Nanowire Heterostructures- Growth, Characterization and Optical Physics

Abstract: This thesis describes growth, processing, characterization and photoluminescence (PL) spectroscopy of nanowire heterostructures. The nanowires were made of III-V semiconductor materials and were produced by Au-particle assisted growth. Axial heterostructures in the form of quantum dots (QDs) in the nanowires as well as radial heterostructures in the form of core-shell nanowires were studied. Axial heterostructures in the InAs-GaAs and InAs-InP material systems were studied and it was found that the latter material system gave the best control of the QD dimensions and spectrum. In this system wurtzite InAs0.85P0.15 QDs with well defined dimensions were grown in wurtzite InP nanowires. QDs with diameters, d, between 9 and 22 nm were produced. By growing the heterostructures using only the In stored in the Au seed particle and AsH3 from the gas phase a well controllable QD height of 0.35d was achieved for nanowires with diameters larger than the critical diameter for the Gibbs-Thomson effect, which was estimated to be 9 nm under the growth conditions used. The size dependence of the QD PL spectrum was studied. We found that the largest QDs were luminescing at the telecommunication wavelength of 1300 nm. With decreasing QD size the emission blue shifted and level splittings as well as the biexciton binding energy increased, illustrating strong confinement. A comparison of the experimental data to a strain dependent k•p model indicated that the band gap of InAs0.85P0.15 is 190 meV larger and the effective mass is a factor two larger in the wurtzite polytype compared to the zinc blende polytype. Radial heterostructures, core-shell nanowires, were grown using low temperature, kinetically limited growth for the core to suppress lateral growth and high temperature growth for the shell. Core-shell nanowires in two material systems, GaAs-GaInP and GaAs-AlInP, were investigated. It was found that the shell increases the emission efficiency of the core two to three orders of magnitude by moving the surface states away from the core. The effect of strain, caused by a lattice mismatched shell, on the PL spectrum was investigated and the core emission was tuned over a range of 240 meV. It was also found that planar defects such as twins and stacking faults in the nanowire propagate into the shell during growth and that new twins and stacking faults are created due to {1,1,1} facetted corrugations on the nanowire surface. A defect free shell therefore requires a defect free core without {1,1,1} micro-facets. Furthermore, phase segregation due to capillarity effects during shell growth was studied. Cross-sectional transmission electron microscopy showed that Al rich domains are formed in the <-2,1,1> directions in the shell. However, PL investigations indicated that In rich domains also exist, these are possibly associated with the corrugated nanowire surface. Two other applications for the core-shell nanowires were also investigated. A growth scheme was developed where the shell growth was extended in order to completely embed the nanowires which could then be cleaved to enable cross-sectional scanning tunneling microscopy of the interior of the nanowires. Such studies of axial heterostructures revealed an asymmetry in the sharpness of heterostructure interfaces as a consequence of large amounts of group III material being stored in the Au particle causing memory effects following material switching. In the second application a process for conversion of core-shell nanowires into tubular nanowires which can be used for cell injection was developed. It was shown that these tubular nanowires can be used to efficiently deliver cell membrane impermeable molecules to macrophages.