Semiconductor Quantum-well Structures Research Articles

Infrared and polarization anomalies in the optical spectra of modulation-doped semiconductor quantum-well structures.

We investigate the physical processes which determine the optical spectra of modulation-doped semiconductor quantum-well structures as a function of doping. We argue that even at low doping concentrations, excitons are heavily dressed by charge- and spin-density excitations of the Fermi sea, implying, already in that limit, a Stokes shift between emission and absorption. For all doping concentrations, the charge and spin polarization of the Fermi sea due to many-body effects leads to an enhancement of the symmetry breaking in quantum-well structures, which may explain polarization anomalies observed in luminescence.

Many-body effects in the absorption, gain, and luminescence spectra of semiconductor quantum-well structures.

Absorption, gain, and luminescence spectra of quasi-two-dimensional electron-hole plasmas in semiconductor quantum-well structures are calculated as functions of the plasma density and temperature. Self-energy corrections and the effects of multiple electron-hole scattering are evaluated for a statically screened Coulomb interaction. The Bethe-Salpeter equation for the electron-hole pair propagator is solved both numerically and analytically using a method developed by Noyes. The resulting spectra deviate considerably from the corresponding free-particle spectra, due to the strong pair fluctuations in two dimensions. Implications for the theory of quantum-well lasers are discussed.

Theory of transient excitonic optical nonlinearities in semiconductor quantum-well structures.

We present theoretical results for the effects of an exciton gas and an electron-hole plasma on the excitonic optical absorption in a two-dimensional semiconductor and compare these with recent experimental results on absorption saturation in single- and multiple-quantum-well structures. A simple theoretical description of the nonlinear optical properties of these microstructures is developed for the case of low-density optical excitation near and above the band edge. We argue that the effects of Coulomb screening of excitons by the plasma are relatively weak in these structures but that the consequences of phase-space filling and exchange are significant in each case. We are able to explain the recent unexpected experimental result that ``cold'' excitons are more effective than ``hot'' carriers in saturating the excitonic absorption. Good agreement with the experimental data is obtained without adjustable parameters.

Room-temperature excitonic nonlinear-optical effects in semiconductor quantum-well structures

We review the nonlinear-optical effects observed at room temperature in semiconductor quantum-well structures photoexcited near the band gap. A comprehensive discussion of optical transitions in these microstructures is given, including excitonic effects and the specific features of room-temperature exciton resonances. Experimental investigations using continuous-wave, picosecond-, and femtosecond-laser sources are presented. They show extremely efficient nonlinear processes. In the case of excitations that are long compared with the exciton-ionization time, the induced changes in absorption and refraction do not depend on the wavelength or on the duration of excitation. These changes depend only on the density of absorbed photons and are interpreted in terms of electron–hole plasma screening and band filling. In contrast, for ultrashort excitation, nonlinear processes depend critically on the excitation wavelength. The selective generation of excitons is found to produce effects larger than a plasma of the same density. This unexpected result is shown to arise from the low temperature of the exciton gas before it interacts with the lattice and from the decrease of screening that is the reduced dimensionality of quantum-well structures.

Observation of resonant impurity states in semiconductor quantum-well structures.

Resonant Raman scattering experiments on GaAs(Si doped)-${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum-well structures show transitions involving the ground state of the donors and narrow resonant donor states derived from higher conduction subbands. These new impurity-related features, which occur at slightly higher energies than the associated conduction intersubband excitations, have been studied as a function of power density, temperature, and well width.

Raman scattering studies of donor transitions in semiconductor quantum-well structures

Raman scattering from bound electrons has been observed in Si‐doped GaAs–A1xGa1−xAs multiple heterostructures. The donor‐related features show broadening due to the dependence of the donor spectrum on the position of the impurity in the well. Transitions from 1s to 2s states and bound excitations associated with intersubband transitions were identified. The latter observation confirms the existence of resonant impurity states in quantum‐well structures.

Shallow-impurity states in semiconductor quantum-well structures.

The ground-state energy of a shallow impurity in a semiconductor quantum well of finite barrier height is calculated using a variational method. The binding energy U is obtained as a function of the well width L, the impurity position ${z}_{i}$, and the height of the potential barrier ${V}_{0}$. The behavior of the variational parameter \ensuremath{\lambda}, which determines the extension of the wave function around the impurity, is also studied. The maximum binding energy corresponds to the minimum value of \ensuremath{\lambda}. Larger values of ${V}_{0}$ result in faster variation of U with ${z}_{i}$. Our results agree very well with published experimental data.

Electric-field-induced refractive index variation in quantum-well structure

The refractive index variation in a quantum-well structure by an electric field is given theoretically. The calculated variation is −1% for an applied field of 3.1×105 V/cm in a 300 Å-thick GaInAsP/InP single quantum well, which is about 39 times larger than the bulk value. A semiconductor quantum-well structure is found theoretically to be a new material with a larger electro-optic coefficient. Application to a new optical switching device is also suggested.

Electron-phonon scattering in the presence of a magnetic field in quasi-two-dimensional quantum-well structures

We have obtained the expressions for the relaxation times due to electron-phonon scattering in the presence of a magnetic field in quasi-two-dimensional semiconductor quantum-well structures. For the electron-acoustic phonon scattering we find that the relaxation rate at low temperature is (i) directly proportional to the density of states in a magnetic field (magnetic field in the $z$ direction) in the $x\ensuremath{-}y$ plane, and (ii) inversely proportional to the thickness of the well in the $z$ direction. For the nonpolar optical phonons similar behavior of the relaxation rate is obtained. In the case of the polar optical-phonon scattering the relaxation rate diverges due to complete quantization of the electronic states in a magnetic field in contrast to finite results in the absence of a magnetic field. Furthermore, in the limiting case of $B\ensuremath{\rightarrow}0$, our results reduce to those in the absence of a magnetic field. Explicit calculations are presented for the variations of the relaxation times as a function of temperature and magnetic field for the acoustic and nonpolar optical-phonon scattering.

Two-dimensional hot-electron transport in quantum-well structures of polar semiconductors

The drift velocity of hot electrons itinerant two dimensionally in a semiconductor quantum-well structure is calculated at different lattice temperatures on a displaced Maxwellian model. Interactions with deformation potential acoustic and polar optic phonons are considered using the analytic expressions obtained on the basis of a momentum conservation approximation. Contrary to a previous prediction, no negative differential resistance is obtained.

The electron-phonon interaction in quasi-two-dimensional semiconductor quantum-well structures

Approximate analytic expressions are obtained for the scattering rates and momentum-relaxation rates of an electron in a quasi-two-dimensional quantum well interacting with acoustic, optical and intervalley modes via the deformation potential and with longitudinal optical modes via the polar interaction. These analytic expressions are obtained using a momentum-conservation approximation. The threshold for optical phonon emission, unlike the case in the bulk, is abrupt. All scattering rates are energy-independent and are inversely proportional to L, the thickness of the well. The momentum-relaxation rate associated with the absorption of polar optical phonons, on the other hand, proves to be proportional to L. These properties are shown to lead to a negative differential resistance for pure polar mode scattering, and to the existence of a runaway field for deformation-potential scattering. The self-energy associated with the emission of polar optical phonons at absolute zero is shown to be divergent unless the polar interaction is screened, and some consequences of this for laser and other optical processes are pointed out. The description of scattering by perturbation theory breaks down in very narrow wells.