A Si-based tunable narrow-band flat-top filter with multiple-step-type Fabry-Perot cavity structure

In the optical network, the quick and accurate alignment with wavelength is an important issue during the channel detection. At this point, a filter having flat-top response characteristic is an effective solution. Based on multiple-step-type Fabry-Perot cavity structure, a novel all-Si-based thermooptical tunable flat-top filter with narrow-band has been fabricated, using our patent silicon-on-reflector bonding technology. The device demonstrated a 1-dB flat-top width of 1 nm, 3-dB band of 3 nm, free spectra range of 8 nm, and the tuning range of 4.6 nm was obtained under the applied voltage of 4 V.

Two-dimensional photonic band-gap defect modes with deformed lattice

A numerical study of the defect modes in two-dimensional photonic crystals with deformed triangular lattice is presented by using the supercell method and the finite-difference time-domain method. We find the stretch or shrink of the lattice can bring the change not only on the frequencies of the defect modes but also on their magnetic field distributions. We obtain the separation of the doubly degenerate dipole modes with the change of the lattice and find that both the stretch and the shrink of the lattice can make the dipole modes separate large enough to realize the single-mode emission. These results may be advantageous to the manufacture of photonic crystal lasers and provide a new way to realize the single-mode operation in photonic crystal lasers.

Pressure behavior of the alloy band edge and nitrogen-related centers in GaAs0.999N0.001

The photoluminescence of a GaAsN alloy with 0.1% nitrogen has been studied under pressures up to 8.5 GPa at 33, 70, and 130 K. At ambient pressure, emissions from both the GaAsN alloy conduction band edge and discrete nitrogen-related bound states are observed. Under applied pressure, these two types of emissions shift with rather different pressure coefficients: about 40 meV/GPa for the nitrogen-related features, and about 80 meV/GPa for the alloy band-edge emission. Beyond 1 GPa, these discrete nitrogen-related peaks broaden and evolve into a broad band. Three new photoluminescence bands emerge on the high-energy side of the broad band, when the pressure is above 2.5, 4.5, and 5.25 GPa, respectively, at 33 K. In view of their relative energy positions and pressure behavior, we have attributed these new emissions to the nitrogen-pair states NN3 and NN4, and the isolated nitrogen state N-x. In addition, we have attributed the high-energy component of the broad band formed above 1 GPa to resonant or near-resonant NN1 and NN2, and its main body to deeper cluster centers involving more than two nitrogen atoms. This study reveals the persistence of all the paired and isolated nitrogen-related impurity states, previously observed only in the dilute doping limit, into a rather high doping level. Additionally, we find that the responses of different N-related states to varying N-doping levels differ significantly and in a nontrivial manner.

Band gap narrowing in heavily B doped Si1-xGex strained layers

This paper presents a comprehensive study of the effect of heavy B doping and strain in Si1-xGex strained layers. On the one hand, bandgap narrowing (BGN) will be generated due to the heavy doping, on the other hand, the dopant boron causes shrinkage in the lattice constant of SiGe materials, thus will compensate for part of the strain. Taking the strain compensation of B into account for the first time and uesing the with semi-empirical method, the Jain-Roulston model is modified. And the real BGN distributed between the conduction and valence bands is calculated, which is important for the accurate design of SiGe HBTs.

Determination of the valence band offset of wurtzite InN/ZnO heterojunction by x-ray photoelectron spectroscopy

The valence band offset (VBO) of the wurtzite InN/ZnO heterojunction is directly determined by x-ray photoelectron spectroscopy to be 0.82 +/- 0.23 eV. The conduction band offset is deduced from the known VBO value to be 1.85 -/+ 0.23 eV, which indicates a type-I band alignment for InN/ZnO heterojunction. (C) 2007 American Institute of Physics.

Effect of interaction between periodic delta-doping in both well and barrier layers on modulation of superlattice band structure

The modulation of superlattice band structure via periodic delta-doping in both well and barrier layers have been theoretically investigated, and the importance of interaction between the delta-function potentials in the well layers and those in the barrier layers on SL band structure have been revealed. It is pointed out that the energy dispersion relation Eq. (3) given in [G. Ihm, S.K. Noh, J.I. Lee, J.-S. Hwang, T.W. Kim, Phys. Rev. B 44 (1991) 6266] is an incomplete one, as the interaction between periodic delta-doping in both well and barrier layers had been overlooked. Finally, we have shown numerically that the electron states of a GaAs/Ga0.7Al0.3As superlattice can be altered more efficiently by intelligent tuning the two delta-doping's positions and heights. (c) 2007 Elsevier B.V. All rights reserved.

A photonic crystal waveguide with silicon on insulator in the near-infrared band

A two-dimensional (2D) photonic crystal waveguide in the Gamma-K direction with triangular lattice on a silicon-on insulator (SOI) substrate in the near-infrared band is fabricated by the combination of electron beam lithography and inductively coupled plasma etching. Its transmission characteristics are analysed from the stimulated band diagram by the effective index and the 2D plane wave expansion (PWE) methods. In the experiment, the transmission band edge in a longer wavelength of the photonic crystal waveguide is about 1590 nm, which is in good qualitative agreement with the simulated value. However, there is a disagreement between the experimental and the simulated results when the wavelength ranges from 1607 to 1630 nm, which can be considered as due to the unpolarized source used in the transmission measurement.

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.

Giant Zeeman splitting in manganese-doped ZnSe quantum spheres: Eight-band effective-mass calculations

We deduce the eight-band effective-mass Hamiltonian model for a manganese-doped ZnSe quantum sphere in the presence of the magnetic field, including the interaction between the conduction and valence bands, the spin-orbit coupling within the valence bands, the intrinsic spin Zeeman splitting, and the sp-d exchange interaction between the carriers and magnetic ion in the mean-field approximation. The size dependence of the electron and hole energy levels as well as the giant Zeeman splitting energies are studied theoretically. We find that the hole giant Zeeman splitting energies decrease with the increasing radius, smaller than that in the bulk material, and are different for different J(z) states, which are caused by the quantum confinement effect. Because the quantum sphere restrains the excited Landau states and exciton states, in the experiments we can observe directly the Zeeman splitting of basic states. At low magnetic field, the total Zeeman splitting energy increases linearly with the increasing magnetic field and saturates at modest field which is in agreement with recent experimental results. Comparing to the undoped case, the Zeeman splitting energy is 445 times larger which provides us with wide freedom to tailor the electronic structure of DMS nanocrystals for technological applications.

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.

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.