Resonant Tunneling in Laterally Confined Quantum Structures
Abstract: In the thesis, three-dimensionally confined resonant tunneling structures were studied experimentally. Two approaches were used for obtaining quantum confinement: gate-defined lateral constriction of double barrier structures, and epitaxial growth of self-assembled quantum dots. In addition, the thesis deals with the basic development of large-area double barrier resonant tunneling diodes, as a starting point for more advanced quantum dot structures. Large area double barrier resonant tunneling diodes in several material systems were investigated: GaAs/Ga0.5In0.5P, GaAs/GaAs0.2P0.8, GaAs/GaP, and Ga0.5In0.5As/InP. Emphasis was placed on GaInP/GaAs structures, which were optimized in terms of well width and of doping concentration. Gate-defined lateral confinement was achieved by a buried metal gate positioned 30 nm above a GaInP/GaAs double barrier. The Schottky depletion from the metal directs the current to a designed opening in the gate. The opening constitutes a conducting channel through the depleted semiconductor, where an applied gate voltage alters the effective width of the channel. Room-temperature transistor action was measured in structures with large opening area, and multiple current peaks in the low-temperature current-voltage characteristics of small-area devices indicate that lateral quantum confinement was achieved. The gate and magnetic-field dependence of the features obtained showed qualitative agreement with calculations performed on a coupled 1D-0D-1D quantum system. Low-temperature electron transport through self-assembled InAs dots in InP barriers showed several distinct current peaks. Utilizing As/P exchange reactions on the InP surface, extremely low dot densities (N=4e6 cm-2) was achieved, corresponding to approximately 150 quantum dots in a macroscopic mesa structure. Transport through single- and double-dot layers have been investigated. In the stacked samples, a peak-to-valley ratio of 85 was obtained.
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