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Structural evolution of boron nitride films grown on diamond buffer-layers
Thin Solid Films 515 (2006) 973 – 978 www.elsevier.com/locate/tsf
Structural evolution of boron nitride films grown on diamond buffer-layers Po-Chih Huang a , Tien-Syh Yang b , Shou-Shu Chu a , Ming-Show Wong a,⁎ a
Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan, ROC b General Educational Center, Tzu-Chi College of Technology, Hualien, Taiwan, ROC Available online 1 September 2006
Abstract Boron nitride films on diamond buffer layers of varying grain size, surface roughness and crystallinity are deposited by the reaction of B2H6 and NH3 in a mixture of H2 and Ar via microwave plasma-assisted chemical vapor deposition. Various forms of boron nitride, including amorphous α-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion E-BN, wurzitic w-BN and cubic c-BN, are detected in the BN films grown on different diamond buffer layers at varying distances from the interface of diamond and BN layers. The c-BN content in the BN films is inversely proportional to the surface roughness of the diamond buffer layers. Cubic boron nitride can directly grow on smooth nanocrystalline diamond films, while precursor layers consisting of various sp2-bonded BN phases are formed prior to the growth of c-BN film on rough microcrystalline diamond films. © 2006 Elsevier B.V. All rights reserved. Keywords: Cubic boron nitride; Nanocrystalline diamond film; Boron nitride
1. Introduction Cubic boron nitride is the second hardest material next to diamond. However, it is superior to diamond in some applications owing to its higher temperature oxidation resistance and higher chemical inertness to ferrous materials. Nevertheless, for most BN films prepared by various vapor deposition methods, the crystallinity of c-BN film is poor, the film stress is high, and non-cubic BN phases tend to form before c-BN is nucleated, rendering the applications of c-BN films impractical [1–3]. Most researchers have reported that the formation of c-BN film is possible only within a well-defined window of process parameters, e.g., negative substrate bias or the energy of bombarding particles needs to exceed a threshold value. Due to very large film stress, adherent c-BN films have been grown to a thickness less than 1 μm. A lot of CVD procedures, ion-assisted PVD and hybrid methods are reported for depositing c-BN and diamond films. The primary difference between the deposition of c-BN and diamond films is that ion bombardment during the film growth is essential for the synthesis of c-BN film. Diamond and c-BN
⁎ Corresponding author. Tel.: +886 3 8634206; fax: +886 3 8634200. E-mail address: [email protected] (M.-S. Wong). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.07.077
are isostructural with only 1.3% lattice mismatch. Therefore, they are ideal templates to each other for epitaxial growth. Besides the well-known c-BN and h-BN phases, other forms or phases of BN have also been reported. These include the sp2bonded BN phases like amorphous BN (α-BN), turbostratic BN (t-BN) and rhombohedral BN (r-BN), and the sp3-bonded phase wurtzitic BN (w-BN), a transformation of c-BN with varied a , b, c axes. In addition, there is another novel BN phase of mixed bonding of sp2 and sp3 called explosion BN (E-BN) [4] or orthorhombic BN (o-BN). Fourier transform infrared spectroscopy (FTIR) has been used extensively to study various BN phases, since their characteristic absorption peaks are quite distinguishable [5,6]. The sp2-bonded BN phases, like h-BN, have two peaks at 1380 cm− 1 and 780 cm− 1, which stem from the in-plane B–N stretching mode and out-of-plane B–N–B deformation mode, respectively. For sp3-bonded BN phases, cBN has a reststrahlen band (TO mode) around 1080 cm− 1 and w-BN phase has IR absorption bands at about 1085, 1125 and 1250 cm− 1. E-BN phase has four peaks in its absorption spectrum occurring at 960, 1250, 1450 and 1600 cm− 1. Most of the vapor deposited BN films were not of high c-BN purity, but contained h-BN, t-BN or α-BN phases, and the c-BN grain size was usually in the nanometer scale. In the past few years, there has been quite significant progress regarding the synthesis of c-BN films. Yamamoto et al.
