Metal catalysis-free, direction-controlled planar growth of single-crystalline alpha-Si3N4 nanowires on Si(100) substrate

Single-crystalline alpha-Si3N4 nanowires are controlled to grow perpendicular to the wet-etched trenches in the SiO0.94 film on the plane of the Si substrate without metal catalysis. A detailed characterization is carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photoluminescence at 600 nm from alpha-Si3N4 nanowires is attributed to the recombination at the defect state formed by the Si dangling bond N3 equivalent to Si-center dot. The growth mechanism is considered to be related to the catalysis and nitridation of SiO nanoclusters preferably re-deposited around the inner corner of the trenches, as well as faster Si diffusion along the slanting side walls of the trenches. This simple direction-controlled growth method is compatible with the CMOS process, and could facilitate the fabrication of alpha-Si3N4 nanoelectronic or nanophotonic devices on the Si platform.

Synthesis and structures of morphology-controlled ZnO nano- and microcrystals

Zinc oxide flower-like bunches were directly synthesized on indium-doped tin oxide (ITO) glass substrates through a simple chemical bath deposition process. By adjusting precursor concentration, other morphologies ( spindles and rods) were also obtained. All of them are hexagonal and single crystalline in nature and grow along the [ 0001] crystallographic direction. The possible growth mechanisms for these nano- and microcrystals were proposed. It was revealed that both the inherent highly anisotropic structure of ZnO and the precursor concentration play crucial roles in determining final morphologies of the products. In addition, vibrational properties of ZnO crystals with different morphologies were investigated by Raman spectroscopy.

Characterization of self-organized InAs/GaAs quantum dots under strain-induced and temperature-controlled nucleation mechanisms by atomic force microscopy and photoluminescence spectroscopy

Atomic force microscopy and photoluminescence spectroscopy (PL) has been used to study asymmetric bilayer InAs quantum dot (QD) structures grow by molecular-beam epitaxy on GaAs (001) substrates. The two InAs layers were separated by a 7-nm-thick GaAs spacer layer and were grown at different substrate temperature. We took advantage of the intrinsic nonuniformity of the molecular beams to grow the seed layer with an average InAs coverage of 2.0 ML. Then the seed layer thickness could be divided into three areas: below, around and above the critical thickness of the 2D-3D transition along the 11101 direction of the substrate. Correspondingly, the nucleation mechanisms of the upper InAs layer (UIL) could be also divided into three areas: temperature-controlled, competition between temperature-controlled and strain-induced, and strain-induced (template-controlled) nucleation. Small quantum dots (QDs) with a large density around 5 x 10(10) cm(-2) are found in the temperature-controlled nucleation area. The QD size distributions undergo a bimodal to a unimodal transition with decreasing QD densities in the strain-induced nucleation area, where the QD densities vary following that of the seed layer (templating effect). The optimum QD density with the UIL thickness fixed at 2.4 ML is shown to be around 1.5 x 10(10) cm(-2), for which the QD size distribution is unimodal and PL emission peaks at the longest wavelength. The QDs in the in-between area exhibit a broad size distribution with small QDs and strain-induced large QDs coexisting.

Controlled growth of III-V compound semiconductor nano-structures and their application in quantum-devices

Various techniques on the growth of self-assembled compound semiconductor nano-structures (quantum dots, QDs) have been tried to enhance the controlling on size, density, emitting wavelength, uniformity in size and ordering in location of the QDs. Optimized growth conditions have been used in the application of the QD materials in opto-electronic devices. High-power long-lifetime quantum-dot laser-diodes (QD-LDs) emitting near 1 mu m, QD-LDs emitting in red-light range, 1.3 mu m QD-LDs on GaAs substrate and quantum-dot super-luminescent diodes (QD-SLDs) have successfully been achieved.

Molecular-beam epitaxial growth of position controlled InAs islands on cleaved edge of InGaAs/GaAs superlattice

A promising approach for positioning of InAs islands on (110)GaAs is demonstrated. By combining self-assembly of quantum dots with solid source molecular beam epitaxy (MBE) on cleaved edge of InGaAs/GaAs superlattice (SL), linear alignment of InAs islands on the InGaAs strain layers have been fabricated The cleaved edge of InGaAs/GaAs SL acts as strain nanopattern for InAs selective growth. Indium atoms incident on the surface will preferentially migrate to InGaAs regions where favorable bonding sites are available. The strain nanopattern's effect is studied by the different indium fraction and thickness of InxGa1-xAs/GaAs SL. The ordering of the InAs islands is found to depend on the properties of the underlying InGaAs strain layers.

Anomalous shift of the beating nodes in illumination-controlled In1-xGaxAs/In1-yAlyAs two-dimensional electron gases with strong spin-orbit interaction

The beating patterns in the Shubnikov-de Haas oscillatory magnetoresistance originating from zero-field spin splitting of two-dimensional electron gases (2DEGs) in In0.52Al0.48As/InxGa1-xAs/In0.52Al0.48As quantum wells with silicon delta doped on the upper barrier layer have been investigated by means of magnetotransport measurements before and after illumination. Contrary to the expectation, after each illumination, the beating nodes induced by the zero-field spin-splitting effect shift to lower and lower magnetic field due to the decrease in the zero-field spin-splitting energy of the 2DEGs. The anomalous phenomenon of the shift of the beating nodes and the decrease in spin-orbit coupling constants after illumination cannot be explained by utilizing the previous linear Rashba model. It is suggested that the decrease in the zero-field spin-splitting energy and the spin-orbit coupling constant arise from the nonlinear Rashba spin splitting.

Controlled growth of silicon nanowires by solid-liquid-solid method and their formation mechanism

High quality silicon nanowires (SiNWs) were grown directly from n-(111) silicon single crystal substrate by using Au film as a metallic catalyst. The diameter and length of the formed nanowires are 30-60 nm and from several micrometers to sereral tens of micrometers, respectively. The effects of Au film thickness, annealing temperature, growth time and N-2 gas flow rate on the formation of the nanowires were experimentally investigated. The results confirmed that the silicon nanowires with controlled diameter, length, shape and orientation can be obtained via reasonably choosing and optimizing various technical conditions. The formation process of the silicon nanowires is analyzed qualitatively based on solid-liquid-solid growth mechanism.

Gate controlled Aharonov-Bohm-type oscillations from single neutral excitons in quantum rings

We report on a magnetophotoluminescence study of single self-assembled semiconductor nanorings which are fabricated by molecular-beam epitaxy combined with AsBr3 in situ etching. Oscillations in the neutral exciton radiative recombination energy and in the emission intensity are observed under an applied magnetic field. Further, we control the period of the oscillations with a gate potential that modifies the exciton confinement. We infer from the experimental results, combined with calculations, that the exciton Aharonov-Bohm effect may account for the observed effects.

CONTROLLED FORMATION AND CHARACTERISTICS OF INTERFACES IN GAAS/ALGAAS SINGLE QUANTUM-WELLS

The influence of heterostructure quality on transport and optical properties of GaAs/AlGaAs single quantum wells with different qualities was studied. In a conventional sample-A, the transport scattering time and the quantum scattering time are small and close to each other. The interface roughness scattering is a dominant scattering mechanism. From comparison between theory and experiment, interface roughness with fluctuation height 2.5 Angstrom and the lateral size of 50-70 Angstrom were estimated. For samples introducing superlattices instead of AlGaAs layers or by utilizing growth interruption, both the transport and PL measurements showed that interfaces were rather smooth in the samples. The two scattering times are much longer. The interface roughness scattering is relegated to an unimportant position. Results demonstrated that it is important to control the formation of heterostructures in order to improve the interface quality.