Characterisation of Complex Interfaces Using Hard X-Ray Photoelectron Spectroscopy

The use of hard X-rays in X-ray photoelectron spectroscopy (XPS) extends this traditionally surface-sensitive technique to probing depths that allow the measurement of bulk samples, buried layers, and, crucial for many device technologies, buried interfaces. By using synchrotron sources, the X-ray properties, including energy and polarisation, can be tuned to allow for varying depth resolution as well as discrimination of different atomic and orbital contributions to spectra, allowing the disentanglement of, for example, complex valence states in extended solids and at interfaces.<br/>This talk will briefly overview the key capabilities of hard X-ray photoelectron spectroscopy (HAXPES) for the comprehensive characterisation and exploration of interfaces. A complex oxide interface designed to create a high room-temperature electron mobility two-dimensional electron gas (2DEG) serves as a prototypical application example of the information gained from buried, strongly localised interfaces. The termination at the interface of the perovskite oxides BaSnO₃ and LaInO₃ can be designed to either suppress or strongly enhance the formation of the 2DEG. Energy-dependent HAXPES delivers a non-destructive depth profile of the chemical states across the multilayer structure as well as provides information on the band alignment, including interfacial band bending key for the existence of a 2DEG. Most importantly, HAXPES directly probes key electronic states, such as the free charge carriers at the Fermi energy resulting from a 2DEG formation. To conclude, the talk will provide an outlook on the most recent technique developments and future applications of the technique across a range of novel materials and device designs.