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Influence of high energy ion irradiation on the field emission characteristics of CVD diamond films
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 244 (2006) 217–220 www.elsevier.com/locate/nimb
Influence of high energy ion irradiation on the field emission characteristics of CVD diamond films P.M. Koinkar a, R.S. Khairnar b, S.A. Khan c, R.P. Gupta d, D.K. Avasthi c, M.A. More
a,*
a
d
Department of Physics, University of Pune, Pune 411 007, Maharashtra, India b Department of Physics, S.R.T.M. University, Nanded, Maharashtra, India c Nuclear Science Centre (NSC), Aruna Asaf Ali Road, New Delhi 68, India Central Electronics Engineering Research Institute (CEERI), Pilani 333 031, Rajasthan, India Available online 10 January 2006
Abstract The field emission characteristics of ion-irradiated CVD diamond thin film deposited on silicon substrate has been studied. The diamond thin films, synthesized by hot filament chemical vapor deposition (HFCVD) method, were irradiated by high energy (100 MeV) silver ion (107Ag+ with charge state 9) in the fluence range of 3 · 1011–1 · 1013 ions/cm2. The CVD diamond films were characterized by Raman spectroscopy. The Raman spectra of irradiated samples clearly reveal structural damage due to ion irradiation, which is observed to be fluence dependent. However complete graphitization is not observed. The field emission current–voltage (I–V) characteristics were recorded in ‘diode’ configuration at base pressure 1 · 10 8 mbar. Upon ion irradiation the field emission current is observed to increase with the reduction in the threshold voltage, required to draw 1 lA current. The results indicate that ion irradiation leads to better emission characteristics and the structural damage caused by ion irradiation plays a significant role in emission behavior of CVD diamond films. 2005 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Ion irradiation; Field emission; Raman spectroscopy
1. Introduction The CVD diamond is considered to be a promising material for the cold cathode in field emission displays (FEDs) because of its low or even negative electron affinity, high surface roughness and outstanding chemical and physical stability at operating pressure [1,2]. There are numerous reports on electron emission from CVD diamond and diamond like carbon (DLC) films and several electron emission models have been proposed in support of the emission behavior of these materials [3–6]. However none of these models is accepted universally till date. The last decade has seen many advances in science and technology of CVD diamond. The unceasing technological interest in the diamond based cold cathodes, despite the *
Corresponding author. Tel.: +91 20 2569 2678x416; fax: +91 20 2569 1684. E-mail address: [email protected] (M.A. More). 0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.020
non-exactness about the nature of apparently low workfunction, has resulted in various in situ and/or ex situ conditioning processes employed to obtain better emission characteristics. The most commonly used processes are acid etching [7,8], doping with impurities such as boron, nitrogen [9,10], use of over-layer beneath or on the top of the CVD diamond films [11,12], thermal annealing [13], in addition to ion implantation/irradiation. Ion implantation of polycrystalline CVD diamond, DLC films and carbon fiber has been studied by several groups [14–18]. It has been shown that ion irradiation can be used to either increase the sp2-phase content or decrease the sp3phase content of CVD diamond. In general most of these reports describe the effect of ion irradiation on electrical conductivity, surface microstructure and/or chemical nature of carbon atom (sp2 to sp3) except a few focused on field emission investigations. Zhu et al. have studied the field emission characteristics of CVD diamond films implanted with boron, sodium and carbon ions at doses
218
P.M. Koinkar et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 217–220
1014–1015 ions/cm2 [19]. The authors have observed reduction in the threshold field from 164 to 42 V/lm after ion implantation, which was attributed to the ‘defects’ generated by ion implantation. They have speculated that these ‘defects’ introduced in the near surface region increase the conductivity, which in turn lowers the workfunction of the implanted samples. Improved field emission from ion irradiated carbon fibers has been reported by Walter et al. [14]. Recently Habermann et al. have investigated the influence of ion-implantation on field emission characteristics of CVD diamond thin films [20]. They have performed C+ ion implantation (50 keV, 1015–1016 ions/cm2) and Si+ ion implantation (4.4 MeV, 1 · 1017 ions/cm2) through a mask consisting of a square array of holes (/ = 8 lm). The authors have observed enhancement in the field emission current and reduction in the threshold field by 30% and 16% for C+ and Si+ ions, respectively. In most of the studies carried out earlier either self ion (C+) or low-Z ions with energy ranging from 100 keV to a few MeV have been used for irradiation purpose. As the ‘damage’ created in near surface region is function of mass and energy of incident ion, we thought to use highZ ions with very high energy for irradiation process. In this paper, we report the investigation on influence of high energy 107Ag+ ion irradiation on the field emission characteristics of CVD diamond films. 2. Experimental 2.1. Specimen preparation Diamond films were deposited by a simple hot filament chemical vapor deposition (HFCVD) method using a mixture of methane and hydrogen gases. The substrates used in the experiments were silicon (1 1 1) wafers (1 cm · 1 cm). Pre-scratching of the silicon substrates was carried out with 1 lm diamond paste to initiate nucleation. The deposition was carried out for 4 h duration. The experimental details are described elsewhere [8].
