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A model of the Elliot process for spin relaxation is developed that explicitly incorporates the Dresselhaus spin-splitting of the conduction band in semiconductors lacking an inversion symmetry. It is found that this model reduces to existing models in bulk if the scattering matrices are constructed from a superposition of eigenstates. It is shown that the amplitude for intra-sub-band spin relaxation disappears in quantum wells on the basis of existing models. However, an amplitude due to the Dresselhaus spin-splitting remains, becoming increasingly important as the well becomes narrower. It is also shown that this component does not disappear for scattering between spin states at the same wavevector. It is concluded that for quantum wells and lower dimensional semiconductors that this modified model should be used in analysis of the spin dynamics.

This letter presents the first calculation of the exciton binding energy (Ex) and oscillator strength (OS) for quantum dots in type II semiconductor systems. These structures consist of a spherical dot of one semiconductor embedded in a second semiconductor. As in the type II exciton systems in quantum wells, the electron is confined in one semiconductor and the hole in the other due to band line-ups in the two materials which make this arrangement energetically favourable. The author uses a variational calculation where the wavefunction of the particle confined to the dot is a spherical Bessel function and the wavefunction of the other particle is a generalized Fang-Howard wavefunction allowing correlation with the confined particle. He has considered the GaAs/AlAs system where the electron is confined in the X state in the AlAs while the hole is confined in the GaAs dot which is indirect in both real and k space for a dot radius of less than 56 AA.

We calculate the finite-temperature inelastic scattering rate for hot electrons injected parallel to an n-doped GaAs/GaAlAs interface or quantum well for various doping densities n, temperatures T and injection energies. We use the Born approximation and a T-dependent two-pole Pade approximation to the full finite T, frequency omega and wavevector q dependent RPA expression for the dielectric function epsilon (q, omega ,T), which contains the screening of the 2D electron gas and the 3D polar LO phonons treated equally. We calculate the mean free path length and average energy loss for a hot electron scattering from the plasmon-coupled LO phonon and single-particle excitation modes. We find that these quantities are very dependent upon T, n and the energy of the hot carrier. We also find that the 2D plasmon and polar LO phonons mutually screen each other so that the modes must be considered together. We will show that this high-T, low-n approximation, which incorporates Maxwell-Boltzmann statistics, is valid for GaAs systems at n

We investigate the reflection spectra of distributed Bragg reflectors (DBRs) and DBR cavities with and without photonic crystal holes fabricated within them. A finite-difference time domain (FDTD) electromagnetic model which is considered to provide the exact solution of Maxwell equations is used as a reference model. Two simplified modelling approaches are compared to the FDTD results: an effective index model where the individual DBR constituent layers penetrated by holes possess an effective index and a spatial loss model where optical losses are introduced spatially where the holes are fabricated. Results of the FDTD and the spatial loss model show that optical loss determines the properties of an etched DBR and DBR cavity when the lattice constant of the holes of exceeds 1 μm and the hole depth is small. The spatial loss model compares well to the FDTD results for holes with a lattice period exceeding 1 μm. We also consider the realistic effect of angling the sides of the etched holes.

Existing problems in the development of vertical-cavity surface-emitting lasers with photonic crystals (PC-VCSELs) for transverse mode control are reviewed. The theoretical background of VCSELs with PCs incorporated within their top distributed Bragg reflector (DBR) (type I PC-VCSELs) and VCSELs with PCs incorporated within their cavity (type II PC-VCSELs) is introduced. Their fabrication using focused ion-beam (FIB) etching is discussed. It is shown that type I single-mode PC-VCSELs suffer from low optical power and poor small-signal modulation performance due to the reduction of the reflectivity of the top DBR introduced by the PC. The problem is expected to be reduced in type II PC-VCSELs that are being investigated now. One approach to the fabrication of type II PC-VCSELs is described.

We present calculations for the band structure of bulk and confined quantum well and quantum wire GaInNAs structures. To treat this non-randomly alloyed material system we follow previous approaches in using an Anderson impurity model where the nitrogen localized states interact with the GaInAs conduction band states. We solve this model using Matsubara Green's functions and the associated self-energies which produce a complex band structure where both the real and imaginary components depend on the concentration of nitrogen. In particular this approach gives a definite nitrogen dependent lifetime broadening and is different from previous work in that no artificial input parameters are used. The density of states of the conduction band, derived from these functions, is strongly altered by interaction with the nitrogen states. The density of states is required for further optical and transport investigations involving this system.

