A novel method for positioning of InAs islands on GaAs(110)

A novel method for positioning of InAs islands on GaAs (110) by cleaved edge overgrowth is reported. The first growth sample contains strained InxGa1-xAs/GaAs superlattice (SL) of varying indium fraction, which acts as a strain nanopattern for the cleaved-edge overgrowth. Atoms incident on the cleaved edge will preferentially migrate to InGaAs regions where favorable bonding sites are available. By this method InAs island chains with lateral periodicity defined by the thickness of InGaAs and GaAs of SL have been realized by molecular beam epitaxy (MBE). They are observed by means of atomic force microscopy (AFM). The strain nanopattern's effect is studied by the different indium fraction of SL and MBE growth conditions. (c) 2005 Elsevier B.V. All rights reserved.

Influence of exotic Mn on magnetic properties of YCo5.0-xMnxGa7.0 (0.05 <= x <= 3.0)

The YCo5.0-xMnxGa7.0 compounds crystallize with the ScFe6Ga6-type structure. The lattice of YCo5.0-xMnxGa7.0 expands with the increase of the Mn content for 0.05 <= x <= 2.5, but the lattice of YCo2.0Mn3.0Ga7.0 shrinks compared with YCo2.5Mn2.5Ga7.0. The shrinkage of the lattice is attributed to the magnetostriction of YCo2.0Mn3.0Ga7.0. The substitution of Mn for Co forms magnetic clusters in the antiferromagnetic matrix. The magnetic frustration results in the spin-glass-like behavior for 0.8 <= x <= 1.5 and the difference between zero-field-cooling (ZFC) and field-cooling (FC) magnetizations for x = 2.0, 2.5, and 3.0. A stable long-range magnetic ordering appears among the Mn-centered magnetic clusters with the ordering temperature 110 K for x = 2.0. The hump in the thermomagnetization of YCo3.0Mn2.0Ga7.0 can be attributed to the competitive effects between the thermal fluctuation and the enhanced magnetic interaction. Both the hump and the bifurcation between the ZFC and the FC magnetizations of YCo3.0Mn2.0Ga7.0 occur at lower temperatures as the applied field increases. On the two-step magnetization curve of YCo3.0Mn2.0Ga7.0, the inflection point at 4000 Oe is due to the coercive field, and the magnetic moments in the clusters are tilted to the applied field above 4000 Oe. The magnetic ordering temperature is further increased to 210 K for x = 2.5 and to 282 K for x = 3.0. The spontaneous magnetization of YCo2.0Mn3.0Ga7.0 is 0.575 mu B/f.u. at 5 K with a canted magnetic structure.

The effect of In content on high-density InxGa1-xAs quantum dots

We have successfully grown self-assembled InxGa1-xAs (x = 0.44, 0.47, 0.50) quantum dots (QDs) with high density (> 10(11)/cm(2)) by MBE. The effect of In content on the high-density QD is investigated by atomic force microscopy (AFM) and photoluminescence (PL) spectra. It is found that sample with In-mole-fraction of 0.5 shows small size fluctuation and high PL intensity. The influence of growth temperature on high-density QD is also investigated in our experiment. (c) 2005 Elsevier B.V. All rights reserved.

Growth of a periodic InP-based heteroepitaxial structure for a quantum cascade laser

The route to grow InP-based heteroepitaxial structure for quantum cascade laser by molecular beam epitaxy is reported. Optimized growth conditions including substrate temperature, V/III ratio, growth rates, doping levels and interface control are summarized. Double crystal Xray diffraction and cross-sectional transmission electron microscopy disclose that our grown InP-based heteroepitaxial structure for quantum cascade laser has excellent periodicity and sharp interfaces. (C) 2005 Elsevier B.V. All rights reserved.

