| Popis: |
In this thesis a variety of photoelectron spectroscopy techniques have been applied to study CdO, in order to gain a more detailed knowledge of both the bulk and surface electronic properties of this material. Hard x-ray photoelectron spectroscopy (HAXPES) has been used, relaxing the surface preparation requirements of conventional photoemission techniques. This allows CdO to be measured in an as-grown condition with the associated higher carrier concentrations. The effects of conduction band filling and final state screening have been observed, and a discussion of the bulk-like sensitivity of HAXPES is given. The use of synchrotron radiation allows variation of the photon energy used to excite photoelectrons, and this is combined with the associated changes in photoionisation cross-sections to study the orbital character of the CdO valence band (VB). These measurements have been compared to density functional theory (DFT) calculations, using three functionals. It was found that using LDA+U and employing a theoretically calculated value of U = 2.34 eV can adequately reproduce the experimentally measured Cd 4d shallow core level position. The photon energy dependence of the shape of the VB, however, was not reproduced by the DFT calculations, indicating that additional Cd 5p character may be present within the VB, and that the O 2p photoionisation cross-sections may by underestimated at higher photon energies. The electron accumulation layer known to exist at the CdO surface has been tuned by the addition of Rb, which allowed the 2D electron density to be almost doubled. The effects of the increased 2D electron density on the surface band gap narrowing have been investigated by simulating the subband dispersion using the Poisson-Schrödinger model. The effective mass of the subband states has been extracted, and was found to be significantly lower than the reported bulk mass, which is discussed in the context of other recent results. |
