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Effects of plasma treatments on the nitrogen incorporated nanocrystalline diamond films
Diamond & Related Materials 17 (2008) 1994–1997
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
Effects of plasma treatments on the nitrogen incorporated nanocrystalline diamond films Sathyaharish Jeedigunta a,c, Zhenqing Xu b,c, Makoto Hirai b,c, Priscila Spagnol d, Ashok Kumar b,c,⁎ a
Department of Electrical Engineering, University of South Florida, Tampa, FL-33620, USA Department of Mechanical Engineering, University of South Florida, Tampa, FL-33620, USA Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, FL-33620, USA d SRI International, SRI St. Petersburg, 140 Seventh Avenue South, COT 100, St. Petersburg, FL 33701, USA b c
A R T I C L E
I N F O
Article history: Received 3 September 2007 Received in revised form 12 May 2008 Accepted 6 June 2008 Available online 10 June 2008 Keywords Nanocrystalline diamond films Nitrogen-plasma treatment MPECVD
A B S T R A C T The nitrogen incorporated nanocrystalline diamond (NCD) films were grown on n-silicon (100) substrates by microwave plasma enhanced chemical vapor deposition (MPECVD) using CH4/Ar/N2 gas chemistry. The effect of surface passivation on the properties of NCD films was investigated by hydrogen and nitrogen-plasma treatments. The crystallinity of the NCD films reduced due to the damage induced by the plasma treatments. From the crystallographic data, it was observed that the intensity of (111) peak of the diamond lattice reduced after the films were exposed to the nitrogen plasma. From Raman spectra, it was observed that the relative intensity of the features associated with the transpolyacetylene (TPA) states decreased after hydrogen-plasma treatment, while such change was not observed after nitrogen-plasma treatment. The hydrogen-plasma treatment has reduced the sp2/sp3 ratio due to preferential etching of the graphitic carbon, while this ratio remained same in both as-grown and nitrogen-plasma treated films. The electrical contacts of the as-grown films changed from ohmic to near Schottky after the plasma treatment. The electrical conductivity reduced from ~ 84 ohm– 1 cm− 1 (as-grown) to ~ 10 ohm– 1 cm− 1 after hydrogen-plasma treatment, while the change in the conductivity was insignificant after nitrogen-plasma treatment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction It is widely known that the electrical resistivity of diamond films can be reduced by several orders of magnitude upon surface treatment in different ambients. The change in the surface conductivity by the hydrogen-plasma treatment has been reported by many researchers [1–4]. It is also known that the hydrogenation of diamond surfaces reduces the electrical resistivity by “surface transfer doping” mechanism. As polycrystalline diamond films have large grains, a higher amount of hydrogen termination and consecutively a greater reduction in the electrical resistivity can be achieved. The electrical properties of nanocrystalline diamond (NCD) films can be different as they have small grains and tight grain boundaries. In fact, it was reported that the electrical conductivity of pure NCD films has changed only from 10− 6 Ω− 1 cm− 1 to 10− 8 Ω− 1 cm− 1 after hydrogenation [4]. It was reported that the electrical conductivity (~ 200 Ω− 1 cm− 1) of the nitrogen incorporated NCD films is due to sp2 bonded carbon, defects and other impurities [5,6]. As the conductivity of these films is via grain boundaries by hopping mechanism, the surface conductivity ⁎ Corresponding author. ENB 118, Department of Mechanical Engineering, 4202 E. Fowler Avenue, Tampa, FL-33620, USA. Tel.: +1 813 974 3942; fax: +1 813 974 3610. E-mail address: [email protected] (A. Kumar). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.06.001
induced by the plasma treatment may be completely different. The nitrogen incorporated NCD films have gained attention as excellent field and thermionic emission sources [7,8]. It will be interesting to understand the effect of hydrogenation and nitrogenation on the surface conductivity of the nitrogen incorporated NCD films, as the surface treatment is known to improve the field emission properties. Firstly, if the conductivity of such films is due to the presence of defects and sp2 bonded carbon, the atomic hydrogen produced during hydrogenation can passivate the surface and reduce the defects, thereby providing insight into the electrical conduction in the nitrogen incorporated NCD films. On the other hand, the nitrogen-plasma treatment has also shown to improve the electrical properties of the films [9–11]. It was reported that after the nitrogen ion-implantation, the field emission characteristics have improved due to the breaking of sp3 bonds, creation of amorphous regions and possible defects. Due to high energies in the conventional implanters, it may be difficult to recover the damaged surface completely. The plasma treatment can be considered as a low energy ion-implantation technique [12]. Therefore, the nitrogen-plasma treatment of the nitrogen incorporated NCD films has drawn our attention. However, a systematic study on the effect of hydrogen and nitrogen-plasma treatments of the nitrogen incorporated NCD films has not been conducted [4]. In this paper, we have conducted an
S. Jeedigunta et al. / Diamond & Related Materials 17 (2008) 1994–1997
1995
