Strain and Charge Transport in InAsP-InP and InP-InAs Core-Shell Nanowires

Sammanfattning: The mechanical, optical, and electrical properties of III-V semiconductor heterostructures are investigated in thisthesis. The semiconductor materials are grown by metal-organic vapor phase epitaxy, yielding wire shaped crystals(nanowires) having a length of ~ 1 μm and diameter of ~ 100 nm. Nanowires are relevant for many applications,such as optical detectors, photovoltaics, light emitting diodes, and transistors. Nanowires are also used in the fieldof quantum devices, for the study of quantum dots and Josephson junctions.In this thesis, InAsP-InP and InP-InAs core-shell nanowires of wurtzite crystal phase are investigated. The InAsPnanowires are grown epitaxially by the method of Au particle assisted vapor-liquid-solid growth. They are thencovered by an InP surface layer to obtain InAsP-InP core-shell nanowires. The mechanical strain is measured in thecore-shell nanowires by use of X-ray diffraction. The atomic plane spacing is obtained and related to the mechanicalstrain which originate from the epitaxial interface between core and shell. The strain is found to be oriented mainlyalong the axis of the nanowires. This axial strain is shown to increase with the thickness of the InP shell layer. Thisincrease of strain is also found in measurements of the bandgap of the InAsP cores in the core-shell nanowires.The growth method selective area epitaxy is applied to produce pure wurtzite crystal phase InP-InAs core-shell nanowires.The InAs shell exhibit triangular cross section and the InP core has hexagonal cross section. The chargecarrier accumulation in the InAs shell enables the formation of a quantum structure that produce conductingchannels located along the corners of the triangular shell. The electrical transport through the InAs shell isinvestigated at temperatures < 1 K. The nanowires are first probed by Coulomb blockade transport. A method withfour contact electrodes connected to the InAs shell is used to investigate the directional dependence of the Coulombblockade, demonstrating that the corners of the shell are highly coupled and that electrons are delocalized over thefull shell volume. Next, transport measurements with low resistance superconducting contacts show inducedsuperconductivity. A gate tunable supercurrent is produced and a directional dependence of the conductance isfound in the InAs shell.