Papers by Author: Ke Wei Xu

Abstract: Micro-particles and nano-wires, small outgrowths were found to appear on upper film surface when metal thin film is confined between two Si3N4 layers deposited by magnetron sputtering and is annealed at an appropriate temperature. The stress evolution during this process is monitored by multi-beam optic stress sensor, and is qualitatively interpreted in terms of elastic and plastic deformation, as well as bulk diffusion. Additionally, the interface constraint effect among different layers is explored. Stress relaxation of nano-sandwiched thin films behaves in different stress modes. As a comparative study, Si3N4/Zn/Si3N4 sandwiches were prepared and studied by the same method. Experimental results show that the pertinent geometry is strongly dependent on material types and stress states of the substrates. Finally, an appropriate mode was suggested to interpret this phenomenon.

Abstract: Abstract. The phonon spectrum of zigzag h-BN nanoribbons with the edges passivated by hydrogen atoms under tensile strain along the axis direction were calculated by first-principle calculations. It is found that the uniaxial strain can lead to a narrow frequency range of lattice vibration modes. But it hardly affects the two highest frequency modes due to the vibration of B-H or N-H bonds. In particular, the strain usually promotes the softening of phonon modes. It means that more phonons should be activated at a given temperature. This may result in the changes of thermal properties, such as, heat capacity and vibration entropy.

Abstract: Si/C multilayer thin films were prepared by magnetron sputtering and post-annealing in N₂ atmosphere at 1100 for 1h. X-ray diffraction (XRD), Raman scattering and high-resolution transmission electron microscopy (HRTEM) were applied to study the microstructures of the thin films. For the case of Si/C modulation ratio smaller than 1,interlayer diffusion is evident, which promotes the formation of α-SiC during thermal annealing. If the modulation ratio is larger than 1, the Si sublayers are partially crystallized, and the thicker the Si sublayers are, the crystallinity increases. To be excited, brick-shaped nc-Si is directly observed by HRTEM. The brick-shaped nc-Si appears to be more regular near the Si (100) substrate but with twin defects. The results are instructive in the application of solar cells.

Abstract: Au/NiCr/Ta multi-layered metallic films were deposited on Al2O3 substrate by magnetron sputtering at different substrate temperature. The effect of substrate temperature on magnetron sputtering Au/NiCr/Ta films in crystal orientation, residual stress and resistivity was investigated. The all magnetron sputtering films were highly textured with dominant Au-(111) orientation or a mixture of Au-(111) and Au-(200) orientation. The residual stress in magnetron sputtering films at different substrate temperature was tensile stress with 155MPa-400MPa. A smallest resistivity of 3.6µΩ.cm was obtained for Au/NiCr/Ta multi-layered metallic films at substrate temperature 180°C. The experiment results reveal that the resistivity increased with the increase of the residual stress of metallic films.

Abstract: The Cu-line patterns with different linewidth were deposited by radio-frequency sputtering and defined by photolithography lift-off process. The residual stress was evaluated with X-ray diffraction technique and the results show that the Cu-line patterns are in a biaxial stress state and the stress values have a great dependence on linewidth. Further analysis reveals that the intrinsic stress has a main effect on changes of residual stress; however, the thermal stress plays a key role on anisotropic contribution of residual stress in directions along and across the Cu line. The variation of intrinsic stress correlates well with crystal orientation of the Cu line, and the linewidth seems to be the most crucial parameter for evolution of both texture and stress.

Abstract: The deformation process of sputtered tungsten films were investigated according to the morphological characteristics and residual stress analysis. Results show that there are four characteristics depending on the substrates and stress state. For thin films on polyimide, the localized plastic deformation is mediated by the alignment of grain boundaries in the case of tension or line bulges in the case of compression. It results from both in-plane and out-of-plane grain rotation. For thin films on silicon substrate, wedge cracks in the case of tension or regular hillocks in the case of compression become the typical morphology. From this point of view, the deformation behaviors of films depended on the substrate constraint and residual stress states.

Abstract: An in-situ investigation was performed on the stress relaxation of sandwiched Si3N4/Al/Si3N4 thin films by using multi-beam optical stress sensor (MOSS), a developed technique for substrate curvature measurement. Furthermore, the microstructures of the thin films were characterized by several analyzing techniques, such as X-ray Photoelectron Spectroscopy (XPS), Field Emission Scanning Electron Microscope (FE-SEM) and X-ray energy dispersive spectroscopy (EDS). The results indicated sharp rise and drop of the residual stress due to the cracks of Si3N4 surface layer or the separation of Al particles during annealing process. An appropriate model was suggested to interpret this phenomenon.

Abstract: Series Ti-Si-C-N coatings with different carbon were deposited on a high-speed-steel substrate by means of pulsed direct current plasma enhanced chemical vapor deposition (PECVD). The structure and hardness were subsequently investigated using X-ray diffraction, transmission electron microscopy and microindentation measurements. It was found that the carbon content had a profound effect on the microstructure and hardness of the Ti-Si-C-N coatings. The results indicated that these coatings consisted of the dominant nanocrystalline Ti(C, N), accompanying with a small amount of silicide (TiSi2, Si3N4 or SiC) dispersed within the dominant phase. An increase in C content resulted in the decrease in the grain size and the increase in fcc-structure lattice parameter. A maximum hardness of 48 GPa was achieved for a two-phase {Ti (C N) + SiC} structure at the C content of 38.6 at.%.

Abstract: The crystal orientation, surface morphology, surface roughness and scratch properties of Au/NiCr/Ta multi-layered metallic films was examined by X-ray diffraction (XRD), atomic force microscopy (AFM) and a scratch test method, respectively. It was clarified that the surface morphology and surface roughness depend on the substrate temperature. The surface roughness decreases from 4.259nm to 2.935nm when substrate temperature changed from 100°C to 180°C, and then increases when substrate temperature above 180°C. The XRD revealed that there are only Au diffraction peaks with highly textured having a Au-(111) or a mixture of Au-(111) and Au-(200) orientation. The micro-scratch test reveals that both modes can be used for conventionally critical load determination, but the friction mode can additionally reflect the changes at different metallic film layers, the critical characteristic load was not sensitive to substrate temperature.

Abstract: The microstructure and surface micromorphology of ZnSe single crystals grown directly from zinc and selenium have been investigated using rotation orientation x-ray diffraction (RO-XRD), atomic force microscope (AFM) and field emission scanning electron microscope (FE-SEM). The ZnSe samples exhibit only the surface leaning to (111) singular face by the angle of 3.13°, which is the buildup of two-dimensional dendritic crystal layers. Numerous nuclei and cavities distribute unevenly across the crystal surface, governing the formation of growth layer, while the dendritic crystal layers develop rapidly by margining the smaller nuclei. The formation of these microstructure and micromorphology on the surface of ZnSe crystals depends on the surface supersaturation and the growth parameters.