In this thesis, the chemical and electronic structure of Ga2O3 is investigated by means of electron and x-ray spectroscopic techniques. Ga2O3 has a wide bandgap (4.4 - 4.9 eV), which means that it does not absorb visible light. It belongs to two classes of materials: the ultrawide-bandgap (UWBG) semiconductors, and the transparent conductive oxides (TCOs). The wide bandgap and optical transparency make Ga2O3 attractive for usage in a variety of applications, e.g., in electronics and photovoltaics.
In the multilayer structure of Cu(In,Ga)Se2 (CIGSe)-based thin-film solar cells, a buffer layer is needed for the formation of a p-n junction, as well as to separate the absorber from the transparent front contact. Ga2O3 has shown promising potential as buffer layer material, offering several advantages, including its high bandgap for reduced visible light absorption and in-vacuum processability for optimal integration in an inline solar-cell production process. Understanding the chemical structure of this layer and its interface with the absorber is crucial for further optimization of the device performance of the corresponding thin-film solar cells. ... mehrIn the first part of this thesis, the chemical structure of the interface between a sputter-deposited Ga2O3 buffer layer and the CIGSe absorber prepared with a state-of-the-art RbF postdeposition treatment, is studied in detail. Particular focus is placed on understanding the impact of the RbF postdeposition treatment and an ammonia-based rising step on the chemical structure of the Ga2O3/CIGSe interface. Using a combination of synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES), and laboratory-based x-ray photoelectron spectroscopy (XPS) and x-ray excited Auger electron spectroscopy (XAES), a detailed and depth-varied picture of the chemical structure is painted. It is found that the ammonia-based rinse has a significant impact on the chemical structure, partially removing Rb and completely removing F, as well as removing Ga-F, Ga-O, and In-O surface bonds, and reducing the Ga/(Ga+In) ratio at the absorber surface. After Ga2O3 deposition, the formation of In oxides is identified, and the diffusion of Rb and small amounts of F into/onto the Ga2O3 buffer layer is observed.
The electronic properties at the surface, i.e., the positions of the valence band maximum (VBM) and conduction band minimum (CBM) with respect to the Fermi level (EF), the band alignment at the Ga2O3/CIGSe interface, and the bandgap (Eg), directly influence the charge transport, and hence the efficiencies of devices. Thus, understanding the electronic structure of Ga2O3 is equally (if not even more) important as understanding its chemical structure. In the next part of the thesis, the valence bands of three differently prepared Ga2O3 surfaces are investigated. The solar cell samples with the highest thickness of Ga2O3 were taken as a model of nanocrystalline Ga2O3, and β-Ga2O3 single crystals were prepared in two different ways: one sample was cleaved under ultra-high vacuum (UHV) to obtain a clean β-Ga2O3 single crystal surface, while another underwent a mild Ar+-ion treatment to introduce defects, such as oxygen vacancies to the surface. By measuring the valence bands for all three samples with photoelectron spectroscopy (PES) at a wide range of photon excitation energies (70 eV - 6.3 keV), both the surface and the near-bulk electronic structure of the samples could be investigated. Density functional theory (DFT)-based calculations were performed for β-Ga2O3, for a better understanding of the experimental results. While the VBM for the UHV-cleaved β-Ga2O3 is determined as 4.8 ± 0.1 eV, independent of the photon excitation energy, the VBM of the nanocrystalline Ga2O3, and the Ar+-ion treated β-Ga2O3 single crystal samples are significantly different depending on whether the bulk or the surface is being probed. Strong tails in the VBM of the surface-sensitive measurements, likely induced by surface defects, and a downwards shift of the VBM from EF with increasing photon excitation energy, likely due to surface adsorbate-induced band-bending, are observed in the latter two samples.
In the last part of this thesis, the electronic structure of the bulk of the 𝛽-Ga2O3 single crystals are studied by means of x-ray emission spectroscopy (XES), x-ray absorption spectroscopy (XAS), and resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. 𝛽-Ga2O3 is highly anisotropic, so its optical and electronic properties are expected to change depending on the orientation of the crystal with respect to the polarization of the incoming x-ray beam. Polarization-dependent measurements allow to selectively excite different regions within the band structure as well. Thus, polarization-dependent measurements were performed by rotating differently oriented 𝛽-Ga2O3 single crystals with respect to the incoming x-ray beam. In addition to DFT calculations for the ground state, calculations for the excited system with a core/valence exciton in XAS and RIXS were performed using the Bethe-Salpeter Equation (BSE) method. The BSE calculations are able to capture all the main features of the calculated spectra correctly. There are clear differences in the measured spectra depending on the polarization direction, which are all reproduced in the calculated spectra as well. Evidence for core-exciton formation are observed in all the experimental RIXS spectra. Finally, an electron-photon scattering lifetime could be determined from the RIXS spectra as 3 ± 2 fs, in agreement with literature results.
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