Browsing by Department "National Physical Laboratory (NPL)"
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- AlOx surface passivation of black silicon by spatial ALD: Stability under light soaking and damp heat exposure
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-03-01) Heikkinen, Ismo T. S.; Koutsourakis, George; Virtanen, Sauli; Yli-Koski, Marko; Wood, Sebastian; Vähänissi, Ville; Salmi, Emma; Castro, Fernando A.; Savin, HeleScientific breakthroughs in silicon surface passivation have enabled commercial high-efficiency photovoltaic devices making use of the black silicon nanostructure. In this study, the authors report on factors that influence the passivation stability of black silicon realized with industrially viable spatial atomic layer deposited (SALD) aluminum oxide (AlOx) under damp heat exposure and light soaking. Damp heat exposure conditions are 85 °C and 85% relative humidity, and light soaking is performed with 0.6 sun illumination at 75 °C. It is demonstrated that reasonably thick (20 nm) passivation films are required for both black and planar surfaces in order to provide stable surface passivation over a period of 1000 h under both testing conditions. Both surface textures degrade at similar rates with 5 and 2 nm thick films. The degradation mechanism under damp heat exposure is found to be different from that in light soaking. During damp heat exposure, the fixed charge density of AlOx is reduced, which decreases the amount of field-effect passivation. Degradation under light soaking, on the other hand, is likely to be related to interface defects between silicon and the passivating film. Finally, a thin chemically grown SiOx layer at the interface between the AlOx film and the silicon surface is shown to significantly increase the passivation stability under both light soaking and damp heat exposure. The results of this study provide valuable insights into surface passivation degradation mechanisms on nanostructured silicon surfaces and pave the way for the industrial production of highly stable black silicon devices. - Josephson penetration depth in coplanar junctions based on 2D materials
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2019-11-07) Li, Tianyi; Gallop, John C.; Hao, Ling; Romans, Edward J.Josephson junctions and superconducting quantum interference devices with graphene or other 2D materials as the weak link between superconductors have become a hot topic of research in recent years, with respect to both fundamental physics and potential applications. We have previously reported ultrawide Josephson junctions (up to 80 μm wide) based on chemical-vapor-deposition graphene where the critical current was found to be uniformly distributed in the direction perpendicular to the current. In this paper, we demonstrate that the unusually large Josephson penetration depth λ J that this corresponds to is enabled by the unique geometric structure of Josephson junctions based on 2D materials. We derive a new expression for the Josephson penetration depth of such junctions and verify our assumptions by numerical simulations. - Qualitative analysis of scanning gate microscopy on epitaxial graphene
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2019-02-28) Mackenzie, David M. A.; Panchal, Vishal; Corte-Leon, Hector; Petersen, Dirch H.; Kazakova, OlgaWe present scanning gate microscopy (SGM) studies of graphene Hall-cross devices where bi-layer graphene (2LG) regions show unexpected signal inversion relative to single-layer graphene (1LG), an observation reproduced via finite element modelling of the current densities. This is attributed to gate-induced charge carrier redistribution between the two layers in 2LG. Hall cross devices were fabricated from epitaxial graphene 6H-SiC(0001) and were covered by 1LG/2LG with the area ratio of 85: 15%, respectively. Local electric-field sensitivity maps of the devices were obtained in two different measurement geometries using electrical SGM with a conductive tip, where it was observed that the voltage of 2LG islands was inverted relative to anticipated reference maps. Finite element modelling of the current densities and voltage response showed good qualitative agreement with the SGM maps when the effect of the gate was reversed for 2LG. The behaviour is attributed to gate-induced charge carrier redistribution between the two layers in 2LG. The model can be used generally as a tool to predict mixed 1LG/2LG response to electric field. Moreover, regions near the corners of the device show the highest sensitivity when the local electric field was applied to the scanning probe microscopy tip. These regions are capable of detecting highly local electric fields down to 110 kV cm(-1).