Ultraviolet and visible Raman spectroscopy characterization of chemical vapor deposition diamond films

Surface and Coatings Technology 151 – 152 (2002) 263–267

Ultraviolet and visible Raman spectroscopy characterization of chemical vapor deposition diamond films S.M. Huanga,*, Z. Sunb, Y.F. Lua, M.H. Honga a

Laser Microprocessing Laboratory, Data Storage Institute and Department of Electrical & Computer Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 117608, Singapore b School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

Abstract Polycrystalline diamond films with different grain sizes (10 nm, 100 nm and 5 mm) were prepared by hot-filament chemical vapor deposition (HFCVD). The diamond films have been studied using ultraviolet (UV, 244 nm), visible (Vis, 514 nm) and (Vis, 633 nm), near-infrared, micro-Raman scattering. The fluorescence in the excited diamond film is found to change considerably with the incident photon energy. The scattering intensity of amorphous sp2 -bonded carbon compared to the strength of the 1331 cmy1 Raman line from sp3-bonded diamond is found to vary considerably as functions of the grain size of diamond film and the incident photon energy. Possible models for the structures of amorphous sp2 -bonded carbon and sp3-bonded diamond phases are discussed on the basis of the present Raman data. It is shown that UV Raman spectroscopy has provided significantly greater information than visible Raman spectroscopy in characterizing diamond phases. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Diamond; Raman; sp3 bonds; sp2 bonds; Resonant enhancement

1. Introduction The recent development of chemical vapor deposition (CVD) diamond film growth technology has created the need for fast qualitative and quantitative methods of determining diamond film composition and quality. Raman spectroscopy is known as an ideal, convenient and non-destructive tool to characterize diamond and other forms of graphitic and amorphous carbon due to its sensitivity to different carbon microstructures, which produce distinctive Raman peaks for various forms of carbon w1–5x. For example, the exact frequencies of the Raman bands of diamond, graphite, and amorphous forms of carbon depend upon the size and stresses present in the different carbon domains w6–8x. There have been a number of Raman studies with excitation in the visible and near-infrared (IR) region. These studies have characterized the Raman spectra of various forms of carbon, and in particular CVD diamond and amorphous carbon films. Unfortunately, the strong fluorescence and photoluminescence that accompanies * Corresponding author. Tel.: q65-874-8204; fax: q65-777-1349. E-mail address: [email protected] (S.M. Huang).

near-IR, visible and even near-ultraviolet (UV) Raman excitation of CVD diamond generally limits the sensitivity that these Raman excitation wavelengths are able to achieve. The purpose of the present study was to investigate UV Raman excitation in and near the diamond band gap (230 nm) and compare the spectral information contents of the UV and visible regions. A variety of CVD diamond films were examined ranging from nanocrystalline diamond to an almost perfect one with several micrometers grain sizes. 2. Experimental details The polycrystalline diamond films examined in the present study were prepared by a conventional hotfilament (HF) CVD system by decomposition of a mixture of CH4 and H2. During sample preparation, the H2 flow rate was 100 sccm, and CH4 flow rate ranged from 0.5 to 8 sccm. The gas pressure was 100 torr. The filament tungsten (W) temperature was 2300 8C. Crystalline silicon wafers were used as substrates. The substrate temperature ranged from 650 to 800 8C, and the distance between the filament and substrate was 8

0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 5 6 6 - 3

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Fig. 1. Raman spectra of a pyrolytic graphite excited at different wavelengths indicated in the figure.

mm. Diamond film samples with approximately 10- and 100-nm grain sizes were deposited at the substrate temperature of 650 8C with a CH4 yH2 ratio of 2 and 8%, respectively. Another diamond film sample of approximately 5-mm crystalline size was deposited at the substrate temperature of 800 8C and CH4 yH2 ratio of 0.5%. The UV and visible Raman spectra were measured using the 244-nm (5.09 eV) excitation wavelength of a frequency-doubled argon ion laser system (Coherent 90 C FreD series), the 514.5-nm line (2.41 eV) of an argon ion laser and the 633-nm line (1.96 eV) of a He–Ne laser, respectively. The UV and visible scattering light were collected in back scattering with UV-enhanced and normal CCD cameras, using a Renishaw micro-Raman System 2000 and 1000 spectrometers, respectively. The spectral resolutions of the UV and visible spectrometers were 4.0 and 2.0 cmy1, respectively. 3. Results and discussion CVD diamond often contains appreciable amounts of non-diamond carbon that exhibits Raman bands characteristic of graphite or highly disordered sp2 and sp3 carbon. Visible Raman spectroscopy of different carbon species has been studied extensively w1–5x. Generally, single crystalline diamond has a single Raman-active first-order phonon mode, which appears as a single sharp line at 1331 cmy1. The first-order Raman spectrum of single-crystal graphite also shows a sharp, intense peak at 1582 cmy1 which was referred to as the G peak. The G mode of graphite has E2g symmetry involving the in-plane bond stretching motion of pairs

