Study on ECR dry etching and selective MBE growth of AlGaN/GaN for fabrication of quantum nanostructures on GaN (0001) substrates

This paper attempts to form AlGaN/GaN quantum wire (QWR) network structures on patterned GaN (0 0 0 1) substrates by selective molecular beam epitaxy (MBE) growth. Substrate patterns were prepared along - and -directions by electron cyclotron resonance assisted reactive-ion beam etching (ECR-RIBE) process. Selective growth was possible for both directions in the case of GaN growth, but only in the -direction in the case of AlGaN growth. A hexagonal QWR network was successfully grown on a hexagonal mesa pattern by combining the -direction and two other equivalent directions. AFM observation confirmed excellent surface morphology of the grown network. A clear cathodoluminescence (CL) peak coming from the embedded AlGaN/GaN QWR structure was clearly identified

Coherent strain evolution at the initial growth stage of AlN on SiC(0001) proved by in-situ synchrotron X-ray diffraction

We carry out real-time in situ synchrotron X-ray diffraction on the growth of GaN and AlN on SiC(0001) substrate by molecular beam epitaxy. At the initial growth stage of GaN/SiC or AlN/SiC heterostructure, the epitaxial overlayer develops affected by defects with the strain interaction between SiC, eventually showing characteristic lattice deformations. GaN was almost relaxed at the initial stage, showing the transition of the growth mode from 3D to 2D at around 4 nm. In contrast, AlN coherently grow on SiC(0001) at the initial stage for 13 nm concomitantly showing lattice deformation with the beneath SiC, subsequently showing gradual lattice relaxation

Designing Semiconductor Nanowires for Efficient Photon Upconversion via Heterostructure Engineering

Energy upconversion via optical processes in semiconductor nanowires (NWs) is attractive for a variety of applications in nano-optoelectronics and nanophotonics. One of the main challenges is to achieve a high upconversion efficiency and, thus, a wide dynamic range of device performance, allowing efficient upconversion even under low excitation power. Here, we demonstrate that the efficiency of energy upconversion via two-photon absorption (TPA) can be drastically enhanced in core/shell NW heterostructures designed to provide a real intermediate TPA step via the band states of the narrow-bandgap region with a long carrier lifetime, fulfilling all the necessary requirements for high-efficiency two-step TPA. We show that, in radial GaAs(P)/GaNAs(P) core/shell NW heterostructures, the upconversion efficiency increases by 500 times as compared with that of the constituent materials, even under an excitation power as low as 100 mW/cm2 that is comparable to the 1 sun illumination. The upconversion efficiency can be further improved by 8 times through engineering the electric-field distribution of the excitation light inside the NWs so that light absorption is maximized within the desired region of the heterostructure. This work demonstrates the effectiveness of our approach in providing efficient photon upconversion by exploring core/shell NW heterostructures, yielding an upconversion efficiency being among the highest reported in semiconductor nanostructures. Furthermore, our work provides design guidelines for enhancing efficiency of energy in NW heterostructures.Funding Agencies|Swedish Research Council [2019-04312]; Swedish Foundation for International Cooperation in Research and Higher Education (STINT) [JA2014-5698]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; KAKENHI from the Japan Society for the Promotion of Science [16H05970, 19H00855, 21KK0068]; Japan Society for the Promotion of Science</p