D.J. Yang

Pulsed photothermal reflectance measurement of the thermal conductivity of sputtered aluminum nitride thin films

We report on measurements of the thermal conductivity of reactively sputtered aluminum nitride (AlN) thin films with different thickness, ranging from 100nm to 1μm, on silicon substrates. The measurements were made at room temperature using the pulsed photothermal reflectance technique. The thermal conductivities of the sample are found to be significantly lower than the single-crystal bulk AlN and increase with an increasing thickness. The thermal resistance at the interface between the AlN film and the silicon substrate is found to be about 7–8×10−8m2K∕W.

Electron field emission enhancement effects of nano-diamond films

Abstract In this paper, electron field emission properties of nano-diamond films, which were prepared using either CH 4 /H 2 /N 2 or CH 4 /Ar microwave plasma enhanced chemical vapor deposition, were studied. X-ray photoelectron spectroscopy detection indicates that nitrogen was incorporated into the nano-diamond film grown in CH 4 /H 2 /N 2 mixture. This nano-diamond film shows a very low threshold electric field of 2.2 V/μm and a high emission current density of 720 μA/cm 2 at applied field of 6.4 V/μm. Nitrogen incorporation, high grain boundary density and sp 2 -bonded non-diamond components in the films are believed to be responsible for the electron emission enhancement.

Electron field emission from carbon nanotubes and undoped nano- diamond

Abstract In this paper, the electron field emission characteristics of carbon nanotubes (CNTs) and undoped nano-diamond film have been investigated and compared by measuring the electron field emission I–V curve, the emission site density and the emission stability. The CNTs and undoped nano-diamond film were grown in the same microwave plasma enhanced chemical vapor deposition reactor using CH4/H2 and CH4/Ar mixtures, respectively. The morphologies and structures of CNTs and nano-diamond were characterized using field emission scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The field emission results show that CNTs exhibit a lower threshold electric field of 1.3 V/μm and a much higher emission current density of 1620 μA/cm2 at 2.72 V/μm compared to nano-diamond, which exhibits 3.8 V/μm and 540 μA/cm2 at 6.65 V/μm, respectively. However, CNTs have slightly lower emission stability than the nano-diamond.

Electron field emission properties of nano-, submicro- and micro-diamond films

Chemical vapour deposition (CVD) diamond is known for its excellent chemical, mechanical and thermal properties, as well as low or negative electron affinity. These unique characteristics make CVD diamond a good material as an electron emitter. In this study, electron field emission has been measured from nano-, submicro- and micro-diamond films deposited by microwave plasma enhanced chemical vapour deposition. It has been found that nano-diamond exhibits threshold electric fields as low as 1.5 V/μm, which is much lower than those of submicro- and micro-diamond (10 V/μm and 25 V/μm, respectively).

Effects of oxygen and nitrogen on carbon nanotube growth using a microwave plasma chemical vapor deposition technique

In this paper, we studied the effects of oxygen (O2) and nitrogen (N2) on the growth of carbon nanotubes (CNTs) prepared using microwave plasma-enhanced chemical vapor deposition (MPECVD). CNTs were prepared with 20–100-nm-sized Ni catalyst grains under three different groups of plasma conditions. Scanning electron microscopy (SEM) results show that the quality of CNTs is improved and number density greatly increased with addition of N2 and O2 in the process. The morphology and Ni composition strongly suggest that the grain size and composition of the Ni catalyst can be modified by the N2 and O2 plasma pretreatments. A nitride-enhanced growth mechanism is also discussed.

Preparation and electron field emission properties of nano-diamond films

Abstract Nano-diamond films with a mean grain size of 20 nm have been successfully prepared on silicon substrate by microwave plasma chemical vapor deposition (MPCVD) technique using gas mixture of nitrogen–methane–hydrogen. The structure and surface morphology of the films are examined using X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Field emission results show that good crystal quality nano-diamond film has a threshold electric field of 3 V/μm, much lower than the typical threshold electric field of diamond films grown using conventional gas mixture of hydrogen–methane, indicating that nano-diamond film is a good candidate for electron emitter material.

