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Investigation of nitrogen addition on hydrogen incorporation in CVD diamond films from polycrystalline to nanocrystalline
Diamond & Related Materials 19 (2010) 404–408
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Investigation of nitrogen addition on hydrogen incorporation in CVD diamond films from polycrystalline to nanocrystalline C.J. Tang a,b,c,⁎, I. Abe c, L.G. Vieira d, M.J. Soares c, J. Grácio a, J.L. Pinto c a
TEMA, University of Aveiro, 3810-193 Aveiro, Portugal Jiangsu Key Laboratory for Advanced Functional Materials, Changshu Institute of Technology, Changshu 215500, People's Republic of China I3N, University of Aveiro, 3810-193 Aveiro, Portugal d Department of Physics, University of Minho, 4710-057 Braga, Portugal b c
a r t i c l e
i n f o
Available online 4 February 2010 Keywords: Hydrogen impurity incorporation Nitrogen addition Nanocrystalline diamond films Polycrystalline diamond films FTIR spectroscopy Microwave plasma CVD
a b s t r a c t The effect of nitrogen addition in the gas phase on hydrogen impurity incorporation into CVD diamond films was investigated. A series of thick diamond films of different morphology and quality ranging from large-grained polycrystalline to fine-grained nanocrystalline were deposited on silicon wafers using a 5 kW microwave plasma assisted CVD system. They were obtained only by changing the small amount of oxygen and nitrogen addition while keeping all other input parameters the same. Bonded hydrogen impurity in these diamond films was studied by using Fourier-transform infrared spectroscopy. It was found that with increasing the amount of nitrogen addition in the gas phase, the produced diamond films from large-grained polycrystalline gradually shift to fine-grained nanocrystalline and their crystalline quality is drastically degraded, while the amount of incorporated hydrogen impurity in the diamond films increases sharply. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed. These results shed light into the growth mechanism of CVD diamond films ranging from polycrystalline to nanocrystalline, and the incorporation mechanism of hydrogen impurity in CVD diamonds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is an important impurity in diamond. For example, recent theoretical works have shown that through hydrogen impurity passivation of donor–acceptor complexes such as H + B complex, the long-standing doping asymmetry problem for wideband gap materials including diamond may be overcome [1,2]. Hydrogen is a ubiquitous impurity in CVD diamond films ranging from large-grained polycrystalline to nanocrystalline because of the rich hydrogen atmosphere during the growth process [3–12]. Many sophisticated techniques such as nuclear magnetic resonance (NMR) [3,4], elastic recoil detection analysis (EDRA) [6,7], and second ion mass spectroscopy (SIMS) [8,9], have been employed to measure the total amount of hydrogen impurity in diamond films namely both free hydrogen or non-bonded and bonded hydrogen, while Fourier-transform infrared (FTIR) spectroscopy still remains as a powerful tool for detecting boned hydrogen, its incorporation form (such as sp2CHx and sp3CHx, x = 1, 2, 3) and content in diamond films [5,10–12].
⁎ Corresponding author. TEMA, University of Aveiro, 3810-193 Aveiro, Portugal. Tel.: + 351 234 370356; fax: + 351 234 424965. E-mail address: [email protected] (C.J. Tang). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.01.030
Recently, the enormous potential of nitrogen-vacancy defect centre on information process and quantum computing applications has enlivened interest in exploiting the effect of nitrogen addition on CVD diamond growth [13–15], although it has been investigated extensively by many researchers. However, very little work has addressed the effect of nitrogen addition on hydrogen impurity incorporation in CVD diamond films, specially using FTIR spectroscopy. For example, the only report on this subject we can find in the literature investigated the effect of nitrogen addition (0–930 ppm) on hydrogen incorporation in thin diamond films of 2.5 μm thick grown at low temperatures between 340 and 740 °C and low microwave power 800 W [16]. It is very important to investigate the role of nitrogen addition on hydrogen impurity incorporation in CVD diamond, because in one hand, this kind of studies can shed light into the growth mechanism of CVD diamond upon nitrogen addition. On the other hand, if we know how the growth process influences hydrogen impurity incorporation, controlled growth of CVD diamond can be achieved, so that for example, the asymmetry doping problem of diamond as semiconductor may be solved. We have carried out a series of experiments to deposit thick diamond films ranging from large-grained polycrystalline to nanocrystalline by only changing the small amount of N2 and O2 addition under a conventional high microwave power condition using CH4/H2 plasma which is suitable for the growth of a high quality transparent large-
C.J. Tang et al. / Diamond & Related Materials 19 (2010) 404–408
grained polycrystalline film. The morphology, grain size, quality, texture and microstructure can be tuned simply by adjusting the amount of O2 and N2 addition [17]. In this work, we further investigate the effect of nitrogen addition on hydrogen impurity incorporation into this series of diamond films by using FTIR spectroscopy. The objectives of this work are to provide some evidences on the growth mechanism of CVD diamonds, for example the shift from “standard model” of CVD diamond [18] to the so-called Zipper growth mechanism upon nitrogen addition [19] proposed in the literature, and shed light into the incorporation mechanism of hydrogen impurity in CVD diamond films. 2. Experimental details A series of six diamond films A to F investigated here were prepared on silicon wafers of 2 in. in diameter and 3.25 mm in thickness using a 5 kW ASTeX PDS-18 MPCVD system. They were grown using different amounts of N2 and O2 addition, while keeping all other input parameters the same (namely microwave power 3 kW, pressure 105 Torr and 4 vol.% methane/hydrogen concentrations). The essential details are as follows: A (4 vol.% CH4/H2 only, namely neither N2 nor O2 addition), B (1sccm O2 addition, namely 0.24 vol.% O2 addition only), C (0.8 sccm O2 and 0.2 sccm N2 addition, namely 0.19%N2/0.05%O2), D (0.5 sccm O2 and 0.5 sccm N2 addition, namely 0.12%N2/0.12%O2), E (0.2 sccm O2 and
