Shi Jun-Jie

Propagating Optical Phonon Modes and Their Electron-Phonon Interaction Hamiltonians in Asymmetric Wurtzite Nitride Semiconductor Quantum Wells

Within the framework of the dielectric continuum model and Loudons uniaxial crystal model, the properties of frequency dispersion of the propagating (PR) optical phonon modes and the coupling functions of electron-PR phonons interaction in an asymmetrical wurtzite quantum well (QW) are deduced and analyzed via the method of electrostatic potential expanding. Numerical calculation on an asymmetrical Al0.25Ga0.75N/GaN/Al0.15Ga0.85N wurtzite QW were performed. The results reveal that there are infinite branches of PR phonon modes in the systems. The behaviors of frequency forbidden of PR modes in the asymmetric QWs have been clearly observed. The mathematical and physical origins for these features have been analyzed in depth. The PR optical phonon branches have been distinguished and labelled reasonably in terms of the oscillating properties of the PR modes in the well-layer material. Moreover, the amplitudes and frequency properties of the electron-PR modes coupling functions in the barrier and well materials have also been analyzed from both of the mathematical and physical viewpoints.

Electronic structure and magnetic properties of substitutional transition-metal atoms in GaN nanotubes

The electronic structure and magnetic properties of the transition-metal (TM) atoms (Sc—Zn, Pt and Au) doped zigzag GaN single-walled nanotubes (NTs) are investigated using first-principles spin-polarized density functional calculations. Our results show that the bindings of all TM atoms are stable with the binding energy in the range of 6–16 eV. The Sc- and V-doped GaN NTs exhibit a nonmagnetic behavior. The GaN NTs doped with Ti, Mn, Ni, Cu and Pt are antiferromagnetic. On the contrary, the Cr-, Fe-, Co-, Zn- and Au-doped GaN NTs show the ferromagnetic characteristics. The Mn- and Co-doped GaN NTs induce the largest local moment of 4μB among these TM atoms. The local magnetic moment is dominated by the contribution from the substitutional TM atom and the N atoms bonded with it.

Linear and Nonlinear Intersubband Optical Absorptions and Refractive Index Changes in InGaN Strained Single Quantum Wells: Strong Built-in Electric Field Effects

Considering the strong built-in electric field (BEF) effects due to the spontaneous and piezoelectric polarizations, the intersubband optical absorptions and refractive index changes for an InxGa1?xN/AlyGa1?yN strained single quantum well are studied theoretically within the framework of the density matrix method and effective-mass approximation. The linear, third-order nonlinear and total absorption coefficients and refractive index changes are calculated as a function of the incident optical intensity and photon energy. Our results show that both the incident optical intensity and the strong BEF have great influence on the total absorptions and refractive index changes. The results are significant for designing some important photodetectors and the photonic crystal devices with adjustable photonic band structures.

Excitons in quantum-dot quantum-well nanoparticles

A variational calculation is presented for the ground-state properties of excitons confined in spherical core-shell quantum-dot quantum-well (QDQW) nanoparticles. The relationship between the exciton states and structure parameters of QDQW nanoparticles is investigated, in which both the heavy-hole and the light-hole exciton states are considered. The results show that the confinement energies of the electron and hole states and the exciton binding energies depend sensitively on the well width and core radius of the QDQW structure. A detailed comparison between the heavy-hole and light-hole exciton states is given. Excellent agreement is found between experimental results and our calculated 1se-1sh transition energies.

Interband Optical Transitions due to Donor Bound Excitons in Wurtzite InGaN Strained Coupled Quantum Dots: Strong Built-in Electric Field Effects

Considering the three-dimensional confinement of the electrons and holes and the strong built-in electric field (BEF) in the wurtzite InGaN strained coupled quantum dots (QDs), the positively charged donor bound exciton states and interband optical transitions are investigated theoretically by means of a variational method. Our calculations indicate that the emission wavelengths sensitively depend on the donor position, the strong BEF, and the structure parameters of the QD system.

Vibration Spectra of Quasi-confined Optical Phonon Modes in an Asymmetric Wurtzite AlxGa1−xN/GaN/AlyGa1−yN Quantum Well

Based on the dielectric continuum model and Loudons uniaxial crystal model, the properties of the quasi-confined (QC) optical phonon dispersions and the electron-QC phonons coupling functions in an asymmetric wurtzite quantum well (QW) are deduced via the method of electrostatic potential expanding. The present theoretical scheme can naturally reduce to the results in symmetric wurtzite QW once a set of symmetric structural parameters are chosen. Numerical calculations on an asymmetric AlN/GaN/Al0.15Ga0.85N wurtzite QW are performed. A detailed comparison with the symmetric wurtzite QW was also performed. The results show that the structural asymmetry of wurtzite QW changes greatly the dispersion frequencies and the electrostatic potential distributions of the QC optical phonon modes.

Electronic Structures of Wurtzite GaN with Ga and N Vacancies

The electronic band structures of wurtzite GaN with Ga and N vacancy defects are investigated by means of the first-principles total energy calculations in the neutral charge state. Our results show that the band structures can be significantly modified by the Ga and N vacancies in the GaN samples. Generally, the width of the valence band is reduced and the band gap is enlarged. The defect-induced bands can be introduced in the band gap of GaN due to the Ga and N vacancies. Moreover, the GaN with high density of N vacancies becomes an indirect gap semiconductor. Three defect bands due to Ga vacancy defects are created within the band gap and near the top of the valence band. In contrast, the N vacancies introduce four defect bands within the band gap. One is in the vicinity of the top of the valence band, and the others are near the bottom of the conduction band. The physical origin of the defect bands and modification of the band structures due to the Ga and N vacancies are analysed in depth.

Ground state of excitons in quantum-dot quantum-well nanoparticles: stochastic variational method

Within the framework of effective mass approximation, the ground state of excitons confined in spherical core-shell quantum-dot quantum-well (QDQW) nanoparticles is solved by using the stochastic variational method, in which the finite band offset and the heavy (light) hole exciton states are considered. The calculated 1se–1sh transition energies for the chosen CdS/HgS/CdS QDQW samples are in good agreement with the experimental measurements. Moreover, some previous theoretical results are improved.

Effect of a Two-Dimensional Periodic Dielectric Background on Complete Photonic Band Gap in Complex Square Lattices

A two-dimensional photonic crystal model with a periodic square dielectric background is proposed. The photonic band modulation effects due to the two-dimensional periodic background are investigated in detail. It is found that periodic modulation of the dielectric background greatly alters photonic band structures, especially for the E-polarization modes. The number, width and position of the photonic band gaps sensitively depend on the dielectric constants of the two-dimensional periodic background. Complete band gaps are found, and the dependence of the widths of these gaps on the structural and material parameters of the two alternating rods/holes is studied.

Exciton States in Wurtzite InGaN Coupled Quantum Dots

Based on the effective-mass approximation, exciton states confined in wurtzite InxGa1−xN/GaN strained coupled quantum dots (QDs) are investigated, in which the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization are considered. We find that the barrier thickness between the two QDs has a considerable influence on the exciton states and the interband optical transitions. If the barrier thickness is increased, the exciton binding energy is decreased, the emission wavelength is increased, and the electron–hole recombination rate is obviously reduced. Our theoretical results are in qualitative agreement with the experimental measurements.