In the present paper we review some of the present technologies of interest for terahertz (THz) applications, and the physics and modeling of ultra-high frequency devices such as high electron mobility transistors (HEMTs) which have achieved THz frequencies. We present results of full band Cellular Monte Carlo (CMC) physics based simulation of InP and GaN based HEMTs of current interest to industry, and in particular, we address the current limitations in their frequency response in terms of the material and device structure, and the ultimate limits of scaling for such technologies.
Third generation concepts in photovoltaic devices depend critically on the dynamics of ultrafast carrier relaxation and
electron-phonon interactions on very short times scales in nanostructures such as quantum wells, wires and dots. Hot
carrier solar cells in particular depend on the reduction in the energy relaxation rate in an absorber material, where hot
carriers are extracted through energy selective contacts. Here we investigate the short time carrier relaxation in quantum
well, hot electron solar cells under varying photoexcitation conditions using ensemble Monte Carlo (EMC) simulation
coupled with rate equation models, to understand the limiting factors affecting cell performance. In particular, we focus
on the potential role of hot phonons in reducing the energy loss rate in order to achieve sufficient carrier temperature for
Here we report on high field transport in GaN and GaN field effect devices, based on the rigid-ion model of the electron-phonon
interaction within the Cellular Monte Carlo (CMC) approach. Using the rigid pseudo-ion method for the
hexagonal wurzite structure, the anisotropic deformation potentials are derived from the electronic structure, the atomic
pseudopotential, and the full phonon dispersion and eigenvectors for both acoustic and optical modes. Piezoelectric as
well as anisotropic polar optical phonon scattering is accounted for as well. In terms of high field transport, the peak
velocity is primarily determined by deformation potential scattering described through the rigid pseudo-ion model. The
calculated velocity is compared with experimental data from pulsed I-V measurements. We simulate the effects of non-equilibrium
hot phonons on the energy relaxation as well, using a detailed balance between emission and absorption
during the simulation, and an anharmonic decay of LO phonons to acoustic phonons, as reported previously. Non-equilibrium
phonons are shown to result in a significant degradation of the velocity field characteristics for high carrier
densities, such as those encountered at the AlGaN/GaN interface due to polarization effects.
High field transport in wide bandgap materials such as ZnS and SrS is of current importance for thin film electroluminescent devices currently used in flat panel display applications. Typically, carriers injected into the phosphor layer of such structures undergo acceleration in fields ranging from 1 - 2 MV/cm, with average carrier energies of 1 - 2 eV, and therefore high field transport is critical to the device operation. A major problem in the understanding of transport in such wide bandgap materials is the relative lack of experimental data for the electron- phonon coupling constants and impact ionization coefficients, particularly under high electric fields, where details of the full bandstructure are important. Hence, first-principles modeling of the electronic and transport properties is required for assessing the technological potential of these materials. In the present work, a review is given on the electronic and transport properties of three wide bandgap materials, ZnS, SrS, and GaN, simulated using full-band ensemble Monte Carlo (EMC) simulations. The impact ionization rates for both electrons and holes were derived directly from bandstructure calculated using the empirical pseudopotential method (EPM). To avoid arbitrary fitting parameters for the electron-phonon coupling, a microscopic rigid-ion model calculation is performed of the electron- phonon scattering rate directly from the EPM bandstructure, and a valence-shell model for the lattice dynamics. The momentum averaged scattering rate is input directly into the full-band EMC simulation. Results for the high field distribution functions for all three materials are calculated and compared. Further, the process of impact excitation of luminescent centers by hot carriers is included, and compared to experimental photo-induced- luminescence versus field data, where good agreement is obtained.
Decay of the longitudinal optical (LO) phonons in wurtzite GaN and AlxGa1-xN (x equals 0.1) has been studied by subpicosecond time-resolved Raman spectroscopy. In contrast to the usually-believed 2LA decay channel for LO phonons in other semiconductors, our experimental results show that, among the various possible decay channels, the LO phonons in wurtzite GaN and AlxGa1-xN (x equals 0.1) decay primarily into a large wavevector TO and a large wavevector LA or TA phonons. These experimental results are consistent with the recent theoretical calculations of the phonon dispersion curves.
Accurate characterization of the electromagnetic radiation arising from photoconducting systems is discussed. A computational technique is presented which combines the finite-difference time domain method with a spatial transformation, the Kirchhoff surface integral formulation. The technique enables incorporation of any number of material parameters while accurately accounting for the potentially wide-band nature of the radiation in an efficient computational method. Results are presented demonstrating a more accurate portrait of the radiation arising from a photoconducting structure than has been previously reported. Based on the simulation results, a simple model incorporating equivalent dipole sources is developed. Good agreement is shown between simulation results and measurements of similar structures.
We discuss the use of ensemble Monte Carlo techniques for the simulation of some opto-electronic devices. This approach has been reasonably successful for device simulation, and the Monte Carlo approach has worked very well for sub-picosecond time scales, but computational time becomes excessive for multi-picosecond simulations. Various nonlinear effects such as carrier-carrier scattering, non- equilibrium phonons, quantization in low-dimensional systems, and finite collision duration have all successfully been incorporated into the Monte Carlo method.
Monte Carlo simulation has been shown to be an effective approach to the study of ultrafast carrier relaxation in semiconductor bulk materials and in microstructures. We review the use of this methodology to study electron-electron and electron-hole interactions, non-equilibrium and confined phonons, and inter-subband relaxation in quantum wells. We also discuss the presence of the collision-duration on the short-time scale, and review the work of some other workers in the field. Finally, we discuss some of the limitations of the Monte Carlo technique.
