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Moderate-pressure microwave plasmas are used to prepare nitrogen-incorporated carbon nanowalls (CNWs) by chemical vapor deposition using acetylene and methane as precursor gases. The growth temperature range for acetylene is shown to be totally lower than that (>1000 °C) for methane, which is attributed to the difference in degree of hydrogenation of radical species in the two plasmas. The structural order and cluster size of sp2 carbon phase in CNWs characterized by the Raman spectroscopy increase initially and, then, decrease with temperature in each temperature range, showing the same trend as the inverse of the sheet resistance of CNWs. The results indicate that the carrier transport in CNWs depends exclusively on the microstructure of sp2 carbon phase, despite a large difference in the growth temperature range depending on the precursor gas.

The Asia–Pacific International Symposium on the Basics and Applications of Plasma Technology (APSPT) is held in Taiwan or Japan every two to four years since 1997. It is an important and great platform for scientists all around the world sharing research findings in the field of plasma science and technology. The 12th APSPT was held on December 9–11, 2021, at the National Taiwan University of Science and Technology (NTUST) in Taipei, Taiwan. There are 260 papers contributed by participants from 13 countries.

Vertically aligned nanographite structures, the so-called carbon nanowalls (CNWs), are decorated with ultrananocrystalline diamond particles by an electrostatic self-assembly seeding technique, followed by short-term growth in plasma chemical vapor deposition, to enhance field emission efficiency and stability. A nanodiamond suspension diluted with a dispersion medium with high wettability on CNWs enables seeding of diamond nanograins consisting of nanoparticles of 3–5 nm in diameter on CNWs with high uniformity and minimal aggregation and control of their number density. The field emission turn-on field depends upon the density of diamond nanograins and decreases from 3.0 V μm–1for bare CNWs to 1.8 V μm–1for diamond-decorated CNWs together with about an order of magnitude increase in current density. Finite element modeling indicates that only a part of decorating diamond located at the top of nanowalls actually contributes to field amplification and emission. The diamond-decorated CNWs show also higher emission stability with much larger time constants of current degradation than the bare CNWs for long-term duration. The enhanced emission efficiency is explained by larger field amplification rather than lowering of the tunneling barrier, while the enhanced emission stability is attributed to the higher robustness of diamond.

The electrical characteristics of metal contacts to vertically-aligned nanographite structures, the so-called carbon nanowalls (CNWs), are examined using nickel as the electrode material. The total resistance between a pair of electrodes on CNWs is regarded as the sum of the serial resistance of CNWs and two metal-CNWs contacts to measure the contact resistance and specific contact resistivity by the transmission line method and transfer line method, respectively. The contact resistance and resistivity are around 2.8 Ω and 0.1 Ω cm2, respectively, at room temperature and decrease gently to 2.4 Ω and 0.06 Ω cm2, respectively, when the temperature is increased up to 300 °C. The apparent activation energy corresponding to the conduction barrier at the contact is in the range of 10−3eV to 10−2eV, which is comparable with that for the sheet resistance of CNWs. The contact resistance occupies a large portion of the total resistance between a pair of electrodes even at high temperatures, thus affecting electrical signal measurement in electronic and electrochemical device applications.

Cubic and hexagonal boron nitride (cBN and hBN) heterojunctions to n-type Si are fabricated under low-energy ion bombardment by inductively coupled plasma-enhanced chemical vapor deposition using the chemistry of fluorine. The sp2-bonded BN/Si heterojunction shows no rectification, while the cBN/sp2BN/Si heterojunction has rectification properties analogue to typical p–n junction diodes despite a large thickness (∼130 nm) of the sp2BN interlayer. The current–voltage characteristics at temperatures up to 573 K are governed by thermal excitation of carriers, and mostly described with the ideal diode equation and the Frenkel–Poole emission model at low and high bias voltages, respectively. The rectification in the cBN/sp2BN/Si heterojunction is caused by a bias-dependent change in the barrier height for holes arising from stronger p-type conduction in the cBN layer and enhanced with the thick sp2BN interlayer for impeding the reverse current flow at defect levels mainly associated with grain boundaries.