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reported that thick c-BN films (up to 2.7 μm) were deposited onto Si substrates by an RF diode apparatus using boron carbide (B4C) targets [7]. Feldermann et al. found that the nucleation of nanocrystalline c-BN film directly onto AlN substrates without the soft turbostratic BN interlayer was indeed possible [8]. Matsumoto and Zhang reported that a thick c-BN film of about 20 μm with high phase purity and low film stress was synthesized on Si substrate by dc-bias-assisted DC-jet CVD through the introduction of the chemical effects of fluorine in an Ar–N2–BF3–H2 atmosphere [9]. In this study, we are able to deposit BN and diamond films in the same microwave plasma-assisted chemical vapor deposition (MPCVD) chamber, allowing consecutive growth of diamond/ BN bilayers or multilayers. The influence of diamond films with different crystallinity and surface roughness as buffer-layers on the growth of BN films is explored. 2. Experimental The synthesis of BN and diamond films is performed in a MPCVD system. The MPCVD system allowed independent control of several critical deposition parameters and the details of the system are described elsewhere [2,10]. The diamond buffer layers are deposited for 60 min by the reaction of a constant 1% CH4 (1 sccm) with various flow ratios of Ar/H2 of 0/99, 20/79, 50/49 and 90/9 (sccm/sccm) [10], which produced diamond films of varying crystallinity and surface roughness values of 32, 27, 13 and 0.4 nm, assigned as Films A, B, C and D, respectively and listed in Table 1. The BN layer is then deposited on the diamond film using the same processing parameters. The BN films are deposited by the reaction of 35% NH3/Ar, 5% B2H6/Ar and 60% H2 mixture, which renders the optimal amount of c-BN content based on our prior study described in Ref. [2]. The substrate-temperature, the microwave power and substrate bias current are controlled at 900 °C, 1000 W and 80 mA, respectively. The deposition is continued for 50 min. The same sample notations of Films A, B, C and D are used for the BN films grown on the diamond buffer layers as listed in Table 1. A Bomem-DA8.3 infrared spectrometer is used to obtain the FTIR spectra of the BN films. As suggested, the sp3-bonded BN or c-BN content can be estimated from the peak area (A) intensity ratio A1080/(A1080 + A1380). We extend the same simple formula to A1080, 1125, 1250/(A1080, 1125 ,1250 + A1380) to estimate all the sp3-bonded BN phases [11]. The morphologies of the BN films are obtained using a Hitachi-S4100 SEM with a field-emission electron gun operating Table 1 Growth condition and surface roughness of the diamond buffer layers and c-BN content in the overgrown BN films Sample ID Growth condition: 1 sccm CH4 film in Ar/H2 flow
Surface roughness (Ra)
c-BN content (%)
A B C D
32 nm 27 nm 13 nm 0.4 nm
45 55 73 90
0/99 sccm 50/49 70/29 90/9
at 15 kVand JEOL JSM-6500F with cold cathode field-emission electron gun. Transmission electron microscopy (TEM) analyses of the BN films were performed using a JEOL JEM 3010 with an acceleration voltage of 300 kV and 0.14 nm point-to-point resolution. Samples were thinned mechanically to a thickness of 10 μm. To complete the sample preparation for TEM, 4.5 keV argon ions with an incident angle of 9° to the sample surface, are used to thin the sample further to electron transparency. 3. Results and discussion Fig. 1 shows the Raman spectra of the diamond films with varying roughness from 0.4 to 32 nm. In the Film A, a characteristic peak of diamond at 1332 cm− 1 and some broad peaks associated with graphite (sp2 carbon) around 1350 and 1500– 1650 cm− 1 are observed. The graphite species usually appear in the grain boundaries of CVD diamond films. With the increasing Ar fraction in the gas mixture, the 1332 cm− 1 peak becomes broader in the Films B and C, and disappears in the Film D. In the meanwhile, the characteristic peak of nanocrystalline diamond around 1120 cm− 1 is found in the Films C and D. The series of diamond films are generally categorized as microcrystalline diamond (MCD) for Films A and B and nanocrystalline diamond (NCD) for Films C and D [10]. Fig. 2 shows the SEM morphologies of the diamond films and the overgrown BN films. The images reveal that the surface grain-size and roughness of the diamond are reduced with the increasing Ar fraction in the gas mixture. The morphologies of the overgrown BN films under the same growth conditions are distinctly different from those of the underlying diamond buffer layers. However, the surface roughness and the grain-size of the BN films follow the same trend as the diamond buffer layers. Fig. 3 shows the FTIR absorption spectra of the BN films deposited on the diamond buffer layers. The c-BN content is increased following the decreasing roughness of the diamond films. The estimated c-BN content of the Film A is 45% and is up to 90% for the Film D as listed in Table 1. An inverse dependence of c-BN content in the BN films on the roughness of the diamond layers is observed. The FTIR spectra in Fig. 3 show not only the major h-BN and c-BN phases but also the existence of w-BN and E-BN. The FTIR spectra also indicate the evolution of BN phases among EBN, w-BN, h-BN and c-BN. As the roughness of diamond buffer layers decreased, the BN films consisting of h-BN, c-BN, E-BN and w-BN changed from h-BN dominant, to more w-BN, to more E-BN, and finally to c-BN dominant BN films. This phenomenon is related to the roughness of the diamond buffer layers which could affect the degree of ion bombardment or the ion to neutral ratio during the ion-assisted deposition of BN films. Fig. 4 shows the plan-view TEM images of the BN layer of Film D. Fig. 4(a) shows the selected area diffraction (SAD) ring pattern which is indexed as (1 1 1), (2 0 0), (2 2 0), (2 2 2), (3 1 1) of c-BN. The dominant c-BN content in the film is evidenced by the SAD pattern and the prior FTIR results. Fig. 4(b) and (c) show the bright-field and the dark-field images, respectively, of the film. The dark-field image used the (1 1 1) reflection of c-BN.
