Growth of low-density InAs/GaAs quantum dots on a substrate with an intentional temperature gradient by molecular beam epitaxy

InAs was deposited by molecular beam epitaxy (MBE) on a GaAs substrate with an intentional temperature gradient from centre to edge. Two-dimensional (2D) to three-dimensional (3D) morphology evolution was found along the direction in which the substrate temperature was decreasing. Quantum dots (QDs) with density as low as similar to 8 x 10(6) cm(-2) were formed in some regions. We attribute the morphological evolution to the temperature-dependent desorption of deposited indium and the intermixing between deposited indium and gallium from the buffer.

Realization of GaAs/AlGaAs quantum-cascade lasers with high average optical power

Quasi-continuous-wave operation of GaAs/AlGaAs quantum-cascade lasers with high average optical power is demonstrated. Double X-ray diffraction has been used to investigate the quality of the epitaxial material. The compositional gradients and the interface quality are controlled effectively. The corrected average power of per facet about 17 mW and temperature tuning coefficient of the gain peak about 0.91 nm/K from 83 K to 140 K is achieved in pulse operation. Best value of threshold current density is less than 3.0 kA/cm(2) at 83 K. (C) 2005 Elsevier Ltd. All rights reserved.

Influence of growth pressure of a GaN buffer layer on the properties of MOCVD GaN

The influence of growth pressure of GaN buffer layer on the properties of MOCVD GaN on alpha-Al2O3 has been investigated with the aid of a home-made in situ laser reflectometry measurement system. The results obtained with in situ measurements and scanning electron microscope show that with the increase in deposition pressure of buffer layer, the nuclei increase in size, which roughens the surface, and delays the coalescence of GaN nuclei. The optical and crystalline quality of GaN epilayer was improved when buffer layer was deposited at high pressure.

Quantum waveguide theory for hole transport in mesoscopic structures

A quantum waveguide theory is proposed for hole transport in the mesoscopic structures, including the band mixing effect. We found that due to the interference between the 'light' hole and 'heavy' wave, the transmission and reflection coefficients oscillate more irregularly as a function of incident wave vector geometry parameters. Furthermore conversion between the heavy hole and light hole states occurs at the intersection. (C) 2003 Elsevier Ltd. All rights reserved.

Photoluminescence of Mg-doped GaN grown by metalorganic chemical vapor deposition

Two Mg-doped GaN films with different doping concentrations were grown by a metalorganic chemical vapor deposition technique. Photoluminescence (PL) experiments were carried out to investigate the optical properties of these films. For highly Mg-doped GaN, the PL spectra at 10 K are composed of a blue luminescence (BL) band at 2.857 eV and two excitonic luminescence lines at 3.342 eV and 3.282 eV, in addition to a L2 phonon replica at 3.212 eV. The intensity of the L1 line decreases monotonously with an increase,in temperature. However, the intensity of the L2 line first slowly increases at first, and then decreases quickly with an increase in temperature. The two lines are attributed to bound excitonic emissions at extended defects. The BL band is most likely due to the transition from deep donor Mg-V-N complex to Mg shallow acceptor. From the temperature dependence of the luminescence peak intensity of the BL band, the activation energy of acceptor Mg was found to be 290 meV. (C) 2003 American Vacuum Society.

The micro-magnetic structures of Mn+ ion-implanted GaSb

The micro-magnetic structures of Mn+ ion-implanted GaSb are studied using a magnetic force microscope (MFM). MFM images reveal that there are many magnetic domains with different magnetization directions in our samples. The magnetic domain structures and the magnetization direction of typical MFM patterns are analyzed by numeric simulation.

Magnetic properties and rectifying behaviour of silicon doped with gadolinium

The magnetic/nonmagnetic p-n junction was prepared by implanting gadolinium into the n-type silicon with low-energy dual-ion-beam epitaxy technology. The magnetic layer GdxSi1-x shows excellent magnetic properties at room temperature. High magnetic moment 10mu(B) per Gd atom is observed, which is interpreted by RKKY mechanism. Magnetic/nonmagnetic p-n junctions show rectifying behaviour, but no magnetoresistance is observed.

Void formation and failure in InGaN/AlGaN double heterostructures

Condensed clusters of point defects within an InGaN/AlGaN double heterostructure grown by metal-organic vapor phase epitaxy on sapphire substrate have been observed using transmission electron microscopy. The existence of voids results in failure of the heterostructure in electroluminescence. The voids are 50-100 nm in diameter and are distributed inhomogeneously within In0.25Ga0.75N/AlGaN active layers. The density of the voids was measured as 10(15) cm(-3), which corresponds to a density of dangling bonds of 10(20) cm(-3). These dangling bonds may fully deplete free carriers in this double heterostructure and result in the heterostructure having high resistivity as confirmed by electrical measurement. (C) 2003 Elsevier Science B.V. All rights reserved.

Abnormal temperature dependence of photoluminescence from self-assembled InAs quantum dots covered by an InAlAs/InGaAs combination layer

In this report we have investigated the temperature dependence of photoluminescence (PL) from self-assembled InAs quantum dots (QDs) covered by an InAlAs/InGaAs combination layer. The ground state experiences an abnormal variation of PL linewidth from 15 K up to room temperature. Meanwhile, the PL integrated intensity ratio of the first excited state to the ground state for InAs QDs unexpectedly decreases with increasing temperature, which we attribute to the phonon bottleneck effect. We believe that these experimental results are closely related to the partially coupled quantum dots system and the large energy separation between the ground and the first excited states. (C) 2003 Elsevier Science Ltd. All rights reserved.

