Kungen Teii

Effect of the sp2 carbon phase on n-type conduction in nanodiamond films

Structural and electrical conduction properties of nitrogen-doped nanocrystalline diamond films are studied as a function of deposition temperature (TD) in a microwave Ar-rich/CH4 plasma with 30%N2 addition. Hall- and Seebeck-effect measurements confirm n-type conduction for TD above 1100 K. For TD from 1100 and 1220 K, the electron concentration increases up to 1020 cm−3 and the electron mobility is in the range of 4–8 cm2 V−1 s−1. For TD above 1250 K, the mobility decreases to ∼1 cm2 V−1 s−1. Low conductivity films deposited at low TD exhibit semiconductorlike thermal activation in the Arrhenius plots, while high conductivity films deposited at high TD are almost temperature independent, indicative of quasimetallic conduction. The nitrogen concentration in the films is about 0.3 at. %, independent of TD. As TD is increased, the sp2 content and order increase. This is responsible for the appearance of midgap states, their delocalization, and the larger distance between diamond grains. The high conductivi...

Study on polymeric neutral species in high-density fluorocarbon plasmas

Production and extinction processes of polymeric neutral species (CmFn;m⩾2) in electron cyclotron resonance C4F8 and CF4 plasmas have been studied by using a quadrupole mass spectrometer (QMS) employing low-energy electron attachment technique. This technique allows the detection of electronegative CmFn species as negative ions by scanning the attaching electron energy in the QMS typically in the range of 0–10 eV. In addition to the most abundant F− and CF3− signals resulting from dissociative attachment to various fluorocarbon species, pronounced attachment resonances of negative ions corresponding to the series of CmF2m±1− such as C3F7−, C4F9−, and C5F9− were primarily observed especially at low microwave powers and high pressures. The C4F8 plasma contained a large amount of polymeric species and a high fraction of reactive F-stripped species as compared to the CF4 plasma, providing evidence of a high potential of gas phase and surface polymerization in a low F/C ratio plasma. The amount and composition...

Synthesis and electrical characterization of n-type carbon nanowalls

Nitrogen-incorporated carbon nanowalls are prepared by microwave plasma-enhanced chemical vapor deposition using acetylene and methane. n-type conduction in the nanowalls is confirmed by Hall- and Seebeck-effect measurements. We show that increasing the amount of C2 radicals by adding Ar enables catalyst-free growth of nanowalls at a high rate up to about 1 μm/min and reduces the deposition temperature (TD) down to around 650 °C. A substrate pretreatment using diamond powder results in a composite of nanowalls and nanocrystalline diamond films, suggesting that the nanowall growth is limited by gas-phase conditions rather than surface conditions. The low conductivity nanowalls for low TD exhibit thermal activation in the Arrhenius plot, indicative of semiconducting conduction, while the high conductivity nanowalls for high TD are almost temperature independent, indicative of quasimetallic conduction. The high conductivity is attributed to a global increase in the sp2 cluster size and crystallinity, which i...

Origin of low threshold field emission from nitrogen-incorporated nanocrystalline diamond films

Highly conductive, nitrogen-incorporated nanocrystalline diamond films with quasimetallic character emit electrons at low turn-on fields (∼3 V μm−1). These films exhibit stronger delocalization of carriers, indicative of smaller energy separation between the defect bands in the band gap. We show that the emission level derived from the measured emission characteristic and electron affinity shifts upward (up to a few eV) with increasing the film conductivity, thereby decreasing the effective potential barrier height for the emission. This is attributed to higher probabilities of electron injection into upper defect levels during the transport process, originating from internal band bending and increasing band continuity.

