The electric field dependence of a donor impurity in graded <Emphasis Type="Bold">GaAs</Emphasis> quantum wires

The effect of the electric field on the binding energy of the ground state of a shallow donor impurity in a graded GaAs quantum-well wire (GQWW) was investigated. The electric field was applied parallel to the symmetry axes of the wire. Within the effective mass approximation, we calculated the binding energy of the donor impurity by a variational method as a function of the wire dimension, applied electric field, and donor impurity position. We show that changes in the donor binding energy in GQWWs strongly depend not only on the quantum confinement, but also on the direction of the electric field and on the impurity position. We also compared our results with those for the square quantum-well wire (SQWW). The results we obtained describe the behavior of impurities in both square and graded quantum wires. PACS 68.65.-k; 71.55.-i; 71.55.Eq     

Stark Effect Dependence on Hydrogenic Impurities in GaAs Parabolic Quantum-Well Wires

The ground-state and lowest excited-state binding energies of a hydrogenic impurity in GaAs parabolic quantum-well wires （Q WWs） subjected to external electric and magnetic fields are investigated using the finite-difference method within the quasi-one-dimensional effective potential model. We define an effective radius Pen of a cylindrical QWW, which can describe the strength of the lateral confinement. For the ground state, the position of the largest probability density of electron in x-y plane is located at a point, while for the lowest excited state, is located on a circularity whose radius is Pen. The point and circularity are pushed along the left haft of the center axis of the quantum-well wire by the electric field dire ted along the right half. When an impurity is located at the point or within the circularity, the ground-state or lowest excited-state binding energies are the largest; when the impurity is apart from the point or circularity, the ground-state or lowest excited-state binding energies start to decrease.     

Magnetic field dependence of the binding energy of shallow donors in GaAs quantum-well wires

Using a variational procedure within the effective-mass approximation we have calculated the binding energies of shallow-donor impurities in cylindrical GaAs quantum-well wires, in an axial magnetic field and an infinite confinement potential. In contrast to the previous results in quantum wells, we have found that, in the magnetic field, the impurity binding energy may be increased or decreased as a function of the impurity location in the quantum wire. On the basis of analysis of the variation of the binding energy with magnetic field strength, a method is proposed for experimentalists to confirm the presence of a shallow donor in the vicinity of the wire boundary.     

Relations between theT 0 values of bulk and quantum-well GaAs

A simple calculation of the threshold current densityJ _th of quantum-well and bulk GaAs lasers shows that it is proportional to the temperatureT in the first case and toT ^3/2 in the second case. As a consequence theT ₀ value of quantum-well GaAs is 3/2 times larger than for the bulk material. Moreover, we get a further doubling ofT ₀ for a one-dimensional quantum-well wire. These results are independent on the magnitude ofJ _th and hold also for other laser materials, provided that the Auger recombination is negligible as in GaAs.     

Stark Effect Dependence on Hydrogenic Impurities in GaAs ParabolicQuantum-Well Wires

The ground-state and lowest excited-state binding energies of ahydrogenic impurity in GaAs parabolic quantum-well wires (QWWs)subjected to external electric and magnetic fields are investigated using the finite-difference method within the quasi-one-dimensional effective potential model. We define an effective radius ρ_eff of a cylindrical QWW, which can describe the strength of the lateral confinement. For the ground state, theposition of the largest probability density of electron in x-y plane is located at a point, while for the lowest excited state, is located on a circularity whose radius is ρ_eff. The point and circularity are pushed along the left half of the center axis of the quantum-well wire by the electric field directed along theright half. When an impurity is located at the point or within the circularity, the ground-state or lowest excited-state binding energies are the largest; when the impurity is apart from the point or circularity, the ground-state or lowest excited-state binding energies start to decrease.     

Simultaneous effects of hydrostatic pressure and electric field on impurity binding energy and polarizability in coupled InAs/GaAs quantum wires

This work is concerned with the theoretical study of the combined effects of applied electric field and hydrostatic pressure on the binding energy and impurity polarizability of a donor impurity in laterally coupled double InAs/GaAs quantum-well wires. Calculations have been made in the effective mass and parabolic band approximations and using a variational method. The results are reported for different configurations of wire and barriers widths, impurity position, and electric field and hydrostatic pressure strengths. Our results show that for symmetrical structures the binding energy is an even function of the impurity position along the growth direction of the structure. Also, we found that for hydrostatic pressure strength up to 38 kbar, the binding energy increases linearly with hydrostatic pressure, while for larger values of hydrostatic pressure the binding energy has a non-linear behavior. Finally, we found that the hydrostatic pressure can increase the coupling between the two parallel quantum-well wires.     

