Diamond Light Source - Annual Review 2022/23

32 33 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 Taming disorderly oxides Surfaces – Technique Development – Materials Science – Artificial Intelligence – Physics – Hard Condensed Matter – Structures – Electronics – Energy Materials – Information & Communication Technologies Gallium oxide (Ga 2 O 3 ) is an interesting electronic material with potential applications in, for example, power electronics, solar-blind UV photodetectors, gas-sensing devices, and solar cells. It exists in a number of different crystal structures (polymorphs), known as alpha, beta, gamma, delta and epsilon. An international team of researchers wanted to understand how structure influences the electronic structure of one particular phase, γ-Ga 2 O 3 , which is highly disordered and, therefore, incredibly challenging for experiment and theory. This work was an international collaboration across disciplines, including experimentalists and theorists, chemists and physicists, and synthesis and characterisation experts and combined a machine-learning theoretical approach with experimental results. They used both soft and hard X-ray Photoelectron Spectroscopy as well as X-ray Absorption Spectroscopy at beamline I09 at Diamond to investigate the electronic properties of γ-Ga 2 O 3 and how this relates to its crystal structure. They combined their results with first principles calculations and machine learning (ML). In addition, they used a range of complementary characterisation techniques, including X-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry and photoluminescence excitation spectroscopy. Using the theoretical approach (including screening one million structures and more than one thousand individual density functional theory (DFT) calculations), they were able to identify a small number of possible structures γ-Ga 2 O 3 could realistically have. They then validated this by directly comparing theory with the photoelectron spectroscopy results. By using this combined approach, they could identify good descriptions of both structure and electronic structure of this complex material. Disordered systems are increasingly interesting for electronic and optical applications. Unlocking their full potential will involve engineering their structure through targeted synthesis in a way that enables finetuning of their electronic and optical behaviour and performance in a device. This is only possible if we have the fundamental knowledge about the relationships between these aspects and the tools to describe and probe them. Photoelectron spectroscopy is crucial to the exploration of the chemistry and electronic structure of condensed matter, and Diamond houses a number of advanced beamlines that allow the scientific community to use this technique. Hard X-ray Photoelectron Spectroscopy at beamline I09 is particularly crucial as it probes deeper into a material, studying the bulk and buried layers and interfaces, which is important in understanding materials in device applications. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step toward exploring how their electronic structure can be understood in terms of local coordination and overall structure. In addition, it showcases a set of tools that the research team successfully applied to a highly challenging material. Related publication: Ratcliff, L. E. et al. Tackling disorder in γ-Ga 2 O 3 . Advanced Materials 34 , (2022). DOI: 10.1002/adma.202204217 Funding acknowledgement: EPSRC Early Career Research Fellowship (EP/P033253/1), (EP/L01551X/1), (EP/P020194/1 and EP/T022213/1). Thomas Young Center under grant number TYC-101. DFG (project number 446185170). Corresponding authors: Dr Anna Regoutz, University College London, [email protected] Dr Laura Ratcliff, University of Bristol, [email protected] Structures and Surfaces Group Beamline I09 Schematic overview of the parallel experimental and theoretical methods applied to γ-Ga 2 O 3 . The bottom box displays both experimental X-ray photoelectron spectra as well as theoretical spectra and density of states for core, semicore and valence states. MBE: Molecular-beam epitaxy.