Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

Spin mapping at the nanoscale and atomic scale

Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoSx) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset o...

Molybdenum sulfide/N-doped CNT forest hybrid catalysts for high-performance hydrogen evolution reaction.

Spintronics is one of the emerging research fields in nanotechnology and has been growing very rapidly. Studies of spintronics were started after the discovery of giant magnetoresistance in 1988, which utilized spin-polarized electron transport across a non-magnetic metallic layer. Within 10 years, this discovery had been implemented into hard disk drives, the most common storage media, followed by recognition through the award of the Nobel Prize for Physics 19 years later. We have never experienced such fast development in any scientific field. Spintronics research is now moving into second-generation spin dynamics and beyond. In this review, we first examine the historical advances in spintronics together with the background physics, and then describe major device applications.

https://iopscience.iop.org/article/10.1088/0022-3727/47/19/193001/pdf

Future perspectives for spintronic devices

Selective growth of wurtzite GaN and AlxGa1-xN on GaN/sapphire substrates by metalorganic vapor phase epitaxy

Coherently strained InAs quantum dots (QDs) with narrow inhomogeneous broadening were grown by molecular beam epitaxy (MBE) using the Stranski-Krastanov (SK) growth mode. The increase in the InAs dot size and decrease in the dot density were induced by surface migration enhancement due to the low arsenic pressure below 6×10-7 Torr. In addition, the low arsenic pressure and the low growth rate produced the self size-limiting effect, which was attributed to the inhibition of indium incorporation. As a result, the narrowest photoluminescence (PL) linewidth of 18.6 meV (14 K) was successfully obtained from the single InAs/GaAs QD layer.

https://iopscience.iop.org/article/10.1143/JJAP.39.L1245/pdf

Stranski-Krastanov Growth of InAs Quantum Dots with Narrow Size Distribution

Self-assembled InAs quantum dots on GaSb/GaAs(0 0 1) layers by molecular beam epitaxy

We have investigated carrier spin dynamics in highly uniform self-assembled InAs quantum dots. The highly uniform quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of inhomogeneous broadening. The spin relaxation times in the ground state and the second state were measured to be 1.0 and 0.6 ns, respectively. Our measurements reveal the absence of the carrier density dependence of the spin relaxation time. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well explained in terms of the mechanism of acoustic phonon emission.

Spin relaxation dynamics in highly uniform InAs quantum dots

Semiconductor quantum dots (QDs) have a potential to increase the power conversion efficiency in photovoltaic operation because of the enhancement of photoexcitation. Recent advances in self-assembled QD solar cells (QDSCs) and colloidal QDSCs are reviewed, with a focus on understanding carrier dynamics. For intermediate-band solar cells using self-assembled QDs, suppression of a reduction of open circuit voltage presents challenges for further efficiency improvement. This reduction mechanism is discussed based on recent reports. In QD sensitized cells and QD heterojunction cells using colloidal QDs well-controlled heterointerface and surface passivation are key issues for enhancement of photovoltaic performances. The improved performances of colloidal QDSCs are presented.

https://www.spiedigitallibrary.org/journals/journal-of-photonics-for-energy/volume-6/issue-4/040901/Recent-progress-on-quantum-dot-solar-cells-a-review/10.1117/1.JPE.6.040901.pdf

Recent progress on quantum dot solar cells: a review

Selective epitaxial growth of GaAs was carried out by atmospheric-pressure metalorganic chemical vapor deposition, and lateral supply mechanisms of reactant species from mask areas to window areas were investigated. Since reactant species arriving on SiO2 surfaces were easily reevaporated, deviation from mass balance was observed. In the case of a SiO2 mask with width of more than 20 µm, lateral vapor-phase diffusion dominated over increment of selective epilayers because of short surface-diffusion length on SiO2 surfaces (<1 µm). It was apparent experimentally and analytically that long ridged tails of selective epilayers were formed by lateral vapor-phase diffusion. Appearance of wavelike tails at mask edges was due to surface diffusion from (111)B facets. Surface-diffusion length on (111)B surfaces was estimated at 3 µm.

Koichi Yamaguchi

Papers

Stranski-Krastanov Growth of InAs Quantum Dots with Narrow Size Distribution

Self-assembled InAs quantum dots on GaSb/GaAs(0 0 1) layers by molecular beam epitaxy

Spin relaxation dynamics in highly uniform InAs quantum dots

Recent progress on quantum dot solar cells: a review

Lateral Supply Mechanisms in Selective Metalorganic Chemical Vapor Deposition