PIXE analysis of Fe content in Fe-implanted GaN film

The diluted magnetic semiconductors (DMSs) were achieved by the ion implantation. Fe+ ions (250 keV) were implanted into n-type GaN at room temperature with doses ranging from 8 X 10(15) cm(-2) to 8 X 10(16) cm(-2) and subsequently rapidly annealed at 800 degrees C for 5 m in N-2 ambient. PIXE was employed to determine the Fe-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters are detected within the sensitivity of XRD. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The relationships between the Fe-implanted content and the ferromagnetic property are discussed. (c) 2006 Elsevier B.V. All rights reserved.

Nonlinear Rashba model and spin relaxation in quantum wells

We find that the Rashba spin splitting is intrinsically a nonlinear function of the momentum, and the linear Rashba model may overestimate it significantly, especially in narrow-gap semiconductors. A nonlinear Rashba model is proposed, which is in good agreement with the numerical results from the eight-band k center dot p theory. Using this model, we find pronounced suppression of the D'yakonov-Perel' spin relaxation rate at large electron densities, and a nonmonotonic dependence of the resonance peak position of the electron spin lifetime on the electron density in [111]-oriented quantum wells, both in qualitative disagreement with the predictions of the linear Rashba model.

Mode softening in charge-density excitations of a two-dimensional electron gas driven by Rashba spin-orbit interaction

The electron density response of a uniform two-dimensional (2D) electron gas is investigated in the presence of a perpendicular magnetic field and Rashba spin-orbit interaction (SOI). It is found that, within the Hartree-Fock approximation, a charge density excitation mode below the cyclotron resonance frequency shows a mode softening behavior, when the spin-orbit coupling strength falls into a certain interval. This mode softening indicates that the ground state of an interacting uniform 2D electron gas may be driven by the Rashba SOI to undergo a phase transition to a nonuniform charge density wave state.

Two opposite gradients of hole density in as-grown and annealed (Ga,Mn)As layers

The depth distribution of the hole density p in 500 nm-thick (Ga,Mn)As layers is investigated. From Raman scattering spectra, it is found that the gradients of p are opposite in the as-grown and annealed layers. At the region around the free surface, with increasing etching depth, p significantly increases in the as-grown layer; however, p decreases distinctly in the annealed layer. Then, in the bulk, p becomes almost homogeneous for both cases. The etching-depth dependence of Curie temperature obtained from magnetic measurements is in agreement with the distribution characterization of p. These results suggest that annealing induces outdiffusion of Mn interstitials towards the free surface, and incomplete outdiffusion during the growth leads to an accumulation of Mn interstitials around the free surface of the as-grown (Ga,Mn)As. (c) 2006 Elsevier B.V. All rights reserved.

Electronic structure and binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/AlxGa1-xAs quantum dot

We calculate the electronic structures and binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/AlxGa1-xAs quantum dot (QD) in the framework of effective-mass envelope-function theory. The variation of the electronic structures and binding energy with the QD structure parameters and the position of the impurity are studied in detail. We find that (1) acceptor impurity energy levels depend more sensitively on the size of the QD than those of a donor impurity; (2) all impurity energy levels strongly depend on the GaAs quantum well (QW) width; (3) a donor impurity in the QD has only one binding energy level except when the GaAs QW is large; (4) an acceptor impurity in the QD has two binding energy levels, which correspond to heavy- and light-hole quantum states; (5) the binding energy has a maximum value when the impurity is located below the symmetry axis along the growth direction; and (6) the binding energy has a minimum value when the impurity is located at the top corner of the QD. (c) 2006 American Institute of Physics.

Semiconductor-metal transition in InSb nanowires and nanofilms under external electric field

The electronic structures, Rashba spin-orbit couplings, and transport properties of InSb nanowires and nanofilms are investigated theoretically. When both the radius of the wire (or the thickness of the film) and the electric field are large, the electron bands and hole bands overlap, and the Fermi level crosses with some bands, which means that the semiconductors transit into metals. Meanwhile, the Rashba coefficients behave in an abnormal way. The conductivities increase dramatically when the electric field is larger than a critical value. This semiconductor-metal transition is observable at the room temperature. (c) 2006 American Institute of Physics.

Electronic structure of InSb quantum ellipsoids in an external magnetic field

The shape dependence of electronic structure, electron g factors in the presence of the external magnetic field of InSb quantum ellipsoids are investigated in the framework of eight-band effective-mass approximation. It is found that as the increasing aspect ratio e, the electron states with P character split into three doublets for the different physical interaction and the light-hole states with S character come up to the top of valence bands at e = 2.6 in comparison with the heavy-hole states. In the presence of the external magnetic field, the energy splits of electron states are different for their wave function distribution direction, and the hole ground state remain optical active for a suitable aspect ratio. The electron g factors of InSb spheres decrease with increasing radius, and have the value of about two for the smallest radius, about -47.2 for sufficiently larger radius, similar to the bulk material case. Actually, the electron g factors decrease as any one of the three dimensions increase. The more dimensions increase, the more g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimensions. (c) 2006 Elsevier B.V. All rights reserved.

Influence of nitridation time on the morphology of GaN nanorods synthesized by nitriding Ga2O3/ZnO films

Gallium nitride (GaN) nanorods were synthesized by nitriding Ga2O3/ZnO films which were deposited in turn on Si (111) substrates using radio frequency (RF) magnetron sputtering system. In the nitridation process, ZnO was reduced to Zn and Zn sublimated at 950 degrees C. Ga2O3 was reduced to Ga2O and Ga2O reacted with NH3 to synthesize GaN nanorods with the assistance of the sublimation of Zn. The morphology and structure of the nanorods were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The composition of GaN nanorods was studied by Fourier-transform infrared spectrophotometer (FTIR). The synthesized nanorods is hexagonal wurtzite structured. Nitridation time of the samples has an evident influence on the morphology of GaN nanorods synthesized by this method. (c) 2006 Elsevier B.V. All rights reserved.

