Ultrafast carrier relaxation and vertical-transport phenomena in semiconductor superlattices: A Monte Carlo analysis

Abstract

The ultrafast dynamics of photoexcited carriers in semiconductor superlattices is studied theoretically on the basis of a Monte Carlo solution of the coupled Boltzmann transport equations for electrons and holes. The approach allows a kinetic description of the relevant interaction mechanisms such as intraminiband and interminiband carrier-phonon scattering processes. The energy relaxation of photoexcited carriers, as well as their vertical transport, is investigated in detail. The effects of the multiminiband nature of the superlattice spectrum on the energy relaxation process are discussed with particular emphasis on the presence of Bloch oscillations induced by an external electric 6eld. The analysis is performed for different superlattice structures and excitation conditions. It shows the dominant role of carrierpolar-optical-phonon interaction in determining the nature of the carrier dynamics in the low-density limit. In particular, the miniband width, compared to the phonon energy, turns out to be a relevant quantity in predicting the existence of Bloch oscillations.

Coherent phenomena in semiconductors

“…This is due to the following reasons: In agreement with recent experimental 107,117 and theoretical [109][110][111] investigations, for superlattices characterized by a miniband width smaller than the LO-phonon energy -as for the structure considered here-and for laser excitations close to the band gap, at low temperature carrier-phonon scattering is not permitted. Moreover, in this low-density regime carrier-carrier scattering plays no role: Due to the quasi-elastic nature of Coulomb collisions, in the low-density limit the majority of the scattering processes is characterized by a very small momentum transfer; As a consequence, the momentum relaxation along the growth direction is negligible.…”

“…In this section, we will review recent simulated experiments of the ultrafast carrier dynamics in semiconductor superlattices [109][110][111][112][113][114] . They are based on a generalized Monte Carlo solution [66][67][68][69][70] of the set of kinetic equations (so-called semiconductor Bloch equations) derived in section II B.…”

“…Such exciton peaks, which are no longer equidistantly spaced, are often referred to as excitonic Wannier-Stark ladders 118 of the superlattice. Since for the superlattice structure considered in this simulated experiment 109,113 the combined miniband width is larger than the typical two-and three-dimensional exciton binding energies, it is possible to investigate the quasi-three dimensional absorption behavior of the delocalized miniband states as well as localization effects induced by the electric field.…”

Ultrafast carrier dynamics in semiconductor nanostructures: interplay between coherence and relaxation

“…This is due to the following reasons: in agreement with recent experimental [14,15] and theoretical [6,13] investigations, for superlattices characterized by a miniband width smaller than the LO phonon energy-as for the structure considered here-and for laser excitations close to the band gap, at low temperatures carrier-phonon scattering is not permitted. Moreover, in this lowdensity regime carrier-carrier scattering plays no role: due to the quasi-elastic nature of Coulomb collisions, in the low-density limit the majority of the scattering processes are characterized by a very small momentum transfer.…”

“…The superlattice model employed in our simulated experiments is described in [13]: the energy dispersion and the corresponding wavefunctions along the growth direction (k ) are computed within the well-known Kronig-Penney model while for the in-plane direction (k ⊥ ) an effective mass model has been used. Starting from these three-dimensional wavefunctions, the various carrier-carrier as well as carrier-phonon matrix elements are numerically computed.…”