Lower pressure limit of diamond growth in inductively coupled plasma

A study of diamond growth at pressures below 20 mTorr by using an inductively coupled radio frequency plasma is presented. Emissive and Langmuir probes, and optical emission spectroscopy were used to examine the plasma, and deposits were obtained on single crystalline silicon and diamond substrates with controlling the bombarding ion energy by the sheath potential (Vsheath). A higher threshold of Vsheath that allowed diamond growth was found in the range of 11–19 V above 20 mTorr, while a shift down of the Vsheath threshold was observed below 10 mTorr, as confirmed by Raman spectroscopy and electron diffraction. The growth at 10 mTorr was successful only when Vsheath was reduced to 2 V, however, the growth at 5 mTorr was no longer possible even when Vsheath was reduced to 2 V. Effects of the pressure decrease on the suppression of diamond growth below 10 mTorr were interpreted in terms of an increase in ion flux relative to radical flux as well as low radical density corresponding to the plasma density of...

Electron field emission from nanostructured cubic boron nitride islands

Nanocrystal-assembled cubic boron nitride (cBN) islands are formed by using low-energy (∼20eV) ion irradiation in an inductively coupled fluorine-containing plasma. The temporal evolution of surface morphology and roughness reveals three-dimensional island growth for initial sp2-bonded BN and subsequent cBN, accompanied by a high frequency of renucleation. The formation of cBN islands enhances the field emission and reduces the turn-on field down to around 9V∕μm due to an increase in the island-related field. The results demonstrate the high potential of cBN for field emitters, comparable to other wide band gap semiconductors.

Synthesis and field emission properties of nanocrystalline diamond/carbon nanowall composite films

Nanostructured composite films consisting of almost vertically aligned graphene layers, so-called “carbon nanowalls” (CNWs), and nanocrystalline diamond films are prepared by plasma-enhanced chemical vapor deposition. The space between the walls for the composite films is widened compared to simple CNWs by interception of in-plane continuity of the wall structures. The nucleation density of diamond is responsible for the spacing and arrangement of the walls. Field emission measurements show that the composite films have lower turn-on fields (∼1 V/μm) and larger field enhancement factors (∼4000) than simple CNWs. The results indicate that electric field screening between neighboring walls is well suppressed.

Effect of enhanced C2 growth chemistry on nanodiamond film deposition

A route to high-purity nanocrystalline diamond films from C2 dimers and related mechanisms have been investigated by enhancing C2 growth chemistry in Ar-rich microwave plasmas. Efficient C2 production by direct dissociation from acetylene causes the micro- to nanocrystal transition with a low threshold Ar concentration of ∼70% and produces films of ∼20nm grains with a distinct visible-Raman peak of diamond. C2 grows nanodiamond on diamond surfaces but rarely initiates nucleation on foreign surfaces. The phase purity can be improved by increasing the dominance of nanodiamond growth from C2 over nondiamond growth from CHx(x=0–3) and large radicals.

Synthesis of cubic boron nitride films with mean ion energies of a few eV

The lowest threshold energy of ion bombardment for cubic boron nitride (cBN) film deposition is presented. cBN films are prepared on positively biased Si (100) substrates from boron trifluoride (BF3) gas in the high-density source region of an inductively coupled plasma with mean ion impact energies from 45 down to a few eV or less. The great decrease in the threshold ion energy is mainly attributed to specific chemical effects of fluorine as well as high ion-to-boron flux ratios. The results show evidence for the existence of a way to deposit cBN films through quasistatic chemical processes under ultralow-energy ion impact.

Precursors of fluorocarbon film growth studied by mass spectrometry

The precursor species of fluorocarbon film growth at the reactor wall irradiated by an electron cyclotron resonance C4F8 plasma have been studied by using a quadrupole mass spectrometer. The amount of polymeric neutral species [CmFn (m⩾2)] and absolute densities of CFx (x=1–3) radicals in the vicinity of the wall were measured by electron attachment and threshold ionization mass spectrometry, respectively. The trends in the film growth rate as a function of gas residence time, diluted hydrogen concentration, and microwave power were well accounted for by the competition between the incorporation of CFx radicals and positive ions and the removal by F and H atoms. The fluxes of CFx radicals and positive ions incident upon the wall were shown to be comparable with the net condensed carbon flux derived from the growth rate. In contrast, the trends in the amount of polymeric neutrals were not well correlated to the growth rate.