Quantum transport in superlattice and quantum dot structures

Chapter 1 contains a summary of the research into superlattices from 1970 when they were initially proposed, up to the present day. It endeavours to cover the main theoretical concepts involved in superlattice work and the basic physics necessary to follow the work in this thesis. Chapter 2 details the apparatus and experimental techniques used to obtain the results presented in this thesis. It includes a comprehensive list of the devices studied along with important device parameters. This chapter will also contain results obtained via x-ray diffraction and optical techniques carried out for characterisation purposes. Chapter 3 is devoted to transport studies of superlattices in the miniband transport regime. Data is presented for a wide range of temperatures (300mK-300K) and applied electric and magnetic fields. The magnetic field is applied in the superlattice growth direction (B//I) for the majority of data presented and ranges in strength from 0 to 23T. In this regime, a new phenomenon is observed, namely the strong suppression of miniband conduction in high magnetic field. A schematic model is presented to explain effect. It is then reinforced by the results of Monte-Carlo simulations incorporating this model. Finally, some results obtained in tilled magnetic field are shown which are not yet understood and may be a focus for future work. The work in Chapter 4 was inspired by a theory paper written by Lyanda-Geller and Leburton. Their work predicts the manifestation of oscillations in the current -voltage characteristics of superlattices in the Wannier-Stark transport regime when strong lateral confinement is provided due either to gaps in the folded phonon spectrum or phonon momentum meeting the condition for Bragg reflection. Current-voltage measurements are shown in this chapter for superlattice devices in the Wannier-Stark regime for a range of electric and magnetic fields (B//I). Many oscillations are clearly observed in the I(V) data presented, the possible origins of which are then fully explored. Moving away from superlattices, data involving tunnelling through quantum dots embedded in the barrier of a GaAs/AIAs/GaAs resonant tunnelling diode are presented in chapter 5. Quasi-hydrostatic pressure is used to tune tunnelling through the dots. These results lead to a new picture for the conduction band potential profile of this device, and give us important new information relating to devices incorporating self-assembled quantum dots. More importantly, at low bias evidence for a phase change in the two dimensional electron gas formed at the tunnel barrier is obtained. (author)