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In the present talk, we discuss the use of Ensemble
Monte Carlo (EMC) techniques for the simulation of ultrafast
phenomena in semiconductor nanostructures and devices. EMC is
essentially a direct solution of the semi-classical Boltzmann
transport equation (BTE), which can be extended to account for
various quantum mechanical and many body effects beyond the BTE
framework. We first discuss the application of this simulation
method to carrier relaxation during ultrafast photoexcitation in
GaAs quantum confined systems, in particular the role of
intercarrier scattering and non-equilibrium phonons in intersubband
relaxation.
We then discuss the simulation of transport in GaN
and AlGaN heterostructures using a full-band Cellular Monte Carlo
simulator developed by our group [1]. The electronic band structure
and phonon spectra are used as direct inputs to the simulator for
both cubic, hexagonal, and strained crystal structures using both
empirical and ab initio methods. The full anisotropic
electron-phonon interaction is calculated from the rigid-ion model
using the electronic structure, the atomic pseudopotential, and the
full phonon dispersion and eigenvectors for both acoustic and
optical modes. Good agreement is obtained between the simulated
results, and experimental pulse I-V measurements of high field
transport measured in-house [2]. The role of nonequilibrium hot
phonons in limiting the frequency response of heterostructure field
effect devices is also discussed.
Finally, we discuss the simulation of terahertz
emission during ultrafast optical excitation in pin diode structure
using the CMC code, and comparison to experimental studies by
Leitensdorfer et al. [3]. Here good agreement is obtained between
the experimentally observed temporal evolution of the terahertz
pulse and the non-stationary dynamics of photoexcited electrons and
holes at various DC field strengths. For these simulations, and for
high frequency and electro-optic devices in general, we have also
coupled the CMC simulator with finite difference time domain (FDTD)
solutions of Maxwell’s equations, and applied it to simulate
terahertz pulse generation directly during electro-optic sampling.
[1] M. Saraniti and S. M. Goodnick, IEEE Trans.
Elec. Dev. 47 (2000) 1909.
[2] J. M.
Barker et al., Physica B314 (2002) 39.
[3] A. Leitenstorfer, S. Hunsche, J. Shah, M.C. Nuss,
and W.H. Knox: Phys. Rev. Lett. 82 5140 (1999). |