III-V compound semiconductor nanowires

GaAs and InP based nanowires were grown epitaxially on GaAs or InP (111)B substrates by metalorganic chemical vapor deposition using Au nanoparticles as catalyst. In this talk, I will give an overview of nanowire research activities in our group. Especially, the effects of growth parameters for GaAs and InP nanowires on the crystal quality have been studied in detail. We demonstrated the ability to obtain defect-free GaAs nanowires and control the crystal structure of InP nanowires, ie, WZ or ZB, by choosing a combination of growth parameters, such as temperature, V/III ratio and nanowire diameter. © 2009 IEEE.

Epitaxy of III-V semiconductor nanowires towards optoelectronic devices

GaAs and InP based nanowires were grown epitaxially on GaAs or InP (111)B substrates by MOCVD via VLS mechanism. In this paper, I will give an overview of nanowire research activities in our group. © 2009 IEEE.

Polarity driven formation of InAs/GaAs hierarchical nanowire heterostructures

The structural and morphological characteristics of InAs/GaAs radial nanowire heterostructures were investigated using transmission electron microscopy. It has been found that the radial growth of InAs was preferentially initiated on the { 112 } A sidewalls of GaAs nanowires. This preferential deposition leads to extraordinarily asymmetric InAs/GaAs radial nanowire heterostructures. Such formation of radial nanowire heterostructures provides an opportunity to engineer hierarchical nanostructures, which further widens the potential applications of semiconductor nanostructures. © 2008 American Institute of Physics.

Direct observation of charge-carrier heating at WZ-ZB InP nanowire heterojunctions.

We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.

Electrically controllable RKKY interaction in semiconductor quantum wires

We demonstrate in theory that it is possible to all-electrically manipulate the RKKY interaction in a quasi-one-dimensional electron gas embedded in a semiconductor heterostructure, in the presence of Rashba and Dresselhaus spin-orbit interaction. In an undoped semiconductor quantum wire where intermediate excitations are gapped, the interaction becomes the short-ranged Bloembergen-Rowland superexchange interaction. Owing to the interplay of different types of spin-orbit interaction, the interaction can be controlled to realize various spin models, e.g., isotropic and anisotropic Heisenberg-like models, Ising-like models with additional Dzyaloshinsky-Moriya terms, by tuning the external electric field and designing the crystallographic directions. Such controllable interaction forms a basis for quantum computing with localized spins and quantum matters in spin lattices.

Dynamics of photo-enhanced magneto-crystalline anisotropy in diluted ferromagnetic GaMnAs

By chopping a pump beam in conventional time-resolved Kerr rotation (TRKR) experiments and measuring the time evolution of M-shaped "major" hysteresis loops of magnetic linear dichroism (Delta MLD = MLDpump-on MLDpump-off), the differential MLD signal in the presence and the absence of the pump beam, we studied the dynamics of photo-enhanced magneto-crystalline anisotropy, and found that its very long recovering time (much longer than 13 ns) might reflect the nature of the coherent coupling between photo-excited holes and localized spins in the d shell of manganese.

Magnetic quantum oscillations for the surface states of topological insulator Bi2Se3

We study quantum oscillations of the magnetization in Bi2Se3 (111) surface system in the presence of a perpendicular magnetic field. The combined spin-chiral Dirac cone and Landau quantization produce profound effects on the magnetization properties that are fundamentally different from those in the conventional semiconductor two-dimensional electron gas. In particular, we show that the oscillating center in the magnetization chooses to pick up positive or negative values depending on whether the zero-mode Landau level is occupied or empty. An intuitive analysis of these features is given and the subsequent effects on the magnetic susceptibility and Hall conductance are also discussed.

Magnetic silicon fullerene

A metal-encapsulating silicon fullerene, Eu@Si-20, has been predicted by density functional theory to be by far the most stable fullerene-like silicon structure. The Eu@Si-20 structure is a dodecahedron with D-2h symmetry in which the europium atom occupies the center site. The calculated results show that the europium atom has a large magnetic moment of nearly 7.0 Bohr magnetons. In addition, it was found that a stable "pearl necklace" nanowire, constructed by concatenating a series of Eu@Si-20 units, with the central europium atom, retains the high spin moment. The magnetic structure of the nanowire indicates potential applications in the fields of spintronics and high-density magnetic storage.

Tunneling magnetoresistance in CoFeB/GaAs/(Ga,Mn)As hybrid magnetic tunnel junctions

We reported the all electronic demonstration of spin injection and detection in the trilayers with hybrid structure of CoFeB/GaAs/(Ga,Mn)As (metal/insulator semiconductor) by probing the magnetoresistance at low temperature from 1.8 to 30 K. Tunneling magnetoresistance (TMR) ratios of 3.8%, 4.7%, 2.9%, and 1.4% at 1.8, 10, 20, and 30 K, respectively, were observed. Bias dependence of both the junction resistance and TMR ratio was studied systematically. V-half at which TMR drops to half of its maximum is 6.3 mV, being much smaller compared to that observed in (Ga,Mn)As/ZnSe/Fe and (Ga,Mn)As/AlAs/MnAs hybrid structures, indicating lower Fermi energy of (Ga,Mn)As.

Magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO: First-principles calculations

The electronic structure and magnetic coupling properties of rare-earth metals (Gd, Nd) doped ZnO have been investigated using first-principles methods. We show that the magnetic coupling between Gd or Nd ions in the nearest neighbor sites is ferromagnetic. The stability of the ferromagnetic coupling between Gd ions can be enhanced by appropriate electron doping into ZnO Gd system and the room-temperature ferromagnetism can be achieved. However, for ZnO Nd system, the ferromagnetism between Nd ions can be enhanced by appropriate holes doping into the sample. The room-temperature ferromagnetism can also be achieved in the n-conducting ZnO Nd sample. Our calculated results are in good agreement with the conclusions of the recent experiments. The effect of native defects (V-Zn, V-O) on the ferromagnetism is also discussed. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3176490]

First-Principles Study of Magnetic Properties of 3d Transition Metals Doped in ZnO Nanowires

The defect formation energies of transition metals (Cr, Fe, and Ni) doped in the pseudo-H passivated ZnO nanowires and bulk are systematically investigated using first-principles methods. The general chemical trends of the nanowires are similar to those of the bulk. We also show that the formation energy increases as the diameter of the nanowire decreases, indicating that the doping of magnetic ions in the ZnO nanowire becomes more difficult with decreasing diameter. We also systematically calculate the ferromagnetic properties of transition metals doped in the ZnO nanowire and bulk, and find that Cr ions of the nanowire favor ferromagnetic state, which is consistent with the experimental results. We also find that the ferromagnetic coupling state of Cr is more stable in the nanowire than in the bulk, which may lead to a higher T (c) useful for the nano-materials design of spintronics.

Magnetization of two-dimensional heavy holes with boundaries in a perpendicular magnetic field

The magnetisation of heavy holes in III-V semiconductor quantum wells with Rashba spin-orbit coupling (SOC) in an external perpendicular magnetic field is studied theoretically. We concentrate on the effects on the magnetisation induced by the system boundary, the Rashba SOC and the temperature. It is found that the sawtooth-like de Haas-van Alphen (dHvA) oscillations of the magnetisation will change dramatically in the presence of such three factors. Especially, the effects of the edge states and Rashba SOC on the magnetisation are more evident when the magnetic field is smaller. The oscillation center will shift when the boundary effect is considered and the Rashba SOC will bring beating patterns to the dHvA oscillations. These effects on the dHvA oscillations are preferably observed at low temperatures. With increasing temperature, the dHvA oscillations turn to be blurred and eventually disappear.

Effects of shape and magnetic field on the optical properties of wurtzite quantum rods

The optical properties of quantum rods in the absence and presence of the magnetic field are studied in the framework of effective-mass envelope function theory. The two-dimensional (2D) and 1D transition dipoles of wurtzite quantum rods are investigated. It is found that the transition dipoles change from 2D to 1D as the aspect ratio of the ellipsoid increases, in agreement with the experimental results. The linear polarization factors of optical transitions of quantum rods with critical aspect ratio are zero at every orientation of the wave propagation. So quantum rods with critical aspect ratio have isotropic transition dipoles. Due to the 2D or 1D transition dipoles, the linear polarization factors of optical transitions of quantum rods change from negative or positive values to zero as the orientation of the wave propagation changes from the x axis of the crystal structure to the z axis, in agreement with the experimental results. Under magnetic field applied along the z axis of the crystal structure, the negative linear polarization factors in the 2D transition dipole case decrease as the magnetic field increases, while under magnetic field applied along the x axis, the negative linear polarization factors increase as the magnetic field increases. The antisymmetric Hamiltonian is very important to these effects of the magnetic field. It is found that quantum rods with a given radius at a given temperature have dark excitons in a range of aspect ratio. The dimensions along the x, y axes of the crystal structure play opposite roles to the dimension along the z axis on the dark exciton phenomenon. Dark excitons become bright under appropriate magnetic field.

Transverse magnetic field effect on the current hysteresis of doped GaAs/AlAs superlattice

The influence of a transverse magnetic field up to 13 T at 1.6 K on the current-voltage, I (V), characteristics of a doped GaAs/AlAs superlattice was investigated. Current hysteresis was observed in the domain formation regions of the I (V) at zero magnetic field while applied bias was swept in both up (0-6 V) and down (6-0 V) directions. The magnitude of current hysteresis was reduced and finally disappeared with increasing transverse magnetic field. The effect is explained as the modification of the current density versus electric field characteristic by transverse magnetic fields. Calculated results based on the tunnelling current formula in a superlattice support our interpretation.

Giant and zero electron g factors of dilute nitride semiconductor nanowires

The electronic structures and electron g factors of InSb1-sNs and GaAs1-sNs nanowires and bulk material under the magnetic and electric fields are investigated by using the ten-band k.p model. The nitrogen doping has direct and indirect effects on the g factors. A giant g factor with absolute value larger than 900 is found in InSb1-sNs bulk material. A transverse electric field can increase the g factors, which has obviously asymmetric effects on the g factors in different directions. An electric field tunable zero g factor is found in GaAs1-sNs nanowires. (C) 2007 American Institute of Physics.