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

The mechanical, optical, and electrical properties of III-V semiconductor heterostructures are investigated in this
thesis. 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 field
of 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 InAsP
nanowires are grown epitaxially by the method of Au particle assisted vapor-liquid-solid growth. They are then
covered by an InP surface layer to obtain InAsP-InP core-shell nanowires. The mechanical strain is measured in the
core-shell nanowires by use of X-ray diffraction. The atomic plane spacing is obtained and related to the mechanical
strain which originate from the epitaxial interface between core and shell. The strain is found to be oriented mainly
along the axis of the nanowires. This axial strain is shown to increase with the thickness of the InP shell layer. This
increase 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 conducting
channels located along the corners of the triangular shell. The electrical transport through the InAs shell is
investigated at temperatures < 1 K. The nanowires are first probed by Coulomb blockade transport. A method with
four contact electrodes connected to the InAs shell is used to investigate the directional dependence of the Coulomb
blockade, demonstrating that the corners of the shell are highly coupled and that electrons are delocalized over the
full shell volume. Next, transport measurements with low resistance superconducting contacts show induced
superconductivity. A gate tunable supercurrent is produced and a directional dependence of the conductance is
found in the InAs shell.