elaborate and a systematic study on the influence of plasma treatments on the structural properties and the electrical conductivity of nitrogen incorporated NCD films. 2. Experimental The nitrogen incorporated nanocrystalline diamond films were grown in a 2.45 GHz Cyrannus I Iplas microwave plasma enhanced chemical vapor deposition (MPECVD) reactor. The n-Si (100) substrates (resistivity 5–15 ohm cm) were seeded by ultrasonic scratching in a nanodiamond powder dispersed in methanol. The process chemistry included CH4/Ar/N2 gases with 1% methane, 79% argon, and 20% nitrogen at a microwave power of 800 W, pressure of 100 Torr, and a substrate temperature ~750 °C for 3 h. The as-grown films were exposed to nitrogen and hydrogen plasmas using a microwave power of 600 W, at a pressure of 30 Torr and a substrate temperature of ~ 700–750 °C for about 30 min. The thickness of both as-grown and the plasma-treated films was found to be ~ 1 μm. A bilayer of Ti (~ 20 nm)/Au (~ 190 nm) was deposited by R.F sputtering and the electrical contacts (area: 1000 µm × 1000 µm) were achieved by conventional photo-lithography technique. The structural properties of the films were characterized by X-ray diffraction (XRD), visible-Raman spectroscopy, scanning electron microscopy (SEM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. XRD measurements were conducted in a Philips X'pert pro diffractometer using Cu-kα radiation, 1° incident slit, 2° diffracted slit and a 15 mm incident beam mask. The Raman spectroscopy was carried out using a Renishaw 1000 Raman spectrometer with an Argon laser at a wavelength of 514.5 nm, and a laser spot size of 1 µm at a power of 25 mW. SEM was performed in a Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 5 kV and a beam current of 10 µA. In order to study the influence of surface plasma treatment on the nature of chemical bonding, NEXAFS study was conducted at Synchrotron Radiation Center at University of Wisconsin, Madison using Hermon beam line in a total electron yield (TEY) mode. The room temperature
Fig. 2. (a–c) Scanning electron micrographs of the as-grown films and the samples exposed to hydrogen and nitrogen plasma.
electrical conductivity of the films was measured in plane by making the electrical contacts through the top metal (Ti/Au) electrodes using a Jandel four point probe resistivity measurement tool. 3. Results and discussion
Fig. 1. X-ray diffraction spectra of the nitrogen incorporated nanocrystalline diamond films grown in 20% N before and after the hydrogen and nitrogen-plasma treatments.
Fig. 1 shows the (111) peak of as-grown, hydrogen-plasma treated and nitrogen-plasma treated nitrogen incorporated NCD films. It can be observed that the diffraction spectra of both as-grown and hydrogen (H) plasma treated films were similar; but the later ones showed decrease in the intensity of (111) peak and increase in the FWHM value. In the nitrogen (N) plasma treated films, the diffraction intensity has decreased further indicating the loss of crystallinity. It was reported earlier that the loss of crystallinity in the nitrogen-plasma treated films could be due to breaking of sp3 bonds [9]. It can be concluded that the nitrogen-plasma treatment has more pronounced effect on the crystallinity than the hydrogen-plasma treatment. From the top-view SEM micrographs of the as-grown and the plasmatreated films, it can be observed that the structure of as-grown nitrogen incorporated NCD films (See Fig. 2 (a)) resembled a “cauliflower”-like morphology. Further, it can be noticed in Fig. 2 (b) and Fig. 2 (c) that such a structure disappeared after the plasma treatment. After the hydrogen-
1996
S. Jeedigunta et al. / Diamond & Related Materials 17 (2008) 1994–1997
plasma treatment, fine needle-like structure was observed (see Fig. 2 (b)) indicating the etching of graphitic carbon by the bombardment of hydrogen ions. The transition in the structural morphology is revealed due to the removal of carbon layer on the surface of the nanodiamond film. It was later confirmed by Raman and NEXAFS spectroscopy that the sp2 bonded carbon was indeed etched after hydrogenation. On the other hand, the surface morphology of nitrogen-plasma treated films showed that the cauliflower-like surface in the as-grown films (see Fig. 2 (c)) has coalesced to form a homogenous layer. It is known that the nitrogen-plasma treatment deposits an amorphous carbon layer [11]. An indistinct amount of this amorphous carbon layer could have covered the surface of the film, which may have slightly changed the surface morphology. The visible-Raman spectra (Fig. 3) of as-grown and plasma-treated NCD films are characterized by four important features: i.e. peaks at ~ 1145 cm− 1, 1348 cm− 1, 1486 cm− 1 and 1564 cm− 1, respectively. The peak at ~1145 cm− 1 was referred to as a signature of nanodiamond [13–15], while A. C. Ferrari et al. have suggested that this peak is always accompanied by another feature at ~ 1450 cm− 1 and is due to the presence of transpolyacetylene (TPA) states at the grain boundaries [16]. The peaks at ~1348 cm− 1 and ~ 1560 cm− 1 correspond to the D (dis-ordered) band and G (graphitic) band respectively [6]. They represent the sp2 bonded carbon located in the grain boundaries of the nanodiamond film. It has been shown that the amount of TPA states present in the films is related to the presence of hydrogen [17]. As TPA states are unstable at high temperatures, these features disappear when NCD films are heated at ~1200 °C [18]. But in this study, the hydrogen-plasma treatment of the as-grown films was done at relatively lower temperatures (~700 °C–750 °C). Hence, the TPA states have not completely disappeared; instead, the relative intensities of the features associated with TPA states (1140 cm− 1 and the shoulder at 1486 cm− 1) decreased along with a shift in the position
Fig. 4. (a) NEXAFS spectra of the 20% nitrogen incorporated nanocrystalline diamond films before and after the plasma treatment, (b) shows the peak corresponding to the π⁎ anti-bonding state of the films before and after the plasma treatment.
