Synthesis and Modification of beta-C 3 N 4 Materials by Ion-Beam Processing
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ABSTRACT Covalent carbon nitride films have been synthesized by laser ablation and ion-beam coprocessing. Different laser ablations give different ablated graphite plasma. Beta- C3N 4 polycrystalline has been found in the deposited films. Under laser energy of 50 mJ per pulse, the adoption of 532 nm laser used for ablation of graphite is proposed for the purpose of production of good carbon nitride films. The C-N crystal structures can be improved by followed ion-beam processing. INTRODUCTION Covalent carbon nitride, beta-C3 N4, was'predicted by Liu and Cohen[1,2] to be a good candidate for extreme hardness. Many groups[3-7] have tried to synthesize carbon nitride materials. Niu[8] deposited covalent solid carbon nitride using atomic-beam- assisted laser ablation, and the experimental evidence supported the beta- C3N4 structure. This led to detailed investigations of this tetrahedral solid from both theoretical and experimental aspects[9-12]. We have also synthesized covalent carbon nitride films[11,12] using laser ablation of graphite under low-energy nitrogen ion beam bombardment. XPS, HEBS, Raman, and electron diffraction measurements have been used to characterize the films. Since the properties of the deposited films are dependent on the precursor radicals ablated from the solid graphite, studies of the influence of the parameters of the laser ablation, such as laser power and wavelength on the mechanism of synthesis of carbon nitride films are relatively important. In our experiment, in order to select a laser with an appropriate wavelength to deposit carbon nitride films with desirable properties, we study the optical emission spectra of the plasma produced by the laser ablation of graphite at different wavelengths: 355, 532 nm and 1.06 micrometer. Followed ion-beam processing is also been carried out to improve the ratio of crystal carbon nitride materials in the films. EXPERIMENT In our experimental set up, as described in a previous paper[ 11], a YAG laser (model YG661-10 by Continuum Corp.) with base ivavelength of 1.06 micrometer is used to generate a laser beam to ablate graphite. A Kaufman broad-beam source is used to generate a nitrogen ion beam. The sputtered carbon species chemically combine with the low-energy nitrogen-ion beam on the Si(100) substrate. The YAG laser provides 80 ns pulses at 10 Hz with energy 50 mJ per pulse. The energy of nitrogen-ion beam ranges 313 Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
from 70 to 200eV. The plasma optical emissior. of the portion about 4-6 mm away from the graphite target is sampled. The signal detected by a photomultiplier tube is sent to a Boxcar analyzer (model 4400 by EG&G) for signal processing. RESULTS AND DISCUSSION The N-content x (defined by CNx) of the 532 nm laser deposited film is 0.7, corresponding to N-percentage about 41%[12]. The film deposited by 355 laser ablation contents less N species. In the TEM and electron diffraction measurements, the deposited films by 532 or 355 nm laser ablation appear to be predomina
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