2020, Kamimura, Jumpei, Budde, Melanie, Bogdanoff, Peter, Tschammer, Carsten, Abdi, Fatwa F., van de Krol, Roel, Bierwagen, Oliver, Riechert, Henning, Geelhaar, Lutz

The photoelectrochemical properties of n-type Ga-polar GaN photoelectrodes covered with NiO layers of different thicknesses in the range 0–20 nm are investigated for aqueous solution. To obtain layers of well-defined thickness and high crystal quality, NiO is grown by plasma-assisted molecular-beam epitaxy. Stability tests reveal that the NiO layers suppress photocorrosion. With increasing NiO thickness, the onset of the photocurrent is shifted to more positive voltages and the photocurrent is reduced, especially for low bias potentials, indicating that hole transfer to the electrolyte interface is hindered by thicker NiO layers. Furthermore, cathodic transient spikes are observed under intermittent illumination, which hints at surface recombination processes. These results are inconsistent with the common explanation of the protection mechanism that the band alignment of GaN/NiO enables efficient hole-injection, thus preventing hole accumulation at the GaN surface that would lead to anodic photocorrosion. Interestingly, the morphology of the etch pits as well as further experiments involving the photodeposition of Ag indicate that photocorrosion of GaN photoanodes is related to reductive processes at threading dislocations. Therefore, it is concluded that the NiO layers block the transfer of photogenerated electrons from GaN to the electrolyte interface, which prevents the cathodic photocorrosion. © 2020 The Authors. Solar RRL published by Wiley-VCH GmbH

2019, Luna, Esperanza, Trampert, Achim, Lu, Jing, Aoki, Toshihiro, Zhang, Yong-Hang, McCartney, Martha R., Smith, David J.

Semiconductor heterostructures are intrinsic to a wide range of modern-day electronic devices, such as computers, light-emitting devices, and photodetectors. Knowledge of chemical interfacial profiles in these structures is critical to the task of optimizing the device performance. This work presents an analysis of the composition profile and strain across the noncommon-atom heterovalent CdTe/InSb interface, carried out using a combination of electron microscopy imaging techniques. Because of the close atomic numbers of the constituent elements, techniques such as high-angle annular-dark-field and large-angle bright-field scanning transmission electron microscopy, as well as electron energy-loss spectroscopy, give results from the interface region that are inherently difficult to interpret. By contrast, use of the 002 dark-field imaging technique emphasizes the interface location by comparing differences in structure factors between the two materials. Comparisons of experimental and simulated CdTe-on-InSb profiles reveal that the interface is structurally abrupt to within about 1.5 nm (10–90% criterion), while geometric phase analysis based on aberration-corrected electron microscopy images reveals a minimal level of interfacial strain. The present investigation opens new routes to the systematic investigation of heterovalent interfaces, formed by the combination of other valence-mismatched material systems. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim