Energy band design for Si/SiGe quantum cascade laser

This paper introduces in detail the working principle of Si/SiGe Quantum cascade laser(QCL). Appropriate parameters are used to calculate the hole subband structure of Si/Si1-xGex quantum well using a six-band k center dot p method. The dispersion relation and energy band for different layer thickness and compositions are investigated. Meanwhile, the energy separations between hole subbands in Si/Si1-xGex/Si quantum wells are also analyzed. Finally the calculated results are used for the Si/SiGe QCL design, which will be beneficial to the structure optimization of Si/SiGe QCL.

Tunable ultra-narrow band-gap of anomalous dispersion photonic crystals

We use the transfer-matrix method to research the band structures in one-dimensional photonic crystals composed of anomalous dispersion material ( saturated atomic cesium vapor). Our calculations show that that type of photonic crystal possesses an ultra-narrow photonic band gap and this band gap is tunable when altering the electron population in the atomic ground state of the anomalous dispersion material by the optical pumping method. Copyright (C) EPLA, 2007.

Influence of N doping on the Rashba coefficient, semiconductor-metal transition, and electron effective mass in InSb1-xNx nanowires: Ten-band k center dot p model

The electronic structures of InSb1-xNx nanowires are investigated using the ten-band k center dot p method. It is found that nitrogen increases the Rashba coefficient of the nanowires dramatically. For thick nanowires, the Rashba coefficient may increase by more than 20 times. The semiconductor-metal transition occurs more easily in InSb1-xNx nanowires than in InSb nanowires. The electronic structure of InSb1-xNx nanowires is very different from that of the bulk material. For fixed x the bulk material is a semimetal, while the nanowires are metal-like. In InSb1-xNx bulk material and thick nanowires, an interesting decrease of electron effective mass is observed near k=0 which is induced by the nitrogen, but this phenomenon disappears in thin nanowires.

Broad-band semiconductor optical amplifiers

Broad-band semiconductor optical amplifiers (SOAs) with different thicknesses and thin bulk tensile-strained active layers were fabricated and studied. Amplified spontaneous emission (ASE) spectra and gain spectra of SOAs were measured and analyzed at different CW biases. A maximal 3 dB ASE bandwidth of 136 nm ranging from 1480 to 1616 nm, and a 3 dB optical amplifier gain bandwidth of about 90 nm ranging from 1510 to 1600 nm, were obtained for the very thin bulk active SOA. Other SOAs characteristics such as saturation output power and polarization sensitivity were measured and compared. (c) 2006 Elsevier B.V. All rights reserved.

Pressure dependence of the near-band-edge photoluminescence from ZnO microrods at low temperature

The temperature and pressure dependences of band-edge photo luminescence from ZnO microrods have been investigated. The energy separation between the free exciton (FX) and its first order phonon replica (FX-1LO) decreases at a rate of k(B)T with increasing temperature. The intensity ratio of the FX-1LO to the bound exciton (BX) emission is found to decrease slightly with increasing pressure. All of the exciton emission peaks show a blue shift with increasing pressure. The pressure coefficient of the FX transition, longitudinal optical (LO) phonon energy, and binding energy of BX are estimated to be 21.4, 0.5, and 0.9 meV/GPa, respectively. (c) 2006 Elsevier Ltd. All rights reserved.

Electronic structure of InSb quantum ellipsoids in an external magnetic field

The shape dependence of electronic structure, electron g factors in the presence of the external magnetic field of InSb quantum ellipsoids are investigated in the framework of eight-band effective-mass approximation. It is found that as the increasing aspect ratio e, the electron states with P character split into three doublets for the different physical interaction and the light-hole states with S character come up to the top of valence bands at e = 2.6 in comparison with the heavy-hole states. In the presence of the external magnetic field, the energy splits of electron states are different for their wave function distribution direction, and the hole ground state remain optical active for a suitable aspect ratio. The electron g factors of InSb spheres decrease with increasing radius, and have the value of about two for the smallest radius, about -47.2 for sufficiently larger radius, similar to the bulk material case. Actually, the electron g factors decrease as any one of the three dimensions increase. The more dimensions increase, the more g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimensions. (c) 2006 Elsevier B.V. All rights reserved.

Design of shallow acceptors in ZnO: First-principles band-structure calculations

p-type doping is a great challenge for the full utilization of ZnO as short-wavelength optoelectronic material. Due to a large electronegative characteristic of oxygen, the ionization energy of acceptors in ZnO is usually too high. By analyzing the defect wave-function character, we propose several approaches to lower the acceptor ionization energy by codoping acceptors with donor or isovalent atoms. Using the first-principles band-structure method, we show that the acceptor transition energies of V-Zn-O-O can be reduced by introducing F-O next to V-Zn to reduce electronic potential, whereas the acceptor transition energy of N-O-nZn(Zn) (n=1-4) can be reduced if we replace Zn by isovalent Mg or Be to reduce the anion and cation kinetic p-d repulsion, as well as the electronic potential.

Biaxial stress dependence of the electrostimulated near-band-gap spectrum of GaN epitaxial film grown on (0001) sapphire substrate

The biaxial piezospectroscopic coefficient (i.e., the rate of spectral shift with stress) of the electrostimulated near-band-gap luminescence of gallium nitride (GaN) was determined as Pi=-25.8 +/- 0.2 meV/GPa. A controlled biaxial stress field was applied on a hexagonal GaN film, epitaxially grown on (0001) sapphire using a ball-on-ring biaxial bending jig, and the spectral shift of the electrostimulated near-band-gap was measured in situ in the scanning electron microscope. This calibration method can be useful to overcome the lack of a bulk crystal of relatively large size for more conventional uniaxial bending calibrations, which has so far hampered the precise determination of the piezospectroscopic coefficient of GaN. The main source of error involved with the present calibration method is represented by the selection of appropriate values for the elastic stiffness constants of both film and substrate. The ball-on-ring calibration method can be generally applied to directly determine the biaxial-stress dependence of selected cathodoluminescence bands of epilayer/substrate materials without requiring separation of the film from the substrate. (c) 2006 American Institute of Physics.

Electronic states in InAs quantum spheres and ellipsoids

The eight-band effective-mass Hamiltonian of the free-standing narrow-gap InAs quantum ellipsoids is developed, and the electron and hole electronic structures as well as optical properties are calculated by using the model. The energies, wave functions and transition probabilities of quantum spheres as functions of the radius of quantum sphere R is presented. It is found that the energy levels do not vary as 1/R-2, which is caused by the coupling between the conduction and valence bands, and by the constant terms correspond to the spin-orbit splitting energy. The blueshifts of hole states depend strongly on the coupling from electron states, so that the order of hole states changes as has been predicted in experiment. The exciton binding energies are calculated, the calculated excitonic gaps as functions of the ground exciton transition energy are in good agreement with the photoluminescence measured spectra in details. Finally, the hole energy levels and the linear polarization factors in InAs quantum ellipsoids as functions of the aspect ratio are presented. The state 1S(Z up arrow)((1/2)) becomes the hole ground state when e is larger than 2.4. The saturation value of the linear polarization factors of the InAs long ellipsoids of diameter 2.0 nm is 0.86, in agreement with the experimental results.

Comparative study for colloidal quantum dot conduction band state calculations

By comparing the results of some well-controlled calculation methods, we analyze the relative importance of bulk band structure, multi-bulk-band coupling, and boundary conditions in determining colloidal quantum dot conduction band eigenenergies. We find that while the bulk band structure and correct boundary conditions are important, the effects of multi-bulk-band coupling are small.

Synthesis and temperature-dependent near-band-edge emission of chain-like Mg-doped ZnO nanoparticles

Chain-like Mg-doped ZnO nanoparticles were prepared using a wet chemical method combined with subsequent heat treatment. The blueshifted near-band-edge emission of the doped ZnO sample with respect to the undoped one was investigated by temperature-dependent photoluminescence. Based on the energy shift of the free-exciton transition, a band gap enlargement of similar to 83 meV was estimated, which seems to result in the equivalent shift of the bound-exciton transition. At 50 K, the transformation from the donor-acceptor-pair to free-to-acceptor emissions was observed for both the undoped and doped samples. The results show that Mg doping leads to the decrease of the acceptor binding energy. (c) 2006 American Institute of Physics.

Band gap renormalization and carrier localization effects in InGaN/GaN quantum-wells light emitting diodes with Si doped barriers

The optical properties of two kinds of InGaN/GaN quantum-wells light emitting diodes, one of which was doped with Si in barriers while the other was not, are comparatively investigated using time-integrated photoluminescence and time-resolved photoluminescence techniques. The results clearly demonstrate the coexistence of the band gap renormalization and phase-space filling effect in the structures with Si doped barriers. It is surprisingly found that photogenerated carriers in the intentionally undoped structures decay nonexponentially, whereas carriers in the Si doped ones exhibit a well exponential time evolution. A new model developed by O. Rubel, S. D. Baranovskii, K. Hantke, J. D. Heber, J. Koch, P. Thomas, J. M. Marshall, W. Stolz, and W. H. Ruhle [J. Optoelectron. Adv. Mater. 7, 115 (2005)] was used to simulate the decay curves of the photogenerated carriers in both structures, which enables us to determine the localization length of the photogenerated carriers in the structures. It is found that the Si doping in the barriers not only leads to remarkable many-body effects but also significantly affects the carrier recombination dynamics in InGaN/GaN layered heterostructures. (c) 2006 American Institute of Physics.

Origin and elimination of spurious solutions of the eight-band k center dot p theory

The origin of spurious solutions in the eight-band envelope function model is examined and it is shown that spurious solutions arise from the additional spurious degeneracies caused by the unphysical bowing of the conduction bands calculated within the eight-band k center dot p model. We propose two approaches to eliminate these spurious solutions. Using the first approach, the wave vector cutoff method, we demonstrate the origin and elimination of spurious solutions in a transparent way without modifying the original Hamiltonian. Through the second approach, we introduce some freedom in modifying the Hamiltonian. The comparison between the results from the various modified Hamiltonians suggests that the wave vector cutoff method can give accurate enough description to the final results.

Violet/blue photoluminescence from CeO2 thin film

CeO2 thin film was fabricated by dual ion beam epitaxial technique. The violet/blue PL at room temperature and lower temperature was observed from the CeO2 thin film. After the analysis of crystal structure and valence in the compound was carried out by the XRD and XPS technique, it was inferred that the origin of CeO2 PL was due to the electrons transition from Ce4f band to O2p band and the defect level to O2p band. And these defects levels were located in the range of 1 eV around Ce4f band.

Study on optical band gap of boron-doped nc-Si : H film

The optical band gap (E-g) of the boron (B)-doped hydrogenated nano-crystalline silicon (nc-Si:H) films fabricated using plasma enhanced chemical vapor deposition (PECVD) was investigated in this work. The transmittance of the films were measured by spectrophotometric and the E-g was evaluated utilizing three different relations for comparison, namely: alphahnu=C(hnu-E-g)(3), alphahnu=B-0(hnu-E-g)(2), alphahnu=C-0(hnu-E-g)(2). Result showed that E-g decreases with the increasing of Boron doping ratio, hydrogen concentration, and substrate's temperature (T-s), respectively. E-g raises up with rf power density (P-d) from 0.45W.cm(-2) to 0.60w.cm(-2) and then drops to the end. These can be explained for E-g decreases with disorder in the films.