The influence of Coulomb enhancement on the modulation propertiesof quantum-well lasers

The phenomenon of Coulomb enhancement, resulting from the consideration of many-body effects, is included in a detailed calculation of the gain of a quantum-well (QW) laser, which is then used to predict the laser's modulation response. Carrier transport in the separate-confinement heterostructure is taken into account. The modulation response is compared to experimental data and to predictions from calculations using only the free-carrier gain. The comparison shows that the inclusion of Coulomb enhancement in the theoretical calculations leads to better agreement between simulated and experimental data     

Binding energy of shallow donors in asymmetrical systems of quantum wells

The dependence of the binding energy of a shallow donor impurity on its position in an asymmetrical system of tunnel-coupled quantum wells is mainly determined by the structure of the one-electron envelope functions and the difference between the dielectric constants of the quantum-well and barrier materials. An effective technique is suggested for calculating the binding energies and envelope functions of the shallow donor states in type-I heterostructures with narrow wells and barriers. We present the results of calculations for Al_xGa_1−x As-GaAs structures with two or more quantum wells without imposing any restrictions on the ratios of their sizes. Fiz. Tekh. Poluprovodn. 31, 302–307 (March 1997)     

Carrier energy relaxation time in quantum-well lasers

Carrier energy relaxation via carrier-polar optical phonon interactions with hot phonon effects in multisubband quantum-well structures is theoretically studied by using both bulk longitudinal optical phonons and confined longitudinal optical phonons. We find that the width and the depth of quantum wells only have moderate effects on carrier energy relaxation rates. Our results also indicate that the difference of energy relaxation rates between the quantum well and the bulk material is not significant. We investigate the effects of longitudinal optical phonon lifetimes on the carrier energy relaxation rate. Neglect of the finite decay time of longitudinal optical phonons will significantly underestimate the carrier energy relaxation time; this not only contradicts the experimental results but also severely underestimates the nonlinear gain coefficient due to carrier heating. The implications of our theoretical results in designing high-speed quantum-well lasers are discussed     

Calculation of the energy levels of shallow acceptors in uniaxially strained germanium

A variational method is used to calculate the positions of resonant and localized energy levels of a shallow acceptor impurity in uniaxially compressed germanium in the high-pressure limit. The dependence of the positions of these levels on the applied pressure is presented. Fiz. Tekh. Poluprovodn. 32, 799–802 (July 1998)     

Possibility of coherent light emission from Bose condensed statesin quantum-well systems

The possibility of coherent light emission in systems with a pairing of electrons and holes in a double quantum well spatially separated by a thin barrier [separated electron hole pairs (SEHPs)] is predicted. Bose condensation of excitons formed from SEHPs in GaAs/Al _xGa_(1-x)As double quantum wells of ~10 Å size are possible at relatively high temperatures. Coherent recombination emission due to the transfer of coherence from the Bose-condensed SEHPs to the photon field is expected     

Binding energy of donors in symmetric triangular quantum wells

The great progressin semiconductor technology,suchas molecular beamepitaxy( MBE) ,made it possible togrow quantum wells of non-square in high precision.Quantum well structures with triangular confinementprofiles have been performed by MBE and the opticalandtransport properties have been alsoinvestigated[1-4].Triangular quantum well (TQW) structures have at-tracted muchattention because of their special propertiesof quantumlevelsinrecent years .Jianget al .[5]calculat-edthe binding energies…     

Investigation of Coulomb scattering on sSi/Si0.5Ge0.5/sSOI quantum-well p-MOSFETs

sSi/Si_0.5Ge_0.5/sSOI quantum-well (QW) p-MOSFETs with HfO₂/TiN gate stack were fabricated and characterized. According to the low temperature experimental results, carrier mobility of the strained Si_0.5Ge_0.5 QW p-MOSFET was mainly governed by phonon scattering from 300 to 150 K and Coulomb scattering below 150 K, respectively. Coulomb scattering was intensified by the accumulated inversion charges in the Si cap layer of this Si/SiGe heterostructure, which led to a degradation of carrier mobility in the SiGe channel, especially at low temperature.     

Tunnelling in quantum-well structures

An exact solution of the tunnelling current through quantum-well barriers is compared with various approximations. Better agreement between theory and experiment follows in certain règimes.     

Wind energy systems

Wind energy has matured to a level of development where it is ready to become a generally accepted utility generation technology. A brief discussion of this development is presented, and the operating and design principles are discussed. Alternative designs for wind turbines and the tradeoffs that must be considered are briefly compared. Development of a wind energy system and the impacts on the utility network including frequency stability, voltage stability, and power quality are discussed. The assessment of wind power station economics and the key economic factors that determine the economic viability of a wind power plant are presented     

Linewidth enhancement in quantum-well lasers

We have developed explicit approximations for the Hnewidth enhancement factor in quantum-well lasers. These simple expressions represent a quick and easy means of calculating the linewidth enhancement factor under a variety of operating conditions, and help us to understand the physics influencing this important parameter.     

Thermal acceptors in irradiated silicon

Comparative analysis of the conditions for the formation of shallow acceptor centers upon high-temperature annealing in silicon irradiated with electrons, neutrons, and energetic ions is performed. The introduction of a sufficiently large (in comparison with the initial concentration of impurities and defects) concentration of radiation-induced distortions of the silicon lattice is shown to lead to the formation of thermal acceptors stable up to annealing temperature of ～650°C. The acceptor formation is supposed to be due to the interaction of background acceptor impurities (supposedly boron) with vacancies “stored” in multivacancy clusters and released upon their breakup.     