The effect of quantum-well intermixing on the material gain of a GaInNAs/GaAs quantum-well laser is investigated theoretically. The diffusion of gallium and indium atoms in the intermixed sample is assumed and their compositional profiles are modelled using Fick's law. The band-anti-crossing model is used to calculate the band structure of the GaInNAs quantum well, which is appropriate for this non-randomly-alloyed material system. The calculated results show good agreement with the observed photoluminescence excitation for both non-intermixed and intermixed samples, which confirms this model. It is found that the strain gradient, the variation of material band gap and the degeneracy between heavy and light holes are the main factors determining the quantized energy levels of the intermixed quantum well. With the increase of diffusion length, the material gain and differential gain decrease due to the increase of the conduction band effective mass and the rapid decrease of the dipole moments. These characteristics of quantum-well intermixing effects will be useful in the design of integrated photonic devices based on this material.

Changes in the optical properties in GaInNAs/GaAs quantum wells after alpha particle bombardment followed by low temperature annealing are reported. Both blue and red shifts of the lasing wavelength are observed under different annealing conditions. This differs from the usually observed blue shift which is found after high-temperature post-grown annealing. Competing processes that result in the lasing wavelength shifts are energetic considerations which act to increase the number of Ga?N and In?As bonds (maximize the cohesive energy), minimizing the strain of the system which increases the number of In?N and Ga?As bonds (large-ion-small-ion links), maximizing the number of N located at lattice sites effective at shrinking the band-gap and moving the N position within the quantum well. For the case of high-temperature post-grown annealing the increase of Ga?As and In?N bonding wins, resulting in the blue shift observed. The wavelength shifts are discussed in terms of these competing mechanisms.

A general group-theoretical method has been derived for the evaluation of Gamma * epsilon Jahn-Teller matrix elements for point- and double-point-group symmetries. In this notation Gamma corresponds to the irreducible representation labelling the degenerate electronic state and epsilon is the doubly degenerate representation labelling the vibrations in the point group of the physical system. This method has been explicitly carried out to construct a general equation of Gamma * epsilon Jahn-Teller matrix elements for site symmetries that contain the factor group C3v such as O, O*, Td, Th, including the Jahn-Teller systems E* epsilon , T* epsilon and Gamma 8* epsilon . The method is shown to be straightforward to apply to other point groups as well, involving the multiplication of three terms, a phase factor, a reduced matrix element and a 6j symbol, all of which can be tabulated. The form of the matrix elements and symmetrised electron phonon bases are very simple and this allows one to calculate matrix elements involving high phonon numbers by following a recipe to combine the three tabulated terms. Another advantage of this method is that it can be extended to include spin-orbit, higher-order and anharmonic terms within the same framework.

In modulation-doped quantum wells (QWS) electrons are introduced into the QW by doping only the barrier region with an undoped spacer layer nearest the quantum well. The electrons flow into the QW forming a two-dimensional (2D) gas which is separated from the ionised donors by the spacer layer. The authors have developed a theoretical model to determine the size and scale of potential fluctuations 2 eta created in the 2D electron gas in the QW by the random distribution of ionised donors modelled as a 2D sheet. The interaction is through a 2D statistically screened coulomb interaction. The size of the fluctuations grow as square root Ns where Ns is the two-dimensional density of ionised donors which compares with a 2D level Fermi energy EFE which grows as n2D, where n2D is the two-dimensional density of electronics. They show that for EFE< eta all electrons are localised at low temperature, while for EFE< eta there are both localised and free electrons. This behaviour manifests itself in experimental results of mobility measurements using persistent photoconductivity which exhibit a mobility edge. Photoluminescence results where the oscillator strength for the localised electrons is suppressed compared with free electrons recombining with the few photo-excited localised holes also support this theory since the fluctuations separate the localised electrons and holes in real space.

