Electron Transport in Semiconductor Nanowires
Abstract: In this thesis, semiconductor nanowires are studied from the point of view of growth and electrical properties. The growth of nanowires is done by chemical beam epitaxy (CBE), an ultra-high vacuum technique allowing a precise control of precursor deposition and low growth rates. In conventional epitaxy, growth is usually two or three- dimensional, depending on growth conditions and material combinations. Here we have used Au metal particles to catalyze one-dimensional growth perpendicular to the substrate surface. By switching the sources during growth, heterojunctions can be formed inside the wires. We have studied the InAs/InP system, which has a sufficiently high lattice mismatch to prevent defect free growth in conventional geometries. However, due to the small diameter of the wires, usually 20-70 nm, the strain can be accomodated by lateral relaxation within a few atomic layers without forming dislocations. Therefore nanowires offer an extended opportunity for bandgap engineering and open prospects for designed quantum components inside nanowires. Electrical measurements on homogeneous InAs nanowires have been performed as a function of applied source-drain voltage, Fermi level position, temperature, and magnetic field. The wires are n-type and function as field effect transistors at room temperature. Magneto-transport measurements at low temperatures showed a crossover from weak localization to weak anti-localization as the Fermi level increases. From the data, elastic scattering length, phase coherence length and spin scattering length were determined to roughly 80, 250, and 200 nm respectively. Finally, measurements on InAs wires containing InP segments have been performed with emphasis on single segments and double barriers. A thick InP potential barrier effectively blocks tunneling at low temperature and bias voltage. Thermal excitation over the barrier was used to deduce a 600 meV high barrier for the electrons. Finally different double barrier structures with varying quantum dot length were fabricated. Large dots where the energy level spacing is much smaller than the charging energy resulted in single electron transistors. As the dot length is reduced, the influence of the energy level spacing becomes more important, and at a dot size of 10 nm the quantum dot is completely empty of electrons. By applying a positive gate voltage electrons can then be added to the dot one by one. The filling of the dot results in a shell structure due to spin and orbital degeneracies of the system. These devices also function as resonant tunneling transistors.
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