N. J. M. Horing

Balance-Equation Approach to Hot-Carrier Transport in Semiconductors

The balance-equation approach to nonlinear hot-carrier transport theory, formulated by Lei and Ting (1984), is addressed in this comprehensive review. A central feature is the role of strong electron-electron interactions in promoting rapid thermalization about the drifted transport state and the concomitant substantial simplification of the transport theory. This physical feature is embodied in the initial density matrix chosen to represent the unperturbed carrier system. Force and energy balance equations are formulated for the dc steady state, ac dynamic and transient cases of charge conduction, including memory effects. The scattering mechanisms include impurity and phonon interactions along with dynamic nonlocal screening effects due to carrier-carrier interactions. Both linear and nonlinear resistivities are discussed in the degenerate and nondegenerate statistical regimes. Interesting phenomena such as electron cooling and thermal noise and diffusion are discussed as well. Semiconductor microstructure transport is described for both linear and nonlinear hot carrier conduction. In this connection, quasi-2D-systems, heterojunctions, and quantum well superlattices are treated with attention to steady state, transient and high frequency transport, including, for example, superlattice plasmon resonance structure. Type-II superlattice transport is reviewed as well as type-I, and electron-hole drag is treated in conjunction with negative minority electron mobility in a quantum well. Multivalley semiconductors are discussed in some detail. Furthermore, attention is also focused on the center-of-mass velocity fluctuation, Langevin-type equation and thermal noise and diffusion for microstructures and multivalley systems. A number of particularly important phenomena are examined from the balance-equation point of view, such as nonequilibrium phonons, higher order scattering effects and weak localization, hydrodynamic equations for weakly nonuniform systems, and the intracollisional field effect. Alternative formulations and interpretations of the balance-equation approach are reviewed. The distinction between this many-particle, isothermal, balance-equation theory and the noninteracting (single-particle) adiabatic transport theory is discussed to clarify issues subject to controversy in the literature. Finally, we give a brief review of recent developments in the balance-equation approach: investigation of the distribution function in balance-equation theory, improved calculations for GaAs/AlGaAs heterojunctions, extension of the balance equations to an abitrary energy band and recent work on superlattice miniband transport.

Nonequilibrium phonon effects on hot carrier transport and thermal noise in semiconductor heterostructures

Hot carrier steady state and transient transport in bulk semiconductors and heterostructures is analyzed here using a general balance equation formulation with nonequilibrium phonon occupation and an ambient magnetic field of arbitrary strength. Carrier-carrier Coulomb interactions are also incorporated dynamically, and for superlattices both intraplanar and interplanar interactions are included. We have also examined the thermal noise temperature for hot electrons at a nonzero bias.

Efficient transition path sampling for systems with multiple reaction pathways

A new algorithm is developed for sampling transition paths and computing reaction rates. To illustrate the use of this method, we study a two-dimensional system that has two reaction pathways: one pathway is straight with a relatively high barrier and the other is roundabout with a lower barrier. The transition rate and the ratio between the numbers of the straight and roundabout transition paths are computed for a wide range of temperatures. Our study shows that the harmonic approximation for fluctuations about the steepest-descent paths is not valid even at relatively low temperatures and, furthermore, that factors related to entropy have to be determined by the global geometry of the potential-energy surface (rather than just the local curvatures alone) for complex reaction systems. It is reasonable to expect that this algorithm is also applicable to higher dimensional systems.

COULOMB DRAG IN DOUBLE QUANTUM WELLS WITH A PERPENDICULAR MAGNETIC FIELD

Momentum transfer due to electron-electron interaction (Coulomb drag) between two quantum wells, separated by a distance

Hot-electron transient transport in GaAs-AlGaAs heterosystems with finite proton relaxation times

d

Positive current noise cross correlations in capacitively coupled double quantum dots with ferromagnetic leads

, in the presence of a perpendicular magnetic field, is studied at low temperatures. We find besides the well known Shubnikov-de Haas oscillations, which also appear in the drag effect, the momentum transfer is markedly enhanced by the magnetic field.

Convective instability of a biased semiconductor superlattice

The effect of nonequilibrium phonon occupation on the transient response of the drift velocity and electron temperature to a step uniform electric field of moderate strength is calculated for single- and multi-layer GaAs-AlGaAs heterosystems, by using a balance equation approach. Interesting ballistic and overshoot behaviour is observed in almost all the cases examined. Although the drift velocity overshoot is only slightly modified by a finite phonon relaxation time, tau p, up to 3.5 ps, the electron temperature is greatly enhanced and the approach to a steady state is significantly prolonged.

Refine treatment of plasmon mediated photoconductivity resonances in a grid-gated double-quantum-well field-effect transistor

We examine cross-correlations (CCs) in the tunneling currents through two parallel interacting quantum dots coupled to four independent ferromagnetic electrodes. We find that when either one of the two circuits is in the parallel configuration with sufficiently strong polarization strength, a new mechanism of dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events, governs transport through the system together with the inter-dot Coulomb interaction, leading to a sign-reversal of the zero-frequency current CC in the dynamical channel blockade regime, and to enhancement of positive current CC in the dynamical channel anti-blockade regimes, in contrast to the corresponding results for the case of paramagnetic leads.

Non-linear generation of alternating current harmonics in quantum dot superlattice miniband transport

A fully three-dimensional analysis of the convective instability of a planar superlattice biased in the regime of negative differential miniband conductance is carried out for the first time with accurate microscopic treatment of phonon and impurity scatterings. For a typical GaAs-based superlattice having period d=10 nm, miniband width Delta =900 K and electron sheet density Ns=1.5*1015 m-2, we find that the convective space-charge waves propagate at a phase velocity ranging from 0.75 upsilon 0 to 0.95 upsilon 0 ( upsilon 0 is the carrier drift velocity) and with an amplitude growth rate only about a few per cent of that predicted by the conventional drift-diffusion model and other existing methods.

Study of lennard-jones clusters: Effects of anharmonicities far from saddle points

Recent observations of THz photoconductivity in a double-quantum-well (DQW) field-effect transistor (FET) with a periodic metal-grid gate indicate that the presence of an asymmetric DQW is essential to produce a large photoresponse. The strongest THz photoresponse occurs when the upper QW (nearest to the gate) is fully depleted under metal portions of the grating gate, while the lower one remains connected, but with a laterally modulated electron density. The positions and strengths of the resonant peaks in the photoresponse are controlled by both the voltage applied to the gate and the period of the grating gate. In this paper, we exhibit a correlation between magnitudes of the plasmon absorption resonances and resonant changes in photoconductance of the DQW-FET channel.