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Fig. 1. Raman spectra of the diamond buffer layers. Fig. 3. FTIR spectra of the overgrown BN films.
According to the complementary contrast of a bright-field and a dark-field in a specific diffraction condition, the average grain size can be determined to be about 5 nm. The structure of the BN film is compact with nanolumps (nanocrystallites) or nanoparticles, as shown in Fig. 4(d). The high resolution TEM (HRTEM) of the nanolumps in Fig. 4(e) shows the lattice fringes with the interlayer distance about 0.21 nm, which matched the (1 1 1) dspacing of c-BN. Herein, it is also observed that some amorphous or hexagonal phases exist in the grain boundaries. Fast Fourier transformation (FFT) was performed to analyze the crystal structure of the BN nanolumps in the film. FFT was applied to the image in Fig. 4(e), and the resultant power spectrum is shown in Fig. 4(f). The interlayer distance was estimated using the silicon substrate image as a standard. The inner ring pattern of the FFT shows (111) facet and the interplane
distance estimated from those spots is 0.208 ± 0.006 nm. The outer ring pattern of the FFT is (200) facet and the interplane distance is 0.1808 ± 0.005 nm. Fig. 5 shows the cross-sectional HRTEM images and the corresponding FFT patterns of the Film D shown in Fig. 4. Fig. 5(a) shows two layers on the Si substrate: the lower layer is the NCD buffer layer, the upper layer is the BN film. Fig. 5(b) shows a HRTEM image of a typical interface between the BN film and the NCD buffer layer. Fig. 5(c) exhibits the FFT pattern acquired from the white-framed area c of the BN film region in Fig. 5(b). The (111) and (200) spots assigned to c-BN structure are indicated by arrows δ and γ, respectively. The angles between these spots are marked with α and β. The interplane
Fig. 2. SEM morphologies of the diamond buffer layers and the overgrown BN films.
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Fig. 4. TEM plan-view of Film D with 90% c-BN content: (a) the SAD pattern matching that for c-BN, (b) the bright field image, (c) the dark field image taken on (1 1 1) of c-BN, (d) a higher magnification of the image in (b), (e) HRTEM of the image in (d), and (f) the FFT pattern of the image in (e).
distances estimated from spots δ and γ are 0.208 ± 0.006 nm and 0.180 ± 0.009 nm, respectively. The angles of α and β are 55.2 ± 0.8° and 71.3 ± 0.6°, respectively. In Fig. 5(d), a FFT pattern was obtained from the white-framed area d of the NCD region in Fig. 5(b). The diamond (111) spots can be seen clearly and the inter-spot distance estimated is 0.205 ± 0.01 nm. Cubic BN phase seems to grow directly on NCD without the precursor layers of sp2-bonded BN phase. In contrast, when a MCD buffer layer is used under the identical growth conditions for BN film, a ∼ 50 nm thick interlayer consisting of sp2-bonded BN phases is needed for c-BN nucleation as depicted in the following. Fig. 6 shows the TEM cross-sectional image of Film A. Fig. 6(a) depicts the continuous structural change during the BN film growth and the rough diamond/BN interfaces and BN top surface. Starting from MCD buffer layer, there are α-BN layer, t-BN layer, r-BN layer, and finally c-BN layer. A series of HRTEM images and power spectra to further verify various BN phases are shown in Fig. 6(b) for diamond, α-BN, t-BN and rBN layers and in Fig. 6(c) for c-BN layer. The inter-spot
distance of the spot ɛ shown in Fig. 6(b) is 0.361 ± 0.02 nm which could be assigned to t-BN (002) plane. The distances estimated from spots θ and η in Fig. 6-(b) are 0.341 ± 0.02 nm and 0.212 ± 0.01 nm, respectively. They are attributed to the r-BN (003) plane and (101) plane. Fig. 6(c) illustrates a clear c-BN crystallite and the corresponding FFT pattern. The results demonstrate that the roughness and the crystallinity of the diamond layer play an important role in the growth of c-BN. It has been proposed that the nucleation and growth of c-BN by energetic species is a subsurface but not a surface process involving direct physical phase transformation, equivalent to the bias-enhanced nucleation of diamond [3]. Substrate or buffer layer roughness shadowed some areas of the growing films from ion irradiation and affected the c-BN content, preventing the optimal condition for c-BN formation [1,12]. For BN film grown on smooth diamond buffer layer, in addition to a smooth surface being easier to achieve a uniform ion bombardment effect, the crystallinity of the NCD buffer layer may provide numerous nucleation sites beneficial for the
Fig. 5. (a) TEM cross-sectional image of Film D, (b) HRTEM image of interface between BN film and nano-diamond film, (c) the FFT pattern acquired from the square area c in (b), and (d) the FFT pattern obtained from the square area d in (b).