diamond film was used as cathode and phosphor coated tin oxide glass plate as an anode. The spacing between anode and cathode was kept 1 mm. The CVD diamond samples were characterized by Raman spectroscopy to confirm whether the ‘structural’ quality of the diamond is still present after irradiation or not. The Raman spectra were recorded by Renishaw Spectrometer (model: Ramanscope 1000), which utilizes wavelength k 514.5 nm of Ar+ ion laser. 3. Results and discussion The Raman spectra of as-deposited and ion-irradiated samples are shown in Fig. 1. A typical Raman spectrum of as-deposited film (Fig. 1(a)) exhibits a well-defined peak at 1333 cm 1, the signature of sp3 phase. Thus the Raman spectrum clearly reveals the formation of diamond phase under the experimental conditions. The Raman spectrum of CVD diamond film irradiated with fluence of 3 · 1011 ions/cm2 (low fluence case) is shown in Fig. 1(b). The spectrum shows a well defined peak at 1331 cm 1, signature of diamond phase along with a broad hump centered at 1550 cm 1, indicative of non-diamond phase. Thus the spectrum clearly reveals presence of both the diamond and non-diamond phases in the sample after ion irradiation. Fig. 1(c) shows a typical Raman spectrum of CVD diamond films irradiated with fluence of 1 · 1013 ions/cm2 (high fluence case). It is interesting to note that the spectral features of Fig. 1(c) are similar to that of Fig. 1(b). However, a careful observation reveals that the intensity and full width at half maximum (FWHM) of the corresponding spectral features are different. From the Raman spectra it is observed that with increase in the fluence, the FWHM of diamond peak and
2.2. Ion irradiation The ion irradiation was carried out using 107Ag9+ ions with energy 100 MeV from 15 MV Pelletron accelerator, Nuclear Science Centre (NSC), New Delhi [21]. The current was 1 particle-nanoampere (pnA) which is equivalent to 6.2 · 109 ions/s. The samples were mounted inside the scattering chamber (/ = 1.5 m) which was evacuated to base pressure of 1 · 10 6 mbar. The samples were irradiated with the fluence in the range 3 · 1011, 1 · 1012, 3 · 1012 and 1 · 1013 ions/cm2. 2.3. Field emission studies The field emission measurements were carried out in a ‘diode’ configuration in all metal ultra high vacuum chamber evacuated to base pressure 1 · 10 8 mbar. The CVD
Fig. 1. Raman spectra of CVD diamond films (a) as-deposited, (b) irradiated with fluence 3 · 1011 ions/cm2 and (c) irradiated with fluence 1 · 1013 ions/cm2.