Erosion resistance of polycrystalline diamond films to atomic oxygen

Abstract Polycrystalline diamond films deposited using hot filament chemical vapor deposition (CVD) technique have been investigated in atomic oxygen simulated as low earth orbit environment to examine their erosion resistance properties. After exposure to the atomic oxygen beam with a flux of 2.6×1016 atoms/cm2 s, the diamond films only show a small mass loss. The reaction efficiency is estimated to be between 6.35×10−26 and 8.28×10−26 cm3/atom. Oxidation mechanism is investigated through the reaction temperature influence on the reaction rate. We suggest that atomic oxygen reacts with diamond surface and forms ether (C–O–C) and carbonyl (>CO) configurations besides eroding the surface.

CVD diamond nucleation enhanced by ultrasonic pretreatment using diamond and mixture of diamond and TaC powders

Abstract Effects of ultrasonic pretreatment on chemical vapor deposition (CVD) diamond nucleation on Si substrates were systematically studied. Pure 1.5–40 μm-diamond powder and mixtures of 1.5–5 μm-diamond as well as 5–20 μm-Tantalum Carbide (TaC) powder were used in ultrasonic pretreatment. The root-mean-square (Rms) surface roughness of the pretreated substrates, residual diamond and TaC powders left on the substrates were examined using atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction (XRD), respectively. It was observed that the Rms surface roughness increases with increasing diamond or TaC powder size, and there is some diamond or TaC powder left on the substrates after ultrasonic pretreatment. Diamond films were deposited using microwave plasma chemical vapor deposition (MPCVD) technique and characterized by field emission scanning electron microscopy (FE-SEM). It was found that CVD diamond nucleation density strongly depends on particle size of diamond or TaC powder used, the nucleation density increases with increasing diamond or TaC powder size. A mixture of diamond and TaC powders enhances CVD diamond nucleation much more significantly than that of pure diamond powder. A mixture of 1.5 μm-diamond and 20 μm-TaC powders has an equivalent nucleation enhancement efficiency, which could be caused by pure 40 μm-diamond powder.

Reaction of diamond thin films with atomic oxygen simulated as low-earth-orbit environment

Diamond thin films deposited using a hot filament chemical vapor deposition (CVD) technique have been explored to atomic oxygen simulated as low-earth-orbit environment to examine their erosion-resistance properties. X-ray photoelectron spectroscopy results suggest that the atomic oxygen reacts with the diamond surface and forms ether (C–O–C) and carbonyl (>C=O) configurations. After a 3 h exposure to the atomic oxygen beam with a flux of 2.6×1016 atoms/cm2s, the diamond films only show a small mass loss with a reaction efficiency of 8.28×10−26 cm3/atom. Such a small reaction efficiency suggests that the CVD diamond is highly erosion resistive to atomic oxygen and it can be used as a passivation material in space.

Diamond-based alpha particle detectors with coplanar geometry

Abstract In this paper, we report the results concerning significant improvement of charge collection efficiency for chemical vapor deposition (CVD) diamond-based alpha particle detectors with coplanar geometry by reducing the inter-electrode spacing from 100 to 25 μm. CVD diamond films used were prepared employing hot-filament CVD technique. Electrical contacts of the detectors were only patterned on the growth surface of the polished free-standing 300 μm thick diamond film using lift-off technology. The performance of the fabricated diamond-based detectors was tested using a 241 Am alpha particle source with a characteristic energy of 5.5 MeV. Average charge collection efficiency of 52% for detector with narrow inter-electrode spacing of 25 μm and of 32% for detector with wide inter-electrode spacing of 100 μm was obtained, indicating that the detector with narrow inter-electrode has a higher charge collection efficiency than the detector with wide inter-electrode spacing. This should be attributed to fewer boundary trap and recombination centers between electrodes in detector with narrow inter-electrode spacing than that with wide inter-electrode spacing.