Free-carrier absorption in a parabolic quantum well with consideration of scattering on ionized impurities

Intra-subband transitions caused by light absorption in a parabolic quantum well is considered taking into account the scattering by ionized impurity centers. To calculate the scattering matrix element, the Born approximation is used and the interaction with the impurity is described by the Coulomb potential. An analytical expression for the absorption coefficient of processes with the initial absorption of photon and further scattering by an ionized impurity center is obtained. For absorption coefficient the frequency characteristics and dependence on the width of quantum well are examined.     

Infra-red free-carrier absorption due to impurity scattering in semiconducting quantum well structures

The theory of free-carrier absorption (FCA) is developed, in the extreme quantum limit when the carriers are assumed to populate only the lowest quantized energy level, for quasi-two and one-dimensional semiconducting quantum well structures where the carriers are scattered by ionized impurities. The radiation field is assumed to be polarized in the plane of the layer in the quasi-two-dimensional case and along the length of the wire in the quasi-one-dimensional case. Expressions for FCA are obtained for the cases where the impurities are either in the well (background impurities) or outside the well (remote impurities). Variation of FCA is numerically studied with photon frequency and well width.     

Interface phonon assisted transition in double quantum well

A detailed calculation of interface phonon assisted electron intersubband transition in double GaAs/AlGaAs quantum well structure is presented. Our calculation concentrates on the lowest two subbands which can be designed to be in resonance with a given interface phonon mode. Various phonon mode profiles display quasi-symmetric or quasi-antisymmetric shapes. The quasi-antisymmetric phonon modes give rise to much larger transition rates than those assisted by quasi-symmetric ones. The transition rate reaches a maximum when the subband separation coincides with a given phonon mode energy. The calculation procedure presented here can be easily applied to the design and simulation of other low dimensional semiconductor structures, such as quantum cascade lasers. Received 22 December 2002 Published online 23 May 2003 RID="a" ID="a"e-mail: bhwu@263.net     

One phonon resonant Raman scattering in free-standing quantum wires

The scattering intensity (SI) of a free-standing cylindrical semiconductor quantum wire for an electron resonant Raman scattering (ERRS) process associated with bulk longitudinal optical (LO) phonon modes and surface optical (SO) phonon modes is calculated separately for T=0 K$T=0\text{K}$. The Fröhlich interaction is considered to illustrate the theory for GaAs and CdS systems. Electron states are confined within a free-standing quantum wire (FSW). Single parabolic conduction and valence bands are assumed. The selection rules are studied. Numerical results and a discussion are also presented for various radii of the cylindrical.     

Exciton linewidth due to scattering from free carriers in semiconducting quantum well structures

The contribution to the exciton linewidth in semiconducting quantum well structures due to the scattering of excitons by free carriers is calculated. It is found that this contribution becomes very important in limiting the exciton linewidth when a high density of free carriers is present or at low temperatures where the scattering of the excitons by optical and acoustic phonons is reduced. This contribution to the linewidth in quantum well structures is found to increase with the free carrier concentration and to extremely broaden and exciton peak at high carrier concentrations. At lower carrier concentrations, where the carriers behave as a nondegenerate gas of particles, the contribution to the exciton linewidth due to scattering by free carriers increases with temperature.     

Bound polaron in quantum dot quantum well structures

The problem of bound polarons in quantum dot quantum well (QDQW) structures is studied theoretically. The eigenfrequencies of bulk longitudinal optical (LO) and surface optical (SO) modes are derived in the framework of the dielectric continuum approximation. The electron--phonon interaction Hamiltonian for QDQW structures is obtained and the exchange interaction between impurity and LO-phonons is discussed. The binding energy and the trapping energy of the bound polaron in CdS/HgS QDQW structures are calculated. The numerical results reveal that there exist three branches of eigenfrequencies of surface optical vibration in the CdS/HgS QDQW structure. It is also shown that the binding energy and the trapping energy increase as the inner radius of the QDQW structure decreases, with the outer radius fixed, and the trapping energy takes a major part of the binding energy when the inner radius is very small.     

Interference effects in phonon scattering across a double atomic well

We develop an analytical and numerical formalism to study the double atomic nanowell configuration influence on coherent transmission and reflection scattering, derived as elements of a Landauer-Büttiker type scattering matrix in quasi-one-dimensional multichannel waveguides. The state densities at neighbourhood of the defect region are investigated. The defect region consists in the presence of a double atomic nanowell occurring in thin film. One solves this problem by the matching method in the harmonic approximation. The theoretical formalism using simultaneously the Green’s function and the matching method is detailed in order to describe the complete evanescent and the propagating fields. The phononic coherent conductance is also studied. Numerical calculations are presented and illustrated as function of the force constants occurring in the model. The fluctuations in the conductance spectra are related to Fano resonances due to the coherent coupling between travelling phonons and the localized vibration modes in the neighbourhood of the nanowell domain.     

Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.     

