R.P. Joshi

Electromagnetic and transport considerations in subpicosecond photoconductive switch modeling

The authors discuss a combination of direct finite-difference time-domain solutions of Maxwells equations and Monte Carlo models of photocarrier transport used to avoid assumptions commonly made in developing equivalent circuit models for transmission lines and in other simplifications commonly made in modeling conductivity. Problems that complicate the development of an accurate model for subpicosecond optoelectronic switching and the measurement of electrical waveforms on microstrip lines are discussed. >

Theoretical and experimental investigations of subpicosecond photoconductivity

Monte Carlo methods are used to study photoconductive transients in gallium arsenide. It is demonstrated that working with presently established ranges for the Γ‐L coupling coefficient, the existence of a velocity overshoot at moderate fields cannot be exactly predicted. The role of negative velocity electrons in the initial transient for short wavelength excitation is also demonstrated. Details of an actual experiment are described and evaluated against a model which incorporates the Monte Carlo simulation into a transmission line structure with a frequency‐dependent characteristic impedance. The results demonstrate that an appropriately designed experiment can observe subpicosecond carrier transport transients.

Hot-phonon and electron-hole scattering effects on the transient transport of photogenerated electrons in GaAs

We investigate the transient response of photogenerated carriers to an external electric field in bulk GaAs. The results of our Monte Carlo simulations indicate that the initial velocity rise times are a strong function of the carrier density. This is caused by a combination of the hot‐phonon effect and the enhanced electron‐hole scattering within the plasma. Contrary to some previous suggestions, the hot‐phonon effect alone is insufficient to explain the initial velocity behavior seen experimentally. The steady‐state velocity is limited by the electron‐hole scattering.

Coupling Maxwell's equation time-domain solution with Monte-Carlo technique to simulate ultrafast optically controlled switches

An accurate model for studying transients in semiconductor devices and optical switches is presented. This approach couples a direct solution of Maxwells equations with an ensemble Monte-Carlo model. This model is capable of simulating very-high-frequency devices and switches on the subpicosecond scale. Optical interaction with semiconductor devices can also be modeled.<<ETX>>

Time-domain finite difference and EMC study of hot carrier transport in GaAs on a picosecond scale

Abstract It is now possible to use optoelectronic techniques to generate and measure electrical waveforms with subpicosecond risetimes. Though experimental methods have become relatively advanced, simulation techniques for analyzing and modelling such ultrafast transients remain inexact and oversimplified. The simplifications commonly made while developing equivalent circuit models for the experimental structures, prevent accurate simulation of the electrical response during the picosecond regime. In order to obtain a better physical picture, it is essential to develop a more physical model for the microstrip circuits routinely used in such experiments. Furthermore, non-Ohmic transport behavior for the photogenerated carriers within the optoelectronic device also needs to be correctly incorporated. We address both the circuit and device response issues through a novel scheme which combines direct time domain solutions to Maxwells equations with the ensemble Monte Carlo model for carrier transport. By coupling the electromagnetics with the EMC, we avoid having to make assumptions whose validity breaks down for picosecond transport.

Monte Carlo analysis of high-field hole diffusion coefficients in nondegenerate GaAs

We examine the field dependence of the carrier diffusion coefficients in GaAs using an ensemble Monte Carlo technique. An analysis for the field dependence of the hole diffusivity is presented for the first time. Unlike for the electrons, no significant interband transfer effects are observed. The hole diffusivity is seen to decrease monotonically with increasing field.

Ultrafast Relaxation Of Hot Photoexcited Carriers In GaAs

In this paper, we examine a number of factors concerning the relaxation of hot photoexcited electron-hole plasmas in semiconductors. Analytical solutions are utilized to probe the influence of the light-holes on the longer time behavior. The role of the electron-hole interaction and dynamic, self-consistent screening is discussed. Then, the scattering to the satellite L and X valleys is examined in the absence of the inter-carrier interactions. Ensemble Monte Carlo calculations used in this latter approach indicate that the time constant for relaxation of the central valley electrons due to inter-valley scattering cannot be faster than 80-100 fs.

Dynamic simulation of a photoconductive switching experiment

Improved modelling of photoconductive switching experiments can be obtained by embedding a bipolar ensemble Monte Carlo model of the photoconductive gap into a time domain solution of Maxwells equations. In addition to making fewer assumptions than previous models, this technique allows for the simulation of a probe pulse used in pump and probe experiments. Thus it is possible to accurately simulate the entire experiment and plot the quantity actually observed in the experiments-the phase shift of the probe beam.

A self consistent Monte Carlo method for the transient response of laser excited photoconductors

Abstract We use a bipolar Ensemble Monte Carlo embedded within a circuit solver to include the nonlinear transport of photogenerated electron-hole plasmas. All the usual carrier-carrier and carrier-phonon scattering mechanisms are included within a static screening model to incorporate non-ohmic behaviour. A one dimensional Poisson solver combines nonuniform field effects brought about by charge redistribution. Unlike previous EMC simulations for photoconductive circuits, we allow for transient changes in the net mobile charge within the photoexcitation region, by appropriately modifying the supercharge associated with each particle. Using this time varying supercharge scheme, we test an N + -N-N + structure under both dc and transient conditions.

Ultrafast relaxation of hot photoexcited carriers in GaAs

Abstract The roles of carrier-carrier interactions and non-equilibrium phonons on the ultrafast relaxation of photoexcited carriers in GaAs are examined. At low carrier concentrations, the e-ph interaction is the main energy loss channel for hot electrons, while at high carrier concentrations, the e-h interaction is the primary energy loss channel. This latter result follows from the high e-h scattering rate, the screening of the e-ph interaction, and the high efficiency of hole-phonon scattering through the unscreened deformation potential interaction. The electron energy-loss rates through the e-h interaction increases as the excitation energies and intensities are increased. In two-dimensional systems, the e-h interaction further complicates the problem since the transverse optical modes out are also driven out of equilibrium by their interaction with the holes.