T.C. Chong

ELECTRONIC BAND STRUCTURES AND EFFECTIVE-MASS PARAMETERS OF WURTZITE GAN AND INN

The electronic band structures of wurtzite GaN and InN are calculated by the empirical pseudopotential method (EPM) with the form factors adjusted to reproduce band features which agree with recent experimental data and accurate first-principles calculations. The electron and hole effective masses at the Γ point are obtained using a parabolic line fit. Further, using the effective-mass Hamiltonian and the cubic approximation for wurtzite semiconductors, band edge dispersion at the Γ point obtained using the k.p method is fitted to that calculated using the EPM by adjusting the effective-mass parameters. Thus, we derived important band structure parameters such as the Luttinger-like parameters for GaN and InN which will be useful for material design in wide-gap nitride-based semiconductor lasers employing InGaN. The results also showed that the cubic approximation is fairly successful in the analysis of valence band structures for wurtzite nitrides.

Electronic properties of zinc‐blende GaN, AlN, and their alloys Ga1−xAlxN

The electronic properties of wide‐energy gap zinc‐blende structure GaN, AlN, and their alloys Ga1−xAlxN are investigated using the empirical pseudopotential method. Electron and hole effective mass parameters, hydrostatic and shear deformation potential constants of the valence band at Γ and those of the conduction band at Γ and X are obtained for GaN and AlN, respectively. The energies of Γ, X, L conduction valleys of Ga1−xAlxN alloy versus Al fraction x are also calculated. The information will be useful for the design of lattice mismatched heterostructure optoelectronic devices based on these materials in the blue light range application.

VALENCE HOLE SUBBANDS AND OPTICAL GAIN SPECTRA OF GAN/GA1-XALXN STRAINED QUANTUM WELLS

The valence hole subbands, TE and TM mode optical gains, transparency carrier density, and radiative current density of the zinc‐blende GaN/Ga0.85Al0.15N strained quantum well (100 A well width) have been investigated using a 6×6 Hamiltonian model including the heavy hole, light hole, and spin‐orbit split‐off bands. At the k=0 point, it is found that the light hole strongly couples with the spin‐orbit split‐off hole, resulting in the so+lh hybrid states. The heavy hole does not couple with the light hole and the spin‐orbit split‐off hole. Optical transitions between the valence subbands and the conduction subbands obey the Δn=0 selection rule. At the k≠0 points, there is strong band mixing among the heavy hole, light hole, and spin‐orbit split‐off hole. The optical transitions do not obey the Δn=0 selection rule. The compressive strain in the GaN well region increases the energy separation between the so1+lh1 energy level and the hh1 energy level. Consequently, the compressive strain enhances the TE mode ...

Effects of tensile strain in barrier on optical gain spectra of GaInNAs/GaAsN quantum wells

The band structures, optical gain spectra, and transparency radiative current densities of compressive-strained GaInNAs quantum wells (QWs) with different tensile-strained GaAsN (N composition from 0 to 3%) barriers are systematically investigated using a modified 6×6u200ak⋅p Hamiltonian including the heavy hole, light hole, and spin-orbit splitting bands. We found that the transition energy decreases when increasing the N composition in the barrier. The optical gain spectra and maximum optical gain as a function of carrier density and radiative current density are obtained for the GaInNAs/GaAsN QWs with well width of 5 nm, InW=28%, and NW=2.66% emitting around 1.55 μm. The transparency carrier density increases with the nitrogen composition in the GaAsN barrier. The transparency radiative current density decreases with more nitrogen being added into the barrier, which is in agreement with the recent experimental observation.

Optical, magnetic, and transport behaviors of Ge1−xMnxTe ferromagnetic semiconductors grown by molecular-beam epitaxy

The optical, magnetic, and transport behaviors of Ge1−xMnxTe (x=0.24 and 0.55) grown by solid-source molecular-beam epitaxy are investigated. X-ray diffraction shows that Ge1−xMnxTe crystallizes in rocksalt structure. The temperature-dependent magnetization (M-T) for x=0.55 sample gives a Curie paramagnetic temperature of θp∼180u2002K, which is consistent with the temperature-dependent resistivity ρ(T) measurement. Anomalous Hall effect is clearly observed in the samples and can be attributed to extrinsic skew scattering based on the scaling relationship of ρxy∝ρxx1.06. The magnetoresistance of Ge1−xMnxTe is isotropic and displays a clear hysterestic loop at low temperature, which resembles that of giant-magnetoresistance granular system in solids.

