Soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) in the photon-energy range around 1 keV combines resolution in electron momentum k with enhanced photoelectron escape depth. These benefits allow previously unthinkable k-resolved electronic structure studies on buried heterostructures and impurities, which are in the heart of solid-state quantum devices.
For example, soft-X-ray ARPES experiments on AlGaN/GaN high-electron-mobility transistor heterostructures find anisotropy of the interfacial quantum-well states, resulting in anisotropic high-field electron transport [Nature Comm. 9 (2018) 2653]. For the oxide interfaces LaAlO3/SrTiO3, resonant photoexcitation of the Ti-derived interfacial charge carriers resolves their multiphonon polaronic nature, fundamentally limiting their mobility [Nature Comm. 7 (2016) 10386]. Experiments on NbN/GaN superconductor/semiconductor heterostructures find that the Fermi states of NbN are well separated from the GaN states in energy and k. Excluding poisoning of the superconductivity, this allows the integration of NbN-based quantum devices into semiconductor technology [Sci. Adv. 7 (2021) eabi5833].
Impurity systems are illustrated by In(Fe)As, where resonant photoexcitation of the Fe-derived states identifies their full integration into the conduction band of GaAs, allowing high mobility of spin-polarized electrons [Phys. Rev. B 103 (2021) 115111]. Other cases include magnetic V impurities in the topological Bi3Se2 competing with the quantum anomalous-Hall effect, etc.