We studied intersubband relaxation of carries during ultrafast photoexcitation in single and coupled quantum wells using ensemble Monte Carlo simulation, Intra- and intersubband scattering due to polar and nonpolar optical phonons, acoustic phonons, and intercarrier scattering are included in the simulation. The polar optical mode description is given in terms of a two-pole dielectric continuum model for the alloy barriers. In the present work we focus on relaxation when the 2-1 subband spacing is smaller than the optical phonon energy so that suppression of the intersubband polar optical phonon scattering rate occurs. Our results for a single well show that intercarrier intersubband scattering dominates over acoustic phonon scattering during the initial relaxation of carriers from 2-1, with a strong contribution due to polar optical phonon emission from the tails of the heated distributions as well. We have studied optical pumping for a 3 level coupled quantum well system in which (Delta) E12 is less than h(omega) 0, and calculate the change in occupancy of the excited subbands through pumping of the 1-3 transition.
We have experimentally studied the time-evolution of the exciton population in a higher subband of GaAs quantum wells, below the free carrier continuum. The lifetime of the exciton formed by an electron of the lowest subband and a heavy hole of the second subband in GaAs quantum wells is determined by time-resolved luminescence at 130 +/- 20 ps. This result is consistent with theoretical estimations of intersubband scattering by acoustic phonon emission. The exciton lifetime in the second heavy-hole subband is considerably longer than reported values of the recombination time in the lowest exciton state at k equals 0. The excitons in the higher subband at k equals 0 can be excited selectively without exciting the lower subband at k > 0. From these findings we conclude that subband transitions of excitons in quantum wells represent a new appealing concept for optically pumped coherent sources in the meV range.
A Monte Carlo solution to the Boltzmann transport equation is used to simulate hot-carrier relaxation and transport in a unipolar superlattice base transistor. Simulated results show that, due to the reduced density of states and wavefunction overlap, interminiband scattering is suppressed and high-energy transport is maintained in the superlattice base longer than in a bulk base region. However, an increased probability of reverse scattering and a lower magnitude of velocity along the superlattice axis result in a reduced transfer ratio across the superlattice base.
We present a Monte Carlo analysis of a `true' GaAs-based quantum wire, whose dimensions correspond to present state-of-the-art technology. Intrasubband and intersubband scattering rates for the electron-polar optical phonon interaction are included in the simulation as well as electron-electron interaction. We have studied the nonequilibrium transport characteristics of the one-dimensional system in two different situations: the response of the electron gas to an external electric field applied along the wire direction, and the cooling dynamics following laser photoexcitation. With respect to 3-D and 2-D systems, we can show that the electron- phonon interaction is not substantially modified, while a strong reduction in the electron- electron scattering rate of the wire is found.
Quantum wires with double bend discontinuities have been fabricated in modulation-doped field-effect transistors. The low temperature conductance shows resonant peaks in the lowest quantized conductance plateau. The double bend constitutes an electron cavity where the number of peaks is directly related to the cavity length. This view is supported by comparison to the theoretical conductance calculated from a generalized mode-matching theory. The experimental peak conductivity decreases with cavity length, which is consistent with elastic scattering due to random disorder in the quantum wire. Magnetic field studies show quenching of the resonance structure when the cyclotron radius approaches the one-dimensional channel width.
Modal expansions of the wave function and a mode-matching technique are used to calculate the transmission
characteristics of semiconductor quantum wire structures assuming hard wall confmement in the transverse directions.
Results for cascaded right-angle bends and periodic structures in a split-gate configuration are presented. A sharp
transition to a plateau of zero conductance is observed for the double bend configuration. For periodic structures in
the split-gate configuration, highly resonant behavior similar to that in tunneling resonant diodes is found. Calculated
current-voltage characteristics for the case of two narrow constrictions are shown, exhibiting a region of negative
Pseudomorphic Al 35Ga 65As/In 1Ga 9As resonant tunneling diodes fabricated with
asymmetric spacer layers adjaceht to the tunnel barrier were characterized via
magneto-transport measurements. Novel tunneling effects (ground vs excited state
tunneling) were observed in the current-voltage characteristics of these devices
which depend upon the bias direction. Shubnikov-de Haas oscillations obtained at
high magnetic fields show a strong asymmetry with bias direction and give evidence
of silicon dopant out diffusion during molecular beam epitaxy.
We use an ensemble Monte Carlo simulation of coupled electrons, holes and polar
optical phonons in multiple quantum well systems to model the intersubband
relaxation of hot carriers measured in ultra-fast optical experiments. Our
simulated results are in good agreement with experimental results in modulation
doped quantum wells and coupled double well structures where we find that the
intersubband relaxation time is controlled by the spatial overlap of the subband
envel ope wavefuncti ons.
We discuss the role of LO-phonons confinement in quantum well systems, by comparing two different
phonon models that have been proposed in the literature. A critical discussion concerning
the use of macroscopic approaches for the description of phonons in two dimensional systems is
presented. We use a Monte Carlo simulation which includes nonequilibrium phonon effects as well
as carrier-carrier scattering to determine the effect of phonon confinent on the relaxation of photoexcited
carriers in A1GaAs-GaAs quantum wells. Good agreement with available experimental
data is found. Even at low excitation densities, intercarrier scattering and phonon reabsorption
are important, and need to be taken into account in the interpretation of experimental data.