Theoretical studies of AlGaAs-GaAs multiple-quantum-well opticalcouplers defined by ion-implantation-induced intermixing

A simple and accurate model is presented for the analysis of ion-implanted AlGaAs-GaAs multiple-quantum-well (MQW) symmetric and asymmetric optical couplers. The modal propagation constants and field profiles of the guided modes of the couplers are solved numerically using a quasi-vector method based on the Galerkin's method. MQW optical couplers defined by ion-implantation-induced intermixing are shown to have similar optical properties as conventional dielectric rib optical couplers. They also provide a more flexible control over the waveguiding and coupling characteristics by changing parameters such as waveguide separation, mask width, ion implant energy, and diffusion time     

Metalorganic Vapor Phase Epitaxial Growth of InAs/InGaAs Multiple Quantum Well Structures on InP Substrates

Device quality InAs/InGaAs multiple quantum well (MQW) structures were grown on InP substrates by metalorganic vapor phase epitaxy (MOVPE) and applied to lasers emitting at wavelengths longer than 2 mum. InAs/InGaAs MQWs with flat interfaces were obtained by adjusting the growth temperature between 460 degC and 510 degC. The photoluminescence peak wavelength of the MQWs increases from 1.93 to 2.47 mum as the thickness of InAs quantum wells increases from 2 to 7 nm. The structural and optical properties remained almost unchanged even after annealing at 620 degC. For 40-mu m-wide stripe broad-area lasers with 5-nm-thick InAs quantum wells, a lasing wavelength longer than 2.3 mum and an output power higher than 10 mW were achieved under continuous-wave operation at a temperature of 25 degC. These results indicate that InAs/InGaAs MQW structures grown by MOVPE are very useful for the active region of 2 mum wavelength lasers.     

Time-development of transient-carrier temperature, density, andgain spectrum in ultrashort optical pulse excited InGaAsmultiquantum-well laser structure

We present a novel transient gain-spectra measurement method based on the traditional variable pump-stripe technique. Using the pump-stripe technique with ultra-short optical pulse excitation, time-resolved amplified spontaneous emission spectroscopy of an InGaAs-InGaAsP multiquantum-well (MQW) laser structure was measured, and time-development of the transient optical net gain spectra was obtained accordingly. By fitting the measured gain spectra with a model for band-to-band transitions including momentum conservation and an energy- and density-dependent lifetime broadening, dynamics of band filling, carrier density, carrier temperature and bandgap renormalization have been obtained. This opens the possibility to study simultaneously the influence of transient-carrier density and, in particular, transient-carrier temperature on the transient optical gain. Strong gain compression in the whole gain-spectra region due to transient high carrier temperature after ultra-short pulse injection is clearly demonstrated for the first time     

Influences of unconfined states on the optical properties ofquantum-well structures

The population of the unconfined states, with energies above the band edge of the barrier layers, can be significant in some regions of the active volume in high power lasers and amplifiers. This paper analyzes the influences of these states on optical properties, such as gain, refractive index, differential gain, and linewidth enhancement factor, for different quantum-well (QW) structures. Our results show that at high excitation levels, the unconfined band contributions to the real part of the optical susceptibility can be significant, especially in structures with weak quantum confinement potentials. This is in agreement with recent measurements of peak gain and carrier-induced refractive index change versus carrier density, for InGaAs-GaAs QW laser structures     

Confined phonon scattering in multivalley Monte Carlo simulation of quantum cascade lasers

We present a detailed investigation of the effects that optical phonon confinement has on the electronic transport properties of GaAs-based multiple-quantum-well (MQW) quantum cascade laser (QCL) structures. Both confined and interface phonon modes are included based on the macroscopic dielectric continuum model. Interface phonon dispersions are obtained using the transfer matrix method with periodic boundary conditions. Scattering rates of both Γ- and X-valley electrons by all the interface and confined phonon modes are calculated and fully incorporated in the multivalley Monte Carlo simulation of a deep-active-well 6.7 μm GaAs-based MQW QCL. We find that the inclusion of phonon confinement enhances the electron-phonon scattering rates and output current to a relatively small extent with respect to the bulk phonon approximation.     

