G-C Wang

Manipulating the column tilt angles of nanocolumnar films by glancing-angle deposition

Nanometre-size cobalt columns with tilt angle ranging from ~0? to ~40? and fixed density on Si(100) were fabricated by combining oblique-angle physical evaporation with controlled substrate motion. The column tilt angle can be controlled by the speed and phase of substrate rotation in addition to the angle of the incident vapour beam. A simple geometrical model is proposed to describe the relationship between the tilt angle of the cobalt columns and the rotational parameters (speed and phase), and is consistent with our experimental results.

Surface texture evolution of polycrystalline and nanostructured films: RHEED surface pole figure analysis

In this topical review, we outline the construction of a reflection high-energy electron diffraction (RHEED) surface pole figure from a polycrystalline film by recording multiple RHEED patterns as the substrate is rotated around the surface normal. Due to the short penetration depth of electrons, the constructed pole figure is a surface pole figure. It is in contrast to the conventional x-ray pole figure which gives the average texture information of the entire polycrystalline film. Examples of the surface pole figure construction processes of a fibre texture and a biaxial texture are illustrated using Ru vertical nanorods and Mg nanoblades, respectively. For a biaxially textured film, there often exists an in-plane morphological anisotropy. Then additional intensity normalization must be applied to compensate for the effects of anisotropic morphology on RHEED surface pole figure construction. Rich information on the texture evolution, such as the change in the tilt angle of the texture axis, has been obtained from the in situ study of oblique angle vapour deposition of Mg nanoblades using RHEED surface pole figures. Finally we make a comparison between the RHEED surface pole figure and the conventional x-ray pole figure techniques.

Novel growth mechanism of single crystalline Cu nanorods by electron beam irradiation

Single-crystal Cu nanorods have been fabricated on carbon films by manipulating electron beam irradiation on a Cu powder charge in a transmission electron microscope (TEM). By adjusting the intensity of the electron beam, we observed lively nucleation and growth of the Cu nanorods with the TEM. Individual Cu nanorods are straight with diameters ranging from 40 to 50 nm. The length of the rods is controlled by e-beam irradiation duration, and can be extended to . Selective area electron diffraction showed that the sidewalls of the Cu nanorods were associated with the (110) planes, while facets on the tips were associated with the (111) planes. We believe that the growth process is mainly controlled by high surface mobility of Cu atoms on the C surface, and surface diffusion of the Cu atoms from high surface energy planes to low surface energy planes of the Cu nanorods.

Size control of Cu nanorods through oxygen-mediated growth and low temperature sintering

Control of the size of Cu nanorods vapor-deposited at an oblique angle (approximately 85 degrees) by oxygen-mediated growth was investigated using scanning electron microscopy (SEM) and x-ray diffraction (XRD). It was observed that exposure of Cu nanorods to the oxygen ambient periodically resulted in a reduction in the diameter of the nanorods as well as an increase in the areal density of the nanorods. This oxygen-induced modification to the nanorod growth is attributed to the higher energy barrier for Cu adatom migration on the oxide surface at room temperature; this reduces the rod diameter. At a low annealing temperature of approximately 300 degrees C, the SEM images show that the nanorods have densified and formed a continuous film structure, which is consistent with the sintering phenomenon. The XRD and SEM analyses show that the coalescent/grain growth rate for Cu nanorods with smaller diameters is enhanced due to the size effect. This low temperature sintering characteristic of the Cu nanorod array has great potential for being utilized in wafer bonding for three-dimensional integration of devices.

Biaxially oriented CdTe films on glass substrate through nanostructured Ge/CaF2 buffer layers

Heteroepitaxial CdTe films were grown by metal organic chemical vapor deposition on glass substrates through nanostructured Ge/CaF2 buffer layers which were biaxially oriented. It allows us to explore the structural properties of multilayer biaxial semiconductor films which possess small angle grain boundaries and to test the principle of a solar cell made of such low-cost, low-growth-temperature semiconductor films. Through the x-ray diffraction and x-ray pole figure analysis, the heteroepitaxial relationships of the mutilayered films are determined as [111] in the out-of-plane direction and 〈10〉CdTe//〈10〉Ge// in the in-plane direction. The I–V curves measured from an ITO/CdS/CdTe/Ge/CaF2/glass solar cell test structure shows a power conversion efficiency of ~η = 1.26%, illustrating the initial success of such an approach. The observed non-ideal efficiency is believed to be due to a low shunt resistance and high series resistance as well as some residual large-angle grain boundary effects, leaving room for significant further improvement.