Fig. 3. Visible-Raman spectra of the 20% N incorporated nanocrystalline diamond films before and post plasma treatments.
of G-band to a higher wave number. It can be noticed that the surface hydrogenation has a significant role in affecting the bonding characteristics of the films. For the as-grown and the nitrogen-plasma treated films, the relative intensity of G-band was higher than D-band, indicating the existence of a higher amount of ordered sp2 carbon in the films. Fig. 4 (a) shows the NEXAFS spectra of as-grown and plasmatreated samples after the normalization and the removal of background signal from any carbon contamination of the optics. It can be observed that the spectrum of the as-grown sample has three well defined features: π⁎ anti-bonding state at ~ 284 eV, σ⁎ exciton peak at ~289 eV and a second band gap of diamond at ~ 302 eV. After the samples were exposed to hydrogen plasma, the σ⁎ exciton peak has become more distinct with a sharp and defined pre-edge, while this feature in the nitrogen-plasma treated samples has not shown any variation. On the other hand, the spectrum of nitrogenated films has an unknown peak at ~286 eV and other features at ~284 eV, ~ 289 eV and ~302 eV, respectively. Fig. 4 (b) shows the π⁎ anti-bonding state of
S. Jeedigunta et al. / Diamond & Related Materials 17 (2008) 1994–1997 Table 1 Effect of the post deposition plasma treatment on the conductivity of the samples. “as-grown” indicates the conductivity of the NCD films grown with 20% nitrogen in the gas chemistry
Ar CH4 N2 H2 Power (W) Pressure (Torr) Substrate temperature (°C) Conductivity (ohm.cm)– 1 Nature of contact
As-grown (20% N) NCD film
H-plasma treated film
N-plasma treated film
79% 1% 20% 0% 800 100 750
100% 600 30 750
600 30 750
84.03 ± 2.4
10.93 ± 1.71
82.67 ± 1.24
Ohmic
Non-ohmic
Non-ohmic
100%
the as-grown, hydrogen-plasma treated and nitrogen-plasma treated NCD films. In order to quantify the amount of sp2 present in the asgrown and plasma-treated films, we have estimated the fraction of sp2 by using Eq. (1).
Fsp2 ¼
⁎
π⁎ ISAMPLE ⁎ π IREFERENCE ⁎
IREFERENCE ðΔEÞ ISAMPLE ðΔEÞ
1997
plasma excitation of nitrogen ions will not change the sp2 bonded carbon content in the nitrogen incorporated NCD films. 4. Conclusions The effects of nitrogen and hydrogen-plasma treatments on the structural properties and the electrical conductivity of nitrogen incorporated NCD films were studied. The diffraction studies showed that there was a reduction in the crystallinity of the as-grown films after the plasma treatments. nitrogen-plasma treatment showed loss of crystallinity to a higher degree than the hydrogenated films. Raman spectra of the hydrogen-plasma treated films showed a reduced intensity of the peaks associated with the TPA states and a shift in the G-band to a higher wave number, suggesting a possible rearrangement of sp2-carbon. It was observed that the sp2/sp3 ratio in the asgrown and the nitrogenated films was ~14%, whereas this ratio decreased to 12% after the hydrogen-plasma treatment. The reduction in the electrical conductivity of the hydrogenated samples was attributed to the decrease in the amount of sp2 bonded carbon. The nitrogen-plasma treatment has an insignificant effect on the electrical conductivity. It was confirmed that the electrical conductivity in the films was predominantly due to the sp2 bonded carbon and the surface defects that are passivated during the hydrogenation.