Thick cubic boron nitride (cBN) films in micrometer-scale are deposited on tungsten carbide–cobalt (WC-Co) substrates without adhesion interlayers by inductively coupled plasma-enhanced chemical vapor deposition (ICP-CVD) using the chemistry of fluorine. The residual film stress is reduced because of very low ion-impact energies (a few eV to ∼25 eV) controlled by the plasma sheath potential. Two types of substrate pretreatment are used successively; the removal of surface Co binder using an acid solution suppresses the catalytic effect of Co and triggers cBN formation, and the surface roughening using mechanical scratching and hydrogen plasma etching increases both the in-depth cBN fraction and deposition rate. The substrate surface condition is evaluated by the wettability of the probe liquids with different polarities and quantified by the apparent surface free energy calculated from the contact angle. The surface roughening enhances the compatibility in energy between the cBN and substrate, which are bridged by the interfacial sp2-bonded hexagonal BN buffer layer, and then, the cBN overlayer is nucleated and evolved easier.

Nitrogen-incorporated, n-type nanocrystalline diamond (NCD) films are deposited on p-type Si(001) and 4H-SiC(0001) substrates by moderate-pressure, microwave plasma-enhanced chemical vapor deposition using a mixture of 1%CH4-30%N2-69%Ar. X-ray diffraction and visible Raman spectroscopy reveal that the structure of the NCD films is identical independent of the substrate materials, such that diamond nanoparticles with apparent crystal sizes of 5-8 nm are embedded in amorphous sp2 carbon matrix. For p-Si/n-NCD heterojunctions in a diode configuration, the rectifying behavior in current-voltage curves depends upon the substrate temperature for film deposition, and the rectification ratio reaches a maximum of about 300 when the film is deposited at 830 °C. For p-4H-SiC/n-NCD heterojunctions, the rectification ratio increases greatly to about 10000 when the film is deposited at 830 °C due exclusively to suppression of the reverse leakage current.

Nanocrystalline diamond (NCD)/3C-SiC layered films are deposited on Si substrates by using a moderate-pressure microwave plasma apparatus. The epitaxial 3C-SiC thin layer is grown on p-type Si(001) above 1200°C in 2%CH4/98%H2 by plasma-assisted carbonization and the n-type NCD overlayer is subsequently grown at 830°C in 1%CH4/30%N2/69%Ar by plasma-enhanced chemical vapor deposition (CVD). According to cross sectional TEM observations, the initial thickness of the 3C-SiC layer (~20 nm) is reduced to 10 nm or less in the beginning of the NCD growth due most likely to etching. A rectifying current-voltage characteristic is obtained for an n-type NCD/epitaxial 3C-SiC/p-type Si(001) junction in a diode configuration.

Fine-grained metals with sizes as small as 10 nm are deposited on nitrogen-incorporated carbon nanowalls to enhance field emission. The amount of metal coating is varied using the nominal thickness to control morphology and coverage of metal nanoparticles. The emission turn-on field decreases down to around 1.6 V um-1 and 1.8 V um-1 with increasing thickness of Ag and Au to 2 nm and 4 nm, respectively, and increases for further increase in thickness. A lower turn-on field corresponds well to a larger field enhancement factor, confirming that additional field enhancement with the morphological evolution of metal nanoparticles governs emission performance.

Cone-like nanostructures are formed on nitrogen-incorporated nanocrystalline diamond films by reactive ion etching (RIE) in a moderate-pressure microwave plasma. A thin Au layer is deposited on the film and followed by heating for transformation of an Au layer into Au nanoparticles as the mask for RIE. The diameter and spacing of Au nanoparticles depend upon the heating temperature and the thickness of Au. The smooth film surface is transformed into a number of cone-like protrusions with diameters, heights, and spacings of tens to a few hundreds of nm by RIE in a hydrogen/argon plasma and removal of Au residue in aqua regia. The formation of sharp nanostructures enhances field emission characteristics, so that the current density at high fields is increased largely up to the order of 10[?]3 A cm[?]2 and the turn-on field is reduced down to around 1.8 V um[?]1, comparable to other carbon nanostructures, due to enhancement of local fields.

Cubic silicon carbide (3C-SiC)/nitrogen-incorporated nanocrystalline diamond layered films are prepared on p-type Si(100) substrates by carbonization and chemical vapor deposition in moderate-pressure microwave plasmas. X-ray diffraction, cross-sectional transmission electron microscopy, and energy dispersive X-ray spectroscopy reveal that epitaxial 3C-SiC thin layers about 10 nm thick with very high phase-purity are grown at the interface of Si and nanocrystalline diamond. The infrared absorption coefficient for the 3C-SiC layer is estimated to be around 420000 cm-1. The p-Si/3C-SiC/n-nanocrystalline diamond junction in a diode configuration shows rectification in the current--voltage measurement. Structural defects and surface roughening of the SiC layers are highly responsible for increasing the reverse leakage current and thus lowering the diode performance.