P.M. Koinkar et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 217–220
3.1. Field emission characteristics The field emission current–voltage (I–V) characteristics of CVD diamond films, as-grown and ion irradiated, are shown in Fig. 2. In comparison with that of as-deposited case, the overall I–V characteristics are not very distinct, except the values of threshold voltages. The threshold voltage defined as the voltage required to draw 1 lA current is found to be 944, 400 and 720 V for as-deposited, irradiated with fluence of 3 · 1011 and 1 · 1013 ions/cm2, respectively. Thus the threshold voltage is observed to decrease upon ion irradiation. The CVD film irradiated with low fluence has lower threshold voltage as compared to as-deposited film. The reduction in the threshold voltage is attributed to the presence of non-diamond phases and various defects in the irradiated film. A detailed description is presented below. The film irradiated with high fluence has threshold value lower than that of as deposited case, but higher than that of low
20 EMISSION CURRENT (µΑ)
intensity of the non-diamond peak increase. The broadening of the diamond peak and increase in intensity of nondiamond phase clearly indicate that ion irradiation leads to graphitization of the sample. However, a presence of well defined diamond peak in Fig. 1(c) suggests that even after irradiation with high fluence 1 · 1013 ions/cm2, the diamond structure still persists in the film and complete graphitization of the sample is not possible with the fluence range used in present studies. The observed Raman spectra are identical to those reported by Terai et al. who have studied the of self-ion irradiation of polycrystalline diamond [15]. In the present case, the Raman spectra suggest that there is ‘structural damage’ to the diamond lattice upon ion irradiation, as seen by the emergence of the non-diamond peak in the corresponding spectra. From TRIM results, when the energy of the incident ion is of few keV then the atomic displacements (nuclear energy transfer) are significant as compared to ion distribution and ionization (electronic energy transfer). In present experiment we have used very high energy ions for irradiation and the electronic energy transfer ˚ ) is dominant over the nuclear energy transfer (1.55 keV/A ˚ ) interactions in the near surface region. In (0.006 keV/A CVD diamond, the incident ions are expected to break the C–C bonds, C–H bonds leading to loss of hydrogen content in the films. The loss of hydrogen results into recreation of dangling bonds or vacancies in the films. The ‘disordered’ carbon atoms will contribute to the internal stress and thus the Raman peaks will be broader rather than of sharp ones. The surface microstructure was investigated by atomic force microscopy (AFM). The AFM images (not shown here) reveal change in surface roughness upon ion irradiation. At high fluence, the surface roughness is observed to be lower than that of as-deposited case, indicating smoothening of the CVD diamond film surface, which is in agreement with the reported data.
219 (c)
(b)
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VOLTAGE (Volt)
Fig. 2. Field emission current–voltage (I–V) characteristics of CVD diamond films (a) as-deposited, (b) irradiated with fluence 3 · 1011 ions/ cm2 and (c) irradiated with fluence 1 · 1013 ions/cm2, at base pressure 1 · 10 8 mbar.
fluence case. When compared with the low fluence case, the higher threshold voltage is attributed to film surface roughness. Field emission is surface sensitive phenomenon and for a rough surface, due to the high ‘local fields’ at surface asperities, the threshold voltage will be considerably low. As the film irradiated with high fluence has smooth surface, its threshold voltage is expected to be higher. In this case, although the non-diamond content present in the film is more, it is speculated that the film smoothness will ‘control’ the field emission characteristics. The overall reduction in the threshold voltage, upon irradiation, is analogous with the observations of Raman spectroscopy and surface roughness. It is expected that the structural damage caused by ion irradiation will create various defects along with formation of non-diamond phases. It is well established by various researchers that the ‘defect’ thus generated lead to formation of intermediate energy states in wide band gap of diamond and such ‘defectstates’ enhances the emission probability. Zhu et al. have explained in detail the mechanism of enhanced emission current from ion-irradiated CVD diamond films [19]. The presence of non-diamond phases in CVD diamond films is also responsible for the reduction in the threshold voltage [8]. Moreover, the emission characteristics are improved with increase in the non-diamond content in the film. Therefore the presence of non-diamond phases and various defects in the irradiated films is expected to reduce the threshold voltage and improve the I–V characteristics. 4. Conclusions The field emission characteristics of CVD diamond films are influenced by high energy ion irradiation. The Raman spectroscopy results clearly reveal formation of non-diamond phases and ‘structural damage’ to the diamond lattice, upon irradiation. The Raman results are observed to be fluence dependent. The ion irradiation is found to improve the field emission characteristics of CVD diamond films. The non-diamond phases and various defects,
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created by irradiation, play significant role in emission characteristics of ion irradiated CVD diamond thin films. Acknowledgements One of the authors, P.M.K. is very grateful to Council of Scientific Industrial Research, New Delhi, India for awarding Senior Research Fellowship. This work was supported by University Grant Commission New Delhi, India (F.10-11/2003, SR). References [1] F.J. Himpsel, J.A. Van Venchten, D.E. Eastmann, Phys. Rev. B 20 (1979) 624. [2] K. Okano, K.K. Glenason, Electron. Lett. 31 (1995) 74. [3] N.S. Xu, R.V. Latham, J. Phys. D 19 (1986) 477. [4] M.W. Geis, J.C. Twinchell, J. Macaulay, K. Okano, Appl. Phys. Lett. 67 (1995) 1328. [5] W. Zhu, G.P. Kochanski, S. Jin, L. Siebles, J. Vac. Sci. Technol. B 14 (1996) 2011. [6] R. Schelesser, M.T. McClure, B.L. McCarson, Z. Sitar, J. Appl. Phys. 82 (1997) 5763.