ð1Þ
π π ISAMPLE ; IREFERENCE ; IREFERENCE ðΔEÞ; ISAMPLE ðΔEÞ denote the areas under the π⁎ peak of both as-deposited and the plasma-treated NCD films and the HOPG reference sample, respectively. IREFERENCE(ΔE), ISAMPLE(ΔE) are the remaining areas under the HOPG reference spectrum, the as-deposited and the plasma-treated spectra respectively. From these calculations, it was estimated that the as-grown films consisted of ~14% sp2. The presence of this order of sp2 bonded carbon in the grain boundaries has been reported [6]. Similarly, the sp2 content in the hydrogen and nitrogen-plasma treated films was found to be ~12% and ~14.2%, respectively. It is known that the hydrogen-plasma treatment can etch the sp2 bonded carbon resulting in the decreased sp2/sp3 ratio. As observed in the Raman spectra, the ratio ID:IG in both as-grown and nitrogenated samples remained the same. Hence, the lower crystallinity of the nitrogen-plasma treated films cannot be due to breaking of sp3 bonds to sp2 and other constituents, but rather could be due to the damage induced onto the crystal lattice. It may not be possible to increase the sp2 content or convert sp3 to sp2 bonded carbon by this method of nitrogen-plasma treatment. This data was consistent with the fact that the electrical conductivity values (see Table 1) of both as-grown and nitrogenated samples was similar. It is known that the electrical conductivity of the as-grown NCD films is due to the creation of electron lone pair induced during the incorporation of nitrogen [6,19–21]. The room temperature electrical conductivity of asgrown, hydrogen-plasma and nitrogen-plasma treated films is shown in Table 1. The highest conductivity of ~84.03 ohm– 1 cm− 1 was obtained on as-grown films, while the hydrogen-plasma treated films showed the least. No significant change in the electrical conductivity of the NCD films was observed after the nitrogen-plasma treatment. It was difficult to obtain a linear relation between current and voltage after the hydrogenplasma treatment. The conductivity of the hydrogen-plasma treated films measured at 10 V showed a huge decrease. The decrease in the electrical conductivity is attributed to the removal of sp2 bonded carbon and the passivation of defects during the hydrogenation. It confirms that the higher values of electrical conductivity in the nitrogen incorporated NCD films is due to the presence of sp2 bonded carbon, and defects in the grain boundaries. On the other hand, the passive role of nitrogen-plasma treatment on the electrical conductivity suggests that the microwave
Acknowledgements Financial support of this work by SMDC, under contract No. W9113 M-06-C-0022 and by NSF NIRT, under grant No. 0404137 is highly acknowledged. The authors would like to thank Mr. Dave Edwards for conducting the SEM. This work is based upon the research conducted at the Synchrotron Radiation Center, University of Wisconsin– Madison, which is supported by the NSF under Contract No. DMR0084402. “Distribution A”. Approved for Public Release; distribution Unlimited.” References [1] S. Albin, L. Watkins, Appl. Phys. Lett 56 (15) (1990) 1454. [2] Y. Yamazaki, K. Ishikawa, N. Mizuochi, S. Yamasaki, Diam. Relat. Mater. 14 (2005) 1939. [3] Y.C. Lee, D. Pradhan, S.J. Lin, C.T. Chia, H.F. Cheng, I.N. Lin, Diam. Relat. Mater 14 (2005) 2055. [4] Y.C. Chen, A.J. Cheng, M. Clark, Y.K. Liu, Y. Tzeng, Diamond Relat. Mater 15 (2006) 440. [5] S. Bhattacharyya, O. Auciello, J. Birrell, J.A. Carlisle, L.A. Curtiss, A.N. Goyette, D.M. Gruen, A.R. Krauss, J. Schlueter, A. Sumant, P. Zapol, Appl. Phys. Lett. 79 (10) (2001) 1441. [6] J. Birrell, J.E. Gerbi, O. Auciello, J.M. Gibson, D.M. Gruen, J.A. Carlisle, J. Appl. Phys. 93 (9) (2003) 5606. [7] R.S. Takalkar, W.P. Kang, J.L. Davidson, B.K. Choi, W.H. Hofmeister, K. Subramanian, Diam. Relat. Mater 15 (2006) 329. [8] Franz A.M. Koeck, Robert J. Nemanich, Diam. Relat. Mater 15 (2006) 217. [9] M.Y. Chen, K.Y. Wu, J.H. Wang, M.T. Chang, L.J. Chou, C.S. Kou, Nanotechnology 18 (2007) 455706. [10] C. Kimura, Y. Yamamuro, H. Aoki, T. Sugino, Diam. Relat. Mater 16 (2007) 1383. [11] F.L. Freire Jr., S.I. Castañeda, R. Prioli, Diam. Relat. Mater 16 (2007) 1282. [12] J.R. Roth, Industrial Plasma Engineering, Applications to Nonthermal Plasma Processing, vol. 2, Institute of Physics Publishing, Bristol, 2001. [13] T. Sharda, T. Soga, T. Jimbo, M. Uembo, Diam. Relat. Mater 10 (2001) 352. [14] T. Lin, Y. Yu, A.T.S. Wee, Z.X. Shen, K.P. Loh, Appl. Phys. Lett 77 (2000) 2692. [15] L. Fayette, B. Marcus, M. Mermoux, G. Tourillon, K. Laffon, P. Parent, F. LeNormand, Phys. Rev. B 57 (1998) 14123. [16] A.C. Ferrari, J. Robertson, Phys. Rev. B 63 (2001) 121405-1. [17] K. Teii, T. Ikeda, A. Fukutomi, K. Uchino, J. Vac. Sci. Technol. B 24 (1) (2006) 263. [18] R. Pfeiffer, H. Kuzmany, P. Knoll, S. Bokova, N. Salk, B. Günther, Diam. Relat. Mater 12 (2003) 268. [19] J.E. Gerbi, J. Birrell, M. Sardela, J.A. Carlisle, Thin Solid Films 473 (2005) 41. [20] P. Zapol, M. Sternberg, L.A. Curtiss, T. Frauenheim, D.M. Gruen, Phys. Rev., B 65 (2001) 045403. [21] Q. Hu, S. Jeedigunta, Z. Xu, M. Hirai, A. Kumar, Extended Abstracts (The 68th Autum Meeting, 2007), vol. 612, The Japan Society Applied Physics II, 2007.
