Electronic and surface properties of Ga-doped In₂O₃ ceramics

Graphical abstract: - Highlights: • The solubility limit of Ga in In₂O₃ was established to be around 6%. • Ga doping causes a reduction in band gap although the band gap of Ga₂O₃ is larger than that of In₂O₃. • The reduction in band gap is attributed to the role of lone pairs at surfaces and grain boundaries. • A pronounced surface segregation of Ga is observed. - Abstract: The limit of solubility of Ga₂O₃ in the cubic bixbyite In₂O₃ phase was established by X-ray diffraction and Raman spectroscopy to correspond to replacement of around 6% of In cations by Ga for samples prepared at 1250 °C. Density functional theory calculations suggest that Ga substitution should lead to widening of the bulk bandgap, as expected from the much larger gap of Ga₂O₃ as compared to In₂O₃. However both diffuse reflectance spectroscopy and valence band X-ray photoemission reveal an apparent narrowing of the gap with Ga doping. It is tentatively concluded that this anomaly arises from introduction of Ga⁺ surface lone pair states at the top of the valence band and structure at the top of the valence band in Ga-segregated samples is assigned to these lone pair states. In addition photoemission reveals a broadening of the valence band edge. Core X-ray photoemission spectra and low energy ion scattering spectroscopy both reveal pronounced segregation of Ga to the ceramic surface, which may be linked to both relief of strain in the bulk and the preferential occupation of surface sites by lone pair cations. Surprisingly Ga segregation is not accompanied by the development of chemically shifted structure in Ga 2p core XPS associated with Ga⁺. However experiments on ion bombarded Ga₂O₃, where a shoulder at the top edge of the valence band spectra provide a clear signature of Ga⁺ at the surface, show that the chemical shift between Ga⁺ and Ga³⁺ is too small to be resolved in Ga 2p core level spectra. Thus the failure to observe chemically shifted structure associated with Ga⁺ is not inconsistent with the proposal that band gap narrowing is associated with lone pair states at surfaces and interfaces