By studying how nanowires lying on a surface bend when pushed by an atomic force microscopy tip we are able to measure the friction between them and the substrate. Here, we show how the friction between InAs nanowires and an insulating silicon nitride layer varies when a dc voltage is applied to the tip during manipulation. The bias charges the capacitor formed by the wire and the grounded silicon back contact. Electrostatic forces increase the contact pressure and allow us to tune the friction between the wire and the silicon nitride surface. Using nanowires of about 40-70 nm diameter and a few microns in length we have applied biases in the range +12 to -12 V. A monotonic increase of the sliding friction with voltage was observed. This increase in friction with the normal force implies that the mesoscopic nanowire-surface system behaves like a macroscopic contact, despite the nanometer size of the contact in the direction of motion. The demonstrated bias-controlled friction has potential applications in MEMS/NEMS devices.

The electrical and structural properties of 111B-oriented InAs nanowires grown using metal-organic precursors have been studied. On the basis of electrical measurements it was found that the trends in carbon incorporation are similar to those observed in the layer growth, where an increased As/In precursor ratio and growth temperature result in a decrease in carbon-related impurities. Our results also show that the effect of non-intentional carbon doping is weaker in InAs nanowires compared to bulk, which may be explained by lower carbon incorporation in the nanowire core. We determine that differences in crystal quality, here quantified as the stacking fault density, are not the primary cause for variations in resistivity of the material studied. The effects of some n-dopant precursors (S, Se, Si, Sn) on InAs nanowire morphology, crystal structure and resistivity were also investigated. All precursors result in n-doped nanowires, but high precursor flows of Si and Sn also lead to enhanced radial overgrowth. Use of the Se precursor increases the stacking fault density in wurtzite nanowires, ultimately at high flows leading to a zinc blende crystal structure with strong overgrowth and very low resistivity.

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.

A controlled method of manipulation of nanowires was found using the tip of an Atomic Force Microscope (AFM). Manipulation is done in the 'Retrace Lift' mode, where feedback is turned off for the reverse scan and the tip follows a nominal path. The effective manipulation force during the reverse scan can be changed by varying an offset in the height of the tip over the surface. Using this method, we have studied InAs nanowires on different substrates. We have also investigated interactions between wires and with gold features patterned onto the substrates.

We report the detection of quantum confinement in single InAs-InP core-shell nanowires. The wires having an InAs core with ~25 nm diameter are characterized by emission spectra in which two peaks are identified under high excitation intensity conditions. The peaks are caused by emission from the ground and excited quantized levels, due to the quantum confinement in the plane perpendicular to the nanowire axis. We have identified in the emission spectra different energy contributions related to the wurtzite structure of the wires, the strain between the wurtzite core and shell, and the confinement energy of the InAs core. Calculations based on 6-band strain-dependent k.p theory allow the theoretical estimation of the confined energy states in such materials and we found these results to be in good agreement with those from the photoluminescence studies.

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.

We have grown Au seeded InAs nanowires using chemical beam epitaxy and report on the growth rate dependence on nanowire diameter. We find a maximum of the growth rate at a nanowire diameter of 25 nm, below which the growth rate decreases due to the Gibbs-Thomson effect. Above the maximum, the growth rate decreases with increasing diameter due to the effect of material diffusion to the growth point. A unified model, which accounts for both the Gibbs-Thomson effect and material diffusion, is presented and successfully compared to the experiments. From the comparison, we extract the diffusion length on the substrate surface and a critical diameter, below which nanowire growth ceases, and show that these physical parameters can be tuned by controlling the supersaturation.

We demonstrate a wrap-gated field effect transistor based on a matrix of vertically standing InAs nanowires (Jensen, et. al., 2004). A lower limit of the mobility, derived from the transconductance, is on the order of 3000 cm2/Vs. The narrow ~100 nm channels show excellent current saturation and a threshold of Vg = -0.15 V. The sub-threshold characteristics show a close to ideal slope of 62mV/decade over two orders of magnitude

Strained nanowires with varying InAs/InP core/shell thicknesses were grown using Chemical Beam Epitaxy. Microphotoluminescence spectroscopy, performed at low temperature, was then used to study the optical properties of single wires. Emission from the InAs core was observed and its dependence on the shell thickness / core diameter ratio was investigated. We found that it is possible to tune the emission energy towards 0.8 eV by controlling this ratio. We have compared the measured energies with calculated energies. Our findings are consistent with the wires having a hexagonal crystal structure.

We report tunneling spectroscopy measurements of the Zeeman spin splitting in InAs few-electron quantum dots. The dots are formed between two InP barriers in InAs nanowires with a wurtzite crystal structure grown by chemical beam epitaxy. The values of the electron g-factors of the first few electrons entering the dot are found to strongly depend on dot size and range from close to the InAs bulk value in large dots |g^*|=13 down to |g^*|=2.3 for the smallest dots. These findings are discussed in view of a simple model.