Studies of Nanowire Devices Enabled by Advanced Nanofabrication

Sammanfattning: This thesis explores the possibility of using advanced device geometries and heterostructure engineering to manipulate, control, and study the electrical and optical properties of semiconductor nanowires. The first part of the thesis investigates the use of different gate-all-around architectures for creating new types of gate-controlled nanowire devices, primarily intended for fundamental studies of semiconductor physics. Complex fabrication schemes for creating laterally oriented single-nanowire transistors with fully wrapped gates, as well as vertically oriented multi-nanowire devices with transparent wrap-gates for studies of gate-controlled photoluminescence, are described. Nanowire transistors with semi-wrapped gates are shown to operate very close to the fundamental thermal limit, and gate-controlled ambipolar conduction is observed. A nanowire field-effect diode is realized using two semi-wrapped gates placed on a single nanowire, creating a device with gate-tunable current rectification properties in which gate-controlled electric fields induce a p-n junction in a doping-free part of the nanowire. The second part of the thesis describes the fabrication and characterization of a nanowire ratchet, in which a stepwise compositionally graded heterostructure sequence is used to produce a sawtooth-shaped electrostatic potential along the nanowire axis, leading to strong rectification of electron transport, in excellent agreement with theory and simulations. This structure is interesting for fundamental studies of ratchet physics, directed charge transport, and hot-carrier dynamics in low-dimensional semiconductor systems, and also as a platform for realizing novel optoelectronic device concepts. When combined, the ability to create designed nanowire potential structures with wrap-gate control over charge carrier behavior opens up many possibilities for future studies of the unique properties of semiconductor nanowires.