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0.8 sccm N2 addition, namely 0.05%N2/0.19%O2) and F (1 sccm N2 addition, namely 0.24% N2 addition only). A Hitachi S-4100 scanning electron microscope (SEM) operated at 25 kV and micro-Raman spectroscopy using a Jobin Yvon Raman spectrometer with a 514.5 nm Ar+ laser were used to characterize morphology and crystalline quality of the diamond samples. The thickness of the samples was measured by using SEM cross-sectional view, and they are 47 μm for A, 76 μm for B and D, 116 μm for C, 80 μm for E and 87 μm for F. The middle infrared transmittance spectra with 4 cm− 1 resolution were obtained at normal incidence in an evacuated chamber of an IFS 66 V FTIR spectrometer manufactured by Bruker Optics. A KBr beam-splitter and a DTGS detector with KBr window were used to cover the frequency range 400–4000 cm− 1. Note that the FTIR spectra were measured for the as-grown diamond samples when they are still on the silicon substrates. 3. Results and discussion 3.1. Film morphology and crystalline quality Fig. 1 shows the large scale SEM micrographs of the six diamond films for comparison. From these images, one can see that films A (Fig. 1a) and B (Fig. 1b) dominantly consist of well-defined pyramid-
Fig. 1. Large scale SEM images of the six diamond samples A to F grown with different amounts of N2 addition: (a) A, (b) B, (c) C, (d) D, (e) E and (f) F.
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3.2. Analysis of the infrared spectra and calculation of bonded H impurity content of the diamond films
Fig. 2. The FWHM of the diamond Raman peak as a function of the concentration of N2 additive in the gas phase.
like small and large crystallites composed mainly by smooth {111} facets, evincing the high crystalline quality and polycrystalline nature of the samples. While for film C (Fig. 1c), the (111) facets of large crystallites become very rough with many small crystallites overlapping on them, indicating degradation of the crystalline quality due to severe secondary nucleation. In contrast, for films D (Fig. 1d), E (Fig. 1e) and F (Fig. 1f), we cannot observe well-defined crystal shape like those observed in films A, B and C, instead large clusters composed by grains of nanometer size are observed. Of course, their nanocrystalline diamond nature was further confirmed by high resolution SEM observation and AFM analysis, and micro-Raman spectra. For the detailed morphology, Raman spectra and orientation or texture characterization by X-ray diffraction of these samples, please refer to our previous work [17]. The full width at half maximum (FWHM) of the diamond Raman peak around 1332 cm− 1 of the films are plotted in Fig. 2 as a function of N2 concentration in the gas phase. It is easy to observe that the FWHM of the diamond peak of the films increases rapidly with the amount of N2 addition in the gas phase. These results clearly demonstrate the effect of N2 addition on degradation of the diamond film quality, which is consistent with the SEM morphology observation shown above.
The FTIR transmittance spectra (T) of the samples were measured together with the silicon substrates. The absorption by the thick silicon substrate (3.25 mm) and scattering due to surface roughness of the diamond films (especially for large-grained polycrystalline diamond films) make it difficult to calculate the absolute absorbance of the diamond films. Anyway, because the silicon substrate has no specific absorption bands in the diamond two-phonon absorption region above 1332 cm− 1, it follows that T ≈ Ce−(α + αs)d, where C is a constant, αs is the scattering coefficient, α is the diamond absorption coefficient and d is the diamond film thickness. Since light scattering due to surface roughness of diamond films varies smoothly with frequency, the absorbance calculated by taking the logarithm of the transmittance of the whole system (film and substrate), after a local baseline correction, is a good estimation of the absorbance of the diamond films. The FTIR absorbance spectra of the diamond samples calculated by this way are shown in Fig. 3. For easy comparison, we divided the FTIR absorbance spectra into two groups: rough largegrained polycrystalline diamond films and smooth nanocrystalline diamond films. For the three large-grained polycrystalline diamond films A, B and C, from Fig. 3a, one can see clearly that there are two absorption bands with some fine features in the FTIR spectra: one is in the region between 1700 and 2650 cm− 1, which is the intrinsic twophonon absorption band of diamond, and the other is the CH stretching band in the three-phonon region between 2800 and 3100 cm− 1. The well-defined diamond absorption feature further confirms the diamond nature of the polycrystalline diamond samples A, B and C. For samples A and B, the CH stretching band is small, while for sample C, the CH stretching band is much bigger than that of films A and B, with sharp peaks at 2820, 2850 and 2920 cm− 1. For the three nanocrystalline diamond films D, E and F, as shown in Fig. 3b, the twophonon absorption of diamond is weak in comparison with that of the large-grained polycrystalline diamond films, which is consistent with their decreasing of grain size and crystalline quality, demonstrated in the SEM micrographs (Fig. 1). In contrast, the CH stretching bands of the three NCD films D, E, and F are much larger than that of the three polycrystalline films A, B and C and show different fine structures. Among the six films, film F has the largest CH stretching band and film B the smallest one in terms of intensity. The fine structures or sharp
Fig. 3. The absorbance FTIR spectra of the six diamond samples: (a) three large-grained polycrystalline diamond films A, B and C; and (b) three nanocrystalline diamond films D, E and F.
