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Field emission characteristics of high-energy ion-irradiated polycrystalline diamond thin films
ARTICLE IN PRESS
Vacuum 72 (2004) 297–305
Field emission characteristics of high-energy ion-irradiated polycrystalline diamond thin films P.T. Pandeya, G.L. Sharmaa, D.K. Awasthib, V.D. Vankara,* a
Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110 016, India b Nuclear Science Centre, New Delhi-110 067, India Received 2 March 2003; received in revised form 18 July 2003; accepted 3 August 2003
Abstract Diamond thin films have been synthesized by hot-filament chemical vapor deposition process using a mixture of methane and hydrogen gases. The samples were subjected to very high-energy ion irradiation (100 MeV Au7+ ions). The field emission characteristics of ion-irradiated samples have been studied. High emission currents and low turn-on and threshold fields were obtained for ion-irradiated samples. The as-deposited and the ion-irradiated samples have been characterized by X-ray Diffraction, Scanning Electron Microscopy and Micro-Raman Spectroscopy techniques and the resulting changes are correlated with field emission results. r 2003 Elsevier Ltd. All rights reserved. Keywords: Diamond; Chemical vapor deposition; Field emission; Ion irradiation
1. Introduction Diamond and diamond like carbon thin films have become the most probable candidates for field emission devices due to their negative electron affinity [1], high mechanical stability, high thermal conductivity (2600 W/mK), large intrinsic breakdown field (107 V/cm) and robust mechanical and chemical properties. Diamond-based cold cathodes have durability for high-temperature applications and high-voltage operation due to large breakdown field. Impressive emission currents are obtainable under low-voltage operation from the *Corresponding author. Tel.: +91-11-2686-1329; fax: +9111-2658-1114. E-mail address: [email protected] (V.D. Vankar).
diamond surface. The traditional field emitters were made up of high work function materials such as tungsten, molybdenum or silicon tips. The field strength required in metallic tips are >1000 V/mm and for Si tips B100 V/mm. Silicon tips are prone to oxidation and to damage by particle bombardment. Enhanced field emission has been demonstrated by diamond-coated Mo tips and Si tips [2–4]. Coatings of polycrystalline diamond have been used in tip and planar configuration, resulting in considerable lowering of applied voltage required for emission. Low threshold fields have been obtained from nanocrystalline diamond thin films [5,6]. Various other treatments such as doping with nitrogen [7], boron [8,9] and phosphorous [10], exposure to O2 plasma and Cs deposition [11], etc., lower the field required for emission to various extents. Effect of
0042-207X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.08.007
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ion implantation [12] using keV energy has been found to reduce the field strength and this has been associated to defects produced in the film. The nature of defects produced by high-energy ions (100 MeV) is totally different and must effect the field emission characteristics. This kind of study has not been reported in the literature. In this communication, we report the effect of very highenergy ion irradiation (100 MeV Au7+ ions) on electron field emission from polycrystalline CVD diamond thin films. The effect of high-energy ion irradiation is to produce defects due to inelastic scattering and is expected to modify the surface/ interface in a much different way as compared to other techniques especially ion implantation. And since surface/interface plays a vital role in the field emission process, this study is extremely important.
2. Experimental details The details of the deposition set up used to grow diamond films studied in this work are described elsewhere [13]. Briefly, diamond films were deposited on silicon substrates by hot-filament chemical vapor deposition process. The reactant gases were a mixture of H2 and 10% CH4+90% Ar. A tungsten filament was employed which was heated to 2200 C. The substrate temperature was measured by a chromel–alumel thermocouple, which was maintained around 750 C. The deposition pressure was 4000 N/m2. The substrates were pretreated by scratching with 0.5 mm diamond powder followed by degreasing and cleaning in trichloroethylene and propanol. The high-energy ion irradiation was performed by subjecting the diamond samples to 100 MeV Au7+ ions from (at Nuclear Science Centre, New Delhi, India) 15 MV Pelletron accelerator. The beam current was maintained at 2 pnA (particle nanoampere, which is defined as the number of particles in a current of 1 nA per second). The samples were subjected to a dose approximately 9.5 1012 ions/cm2. High-energy heavy ions were chosen due to higher electronic stopping power. The 100 MeV Au7+ have electronic losses of the ( while the nuclear stopping is order of 1667 eV/A
( The irradiation-induced effects are 2.44 eV/A. expected to be due to electronic stopping only. The damage effects of nuclear stopping are negligible. This is the major difference when high-energy ions are used as compared to the effects due to low-energy ions of keV range. The projected range of ions as calculated by ‘‘TRIM’’ (transport of ions in matter) analysis was about 10.36 mm, which is higher than the film thickness. Electron field emission measurements were performed using a specially fabricated diode assembly in a vacuum system maintained at 106 Torr. The cathode and anode were separated by 30 mm thick spacer. The anode was a copper plate, which was biased positively, and the current was measured using Keithley electrometer. The I– V measurements were analyzed using Fowler– Nordheim model [14] which follows the relation I ¼ aV 2 expðbf3=2 =bV Þ; where a and b are constants, I is the emission current, V is the applied voltage to the material, f is the work function of the cathode material and b is the field enhancement factor. The negative slope of the Fowler–Nordheim plot is proportional to f3=2 =b: The as-deposited and irradiated samples were characterized by X-ray Diffraction, Scanning Electron Microscopy and Micro-Raman Spectroscopy and the resulting changes observed were correlated with its field emission property. The XRD diffractograms were recorded using CuKa radiation. (Giegerflex D/MAX-RB 300 from Rigaku Corporation, Japan). The surface morphological studies of diamond thin films were carried out using a scanning electron microscope (Cambridge Instruments, Model S320). A 514.6 nm line of 15 W Argon ion laser (model 2030, Spectra Physics) was used to record the micro-Raman spectra at room temperature.
