Diamond and Related Materials 10 Ž2001. 900᎐904

Influence of ambient gas on diamond-like carbon films prepared by KrF pulsed laser deposition Kenji Ebihara a,U , Toshiyuki Nakamiyaa , Tamiko Ohshimaa , Tomoaki Ikegami a , Shin-ichi Aoqui b a

Department of Electrical and Computer Engineering, Graduate School of Science and Technology, Kumamoto Uni¨ ersity, Kurokami 2-39-1, Kumamoto 860-8555, Japan b Department of Electrical Engineering, Kumamoto Institute of Technology, Ikeda, Kumamoto 860-0082, Japan

Abstract We have studied the diamond-like carbon ŽDLC. and carbon nitride ŽCN. thin films deposited by KrF excimer laser Ž248 nm. ablation under various ambient gas conditions. DLC thin films were deposited on quartz and SiŽ100. substrates in pure hydrogen and helium gas at laser fluence of 2᎐10 Jrcm2. Optical absorption measurements show that the DLC films prepared in a H 2 atmosphere of 800 mTorr with 8 Jrcm2 have an optical band gap of 2.0 eV. The tetrahedral amorphous carbon Žta-C. films deposited in helium gas have an optical band gap of approximately 1.0 eV. Incorporation of nitrogen atoms in the DLC films showed marked change in the optical properties when the N2 mixture ratio in hydrogen exceeded 50%. The composite layered structures consisting of DLC and CN were formed to investigate the optical property in stacked CN films. Laser-induced fluorescent technique was used to study the dynamics of C 2 molecules produced by pulsed laser deposition at various ambient gas conditions. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Diamond-like carbon; CN; Laser ablation; Optical property

1. Introduction The interaction of laser ablated plasma plume with background gases is receiving considerable attention to understand the dynamics of carbon-based thin film growth as well as nanoparticle formation. The dynamics of laser ablated carbon plume used for pulsed laser deposition ŽPLD. is influenced by conditions such as the ambient gas and the laser fluence. Films of tetrahedral amorphous carbon Žta-C. have been deposited using PLD under high vacuum atmosphere Žapprox.

U

Correspondence author. Tel.: q81-96-342-3628; fax: q81-96342-3630. E-mail address: [email protected] y u.ac.jp ŽK. Ebihara ..

10y7 Torr. w1᎐5x. The ta-C films prepared on Si substrate showed high sp 3 content over 95% w1x. Diamondlike carbon ŽDLC. films have been formed under hydrogen atmosphere at room temperature. It is well known that optical property of DLC depends on the deposition conditions such as hydrogen pressure and substrate temperature. Incorporation of nitrogen atoms in ta-C and production of carbon nitride CN alloy in nitrogen atmosphere have attracted much attention for its use in the preparation of n-type doping carbon and electron field emission. In this paper, we studied the effect of ambient gases on the optical, structural and physical properties of carbon-based thin films prepared by KrF excimer laser ablation technique. The hydrogenated amorphous carbon Ža-C:H. film and nitrogen incorporated DLC film

0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 4 4 5 - 3

K. Ebihara et al. r Diamond and Related Materials 10 (2001) 900᎐904

have been investigated using optical absorption, FTIR measurement and atomic force microscopy ŽAFM..

2. Experimental Fig. 1 shows a schematic diagram of the pulsed laser deposition system. The carbon-based thin films were deposited by KrF excimer laser Ž ␭ s 248 nm, pulse width 25 ns: Lambda Physik Model LPX305icc, maximum energy 1200 mJ. ablation of a rotating graphite target Ž ␾ 30 mm= 5 mm, 99.999% purity. w6,7x. The pure hydrogen gas and its mixture with nitrogen gas were fed into the stainless chamber Ž ␾ 500 mm= 400 mm. after evacuating to a base pressure of 5 = 10y7 Torr. KrF laser beam was steered into the chamber using lenses and fused quartz window. The laser fluence incident on a small area Ž2 = 5 mm2 . of the pellet at an angle of 45⬚ was varied from 2 Jrcm2 to 10 Jrcm2 . The films were deposited on quartz and SiŽ100. at room temperature ŽRT.. Optical transparency and the optical band gap of the films were measured by using a visible light spectrometer ŽSHIMAZU UV-160.. The bonding structure was also investigated with a FTIR spectrometer ŽBIO-RAD FT-IR Excalibur FTS 3000.. Surface morphology of the as-grown films was studied by AFM ŽSPI 3800N, Seiko Instrument . analysis. Laser induced fluorescent ŽLIF. technique was used to study C 2 species in the plasma plume during the carbon film deposition.