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Fig. 6. (a) TEM cross-sectional image of Film A showing continuous structural evolution, (b) and (c) selected HETEM and power spectra of t-BN layer, r-BN layer and c-BN layer.
nucleation of c-BN on diamond. Our results indicate that a smooth NCD buffer layer favors the deposition of BN films of high c-BN content. The crystallinity of diamond with various defects including sp2-bonded carbon content can influence the types of the BN nucleated [13]. Amorphous α-BN was easy to deposit on carbon of sp2 and of mixed sp2 and sp3 bonding. Subsequently, t-BN was able to grow on α-BN, followed by an ordered structure of r-BN-like and finally by c-BN as a result of ion-assisted growth. Bending and elimination of BN atomic planes during the ionassisted growth of oriented t-BN from vapor phase might have resulted in the formation of r-BN-like structure [14]. Under a compressive stress, the sp2-bonded material (r-BN-like) would transform locally to the sp3-bonded phase (c-BN) of the closest structural similarity. Although the large activation energy for cBN transformation prevents a diffusive transformation at reasonable rates for bulk static compression at temperatures below 1200 °C–1500 °C [15], the ion bombardment effect and
the high defect concentrations that exist during ion-assisted deposition would likely enhance the rate of c-BN formation. Therefore, most of the sp2-bonded BN (r-BN-like) configuration forms initially in and thereby can transform to c-BN directly. 4. Conclusions The surface roughness and the crystallinity of a diamond buffer layer can influence the formation of various BN phases. When the surface roughness of diamond films is decreased from 32 to 0.4 nm, the c-BN content is increased from 45% to 90%. The c-BN can grow directly on NCD buffer layer without precursor layers while a thick sp2-bonded BN interlayer is needed for c-BN nucleation on MCD buffer layer. A series of HRTEM images and power spectra revealed continuous structural evolution of the BN film grown on MCD buffer layer, starting with α-BN, then followed by t-BN, r-BN and finally c-BN layers.
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Acknowledgement The National Science Council of Taiwan ROC supported this work under grant No. NSC 92-2216-E-259-004 and 94-2120M-259-001. References [1] J. Pascallon, V. Stambouli, S. Ilias, D. Bouchier, G. Nouet, F. Silva, A. Gicquel, Diamond Relat. Mater. 8 (1999) 325. [2] T.S. Yang, Y.P. Cheng, C.L. Cheng, M.S. Wong, Thin Solid Films 441 (2004) 136. [3] Y. Lifshitz, Th. Köhler, Th. Frauenheim, I. Guzmann, A. Hoffman, R.Q. Zhang, X.T. Zhou, S.T. Lee, Science 297 (2002) 1531. [4] A. Olszyna, J. Konwerska-Hrabowska, M. Lisicki, Diamond Relat. Mater. 6 (1997) 617. [5] H. Hofsäss, H. Feldermann, S. Eyhusen, C. Ronning, Phys. Rev., B 65 (2002) 115410.