of C sp2 atoms w3x. This mode does not require the presence of sixfold rings, and so it occurs at all sp2 sites, not only those in rings. In the well-crystallized graphites with small particle size, another peak emerges at approximately 1355 cmy1, referred to as the D peak. The D peak of graphite was first attributed to an A1g breathing mode involving phonons near the K zone boundary, activated by disorder and breakdown of the qs0 selection rule w3x. Recently Ferrari and Robertson w9x developed another model of the Raman spectra of disordered carbons. This model suggests that the intensity of the D peak arises from clusters of sp2 sites in sixfold aromatic rings. As the crystallite size decreases, the intensity of the D band increases as compared with the G band w1x. Simultaneously, the latter starts to develop a shoulder in the high-energy side, which grows into a D9 band at approximately 1620 cmy1 as the disorder further increases w4x. The appearance of the D9 line is also activated by disorder and breakdown of the qs0 selection rule and explained by the large density of states peaks at q/0 points in the Brillouin zone. Fig. 1 shows 633-, 514.5- and 244-nm excited Raman features of a pyrolytic graphite sample. The first-order G band appears at 1586, 1582 and 1580 cmy1 for 633-, 514- and 244-nm excitation, respectively. The G peak position is almost kept constant with the change of the excitation wavelength. The spectrum excited at 633 nm shows a D band at 1327 cmy1, and D9 band at 1616 cmy1 shown in Fig. 1a. When the excitation wavelength decreases to 514.5 nm, the D peak shifts to 1352 cmy1, and D9 peak shifts to 1621 cmy1 as shown in Fig. 1b. The relative intensity of D and G bands decreases as the excitation wavelength decreases. The

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Fig. 2. Raman spectra of the polycrystalline diamond film consisting of approximately 10-nm crystals excited at different wavelengths indicated in the figure.

high frequency shift and the reduction in the relative intensity of the D band with the excitation wavelength decreasing are consistent with the results reported by Wagner w10x and Vidano w11x from visible Raman study of thermal annealing hydrogenated amorphous carbon (a-C:H) films and other graphitic materials, respectively. The slight up-shift of the D9 peak with higher excitation energy could be attributed to the variation in the energy resolution of the 514.5-nm Raman measurement system compared to the 633-nm measurement system. When the excitation wavelength decreases to 244 nm, both the D and the D9 lines disappear. The absence of the D band in the 244-nm excited Raman spectrum follows the trend shown in previous visible Raman measurements. Since the D band from the graphitic carbon does not appear in the UV Raman spectra, the diamond Raman first-order phonon spectrum is easily obtained without interference even for the poor quality CVD diamond films with relatively large non-diamond concentration. Fig. 2 displays 633-, 514.5- and 244-nm excited Raman spectra of the CVD diamond film consisting of approximately 10-nm grains. Both 633- and 514.5-nm excitation Raman spectra show two broad D and G bands, corresponding to the relatively strong scattering from the amorphous graphitic carbon phase in the film. For both of them, the whole Raman spectrum is superimposed on a relatively intense photoluminescence background shown in Fig. 2a and 2b. A weaker shoulder at ;940 cmy1 is present in the 633-nm excited Raman spectrum. The shoulder band is developed into a pronounced peak at ;1140 cmy1 in the 514.5-nm excited

Raman spectrum. In the Raman spectra excited at both 633 and 514.5 nm, the 1331 cmy1 diamond signal is not present at all. In contrast to the 633- and 514.5-nm excitation Raman spectra, the 244 excited Raman spectrum shows a relatively sharp line at 1331 cmy1 with a full width at half maximum (FWHM) of 19 cmy1 shown in Fig. 2c, and displays a distinct diamond phase. The UV Raman spectrum is also dominated by a broad G band at ;1576 cmy1, arising from non-diamond carbon in the film. Therefore, in the UV spectrum, nondiamond carbon features appear as the G band without the sp2 carbon D band. Fig. 3 shows Raman spectra excited at different excitation energies for the CVD diamond film consisting of larger diamond crystals, approximately 100 nm in diameter. Drastic changes upon variation of the excitation energy were also observed. For the excitation energies F2.41 eV, the typical diamond Raman signal at 1331 cmy1 is discernible but not easily resolved. The spectra consist of three broad bands arising from amorphous carbon phases or crystalline materials with extremely small grain sizes w5x. Both spectra show an intense photoluminescence background. For 5.09-eV excitation the Raman spectrum clearly shows a dominant diamond phase with a sharp line at 1331 cmy1 with a FWHM width of 17 cmy1. It only shows a relatively weak G band at ;1583 cmy1, resulting from amorphous carbon phases in the film. In Fig. 4a sequence of Raman spectra excited at different excitation energies ranging from 1.96 to 5.09 eV are plotted for the CVD diamond film consisting of

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Fig. 3. Raman spectra of the polycrystalline diamond film consisting of approximately 100-nm crystals excited at different wavelengths indicated in the figure.

