Postgraduate OpportunitiesCarrier capture by quantum wells has been the subject of a number of experimental and theoretical studies, and is of interest because the modulation response of QW lasers is sensitive to the efficiency of the capture process. Experiment has confirmed theoretical predictions of pronounced resonances in the capture rate as a function of well width.
For resonances to occur, the phase coherence and the amplitude of the carrier wave functions must both be maintained across the width of the quantum well. Loss of phase coherence is associated with the transition from the quantum transport regime to the classical regime decribed by the Boltzmann transport equation. Theoretical description of this transitional regime is a challenge: neither Fermi's golden rule nor the Boltzmann equation are applicable. Recent microscopic treatments of capture, based on the Schrödinger-equation Monte Carlo method or greatly simplified Green's-function techniques, have included only the effects of the interaction between carriers and optical phonons. In real systems, scattering by acoustic phonons, impurities and other carriers all play their part in de-phasing the carriers. The work proposed is to make a quantitative assessment of the effects of some of these additional scattering mechanisms, by use of non-equilibrium many-body Green's-function or density-matrix methods.
- Semiconductor theory, including:
- dynamics of carrier relaxation, localization and recombination in semiconductor microstructures
- carrier capture into (and thermionic emission from) quantum wells and dots
- effective mass/k·p approximations
- many-body effects in dense gases of carriers.
- Quantum theory of mechanical systems with holonomic and nonholonomic constraints.