Effects of different modified underlayer surfaces on growth and optical properties of InGaN quantum dots

InGaN/GaN quantum dots were grown on the sapphire (0 0 0 1) substrate in a metalorganic chemical vapor deposition system. The morphologies of QDs deposited on different modified underlayer (GaN) surfaces, including naturally as grown, Ga-mediated, In-mediated, and air-passivated ones, were investigated by atomic force microscopy (AFM). Photo luminescence (PL) method is used to evaluate optical properties. It is shown that InGaN QDs can form directly on the natural GaN layer. However, both the size and distribution show obvious inhomogeneities. Such a heavy fluctuation in size leads to double peaks for QDs with short growth time, and broad peaks for QDs with long growth time in their low-temperature PL spectra. QDs grown on the Ga-mediated GaN underlayer tends to coalesce. Distinct transform takes place from 3D to 2D growth on the In-mediated ones, and thus the formation of QDs is prohibited. Those results clarify Ga and In's surfactant behavior. When the GaN underlayer is passivated in the air, and together with an additional low-temperature-grown seeding layer, however, the island growth mode is enhanced. Subsequently, grown InGaN QDs are characterized by a relatively high density and an improved Gaussian-like distribution in size. Short surface diffusion length at low growth temperature accounts for that result. It is concluded that reduced temperature favors QD's 3D growth and surface passivation can provide another promising way to obtain high-density QDs that especially suits MOCVD system. (c) 2004 Elsevier Ltd. All rights reserved.

Investigation of Ge-Si atomic interdiffusion in Ge nano-dots multilayer structure by double crystal X-ray diffraction

The fluctuations of the strained layer in a superlattice or quantum well can broaden the width of satellite peaks in double crystal X-ray diffraction (DCXRD) pattern. It is found that the width of the 0(th) peak is directly proportional to the fluctuation of the strained layer if the other related facts are ignored. By this method, the Ge-Si atomic interdiffusion in Ge nano-dots and wetting layers has been investigated by DCXRD. It is found that thermal annealing can activate Ge-Si atomic interdiffusion and the interdiffusion in the nano-dots area is much stronger than that in the wetting layer area. Therefore the fluctuation of the Ge layer decreases and the distribution of Ge atoms becomes homogeneous in the horizontal Ge (GeSi actually) layer, which make the width of the 0(th) peak narrow after annealing.

Stabilized and tunable single-frequency erbium-doped fiber ring laser employing external injection locking

A stabilized and tunable single-longitudinal-mode erbium-doped fiber ring laser has been proposed and experimentally demonstrated. The laser is structured by combining the compound cavity with a fiber Fabry-Perot tunable filter. An injection-locking technique has been used to stabilize the wavelength and output power of the laser. One of the longitudinal modes is stimulated by the injected continuous wave so that this mode is able to win the competition to stabilize the system. A minimum output power of 0.6 dBm and a signal-to-noise ratio of over 43 dB within the tuning range of 1527-1562 nm can be achieved with the proposed technique. A wavelength variation of less than 0.01 nm, a power fluctuation of less than 0.02 dB, and a short-term linewidth of about 1.4 kHz have also been obtained.

Dynamics of quantum dissipation systems interacting with fermion and boson grand canonical bath ensembles: Hierarchical equations of motion approach

A hierarchical equations of motion formalism for a quantum dissipation system in a grand canonical bath ensemble surrounding is constructed on the basis of the calculus-on-path-integral algorithm, together with the parametrization of arbitrary non-Markovian bath that satisfies fluctuation-dissipation theorem. The influence functionals for both the fermion or boson bath interaction are found to be of the same path integral expression as the canonical bath, assuming they all satisfy the Gaussian statistics. However, the equation of motion formalism is different due to the fluctuation-dissipation theories that are distinct and used explicitly. The implications of the present work to quantum transport through molecular wires and electron transfer in complex molecular systems are discussed. (c) 2007 American Institute of Physics.

Room-temperature photoluminescence of ZnO/MgO multiple quantum wells deposited by reactive magnetron sputtering

Wurtzite ZnO/MgO superlattices were successfully grown on Si (001) substrates at 750 degrees C using radio-frequency reactive magnetron sputtering method. X-ray reflection and diffraction, electronic probe and photoluminescence analysis were used to characterize the multiple quantum wells (MQWs). The results showed the periodic layer thickness of the MQWs to be 1.85 to 22.3 nm. The blueshift induced by quantum confinement was observed. Least square fitting method was used to deduce the zero phonon energy of the exciton from the room-temperature photoluminescence. It was found that the MgO barrier layers has a much larger offset than ZnMgO. The fluctuation of periodic layer thickness of the MQWs was suggested to be a possible reason causing the photoluminescence spectrum broadening.

