Simulational studies of epitaxial semiconductor superlattices: Quantum dynamical phenomena in ac and dc electric fields

Reynolds, Joseph
Major Professor
Marshall Luban
Committee Member
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Physics and Astronomy
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Physics and Astronomy

Using high-accuracy numerical methods we investigate the dynamics of independent electrons in both ideal and realistic superlattices subject to arbitrary ac and/or dc electric fields. For a variety of superlattice potentials, optically excited initial wave packets, and combinations of ac and dc electric fields, we numerically solve the time-dependent Schrodinger equation. In the case of ideal periodic superlattice potentials, we investigate a long list of dynamical phenomena involving multiple miniband transitions and time-dependent electric fields. These include acceleration effects associated with interminiband transitions in strong fields, Zener resonances between minibands, dynamic localization with ac fields, increased single-miniband transport with an auxiliary resonant ac field, and enhanced or suppressed interminiband probability exchange using an auxiliary ac field. For all of the cases studied, the resulting time-dependent wave function is analyzed by projecting the data onto convenient orthonormal bases. This allows a detailed comparison with approximate analytic treatments;In an effort to explain the rapid decay of experimentally measured Bloch oscillation (BO) signals we incorporate a one-dimensional representation of interface roughness (IR) into our superlattice potential. We show that as a result of IR, the electron dynamics can be characterized in terms of many discrete, incommensurate frequencies near the Bloch frequency. The interference effects associated with these frequencies cause a substantial decrease in amplitude of the signal after several Bloch periods. We suggest that this is an important source of coherence loss in BO signals at low temperature and low carrier density. We also propose an experimental method that should significantly reduce the effects of IR by exciting electrons to only a single layer of the superlattice. This is accomplished by doping the central GaAs layer with a very small amount (<1%) of In, thus reducing the energy gap for this layer. Thus, a laser excitation pulse tuned somewhat below the nominal electron-hole excitation energy, will only excite a few Wannier-Stark eigenstates associated with this In-doped layer. Our numerical simulations show that the THz signal from electrons optically excited using this novel procedure is nearly free from all inhomogeneous broadening associated with IR.