Intersubbands carrier dynamics in quantum wire structures

Monte Carlo simulation of real and reciprocal space transfer of electrons in a pseudomorphically strained GaAs/In 0.15Ga 0.85As/GaAs quantum well

Intersubband optical transitions in one dimensional quantum wire structures

Ensemble Monte Carlo simulation of real space transfer (NERFET/CHINT) devices

A unified macroscopic continuum theory for the treatment of optical-phonon modes in quantum-wire structures is established. The theory is based on a Lagrangian formalism from which the equations of motion are rigorously derived. They consist of four coupled second-order differential equations for the vibrational amplitude and electrostatic potential. The matching boundary conditions are obtained from the fundamental equations. It is shown that no incompatibility exists between mechanical and electrostatic matching boundary conditions when a correct mathematical treatment of the problem is given. The particular case of a GaAs quantum wire buried in AlAs, where the phonons can be considered completely confined, is analyzed and the vector displacement and electron-phonon interaction potential are illustrated for several modes.

The interaction between holes confined in a strained Si1−xGex quantum well and acoustic and optical phonons is investigated in an effort to elucidate in-plane carrier transport. This treatment utilizes conventional deformation potential theory within the context of the momentum conservation approximation in the confinement direction to obtain analytical expressions for both acoustic and optical phonon scattering rates as functions of carrier energy. The optical phonon interaction explicitly accounts for the three vibrational modes: Si-Si, Si-Ge, and Ge-Ge by incorporating experimentally determined longitudinal optical (LO) phonon frequency shifts in the calculation of the matrix elements. The oscillator strengths for each of these LO (short wavelength) modes are approximated using a binomial distribution to describe the local atomic arrangement of a unit cell. The two-dimensional scattering rates are evaluated and compared with bulk scattering calculations for a well of fixed width and varying Ge content. It is found that the overall hole-phonon scattering rate in a SiGe well is higher than that of holes in strained SiGe layers due to the combined effects of similar two- and three-dimensional density of states and large inter-sub-band scattering. A qualitative description of high field transport within the well is obtained by calculating the relative contributions of particular optical phonon modes to the overall scattering rate at high carrier energies.

In this work, we have fabricated delta doped quantum wire structures with sharp tips and smooth side walls. Si delta doped InGaAs/GaAs quantum wire structures were grown on SiO/sub 2/ masked GaAs substrate by low pressure metalorganic chemical vapor deposition by using selective area epitaxy. The effects of the growth parameters such as growth rate, V/III ratio, growth temperature in selective epitaxy were investigated for GaAs/Al/sub 0.5/Ga/sub 0.5/As multilayer structures. To characterize and analyze the selectively grown structures, scanning electron microscopy and temperature dependent photoluminescence were used. The emission peak from quantum wires was observed at 975 nm. With increasing of temperature the emission intensity from side wall quantum wells decreased abruptly. But the intensity from quantum wires decreased slowly compared to that of side wall quantum wells and it became even stronger at about 50 K. The carrier capture processes also discussed.

Line shape functions in quantum-well and quantum-wire structures are theoretically analyzed taking non-Markovian relaxation processes into account. For high carrier density (as in laser operation), the line shape functions in low-dimensional systems have a strong convergent characteristic because the carrier-carrier coupling, i.e. the system-reservoir coupling, becomes stronger in the lower-dimensional systems. In particular, the line shape of the quantum wire can be approximated by the Gaussian function. In the case of low carrier density, the lower-dimensional systems have smaller homogeneous broadening widths because of longitudinal optical phonon localization; the transverse relaxation times are 0.1 ps (bulk), 0.3 ps (quantum well), and 0.7 ps (quantum wire).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantum wire self-ordered growth seeded by electron-beam lithography

InGaAs/InAlAs quantum wire structures on V-grooved substrates have been fabricated under As/sub 2/ flux by molecular beam epitaxy. Under As/sub 2/ flux, a smaller number of In atoms migrate than those under As/sub 4/ flux to the V-groove bottom from the sidewall surface. The InAlAs layer on the V-grooved InP substrates grown under As/sub 2/ flux preserves the V-shape, whereas the V-shape cannot be preserved and the quantum wire structures cannot be fabricated under As/sub 2/ flux. The InGaAs quantum wires grown under As/sub 2/ flux have good optical property.