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
Effects of plasma treatments on the nitrogen incorporated nanocrystalline diamond films Sathyaharish Jeedigunta a,c, Zhenqing Xu b,c, Makoto Hirai b,c, Priscila Spagnol d, Ashok Kumar b,c,⁎ a
Department of Electrical Engineering, University of South Florida, Tampa, FL-33620, USA Department of Mechanical Engineering, University of South Florida, Tampa, FL-33620, USA Nanomaterials and Nanomanufacturing Research Center, University of South Florida, Tampa, FL-33620, USA d SRI International, SRI St. Petersburg, 140 Seventh Avenue South, COT 100, St. Petersburg, FL 33701, USA b c
A R T I C L E
I N F O
Article history: Received 3 September 2007 Received in revised form 12 May 2008 Accepted 6 June 2008 Available online 10 June 2008 Keywords Nanocrystalline diamond films Nitrogen-plasma treatment MPECVD
A B S T R A C T The nitrogen incorporated nanocrystalline diamond (NCD) films were grown on n-silicon (100) substrates by microwave plasma enhanced chemical vapor deposition (MPECVD) using CH4/Ar/N2 gas chemistry. The effect of surface passivation on the properties of NCD films was investigated by hydrogen and nitrogen-plasma treatments. The crystallinity of the NCD films reduced due to the damage induced by the plasma treatments. From the crystallographic data, it was observed that the intensity of (111) peak of the diamond lattice reduced after the films were exposed to the nitrogen plasma. From Raman spectra, it was observed that the relative intensity of the features associated with the transpolyacetylene (TPA) states decreased after hydrogen-plasma treatment, while such change was not observed after nitrogen-plasma treatment. The hydrogen-plasma treatment has reduced the sp2/sp3 ratio due to preferential etching of the graphitic carbon, while this ratio remained same in both as-grown and nitrogen-plasma treated films. The electrical contacts of the as-grown films changed from ohmic to near Schottky after the plasma treatment. The electrical conductivity reduced from ~ 84 ohm– 1 cm− 1 (as-grown) to ~ 10 ohm– 1 cm− 1 after hydrogen-plasma treatment, while the change in the conductivity was insignificant after nitrogen-plasma treatment. © 2008 Elsevier B.V. All rights reserved.
1. Introduction It is widely known that the electrical resistivity of diamond films can be reduced by several orders of magnitude upon surface treatment in different ambients. The change in the surface conductivity by the hydrogen-plasma treatment has been reported by many researchers [1–4]. It is also known that the hydrogenation of diamond surfaces reduces the electrical resistivity by “surface transfer doping” mechanism. As polycrystalline diamond films have large grains, a higher amount of hydrogen termination and consecutively a greater reduction in the electrical resistivity can be achieved. The electrical properties of nanocrystalline diamond (NCD) films can be different as they have small grains and tight grain boundaries. In fact, it was reported that the electrical conductivity of pure NCD films has changed only from 10− 6 Ω− 1 cm− 1 to 10− 8 Ω− 1 cm− 1 after hydrogenation [4]. It was reported that the electrical conductivity (~ 200 Ω− 1 cm− 1) of the nitrogen incorporated NCD films is due to sp2 bonded carbon, defects and other impurities [5,6]. As the conductivity of these films is via grain boundaries by hopping mechanism, the surface conductivity ⁎ Corresponding author. ENB 118, Department of Mechanical Engineering, 4202 E. Fowler Avenue, Tampa, FL-33620, USA. Tel.: +1 813 974 3942; fax: +1 813 974 3610. E-mail address: [email protected] (A. Kumar). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.06.001
induced by the plasma treatment may be completely different. The nitrogen incorporated NCD films have gained attention as excellent field and thermionic emission sources [7,8]. It will be interesting to understand the effect of hydrogenation and nitrogenation on the surface conductivity of the nitrogen incorporated NCD films, as the surface treatment is known to improve the field emission properties. Firstly, if the conductivity of such films is due to the presence of defects and sp2 bonded carbon, the atomic hydrogen produced during hydrogenation can passivate the surface and reduce the defects, thereby providing insight into the electrical conduction in the nitrogen incorporated NCD films. On the other hand, the nitrogen-plasma treatment has also shown to improve the electrical properties of the films [9–11]. It was reported that after the nitrogen ion-implantation, the field emission characteristics have improved due to the breaking of sp3 bonds, creation of amorphous regions and possible defects. Due to high energies in the conventional implanters, it may be difficult to recover the damaged surface completely. The plasma treatment can be considered as a low energy ion-implantation technique [12]. Therefore, the nitrogen-plasma treatment of the nitrogen incorporated NCD films has drawn our attention. However, a systematic study on the effect of hydrogen and nitrogen-plasma treatments of the nitrogen incorporated NCD films has not been conducted [4]. In this paper, we have conducted an
S. Jeedigunta et al. / Diamond & Related Materials 17 (2008) 1994–1997
1995