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peaks within the CH stretching band of the nanocrystaline films mainly arise from different types of carbon–hydrogen bonds [5,10] and will be further analyzed in detail later since it is not the scope of this paper. Because the silicon substrate and diamond have no specific absorption band in the CH stretching band region between 2800 and 3000 cm− 1, we can use a local base line fitting procedure to calculate the area or integrated intensity of the CH stretching band in CVD diamond films from the FTIR absorbance spectra. We have shown that the integrated intensity of the CH stretching band ACH divided by the film thickness d, can serve as an indicator for qualitative comparison of the bonded hydrogen content among different CVD diamond films [20]. We therefore calculated the bonded hydrogen content of the six films by this way and plot them in Fig. 4 as a function of the amount of N2 additive in the gas phase for further analysis. Note that so far, we did not consider the difference on transition dipole moments (or absorption strengths) for the individual C–H vibrations (such as sp3CH3, sp3CH2, and sp3CH). Two important remarks upon Fig. 4 can be made. Firstly, the bonded hydrogen content of the diamond films ranging from polycrystalline to nanocrystalline increase rapidly with the amount of nitrogen additive, the same trend as found for their FWHM of the diamond Raman peak. Secondly, the bonded hydrogen content of the nanocrystalline diamond films is much higher (more than one order of magnitude) than that of the polycrystalline films. For instance, the hydrogen content of film F is about 28 times that of film A which was grown without nitrogen addition. These results clearly show that nitrogen addition provoke hydrogen incorporation in CVD diamond films. In summary, our results clearly evidence that a small amount of nitrogen addition not only drastically degrades the crystalline quality of diamond films and lead to the production of diamond film from polycrystalline to nanocrystalline, but also increases rapidly the incorporation of hydrogen impurity into CVD diamond films. 3.3. On the role of nitrogen and oxygen addition on diamond growth and hydrogen incorporation At this point we discuss our results in order to enlighten the role of nitrogen and oxygen addition on diamond growth and hydrogen incorporation. Firstly, in this work, the other growth parameters besides N2 and O2 concentration, such as temperature, CH4 concentration in H2, pressure, and microwave power, were kept constant. Therefore, their roles on complicating the effect of N2 and O2 addition on diamond growth were clearly ruled out. In one hand, the fact that film A (grown by only using CH4/H2 plasma) is a high quality transparent largegrained polycrystalline diamond film, evinces that the set of growth parameters used in this case are conventional conditions for growth of
Fig. 4. The H impurity content calculated from the area of the CH stretching band as a function of the concentration of N2 additive in the gas phase.
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high quality transparent large-grained polycrystalline diamond film. This implies that “standard model” of CVD diamond [18] can be applied to explain the growth mechanism. On the other hand, since film A was grown without N2 or O2 addition, this sample can serve as reference for clarifying the effect of N2 and O2 addition on diamond growth under the conditions employed here. Secondly, by comparing samples A and B, one can conclude that 1 sccm (0.24%) O2 addition improves the crystalline quality of diamond film and largely suppresses hydrogen impurity incorporation. It is worth to note that for sample B the Raman peak FWHM of 4.8 cm− 1 is comparable to that of a high quality IIa natural single diamond crystal (FWHM of 4.0 cm− 1) measured under the same condition, indicating quite high quality of the diamond film. Therefore, it will be expected that under these conditions, O2 addition between 0 and 1 sccm will result in a large-grained polycrystalline diamond film. Thirdly, by comparing samples A and F, one can see that when nitrogen of 1 sccm (namely 0.24%) was added into the gas phase, a drastic change from large-grained polycrystalline to fine-grained NCD happens. Hence, one can draw a conclusion that the same amount of nitrogen addition (0.24%) as that of O2 addition tends to favor the growth of NCD films instead of polycrystalline diamond and strongly degrade the crystalline quality of the diamond films and heavily provoke large amount of hydrogen incorporation into NCD films. In other words, the crystalline quality of diamond films is very sensitive to the amount of N2 addition. This also indicates that there exists a critical value above which NCD will grow instead of polycrystalline diamond when nitrogen addition is between 0 and 1 (SCCM). By comparing samples B and F, one can see that the effect of only nitrogen addition is opposite to that of oxygen addition only. Therefore, for the other three samples C, D and E, when both O2 and N2 are added simultaneously into the CH4/H2 plasma, the results are in between the above two cases, and strongly depends on the relative amount of O2 to N2 