[6] D.L. Medlin, T.A. Friedmann, P.B. Mirkarimi, P. Rez, M.J. Mills, K.F. McCarty, J. Appl. Phys. 76 (1994) 295. [7] K. Yamamoto, M. Keunecke, K. Bewilogua, Zs. Czigany, L. Hultman, Surf. Coat. Technol. 142 (2001) 881. [8] H. Feldermann, C. Ronning, H. Hofsäss, Y.L. Huang, M. Seibt, J. Appl. Phys. 90 (2001) 3248. [9] S. Matsumoto, W. Zhang, Jpn. J. Appl. Phys. 39 (2000) 442. [10] T.S. Yang, J.Y. Lai, C.L. Cheng, M.S. Wong, J. Appl. Phys. 92 (2002) 4912. [11] C.W. Ong, X.A. Zhao, K.F. Chan, Y.M. Ng, P.W. Chan, C.L. Choy, R.W.M. Kwok, Thin Solid Films 307 (1997) 152. [12] X.W. Zhang, H.G. Boyen, N. Deyneka, P. Ziemann, F. Banhart, M. Schreck, Nat. Matters 2 (2003) 312. [13] W. Zhu, PhD Thesis, Pennsylvania State University, USA, 1990. [14] Y. Yamada-Takamura, O. Tsuda, H. Ichinose, T. Yoshida, Phys. Rev. B59 (1999) 10351. [15] F.R. Corrigan, F.P. Bundy, J. Chem. Phys. 63 (1975) 3812.
Structural evolution of boron nitride films grown on diamond buffer-layers Po-Chih Huang a , Tien-Syh Yang b , Shou-Shu Chu a , Ming-Show Wong a,⁎ a
Department of Materials Science and Engineering, National Dong Hwa University, Hualien, Taiwan, ROC b General Educational Center, Tzu-Chi College of Technology, Hualien, Taiwan, ROC Available online 1 September 2006
Abstract Boron nitride films on diamond buffer layers of varying grain size, surface roughness and crystallinity are deposited by the reaction of B2H6 and NH3 in a mixture of H2 and Ar via microwave plasma-assisted chemical vapor deposition. Various forms of boron nitride, including amorphous α-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion E-BN, wurzitic w-BN and cubic c-BN, are detected in the BN films grown on different diamond buffer layers at varying distances from the interface of diamond and BN layers. The c-BN content in the BN films is inversely proportional to the surface roughness of the diamond buffer layers. Cubic boron nitride can directly grow on smooth nanocrystalline diamond films, while precursor layers consisting of various sp2-bonded BN phases are formed prior to the growth of c-BN film on rough microcrystalline diamond films. © 2006 Elsevier B.V. All rights reserved. Keywords: Cubic boron nitride; Nanocrystalline diamond film; Boron nitride
1. Introduction Cubic boron nitride is the second hardest material next to diamond. However, it is superior to diamond in some applications owing to its higher temperature oxidation resistance and higher chemical inertness to ferrous materials. Nevertheless, for most BN films prepared by various vapor deposition methods, the crystallinity of c-BN film is poor, the film stress is high, and non-cubic BN phases tend to form before c-BN is nucleated, rendering the applications of c-BN films impractical [1–3]. Most researchers have reported that the formation of c-BN film is possible only within a well-defined window of process parameters, e.g., negative substrate bias or the energy of bombarding particles needs to exceed a threshold value. Due to very large film stress, adherent c-BN films have been grown to a thickness less than 1 μm. A lot of CVD procedures, ion-assisted PVD and hybrid methods are reported for depositing c-BN and diamond films. The primary difference between the deposition of c-BN and diamond films is that ion bombardment during the film growth is essential for the synthesis of c-BN film. Diamond and c-BN
⁎ Corresponding author. Tel.: +886 3 8634206; fax: +886 3 8634200. E-mail address: [email protected] (M.-S. Wong). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.07.077
are isostructural with only 1.3% lattice mismatch. Therefore, they are ideal templates to each other for epitaxial growth. Besides the well-known c-BN and h-BN phases, other forms or phases of BN have also been reported. These include the sp2bonded BN phases like amorphous BN (α-BN), turbostratic BN (t-BN) and rhombohedral BN (r-BN), and the sp3-bonded phase wurtzitic BN (w-BN), a transformation of c-BN with varied a , b, c axes. In addition, there is another novel BN phase of mixed bonding of sp2 and sp3 called explosion BN (E-BN) [4] or orthorhombic BN (o-BN). Fourier transform infrared spectroscopy (FTIR) has been used extensively to study various BN phases, since their characteristic absorption peaks are quite distinguishable [5,6]. The sp2-bonded BN phases, like h-BN, have two peaks at 1380 cm− 1 and 780 cm− 1, which stem from the in-plane B–N stretching mode and out-of-plane B–N–B deformation mode, respectively. For sp3-bonded BN phases, cBN has a reststrahlen band (TO mode) around 1080 cm− 1 and w-BN phase has IR absorption bands at about 1085, 1125 and 1250 cm− 1. E-BN phase has four peaks in its absorption spectrum occurring at 960, 1250, 1450 and 1600 cm− 1. Most of the vapor deposited BN films were not of high c-BN purity, but contained h-BN, t-BN or α-BN phases, and the c-BN grain size was usually in the nanometer scale. In the past few years, there has been quite significant progress regarding the synthesis of c-BN films. Yamamoto et al.
