Exciton spectra of semiconductor superlattices in a parallel magnetic field

Low-temperature photoluminescence and photoluminescence excitation spectra of GaAs/AlGaAs semiconductor superlattices having different potential barrier widths (b=20, 30, 50, and 200 Å), i.e., degrees of tunnel coupling between quantum wells, are studied in magnetic fields up to 5 T oriented parallel and perpendicular to the layers of the structure. The changes in the qualitative character of the photoluminescence excitation spectra observed in a parallel magnetic field with increasing tunnel transparency of the barrier correspond to a transition from a quasi-two-dimensional to a quasi-three-dimensional electronic spectrum as a miniband develops in the superlattice. In the photoluminescence excitation spectra of the superlattice with b=50 Å, as the parallel magnetic field is increased, a new line appears in the violet wing of the spatially indirect exciton excitation line, which is absent in a perpendicular field. A similar line was also observed to arise in the photoluminescence spectra. It is shown that the indirect exciton luminescence line can be suppressed by both parallel and perpendicular magnetic fields.     

Dynamics of electromagnetic pulses with wide spectra in semiconductor superlattices

We considered the Maxwell equations for electromagnetic-field propagation in a solid with a one-dimensional semiconductor superlattice of quantum dots in the case where the spectral width of the electromagnetic pulse is sufficient to excite transitions between different minizones. A phenomenological equation was obtained in the form of the classical one-dimensional sine-Gordon equation with the perturbation caused by quantum transitions between the minizones. Quantum behavior of electrons was considered using the microscopic Hamiltonian, in the assumption that the pulse duration is small enough for the phonon effects to be neglected. The equation obtained was analyzed numerically, and cases where the adiabatic perturbation theory for the sine-Gordon equation can be used were found. Numerical solutions were obtained, and the domain where transitions between the minizones play a significant role in the electromagnetic-pulse dynamics was found. Talk presented at the oral issue of J. Russ. Laser Res. dedicated to the memory of Professor Vladimir A. Isakov, Professor Alexander S. Shumovsky, and Professor Andrei V. Vinogradov held in Moscow February 21–22, 2008.     

Line shape in collision-induced absorption spectra

A new and improved method is presented for computer simulation of collision-induced far-infrared absorption spectra of atomic and molecular species. The improvements involve (a) eliminating redundancy in the statistical sampling of the molecular trajectories, and (b) transforming the integrals for the orbital variables so that the singularities of the integrands at the turning points are eliminated. An asymptotic theory of the line shape developed earlier for atomic spectra is extended to molecular spectra. Results are given for the atomic species, due to overlap induction, at the reduced temperatures T* = 1, 2, 4, 5, 10, 15, and for the molecular species, due to quadrupolar, octopolar and hexadecapolar induction, at T* = 1, 2, 4. The agreement between the simulated and theoretical spectra is excellent.     

Interband critical-point symmetry from electroreflectance spectra

The theoretical basis for electroreflectance symmetry analysis is reviewed, with emphasis on its application to common diamond, zinc blende, rock salt, and wurtzite type materials. Both longitudinal and transverse geometries are considered. The use of atomic-state selection rules in determining electroreflectance polarization dependences is reviewed. Symmetry analysis of excitonic transitions and transitions involving degenerate states is discussed, along with the importance of low-field spectra, and the roles of thermal and lifetime broadening. The phenomenological theory of linear and quadratic low-field electroreflectance is presented for both longitudinal and transverse geometries, and the expected polarization dependences of these are analyzed in detail. The relationship of tensorial polarization dependence to symmetry analysis is also examined along with the measurement of individual tensor components. Experimental considerations such as photocarriers and surface barriers are reviewed, as are techniques for separately observing linear and quadratic low-field spectra. The utility of linear low-field spectra in symmetry analysis of wurtzite type materials is pointed out.     

Self-oscillations in semiconductor superlattices

An investigation is made of nonlinear oscillations of the field and current in semiconductor superlattices driven by strong terahertz radiation. Regimes of periodic, quasi-periodic, and stochastic self-oscillations are determined and mechanisms for their formation are discussed. It is shown that the self-oscillation spectra are many-valued functions of the external field amplitude and the static field in them is either absent, weak, or fractionally quantized. Previously predicted states of self-induced superlattice transparency and dynamic electron localization are destroyed as a result of the evolution of dissipative and parametric instabilities and can only be observed in transient processes whose duration decreases with increasing electron concentration.     