Atomistic tight-binding theory for acceptor states (C,Be, Mg,Zn, Si and Cd) of GaAs nanocrystals

The atomistic tight-binding theory is introduced as a new way to numerically calculate the binding energies of acceptors (C, Be, Mg, Zn, Si and Cd) in GaAs nanocrystals. In the present paper, the electronic structures and optical properties of spherical GaAs nanocrystals with a single substitutional acceptor impurity at the center and off-center sites are studied. The optical band gaps of undoped GaAs nanocrystals are larger for smaller nanocrystals, due to quantum confinement effects. Numerical results show that the acceptor binding energies are highly dependent on the nanoparticle sizes, impurity types and positions. Larger acceptor binding energies in doped GaAs nanocrystals are also seen for smaller nanocrystals. In term of acceptor varieties, the high-to-low order of binding energies is found in Cd, Si, Zn, Mg, Be and C acceptors, respectively. In term of positions, the acceptor binding energies are reduced with the increasing distances from the center of nanocrystals. In addition, the optical property is mainly improved in the presence of the substitutional acceptor impurity. Finally, these computational results provide an inroad to new materials not accessible via other means with the aim to implement novel applications based on the substitutional acceptor impurity in the nanostructures.     

InGaN-based ultraviolet emitting heterostructures with quaternary AlInGaN barriers

Nitride-based ultraviolet (UV) heterostructures with InGaN quantum wells and AlInGaN barrier layers have been grown by metal-organic chemical vapor deposition on sapphire substrates. The emission band was at 3.307 eV (375 nm) at room temperature (RT) and its full-width at half-maximum was /spl sim/82meV. In addition to the UV band, some blue emission admixture was found in a single-quantum-well (SQW) structure, which the authors attribute to recombination of injected electrons that are not captured into the SQW and enter the p-side of the structure. The authors demonstrate a significant advantage in utilizing multiple-quantum-well (MQW) structures that provide a more effective capture of injected carriers into wells and predominance of UV emission. Temperature-sensitive competition between two emission mechanisms in MQW structures has been observed. Below /spl sim/170 K, the blue impurity-related emission dominated. In the 170-190 K range, an anomalous temperature-induced "blue jump" by over /spl sim/340 meV to UV region occurred, with UV emission dominating above 190 K.     

A universal optical heterostructure for photonic integratedcircuits: a case study in the AlGaAs material system

An approach for the realization of a single heterostructure for multiple optical device applications in a photonic integrated circuit environment is addressed from the standpoint of building into the heterostructure separate device features. In particular, a low compositionally graded depressed cladding laser structure is shown to have a large mode, compared to a structure optimized for maximum confinement factor, and a radiation loss an order of magnitude lower than for a structure without the inner cladding, features which are important for fiber coupling and for compact bends. By incorporating a narrow gap section into the barrier layer, a bulk electroabsorption modulator can be integrated into the same structure that contains quantum wells for optical amplifiers. Data are presented on the device properties of a single heterostructure when used as large optical cavity lasers, s-bends, and bulk electroabsorption modulators. The only laser parameter affected by the incorporation of the modulator is the transparency current density which is increased 25% over standard structures. The performance of rib delineated s-bends is exceeded only by native oxide defined small optical cavity laser structures. The electroabsorption modulator, which was activated in a disordered section under reverse bias, had a modulation depth approximately 1/3 that predicted by model studies which was attributed to the depletion electric field profile. Methods for improving the modulation depth while lowering the transparency current density are outlined     

Interband and Intersubband Optical Properties of Doped n- $hbox{Zn}_{bf 0.46}hbox{Cd}_{bf 0.54}hbox{Se/Zn}_{bf 0.24}hbox{Cd}_{bf 0.25}hbox{Mg}_{bf 0.51}hbox{Se}$ Multiple Quantum Wells for Intersubband Device Applications

