Exciton Localization and Delocalization in GaNAs/GaAs Quantum Wells

We have studied exciton localization and delocalization effect in GaNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL) and timeresolved PL measurements. Studied results suggest that, at low temperature and under a conventional CW excitation, measured PL spectra were dominated by localized exciton (LE) emission caused by potential fluctuations in GaNAs layer. However, under short pulse laser excitation, it is different. An extra high-energy PL peak comes out from GaNAs/GaAs QWs and dominates the PL spectra under high excitation and/or at high temperature. By investigation, we have attributed the new PL peak to the recombination of delocalized excitons in QWs. This recombination process competes with the localized exciton emission, which, we believe, constitutes the "S-shaped" temperature-dependent emission shift often reported in ternary nitrides of InGaN and AlGaN in the literature.

High Power 808nm AlGaAs/GaAs Quantum Well Laser Diodes with Broad Waveguide

The 808nm laser diodes with a broad waveguide are designed and fabricated. The thickness of the Al_(0.35)-Ga_(0.65)As waveguide is increased to 0.9μm. In order to suppress the super modes, the thickness of the Al_(0.55)Ga_(0.45)As cladding layers is reduced to only 0.7μm while keeping the transverse radiation losses of the fundamental mode below 0.2cm~(-1). The structures are grown by metal organic chemical vapour deposition. The devices show excellent performances. The maximum output power of 10.2W in the 100μm broad-area laser diodes is obtained.

Etching Behavior of GaN/GaAs(001) Epilayers Grown by MOVPE

Wet etching characteristics of cubic GAN (c-GaN) thin films grown on GaAs(001) by metalorganic vapor phase epitaxy (MOVPE) are investigated. The samples are etched in HCl, H_3PO_4, KOH aqueous solutions, and molten KOH at temperatures in the range of 90～300 ℃. It is found that different solution produces different etch figure on the surfaces of a sample. KOH-based solutions produce rectangular pits rather than square pits. The etch pits elongate in [1(1-bar)0] direction, indicating asymmetric etching behavior in the two orthogonal <110> directions. An explanation based on relative reactivity of the various crystallographic planes is employed to interpret qualitatively the asymmetric etching behavior. In addition, it is found that KOH aqueous solution would be more suitable than molten KOH and the two acids for the evaluation of stacking faults in c-GaN epilayers.

Small Signal Equivalent Circuit Model and Modulation Properties of Vertical Cavity-Surface Emitting Lasers

Small signal equivalent circuit model and modulation properties of vertical cavity-surface emitting lasers (VCSEL's) are presented. The modulation properties both in analytic-equation calculation and in circuit model simulation are studied. The analytic-equation calculation of the modulation properties is calculated by using Mathcad program and the circuit model simulation is simulation is simulated by using Pspice program respectively. The results of calculation and the simulation are in good agreement with each other. Experiment is performed to testify the circuit model.

Fiber Coupling of Laser Diode Bar to Multimode Fiber Array

A piece of multimode optical fiber with a low numerical aperture (NA) is used as an inexpensive microlens to collimate the output radiation of a laser diode bar in the high numerical aperture (NA) direction. The emissions of the laser diode bar are coupled into multimode fiber array. The radiation from individual ones of emitter regions is optically coupled into individual ones of fiber array. Total coupling efficiency and fiber output power are 75% and 15W, respectively.

Spotsize Converter Integrated DFB Laser Diode Using Selective Area Growth of MOCVD

The characteristics of thickness enhancement factor and bandgap wavelength of selectively grown In-GaAsP are investigated. A high thickness enhancement factor of 2.9 is obtained. Spotsize converter integrated DFB lasers are fabricated by using the technique of SAG. The threshold current is as low as 10.8mA. The output power is 10m W at 60mA without coating and the SMSR is 35.8dB. The vertical far field angle (FWHM) is decreased from 34 °to 9 °. The tolerance of 1dBm misalignment is 3.4μm vertically.

Calculation of Valence Subband Structures for Strained Quantum-Wells by Plane Wave Expansion Method Within 6 * 6 Luttinger-Kohn Model

The valence subband energies and wave functions of a tensile strained quantum well are calculated by the plane wave expansion method within the 6 * 6 Luttinger-Kohn model. The effect of the number and period of plane-waves used for expansion on the stability of energy eigenvalues is examined. For practical calculation, it should choose the period large sufficiently to ensure the envelope functions vanish at the boundary and the number of plane waves large enough to ensure the energy eigenvalues keep unchanged within a prescribed range.

Growth of Cubic GaN by MOCVD at High Temperature

High quality cubic GaN (c-GaN) is grown by metalorganic vapor deposition (MOCVD) at an increased growth temperature of 900 ℃, with the growth rate of 1.6 μm/h. The full width at half maximum (FWHM) of room temperature photoluminescence (PL) for the high temperature grown GaN film is 48meV. It is smaller than that of the sample grown at 830 ℃. In X-ray diffraction (XRD) measurement, the high temperature grown GaN shows a (002) peak at 20° with a FWHM of 21'. It can be concluded that, although c-GaN is of metastable phase, high growth temperature is still beneficial to the improvement in its crystal quality. The relationship between the growth rate and growth temperature is also discussed.

