Q-switched and mode-locked diode-pumped Nd : GdVO4 laser with low temperature GaAs saturable absorber

Low temperature GaAs (LT-GaAs) was successfully grown at the temperature of 550 degrees C by metal organic vapor phase epitaxy on a semi-insular GaAs substrate. With such an absorber as well as an output coupler we obtain Q-switched mode-locked (QML) 1064 nm Nd:GdVO4 laser pumped by diode laser with high repetition rate, formed with a simple flat-flat cavity. The repetition rate of the Q-switched envelope increased from 100 to 660 kHz as the pump power increased from 2.28 to 7.29 W. The mode-locked pulses inside the Q-switched pulse envelope had a repetition rate of similar to 1.36 GHz. A maximum average output power of 953 mW was obtained. The dependence of the operational parameters on the pump power was also investigated experimentally. (C) 2005 Elsevier B.V. All rights reserved.

Growth of InAs quantum dots on vicinal GaAs (100) substrates by metalorganic chemical vapor deposition and their optical properties

The growth of InAs quantum dots on vicinal GaAs (100) Substrates was systematically studied using low-pressure metalorganic chemical vapor deposition (MOCVD). The dots showed a clear bimodal size distribution on vicinal substrates. The way of evolution of this bimodal size distribution was studied as a function of growth temperature, InAs layer thickness and InAs deposition rate. The optical properties of dots grown on vicinal substrates were also studied by photoluminescence (PL). It was found that, compared with dots on exact substrates, dots on vicinal substrates had better optical properties such as a narrower PL line width, a longer emission wavelength, and a larger PL intensity. (c) 2006 Elsevier B.V. All rights reserved.

Field-aided collection in GaInP2 top solar cells

This paper reports the study on a field-aided collection in p-on-n GaInP2 top cells. The cells were produced by metalorganic vapor phase epitaxy at a low gas pressure. In order to optimize the device configuration, numerical simulations have been performed for the impacts of field-aided collection on the performance of the top cells. On the basis of the modeling results, a modified p(+)-p(-)-n(-)-n(+) configuration is introduced for GaInP2 top cells. This modification has brought out improved photovoltaic performance of the top cells, with conversion efficiency EFF = 14.26% (AM0, 2 x 2 cm(2), 25degreesC). (C) 2003 Elsevier B.V. All rights reserved.

Stress and its effect on optical properties of GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire

The stress states in unintentionally doped GaN epilayers grown on Si(111), 6H-SiC(0001), and c-plane sapphire, and their effects on optical properties of GaN films were investigated by means of room-temperature confocal micro-Raman scattering and photoluminescence techniques. Relatively large tensile stress exists in GaN epilayers grown on Si and 6H-SiC while a small compressive stress appears in the film grown on sapphire. The latter indicates effective strain relaxation in the GaN buffer layer inserted in the GaN/sapphire sample, while the 50-nm-thick AlN buffer adopted in the GaN/Si sample remains highly strained. The analysis shows that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual strain in the films. Finally, a linear coefficient of 21.1+/-3.2 meV/GPa characterizing the relationship between the luminescent bandgap and the biaxial stress of the GaN films is obtained. (C) 2003 American Institute of Physics.

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.

X-ray diffraction analysis of MOCVD grown GaN buffer layers on GaAs(001) substrates

In order to understand the growth feature of GaN on GaAs (0 0 1) substrates grown by metalorganic chemical vapor deposition (MOCVD), the crystallinity of GaN buffer layers with different thicknesses was investigated by using double crystal X-ray diffraction (DCXRD) measurements. The XRD results showed that the buffer layers consist of predominantly hexagonal GaN (h-GaN) and its content increases with buffer layer thickness. The nominal GaN (111) reflections with chi at 54.74degrees can be detected easily, while (0 0 2) reflections are rather weak. The integrated intensity of reflections from (111) planes is 4-6 times that of (0 0 2) reflections. Possible explanations are presented. (C) 2003 Elsevier Science B.V. All rights reserved.

High-resolution X-ray diffraction analysis of the Bragg peak integrated intensity in highly mismatched III-N epilayers

A new method of measuring the thickness of GaN epilayers on sapphire (0 0 0 1) substrates by using double crystal X-ray diffraction was proposed. The ratio of the integrated intensity between the GaN epilayer and the sapphire substrate showed a linear relationship with the GaN epilayer thickness up to 2.12 mum. It is practical and convenient to measure the GaN epilayer thickness using this ratio, and can mostly eliminate the effect of the reabsorption, the extinction and other scattering factors of the GaN epilayers. (C) 2003 Elsevier Science B.V. All rights reserved.

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.

Luminescence study of (11(2)over-bar0) GaN film grown by metalorganic chemical-vapor deposition

Investigations on photoluminescence properties of (11 (2) over bar0) GaN grown on (1 (1) over bar 02) Al2O3 substrate by metalorganic chemical-vapor deposition are reported. Several emission lines not reported before are observed at low temperature. The sharp peak at 3.359 eV is attributed to the exciton bound to the neutral acceptor. Another peak at 3.310 eV represents a free-to-bound, probably a free electron-to-acceptor, transition. The 3.241 and 3.170 eV lines are interpreted as phonon replica lines of the 3.310 eV line. The phonon energy is 70 meV, consistent with the energy of transverse optical E-1 phonon. The optical properties of the lines are analyzed. (C) 2003 American Institute of Physics.

