Bandgap Engineering and Doping of CdO
The unique properties of metal-oxide semiconductors make them well suited to a variety of optoelectronic applications. Metal oxides comprise all industrially-relevant transparent conductors (TCs), and there is significant interest in their development as photovoltaic (PV) absorbers, light-emitting diodes, and electrodes in photoelectrochemical (PEC) devices. TCs are heavily-doped, wide-bandgap semiconductors that must simultaneously exhibit high conductivity and high optical transparency across (and beyond) the visible spectral range, but such materials face severe fundamental tradeoffs between transparency and conductivity that preclude the use of TCs in a number of applications that require infrared and near-infrared transparency. High-mobility TC materials such as CdO can acheive a better balance of transparency and conductivity, but their short-wavelength transparency is limited by a low intrinsic direct bandgap of 2.2 eV.
This dissertation explores how the properties of CdO can be tuned by bandgap engineering and doping and discusses their use in TC and photoelectrochemical applications. Alloys of the structurally-mismatched endpoint compounds CdO--ZnO have attracted considerable interest within the field of oxide bandgap engineering as they allow the bandgap of ZnO (3.3 eV) to be tuned across the visible range with the incorporation of Cd into hexagonal ZnO. Little is known about the properties of cubic alloys based on Zn in CdO, however.
The correlated structural, optical, and electrical properties of CdxZn1-x thin film alloys (0xZn1-x alloys are determined by three complementary techniques: ion-irradiation-induced pinning of the Fermi level at the Fermi-level stabilization energy, X-ray photoelectron spectroscopy, and soft X-ray absorption and emission spectroscopy. The three techniques find consensus in explaining the origin of compositional trends in the optical-bandgap narrowing upon Cd incorporation in wurtzite ZnO and widening upon Zn incorporation in rocksalt CdO. The conduction band minimum is found to be stationary for both wurtzite and rocksalt alloys, and a significant upward rise of the valence band maximum accounts for the majority of these observed bandgap changes. These band alignment details, combined with the unique optical and electrical properties of the two phase regimes, make CdxZn1-x alloys attractive candidates for PEC water splitting applications. A proposed device structure of a CdxZn1-xO-Si tandem PEC is presented.
Finally, the unique electrical properties and doping behavior of CdO are examined using simulations based on the amphoteric defect model. The calculated doping and mobility limits are compared with experimental values for Ga-doped CdO.