addition, and transition from polycrystalline to nanocrystalline happens, since the enhancement effect of oxygen addition counteracts the deleterious effect of nitrogen addition to some extent. Fourthly, we briefly discuss about the growth mechanism and hydrogen incorporation mechanism with nitrogen addition based on our results. In our case, the 3 kW microwave power we employed for the diamond growth is high, correspondingly the gas temperature in the plasma is very high above 3000 K according to the plasma simulation work for a similar case by Lombard et al. [21], and thus the dissociation of the feeding gases H2, CH4, N2 and O2 is very effective. When only CH4/H2 plasma is used, CH3 growth species are dominant and the “standard model” of CVD diamond growth mechanism can be expected. When a small amount of N2 or O2 or both is introduced into the growth chamber, many additional reaction paths are introduced and new intermediate reaction species containing N or O or both are created, which definitely modify the gas phase chemistry and surface reactions and hence result in the formation of either polycrystalline or nanocrystalline diamond film upon to the different amounts of N2 and O2 addition. For example, Kaukonen et al. [19] proposed the Zipper growth mechanism upon nitrogen doping, a modification of the “standard model” of CVD diamond, in which the step consisting of CH3 group attaching to a surface dangling bond site is severely hindered by the presence of subsurface nitrogen, a CH2 can be directly inserted into the bridging position of a dimmer bond at the diamond (100): H (2 × 1) surface, and growth of a whole layer may be catalyzed by the presence of one N electron. Furthermore, T. Van Regemorter and K. Larsson have recently studied the effect of a coadsorbed NH species on the binding of CH3 (or CH2) next to a step edge on the H-terminated (100)−2 × 1 surface, using density functional theory (DFT) [22]. They found that in the two types of monatomic step edges on the diamond (100) surface, the CH2 adsorption reaction was always found to be significantly favored by the presence of NH. Therefore, the
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achievement of nanocrystalline diamond film F of higher growth rate with a small amount of N2 addition into a conventional condition for growth of high quality polycrystalline diamond (standard model of CVD diamond) can be explained by this Zipper growth mechanism or in turn confirms the suitability of this Zipper growth mechanism with nitrogen addition. We propose the following mechanism for hydrogen incorporation in CVD diamond. There are four main steps in the standard model CVD diamond: (i) Removal of H from the surface, (ii) Methyl absorption, (iii) H abstraction and surface rearrangement, and (iv) Migration of CH2 to bridging position. One can see that H is involved in all the steps of CVD diamond growth. If any of these steps does not run thoroughly, then H will be buried into the diamond lattice as an impurity. In other words, the probability for H remain in a CVD diamond film as an impurity during the growth process, strongly depends on the rate of removal of H from the surface and the rate of abstraction of H from CHx radicals adsorbed on the diamond lattice. This point will be further studied later. Since one N may stimulate the growth of a whole layer or the presence of NH species can strongly favor the adsorption of CH2 species, the chance for H being buried as impurity in the diamond lattice is very much higher compared to that without nitrogen addition. This can explain well why with nitrogen addition, the amount of hydrogen impurity significantly increase with the amount of nitrogen addition in the gas phase. 4. Conclusions In this work, various thick diamond films ranging from high quality transparent large-grained polycrystalline to nanocrystalline were prepared only by changing the small amount of nitrogen and oxygen addition into a conventional condition suitable for the growth of a high quality transparent large-grained polycrystalline diamond film. Gradually increasing the amount of nitrogen addition in the gas phase, leads to a significant decrease of grain size and strong degradation of the crystalline quality (increase of secondary nucleation and loss of crystallite shape or facet), and provokes a large amount of hydrogen impurity (measured by FTIR spectroscopy) incorporated into the produced diamond films, which change from high quality transparent large-grained polycrystalline down to black nanocrystalline. These results clearly demonstrate the effect of nitrogen induced nanocrystallinity. On the other hand nitrogen provoked hydrogen impurity incorporation, therefore supporting the zipper growth mechanism proposed with nitrogen addition in the literature, a modification of the