Investigation of Residual Donor Defects in Undoped and Fe-Doped LEC InP

The free electron concentration of as-grown liquid encapsulated Czochralski (LEC) InP measured by Hall effect is much higher than the concentration of net donor impurity determined by glow discharge mass spectroscopy. Evidence of the existence of a native donor hydrogen-indium vacancy complex in LEC undoped and Fe-doped InP materials can be observed with infrared absorption spectra. The concentration increase of the donor complex correlates with the increase of ionized deep acceptor iron impurity Fe~(2+) concentration in Fe-doped semi-insulating (SI) InP. These results indicate that the hydrogen-indium vacancy complex is an important donor defect in as-grown LEC InP, and that it has significant influence on the compensation in Fe-doped SI InP.

Growth and photoluminescence of InAlGaN films

Two quaternary InAlGaN films were grown by metal-organic chemical-vapor deposition (MOCVD) on sapphire (0001) substrates with and without high-temperature GaN interlayer, respectively. The structural and optical properties of the quaternary films were investigated by high-resolution X-ray diffraction (HRXRD), high-resolution electron microscopy (HREM), temperature-dependent photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy. According to the HRXRD and PL results, it is demonstrated that two samples have the same crystal quality. The TRPL signals of both samples were fitted well as a stretched exponential decay from 14 K to 250 K, indicating significant disorder in the materials, which is attributed to recombination of excitons localized in disorder quantum nanostructures such as quantum dots or quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-section HREM measurement further proves that there exist disorder quantum nanostructures in the quaternary. By investigating the temperature dependence of the dispersive exponent beta, it is shown that the stretched exponential decays of the two samples originate from different mechanisms. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Reduction in crystallographic tilt of lateral epitaxial overgrown GaN by using new patterned shape mask

We proposed a new method to suppress the crystallographic tilt in the lateral epitaxial overgrowth of GaN by using an oxide mask with a newly designed pattern. A rhombus mask with edges oriented in the direction of <10 - 10>(GaN) was used instead of the traditional stripe mask. The morphology evolution during the LEO GaN with the rhombus mask was investigated by SEM, and the crystallographic tilt in the LEO GaN was measured by DC-XRD. It is found that using the new rhombus mask can decrease the crystallographic tilt in the LEO GaN. In addition, this method makes the ELO GaN stripes easy to coalesce. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Single steady frequency and narrow line width external cavity semiconductor laser

A single longitudinal mode and narrow line width external cavity semiconductor laser is proposed. It is constructed with a semiconductor laser, collimator, a flame grating, and current and temperature control systems. The one facet of semiconductor laser is covered by high transmission film, and another is covered by high reflection film. The flame grating is used as light feedback element to select the mode of the semiconductor laser. The temperature of the constructed external cavity semiconductor laser is stabilized in order of 10(-3)degreesC by temperature control system. The experiments have been carried out and the results obtained-the spectral fine width of this laser is compressed to be less than 1.4MHz from its original line-width of more than 1200GHz and the output stability (including power and mode) is remarkably enhanced.

The influence of nitrogen clustering effect on optical transitions in GaInNAs/GaAs quantum wells

Photoluminescence (PL) spectra of GaInNAs/GaAs multiple quantum wells and GaInNAs epilayers grown on GaAs substrate show an apparent "S-shape" temperature-dependence of the of dominant luminescence peak. At low temperature and weak excitation conditions, a PL peak related to nitrogen cluster-induced bound states can be well resolved in the PL spectra. It displays a remarkable red shift of up to 60 meV and is thermally quenched below 100 K with increasing temperature, being attributed to N-cluster induced bound states. The indium incorporation exhibits significant effect on the cluster formation. The rapid thermal annealing treatment at 750 C can essentially remove the bound states-induced peak.

Photoluminescence characterization of 1.3 mu m In(Ga)As/GaAs islands grown by molecular beam epitaxy

1.3 mum wavelength In(Ga)As/GaAs nanometer scale islands grown by molecular beam epitaxy (MBE) were characterized by photoluminescence (PL) and atomic force microscopy (AFM) measurements. It is shown that inhomogeneous broadening of optical emission due to fluctuation of the In0.5Ga0.5As islands both in size and in compositions can be effectively suppressed by introducing a AlAs layer and a strain reduction In0.2Ga0.8As layer overgrown on top of the islands, 1.3mum emission wavelength with narrower line-width less than 20meV at room temperature was obtained.

Optical transitions in GaNAs/GaAs single quantum well

We have investigated GaNAs/GaAs single quantum wells (SQWs) grown by molecular beam epitaxy (MBE) using photoluminescence (PL), time-resolved PL (TRPL) and photovoltaic (PV) techniques. The low temperature PL is dominated by spatially direct transitions involving electrons confined in GaNAs well and holes localized in the same GaNAs layer. This assignment was supported by PL decay time measurements and absorption line-shape analysis derived from the PV measurements. By fitting the experimental data with a simple calculation, the band offset of the GaN0.015As0.985/GaAS heterostructure was estimated, and a type II band lineup in GaN0.015As0.985/GaAs QWs was suggested. Moreover, DeltaE(C), the discontinuity of conductor band, is found to be a nonlinear function of the nitrogen (N) composition (x), and the average variation of DeltaE(C) is about 0.110eV per %N, The measured band bowing coefficient shows a strong function of x, giving an experimental support to the theoretic calculation of Wei et al [Ref.2].