[7] P.W. May, J.C. Stone, M.N.R. Ashfold, K.R. Hallam, W.N. Wang, N.A. Fox, Diamond Relat. Mater. 7 (1998) 671. [8] P.M. Koinkar, P.P. Patil, M.A. More, V.N. Tondare, D.S. Joag, Vacuum 72 (3) (2004) 321. [9] N.A. Fox, S. Marry, T.J. Davis, W.N. Wang, P.W. May, A. Bewick, J.W. Steeds, J.E. Butler, Diamond Relat. Mater. 6 (1997) 1135. [10] K. Okano, S. Koizumi, S.R.P. Silva, G.A.J. Amaratunga, Nature 381 (1996) 140. [11] A. Lomouri, Y. Wang, G.T. Mearini, I.L. Krainsky, J.A. Dayton Jr., W. Muller, J. Vac. Sci. Technol. B 14 (3) (1996) 2046. [12] D.S. Mao, X. Wang, W. Li, X.H. Liu, Q. Li, J.F. Xu, K. Okano, J. Vac. Sci. Technol. B 18 (5) (2000) 2420. [13] S. Sahli, D. Aslam, Appl. Phys. Lett. 69 (4) (1996) 2051. [14] K.C. Walter, H.H. Kung, C.J. Maggoire, Appl. Phys. Lett. 71 (10) (1997) 1320. [15] T. Terai, T. Kobayashi, Nucl. Instr. and Meth. B 141 (1998) 140. [16] R. Kalish, A. Rernick, K.W. Nugent, S. Prawer, Nucl. Instr. and Meth. B 148 (1999) 626. [17] H. Kakiuchi, T. Kobayashi, T. Terai, Nucl. Instr. and Meth. B 166 (2000) 415. [18] J. Wang, Wen-Zhi Li, Heng-De Li, J. Mater. Sci. Lett. 18 (1999) 1481. [19] W. Zhu, G.P. Kochanski, S. Jin, L. Siebles, D.C. Jacobson, M. McCormack, A.E. White, Appl. Phys. Lett. 67 (1995) 1157. [20] T. Habermann, A. Gohl, D. Nau, M. Wedel, G. Muller, M. Christ, M. Schreck, B. Strizker, J. Vac. Sci. Technol. B 16 (2) (1998) 693. [21] G.K. Mehta, A.P. Patro, Nucl. Instr. and Meth. A 268 (1998) 334.
