Studies of CN 1 Films Deposited by Ion-Beam-Assisted Laser Ablation of Graphite
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China ABSTRACT CKN thin films have been synthesized by ion-beam-assisted laser ablation of graphite. Films with N-concentration of 45% are obtained, indicated by high energy backscattering spectrum (HEBS). Raman and X-ray photoelectron spectroscopy (XPS) data confirm the existence of carbon-nitrogen bonds. Polycrystallites beta-CýN4 structure has been detected in the amorphous matrix of the films, as indicated by transmission electron microscopy (TEM) and electron diffraction. Qualitative tests indicate that the films are relatively hard and adhesive. INTRODUCTION Covalent carbon nitride, beta-CIN4 , was predicted by Liu and Cohen[ 1,2] to be a good candidate for extreme hardness. Recently, Niu[3] deposited covalent solid carbon nitride using atomic-beam-assisted laser ablation, and the experimental evidence supported the beta-C 3 N 4 structure. This led to detailed investigations of this tetrahedral solid both at theoretically and experimentally[4,5]. In our experiment, we synthesize carbon nitride films using laser ablation of graphite under low-energy nitrogen ion beam bombardment, in which the nitrogen ion beam supplies energy for chemical combination higher than that by atomic beam proposed by Niu et al. [3]. X-ray photoemission spectroscopy (XPS), Raman spectra, high energy non-Rutherford backscattering (HEBS), and electron diffraction measurements are used to characterize obtained films. EXPERIMENTAL In our experimental set up, as described in previous papers[6,71, a pulsed YAG laser with wavelength of 532 nm is used to sputter a graphite target with spectrum purity. The sputtered carbon species chemically combine with a low-energy nitrogen-ion beam irradiation on the Si(100) substrate. The polished Si(100) wafers, which have been chemically cleaned to remove an{ impurities and native oxide layers on the surfaces, are bombarded by an Ar beam to make further cleaning before the deposition process. The energy of per laser irradiation pulse is about 100 mJ. The energy of nitrogen-ion beam ranges from 70 to 200eV. HEBS analysis is performed on films with a Model 2*3 MV 9DSH (NEC) tandem Van de Graaf accelerator with He ions at energy of 3.5 MeV to quantitatively measure the C and N species ratio. Raman studies are carried on SPEX spectrometer. 665 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
12 0C0S01000
1020
r4
,,,
S800 z D600
800 6600
400 200
050
400 3.5MeV
250
e
*He
200
1,65
450 C-HANNEL
650
0
850
Fig. 1. HMS results of carbon nitride film deposited under the conditions of 100eV nitrogen bocbdema t with 50 mA current, 600-K substrate temperature. The C-particles used in the weasuremez have an energy of 3.5 MeV. The N-contents are found to be 41%.
RESULTS AND DISCUSSIONS Fig. 1 shows the HEBS result for carbon nitride films deposited under the bombardment of a 100eV nitrogen ion beam. The N-content x (defined by CNM) for is found to be 0.71*0.07, corresponding to N-percentage of about 36.A% to 45.1% with an average value of 41.2%. In the TEM and electron diffraction
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