B. Kleinsorge

Stress reduction and bond stability during thermal annealing of tetrahedral amorphous carbon

A comprehensive study of the stress release and structural changes caused by postdeposition thermal annealing of tetrahedral amorphous carbon (ta-C) on Si has been carried out. Complete stress relief occurs at 600–700 °C and is accompanied by minimal structural modifications, as indicated by electron energy loss spectroscopy, Raman spectroscopy, and optical gap measurements. Further annealing in vacuum converts sp3 sites to sp2 with a drastic change occurring after 1100 °C. The field emitting behavior is substantially retained up to the complete stress relief, confirming that ta-C is a robust emitting material.

Low-temperature growth of carbon nanotubes by plasma-enhanced chemical vapor deposition

Vertically aligned carbon nanotubes were grown at temperatures as low as 120 °C by plasma-enhanced chemical vapor deposition. A systematic study of the temperature dependence of the growth rate and the structure of the as-grown nanotubes is presented using a C2H2/NH3 system and nickel as the catalyst. The activation energy for the growth rate was found to be 0.23 eV, much less than for thermal chemical vapor deposition (1.2–1.5 eV). This suggests growth occurs by surface diffusion of carbon on nickel. The result could allow direct growth of nanotubes onto low-temperature substrates like plastics, and facilitate the integration in sensitive nanoelectronic devices.

Direct growth of aligned carbon nanotube field emitter arrays onto plastic substrates

The direct growth of vertically aligned carbon nanotubes onto flexible plastic substrates using plasma-enhanced chemical vapor deposition is reported. We show that individual lines and dots of free-standing 20–50 nm diameter nanotubes can be grown onto chromium covered commercially available polyimide foil. The scalable deposition method allows large area coverage without degrading or bending the sensitive substrate material. Field emission measurements show a low turn-on field (3.2 V/μm) and a low threshold field (4.2 V/μm). The result establishes a method of flexible field emitter fabrication, which is well suited for display production and integration of nanotubes into plastic electronics.

Effects of catalyst film thickness on plasma-enhanced carbon nanotube growth

A systematic study is presented of the influence of catalyst film thickness on carbon nanostructures grown by plasma-enhanced chemical-vapor deposition from acetylene and ammonia mixtures. We show that reducing the Fe∕Co catalyst film thickness below 3nm causes a transition from larger diameter (>40nm), bamboolike carbon nanofibers to small diameter (∼5nm) multiwalled nanotubes with two to five walls. This is accompanied by a more than 50 times faster growth rate and a faster catalyst poisoning. Thin Ni catalyst films only trigger such a growth transition when pretreated with an ammonia plasma. We observe a limited correlation between this growth transition and the coarsening of the catalyst film before deposition. For a growth temperature of ⩽550°C, all catalysts showed mainly a tip growth regime and a similar activity on untreated silicon, oxidized silicon, and silicon nitride support.

Influence of nitrogen and temperature on the deposition of tetrahedrally bonded amorphous carbon

The effect of nitrogen addition on the properties of tetrahedral amorphous carbon (ta-C) has been studied. The ta-C is deposited by a filtered cathodic vacuum arc. The effect of introducing nitrogen on its plasma was measured by a retarding field analyzer and optical emission spectroscopy. The ta-C:N films were studied as a function of nitrogen content, ion energy, and deposition temperature. The incorporation of nitrogen was measured over the range of 10−2–10 at. % by secondary ion mass spectrometry and elastic recoil detection analysis. The N content was found to vary slightly sublinearly with the N2 partial pressure during deposition. A doping regime was found for N contents of up to 0.4 at. %, in which the conductivity changes while the sp3 content and the optical band gap remain constant. For 0.4%–8% N, the sp3 fraction remains above 80% but the optical gap closes due to a clustering of sp2 sites. Only above about 10% N, the sp3 fraction falls. The influence of nitrogen on the a-C was found to be ind...

