Ann Persson

Growth and characterization of GaAs and InAs nano-whiskers and InAs/GaAs heterostructures

Semiconducting InAs and GaAs nano-whiskers have been grown using a chemical beam epitaxy approach in combination with size-selected catalytic Au aerosol particles. The characterization of InAs and GaAs whiskers shows high crystalline quality as seen by transmission electron microscopy. Gate-dependent transport measurements suggests a diffusive electronic transport mechanism. We have also combined these two material systems by growing a very abrupt heterostructure interface within the whiskers, allowing the growth of highly mismatched structures without misfit dislocations

Surface diffusion effects on growth of nanowires by chemical beam epitaxy

Surface processes play a large role in the growth of semiconductor nanowires by chemical beam epitaxy. In particular, for III-V nanowires the surface diffusion of group-III species is important to understand in order to control the nanowire growth. In this paper, we have grown InAs-based nanowires positioned by electron beam lithography and have investigated the dependence of the diffusion of In species on temperature, group-III and -V source pressure and group-V source combinations by measuring nanowire growth rate for different nanowire spacings. We present a model which relates the nanowire growth rate to the migration length of In species. The model is fitted to the experimental data for different growth conditions, using the migration length as fitting parameter. The results show that the migration length increases with decreasing temperature and increasing group-V/group-III source pressure ratio. This will most often lead to an increase in growth rate, but deviations will occur due to incomplete decomposition and changes in sticking coefficient for group-III species. The results also show that the introduction of phosphorous precursor for growth of InAs1−xPx nanowires decreases the migration length of the In species followed by a decrease in nanowire growth rate. (Less)

Measurements of the band gap of wurtzite InAs1−xPx nanowires using photocurrent spectroscopy

We report measurements of the band gap of InAs1−xPx nanowires having wurtzite crystal structure as a function of the composition for 0.14<x<0.48. The band gap is measured by photocurrent spectroscopy on single InAs nanowires with a centrally placed InAs1−xPx segment. The photocurrent measurements are performed at a temperature of 5 K. The data fit well with a quadratic dependence of the band gap on the composition. Using a bowing parameter of 0.2 eV the extracted values for the band gaps are 0.54 eV for InAs and 1.65 eV for InP. These values are larger than the corresponding zinc blende band gaps. We attribute this increase to the fact that the crystal structure is wurtzite rather than zinc blende.

Thermal Conductance of InAs Nanowire Composites

The ability to measure and understand heat flow in nanowire composites is crucial for applications ranging from high-speed electronics to thermoelectrics. Here we demonstrate the measurement of the thermal conductance of nanowire composites consisting of regular arrays of InAs nanowires embedded in PMMA using time-domain thermoreflectance (TDTR). On the basis of a proposed model for heat flow in the composite, we can, as a consistency check, extract the thermal conductivity Lambda of the InAs nanowires and find Lambda(NW) = 5.3 +/- 1.5 W m(-1) K(-1), in good agreement with theory and previous measurements of individual nanowires.

Measuring Temperature Gradients over Nanometer Length Scales.

When a quantum dot is subjected to a thermal gradient, the temperature of electrons entering the dot can be determined from the dots thermocurrent if the conductance spectrum and background temperature are known. We demonstrate this technique by measuring the temperature difference across a 15 nm quantum dot embedded in a nanowire. This technique can be used when the dots energy states are separated by many kT and will enable future quantitative investigations of electron-phonon interaction, nonlinear thermoelectric effects, and the efficiency of thermoelectric energy conversion in quantum dots.

The fabrication of dense and uniform InAs nanowire arrays

Nanowires are important candidates for use in future electronics, photonics and thermoelectrics applications. We focus here in particular on nanowires for use in thermoelectric power generation and present a method of fabricating dense uniform InAs nanowire arrays amenable to future incorporation of advanced heterostructures that could further increase the thermoelectric performance of these nanowires. In these applications it will be important to have the nanowires densely packed in order to give an appreciable amount of power output. Here we present the fabrication of such dense arrays, using metal-particle seeded growth and chemical beam epitaxy, where the metal particles are defined by electron beam lithography, metal evaporation and lift-off. We evaluate the potential of chemical beam epitaxy for the growth of dense, freestanding InAs nanowire arrays and describe the process that enabled us to achieve areal packing densities of up to 19% with a variation of only a few per cent in nanowire diameter and height. We close by discussing how even higher areal packing densities can be achieved.

Quantum-dot thermometry

We present a method for the measurement of a temperature differential across a single quantum dot that has transmission resonances that are separated in energy by much more than the thermal energy. We determine numerically that the method is accurate to within a few percent across a wide range of parameters. The proposed method measures the temperature of the electrons that enter the quantum dot and will be useful in experiments that aim to test theory which predicts that quantum dots are highly efficient thermoelectrics.

Nanoscale thermoelectric power generation

Nanoscale thermoelectric materials are at the center of current thermoelectric research because they offer higher efficiency than bulk thermoelectrics, which has been attributed to energy selectivity via a strongly modulated electron density of states and lattice thermal conductivity reduction. Our research aims to understand better the efficiency of thermally-induced electron transport using a quantum dot as a model system and to progress toward engineering high-efficiency nanoscale thermoelectric devices for power generation. We discuss here a device used to measure the electronic efficiency of nanoscale thermoelectric processes.