Aarnoud Laurens Roest

Tunable supercurrent through semiconductor nanowires

Nanoscale superconductor/semiconductor hybrid devices are assembled from indium arsenide semiconductor nanowires individually contacted by aluminum-based superconductor electrodes. Below 1 kelvin, the high transparency of the contacts gives rise to proximity-induced superconductivity. The nanowires form superconducting weak links operating as mesoscopic Josephson junctions with electrically tunable coupling. The supercurrent can be switched on/off by a gate voltage acting on the electron density in the nanowire. A variation in gate voltage induces universal fluctuations in the normal-state conductance, which are clearly correlated to critical current fluctuations. The alternating-current Josephson effect gives rise to Shapiro steps in the voltage-current characteristic under microwave irradiation.

Position-controlled epitaxial III-V nanowires on silicon

We show the epitaxial integration of III–V semiconductor nanowires with silicon technology. The wires are grown by the VLS mechanism with laser ablation as well as metal–organic vapour phase epitaxy. The hetero-epitaxial growth of the III– Vn anowires on silicon was confirmed with x-ray diffraction pole figures and cross-sectional transmission electron microscopy. We show preliminary results of two-terminal electrical measurements of III–V nanowires grown on silicon. E-beam lithography was used to predefine the position of the nanowires. (Some figures in this article are in colour only in the electronic version)

Large redshift in photoluminescence of p-doped InP nanowires induced by Fermi-level pinning

We have studied the effect of impurity doping on the optical properties of indium phosphide (InP) nanowires. Photoluminescence measurements have been performed on individual nanowires at low temperatures (5–70 K) and at low excitation intensities (0.5–10W∕cm2). We show that the observed redshift (200 meV) and the linewidth (70 meV) of the emission of p-type InP wires are a result of a built-in electric field in the nanowires. This bandbending is induced by Fermi-level pinning at the nanowire surface. Upon increasing the excitation intensity, the typical emission from these p-InP wires blueshifts with 70meV∕decade, due to a reduction of the bandbending induced by an increase in the carrier concentration. For intrinsic and n-type nanowires, we found several impurity-related emission lines.

Quantum Interference Effects in InAs Semiconductor Nanowires

We report quantum interference effects in InAs semiconductor nanowires strongly coupled to superconducting electrodes. In the normal state, universal conductance fluctuations are investigated as a function of magnetic field, temperature, bias and gate voltage. The results are found to be in good agreement with theoretical predictions for weakly disordered one-dimensional conductors. In the superconducting state, the fluctuation amplitude is enhanced by a factor up to ~ 1.6, which is attributed to a doubling of charge transport via Andreev reflection. At a temperature of 4.2 K, well above the Thouless temperature, conductance fluctuations are almost entirely suppressed, and the nanowire conductance exhibits anomalous quantization in steps of e^{2}/h.

Cross-sectional studies of epitaxial growth of InP and GaP nanowires on Si and Ge

Heteroepitaxial growth of III-V wires on Si and Ge was performed using either MOVPE or laser ablation. The epitaxial relation between wire and substrate was studied using cross-sectional TEM and X-ray diffraction. For both GaP wires on Si and InP wires on Ge perfect epitaxy was observed. In the initial stage of the growth process, the gold particles partly sink into the Si substrate. During subsequent wire growth, the Si that had been dissolved in the gold particle is excreted above the Si surface. Due to the interaction of the gold particle with the substrate, the final substrate/wire interface displays a roughness on the nanometer scale.