elaborate and a systematic study on the influence of plasma treatments on the structural properties and the electrical conductivity of nitrogen incorporated NCD films. 2. Experimental The nitrogen incorporated nanocrystalline diamond films were grown in a 2.45 GHz Cyrannus I Iplas microwave plasma enhanced chemical vapor deposition (MPECVD) reactor. The n-Si (100) substrates (resistivity 5–15 ohm cm) were seeded by ultrasonic scratching in a nanodiamond powder dispersed in methanol. The process chemistry included CH4/Ar/N2 gases with 1% methane, 79% argon, and 20% nitrogen at a microwave power of 800 W, pressure of 100 Torr, and a substrate temperature ~750 °C for 3 h. The as-grown films were exposed to nitrogen and hydrogen plasmas using a microwave power of 600 W, at a pressure of 30 Torr and a substrate temperature of ~ 700–750 °C for about 30 min. The thickness of both as-grown and the plasma-treated films was found to be ~ 1 μm. A bilayer of Ti (~ 20 nm)/Au (~ 190 nm) was deposited by R.F sputtering and the electrical contacts (area: 1000 µm × 1000 µm) were achieved by conventional photo-lithography technique. The structural properties of the films were characterized by X-ray diffraction (XRD), visible-Raman spectroscopy, scanning electron microscopy (SEM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. XRD measurements were conducted in a Philips X'pert pro diffractometer using Cu-kα radiation, 1° incident slit, 2° diffracted slit and a 15 mm incident beam mask. The Raman spectroscopy was carried out using a Renishaw 1000 Raman spectrometer with an Argon laser at a wavelength of 514.5 nm, and a laser spot size of 1 µm at a power of 25 mW. SEM was performed in a Hitachi S-4800 field emission scanning electron microscope at an accelerating voltage of 5 kV and a beam current of 10 µA. In order to study the influence of surface plasma treatment on the nature of chemical bonding, NEXAFS study was conducted at Synchrotron Radiation Center at University of Wisconsin, Madison using Hermon beam line in a total electron yield (TEY) mode. The room temperature
Fig. 2. (a–c) Scanning electron micrographs of the as-grown films and the samples exposed to hydrogen and nitrogen plasma.
electrical conductivity of the films was measured in plane by making the electrical contacts through the top metal (Ti/Au) electrodes using a Jandel four point probe resistivity measurement tool. 3. Results and discussion
Fig. 1. X-ray diffraction spectra of the nitrogen incorporated nanocrystalline diamond films grown in 20% N before and after the hydrogen and nitrogen-plasma treatments.
Fig. 1 shows the (111) peak of as-grown, hydrogen-plasma treated and nitrogen-plasma treated nitrogen incorporated NCD films. It can be observed that the diffraction spectra of both as-grown and hydrogen (H) plasma treated films were similar; but the later ones showed decrease in the intensity of (111) peak and increase in the FWHM value. In the nitrogen (N) plasma treated films, the diffraction intensity has decreased further indicating the loss of crystallinity. It was reported earlier that the loss of crystallinity in the nitrogen-plasma treated films could be due to breaking of sp3 bonds [9]. It can be concluded that the nitrogen-plasma treatment has more pronounced effect on the crystallinity than the hydrogen-plasma treatment. From the top-view SEM micrographs of the as-grown and the plasmatreated films, it can be observed that the structure of as-grown nitrogen incorporated NCD films (See Fig. 2 (a)) resembled a “cauliflower”-like morphology. Further, it can be noticed in Fig. 2 (b) and Fig. 2 (c) that such a structure disappeared after the plasma treatment. After the hydrogen-
1996
S. Jeedigunta et al. / Diamond & Related Materials 17 (2008) 1994–1997
plasma treatment, fine needle-like structure was observed (see Fig. 2 (b)) indicating the etching of graphitic carbon by the bombardment of hydrogen ions. The transition in the structural morphology is revealed due to the removal of carbon layer on the surface of the nanodiamond film. It was later confirmed by Raman and NEXAFS spectroscopy that the sp2 bonded carbon was indeed etched after hydrogenation. On the other hand, the surface morphology of nitrogen-plasma treated films showed that the cauliflower-like surface in the as-grown films (see Fig. 2 (c)) has coalesced to form a homogenous layer. It is known that the nitrogen-plasma treatment deposits an amorphous carbon layer [11]. An indistinct amount of this amorphous carbon layer could have covered the surface of the film, which may have slightly changed the surface morphology. The visible-Raman spectra (Fig. 3) of as-grown and plasma-treated NCD films are characterized by four important features: i.e. peaks at ~ 1145 cm− 1, 1348 cm− 1, 1486 cm− 1 and 1564 cm− 1, respectively. The peak at ~1145 cm− 1 was referred to as a signature of nanodiamond [13–15], while A. C. Ferrari et al. have suggested that this peak is always accompanied by another feature at ~ 1450 cm− 1 and is due to the presence of transpolyacetylene (TPA) states at the grain boundaries [16]. The peaks at ~1348 cm− 1 and ~ 1560 cm− 1 correspond to the D (dis-ordered) band and G (graphitic) band respectively [6]. They represent the sp2 bonded carbon located in the grain boundaries of the nanodiamond film. It has been shown that the amount of TPA states present in the films is related to the presence of hydrogen [17]. As TPA states are unstable at high temperatures, these features disappear when NCD films are heated at ~1200 °C [18]. But in this study, the hydrogen-plasma treatment of the as-grown films was done at relatively lower temperatures (~700 °C–750 °C). Hence, the TPA states have not completely disappeared; instead, the relative intensities of the features associated with TPA states (1140 cm− 1 and the shoulder at 1486 cm− 1) decreased along with a shift in the position
Fig. 4. (a) NEXAFS spectra of the 20% nitrogen incorporated nanocrystalline diamond films before and after the plasma treatment, (b) shows the peak corresponding to the π⁎ anti-bonding state of the films before and after the plasma treatment.