Nuclear Instruments and Methods in Physics Research B 244 (2006) 217–220 www.elsevier.com/locate/nimb
Influence of high energy ion irradiation on the field emission characteristics of CVD diamond films P.M. Koinkar a, R.S. Khairnar b, S.A. Khan c, R.P. Gupta d, D.K. Avasthi c, M.A. More
a,*
a
d
Department of Physics, University of Pune, Pune 411 007, Maharashtra, India b Department of Physics, S.R.T.M. University, Nanded, Maharashtra, India c Nuclear Science Centre (NSC), Aruna Asaf Ali Road, New Delhi 68, India Central Electronics Engineering Research Institute (CEERI), Pilani 333 031, Rajasthan, India Available online 10 January 2006
Abstract The field emission characteristics of ion-irradiated CVD diamond thin film deposited on silicon substrate has been studied. The diamond thin films, synthesized by hot filament chemical vapor deposition (HFCVD) method, were irradiated by high energy (100 MeV) silver ion (107Ag+ with charge state 9) in the fluence range of 3 · 1011–1 · 1013 ions/cm2. The CVD diamond films were characterized by Raman spectroscopy. The Raman spectra of irradiated samples clearly reveal structural damage due to ion irradiation, which is observed to be fluence dependent. However complete graphitization is not observed. The field emission current–voltage (I–V) characteristics were recorded in ‘diode’ configuration at base pressure 1 · 10 8 mbar. Upon ion irradiation the field emission current is observed to increase with the reduction in the threshold voltage, required to draw 1 lA current. The results indicate that ion irradiation leads to better emission characteristics and the structural damage caused by ion irradiation plays a significant role in emission behavior of CVD diamond films. 2005 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Ion irradiation; Field emission; Raman spectroscopy
1. Introduction The CVD diamond is considered to be a promising material for the cold cathode in field emission displays (FEDs) because of its low or even negative electron affinity, high surface roughness and outstanding chemical and physical stability at operating pressure [1,2]. There are numerous reports on electron emission from CVD diamond and diamond like carbon (DLC) films and several electron emission models have been proposed in support of the emission behavior of these materials [3–6]. However none of these models is accepted universally till date. The last decade has seen many advances in science and technology of CVD diamond. The unceasing technological interest in the diamond based cold cathodes, despite the *
Corresponding author. Tel.: +91 20 2569 2678x416; fax: +91 20 2569 1684. E-mail address: [email protected] (M.A. More). 0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.020
non-exactness about the nature of apparently low workfunction, has resulted in various in situ and/or ex situ conditioning processes employed to obtain better emission characteristics. The most commonly used processes are acid etching [7,8], doping with impurities such as boron, nitrogen [9,10], use of over-layer beneath or on the top of the CVD diamond films [11,12], thermal annealing [13], in addition to ion implantation/irradiation. Ion implantation of polycrystalline CVD diamond, DLC films and carbon fiber has been studied by several groups [14–18]. It has been shown that ion irradiation can be used to either increase the sp2-phase content or decrease the sp3phase content of CVD diamond. In general most of these reports describe the effect of ion irradiation on electrical conductivity, surface microstructure and/or chemical nature of carbon atom (sp2 to sp3) except a few focused on field emission investigations. Zhu et al. have studied the field emission characteristics of CVD diamond films implanted with boron, sodium and carbon ions at doses
218
P.M. Koinkar et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 217–220
1014–1015 ions/cm2 [19]. The authors have observed reduction in the threshold field from 164 to 42 V/lm after ion implantation, which was attributed to the ‘defects’ generated by ion implantation. They have speculated that these ‘defects’ introduced in the near surface region increase the conductivity, which in turn lowers the workfunction of the implanted samples. Improved field emission from ion irradiated carbon fibers has been reported by Walter et al. [14]. Recently Habermann et al. have investigated the influence of ion-implantation on field emission characteristics of CVD diamond thin films [20]. They have performed C+ ion implantation (50 keV, 1015–1016 ions/cm2) and Si+ ion implantation (4.4 MeV, 1 · 1017 ions/cm2) through a mask consisting of a square array of holes (/ = 8 lm). The authors have observed enhancement in the field emission current and reduction in the threshold field by 30% and 16% for C+ and Si+ ions, respectively. In most of the studies carried out earlier either self ion (C+) or low-Z ions with energy ranging from 100 keV to a few MeV have been used for irradiation purpose. As the ‘damage’ created in near surface region is function of mass and energy of incident ion, we thought to use highZ ions with very high energy for irradiation process. In this paper, we report the investigation on influence of high energy 107Ag+ ion irradiation on the field emission characteristics of CVD diamond films. 2. Experimental 2.1. Specimen preparation Diamond films were deposited by a simple hot filament chemical vapor deposition (HFCVD) method using a mixture of methane and hydrogen gases. The substrates used in the experiments were silicon (1 1 1) wafers (1 cm · 1 cm). Pre-scratching of the silicon substrates was carried out with 1 lm diamond paste to initiate nucleation. The deposition was carried out for 4 h duration. The experimental details are described elsewhere [8].
