We experimentally investigate the room temperature ferromagnetism (RTFM) observed in Co implanted anatase TiO2 thin films. TiO2 thin films were prepared by RF sputtering onto thermally grown oxide layers on Si substrates. Cobalt implantation was performed using a metal vapor vacuum arc ion source at an implant dose of 4 x1016 cm-2. Post annealing was performed in a vacuum chamber at various temperatures up to 700°C for 2 or 4 hours. Characterization of these films as-implanted and after thermal annealing under various conditions was performed using Rutherford backscattering spectrometry, transmission electron microscopy, x-ray diffractometry and vibrating sample magnetometry. Clear RTFM properties were observed in all samples. The MS value showed a general increase trend with increasing annealing time with higher values at higher annealing temperatures. Quite a number of samples showed Ms values exceeding the bulk Co value of 1.69μB/Co atom after annealing. The maximum MS value observed is about 3.16μB/Co atom for the sample annealed at 700°C for 4 hours. Such high MS values indicate that the RTFM must not come from Co clusters alone. Possible origins of the RTFM properties are discussed in conjunction with the structural properties.
A WC-SiC nanocomposite thin layer structure consisting of nano-grains of WC embedded in SiC has been fabricated on an n-type Si substrate by ion beam synthesis (IBS) using a metal vapor vacuum arc ion source. A SiC layer was first formed by high dose carbon implantation into the silicon substrate. Subsequent W implantation was preformed to form the WC-SiC nanocomposite structure that can be achieved under appropriate implantation and annealing conditions. Characterization of the implanted samples was performed using atomic force microscopy (AFM), conducting AFM, x-ray photoelectron spectroscopy, and transmission electron microscopy. Excellent field emission properties with an ultra-low turn-on field of 0.35 V/μm from such a nanocomposites structure have been achieved. While it has been demonstrated that the surface morphology effect and the local electrical inhomogeneity are two field enhancement mechanisms for IBS SiC/Si structures, they are not sufficient to account for the excellent emission properties of these WC-SiC nanocomposite layers. To explain the excellent field emission properties from these structures, a proximity field enhancement effect between closely spaced conducting grains in a dielectric medium and an internal quantum tunneling mechanism are proposed.
Semiconducting FeSi2 has attracted considerable amount of research interest in the past decade for its potential applications as a silicon-based light emitting material. In this work, FeSi2 precipitates were formed in Si by iron implantation into silicon using a metal vapor vacuum arc ion source. The structures and light emitting properties of these ion-beam-synthesized FeSi2 precipitates were studied in details using various characterization techniques, including transmission electron microscopy (TEM) and photoluminescence measurements. It was found that the implantation temperature played an important role on the dislocation loop formation and hence the FeSi2 phase formation during the subsequent thermal annealing. Photoluminescence (PL) spectra were measured as a function of temperature from 80 to 300 K. Combining the TEM and PL results, the origins of the PL could be distinguished to be either from the defect-related emission of Si or from the FeSi2 precipitates. The FeSi2 precipitates were found to be highly-strained or relaxed depending on the implantation and annealing conditions. The band gap energy of the relaxed samples was determined to be about 11 meV higher than that of the highly strained samples. Simple metal-oxide-semiconductor (MOS) diode structures were fabricated to study the electroluminescence (EL) properties from this FeSi2/Si system. Preliminary results showed that clear EL signals were obtained even at room temperature for samples prepared at appropriate conditions. There are significant differences between the EL and PL spectra and the mechanisms of the EL emission has yet to be further investigated.
Diamond-like-carbon (DLC) films have an amorphous structure comprising a sophistical carbon matrix and have attracted a great deal of scientific interest. Metal-doped DLC (Me-DLC) can possess superior properties as metal nanoclusters or nanocrystalline metallic carbides can be embedded in the carbon network. Therefore, Me-DLC exhibits good adhesion to the substrate, high hardness, low friction coefficient and high magnitude of conductivity. In this work, a metal cathodic vacuum arc and acetylene dual plasma are synchronized to produce Me-DLC. We systematically fabricate the Me-DLC films by varying the acetylene flow rate and substrate bias voltage. Our results acquired by Rutherford backscattering spectrometry (RBS) show that the film thickness and metal content can be controlled by the process windows. Four point probe measurements illustrate a decrease in the resistivity with increasing metal content whereas x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) results show the formation of carbide phases in the carbon matrix. To evaluate the thermal stability of the thin film, both undoped DLC and Me-DLC films are annealed at a series of temperature in argon ambient. Raman scattering results reveal that the Me-DLC films can tolerate a high annealing temperature without serious graphitization. It is believed that metals incorporation retards the restructuring of the carbon matrix during the annealing processes.