“standard model” of CVD diamond by a small amount of nitrogen addition in the gas phase. Acknowledgements C. J. Tang is grateful for the support from FSE of Portugal and POPH of European Union. A. J. S. Fernandes at I3N and Department of Physics of Aveiro University is acknowledged for his help on preparing the diamond samples. The financial funding by the National Science Foundation (NSF) of China under grant No. 10874021 and Natural Science Foundation of Educational Department of Jiangsu Province under grant No. 06kja43014 is acknowledged. References [1] Y.F. Yan, J.B. Li, S.H. Wei, M.M. Al-Jassim, Phys. Rev. Lett. 98 (2007) 135506. [2] C.X. Yan, Y. Dai, R. Long, H. Jiang, B.B. Huang, R.Q. Zhang, W.J. Zhang, I. Bello, J. Phys. Chem. Solids 70 (2009) 307. [3] K.M. McNamara, D.H. Levy, K.K. Gleason, C.J. Robinson, Appl. Phys. Lett. 60 (5) (1992) 580. [4] K.M. McNamara, K.K. Gleason, C.J. Robinson, J. Vac. Sci. Technol. A 10 (5) (1992) 3143. [5] B. Dischler, C. Wild, W. Müller-Sebert, P. Koidl, Physica B 185 (1993) 217. [6] Ayako Kimura, Yujiro Nakatani, Kunihiro Yamada, Tetsuya Suzuki, Diamond Relat. Mater. 8 (1999) 37. [7] S.A. Rakha, Jianqing Cao, Huihao Xia, Guojun Yu, Dezhang Zhu, Jinlong Gong, Diamond Relat. Mater. 18 (2009) 1247. [8] Sh. Michaelson, O. Ternyak, A. Hoffman, Y. Lifshitz, Appl. Phys. Lett. 90 (2007) 031914. [9] X. Jiang, P. Willich, M. Paul, C.-P. Klages, J. Mater. Res. 14 (1999) 3213. [10] K.M. McNamara, B.E. Williams, K.K. Gleason, B.E. Scruggs, J. Appl. Phys. 76 (1994) 2466. [11] M.S. Haque, H.A. Naseem, J.L. Shultz, W.D. Brown, J. Appl. Phys. 83 (1998) 4421. [12] R. Erz, W. Dőtter, K. Jung, H. Ehrhardt, Diamond Relat. Mater. 4 (1995) 469. [13] S. Dunst, H. Sternschulte, M. Schreck, Appl. Phys. Lett. 94 (2009) 224101. [14] T. Liu, D. Raabe, Appl. Phys. Lett. 94 (2009) 021119. [15] W. Muller-Sebert, E. Worner, F. Fuchs, C. Wild, P. Koidl, Appl. Phys. Lett. 68 (1996) 795. [16] J. Stiegler, J. Michler, E. Blank, Diamond Relat. Mater. 8 (1999) 651. [17] C.J. Tang, A.J. Neves, S. Pereira, A.J.S. Fernandes, J. Grácio, M.C. Carmo, Diamond Relat. Mater. 17 (2008) 78. [18] J.E. Butler, Y.A. Mankelevich, A. Cheesman, Jie Ma, M.N.R. Ashfold, J. Phys.: Condens. Matter 21 (2009) 364201. [19] M. Kaukonen, P.K. Sitch, G. Jungnickel, R.M. Nieminen, Sami Poykko, D. Porezag, Th. Frauenheim, Phys. Rev. B 57 (1998) 9965. [20] C.J. Tang, M.A. Neto, M.J. Soares, A.J.S. Fernandes, A.J. Neves, J. Grácio, Thin Solid Films 515 (2007) 3539. [21] G. Lombardi, K. Hassouni, G.D. Stancu, L. Mechold, J. Röpcke, A. Gicquel, J. Appl. Phys. 98 (2005) 053303. [22] T. Van Regemorter, K. Larsson, J. Phys. Chem. C 113 (2009) 19891.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Investigation of nitrogen addition on hydrogen incorporation in CVD diamond films from polycrystalline to nanocrystalline C.J. Tang a,b,c,⁎, I. Abe c, L.G. Vieira d, M.J. Soares c, J. Grácio a, J.L. Pinto c a
TEMA, University of Aveiro, 3810-193 Aveiro, Portugal Jiangsu Key Laboratory for Advanced Functional Materials, Changshu Institute of Technology, Changshu 215500, People's Republic of China I3N, University of Aveiro, 3810-193 Aveiro, Portugal d Department of Physics, University of Minho, 4710-057 Braga, Portugal b c
a r t i c l e
i n f o
Available online 4 February 2010 Keywords: Hydrogen impurity incorporation Nitrogen addition Nanocrystalline diamond films Polycrystalline diamond films FTIR spectroscopy Microwave plasma CVD
a b s t r a c t The effect of nitrogen addition in the gas phase on hydrogen impurity incorporation into CVD diamond films was investigated. A series of thick diamond films of different morphology and quality ranging from large-grained polycrystalline to fine-grained nanocrystalline were deposited on silicon wafers using a 5 kW microwave plasma assisted CVD system. They were obtained only by changing the small amount of oxygen and nitrogen addition while keeping all other input parameters the same. Bonded hydrogen impurity in these diamond films was studied by using Fourier-transform infrared spectroscopy. It was found that with increasing the amount of nitrogen addition in the gas phase, the produced diamond films from large-grained polycrystalline gradually shift to fine-grained nanocrystalline and their crystalline quality is drastically degraded, while the amount of incorporated hydrogen impurity in the diamond films increases sharply. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed. These results shed light into the growth mechanism of CVD diamond films ranging from polycrystalline to nanocrystalline, and the incorporation mechanism of hydrogen impurity in CVD diamonds. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is an important impurity in diamond. For example, recent theoretical works have shown that through hydrogen impurity passivation of donor–acceptor complexes such as H + B complex, the long-standing doping asymmetry problem for wideband gap materials including diamond may be overcome [1,2]. Hydrogen is a ubiquitous impurity in CVD diamond films ranging from large-grained polycrystalline to nanocrystalline because of the rich hydrogen atmosphere during the growth process [3–12]. Many sophisticated techniques such as nuclear magnetic resonance (NMR) [3,4], elastic recoil detection analysis (EDRA) [6,7], and second ion mass spectroscopy (SIMS) [8,9], have been employed to measure the total amount of hydrogen impurity in diamond films namely both free hydrogen or non-bonded and bonded hydrogen, while Fourier-transform infrared (FTIR) spectroscopy still remains as a powerful tool for detecting boned hydrogen, its incorporation form (such as sp2CHx and sp3CHx, x = 1, 2, 3) and content in diamond films [5,10–12].
