XL（雷啸霖） Lei

Balance-equation approach to hot-electron transport in semiconductors irradiated by an intense terahertz field

We investigate the effect of a uniform intense terahertz radiation on hot-electron transport in semiconductors driven by a dc or slowly varying electric field of arbitrary strength. Using a vector potential for the high-frequency field and a scalar potential for the dc or slowly varying field, we are able to separate the center-of-mass motion from relative motion of electrons and to distinguish the slowly varying part from the rapidly oscillating part of the center-of-mass velocity. Considering the fact that relevant transport quantities are measured over a time interval much longer than the period of the terahertz radiation field, we obtain a set of momentum and energy balance equations, without invoking a perturbational treatment of the electron-photon interaction. These equations, which include all the multiphoton processes, are applied to the examination of hot-carrier transport in a GaAs-based quantum well subjected to a weak or a strong dc bias and irradiated by a terahertz radiation of various freq...

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.

Microwave evaluation of Pb0.4Sr0.6TiO3 thin films prepared by magnetron sputtering on silicon: Performance comparison with Ba0.3Sr0.7TiO3 thin films

Pb0.4Sr0.6TiO3 (PST) thin films were deposited on high resistivity silicon substrate by radio frequency magnetron sputtering. A pure perovskite phase was obtained at a low post annealing temperature of 650 °C. The relative dielectric constant, loss factor, tenability, and figure of merit were determined over a large frequency range of 1 GHz to 60 GHz. A large tunability about 60% and a relatively low loss of 16% at 60 GHz were obtained. PST is an alternative material for microwave agile devices integrated with silicon and this is discussed from the standpoint of monolithic integration with a low thermal budget.

Current suppression and harmonic generation by intense terahertz fields in semiconductor superlattices

The time-dependent current response of superlattice miniband conduction to a strong submillimeter wave electric field of varying frequency and strength is investigated at lattice temperature T=77 and 300 K by means of the three-dimensional balance-equation method with accurate microscopic treatment of impurity and phonon scatterings. The terahertz-radiation-induced current suppression and the generated third harmonic power as functions of the input power of the high-frequency field predicted theoretically are in reasonable agreement with recent experimental findings in heavily doped GaAs superlattices.

The effect of an intense terahertz irradiation on electron transport in two-dimensional semiconductors

We study theoretically the electron transport in GaAs-based quasi-two-dimensional systems under the influence of an intense terahertz electromagnetic irradiation, using a balance equation approach in which the slowly varying part of the centre-of-mass velocity is distinguished from the rapidly oscillating part of it. Electron scatterings by charged impurities, and acoustic and polar optical phonons are considered and up to as many as |n|=60 multiphoton channels are taken into account. The carrier mobility and the electron temperature of a typical GaAs quantum well system are calculated in the limit of small dc drift velocity (small dc field) as functions of the radiation-field strength for various frequencies in the range from 1 to 10 THz at lattice temperature T = 10, 77, 150 and 300 K. We find that at low lattice temperature (T = 10 K), dc mobility decreases monotonically with increasing strength of the radiation field, and lower frequency generally has a stronger effect in suppressing the mobility, in agreement with the experimental observation. At room temperature, on the other hand, the present theory predicts an enhancement of the dc mobility due to irradiation with a THz field.

Thermoelectric power in the balance equation theory

The thermoelectric power of bulk semiconductors and quantum well structures is investigated for the first time using the balance equation transport theory extended to weakly non-uniform systems. In the linear transport limit the thermopower expressions obtained are independent of scattering and equivalent to the results derived from the Boltzmann equation with the relaxation time approximation. Effects of carrier heating (due to a current flow or due to an applied electric field in the crossed direction) are examined, showing that thermoelectric power is very sensitively dependent on the method of heating the electrons, and can change sign at low temperatures in the case of a strong electric field bias.

Balance equations for electron transport in a general energy band

The conventional way of developing a momentum balance equation by generating the first-order moment (with the momentum k as the moment operator) from the Boltzmann transport equation relies on the fact that the distribution function f(k) approaches zero rapidly enough that the integral integral Del (k)f(k) d3k is negligible. This is not valid for an energy band (e.g. a superlattice miniband) whose width (in at least one direction of the k-space) is comparable with kBTe (Te is the electron temperature). It is demonstrated that by choosing the moment operator to be the velocity function upsilon (k) identical to Del epsilon (k) a compact moment equation, which is valid for a general energy band and represents an effective momentum balance of the carrier system, can be derived for the distribution function f(k) which is a continuous function of L in the periodic zone scheme. The effective momentum balance equation and the energy balance equation thus obtained share the same formal expressions as the acceleration and energy-balance equations in Leis non-parabolic method (1992). These equations, with the distribution function suggested by Huang and Wu (1994), are applied to the discussion of high electric field transport of electrons in non-parabolic Kane bands and superlattice minibands. The results are compared with the predictions from Leis non-parabolic method and from a carrier temperature model.

ELECTRON-TRANSPORT IN NON-PARABOLIC KANE BANDS

Linear and non-linear transport properties of carriers in non-parabolic Kane bands are investigated using the two extended balance equations introduced by Magnus, Sala and De Meyer (MSD) (1990) and by Lei (1992,1994). Formally, the MSD equations describe the motion of a single particle with fixed charge and fixed mass, while the Lei equations describe the motion of a single particle having fixed charge but variable effective mass. In the parabolic limit these two sets of equations are identical and reduce to the original Lei-Ting balance equations. In the non-parabolic case, although the linear resistivities predicted by the Lei and MSD equations are nearly the same for weakly to medially non-parabolic systems, non-linear drift-velocity-field behaviour obtained from these two sets of equations shows marked differences for medial and strong non-parabolicity. The reasons for these differences are discussed.

The inter-layer local-field corrections of weakly coupled electron-electron and electron-hole layers

We propose a simple analytical expression for the inter-layer local-field correction (LFC) for weakly coupled two-layer systems and develop a sum-rule version of the self-consistent approach of Singwi, Tosi, Land, and Sjolander. On the basis of the approximate approach, the inter-layer LFC is investigated for electron-electron and electron-hole layers with different carrier densities and layer spacings. Using this parametrized inter-layer LFC, we calculated the transresistance in the electron-electron and electron-hole layers. The theoretical results are in good agreement with the available experimental data.

Investigation of the Buttiker-Thomas momentum balance equation from the Heisenberg equation of motion for Bloch electrons

Using an appropriate momentum function for Bloch electrons moving in a single energy band, we are able to obtain the effective force balance equation with the Buttiker-Thomas reduction factor by calculating the rate of change of the total momentum from the Heisenberg equation of motion.