无限深量子阱量子比特及其声子效应

采用Peaker变分法,研究无限深量子阱中量子比特及其声子效应。量子阱中这样的二能级体系可作为一个量子比特。当阱中电子处于基态和第一激发态的叠加态时,电子的概率密度在空间作周期性震荡,得出了振荡周期随耦合强度的增加而减小,随振动频率的增加而增大。     

无限深量子阱量子比特及其声子效应

采用Peaker变分法,研究具有束缚势的无限深量子阱中量子比特及其声子效应。量子阱中这样的二能级体系可作为一个量子比特。当阱中电子处于基态和第一激发态的叠加态时,电子的概率密度在空间作周期性震荡,得出了振荡周期随耦合强度的增加而减小,随振动频率的增加而增大。     

Luminescence rings in quantum well structures

We review the results of an extensive study of the novel luminescence rings found in GaAs and InGaAs double quantum well structures. We propose that the rings define the edge of a metastable population of carriers between the laser excitation spot and the ring. This population carries wavelike excitations at supersonic speeds. Evidence for and against associating this effect with excitonic superfluidity is reviewed. The effect cannot be easily understood in terms of a Kosterlitz-Thouless transition to a superfluid state. We examine the effects of free carriers in the system and suggest a possible mechanism for the luminescence ring effect.     

Dephasing of excitons in quantum well structures studied by picosecond time-resolved resonant Rayleigh scattering

Electronic states in solids with disorder give rise to an elastic (Rayleigh) contribution to the scattering spectrum which becomes resonantly enhanced for excitation in the electronic transition. It is shown theoretically that from this resonant Rayleigh process, if temporally resolved, the coherence time of the electronic states may be deduced. Experimentally this is demonstrated for the first time by studying the n = 1 heavy-hole exciton in GaAs/AlGaAs quantum well structures. Employing picosecond time-resolved spectroscopy and analyzing the data within the developed theory, coherence times are found between 5 and 30 ps in agreement with earlier results obtained by non-linear optical techniques.     

The energy-loss rate via polar-optical phonon scattering in quantum wires

The hot-electron energy-loss rate (ELR) conditioned by confined and interface polar-optical (PO) phonons for a quasi-one-dimensional cylindrical quantum wire embedded in a dielectric medium is investigated analytically. It is shown that the inclusion of the PO-phonon confinement effects is crucial for accurate calculation of the ELR in quantum wire. Taking into account the nonequilibrium phonon populations, the hot-electron ELR is derived by a model, which includes the lowest subband occupation and the phonon confinement effects. The contribution of intersubband transitions to electron ELR for the GaAs quantum wire embedded in Al_xGa_1−xAs is estimated. The extrema on the ELR dependences on electron density are obtained.     

Impurity optical absorption in parabolic quantum well

Optical absorption in GaAs parabolic quantum well in the presence of hydrogenic impurity is considered. The absorption coefficient associated with the transitions between the upper valence subband and donor ground state is calculated. The impurity ground state wave function and energy are obtained using the variational method. Dependence of the absorption spectra on impurity position in quantum well was investigated. It is shown, that along with quantum well width decrease the absorption threshold shifts to higher frequencies. Results obtained within frames of parabolic approximation are compared with results for rectangular infinite-barrier quantum well case. The acceptor state → conduction band transitions considered as well.     

Two-electron absorption in a biased quantum well

Linear light absorption of 2D electrons confined within a biased quantum well is studied theoretically. We demonstrate that for light polarization perpendicular to the 2D plane, in addition to conventional absorption peak at frequency ω≈Δ, where Δ is the intersubband energy distance, there exists a peak around a double frequency ω≈2Δ. This additional peak is entirely due to electron–electron interactions, and corresponds to excitation of two electrons by one photon. The magnitude of two-electron absorption is proportional to U², where U is the applied bias.     

Optical absorption coefficients in asymmetric quantum well

The optical absorption coefficients considering electron-phonon interaction in asymmetrical quantum wells are theoretically studied. The result shows that the optical absorption coefficients depend strongly on the parameters of quantum well. Interestingly, the theoretical values of the optical absorption coefficients obviously increase with considering the electron-phonon interaction.     

Free-carrier multiphoton laser absorption in magnetic semiconductors

The magnon scattering by free carriers in the presence of an intense laser field is discussed. A kinetic equation for electrons is derived from which the multiphonon absorption coefficient is calculated. It is found that for very large fields the absorption coefficient is vanishing small. A physical explanation of this result is also available.     

Bragg suppression for optical absorption in multi-quantum well structures

Direct calculations of absorption spectra for multi-quantum well structures by extracting field distributions at well positions are performed. Results demonstrate the previously reported Bragg suppression, and agree exactly with the indirect calculation by linear dispersion theory. This reveals that Bragg suppression effect in fact originates from the remarkably decreased intensities at well positions by Bragg interference condition, rather than from the formation of supperradiant modes.     

Interface phonons in asymmetric quantum well structures

In semiconductor microstructures with many layers, the phonon modes change from their bulk form and split into ‘confined LO phonons’ (LC) and ‘interface phonons’ (IF), the number and variety of which depends on both the number of layers and the number of different materials in the structure. This affects the electron–phonon scattering rates. Because of the current interest in inter-subband THz emitters, we use these LC and IF modes to evaluate the inter-subband electron–phonon scattering rate in THz emitter prototypes that are based on four-subband stepped quantum wells. These scattering rates in turn affect the population inversion predicted for these devices, so we compare the predicted population inversions for the most promising prototypes against those obtained using bulk phonon scattering rates.