Thermally and Optically Excited Electron Transport in Semiconductor Nanowires

Sammanfattning: This thesis explores the transport of thermally and optically excited electrons invarious nanowire structures. On one hand, electrons are thermally excited when thetemperature is nonzero, and the thermal energy help them surmount energy barriersthat are present in the material. On the other hand, when the electron distributionsat different part of the material are out-of-equilibrium due to thermal or opticalexcitations, an electrical current is created, converting the thermal and opticalenergy into electricity. Thus, in this thesis, the transport of thermally and opticallyexcited electrons is studied to extract the electronic properties of nanowireheterostructures and to investigate the limit of energy conversion in thermoelectricand photovoltaic devices.First, the measurement of thermionic emission current, which is the thermallyinduced electron flow over energy barriers, is used to study the electronic propertiesof InAs crystal phase heterostructures. The band offset, polarization charges, andcarrier density differences between the zinc blende and wurtzite crystal phases areinvestigated. In addition, quantum dot states can be formed within a wurtzitesegment or between two closely spaced wurtzite segments in an otherwise zincblende nanowire. The quantum dot formed between two wurtzite segments can befurther split into two parallel coupled quantum dots. Numerical simulations are usedto understand the formation and the interaction between the two quantum dots.Secondly, the thermoelectric response of pure zinc blende InAs nanowires isstudied. At low temperatures, the quantum confinement effect can be observed, andthe electrons exhibit quasi-1D transport. Conductance quantization and Seebeckcoefficient oscillation as a function of gate voltages, characteristic of quasi-1Dsystem, are observed. More importantly, a theoretical limit for the power factor ofnon-ballistic 1D channels is found and tested experimentally.Finally, the transport of optically excited electrons in InAs-InP-InAsheterostructure nanowires is studied. Electron distributions that are out-of- thermal equilibrium with, more specifically hotter than, the lattice and the environment arecreated through optical excitation with photon energies significantly larger than theband gap. An energy barrier formed by the InP segment is used to selectively extracthigh energy electrons and convert their kinetic energy into electrical potential.Nanophotonic effects including optical resonances in nanowires and localizedsurface plasmon resonances in metal nanostructures are used to create a nonuniformhot-electron distribution around the InP barrier. In particular, the hot-electrons canbe generated locally near the controlled position of the plasmonic metalnanostructures, which facilitates an in-depth study of their transport.