Temperature evolution of domains in potassium niobate single crystals

The behavior of domain walls in KNbO3 single crystals with temperature variation from room temperature to 300u2009°C has been investigated in situ by a heating visualization system. It has been observed that domain walls show active behavior in a small temperature range from the phase transition temperature of 225u2009°C, the range being about 10u2009°C in the orthorhombic phase and 15u2009°C in the tetragonal phase. The 90° domain walls are generated randomly and extend themselves rapidly within the crystal in high density. The 60° domain walls, which are only observed in orthorhombic phase, do not appear randomly but are formed along the boundary of intersecting 90° domain walls. The results suggest that the 90° domain walls are most likely caused by microdefects within the crystal, and that dislocations at the junction of intersecting 90° domain walls supply nucleation sites for the 60° domain walls.

Investigation of optical gain of GaInNAs/GaAs compressive-strained quantum wells

Abstract The band structures of Ga 1− x In x N y As 1− y /GaAs compressive-strained quantum wells (QWs) are investigated using 6×6 k · p Hamiltonian including the heavy hole, light hole and spin-orbit splitting bands. By varying the well width and N composition, the effects of quantum confinement and compressive strain are examined. The valence subband energy dispersion curves, TE and TM optical gain spectra of three possible QW structures emitting at 1.3xa0μm wavelength are given. Our calculations show that the Ga 0.7 In 0.3 N 0.016 As 0.984 /GaAs QW with well width of 43xa0A emitting at 1.3xa0μm has maximum optical gain 3270xa0cm −1 and differential gain up to about 0.88×10 −15 xa0cm 2 at the carrier density N =6×10 18 xa0cm −3 . It is suitable for high-speed laser emitting at 1.3xa0μm wavelength.

Pseudo-dendritic growth in lead molybdate single crystal by Czochralski technique

A pseudo-dendritic crystal growth of lead molybdate (PbMoO4) has been studied with the Czochralski technique. The observed dendritic growth is iso-directional, projecting from the circumference of the growth front, where the content of impurities, melt stoichiometry and temperature are expected to be varied. The dendrite arm length and inter-dendrite angle are closely correlated. It is found that the grown dendrite arms are bounded only by ten low-index crystallographic planes (001), (001), (101), (101), (101), (101), (011), (011), (011), and (011). Based on this work, a facet transition pattern existing among the dendritic arms has been revealed. Further analysis for crystal grains in as-grown PbMoO4 single crystals indicates that a similar dendritic growth can also occur under conventional growth conditions. It is further suggested that the constitutional supercooling is a major factor causing the dendritic growth in PbMoO4 single crystals.

Optical limiting phenomena of carbon nanoparticles prepared by laser ablation in liquids

We report optical limiting properties of carbon nanoparticles, which were made in liquids by laser ablation of a bulk carbon target. The carbon nanoparticles were analyzed with micro-Raman spectroscopy, UV-Vis spectroscopy and Electron microscopy. Optical limiting responses towards 532-nm wavelength were measured with a 7-ns Nd:YAG laser. Nanoparticle size and laser pulse repetition rate effects on optical limiting behaviour were studied. A model was proposed to explain the physical origin of this nonlinear optical process. This work can provide useful information for designing carbon nanoparticle based optical limiters.

Band structure parameters of zinc-blende GaN, AlN and their alloys Ga1−xAlxN

The electronic properties of wide energy gap zinc-blende structure GaN, AlN and their alloys Ga1-xAlxN are investigated using the empirical pseudopotential method. Electron and hole Effective mass parameters, hydrostatic and shear deformation potential constants of the valence band at Gamma and those of the conduction band at Gamma and X are obtained. The energies of Gamma, X, L conduction valleys of Ga1-xAlxN alloy versus Al fraction x are also calculated. The information will be useful for the design of lattice mismatched heterostructure optoelectronic devices in the blue light range.