Fig. 3. Visible-Raman spectra of the 20% N incorporated nanocrystalline diamond films before and post plasma treatments.
of G-band to a higher wave number. It can be noticed that the surface hydrogenation has a significant role in affecting the bonding characteristics of the films. For the as-grown and the nitrogen-plasma treated films, the relative intensity of G-band was higher than D-band, indicating the existence of a higher amount of ordered sp2 carbon in the films. Fig. 4 (a) shows the NEXAFS spectra of as-grown and plasmatreated samples after the normalization and the removal of background signal from any carbon contamination of the optics. It can be observed that the spectrum of the as-grown sample has three well defined features: π⁎ anti-bonding state at ~ 284 eV, σ⁎ exciton peak at ~289 eV and a second band gap of diamond at ~ 302 eV. After the samples were exposed to hydrogen plasma, the σ⁎ exciton peak has become more distinct with a sharp and defined pre-edge, while this feature in the nitrogen-plasma treated samples has not shown any variation. On the other hand, the spectrum of nitrogenated films has an unknown peak at ~286 eV and other features at ~284 eV, ~ 289 eV and ~302 eV, respectively. Fig. 4 (b) shows the π⁎ anti-bonding state of
S. Jeedigunta et al. / Diamond & Related Materials 17 (2008) 1994–1997 Table 1 Effect of the post deposition plasma treatment on the conductivity of the samples. “as-grown” indicates the conductivity of the NCD films grown with 20% nitrogen in the gas chemistry
Ar CH4 N2 H2 Power (W) Pressure (Torr) Substrate temperature (°C) Conductivity (ohm.cm)– 1 Nature of contact
As-grown (20% N) NCD film
H-plasma treated film
N-plasma treated film
79% 1% 20% 0% 800 100 750
100% 600 30 750
600 30 750
84.03 ± 2.4
10.93 ± 1.71
82.67 ± 1.24
Ohmic
Non-ohmic
Non-ohmic
100%
the as-grown, hydrogen-plasma treated and nitrogen-plasma treated NCD films. In order to quantify the amount of sp2 present in the asgrown and plasma-treated films, we have estimated the fraction of sp2 by using Eq. (1).
Fsp2 ¼
⁎
π⁎ ISAMPLE ⁎ π IREFERENCE ⁎
IREFERENCE ðΔEÞ ISAMPLE ðΔEÞ
1997
plasma excitation of nitrogen ions will not change the sp2 bonded carbon content in the nitrogen incorporated NCD films. 4. Conclusions The effects of nitrogen and hydrogen-plasma treatments on the structural properties and the electrical conductivity of nitrogen incorporated NCD films were studied. The diffraction studies showed that there was a reduction in the crystallinity of the as-grown films after the plasma treatments. nitrogen-plasma treatment showed loss of crystallinity to a higher degree than the hydrogenated films. Raman spectra of the hydrogen-plasma treated films showed a reduced intensity of the peaks associated with the TPA states and a shift in the G-band to a higher wave number, suggesting a possible rearrangement of sp2-carbon. It was observed that the sp2/sp3 ratio in the asgrown and the nitrogenated films was ~14%, whereas this ratio decreased to 12% after the hydrogen-plasma treatment. The reduction in the electrical conductivity of the hydrogenated samples was attributed to the decrease in the amount of sp2 bonded carbon. The nitrogen-plasma treatment has an insignificant effect on the electrical conductivity. It was confirmed that the electrical conductivity in the films was predominantly due to the sp2 bonded carbon and the surface defects that are passivated during the hydrogenation.
