Fu-Ming Li

Electronic and mechanical properties of carbon nitride films prepared by laser ablation graphite under nitrogen ion beam bombardment

Carbon nitride films have been formed on Si(100) substrates by laser ablation of graphite under a low energy nitrogen ion beam bombardment. Data of Raman shift and x‐ray photoelectron spectroscopy indicate the existence of carbon‐nitrogen bonds in the films. Time‐of‐flight measurements suggest the existence of paracyanogen‐like materials, such as C4N4, in the films. High energy backscattering spectrometry has shown that the percentage of N content in the film is 41% or so. The x‐ray diffraction and transmission electron micrograph measurements have also been taken to characterize the crystal properties of the obtained films. Qualitative tests indicate the films of high Vickers hardness Hv, and of good adhesion to the silicon substrates.

An arc discharge nitrogen atom source

An intense nitrogen atom beam source of simple construction, with easy handling and maintenance was built and tested. Nitrogen atom beams with an intensity estimated to be 1019 atom/sr s and with an average kinetic energy of 0.8–2 eV in the forward direction were obtained. This novel atom source can be successfully ignited using pure nitrogen gas and operated stably during several hours of continuous performance. The temperature-rise effect of calorimetric sensors due to the bombardment of the N atom beam was used to analyze the intensities and kinetic energies of nitrogen atom beams. The emission spectra from the arc also show that a high concentration of atomic nitrogen was produced using this source. Experiments such as the nitrogen atom beams interacting with substrates to form a TiON film and a carbon nitride film indicate the high concentration of atomic nitrogen in the beam.

Effects of a native oxide layer on the conductive atomic force microscopy measurements of self-assembled Ge quantum dots

The electrical properties of self-assembled Ge quantum dots were investigated by using conductive atomic force microscopy (CAFM). It was found that the conductive properties of quantum dots were strongly affected by the native oxide layer formed on the quantum dots. With the existing oxide layer, the current–voltage curves of the quantum dots obviously depended on the applied normal forces, and all the curves could be well fitted by the Fowler–Nordheim tunnelling model by changing the oxide layer thicknesses. After the oxide layer was removed by HF etching, the current–voltage characteristics could be well explained by the Schottky emission model. But when the etched sample was exposed to air for more than 150 min, its current–voltage behaviour returned back to that before the etching. Despite the significant effects of the oxide layer on the current–voltage characteristics, the current distributions of individual quantum dots remained almost the same for different thicknesses of the oxide layer or different normal forces applied. Therefore, the effects of the oxide layer on the current distribution can be ignored, though the absolute current values were strongly affected. Our results thus provide important evidence for the reliability of CAFM measurements in ambient conditions.

Shape change of SiGe islands with initial Si capping

The morphologies of self-assembled Ge/Si(001) islands with initial Si capping at a temperature of 640 °C are investigated by atomic force microscopy. Before Si capping, the islands show a metastable dome shape with very good size uniformity. This dome shape changes to a pyramid shape with {103} facets at a Si capping thickness of 0.32 nm, and then changes to pyramid shapes with {104} and {105} facets at Si capping thicknesses of 0.42 and 0.64 nm, respectively. Noteworthy is that islands with one side retained their dome shape while the other three sides that changed to {103} facets are observed at a Si capping thickness of 0.18 nm. These observations indicate that island shape change with Si capping is a kinetic rather than thermodynamic process. The atomic processes associated with this island shape change are kinetically limited at a low temperature of 400 °C, and no significant change in size and shape of islands is observed when Si capping layers are deposited at this temperature.

Atomic composition profile change of SiGe islands during Si capping

The 6% Ge isocomposition profile change of individual SiGe islands during Si capping at 640 degrees C is investigated by atomic force microscopy combined with a selective etching procedure. The island shape transforms from a dome to a {103}-faceted pyramid at a Si capping thickness of 0.32 nm, followed by the decreasing of pyramid facet inclination with increasing Si capping layer thickness. The 6% Ge isocomposition profiles show that the island with more highly Si enriched at its one base corner before Si capping becomes to be more highly Si intermixed along pyramid base diagonals during Si capping. This Si enrichment evolution inside an island during Si capping can be attributed to the exchange of capped Si atoms that aggregated to the island by surface diffusion with Ge atoms from inside the island by both atomic surface segregation and interdiffusion rather than to the atomic interdiffusion at the interface between the island and the Si substrate. In addition, the observed Si enrichment along the island base diagonals is attempted to be explained on the basis of the elastic constant anisotropy of the Si and Ge materials in (001) plane. (c) 2006 American Institute of Physics.