3. Results and discussion 3.1. Diamond-like films and tetrahedral carbon film deposition We have earlier reported optimum deposition conditions for the DLC and ta-C films under various parameters of laser fluence, repetition frequency, ambient gas pressure and the distance between the target and the substrate w8,9x. Table 1 lists present experimental results of DLC and ta-C films with various deposi-

Fig. 1. Schematic diagram of pulsed laser deposition system.

901

tion conditions. The optical band gap Eopt was determined according to the Tauc equation. Fig. 2 shows typical optical properties of the DLC film and ta-C films. Most of the DLC films Žfilm thickness: approx. 200 nm. were prepared by 12 000 laser shots Žrepetition frequency: 10 Hz. at RT. The DLC film deposited on the quartz substrate in hydrogen atmosphere Ž800 mTorr. has an Eopt of 2.0 eV at laser fluence of 8 Jrcm2 . The Eopt for the DLC deposited at 50 mTorr and 300 mTorr was 1.0 eV and 1.7 eV at the same laser fluence, respectively. Eopt is found to change on changing the laser fluence at constant ambient pressure of 200 mTorr H 2 ; 1.4 eV for 5 Jrcm2 , 1.8 eV for 8 Jrcm2 and 1.7 eV for 10 Jrcm2 . On increasing ambient gas H 2 pressure with fixed laser fluence of 8 Jrcm2 , the optical property also changed; Eopt of 1.0 eV at 100 mTorr, Eopt of 1.7 eV at 200 mTorr and Eopt of 2.0 eV for 800 mTorr. These results suggest that the DLC films with high optical transparency can be prepared at the optimum deposition condition of 800 mTorr and 8 Jrcm2 . The optical energy band is consistent with the variation of the sp 3 fraction in the carbon-based materials. Our sample of Eopt of approximately 2.0 eV is expected to have sp 3 content of approximately 70%, similar to the published results w10x. The FTIR absorption measurements showed that the DLC film deposited on SiŽ100. substrate has strong absorption peak of sp 3 CH 2 mode at 2925 cmy1. In high vacuum Ž10y7 Torr. the deposited film showed dark colored and lower optical transparency. In the case of the He atmosphere of 100᎐300 mTorr, Eopt varied from 0.8 to 1.1 eV at laser fluence of 8 Jrcm2 . We observed strong carbon emission spectra Ž ␭ s 495.4 nm CII, 588.3 nm CII. from the carbon plasma plume generated in He atmosphere. The existence of these high energetic carbon ions enhances the formation of sp 3 bond in the tetrahedral carbon films. 3.2. Nitrogen incorporated diamond-like carbon films Incorporation of nitrogen atoms in carbon films has shown wide variety of optical, electrical and structural properties. It has been shown that N does act as an effective n-type dopant below and up to a threshold of 1% w11x. At higher concentration of N, carbon nitride CN films are formed. Fig. 3 shows the effect of nitrogen gas on the optical properties of a-C:H films. These films were deposited at various mixture ratios of H 2 and N2 pressure of 150 mTorr. The laser fluence used was 6 Jrcm2 and a negative dc voltage of y150 V was applied to the substrate. A low amount of N2 addition gives no noticeable change in the optical property. However, with a mixture ratio Rs N2rŽN2 q H 2 . of 50%, the optical band gap of 1.9 eV is obtained. The increase of N2 ratio decreases optical transparency. The 97% N2 sample shows Eopt of 1.4 eV, while the

K. Ebihara et al. r Diamond and Related Materials 10 (2001) 900᎐904

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Table 1 Optical property of DLC and ta-C films deposited by KrF excimer laser

Ambient gas

ŽmTorr.

Eopt ŽeV.

8

H2

50 200 600 800

1.0 1.7 1.8 2.0

5 8 10

H2

200

1.4 1.8 1.7

8

He

100 200 300

0.8 1.0 1.1

1.0= 10y6

1.0

Deposition condition Laser fluence ŽJrcm2 .

6

sample deposited in pure N2 was opaque. The FTIR absorption measurement indicated that there are C᎐H stretching modes from 2800 to 3000 cmy1 , C⬅N triple stretching modes from 2065 to 2260 cmy1 , and C⫽N double bond stretching mode at approximately 1600 cmy1 . The result showed that the intensity of the C⫽N double mode of 1600 cmy1 increases with increasing N2 pressure. Large compressive stress of a-C films rich in sp 3 bond is a disadvantage for practical device applications because they can cause adhesion failure at the film᎐substrate interface or prevent the growth of thick films. Laser thermal annealing, energetic carbon bombardment, and adding a small percent of metal or

Fig. 2. Tauc plots of the hydrogenated a-C film and ta-C films deposited in H 2 and He gas.