A theory based on the Keldysh formalism is developed to study carrier transport in inhomogeneous quantum effects devices that operate at higher temperatures under large applied bias voltages. The scattering rates due to dissipative processes within devices are estimated self-consistently from the nonequilibrium particle density and the density of states. Unlike many existing models, the present model guarantees the conservation of the current and the number of particles in active devices. We have applied our model to study carrier transport in GaAs quantum wire devices and report several interesting results. It is found that a sudden increase in the polar-optical phonon scattering rates may result in a negative current at some critical energies when the bias voltage is positive. At low temperatures, the conductance of quantum wires shows quantized steps as a function of the applied bias voltage. Moreover, a negative differential conductance (NDC) is observed in the current–voltage characteristics of devices containing a single tunnel barrier. Such NDC disappears in the presence of strong inelastic scattering. Our results show that it is not possible to simulate many of the novel transport effects without explicitly incorporating the appropriate energy and the position dependences of the scattering rates.

Quantum-confined Stark effects are investigated theoretically in ${\mathrm{GaAs}\mathrm{}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wires formed in V-grooved structures. The electronic structures of the V-shaped quantum wires are calculated within the effective mass envelope function theory in the presence of electric field. The binding energies of excitons are also studied by two-dimensional Fourier transformation and variational method. The blue Stark shifts are found when the electric field is applied in the growth direction. A possible mechanism in which the blueshifts of photoluminescence peaks are attributed to two factors, one factor comes from the asymmetric structure of quantum wire along the electric field and another factor arises from the electric-field-induced change of the Coulomb interaction. The numerical results are compared with the recent experiment measurement.

We have investigated the elementary electronic excitations of the quasi-one-dimensional (1D) magnetoplasma in GaAs-AlGaAs quantum wires by resonant inelastic light scattering. In narrow quantum wire samples we observe the 1D intrasubband plasmon if a wave vector is transferred parallel to the wire direction. This 1D plasmon shows a negative dispersion with magnetic field, which arises from skipping orbit motion of the individual electrons at the edges of the wires. We find evidence for a coupling between adjacent wires in these samples with relatively small periods. Furthermore, we demonstrate theoretically and experimentally that in Raman experiments on confined magnetoplasmons in quantum wire structures with parabolic confinement the generalized Kohn theorem can be broken by using wave vector transfers q > 0.

An overview is given on the Rashba effect in GaxIn1-xAs/InP quantum wires. First, the effect of Rashba spin–orbit coupling on the energy level spectrum of quantum wires with different shapes of the confining potential is theoretically investigated. The wave functions as well as the spin densities in the quantum wire are analyzed for different magnetic fields. It is found that, owing to the additional geometrical confinement, a modification of the characteristic beating pattern in the magnetoresistance can be expected. The theoretical findings are compared to measurements on two different types of wires: First, single wires and, second, sets of parallel wires. A characteristic beating pattern in the Shubnikov–de Haas oscillations is observed for wires with an effective width down to approximately 400 nm. The beating pattern is significantly better resolved for the samples with sets of parallel wires, owing to the effective suppression of conductance fluctuations. A comparison with theoretical simulations confirms that the strength of the Rashba effect is basically not affected by the geometrical confinement of the wires. However, for wires with a very small effective width the strong carrier confinement leads to a suppression of the characteristic beating pattern in the Shubnikov–de Haas oscillations.

We present an extensive characterization of a quantum wire (QWR) structure using transmission electron microscopy (TEM). The structure consisted of a single GaAs layer in between AlGaAs barriers, grown on a GaAs substrate patterned with a submicrometre grating of V-grooves. For reference we also studied other QWR, as well as, quantum well (QW) samples, fabricated under similar conditions. We used bright field and dark field imaging to study the overall structure, high-resolution TEM to study the layer thickness and the interface quality, and chemical lattice imaging to study the compositional variations across the interfaces. In the QWR sample, there were mainly two distinctly different areas of the QW: on the (100) planes between the V-grooves, the QW was flat, whereas the QW on the near side walls of the V-grooves had a flat lower interface and a saw-tooth shaped upper interface. The QWRs at the bottom of the V-grooves were crescent shaped. We also observed a fundamental difference in growth of the GaAs and the AlGaAs on the side wall, where the AlGaAs formed straight interfaces, determined by high-index planes, whereas the GaAs tended to form alternating low-index planes giving a saw-tooth appearance of the GaAs QW.

We provide an analysis of nonlinear transport in diamond and cubic GaN (c-GaN) with emphasis on the different types of optical phonon scattering i.e., optical deformation potential (ODP) scattering versus polar optic phonon (POP) scattering limiting the carrier velocity. Both types of carrier mobilities and carrier saturation (peak) velocities in diamond and c-GaN as functions of different doping types and concentrations are obtained by directly solving the Boltzmann equation. Our model indicates that the nonrandomizing nature of POP scattering causes carrier temperature cooling, resulting in higher carrier drift velocity than with ODP scattering. This effect, in addition to the small carrier effective masses and large optical phonon energies, is responsible for the higher peak velocities in c-GaN, compared to carrier drift velocity in diamond.