⁎ Corresponding author. TEMA, University of Aveiro, 3810-193 Aveiro, Portugal. Tel.: + 351 234 370356; fax: + 351 234 424965. E-mail address: [email protected] (C.J. Tang). 0925-9635/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2010.01.030
Recently, the enormous potential of nitrogen-vacancy defect centre on information process and quantum computing applications has enlivened interest in exploiting the effect of nitrogen addition on CVD diamond growth [13–15], although it has been investigated extensively by many researchers. However, very little work has addressed the effect of nitrogen addition on hydrogen impurity incorporation in CVD diamond films, specially using FTIR spectroscopy. For example, the only report on this subject we can find in the literature investigated the effect of nitrogen addition (0–930 ppm) on hydrogen incorporation in thin diamond films of 2.5 μm thick grown at low temperatures between 340 and 740 °C and low microwave power 800 W [16]. It is very important to investigate the role of nitrogen addition on hydrogen impurity incorporation in CVD diamond, because in one hand, this kind of studies can shed light into the growth mechanism of CVD diamond upon nitrogen addition. On the other hand, if we know how the growth process influences hydrogen impurity incorporation, controlled growth of CVD diamond can be achieved, so that for example, the asymmetry doping problem of diamond as semiconductor may be solved. We have carried out a series of experiments to deposit thick diamond films ranging from large-grained polycrystalline to nanocrystalline by only changing the small amount of N2 and O2 addition under a conventional high microwave power condition using CH4/H2 plasma which is suitable for the growth of a high quality transparent large-
C.J. Tang et al. / Diamond & Related Materials 19 (2010) 404–408
grained polycrystalline film. The morphology, grain size, quality, texture and microstructure can be tuned simply by adjusting the amount of O2 and N2 addition [17]. In this work, we further investigate the effect of nitrogen addition on hydrogen impurity incorporation into this series of diamond films by using FTIR spectroscopy. The objectives of this work are to provide some evidences on the growth mechanism of CVD diamonds, for example the shift from “standard model” of CVD diamond [18] to the so-called Zipper growth mechanism upon nitrogen addition [19] proposed in the literature, and shed light into the incorporation mechanism of hydrogen impurity in CVD diamond films. 2. Experimental details A series of six diamond films A to F investigated here were prepared on silicon wafers of 2 in. in diameter and 3.25 mm in thickness using a 5 kW ASTeX PDS-18 MPCVD system. They were grown using different amounts of N2 and O2 addition, while keeping all other input parameters the same (namely microwave power 3 kW, pressure 105 Torr and 4 vol.% methane/hydrogen concentrations). The essential details are as follows: A (4 vol.% CH4/H2 only, namely neither N2 nor O2 addition), B (1sccm O2 addition, namely 0.24 vol.% O2 addition only), C (0.8 sccm O2 and 0.2 sccm N2 addition, namely 0.19%N2/0.05%O2), D (0.5 sccm O2 and 0.5 sccm N2 addition, namely 0.12%N2/0.12%O2), E (0.2 sccm O2 and
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0.8 sccm N2 addition, namely 0.05%N2/0.19%O2) and F (1 sccm N2 addition, namely 0.24% N2 addition only). A Hitachi S-4100 scanning electron microscope (SEM) operated at 25 kV and micro-Raman spectroscopy using a Jobin Yvon Raman spectrometer with a 514.5 nm Ar+ laser were used to characterize morphology and crystalline quality of the diamond samples. The thickness of the samples was measured by using SEM cross-sectional view, and they are 47 μm for A, 76 μm for B and D, 116 μm for C, 80 μm for E and 87 μm for F. The middle infrared transmittance spectra with 4 cm− 1 resolution were obtained at normal incidence in an evacuated chamber of an IFS 66 V FTIR spectrometer manufactured by Bruker Optics. A KBr beam-splitter and a DTGS detector with KBr window were used to cover the frequency range 400–4000 cm− 1. Note that the FTIR spectra were measured for the as-grown diamond samples when they are still on the silicon substrates. 3. Results and discussion 3.1. Film morphology and crystalline quality Fig. 1 shows the large scale SEM micrographs of the six diamond films for comparison. From these images, one can see that films A (Fig. 1a) and B (Fig. 1b) dominantly consist of well-defined pyramid-
Fig. 1. Large scale SEM images of the six diamond samples A to F grown with different amounts of N2 addition: (a) A, (b) B, (c) C, (d) D, (e) E and (f) F.
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3.2. Analysis of the infrared spectra and calculation of bonded H impurity content of the diamond films
Fig. 2. The FWHM of the diamond Raman peak as a function of the concentration of N2 additive in the gas phase.
like small and large crystallites composed mainly by smooth {111} facets, evincing the high crystalline quality and polycrystalline nature of the samples. While for film C (Fig. 1c), the (111) facets of large crystallites become very rough with many small crystallites overlapping on them, indicating degradation of the crystalline quality due to severe secondary nucleation. In contrast, for films D (Fig. 1d), E (Fig. 1e) and F (Fig. 1f), we cannot observe well-defined crystal shape like those observed in films A, B and C, instead large clusters composed by grains of nanometer size are observed. Of course, their nanocrystalline diamond nature was further confirmed by high resolution SEM observation and AFM analysis, and micro-Raman spectra. For the detailed morphology, Raman spectra and orientation or texture characterization by X-ray diffraction of these samples, please refer to our previous work [17]. The full width at half maximum (FWHM) of the diamond Raman peak around 1332 cm− 1 of the films are plotted in Fig. 2 as a function of N2 concentration in the gas phase. It is easy to observe that the FWHM of the diamond peak of the films increases rapidly with the amount of N2 addition in the gas phase. These results clearly demonstrate the effect of N2 addition on degradation of the diamond film quality, which is consistent with the SEM morphology observation shown above.
