Ultrafast dynamics in semiconductor quantum wells

In this thesis, the recombination dynamics within ZnO/MgZnO quantum wells under the influence of the quantum confined Stark effect, and the coherent dynamics of excitons within GaAs/AlGaAs asymmetric double quantum wells, are studied using a wide range of ultrafast spectroscopy techniques. These two topics are presented in two separate and self contained sections. In the ZnO section, the recombination dynamics within ZnO/Mg0.3Zn0.7O quantum wells under the influence of the quantum confined Stark effect quantum wells are examined from the μs to the fs timescale. Generating a high density of carriers within these quantum wells screens the internal electric field and reduces the impact of the quantum confined Stark effects. Following excitation, the decay of the excited state population restores the internal electric field. This produces a time varying interband transition energy and a time varying recombination lifetime. The time-dependent properties of the quantum well are revealed by pump-probe, time-resolved photoluminescence and modeling. By combining the measured recombination dynamics and the calculated recombination dynamics it is possible to determine the carrier density, the transition energy, the overlap integral of the electron and hole wavefunctions and the recombination lifetime at any time following excitation. In a separate investigation a range of graded barrier ZnO/Mg0.3Zn0.7O quantum well samples are examined. Such structured quantum wells may be used to control the electron and hole wavefunction overlap within quantum wells with a built in electric field. In this thesis I report on the first graded barrier quantum wells based on the the ZnO/Mg0.3Zn0.7O material system. The samples show that the overlap integral of the wavefunctions for the lowest energy electron and hole states may be controlled over a range from 0.25 to 0.86. The capacity to predictably control the transition energy and wavefunction overlap integral in QWs under the influence of the quantum confined Stark effect by using graded barriers adds to the commonly used techniques for tuning quantum well properties, including well width, band gap, and barrier height. The coherent dynamics of coherently coupled excitons within GaAs/Al0.35Ga0.65As asymmetric double quantum wells are examined in a separate section. The dynamics are measured using four-wave mixing and analysed using two-dimensional Fourier transform spectroscopy. Two sample sets with barrier widths of 2, 4, 6, 20 and 50 nm are examined. The first sample set is designed so that the energy difference between the two lowest energy and bright heavy-hole excitons is equal to the longitudinal optical phonon energy. The purpose of this design is to explore the effect of a longitudinal optical phonon resonance on the coherent dynamics of coherently coupled excitons localised to different quantum wells. The second sample set is designed so that the energy difference between the same two heavy-hole excitons is not equal to the longitudinal optical phonon and may be used for comparison with the coherent dynamics measured in the first sample set. In practice the energy separation between the two heavy-hole transitions vary by up to 25 %, making any association with an optical phonon resonance difficult. Despite this, it is still possible to systematically examine the barrier width dependence of the coherent dynamics and coupling between spatially separated excitons. The calculations and experimental results suggest that the two lowest energy and bright heavy-hole excitons are localised to opposite quantum wells for barrier widths greater than or equal to 4 nm. Using two-dimensional Fourier transform spectroscopy we observe coherent coupling between these excitons for barrier widths of 4, 6 and 20 nm. The possible coupling mechanisms are discussed. Through a process of elimination, dipoledipole coupling is suggested as being the most likely coupling mechanism. The investigation presented in this thesis provides a first examination of coherently coupled and spatially separated excitons in an asymmetric double quantum well system.