ð1Þ
π π ISAMPLE ; IREFERENCE ; IREFERENCE ðΔEÞ; ISAMPLE ðΔEÞ denote the areas under the π⁎ peak of both as-deposited and the plasma-treated NCD films and the HOPG reference sample, respectively. IREFERENCE(ΔE), ISAMPLE(ΔE) are the remaining areas under the HOPG reference spectrum, the as-deposited and the plasma-treated spectra respectively. From these calculations, it was estimated that the as-grown films consisted of ~14% sp2. The presence of this order of sp2 bonded carbon in the grain boundaries has been reported [6]. Similarly, the sp2 content in the hydrogen and nitrogen-plasma treated films was found to be ~12% and ~14.2%, respectively. It is known that the hydrogen-plasma treatment can etch the sp2 bonded carbon resulting in the decreased sp2/sp3 ratio. As observed in the Raman spectra, the ratio ID:IG in both as-grown and nitrogenated samples remained the same. Hence, the lower crystallinity of the nitrogen-plasma treated films cannot be due to breaking of sp3 bonds to sp2 and other constituents, but rather could be due to the damage induced onto the crystal lattice. It may not be possible to increase the sp2 content or convert sp3 to sp2 bonded carbon by this method of nitrogen-plasma treatment. This data was consistent with the fact that the electrical conductivity values (see Table 1) of both as-grown and nitrogenated samples was similar. It is known that the electrical conductivity of the as-grown NCD films is due to the creation of electron lone pair induced during the incorporation of nitrogen [6,19–21]. The room temperature electrical conductivity of asgrown, hydrogen-plasma and nitrogen-plasma treated films is shown in Table 1. The highest conductivity of ~84.03 ohm– 1 cm− 1 was obtained on as-grown films, while the hydrogen-plasma treated films showed the least. No significant change in the electrical conductivity of the NCD films was observed after the nitrogen-plasma treatment. It was difficult to obtain a linear relation between current and voltage after the hydrogenplasma treatment. The conductivity of the hydrogen-plasma treated films measured at 10 V showed a huge decrease. The decrease in the electrical conductivity is attributed to the removal of sp2 bonded carbon and the passivation of defects during the hydrogenation. It confirms that the higher values of electrical conductivity in the nitrogen incorporated NCD films is due to the presence of sp2 bonded carbon, and defects in the grain boundaries. On the other hand, the passive role of nitrogen-plasma treatment on the electrical conductivity suggests that the microwave
Acknowledgements Financial support of this work by SMDC, under contract No. W9113 M-06-C-0022 and by NSF NIRT, under grant No. 0404137 is highly acknowledged. The authors would like to thank Mr. Dave Edwards for conducting the SEM. This work is based upon the research conducted at the Synchrotron Radiation Center, University of Wisconsin– Madison, which is supported by the NSF under Contract No. DMR0084402. “Distribution A”. Approved for Public Release; distribution Unlimited.” References [1] S. Albin, L. Watkins, Appl. Phys. Lett 56 (15) (1990) 1454. [2] Y. Yamazaki, K. Ishikawa, N. Mizuochi, S. Yamasaki, Diam. Relat. Mater. 14 (2005) 1939. [3] Y.C. Lee, D. Pradhan, S.J. Lin, C.T. Chia, H.F. Cheng, I.N. Lin, Diam. Relat. Mater 14 (2005) 2055. [4] Y.C. Chen, A.J. Cheng, M. Clark, Y.K. Liu, Y. Tzeng, Diamond Relat. Mater 15 (2006) 440. [5] S. Bhattacharyya, O. Auciello, J. Birrell, J.A. Carlisle, L.A. Curtiss, A.N. Goyette, D.M. Gruen, A.R. Krauss, J. Schlueter, A. Sumant, P. Zapol, Appl. Phys. Lett. 79 (10) (2001) 1441. [6] J. Birrell, J.E. Gerbi, O. Auciello, J.M. Gibson, D.M. Gruen, J.A. Carlisle, J. Appl. Phys. 93 (9) (2003) 5606. [7] R.S. Takalkar, W.P. Kang, J.L. Davidson, B.K. Choi, W.H. Hofmeister, K. Subramanian, Diam. Relat. Mater 15 (2006) 329. [8] Franz A.M. Koeck, Robert J. Nemanich, Diam. Relat. Mater 15 (2006) 217. [9] M.Y. Chen, K.Y. Wu, J.H. Wang, M.T. Chang, L.J. Chou, C.S. Kou, Nanotechnology 18 (2007) 455706. [10] C. Kimura, Y. Yamamuro, H. Aoki, T. Sugino, Diam. Relat. Mater 16 (2007) 1383. [11] F.L. Freire Jr., S.I. Castañeda, R. Prioli, Diam. Relat. Mater 16 (2007) 1282. [12] J.R. Roth, Industrial Plasma Engineering, Applications to Nonthermal Plasma Processing, vol. 2, Institute of Physics Publishing, Bristol, 2001. [13] T. Sharda, T. Soga, T. Jimbo, M. Uembo, Diam. Relat. Mater 10 (2001) 352. [14] T. Lin, Y. Yu, A.T.S. Wee, Z.X. Shen, K.P. Loh, Appl. Phys. Lett 77 (2000) 2692. [15] L. Fayette, B. Marcus, M. Mermoux, G. Tourillon, K. Laffon, P. Parent, F. LeNormand, Phys. Rev. B 57 (1998) 14123. [16] A.C. Ferrari, J. Robertson, Phys. Rev. B 63 (2001) 121405-1. [17] K. Teii, T. Ikeda, A. Fukutomi, K. Uchino, J. Vac. Sci. Technol. B 24 (1) (2006) 263. [18] R. Pfeiffer, H. Kuzmany, P. Knoll, S. Bokova, N. Salk, B. Günther, Diam. Relat. Mater 12 (2003) 268. [19] J.E. Gerbi, J. Birrell, M. Sardela, J.A. Carlisle, Thin Solid Films 473 (2005) 41. [20] P. Zapol, M. Sternberg, L.A. Curtiss, T. Frauenheim, D.M. Gruen, Phys. Rev., B 65 (2001) 045403. [21] Q. Hu, S. Jeedigunta, Z. Xu, M. Hirai, A. Kumar, Extended Abstracts (The 68th Autum Meeting, 2007), vol. 612, The Japan Society Applied Physics II, 2007.