diamond film was used as cathode and phosphor coated tin oxide glass plate as an anode. The spacing between anode and cathode was kept 1 mm. The CVD diamond samples were characterized by Raman spectroscopy to confirm whether the ‘structural’ quality of the diamond is still present after irradiation or not. The Raman spectra were recorded by Renishaw Spectrometer (model: Ramanscope 1000), which utilizes wavelength k 514.5 nm of Ar+ ion laser. 3. Results and discussion The Raman spectra of as-deposited and ion-irradiated samples are shown in Fig. 1. A typical Raman spectrum of as-deposited film (Fig. 1(a)) exhibits a well-defined peak at 1333 cm 1, the signature of sp3 phase. Thus the Raman spectrum clearly reveals the formation of diamond phase under the experimental conditions. The Raman spectrum of CVD diamond film irradiated with fluence of 3 · 1011 ions/cm2 (low fluence case) is shown in Fig. 1(b). The spectrum shows a well defined peak at 1331 cm 1, signature of diamond phase along with a broad hump centered at 1550 cm 1, indicative of non-diamond phase. Thus the spectrum clearly reveals presence of both the diamond and non-diamond phases in the sample after ion irradiation. Fig. 1(c) shows a typical Raman spectrum of CVD diamond films irradiated with fluence of 1 · 1013 ions/cm2 (high fluence case). It is interesting to note that the spectral features of Fig. 1(c) are similar to that of Fig. 1(b). However, a careful observation reveals that the intensity and full width at half maximum (FWHM) of the corresponding spectral features are different. From the Raman spectra it is observed that with increase in the fluence, the FWHM of diamond peak and
2.2. Ion irradiation The ion irradiation was carried out using 107Ag9+ ions with energy 100 MeV from 15 MV Pelletron accelerator, Nuclear Science Centre (NSC), New Delhi [21]. The current was 1 particle-nanoampere (pnA) which is equivalent to 6.2 · 109 ions/s. The samples were mounted inside the scattering chamber (/ = 1.5 m) which was evacuated to base pressure of 1 · 10 6 mbar. The samples were irradiated with the fluence in the range 3 · 1011, 1 · 1012, 3 · 1012 and 1 · 1013 ions/cm2. 2.3. Field emission studies The field emission measurements were carried out in a ‘diode’ configuration in all metal ultra high vacuum chamber evacuated to base pressure 1 · 10 8 mbar. The CVD
Fig. 1. Raman spectra of CVD diamond films (a) as-deposited, (b) irradiated with fluence 3 · 1011 ions/cm2 and (c) irradiated with fluence 1 · 1013 ions/cm2.
P.M. Koinkar et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 217–220
3.1. Field emission characteristics The field emission current–voltage (I–V) characteristics of CVD diamond films, as-grown and ion irradiated, are shown in Fig. 2. In comparison with that of as-deposited case, the overall I–V characteristics are not very distinct, except the values of threshold voltages. The threshold voltage defined as the voltage required to draw 1 lA current is found to be 944, 400 and 720 V for as-deposited, irradiated with fluence of 3 · 1011 and 1 · 1013 ions/cm2, respectively. Thus the threshold voltage is observed to decrease upon ion irradiation. The CVD film irradiated with low fluence has lower threshold voltage as compared to as-deposited film. The reduction in the threshold voltage is attributed to the presence of non-diamond phases and various defects in the irradiated film. A detailed description is presented below. The film irradiated with high fluence has threshold value lower than that of as deposited case, but higher than that of low
20 EMISSION CURRENT (µΑ)
intensity of the non-diamond peak increase. The broadening of the diamond peak and increase in intensity of nondiamond phase clearly indicate that ion irradiation leads to graphitization of the sample. However, a presence of well defined diamond peak in Fig. 1(c) suggests that even after irradiation with high fluence 1 · 1013 ions/cm2, the diamond structure still persists in the film and complete graphitization of the sample is not possible with the fluence range used in present studies. The observed Raman spectra are identical to those reported by Terai et al. who have studied the of self-ion irradiation of polycrystalline diamond [15]. In the present case, the Raman spectra suggest that there is ‘structural damage’ to the diamond lattice upon ion irradiation, as seen by the emergence of the non-diamond peak in the corresponding spectra. From TRIM results, when the energy of the incident ion is of few keV then the atomic displacements (nuclear energy transfer) are significant as compared to ion distribution and ionization (electronic energy transfer). In present experiment we have used very high energy ions for irradiation and the electronic energy transfer ˚ ) is dominant over the nuclear energy transfer (1.55 keV/A ˚ ) interactions in the near surface region. In (0.006 keV/A CVD diamond, the incident ions are expected to break the C–C bonds, C–H bonds leading to loss of hydrogen content in the films. The loss of hydrogen results into recreation of dangling bonds or vacancies in the films. The ‘disordered’ carbon atoms will contribute to the internal stress and thus the Raman peaks will be broader rather than of sharp ones. The surface microstructure was investigated by atomic force microscopy (AFM). The AFM images (not shown here) reveal change in surface roughness upon ion irradiation. At high fluence, the surface roughness is observed to be lower than that of as-deposited case, indicating smoothening of the CVD diamond film surface, which is in agreement with the reported data.