Electric field and impurity concentration effects on the ionization energy of impurities. Application to acceptors in ZnTe

The ionization energies of donor and acceptor centers measured by thermal stimulation methods are systematically lower than the energies deduced from optical measurements. This result can be explained by the perturbations of the emission probability due to tunneling, Poole-Frenkel effect and coulombic interaction between impurities. The effect of the last two terms is calculated in realistic situations corresponding to DLTS or admittance spectroscopy experiments and compared with experimental results obtained for three impurities (Au, Cu, Ag) in ZnTe. After correction the thermal ionization energies are well identified with the optical ones if the deep acceptor concentration does not exceed too much that of the shallow ones.     

Gain of TE-TM modes in quantum-well lasers

Semiclassical laser theory is rigorously applied to semiconductor lasers in order to obtain both the complete TE and TM linear gain. The resulting expressions for the modal gain in heterostructure lasers differ in form from those conventionally accepted. In particular, the conventional modal gain written as the product of a confinement factor and a bulk gain is only an approximation of the true modal gain derived. The conventional expression relies on an explicit definition of the active region of the laser, which can be ambiguous when certain heterostructures, such as parabolic quantum wells, are to be treated. This ambiguity is eliminated by the gain expressions as a more natural active region defined by the product of electron and hole wave functions emerges. The relevant approximations which allow the newly derived gain equations to be written in forms similar to the conventional expressions for single quantum well, multiquantum well (MQW), and in wide active region lasers are explicitly shown     

Some energy relations in linear dynamic systems

It is well known that the two-port reactance function of a lossless system is uniquely related to the stored energy in the system. It is here demonstrated that the analogous reactance function for an active and/or lossy system characterizes the power exchange between the system and its surroundings. It is pointed out that the true stored energy of such a system cannot be uniquely related to its two-port impedance function.     

Intraband relaxation time in quantum-well lasers

Intraband relaxation time, which causes spectral broadening of optical gain and spontaneous emission spectra, is estimated theoretically for quantum-well lasers. Carrier-carrier and carrier-longitudinal-optical (LO) phonon scattering mechanisms are considered, and it is shown that hole-hole, electron-hole, and hole-LO phonon scattering are dominant in spectral broadening. Intraband relaxation time determined by all of these mechanisms increases slightly with the decrease of well width. The dependence of intraband relaxation time on temperature, carrier density, and energy of electron and hole is also shown. Spectral line shape is discussed as an extension of the above calculation, and an approximated formula is given     

Power scaling in quantum-well laser amplifiers

The effects of increasing excitation on the performance of quantum-well semiconductor laser amplifiers were investigated. Amplified spontaneous emission (ASE) and gain roll over at high injected carrier densities are two limitations to the power scaling of these devices. A Rigrod analysis was used to study the effects of these limitations on the gain, ratio of signal to ASE power, and efficiency for different values of injection current, facet reflectivity, and input laser intensity. Comparisons are made with an equivalent amplifier operating with a bulk semiconductor gain medium. This analysis suggests that quantum-well semiconductor amplifier performance improves with a double-pass configuration     

Polarization-dependent gain saturations in quantum-well lasers

Theoretical analyses of polarization-dependent optical gain saturation are given for semiconductor quantum-well (QW) lasers to investigate the conditions of polarization switching and bistable operations. Nonlinear susceptibilities, which give saturation coefficients, are obtained in the perturbative analyses of density matrices, where the relevant electronic states in the QW are calculated by diagonalizing Luttinger's Hamiltonian, thus including valence band mixing. The present formulation is applied to InGaAsP QW lasers with edge-emitting and vertical-cavity surface-emitting laser (VCSEL) structures, and the self- and cross-saturation coefficients with parallel and orthogonal optical polarizations are numerically calculated, which are compared with those of bulk lasers. For the edge-emitting case, the saturation coefficients are strongly dependent on the photon energies, and the bistable operation condition is not satisfied in the gain peak, different from a bulk laser which showed only a slight energy dependence. In a VCSEL, the saturation coefficients are also dependent on the photon energies but the bistable operation condition is always satisfied     

Exciton polaritons in long-period quantum-well structures

The distribution of the normal frequencies in the complex plane for the exciton polaritons in a finite periodic quantum-well structure is analyzed by matrix-theory methods. Relations governing the sums of the normal frequencies for polariton modes that are even and odd relative to the center of the structure are derived. It is shown in an anti-Bragg structure, whose period equals a quarter of the optical wavelength at the excitonic resonance frequency ω₀, that the sets of the natural frequencies corresponding to even and odd solutions transform into one another upon reflection relative to the vertical line ω=ω ₀. Approximate analytical expressions are found for the natural frequencies of the “long-lived” and “short-lived” polariton modes. The relation between the shape of the optical reflection spectrum and the set of natural frequencies of the system is elucidated. Fiz. Tekh. Poluprovodn. 32, 101–107 (January 1998)     

智能预测在能源管理系统中的应用

对于能源的管理方面的问题,一直是众多企业所面临的很重要的功课.如何能够有效地解决企业能源管理建设中的问题,彻底解决功能缺失等一系列弊端,需要结合智能预测在能源管理系统中的具体应用,并且将其对号入座,实现其作用的最大化应用.本论文对此进行了具体的阐述与分析.