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reported that thick c-BN films (up to 2.7 μm) were deposited onto Si substrates by an RF diode apparatus using boron carbide (B4C) targets [7]. Feldermann et al. found that the nucleation of nanocrystalline c-BN film directly onto AlN substrates without the soft turbostratic BN interlayer was indeed possible [8]. Matsumoto and Zhang reported that a thick c-BN film of about 20 μm with high phase purity and low film stress was synthesized on Si substrate by dc-bias-assisted DC-jet CVD through the introduction of the chemical effects of fluorine in an Ar–N2–BF3–H2 atmosphere [9]. In this study, we are able to deposit BN and diamond films in the same microwave plasma-assisted chemical vapor deposition (MPCVD) chamber, allowing consecutive growth of diamond/ BN bilayers or multilayers. The influence of diamond films with different crystallinity and surface roughness as buffer-layers on the growth of BN films is explored. 2. Experimental The synthesis of BN and diamond films is performed in a MPCVD system. The MPCVD system allowed independent control of several critical deposition parameters and the details of the system are described elsewhere [2,10]. The diamond buffer layers are deposited for 60 min by the reaction of a constant 1% CH4 (1 sccm) with various flow ratios of Ar/H2 of 0/99, 20/79, 50/49 and 90/9 (sccm/sccm) [10], which produced diamond films of varying crystallinity and surface roughness values of 32, 27, 13 and 0.4 nm, assigned as Films A, B, C and D, respectively and listed in Table 1. The BN layer is then deposited on the diamond film using the same processing parameters. The BN films are deposited by the reaction of 35% NH3/Ar, 5% B2H6/Ar and 60% H2 mixture, which renders the optimal amount of c-BN content based on our prior study described in Ref. [2]. The substrate-temperature, the microwave power and substrate bias current are controlled at 900 °C, 1000 W and 80 mA, respectively. The deposition is continued for 50 min. The same sample notations of Films A, B, C and D are used for the BN films grown on the diamond buffer layers as listed in Table 1. A Bomem-DA8.3 infrared spectrometer is used to obtain the FTIR spectra of the BN films. As suggested, the sp3-bonded BN or c-BN content can be estimated from the peak area (A) intensity ratio A1080/(A1080 + A1380). We extend the same simple formula to A1080, 1125, 1250/(A1080, 1125 ,1250 + A1380) to estimate all the sp3-bonded BN phases [11]. The morphologies of the BN films are obtained using a Hitachi-S4100 SEM with a field-emission electron gun operating Table 1 Growth condition and surface roughness of the diamond buffer layers and c-BN content in the overgrown BN films Sample ID Growth condition: 1 sccm CH4 film in Ar/H2 flow
Surface roughness (Ra)
c-BN content (%)
A B C D
32 nm 27 nm 13 nm 0.4 nm
45 55 73 90
0/99 sccm 50/49 70/29 90/9
at 15 kVand JEOL JSM-6500F with cold cathode field-emission electron gun. Transmission electron microscopy (TEM) analyses of the BN films were performed using a JEOL JEM 3010 with an acceleration voltage of 300 kV and 0.14 nm point-to-point resolution. Samples were thinned mechanically to a thickness of 10 μm. To complete the sample preparation for TEM, 4.5 keV argon ions with an incident angle of 9° to the sample surface, are used to thin the sample further to electron transparency. 3. Results and discussion Fig. 1 shows the Raman spectra of the diamond films with varying roughness from 0.4 to 32 nm. In the Film A, a characteristic peak of diamond at 1332 cm− 1 and some broad peaks associated with graphite (sp2 carbon) around 1350 and 1500– 1650 cm− 1 are observed. The graphite species usually appear in the grain boundaries of CVD diamond films. With the increasing Ar fraction in the gas mixture, the 1332 cm− 1 peak becomes broader in the Films B and C, and disappears in the Film D. In the meanwhile, the characteristic peak of nanocrystalline diamond around 1120 cm− 1 is found in the Films C and D. The series of diamond films are generally categorized as microcrystalline diamond (MCD) for Films A and B and nanocrystalline diamond (NCD) for Films C and D [10]. Fig. 2 shows the SEM