In the impurity system V2+:MgO, a d3 ion replaces a Mg2+ ion at an octahedral site. The resulting Zeeman spectra have been a puzzle for many years even with techniques employing high magnetic fields and low temperatures. In this paper, the system is treated as a Jahn-Teller 4T2g*( epsilon 1+ epsilon 2) system. In this treatment the triply degenerate electronic state designated by the group theoretical irreducible representation T couples to a spin state of J=3/2 to form spin-orbit states which couple through the electron-phonon interaction to doubly degenerate distortions of the lattice denoted epsilon which break the electronic degeneracy. While the electronic states can couple to the entire phonon spectrum of epsilon -distortions, the cluster approximation is usually employed which approximates by coupling to one suitably chosen frequency mode. In 4T2g*( epsilon 1+ epsilon 2) coupling is taken to two different modes epsilon 1 and epsilon 2 whose frequencies and coupling strengths are varied to probe the phonon density of states. This multimode treatment is found to yield better agreement with experimental results due to the sensitivity of the intensity quenching to a weakly coupled low-frequency mode.

The authors use a temperature-dependent Green function approach within the random-phase approximation to evaluate the imaginary self-energy and hence to derive a path length due to electron-electron interaction. This total path length includes plasmon and single-particle absorption processes (energy gain) as well as plasmon emission, and single-particle excitation processes (energy loss). They can separate this total path length into one due to effective energy loss (energy loss-energy gain), which is relevant to the working of hot-electron transistors, and one due to the individual processes, which can be compared with the commonly used expression for plasmons due to Ferrell (1956, 1957) and the Born approximation for single-particle behaviour. They find that the Bohm and Pines and Ferrell approach of introducing a wave-vector cut-off to separate collective and single-particle behaviour breaks down as the density is lowered and therefore these commonly used expressions break down. They derive shorter path lengths for plasmon emission and longer path lengths for single-particle excitation for low energies of the incident electrons and low doping. They also find that for low doping and low energies of the incident electrons the majority of the contribution for the single particle as well as the plasmon comes from q

Well resolved structure in the neutral donor bound exciton (D0X) recombination in InP, observed at high magnetic fields and low temperatures, is presented. This structure is explained using a model of a donor bound exciton in a cubic semiconductor in a magnetic field in which p-like zero-field excited states are forced to lower energy than the s-like states which form the degenerate zero-field ground states. Account is taken of the degeneracy and anisotropy of the valence bands and diamagnetic effects in second-order perturbation theory. The selection rules for the electric dipole transitions for magnetic fields along the (100), (111) and (110) directions are found to be in good overall agreement with the observed donor bound exciton spectra in InP at magnetic fields of 9.7 T. High-magnetic field transitions of the (D0X) complex to both the 1s neutral donor final state, and to the 2s, 2p-1, 2p0 and 2p-1 excited states (the 'two-electron' transitions) are analysed.

Recently observed luminescence spectra in the alloy semiconductor quantum-well system InGaAs/InP show an enhancement of the electron-hole (e-h) recombination oscillator strength at the electron Fermi energy (EFe). This enhancement is explained, using a generalized e-h pair propagator and static screening, as due to multiple e-h scattering and recombination of the electrons at EFe with the small number of optically excited holes which have wave-vector components of the order of kFe (the electron Fermi wave-vector) due to localisation effects. The localisation is taken to rise from alloy disorder.

Recent experiments by Levi et al. (1985) and Long et al. (1986) in n-doped GaAs have produced path lengths in the range 300-500 AA at 77K for hot-electron decay. The scattering has been attributed to electron-electron and electron-phonon interactions. The authors have made a detailed theoretical study of electron-electron and electron-phonon effects in GaAs in the doping range n=1017-1018 cm-3 where polar LO phonon and plasmon mode mixing effects are expected to be largest. In a previous study on electron-electron interaction where the authors neglected plasmon-LO-phonon mode mixing, they have shown that plasmon dispersion effects are important in the calculation of path lengths for hot-electron decay. Thus altered dispersion in the initial unmixed modes, mode dispersion in the final modes due to mode-mixing and interaction effects and dynamical screening by the electrons and the phonons will influence the damping of hot electrons in these structures. They use a temperature-dependent Green function approach within the RPA approximation to include lattice polarisation and electron-electron effects and evaluate a wave-vector and frequency-dependent screening function epsilon (q, omega ) which they can then use to evaluate the imaginary part of the electron self-energy and hence a path length due to combined electron-electron and electron-LO-phonon interaction.