micro-crystalline crystals, approximately 5 mm in diameter. For the excitation energies G2.41 eV no trace of scattering from amorphous sp2-bonded carbon is found. The 2.41-eV excitation Raman shows a typical diamond line at 1331.8 cmy1 with a FWHM width of 4.6 cmy1 shown in Fig. 4b. The 5.09-eV excitation Raman spec-

trum displays a diamond line at 1333 cmy1 with a FWHM width of 11.8 cmy1 shown in Fig. 4c. For excitation at 1.96 eV, the Raman spectrum shows a discernible and resolved diamond peak at 1331.6 cmy1 with a FWHM width of 4.3 cmy1, however, the whole near IR excitation Raman spectrum is superimposed on

Fig. 4. Raman spectra of the polycrystalline diamond film consisting of approximately 5-mm crystals excited at different wavelengths indicated in the figure.

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an intense photoluminescence background. The apparent increase in the linewidth of the diamond line with increasing excitation energy is caused by the decrease in energy resolution of the used spectrometer. From the above results and analyses, it is shown that the intensity of the 1331 cmy1 diamond band relative to that of sp2 nanodiamond band dramatically increases with UV excitation compared to the visible excitation. The result follows the trend shown by other researchers in which the D band intensity decreases as the excitation energy increases w10,12–15x. In the visible Raman measurement of CVD diamond films, the excitation energy (1.96 eV for 633 nm and 2.4 1 eV for 514 nm) is comparable with the p–p* transition energy at sp2 sites, and this leads to a resonant enhancement of their Raman cross-section w5,10,14,16x. In the UV Raman measurement, UV excitation at 244 nm (5.09 eV) is sufficient to excite the s states of both sp2 and sp3 sites in the CVD diamond films. This allows Raman to provide a more equally weighted view of the vibrational density of states (VDOS) in the CVD films. Therefore, the increase in the relative intensity of the diamond band to the non-diamond band in the UV Raman spectra does not result from the resonant enhancement of the first-order diamond phonon band but rather from the decreased relative Raman cross-section for the nondiamond carbon band. Additionally, the visible Raman spectra of the CVD films used in this study are complicated by the presence of a strong fluorescence signal shown in Figs. 2–4. The strong fluorescence signal required a base-line correction that was not always linear for determining peak areas and limited the maximum signal to noise (SyN) ratio of the Raman measurement. The most striking advantage of UV Raman excitation is the lack of fluorescence. This lack of fluorescence allows the monitoring of the diamond and nanodiamond carbon Raman spectrum in the CVD diamond films. 4. Conclusion Visible Raman spectroscopy has been an essential tool for characterization and identification of CVD

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diamond and diamond like films, but the signal to noise ratio of the visible Raman data is often decreased by strong fluorescence of the nanocrystalline and even microcrystalline diamond, or the highly defective diamond. It is shown that UV laser allows the acquisition of the Raman spectra where fluorescence and photoluminescence do not interfere, and equally weighted view of the vibrational density states (VDOS) of sp2 and sp3 sites in the CVD diamond increases the information content of the Raman data. The ability to measure fluorescence free spectra make UV Raman spectroscopy the method of choice for monitoring and characterizing both the nanocrystalline and microcrystalline CVD diamond, or the high and low quality CVD diamond, specially for the low quality nano-crystalline CVD films. This approach will be ideal for in situ measurements of growing diamond films. References w1x D.S. Knight, W.B. White, J. Mater. Res. 4 (2) (1989) 385. w2x R.O. Dillon, A. Woollam, V. Katkanant, Phys. Rev. B 29 (6) (1984) 3482. w3x F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. w4x P. Lespade, R. Al-Jishi, M. Dresselhaus, Carbon 20 (1982) 427. w5x R.J. Nemanich, J.T. Glass, G. Lucovsky, R.E. Shroder, J. Vac. Sci. Technol. A 6 (1988) 1783. w6x R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. w7x M. Yoshikawa, Y. Mori, H. Obata, et al., Appl. Phys Lett. 67 (1995) 694. w8x Y. Namba, E. Heidarpour, M. Nakayama, J. Appl. Phys. 72 (1991) 1748. w9x A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. w10x J. Wagner, M. Ramsteiner, C. Wild, P. Koidl, Phys. Rev. B 40 (1989) 1817. w11x R.P. Vidano, D.B. Fischbach, L.J. Willis, T.M. Loehr, Solid State Commun. 39 (1981) 341. w12x Y. Wang, D.C. Alsmeyer, R.R. McCreery, Materials 2 (1990) 557. w13x J. Wagner, C. Wild, P. Koidl, Appl. Phys. Lett. 59 (1991) 779. w14x M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani, J. Appl. Phys. 64 (1988) 6464. w15x M. Ramsteiner, J. Wagner, Appl. Phys. Lett. 51 (1987) 1355. w16x M. Yoshikawa, N. Nagai, M. Matsuki, et al., Phys. Rev. B 46 (1992) 7169.