Effect of GaAS(100) 2 degrees surface misorientation on the formation and optical properties of MOCVD grown InAs quantum dots

The influence of GaAS(1 0 0)2 degrees substrate misorientation on the formation and optical properties of InAs quantum dots (QDs) has been studied in compare with dots on exact GaAs(1 0 0) substrates. It is shown that, while QDs on exact substrates have only one dominant size, dots on misoriented substrates are formed in lines with a clear bimodal size distribution. Room temperature photoluminescence measurements show that QDs on misoriented substrates have narrower FWHM, longer emission wavelength and much larger PL intensity relative to those of dots on exact substrates. However, our rapid thermal annealing (RTA) experiments indicate that annealing shows a stronger effect on dots with misoriented substrates by greatly accelerating the degradation of material quality. (c) 2005 Elsevier B.V All rights reserved.

Interplay between s-d exchange interaction and Rashba effect: Spin-polarized transport

The authors investigate the spin-polarized transport properties of a two-dimensional electron gas in a n-type diluted magnetic narrow gap semiconductor quantum well subjected to perpendicular magnetic and electric fields. Interesting beating patterns in the magnetoresistance are found which can be tuned significantly by varying the electric field. A resonant enhancement of spin-polarized current is found which is induced by the competition between the s-d exchange interaction and the Rashba effect [Y. A. Bychkov and E. I. Rashba, J. Phys. C 17, 6039 (1984)]. (c) 2006 American Institute of Physics.

Structural characterization of AlGaN/GaN superlattices by x-ray diffraction and Rutherford backscattering

We report on structural characterization of AlGaN/GaN superlattices grown on sapphire. The superlattice formation is evidenced by high-resolution x-ray diffraction and transmission electron microscopy. The high resolution x-ray diffraction spectra exhibit a pattern of satellite peaks. The in-plane lattice constants of the superlattices indicate the coherent growth of the AlGaN layer onto GaN. The average At composition in the superlattices is determined to be 0.08 by Rutherford backscattering spectroscopy. The average parallel and perpendicular elastic strains for the SLs are determined to be (e(parallel to)) = +0.25% and (e(perpendicular to)) = -0.17%. (c) 2006 Elsevier Ltd. All rights reserved.

Design of shallow acceptors in ZnO: First-principles band-structure calculations

p-type doping is a great challenge for the full utilization of ZnO as short-wavelength optoelectronic material. Due to a large electronegative characteristic of oxygen, the ionization energy of acceptors in ZnO is usually too high. By analyzing the defect wave-function character, we propose several approaches to lower the acceptor ionization energy by codoping acceptors with donor or isovalent atoms. Using the first-principles band-structure method, we show that the acceptor transition energies of V-Zn-O-O can be reduced by introducing F-O next to V-Zn to reduce electronic potential, whereas the acceptor transition energy of N-O-nZn(Zn) (n=1-4) can be reduced if we replace Zn by isovalent Mg or Be to reduce the anion and cation kinetic p-d repulsion, as well as the electronic potential.

Study on in-plane optical anisotropy of semiconductor materials by reflectance difference spectroscopy

In the present review, the measuring principle of reflectance difference spectroscopy (RDS) is given. As a powerful tool in the surface and interface analysis technologies, the application of RDS to the research on semiconductor materials is summarized. along with the origins of the in-plane optical anisotropy of semiconductors. And it is believed that RDS will play an important role in the electrooptic modification of Si-based semiconductor materials.

Electrical properties of undoped In0.53Ga0.47As grown on InP substrates by molecular beam epitaxy

A series of 1-mu m-thick undoped In0.53Ga0.47As with different substrate growth temperature (T-g) or different beam flux pressure (BFP) of As were grown on lattice-matched semi-insulating InP (001) substrates by molecular beam epitaxy (MBE). Van der Pauw Hall measurements were carried out for these In0.53Ga0.47As samples. The residual electron concentration decreased with increasing temperature from 77 to 140 K, but increased with increasing temperature from 140 to 300 K. Rapid thermal annealing (RTA) can reduce the residual electron concentration. The residual electron mobility increased with increasing temperature from 77 to 300 K. All these electrical properties are associated with As antisite defects. (c) 2006 Elsevier B.V. All rights reserved.

Al2O3 : Cr3+ nanotubes synthesized via homogenization precipitation followed by heat treatment

Cr3+-doped NH4Al(OH)(2)CO3 nanotubes, templated by surfactant assemblies, were successfully synthesized via the homogenization precipitation method, and various crystallographic phase Al2O3:Cr3+ nanotubes were also obtained by postannealing at different temperatures. The characteristic R-1, R-2 doublet line transitions of ruby can be observed in the high crystalline alpha-Al2O3 nanotubes calcined at temperatures higher than 1200 degrees C. The results also indicate that the formation mechanism of the tubular nanostructures should result from the self-rolling action of layered compound NH4Al(OH)(2)CO3 under the assistance of the surfactant soft-template. The convenient synthetic procedure, excellent reproducibility, clean reactions, high yield, and fine quality of products in this work make the present route attractive and significant. Aluminum oxide nanotubes with high specific surface area could be used as fabricating nanosized optical devices doped with different elements and stable catalyst supports of metal clusters.