Two heavily doped n-type Zn_0.46Cd_0.54Se/Zn_0.24 Cd_0.25Mg_0.51Se multiple quantum well (MQW) structures have been grown on InP (0 0 1) substrates by molecular beam epitaxy. Photoluminescence (PL), time-resolved PL, and Fourier transform infrared (FTIR) spectroscopy were performed to characterize their interband and intersubband (ISB) properties. These two MQW samples have similar structures except for different well widths and a different number of periods. Excitation-intensity-dependent PL shows no electronic coupling between the multiquantum wells. The integrated PL intensities and the PL decay times of the MQWs were measured as functions of temperature in the range from 77 to 290 K. Theoretical fittings of temperature dependences of integrated PL intensities and PL decay times indicate that the nonradiative recombination processes observed in our samples can be well described by hole capture by acceptor-like defect centers through multiphonon emissions. ISB absorption spectra of the samples were measured by FTIR and show peak absorption at wavelengths of 3.99 and 5.35 mum for the MQWs with well widths of 28 and 42 A, respectively. Theoretical calculations based on the envelope function approximation confirm that these peaks are due to the transitions from the ground state E₁ to the first excited state E₂.     

1.3-μm InGaAsP-InP n-type modulation-doped strainedmultiquantum-well lasers

The use of n-type modulation doping to reduce the threshold current, the carrier lifetime, and the internal loss in 1.3-μm InGaAsP-InP strained multiquantum-well (MQW) lasers is experimentally demonstrated. The threshold current density, the carrier lifetime, and the internal loss were reduced by about 33%, 36%, and 28%, respectively, as compared with an undoped MQW laser. Moreover, the turn-on delay time in the n-type modulation-doped MQW lasers with a low-leakage buried heterostructure was reduced by about 35%. These results confirm the suitability of this type of laser for use in the basic structure of a monolithic laser array used as a light source for high-density parallel optical interconnection     

A new approach to analyzing anisotropic and non-parabolic effects on quantum wires

In this work we present a new approach to obtaining the subband structures of arbitrary-shape cross-section quantum wires. The method can be implemented for a general band structure, thus making it possible to introduce the anisotropic and non-parabolic features of the band into the analysis. Finally we compare the results obtained using anisotropic and non-parabolic band models with isotropic and parabolic ones in a triangular cross-section Si quantum wire with several crystalline orientations.     

The metal-organic chemical vapor deposition growth and propertiesof InAsSb mid-infrared (3-6-μm) lasers and LEDs

We describe the metal-organic chemical vapor deposition (MOCVD) growth of AlAs_1-xSb_x cladding layers and InAsSb-InAs multiple-quantum well (MQW) and InAsSb-InAsP strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. The AlAs_1-xSb_x cladding layers were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. By changing the layer thickness and composition of SLSs and MQWs, we have prepared structures with low temperature (<20 K) photoluminescence wavelengths ranging from 3.2 to 6.0 μm. We have made gain-guided injection lasers using undoped p-type AlAs_0.16Sb_0.84 for optical confinement and both strained InAsSb-InAs MQW and InAsSb-InAsP SLS active regions. The lasers and light emitting diodes (LEDs) utilize the semi-metal properties of a GaAsSb(p)-InAs(n) heterojunction as a source for electrons injected into active regions. A multiple-stage LED utilizing this semi-metal injection scheme is reported. Gain-guided, injected lasers with a strained InAsSb-InAs MQW active region operated up to 210 K in pulsed mode with an emission wavelength of 3.8-3.9 μm and a characteristic temperature of 29-40 K. We also present results for both optically pumped and injection lasers with InAsSb-InAsP SLS active regions. The maximum operating temperature of an optically pumped 3.7-μm strained-layer superlattice (SLS) laser was 240 K. An SLS LED emitted at 4.0 μm with 80 μW of power at 300 K     