The FTIR transmittance spectra (T) of the samples were measured together with the silicon substrates. The absorption by the thick silicon substrate (3.25 mm) and scattering due to surface roughness of the diamond films (especially for large-grained polycrystalline diamond films) make it difficult to calculate the absolute absorbance of the diamond films. Anyway, because the silicon substrate has no specific absorption bands in the diamond two-phonon absorption region above 1332 cm− 1, it follows that T ≈ Ce−(α + αs)d, where C is a constant, αs is the scattering coefficient, α is the diamond absorption coefficient and d is the diamond film thickness. Since light scattering due to surface roughness of diamond films varies smoothly with frequency, the absorbance calculated by taking the logarithm of the transmittance of the whole system (film and substrate), after a local baseline correction, is a good estimation of the absorbance of the diamond films. The FTIR absorbance spectra of the diamond samples calculated by this way are shown in Fig. 3. For easy comparison, we divided the FTIR absorbance spectra into two groups: rough largegrained polycrystalline diamond films and smooth nanocrystalline diamond films. For the three large-grained polycrystalline diamond films A, B and C, from Fig. 3a, one can see clearly that there are two absorption bands with some fine features in the FTIR spectra: one is in the region between 1700 and 2650 cm− 1, which is the intrinsic twophonon absorption band of diamond, and the other is the CH stretching band in the three-phonon region between 2800 and 3100 cm− 1. The well-defined diamond absorption feature further confirms the diamond nature of the polycrystalline diamond samples A, B and C. For samples A and B, the CH stretching band is small, while for sample C, the CH stretching band is much bigger than that of films A and B, with sharp peaks at 2820, 2850 and 2920 cm− 1. For the three nanocrystalline diamond films D, E and F, as shown in Fig. 3b, the twophonon absorption of diamond is weak in comparison with that of the large-grained polycrystalline diamond films, which is consistent with their decreasing of grain size and crystalline quality, demonstrated in the SEM micrographs (Fig. 1). In contrast, the CH stretching bands of the three NCD films D, E, and F are much larger than that of the three polycrystalline films A, B and C and show different fine structures. Among the six films, film F has the largest CH stretching band and film B the smallest one in terms of intensity. The fine structures or sharp
Fig. 3. The absorbance FTIR spectra of the six diamond samples: (a) three large-grained polycrystalline diamond films A, B and C; and (b) three nanocrystalline diamond films D, E and F.
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peaks within the CH stretching band of the nanocrystaline films mainly arise from different types of carbon–hydrogen bonds [5,10] and will be further analyzed in detail later since it is not the scope of this paper. Because the silicon substrate and diamond have no specific absorption band in the CH stretching band region between 2800 and 3000 cm− 1, we can use a local base line fitting procedure to calculate the area or integrated intensity of the CH stretching band in CVD diamond films from the FTIR absorbance spectra. We have shown that the integrated intensity of the CH stretching band ACH divided by the film thickness d, can serve as an indicator for qualitative comparison of the bonded hydrogen content among different CVD diamond films [20]. We therefore calculated the bonded hydrogen content of the six films by this way and plot them in Fig. 4 as a function of the amount of N2 additive in the gas phase for further analysis. Note that so far, we did not consider the difference on transition dipole moments (or absorption strengths) for the individual C–H vibrations (such as sp3CH3, sp3CH2, and sp3CH). Two important remarks upon Fig. 4 can be made. Firstly, the bonded hydrogen content of the diamond films ranging from polycrystalline to nanocrystalline increase rapidly with the amount of nitrogen additive, the same trend as found for their FWHM of the diamond Raman peak. Secondly, the bonded hydrogen content of the nanocrystalline diamond films is much higher (more than one order of magnitude) than that of the polycrystalline films. For instance, the hydrogen content of film F is about 28 times that of film A which was grown without nitrogen addition. These results clearly show that nitrogen addition provoke hydrogen incorporation in CVD diamond films. In summary, our results clearly evidence that a small amount of nitrogen addition not only drastically degrades the crystalline quality of diamond films and lead to the production of diamond film from polycrystalline to nanocrystalline, but also increases rapidly the incorporation of hydrogen impurity into CVD diamond films. 3.3. On the role of nitrogen and oxygen addition on diamond growth and hydrogen incorporation At this point we discuss our results in order to enlighten the role of nitrogen and oxygen addition on diamond growth and hydrogen incorporation. Firstly, in this work, the other growth parameters besides N2 and O2 concentration, such as temperature, CH4 concentration in H2, pressure, and microwave power, were kept constant. Therefore, their roles on complicating the effect of N2 and O2 addition on diamond growth were clearly ruled out. In one hand, the fact that film A (grown by only using CH4/H2 plasma) is a high quality transparent largegrained polycrystalline diamond film, evinces that the set of growth parameters used in this case are conventional conditions for growth of
Fig. 4. The H impurity content calculated from the area of the CH stretching band as a function of the concentration of N2 additive in the gas phase.