Fig. 3. Effect of nitrogen gas pressure on the transparency of the DLC films. The mixture ratio is Rs N2 rŽH 2 q N2 ..

boron to the film have been used to achieve stress relief in a-C films. The layered structure is an interesting alternative to release the compressive stress in the carbon-based films. The DLC film has better adhesion to the substrate than the CN film. The mechanical hardness of the laser-produced DLC and a-C films has been reported in the range of 50᎐60 GPa and 30᎐38 GPa, respectively w1,2,5x. On the other hand, the nitrogen incorporated CN thin films show lower hardness of 1.2 GPa w12x, while the ion-assisted PLD CN films are considerably harder than diamond-like carbon films w13x. We prepared the composite layered structures consisting of the DLC film and the CN film on the quartz substrate at a laser fluence of 6 Jrcm2 . Initially, the DLC layer Ž200 nm. was deposited on the quartz substrate at hydrogen atmosphere Ž400 mTorr.. In the second procedure, the CN film Ž100᎐300 nm. was formed on the DLC film in nitrogen atmosphere Ž100 mTorr.. The thick CN thin film deposited directly on

Fig. 4. Optical property of the composite layered structures consisting of the DLC and the CN film.

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the quartz substrate in nitrogen is black in color and opaque. Fig. 4 shows the optical property of the layered structures when the thickness of the CN layer was changed from 100 nm to 300 nm. Although increasing the CN layer thickness decreases the overall optical transparency, the adhesion to the substrate was improved compared to the single CN thin film on a quartz substrate. Optical band gap Eopt changed from 2.1 eV ŽCN: 100 nm. to 1.6 eV ŽCN: 300 nm.. 3.3. Surface morphology of the carbon-based films at ¨ arious ambient gases

Fig. 5 shows AFM images of as grown DLC film and ta-C film. DLC film deposited at 800 mTorr H 2 has cubic structure of 150 nm = 150 nm as shown in Fig. 5a. The root mean square ŽRMS. surface roughness is 13 nm. On the other hand, the ta-C film prepared in helium atmosphere Ž300 mTorr. at 8 Jrcm2 shows small cubes of average size of 80 nm = 80 nm in Fig. 5b. The surface of He ambient sample is very smooth with RMS roughness of approximately 2 nm. Since the ionization energy of He gas at 24.59 eV is higher than that of H Ž13.6 eV., small surface roughness in He is attributed to the presence of high energetic He species ŽHe radical, He ion. impinging onto the deposited films that results in a large number of nucleation sites. 3.4. Dynamics of KrF excimer laser plasma at ¨ arious ambient conditions Two-dimensional C 2 density profiles were measured using LIF method. C 2 molecules were excited by the YAG laser pumped dye laser ŽLambda Physik, SCANmate 2EC-400., which was tuned to a3 ⌸ u ᎐d 3 ⌸ g Ž0,0. Ž ␭ s 516.5 nm, ; 3 mJ. and LIF from d 3 ⌸ g ᎐a3 ⌸ u Ž0,1. Ž ␭ s 563.5 nm. was detected by an ICCD camera. Fig. 6 shows time evolution of 2D-LIF images of C 2 measured in vacuum, 300 mTorr and 700 mTorr of H 2 ambient gas. In a vacuum C 2 exists near the target and diffuses along the target surface, while in H 2 gas, C 2 production due to collision is enhanced at the shock front and C 2 diffusion is suppressed. It implies that C 2

Fig. 6. Time evolution of 2D-LIF images of C 2 at different H 2 pressure.

generation is accompanied by target surface ablation and recombination of neutral carbon in gaseous phase.

4. Conclusion DLC films have been deposited in H 2 , helium and N2 using KrF excimer laser ablation technique. The DLC film of optical band gap of 2.0 eV is obtained at 800 mTorr H 2 and a laser fluence of 8 Jrcm2 . The tetrahedral amorphous carbon films prepared in He gas have optical energy band of approximately 1.0 eV. Nitrogen incorporation in the DLC films deposited at high mixture ratio over 50% affects the optical property of the films. The layered structures are promising composite materials having optical versatility and improving the adhesion failure at the film᎐substrate interface. LIF measurement showed that in the carbon ablated plasma plume the C 2 molecules originate in the carbon target ablation and recombination of C in gaseous phase.

Acknowledgements This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture. References Fig. 5. AFM images of the DLC and ta-C films.

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