studies of Hydrogen Defects and Free-Carrier Absorption in Transparent Conducting Oxides using Fourier Transform Infrared Spectroscopy" (2017). Theses and Dissertations. 2872.
Available for download on Friday, June 01, 2018