morphologies of the diamond films and the overgrown BN films. The images reveal that the surface grain-size and roughness of the diamond are reduced with the increasing Ar fraction in the gas mixture. The morphologies of the overgrown BN films under the same growth conditions are distinctly different from those of the underlying diamond buffer layers. However, the surface roughness and the grain-size of the BN films follow the same trend as the diamond buffer layers. Fig. 3 shows the FTIR absorption spectra of the BN films deposited on the diamond buffer layers. The c-BN content is increased following the decreasing roughness of the diamond films. The estimated c-BN content of the Film A is 45% and is up to 90% for the Film D as listed in Table 1. An inverse dependence of c-BN content in the BN films on the roughness of the diamond layers is observed. The FTIR spectra in Fig. 3 show not only the major h-BN and c-BN phases but also the existence of w-BN and E-BN. The FTIR spectra also indicate the evolution of BN phases among EBN, w-BN, h-BN and c-BN. As the roughness of diamond buffer layers decreased, the BN films consisting of h-BN, c-BN, E-BN and w-BN changed from h-BN dominant, to more w-BN, to more E-BN, and finally to c-BN dominant BN films. This phenomenon is related to the roughness of the diamond buffer layers which could affect the degree of ion bombardment or the ion to neutral ratio during the ion-assisted deposition of BN films. Fig. 4 shows the plan-view TEM images of the BN layer of Film D. Fig. 4(a) shows the selected area diffraction (SAD) ring pattern which is indexed as (1 1 1), (2 0 0), (2 2 0), (2 2 2), (3 1 1) of c-BN. The dominant c-BN content in the film is evidenced by the SAD pattern and the prior FTIR results. Fig. 4(b) and (c) show the bright-field and the dark-field images, respectively, of the film. The dark-field image used the (1 1 1) reflection of c-BN.
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Fig. 1. Raman spectra of the diamond buffer layers. Fig. 3. FTIR spectra of the overgrown BN films.
According to the complementary contrast of a bright-field and a dark-field in a specific diffraction condition, the average grain size can be determined to be about 5 nm. The structure of the BN film is compact with nanolumps (nanocrystallites) or nanoparticles, as shown in Fig. 4(d). The high resolution TEM (HRTEM) of the nanolumps in Fig. 4(e) shows the lattice fringes with the interlayer distance about 0.21 nm, which matched the (1 1 1) dspacing of c-BN. Herein, it is also observed that some amorphous or hexagonal phases exist in the grain boundaries. Fast Fourier transformation (FFT) was performed to analyze the crystal structure of the BN nanolumps in the film. FFT was applied to the image in Fig. 4(e), and the resultant power spectrum is shown in Fig. 4(f). The interlayer distance was estimated using the silicon substrate image as a standard. The inner ring pattern of the FFT shows (111) facet and the interplane
distance estimated from those spots is 0.208 ± 0.006 nm. The outer ring pattern of the FFT is (200) facet and the interplane distance is 0.1808 ± 0.005 nm. Fig. 5 shows the cross-sectional HRTEM images and the corresponding FFT patterns of the Film D shown in Fig. 4. Fig. 5(a) shows two layers on the Si substrate: the lower layer is the NCD buffer layer, the upper layer is the BN film. Fig. 5(b) shows a HRTEM image of a typical interface between the BN film and the NCD buffer layer. Fig. 5(c) exhibits the FFT pattern acquired from the white-framed area c of the BN film region in Fig. 5(b). The (111) and (200) spots assigned to c-BN structure are indicated by arrows δ and γ, respectively. The angles between these spots are marked with α and β. The interplane
Fig. 2. SEM morphologies of the diamond buffer layers and the overgrown BN films.
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Fig. 4. TEM plan-view of Film D with 90% c-BN content: (a) the SAD pattern matching that for c-BN, (b) the bright field image, (c) the dark field image taken on (1 1 1) of c-BN, (d) a higher magnification of the image in (b), (e) HRTEM of the image in (d), and (f) the FFT pattern of the image in (e).