Electronic properties of tetrahedral amorphous carbon investigated by scanning tunneling microscopy

Scanning tunneling spectroscopy (STS) has been used to investigate the electronic structure of tetrahedral amorphous carbon (ta-C) films. The density of states (DOS) was derived from the normalized conductivity spectra at fixed tip heights. Band edges were defined by extrapolating the generalized conductivity (dI/dV) within ±2 eV of the Fermi level. The resulting band gap was found to be similar to that measured optically for ta-C films of different sp3 content. The Fermi level of undoped ta-C was found to lie below midgap, confirming ta-C to be a weakly p-type semiconductor. The valence band tail is found to be steeper than the conduction band tail, the opposite of the situation in a-Si:H. With nitrogen addition, the Fermi level is found to move above midgap, confirming that n-type doping is occurring. The band gap is found to decline at higher N contents. The DOS at larger tip heights and as a function of surface etching shows evidence of a surface layer with a narrower band gap, which we identify as th...

Determination of bonding in amorphous carbons by electron energy loss spectroscopy, Raman scattering and X-ray reflectivity

X-ray reflectivity (XRR) and Raman scattering are developed as non-destructive methods to find the density and sp 3 content of unhydrogenated and hydrogenated amorphous carbon films. An empirical relationship is found to derive the sp 3 fraction from visible Raman spectra, while ultraviolet (UV) Raman is able to directly detect sp 3 sites. The sp 3 fraction and density are linearly correlated in amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) films. ” 2000 Elsevier Science B.V. All rights reserved.

Electrical and optical properties of boronated tetrahedrally bonded amorphous carbon (ta-C:B)

Tetrahedrally bonded amorphous carbon (ta-C) is a predominately sp3-bonded semiconductor with a band gap of order 2 eV. It can be doped n-type using nitrogen but no successful p-type doping has been reported until now. On the other hand, it has recently been shown that the incorporation of boron reduces the intrinsic compressive stress of ta-C, while still maintaining its high fraction of sp3 sites. This paper reports a detailed study of the electrical properties of boron-doped ta-C (ta-C:B). The ta-C:B films are deposited in a filtered cathodic vacuum arc system using a pressed cathode of graphite and boron powder. The composition and structure of the films are examined by electron energy loss spectroscopy. We find that the room temperature conductivity of the films increases by five orders of magnitude with a boron concentration from 0 to 8%. The conductivity activation energy decreases for the same boron concentration, while the E04 gap remains constant. N-type silicon/ta-C:B heterojunctions show a rectifying behaviour as a function of the boron concentration of the films. The films show photo-conductivity. The combined results indicate p-type doping of ta-C.

Electronic structure studies of undoped and nitrogen-doped tetrahedral amorphous carbon using high-resolution electron energy-loss spectroscopy

The electronic structure of undoped and nitrogen-doped tetrahedral amorphous carbon (ta-C) has been studied using high-resolution electron energy-loss spectroscopy in transmission to determine the local bonding of the carbon and nitrogen atoms. We found that the carbon and nitrogen atoms are mainly sp3-bonded for nitrogen contents up to 10 at. %. However, the data analysis is also consistent with some N atoms occupying trivalent σ or pyrrole configurations. The real (e1) and imaginary parts (e2) of the dielectric function were derived over a wide energy range carrying out a Kramers–Kronig analysis of the loss spectra. The e2 spectra show two features due to the excitation of the π and σ valence states. The π feature is very small in ta-C deposited at 100 eV, consistent with its large sp3 content (∼85%). We have determined the ratios of sp2/sp3-hybridized atoms in the samples by both the C 1s edge and the optical sum rules.

Stationary carbon cathodic arc: Plasma and film characterization

The plasma characteristics of a new arc mode on carbon referred to as the ‘‘stationary cathodic arc’’ are reported. Particularly, optical emission spectroscopy, probe and ion energy distribution measurements are used to compare the properties of the stationary arc with the normal ‘‘random cathodic arc’’ on carbon. The measurements revealed that the plasma properties of both arc modes are similar. In addition, we have correlated the plasma characteristics to the film properties. Carbon films deposited using the stationary arc were found to have a surface particle density equivalent to those deposited using the filtered cathodic vacuum arc. The macro‐particle free films were found to be highly tetrahedral and compressively stressed. Both the sp3 fraction and stress values were strongly dependent on the ion energy with maximum values of 85% and 9.4 GPa, respectively, occurring at ≊50 eV.