High‐responsivity AlGaN–GaN multi‐quantum well UV photodetector

In this paper, we propose a set of AlGaN–GaN multi‐quantum well (MQW) photodetectors based on p‐i‐n heterostructures with 14 AlGaN–GaN MQW structures in i‐region, where GaN quantum well has 6 nm thickness and Al_xGa_1−xN barrier thickness is 3 nm. In this structure, the peak responsivity of 0.19 A/W at 246 nm is reported. In addition, we investigate effects of various parameters on responsivity, and we show that responsivity of MQW‐based photodetectors strongly depends on proper device design, that is, number of quantum wells, well thickness, barrier thickness, and mole fraction. We also show that increasing number of quantum wells, thickness of wells, and mole fraction as well as decreasing thickness of barriers, increase the responsivity. Using obtained results, we proposed optimal structure. Copyright © 2013 John Wiley & Sons, Ltd.     

Simulation of static and dynamic properties of edge-emitting multiple-quantum-well lasers

This paper demonstrates simulation tools for edge-emitting multiple-quantum-well (MQW) lasers. Properties of the strained MQW active region are simulated by eight-band kp calculations. Then, a two-dimensional simulation along the transverse cross section of the device is performed based on a drift-diffusion model, which is self-consistently coupled to equations for the optical field. Furthermore, a method is described that allows for an efficient quasi-three-dimensional simulation of dynamic properties of multisection edge-emitting lasers.     

DFT study on structural,electronic, and optical properties of cubic and monoclinic CuO

First-principles calculations were made to explore the structural, electronic, and optical properties of copper oxide (CuO) with monoclinic (m-CuO) and cubic (c-CuO) structures. We calculated the equilibrium structural parameters: lattice parameters (a, b, and c), angle \(\beta \), and volume V. The obtained results were in good agreement with the experimental data reported in the literature. The cohesive energy showed that m-CuO is more stable than c-CuO. The band structure indicated that c-CuO is an indirect band gap semiconductor with a band gap of 0.87 eV along R–G, while m-CuO has a metallic behavior. Furthermore, electrovalent and covalent bonds were observed in both c-CuO and m-CuO. The linear optical properties were calculated and analyzed along different polarization directions of the incident light. The results indicated that m-CuO possesses optical anisotropic properties. In particular, c-CuO can be used as a potential UV detector material because of its high absorption coefficient (356351.3).     

光电集成强电场测量系统及其应用研究

为研发高性能强电场测量系统,在分析高压电场测量的特殊要求后,介绍了光电电场测量系统的基本原理、最新进展及其在高压测量领域的优势。通过对光电集成电场测量系统及传感器体系结构、关键参数的分析,研制了一种光电集成强电场传感器并测量它的输入输出特性。用研制的测量系统测量棒板间隙击穿过程冲击电场与复合绝缘子工频场,证明光电集成电场测量系统在高压测量研究领域的广阔应用前景。     

Analysis of saturation velocity and energy relaxation time of electrons in Si using full-band Monte Carlo simulation

In this paper, we have investigated the influence of optical phonon energy on electron saturation velocity and energy relaxation time by using a full band Monte Carlo simulator. The energy band structure is obtained using the first principle calculation. The scattering probability is calculated so as to conserve the energy and momentum in the full band structure. It is found that the range of optical phonon energy from 56.2 to 63.9 meV allows experimental saturation velocity and energy relaxation time. Electron saturation velocity is more sensitive than relaxation time to the optical phonon energy.     

Effect of band parameters on interband optical absorption in quantum wire structure of low band gap III–V semiconductors

A generalized theory is presented to study the effect of band parameters on inter band optical absorption in quantum wire structure of III–V compound semiconductors considering the wave-vector ( $\vec{k}$ ) dependence of the optical transition matrix element (OME). The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted to calculate the inter band optical absorption coefficient (IOAC) for a wide range of III–V compound semiconductors like, InAs, InSb, Hg_1?x Cd _x Te and In_1?x Ga _x As _y P_1?y lattice matched to InP, having varied split-off energy band compared to their energy band gap. It has been found that IOAC for quantum wires (QWRs) increases in oscillatory manner with increasing incident photon energy and the positions of peaks of oscillation of the coefficient are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of band energy constants. This effect of band parameters is better revealed from the study of light polarization dependence of the absorption coefficient.