Destruction of C60 films by boron ion bombardment

Abstract C 60 films are bombarded by 100 keV boron ion beams at doses ranging from 3 × 10 14 to 1 × 10 16 /cm 2 . The bombarded films are analyzed using Fourier transform infrared spectroscopy (FTIR), Raman spectra and X-ray diffraction (XRD) measurements. Most C 60 soccer-balls in the implanted region in the films are found to be broken at a dose over 1 × 10 15 /cm 2 , while at a dose less than 6 × 10 14 /cm 2 a few C 60 molecules remain undestroyed and maintain some crystal structure. The results of the analyses suggest a complete disintegration of a C 60 molecule under B + bombardment.

Production of intense atomic nitrogen beam used for doping and synthesis of nitride film

An arc‐heated source for producing an intense nitrogen atom beam with intensity of 1019 atoms/sr s and kinetic energies of 0.5–4 eV is presented. The arc discharge has been carried out in pure nitrogen gas and maintained stable in an arc operating pressure of 30–300 Torr. The beam kinetic energy changes with the arc pressure, and is insensitive to the arc current. Auger electron spectroscopy analysis showed that a TiNO layer with a thickness of about 100 A was formed on the smooth Ti wafer at room temperature with interaction of the atomic nitrogen beam.An arc‐heated source for producing an intense nitrogen atom beam with intensity of 1019 atoms/sr s and kinetic energies of 0.5–4 eV is presented. The arc discharge has been carried out in pure nitrogen gas and maintained stable in an arc operating pressure of 30–300 Torr. The beam kinetic energy changes with the arc pressure, and is insensitive to the arc current. Auger electron spectroscopy analysis showed that a TiNO layer with a thickness of about 100 A was formed on the smooth Ti wafer at room temperature with interaction of the atomic nitrogen beam.

Study of the growth of thin nitride films under low-energy nitrogen-ion bombardment

Bombardment of silicon surfaces by low-energy (300–1000 eV) nitrogen ions has been investigated as a potential process for growing ultrathin films at relatively low temperatures (<500°C). The thicknesses and chemical states of the obtained films are analysed using ellipsometry, X-ray Photoelectron Spectroscopy (XPS), and Auger Electron Spectroscopy (AES). All the analyses show that ultrathin (∽ 60 Å) silicon nitride films have been directly grown on silicon substrates. Detailed studies of the influence of different process parameters on the obtained films have been carried out. The thicknesses of the obtained films appear to increase with ion energy. The nitridation is found to be a rapid process which can be divided into two steps. The thicknesses are also observed to vary slightly with substrate temperature. The average active energy of nitridation rates is about 3.5 meV which indicates nonthermal process kinetics. For AES analysis, the films are found to be nitrogen-rich ones with the stoichiometric factor x≈1.7 larger than that of pure silicon nitride (x=1.33). In both AES and XPS studies, the chemical state of the silicon atoms resembles the existence of silicon oxynitride films of low oxygen concentration. The growth mechanism is also discussed briefly.

Boron-carbon-nitrogen films synthesized by laser ablation under ion beam bombardment

Abstract Boron-carbon-nitrogen thin films were deposited by laser ablation of boron carbide (B 4 C) under nitrogen ion beam bombardment. The as-deposited films were analyzed by X-ray photoelectron spectroscopy and Raman measurements. The results showed that carbon, nitrogen and boron species were chemically bonded to each other instead of simple mixtures. Detailed analyses showed that the ion beam bombardment had led to breaking of B-C bonds and formation of C-N and B-N bonds. An appropriate ion beam energy (probably less than 1 keV) was proposed for the purpose of the synthesis of high quality boron-carbon-nitrogen films.

Structural and electronic studies of C60 films deposited using ionized cluster beam deposition

Films of a new form of carbon allotrope, C60, also known as fullerenes are deposited on Si(111) substrates by the ionized cluster beam deposition technique under an accelerating field less than 100 V. Raman spectra and X-ray photoelectron spectroscopy measurements are carried out to analyse the electronic properties of the films, to indicate the existence of C60 soccer-balls in the films. The resistance of this C60 film deposited here to oxygen contamination is better than that deposited by molecular beam epitaxy. Binding energies of C 1s peaks for C60 and highly oriented pyrolytic graphite are 284.7 and 284.3 eV, respectively. X-ray theta -2 theta diffraction investigations show that C60 films deposited under Va=0 V have highly textured close-packed structure with X-ray diffraction assignment (110), while those deposited under Va=65 V turn out to be more polycrystalline. C60 soccer-balls are found to be broken into fragments as accelerating field exceeds about 400 V, indicated by the results of X-ray photoelectron spectroscopy, Raman spectra, X-ray diffraction, and ultraviolet visible absorption spectra.