In this work, TiO2 thin films were prepared by RF sputtering onto thermally grown oxide layers on Si substrates. Cobalt and iron implantation into the TiO2 films was performed using a metal vapor vacuum arc ion source. The as-implanted and annealed films were characterized using Rutherford backscattering spectrometry, transmission electron microscopy, x-ray diffractometry, x-ray photoelectron spectroscopy, spectroscopic ellipsometry, and vibrating sample magnetometry. The dependence of the magnetic properties on the implantation and annealing conditions were studied in detail. Clear room temperature ferromagnetic properties (RT FM) were observed. The saturation magnetization (Ms) values per implanted Co or Fe atom exhibit an oscillatory dependence on the implantation dose. The maximum Ms in one Co implanted samples was determined to be 2.3 μB/Co, exceeding the bulk Co value. The possible origins of the RT FM properties are discussed.
In this paper, the authors present a new low-level birefringence detection (LLBD) system using photoelastic modulation technology. The LLBD system, operating at wavelength 1152 nm, determines point-by-point the magnitude and orientation (angle of fast axis) of birefringence therefore stress states in the sample. The principle of this LLBD system was described and experimentally verified. The resultant instrument shows high sensitivity and good repeatability for measurement. A sensitivity of magnitude of birefringence about 0.03° (or 0.096 nm at 1152 nm) was established for measurement. Several samples with different levels of birefringence were studied using LLBD system.
Diamond-like carbon film (DLC) is well known for its optical, electrical and mechanical properties. However, its properties can vary widely from graphite to diamond-like, strongly depending on the film preparation. So the determination of sp3/sp2 ratios of the DLC films is very important. In this work, a series of DLC films have been prepared on silicon substrates by Filtered Arc Deposition (FAD) and an optical method - spectroscopic ellipsometry (SE) has been applied to study these samples in the visible wavelength range. In the analysis of SE spectra, a single DLC layer on Si substrate model has been used and the DLC layer simply consists of sp3, sp2 and void constituents. The Bruggeman effective medium approximation has been applied to calculate the dielectric response of these three components. From the interpretation of SE spectra, the film thicknesses and the volume fractions of sp3, sp2 and void have been derived. SE results show: the fitted film thicknesses are consistent with those of Rutherford Backscattering spectroscopy; the fractions of sp3 correlate wit results of electron energy loss spectroscopy studies; for all these samples, the concentrations of void are very small which implies the compact DLC layers. It shows that SE is a very useful and promising optical method to determine the sp3/sp2 ratios of the DLC films.
Cobalt ion implantation of silicon substrate by Metal Vapor Vacuum Arc ion source has been studied by spectroscopic ellipsometry (SE) over the wavelength range of 400-2000nm. A series of annealed samples with different substrate temperatures during implantation have been investigated. From the interpretation of the ellipsometric data, the depth profiles of the samples can be derived. And the derived layer thicknesses are found to be consistent with the results of RBS and XTEM studies. At the same time, the optical properties of the buried CoSi2 layers in Si can be derived. To represent the optical properties of these buried CoSi2 layers, a three-term model is used with two classical Lorentz oscillators representing the electronic transitions, and one Drude term modeling the effect of free electrons. The derived Drude parameters can be used to calculate the optical resistivities of these buried CoSi2 layers. And the calculated optical resistivities are consistent with the dc electrical resistivities deduced from electrical measurements.
SiC material is of intense interest because of its unique features. Two samples of SiC/Si heterostructures were prepared by ion beam synthesis (IBS) with metal vapor vacuum arc ion source at an energy of 65keV and a dose of 1.0 by 1018 cm-2. After implantation, one sample was annealed in nitrogen ambient at 1250 degrees C for 10 hours. Spectroscopic ellipsometry (SE) was then performed to study these two samples with the wavelength range of 400-2000nm at a fixed angle of incidence. For the interpretation of SE spectra, the material and optical properties of the annealed sample had been derived. SE result confirmed the formation of a thick buried SiC layer for the annealed sample, but the optical property of this buried layer was found to be different from that of bulk SiC material even if a long time annealing at high temperature was performed. These results were further substantiated by other techniques, including FTIR, XPS and RBS. For the as-implanted sample, different models had been tried but they were not reasonable enough to fit the SE spectra well, the point was how to simulate the optical response of the free carbon atoms distributed in the sample. Further investigation is needed.
ABCD transfer matrix theory was utilized to study the stability of a solid state laser with a Brewster-angle-cut active medium in a Z-fold cavity consists of two spherical and two flat mirrors. Analytic solutions of stability regions for CW operation have been obtained. These solutions allow easy analysis of the behavior of the stability regions when the resonator parameters are varied.
This paper has studied the interface trap distributions and characteristics of novel thin rapid thermal nitrided SiOxNy film (RTNF) used for VLSI by the technique of avalanche hot-electron injection and the measurements of high frequency C-V and quasistatic C-V characteristics. The research results gave that the distribution relationship of midgap interface trap density of the thin RTNF with nitridation time presented 'turnaround effect'. The different kinds of electronic traps, existing in the RTNF and having disparity densities, have been observed. Results indicated that two kinds of fast interface traps, which have different properties, were generated in the Si/SiOxNy interface during avalanche hot-electron injection. The distribution on the density of two interface traps with forbidden band position is provided. A weakly 'N' type distribution relationship of the midgap interface trap density with the avalanche injection dose is also given. The theoretical analyses and discussions of these results are also made in this paper.
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