Study on the reverse characteristics of Ti/6H-SiC Schottky contacts

The reverse I(V) measurement and analytic calculation of the electron transport across a Ti/6H-SiC Schottky barrier are presented. Based on the consideration of the barrier fluctuations and the barrier height shift caused by image charge and the applied voltage drop across Ti/SiC interfical layer, a comprehensive analytical model for the reverse tunneling current is developed using a WKB calculation of the tunneling probability through a reverse biased Schottky barrier. This model takes into account the main reverse conduction mechanism, such as field emission, thermionic field emission and thermionic emission. The fact that the simulated results are in good agreement with the experimental data indicates that the barrier height shift and barrier fluctuation can lead to reverse current densities orders of magnitude higher than that obtained from a simple theory. It is shown that the field and thermionic field emission processes, in which carries can tunnel through the barrier but cannot surmount it with insufficient thermal energy, dominate the reverse characteristics of a SiC Schottky contacts in a normal working condition.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

The growth morphologies of GaN layer on Si(111) substrate

The growth morphologies of metalorganic chemical vapor deposition (MOCVD) grown GaN layer on Si(111) substrate were studied using atomic force microscopy and transmission electron microscopy. It was found that the growth process of GaN/Si(111) consisted of two cycles of island growth and coalescence. These two cycles process differs markedly from that of one cycle process reported. The stress of evolving GaN layers on Si(111) was characterized by measuring the lattice constant c of GaN using X-ray diffraction (XRD) technique. It was proposed that the large tensile stress within the film during growth initiated this second island growth cycle, and the interaction between the GaN islands with high orientational fluctuation on the buffer layer induced this large tensile growth stress when coalescence occurred. (C) 2002 Elsevier Science B.V. All rights reserved.

Dephasing rate in an InAs/GaAs single-electron quantum dot qubit

We have obtained the parameter-phase diagram, which unambiguously defines the parameter region for the use of InAs/GaAs quantum dot as two-level quantum system in quantum computation in the framework of the effective-mass envelope function theory. Moreover, static electric field is found to efficiently prolong decoherence time. As a result, decoherence time may reach the order of magnitude of milli-seconds as external static electric field goes beyond 20 kV/cm if only vacuum fluctuation is taken as the main source for decoherence. Our calculated results are useful for guiding the solid-state implementation of quantum computing.

Analysis of mode quality factors for equilateral triangle semiconductor microlasers with rough sidewalls

The eigenmode characteristics for equilateral triangle resonator (ETR) semiconductor microlasers are analysed by the finite-difference time-domain technique and the Pade approximation. The random Gaussian correlation function and sinusoidal function are used to model the side roughness of the ETR. The numerical results show that the roughness can cause the split of the degenerative modes, but the confined modes can still have a high quality factor. For the ETR with a 3 mum side length and the sinusoidal fluctuation, we can have a quality factor of 800 for the fundamental mode in the wavelength of 1500 nm, as the amplitude of roughness is 75 mn.

1.35 mu m photoluminescence from In0.5Ga0.5As/GaAs islands grown by molecular beam epitaxy via cycled (InAs)(1)/(GaAs)(1) monolayer deposition

1.35 mum photoluminescence (PL) with a narrow linewidth of only 19.2 meV at room temperature has been achieved in In0.5Ga0.5As islands structure grown on GaAs (1 0 0) substrate by solid-source molecular beam epitaxy. Atomic force microscopy (AFM) measurement reveals that the 16-ML-thick In0.5Ga0.5As islands show quite uniform InGaAs mounds morphology along the [ 1(1) over bar 0] direction with a periodicity of about 90 nm in the [1 1 0] direction. Compared with the In0.5Ga0.5As alloy quantum well (QW) of the same width, the In0.5Ga0.5As islands structure always shows a lower PL peak energy and narrower full-width at half-maximum (FWHM), also a stronger PL intensity at low excitation power and more efficient confinement of the carriers. Our results provide important information for optimizing the epitaxial structures of 1.3 mum wavelength quantum dots devices. (C) 2000 Elsevier Science B.V. All rights reserved.