distances estimated from spots δ and γ are 0.208 ± 0.006 nm and 0.180 ± 0.009 nm, respectively. The angles of α and β are 55.2 ± 0.8° and 71.3 ± 0.6°, respectively. In Fig. 5(d), a FFT pattern was obtained from the white-framed area d of the NCD region in Fig. 5(b). The diamond (111) spots can be seen clearly and the inter-spot distance estimated is 0.205 ± 0.01 nm. Cubic BN phase seems to grow directly on NCD without the precursor layers of sp2-bonded BN phase. In contrast, when a MCD buffer layer is used under the identical growth conditions for BN film, a ∼ 50 nm thick interlayer consisting of sp2-bonded BN phases is needed for c-BN nucleation as depicted in the following. Fig. 6 shows the TEM cross-sectional image of Film A. Fig. 6(a) depicts the continuous structural change during the BN film growth and the rough diamond/BN interfaces and BN top surface. Starting from MCD buffer layer, there are α-BN layer, t-BN layer, r-BN layer, and finally c-BN layer. A series of HRTEM images and power spectra to further verify various BN phases are shown in Fig. 6(b) for diamond, α-BN, t-BN and rBN layers and in Fig. 6(c) for c-BN layer. The inter-spot
distance of the spot ɛ shown in Fig. 6(b) is 0.361 ± 0.02 nm which could be assigned to t-BN (002) plane. The distances estimated from spots θ and η in Fig. 6-(b) are 0.341 ± 0.02 nm and 0.212 ± 0.01 nm, respectively. They are attributed to the r-BN (003) plane and (101) plane. Fig. 6(c) illustrates a clear c-BN crystallite and the corresponding FFT pattern. The results demonstrate that the roughness and the crystallinity of the diamond layer play an important role in the growth of c-BN. It has been proposed that the nucleation and growth of c-BN by energetic species is a subsurface but not a surface process involving direct physical phase transformation, equivalent to the bias-enhanced nucleation of diamond [3]. Substrate or buffer layer roughness shadowed some areas of the growing films from ion irradiation and affected the c-BN content, preventing the optimal condition for c-BN formation [1,12]. For BN film grown on smooth diamond buffer layer, in addition to a smooth surface being easier to achieve a uniform ion bombardment effect, the crystallinity of the NCD buffer layer may provide numerous nucleation sites beneficial for the
Fig. 5. (a) TEM cross-sectional image of Film D, (b) HRTEM image of interface between BN film and nano-diamond film, (c) the FFT pattern acquired from the square area c in (b), and (d) the FFT pattern obtained from the square area d in (b).
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Fig. 6. (a) TEM cross-sectional image of Film A showing continuous structural evolution, (b) and (c) selected HETEM and power spectra of t-BN layer, r-BN layer and c-BN layer.
nucleation of c-BN on diamond. Our results indicate that a smooth NCD buffer layer favors the deposition of BN films of high c-BN content. The crystallinity of diamond with various defects including sp2-bonded carbon content can influence the types of the BN nucleated [13]. Amorphous α-BN was easy to deposit on carbon of sp2 and of mixed sp2 and sp3 bonding. Subsequently, t-BN was able to grow on α-BN, followed by an ordered structure of r-BN-like and finally by c-BN as a result of ion-assisted growth. Bending and elimination of BN atomic planes during the ionassisted growth of oriented t-BN from vapor phase might have resulted in the formation of r-BN-like structure [14]. Under a compressive stress, the sp2-bonded material (r-BN-like) would transform locally to the sp3-bonded phase (c-BN) of the closest structural similarity. Although the large activation energy for cBN transformation prevents a diffusive transformation at reasonable rates for bulk static compression at temperatures below 1200 °C–1500 °C [15], the ion bombardment effect and
the high defect concentrations that exist during ion-assisted deposition would likely enhance the rate of c-BN formation. Therefore, most of the sp2-bonded BN (r-BN-like) configuration forms initially in and thereby can transform to c-BN directly. 4. Conclusions The surface roughness and the crystallinity of a diamond buffer layer can influence the formation of various BN phases. When the surface roughness of diamond films is decreased from 32 to 0.4 nm, the c-BN content is increased from 45% to 90%. The c-BN can grow directly on NCD buffer layer without precursor layers while a thick sp2-bonded BN interlayer is needed for c-BN nucleation on MCD buffer layer. A series of HRTEM images and power spectra revealed continuous structural evolution of the BN film grown on MCD buffer layer, starting with α-BN, then followed by t-BN, r-BN and finally c-BN layers.
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Acknowledgement The National Science Council of Taiwan ROC supported this work under grant No. NSC 92-2216-E-259-004 and 94-2120M-259-001. References [1] J. Pascallon, V. Stambouli, S. Ilias, D. Bouchier, G. Nouet, F. Silva, A. Gicquel, Diamond Relat. Mater. 8 (1999) 325. [2] T.S. Yang, Y.P. Cheng, C.L. Cheng, M.S. Wong, Thin Solid Films 441 (2004) 136. [3] Y. Lifshitz, Th. Köhler, Th. Frauenheim, I. Guzmann, A. Hoffman, R.Q. Zhang, X.T. Zhou, S.T. Lee, Science 297 (2002) 1531. [4] A. Olszyna, J. Konwerska-Hrabowska, M. Lisicki, Diamond Relat. Mater. 6 (1997) 617. [5] H. Hofsäss, H. Feldermann, S. Eyhusen, C. Ronning, Phys. Rev., B 65 (2002) 115410.
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