219 (c)
(b)
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VOLTAGE (Volt)
Fig. 2. Field emission current–voltage (I–V) characteristics of CVD diamond films (a) as-deposited, (b) irradiated with fluence 3 · 1011 ions/ cm2 and (c) irradiated with fluence 1 · 1013 ions/cm2, at base pressure 1 · 10 8 mbar.
fluence case. When compared with the low fluence case, the higher threshold voltage is attributed to film surface roughness. Field emission is surface sensitive phenomenon and for a rough surface, due to the high ‘local fields’ at surface asperities, the threshold voltage will be considerably low. As the film irradiated with high fluence has smooth surface, its threshold voltage is expected to be higher. In this case, although the non-diamond content present in the film is more, it is speculated that the film smoothness will ‘control’ the field emission characteristics. The overall reduction in the threshold voltage, upon irradiation, is analogous with the observations of Raman spectroscopy and surface roughness. It is expected that the structural damage caused by ion irradiation will create various defects along with formation of non-diamond phases. It is well established by various researchers that the ‘defect’ thus generated lead to formation of intermediate energy states in wide band gap of diamond and such ‘defectstates’ enhances the emission probability. Zhu et al. have explained in detail the mechanism of enhanced emission current from ion-irradiated CVD diamond films [19]. The presence of non-diamond phases in CVD diamond films is also responsible for the reduction in the threshold voltage [8]. Moreover, the emission characteristics are improved with increase in the non-diamond content in the film. Therefore the presence of non-diamond phases and various defects in the irradiated films is expected to reduce the threshold voltage and improve the I–V characteristics. 4. Conclusions The field emission characteristics of CVD diamond films are influenced by high energy ion irradiation. The Raman spectroscopy results clearly reveal formation of non-diamond phases and ‘structural damage’ to the diamond lattice, upon irradiation. The Raman results are observed to be fluence dependent. The ion irradiation is found to improve the field emission characteristics of CVD diamond films. The non-diamond phases and various defects,
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P.M. Koinkar et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 217–220
created by irradiation, play significant role in emission characteristics of ion irradiated CVD diamond thin films. Acknowledgements One of the authors, P.M.K. is very grateful to Council of Scientific Industrial Research, New Delhi, India for awarding Senior Research Fellowship. This work was supported by University Grant Commission New Delhi, India (F.10-11/2003, SR). References [1] F.J. Himpsel, J.A. Van Venchten, D.E. Eastmann, Phys. Rev. B 20 (1979) 624. [2] K. Okano, K.K. Glenason, Electron. Lett. 31 (1995) 74. [3] N.S. Xu, R.V. Latham, J. Phys. D 19 (1986) 477. [4] M.W. Geis, J.C. Twinchell, J. Macaulay, K. Okano, Appl. Phys. Lett. 67 (1995) 1328. [5] W. Zhu, G.P. Kochanski, S. Jin, L. Siebles, J. Vac. Sci. Technol. B 14 (1996) 2011. [6] R. Schelesser, M.T. McClure, B.L. McCarson, Z. Sitar, J. Appl. Phys. 82 (1997) 5763.
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