Effects of annealing ambient on the formation of compensation defects in InP

Positron annihilation lifetime (PAL) and photoinduced current transient spectroscopies (PICTS) have been employed to study the formation of compensation defects in undoped InP under different annealing processes with pure phosphorus (PP) ambience and iron phosphide (IP) ambience, respectively. The different annealing ambiences convert the as-grown n-type undoped InP into two types of semi-insulating (SI) states. The positron average lifetimes of as-grown InP, PP SI-InP, and IP SI-InP are found to be 246, 251, and 243 ps, respectively, which are all longer than the bulk lifetime of 240 ps, indicating the existence of vacancy-type positron-trapping defects. For as-grown InP, VInH4 complexes are the dominant defects. They dissociate into VInHn(0less than or equal tonless than or equal to3) acceptor vacancies under PP ambience annealing, compensating the residual shallow donors and turning the material semi-insulating. In forming IP SI-InP, diffusion of iron into V-In complexes under IP ambience annealing produces the substitutional compensation defect Fe-In, causing a shorter positron average lifetime. The PICTS measurements show that a group of vacancy-type defects has been suppressed by iron diffusion during the annealing process, which is in good agreement with the PAL results. (C) 2003 American Institute of Physics.

Mechanisms of the sidewall facet evolution in lateral epitaxial overgrowth of GaN by MOCVD

The lateral epitaxial overgrowth of GaN was carried out by low-pressure metalorganic chemical vapor deposition, and the cross section shape of the stripes was characterized by scanning electron microscopy. Inclined {11-2n} facets (n approximate to 1-2.5) were observed in the initial growth, and they changed gradually into the vertical {11-20} sidewalls in accordance with the process of the lateral overgrowth. A model was proposed utilizing diffusion equations and boundary conditions to simulate the concentration of the Ga species constituent throughout the concentration boundary layer. Solutions to these equations are found using the two-dimensional, finite element method. We suggest that the observed evolution of sidewall facets results from the variation of the local V/III ratio during the process of lateral overgrowth induced by the lateral supply of the Ga species from the SiNx mask regions to the growing GaN regions.

Comparative study on the broadening of exciton luminescence linewidth due to phonon in zinc-blende and wurtzite GaN epilayers

Linewidth broadening of exciton luminescence in wurtzite and zinc-blende GaN epilayers was investigated as a function of temperature with photoluminescence. A widely accepted theoretical model was used to fit the experimental data, so that the coupling parameters between exciton and acoustic and longitudinal optical phonons were obtained for both structures. It was found that the coupling constants of both exciton-acoustic optical phonon coupling and exciton-longitudinal optical phonon coupling for zinc-blende GaN are almost twice as much as the corresponding values of wurtzite GaN. These results show that the relatively strong exciton-phonon scattering seems to be characteristic to zinc-blende GaN film. (C) 2002 American Institute of Physics.

Structural characterization of epitaxial lateral overgrown GaN on patterned GaN/GaAs(001) substrates

Epitaxial lateral overgrown (ELO) cubic GaN (c-GaN) on SiO2 patterned GaN/GaAs(0 0 1) substrates by metalorganic vapor phase epitaxy was investigated using transmission electron microscopy and X-ray diffraction (XRD) measurements. The density of stacking faults (SFs) in ELO c-GaN was similar to6 x 10(8) cm(-2), while that in underlying GaN template was similar to5 x 10(9) cm(-2). XRD measurements showed that the full-width at half-maximum of c-GaN (0 0 2) rocking curve decreased from 33 to 17.8 arcmin, indicating the improved crystalline quality of ELO c-GaN. The mechanism of SF reduction in ELO c-GaN was also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

Excitonic photoluminescence properties of nanocrystalline GaSb and Ga0.62In0.38Sb embedded in silica films

The GaSb and Ga0.62In0.38Sb nanocrystals were embedded in the SiO2 films by radio-frequency magnetron co-sputtering and were grown on GaSb and Si substrates at different temperatures. We present results on the 10K excitonic photoluminescence (PL) properties of nanocrystalline GaSb and Ga0.62In0.38Sb as a function of their size. The measurements show that the PL of the GaSb and Ga0.62In0.38Sb nanocrystallites follows the quantum confinement model very closely. By using deconvolution of PL spectra, origins of structures in PL were identified. (C) 2002 Elsevier Science B.V. All rights reserved.

Strain relaxation of InP film directly grown on GaAs patterned compliant substrate

In order to overcome the large lattice mismatch in the heteroepitaxy, a new patterned compliant substrate method has been introduced, which has overcome the disadvantages of previously published methods. InP film of thickness 800 nm was directly grown on this substrate. Scanning electron microscopy (SEM) has shown that good surface morphology has been obtained. In addition, Photoluminescence (PL) and double crystal X-ray diffraction (DCXRD) study have shown that the residual strain has been reduced, and that the structure quality has been improved. (C) 2002 Elsevier Science B.V. All rights reserved.