Raman scattering from confined phonons in GaAs/AlGaAs quantum wires

We report on photoluminescence and Raman scattering performed at low temperature (T =  10 K) on GaAs/Al_0.3Ga_0.7As quantum-well wires with effective wire widths ofL =  100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ο_L10 =  285.6 cm⁻¹forL =  11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderleinas applied to the GaAs/Al_0.3Ga_0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques.     

Uniform Descriptions of Electron-IO Phonon Interaction in Structures of Multi-layer Coupling Low-dimensional Systems

By using the transfer matrix method, within the framework of the dielectric continuum approximation,uniform forms for the interface optical (IO) phonon modes as well as the corresponding electron-IO phonon interaction Hamiltonians in n-layer coupling low-dimensional systems (including the coupling quantum well (CQ W), coupling quantum-well wire (CQWW), and coupling quantum dot (CQD)) have been presented. Numerical calculations on the three-layer asymmetrical AIGaAs/GaAs systems are performed, and the analogous characteristics for limited frequencies of IO phonon in the three types of systems (CQW, CQWW, and CQD) when the wave-vector and the quantum number approach zero or infinity are analyzed and specified.     

有机量子阱电致发光器件

制备了普通的有机量子阱结构,并对结构进行了表征.在此基础上,制备了量子阱结构的白光电致发光器件.在分析了制作工艺对有机量子阱结构特性可能产生的影响之后,为了减少垒、阱界面互扩散效应的影响,提出了有机掺杂量子阱的概念,即垒与阱的母体是相同材料,只是在生长垒层的过程中同时掺入少量发光剂.由于掺杂剂的浓度梯度只有百分之零点几,因此,界面互扩散的影响很小,实际上我们用这种办法制备的有机量子阱器件的亮度、效率均有明显提高.在研究了阱数对器件特性的影响之后,我们发现一般情况下,两个阱是最好的.进一步研究了阱母体材料对有机量子阱器件特性的影响,结果发现,用NPB作母体比Alq作母体更好,这时器件的效率(cd/A)在45～13V工作电压范围内变化不大.     

Uniform Descriptions of Electron-IO Phonon Interaction in Structures of Multi-layer Coupling Low-dimensional Systems

By using the transfer matrix method, within the framework of the dielectric continuum approximation, uniform forms for the interface optical (IO) phonon modes as well as the corresponding electron-IO phonon interaction Hamiltonians in n-layer coupling low-dimensional systems (including the coupling quantum well (CQW), coupling quantum-well wire (CQWW), and coupling quantum dot (CQD)) have been presented. Numerical calculations on the three-layer asymmetrical AlGaAs/GaAs systems are performed, and the analogous characteristics for limited frequencies of IO phonon in the three types of systems (CQW, CQWW, and CQD) when the wave-vector and the quantum number approach zero or infinity are analyzed and specified.     

Effect of electron-electron interaction on spin relaxation of charge carriers in semiconductors

An analysis of spin dynamics is presented for semiconductor systems without inversion symmetry that exhibit spin splitting. It is shown that electron-electron interaction reduces the rate of the Dyakonov-Perel (precession) mechanism of spin relaxation both via spin mixing in the momentum space and via the Hartree-Fock exchange interaction in spin-polarized electron gas. The change in the Hartree-Fock contribution with increasing nonequilibrium spin polarization is analyzed. Theoretical predictions are compared with experimental results on spin dynamics in GaAs/AlGaAs-based quantum-well structures. The effect of electron-electron collisions is examined not only for two-dimensional electron gas in a quantum well, but also for electron gas in a bulk semiconductor and a quantum wire.     

Optimized minigaps for negative differential resistance creation in strongly δ-doped (1D) superlattices

The ‘atomic saw method’ uses the passage of dislocations in two-dimensional (2D) quantum-well superlattices to create periodic slipping layers and one-dimensional (1D) quantum wire superlattices. The effects of this space structuring of the samples on the allowed energies are analysed in the case of GaAs δ-doped superlattices. If they are sufficiently large, the various minigaps appearing in the 1D band structure could be responsible for the presence of negative differential resistance (NDR) with high critical current in these systems. The purpose is to determine the evolution of the minigaps in terms of the sample parameters and to obtain the means to determine both the 2D and 1D structural characteristics where NDR could appear.     