Amorphous semiconductor superlattices

The structural and electronic properties of a new class of superlattices consisting of layers of a-Si:H 8–1200 thick interleaved with a-Ge:H, a-Si_1?xC_x:H or a-SiN_x:H are reviewed.     

Line shape of phonon-broadened spin-12 absorption spectra

A scheme for factorizing spin correlation functions in the time-domain is applied to the calculation of dynamic susceptibilities of a linearly coupled spin-lattice system.     

Helicon waves in semiconductor superlattices

The dispersion relation for the helicon frequency of a system consisting of a periodic array of two-dimensional gas layers is studied as a function of both q and q_z, the components of the wavevector parallel and perpendicular to the layers. The result is compared with that for a homogeneous three-dimensional electron gas.     

Hopping mobility in semiconductor superlattices

We present results on the electrical transport perpendicular to interfaces in GaAs/Al_xGa_1?xAs superlattices. We have measured the current-voltage characteristics on a series of superlattices. This has been simulated numerically, the superlattice being replaced by an effective medium. Using this model we obtain the values of the effective mobility as a function of the superlattice period. Our data are in good agreement with a theory of phonon-assisted hopping transport between localized states, rather than the theory of phonon-limited band transport of Bloch waves.     

Collective excitations in semiconductor superlattices

Collective excitations in semiconductor superlattices are studied beyond tbe random-phase approximation (RPA). The Singwi, Tosi, Land and Sjölander (STLS) theory, which accounts for exchange and short-range correlations effects through an effective potential depending on the structurc factor, is generalized to the layered electron system described by the model of Visscher and Falicov. The exact numerical solution of the STLS self-consistent equations provides information about intraplane and interplane correlations. The plasmon dispersion curves are evaluated for some typical values of the coupling constant r_s of the electron system and the distance between the planes for GaAs/AlGaAs semiconductor superlattices. For comparison, the RPA and the Hubbard approximation are also considered.     

Collective modes in semiconductor superlattices

Electronic collective modes in a semiconductor superlattice structure are studied within the self-consistent field approach. Plasmon and magneto-plasmon dispersion relations are obtained for the cases of strong and weak coupling between layers. The interaction of these collective modes with optical phonons is also investigated.     

Polariton modes in semiconductor superlattices

Polariton electric fields and dispersion relations of some important complex-basis superlattices have been derived by means of electromagnetic theory and the Bloch's theorem. The spatial distribution of the polariton electric fields shows an interesting physical picture: the polaritons are mainly bulk modes as the wavenumber k is small and become typical interface modes with very strong peak intensities as k increases.     

Fano resonances in semiconductor superlattices

We use semiconductor superlattices as a model system for the investigation of Fano resonances. In absorption the excitonic transitions of the Wannier–Stark ladder show the typical asymmetric line shape due to coupling to the continuum of lower-lying transitions. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter – the coupling strength Γ between the discrete state and the degenerate continuum – by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare with an extensive theoretical model of the resonances.     

Forcing of chaos in semiconductor superlattices

The chaotic behavior of high-field transport in weakly-coupled narrow-miniband GaAs/AlAs superlattices (SL) under ac + dc biases has been numerically studied within a self-consistent discrete model. It is shown that the regions of entrainment and quasiperiodicity form the Arnol'd tongues on the driving frequency–driving amplitude parameter plane. Chaos is demonstrated to appear at the boundaries of the tongues and in the regions where they overlap. Numerical simulation shows that ac driving can lead to chaos for different regimes: (i) when the electric-field domains in the SL are stable; and (ii) when they are unstable (periodically oscillating).     

Band mixing effect in semiconductor superlattices

The band mixing effect on the electronic and optical properties of semiconductor superlattices is studied within the framework of the empirical tight-binding model. It is found that the superlattice periodic potentials mix the bulk heavy hole, light hole and spin-orbit-split bands in the valence band states. As a consequence, the optical matrix elements associated with various valence-to-conduction subband transitions are very sensitive to the variation of the wavevector in directions parallel to the interface ( _t). We find that band mixing in conjunction with the exciton effect can account for the Δn≠0 forbidden transitions observed in several recent experiments.     

Phonon polariton modes in semiconductor superlattices

Phonon polariton modes in semiconductor superlattices are studied. Polariton electric fields and the dispersion relation are derived by electromagnetic theory, and due to periodicity in the direction normal to the superlattice layers, Bloch's theorem is applied. Polariton modes are found to exist between the TO and LO phonon frequencies, and approach the surface polariton frequency in the limit of large tangential wave vectors. The frequencies are also strongly dependent on the ratio of the layer thicknesses. Results are illustrated by a GaAsGaP superlattice.