According to Fourier’s law, a temperature difference across a material results in a linear temperature profile and a thermal conductance that decreases inversely proportional to the system length. These are the hallmarks of diffusive heat flow. Here, we report heat flow in ultrathin (25 nm) GaP nanowires in the absence of a temperature gradient within the wire and find that the heat conductance is independent of wire length. These observations deviate from Fourier’s law and are direct proof of ballistic heat flow, persisting for wire lengths up to at least 15 μm at room temperature. When doubling the wire diameter, a remarkably sudden transition to diffusive heat flow is observed. The ballistic heat flow in the ultrathin wires can be modeled within Landauer’s formalism by ballistic phonons with an extraordinarily long mean free path.

In this work we present a method to quantitatively measure the optical absorption of single nanowires that can be applied over a wide range of temperatures and with a high enough sensitivity to enable the measurement of below-band-gap absorption (as well as the absorption of single molecules). The method is based on accurately measuring the heat flow coming from a nanowire when it is illuminated by a laser beam. We experimentally verify this method by measuring the absorption of both a zincblende and a wurtzite GaAs, a wurtzite GaP, and a superlattice Zn 3 P 2 nanowire. Furthermore, we find that the Zn 3 P 2 nanowires have the largest absorption of all these materials. We analyze the advantages and disadvantages of the method and study its range of applicability.

Reactive ion etching (RIE) of silicon and germanium in CF4-02 radio frequency plasmas has been investigated by means of optical emission spectroscopy, mass spectrometry and quasi in situ x-ray photoelectron spectroscopy. Above 10% 02, germanium etching is selective with respect to silicon. In the gas phase, two etching products are detected SiFd and GeF4. Their concentration is shown to be correlated with the amount of atomic fluorine and oxygen as measured by actinometry. In agreement with the plasma characterization, XPS analysis reveals that the S i etch rate appears to be controlled primarily by the thickness of the SiOxFy supelficial layer whereas the formation of GeO,Fy does not inhibit Ge etching. A simpie model Is used to express the surface reactivity in function of the experimental data. Application of this model to experimental CF4-02 plasma etching suggests that sulface composition controls silicon etching, whereas germanium etching depends mainly on the fluorine flux on the surface;

The band offsets occurring at the abrupt heterointerfaces of suitable material combinations offer a powerful design tool for high performance or even new kinds of devices. Because of a large variety of applications for metal− semiconductor heterostructures and the promise of low-dimensional systems to present exceptional device characteristics, nanowire heterostructures gained particular interest over the past decade. However, compared to those achieved by mature twodimensional processing techniques, quasi one-dimensional (1D) heterostructures often suffer from low interface and crystalline quality. For the GaAs−Au system, we demonstrate exemplarily a new approach to generate epitaxial and single crystalline metal−semiconductor nanowire heterostructures with atomically sharp interfaces using standard semiconductor processing techniques. Spatially resolved Raman measurements exclude any significant strain at the lattice mismatched metal− semiconductor heterojunction. On the basis of experimental results and simulation work, a novel self-assembled mechanism is demonstrated which yields one-step reconfiguration of a semiconductor−metal core−shell nanowire to a quasi 1D axially stacked heterostructure via flash lamp annealing. Transmission electron microscopy imaging and electrical characterization confirm the high interface quality resulting in the lowest Schottky barrier for the GaAs−Au system reported to date. Without limiting the generality, this novel approach will open up new opportunities in the syntheses of other metal−semiconductor nanowire heterostructures and thus facilitate the research of high-quality interfaces in metal−semiconductor nanocontacts.

Reactive ion etching of silicon and germanium in CF4 -O2 was investigated. Above 20% O2 germanium etching is selective with respect to silicon. In agreement with the evolution of the fluorine and oxygen concentration in the plasma and of the etch products formation rate, surface analysis reveals that the growth of a SiOxFy layer slows down the silicon etching whereas the formation of GeOxFy does not inhibit germanium etching. Using a simple kinetic model, the silicon and germanium reactivity and its dependency with respect to the plasma composition are expressed in function of the experimental data. Results suggest that surface composition controls silicon etching, whereas germanium etching depends only on the fluorine flux on the surface.