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high quality transparent large-grained polycrystalline diamond film. This implies that “standard model” of CVD diamond [18] can be applied to explain the growth mechanism. On the other hand, since film A was grown without N2 or O2 addition, this sample can serve as reference for clarifying the effect of N2 and O2 addition on diamond growth under the conditions employed here. Secondly, by comparing samples A and B, one can conclude that 1 sccm (0.24%) O2 addition improves the crystalline quality of diamond film and largely suppresses hydrogen impurity incorporation. It is worth to note that for sample B the Raman peak FWHM of 4.8 cm− 1 is comparable to that of a high quality IIa natural single diamond crystal (FWHM of 4.0 cm− 1) measured under the same condition, indicating quite high quality of the diamond film. Therefore, it will be expected that under these conditions, O2 addition between 0 and 1 sccm will result in a large-grained polycrystalline diamond film. Thirdly, by comparing samples A and F, one can see that when nitrogen of 1 sccm (namely 0.24%) was added into the gas phase, a drastic change from large-grained polycrystalline to fine-grained NCD happens. Hence, one can draw a conclusion that the same amount of nitrogen addition (0.24%) as that of O2 addition tends to favor the growth of NCD films instead of polycrystalline diamond and strongly degrade the crystalline quality of the diamond films and heavily provoke large amount of hydrogen incorporation into NCD films. In other words, the crystalline quality of diamond films is very sensitive to the amount of N2 addition. This also indicates that there exists a critical value above which NCD will grow instead of polycrystalline diamond when nitrogen addition is between 0 and 1 (SCCM). By comparing samples B and F, one can see that the effect of only nitrogen addition is opposite to that of oxygen addition only. Therefore, for the other three samples C, D and E, when both O2 and N2 are added simultaneously into the CH4/H2 plasma, the results are in between the above two cases, and strongly depends on the relative amount of O2 to N2 addition, and transition from polycrystalline to nanocrystalline happens, since the enhancement effect of oxygen addition counteracts the deleterious effect of nitrogen addition to some extent. Fourthly, we briefly discuss about the growth mechanism and hydrogen incorporation mechanism with nitrogen addition based on our results. In our case, the 3 kW microwave power we employed for the diamond growth is high, correspondingly the gas temperature in the plasma is very high above 3000 K according to the plasma simulation work for a similar case by Lombard et al. [21], and thus the dissociation of the feeding gases H2, CH4, N2 and O2 is very effective. When only CH4/H2 plasma is used, CH3 growth species are dominant and the “standard model” of CVD diamond growth mechanism can be expected. When a small amount of N2 or O2 or both is introduced into the growth chamber, many additional reaction paths are introduced and new intermediate reaction species containing N or O or both are created, which definitely modify the gas phase chemistry and surface reactions and hence result in the formation of either polycrystalline or nanocrystalline diamond film upon to the different amounts of N2 and O2 addition. For example, Kaukonen et al. [19] proposed the Zipper growth mechanism upon nitrogen doping, a modification of the “standard model” of CVD diamond, in which the step consisting of CH3 group attaching to a surface dangling bond site is severely hindered by the presence of subsurface nitrogen, a CH2 can be directly inserted into the bridging position of a dimmer bond at the diamond (100): H (2 × 1) surface, and growth of a whole layer may be catalyzed by the presence of one N electron. Furthermore, T. Van Regemorter and K. Larsson have recently studied the effect of a coadsorbed NH species on the binding of CH3 (or CH2) next to a step edge on the H-terminated (100)−2 × 1 surface, using density functional theory (DFT) [22]. They found that in the two types of monatomic step edges on the diamond (100) surface, the CH2 adsorption reaction was always found to be significantly favored by the presence of NH. Therefore, the
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achievement of nanocrystalline diamond film F of higher growth rate with a small amount of N2 addition into a conventional condition for growth of high quality polycrystalline diamond (standard model of CVD diamond) can be explained by this Zipper growth mechanism or in turn confirms the suitability of this Zipper growth mechanism with nitrogen addition. We propose the following mechanism for hydrogen incorporation in CVD diamond. There are four main steps in the standard model CVD diamond: (i) Removal of H from the surface, (ii) Methyl absorption, (iii) H abstraction and surface rearrangement, and (iv) Migration of CH2 to bridging position. One can see that H is involved in all the steps of CVD diamond growth. If any of these steps does not run thoroughly, then H will be buried into the diamond lattice as an impurity. In other words, the probability for H remain in a CVD diamond film as an impurity during the growth process, strongly depends on the rate of removal of H from the surface and the rate of abstraction of H from CHx radicals adsorbed on the diamond lattice. This point will be further studied later. Since one N may stimulate the growth of a whole layer or the presence of NH species can strongly favor the adsorption of CH2 species, the chance for H being buried as impurity in the diamond lattice is very much higher compared to that without nitrogen addition. This can explain well why with nitrogen addition, the amount of hydrogen impurity significantly increase with the amount of nitrogen addition in the gas phase. 4. Conclusions In this work, various thick diamond films ranging from high quality transparent large-grained polycrystalline to nanocrystalline were prepared only by changing the small amount of nitrogen and oxygen addition into a conventional condition suitable for the growth of a high quality transparent large-grained polycrystalline diamond film. Gradually increasing the amount of nitrogen addition in the gas phase, leads to a significant decrease of grain size and strong degradation of the crystalline quality (increase of secondary nucleation and loss of crystallite shape or facet), and provokes a large amount of hydrogen impurity (measured by FTIR spectroscopy) incorporated into the produced diamond films, which change from high quality transparent large-grained polycrystalline down to black nanocrystalline. These results clearly demonstrate the effect of nitrogen induced nanocrystallinity. On the other hand nitrogen provoked hydrogen impurity incorporation, therefore supporting the zipper growth mechanism proposed with nitrogen addition in the literature, a modification of the
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