Combined effects of hydrostatic pressure and electric field on the donor binding energy and polarizability in laterally coupled double InAs/GaAs quantum-well wires

This work is concerned with the theoretical study of the combined effects of applied electric field and hydrostatic pressure on the binding energy and impurity polarizability of a donor impurity in laterally coupled double InAs/GaAs quantum-well wires. calculations have been made in the effective mass and parabolic band approximations and using a variational method. The results are reported for different configurations of wire and barriers widths, impurity position, and electric field and hydrostatic pressure strengths. Our results show that for symmetrical structures the binding energy is an even function of the impurity position along the growth direction of the structure. Also, we found that for hydrostatic pressure strength up to 38 kbar, the binding energy increases linearly with hydrostatic pressure, while for larger values of hydrostatic pressure the binding energy has a nonlinear behavior. Finally, we found that the hydrostatic pressure can increase the coupling between the two parallel quantum well wires.     

Quantum-well wave-function localization and the electron-phonon interaction in thin Ag nanofilms

The electron-phonon interaction in thin Ag nanofilms epitaxially grown on Cu(111) is investigated by temperature-dependent and angle-resolved photoemission from silver quantum-well states. Clear oscillations in the electron-phonon coupling parameter as a function of the silver film thickness are observed. Different from other thin film systems where quantum oscillations are related to the Fermi-level crossing of quantum-well states, we can identify a new mechanism behind these oscillations, based on the wave-function localization of the quantum-well states in the film.     

InGaAs/GaAs应变量子阱结构在1054nm激光器中的应用

采用金属有机物化学气相淀积(MOCVD)方法生长了InGaAs/GaAs应变量子阱,通过优化生长条件和采用应变缓冲层结构获得量子阱,将该量子阱结构应用于1 054 nm激光器的制备。经测试该器件具有9 mA低阈值电流和0.4 W/A较高的单面斜率效率,在驱动电流为50 mA时测得该应变量子阱光谱半宽为1.6nm,发射波长为1 054 nm。实验表明:通过优化工艺条件和采用应变缓冲层等手段,改善了应变量子阱质量,该结果应用于1 054 nm激光器的制备,取得了较好的结果。     

Mechanisms of Auger recombination in quantum wells

The main mechanisms for the Auger recombination of nonequilibrium carriers in semiconductor quantum-well heterostructures are investigated. It is shown for the first time that there are three fundamentally different Auger recombination mechanisms in quantum wells: 1) a threshold-free mechanism, 2) a quasithreshold mechanism, and 3) a threshold mechanism. The rate of the threshold-free process has a weak temperature dependence. The rate of the quasithreshold Auger process exhibits an exponential temperature dependence. However, the threshold energy depends significantly on the quantum-well width and is close to zero for narrow quantum wells. It is shown that the threshold-free and quasithreshold processes are dominant in fairly narrow quantum wells, while the quasithreshold and threshold Auger processes are dominant in wide quantum wells. The limiting transition to a three-dimensional Auger process is accomplished for a quantum-well width tending to infinity. The value of the critical quantum-well width, at which the quasithreshold and threshold Auger processes combine to form a single three-dimensional Auger recombination process, is found. Zh. éksp. Teor. Fiz. 113, 1491–1521 (April 1998)     

Specific features in reflectance and absorbance spectra of one-dimensional resonant photonic crystals

The optical spectra of resonant Bragg and quasi-Bragg quantum-well structures are studied theoretically. The existence of special frequencies in the reflectance and absorbance spectra at which the reflectance or absorbance does not depend on the number of quantum wells in a structure is explained analytically. It is shown that the reflectance and absorbance spectra of structures in which the quantum-well width substantially exceeds the exciton Bohr radius also contain special frequencies and that the smooth spectral component is determined primarily by the interaction of the light wave with the quantum-well ground-state exciton.     

Quantum-well states and spin polarization in thin Ni films on Cu(0 0 1)

The unoccupied quantum-well states in thin Ni films on Cu(0 0 1) have been studied by spin- and angle-resolved inverse photoemission. Three quantum-well features are clearly resolved with exchange splittings of up to 70 meV. As a function of the wave vector parallel to the surface, the quantum-well states follow the corresponding sp band dispersion and evolve into surface resonances upon approaching the band-gap boundary.