Selective area growth of GaN on GaAs(001) substrates by metalorganic vapor-phase epitaxy

Selective area growth (SAG) of GaN on SiO2 stripe-patterned GaN/GaAs(001) substrates was carried out by metalorganic vapor-phase epitaxy. The SAG samples were investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). SEM observations showed that the morphology of SAG GaN is strongly dependent on the window stripe orientation and slightly affected by the orientation relationship between the window stripes and the gas flow. The (I 1 1)B sidewalls formed on the SAG GaN stripes are found to be stable. XRD measurements indicated the full-widths at half-maximum (FWHMs) of cubic GaN (0 0 2) rocking curves are reduced after SAG. The measured FWHMs with omega-axis parallel to [1(1) over bar 0] are always larger than the FWHM values obtained with omega-axis parallel to [I 10], regardless of the orientation relationship between the w-axis and the GaN stripes. (C) 2003 Elsevier Science B.V. All rights reserved.

(Ga,Mn,N) compounds growth with mass-analyzed low energy dual ion beam deposition

The (Ga,Mn,N) samples were grown by the implantation of low-energy Mn ions into GaN/Al2O3 substrate at different elevated substrate temperatures with mass-analyzed low-energy dual ion beam deposition system. Auger electron spectroscopy depth profile of samples grown at different substrate temperatures indicates that the Mn ions reach deeper in samples with higher substrate temperatures. Clear X-ray diffraction peak from (Ga,Mn)N is observed in samples grown at the higher substrate temperature. It indicates that under optimized substrate temperature and annealing conditions the solid solution (Ga,Mn)N phase in samples was formed with the same lattice structure as GaN and different lattice constant. (C) 2003 Elsevier Science B.V. All rights reserved.

Room-temperature ferromagnetic semiconductor MnxGa1-xSb

Ferromagnetic semiconductor MnxGa1-xSb single crystals were fabricated by Mn-ions implantation, deposition, and the post annealing. Magnetic hysteresis-loops in the MnxGa1-xSb single crystals were obtained at room temperature (300 K). The structure of the ferromagnetic semiconductor MnxGa1-xSb single crystal was analyzed by Xray diffraction. The distribution of carrier concentrations in MnxGa1-xSb was investigated by electrochemical capacitance-voltage profiler. The content of Mn in MnxGa1-xSb varied gradually from x = 0.09 near the surface to x = 0 in the wafer inner analyzed by X-ray diffraction. Electrochemical capacitance-voltage profiler reveals that the concentration of p-type carriers in MnxGa1-xSb is as high as 1 X 10(21) cm(-3), indicating that most of the Mn atoms in MnxGa1-xSb take the site of Ga, and play a role of acceptors.

Study on the Raman spectra of GaP nanorods synthesized within carbon nanotube templates

The Raman spectra of GaP nanorods grown in carbon nanotube templates have been reported. The red shifts of the TO and LO modes were observed due to phonon confinement effect in GaP nanorods. The measured red shifts range from 2 to 10cm(-1) depending on the size of the measured nanorods. It has been found that the polarization properties, which cannot be well explained by the selection rules of single nanorod, result from the direction disorder of nanorods in the measured area. The more the disorder is, the weaker the directionality of polarization properties is. The decrease of the Raman frequency of the TO and LO mode of the nanorods with the increasing power of the exciting laser suggests that the heating effect of the nanorods is far stronger than the bulk material. In addition, the saturation and then decrease of the Raman intensity with the increasing laser power indicate the rapid increase of the defects in the nanorods exposed to a strong exciting laser.

Optical constants of cubic GaN/GaAs(001): Experiment and modeling

The optical constants epsilon(E)=epsilon(1)(E)+iepsilon(2)(E) of unintentionally doped cubic GaN grown on GaAs(001) have been measured at 300 K using spectral ellipsometry in the range of 1.5-5.0 eV. The epsilon(E) spectra display a structure, associated with the critical point at E-0 (direct gap) and some contribution mainly coming from the E-1 critical point. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden-Munoz model dielectric function [M. Munoz et al., J. Appl. Phys. 92, 5878 (2002)]. This model is based on the electronic energy-band structure near critical points plus excitonic and band-to-band Coulomb-enhancement effects at E-0, E-0 + Delta(0) and the E-1, E-1 + Delta(1), doublet. In addition to evaluating the energy of the E-0 critical point, the binding energy (R-1) of the two-dimensional exciton related to the E-1 critical point was estimated using the effective mass/k.p theory. The line, shape of the imaginary part of the cubic-GaN dielectric function shows excitonic effects at room temperature not withstanding that the exciton was not resolved. (C) 2003 American Institute of Physics.

Ultrafast geometric manipulation of electron spin and detection of the geometric phase via Faraday rotation spectroscopy

Time-resolved Faraday rotation spectroscopy is currently exploited as a powerful technique to probe spin dynamics in semiconductors. We propose here an all-optical approach to geometrically manipulate electron spin and to detect the geometric phase by this type of extremely sensitive experiment. The global nature of the geometric phase can make the quantum manipulation more stable, which may find interesting applications in quantum devices.