3. Results and discussion A number of samples deposited under different deposition conditions were studied for structure and field emission properties. Representative data for three sets of samples (called A, B and C) is presented. The details of the deposition parameters are listed in Table 1. The variation in the
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Table 1 Deposition details of the samples used in the present study Sample name
CH4/H2 ratio (sccm)
Working pressure (N/m2)
Substrate temperature ( C)
A B C
2.5:145 2.5:135 2.5:115
4000 4000 4000
750 750 750
deposition condition is seen to reflect in crystalline quality of the coating. The entire data for all the studies carried out is presented in Table 2. 3.1. Structural observations The X-ray diffraction pattern of one of the asdeposited sample is shown in Fig. 1. All samples show peaks at 2y ¼ 43:9 and 75.3 corresponding to diamond (1 1 1) and (2 2 0) planes, respectively. The X-ray diffraction pattern of ion-irradiated samples showed similar peaks. This suggested that no phase change or any visible major microstructural variation was introduced because of ion irradiation. The X-ray diffraction patterns of all samples (both as-deposited and ion irradiated) showed similar peaks, hence they are not shown here. No amorphous phase was detected within the limits of the experiment. Fig. 2(a–c) show the SEM micrographs of samples A–C. The sample A shows faceted, dense and uniform growth of diamond particles. The SEM micrograph of as-deposited sample B is shown in Fig. 2(b). The sample B also shows faceted growth of diamond particles. The sample C was grown with more CH4/H2 ratio and shows a slight deterioration in the morphology as compared to samples A and B. The surface morphology of any of these samples does not show remarkable change in the microstructure after irradiation. Hence, they are not shown here. 3.2. Micro-Raman analysis The Raman spectra of as-deposited samples A–C are shown in Fig. 3. In all these figures main features corresponding to cubic diamond phase and ‘G’ band of amorphous carbon are observable. The ‘G’ band peak is attributed to highly
disordered carbon atoms (non-diamond carbon) in sp2 hybridization [15]. The details of the voigt analysis i.e. peak position, peak line width and Id =Ind ratio are listed in Table 2. The post-irradiation Raman spectra of all the samples studied also shows a cubic diamond peak and ‘G’ band peak of amorphous carbon (Fig. 3). The ion–irradiated Raman spectrum of samples B and C show additional bands at 1440 and 1447 cm1, respectively. This band is attributed to amorphous carbon phase [16]. Absence of this band in ion-irradiated sample A is due to less nondiamond content in the as-deposited sample A as it was grown with lowest CH4/H2 ratio. Sample A showed minimum radiation damage. The sample C shows more radiation damage as compared to samples A and B. This may be because of more non-diamond content in sample C. An increase in the line width after high-energy ion irradiation reflects the introduction of defects in the coating. A decrease in the value of Id =Ind is also observed after irradiation. This showed an increase in the relative abundance of amorphous carbon phases present in the coating after highenergy ion irradiation. Ager et al. [17] have argued that in natural diamonds, the Raman line broadening is related to the decay of optical phonon created in the Raman process, into two acoustic phonons with opposite wave vectors. In CVD grown polycrystalline diamond films, the phonon may also decay at defect sites and grain boundaries, leading to the broadening of the line widths. Hence one would expect the line widths to increase with increasing defect density. Hyer et al. [18] and Faniculli and coworkers [19] also attributed broadening of Raman lines to reduction of phonon lifetime due to production of point defects. Jamieson and coworkers [20] formulated a linear correlation
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SILICON 40
60
D (220)
D (111)
INTENSITY (a.u.)
10.10 8.33
0.31
20.83 11.90
1.81 16.00 13.33
2.63 33.33 18.31
4.80 40.13 26.67
5.60 61.72 40.16
Slope Threshold field (V/mm) Turn on field (V/mm)
2.18
3.95
2.68
5.85
8.34
15.40 44.27 20.19 120.23 17.70 64.10 28.76 70.19 139.08 20.27 106.48 33.42 179.77 73.30 Highly faceted morphology
Highly faceted morphology
Faceted morphology
Faceted morphology
Deteriorated morphology
Deteriorated morphology
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
A (as-deposited)
A (irradiated)
B (as-deposited)
B (irradiated)
C (as-deposited)
C (irradiated)
1333 1574 1335 1562 1334 1562 1330 1440 1569 1334 1551 1336 1447 1566
3.30
Id =Ind Line width (cm1) SEM
Peak position (cm1)
20
80
2θ (DEG.)
X-ray diffraction
Field emission studies Raman analysis Structural observations Sample name
Table 2 Summary of the various studies used for the present work
1.33
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Fig. 1. X-ray diffraction pattern of a representative diamond thin film.
between increased defect density due to highenergy ion irradiation and the Raman line width. The decrease in the value of Id =Ind indicates the onset of amorphization due to breaking of bonds by high-energy ion irradiation. Wong et al. [21] have also observed that diamond powders irradiated with MeV ions were a mixture of diamond and non-diamond carbon phases. Maeta et al. [22] have suggested that most of the radiation damage in their ion-irradiated diamonds occurred in the form of interstitials dislocation loops and vacancies. Our micro-Raman results indicate the formation of non-diamond structure as well introduction of defects in diamond crystallites as a result of high-energy irradiation, which is consistent with the reported data. 3.3. Field emission characteristics Field emission studies were carried out of asdeposited and the ion-irradiated samples. Fowler– Nordheim plot of all the samples studied showed a straight line of negative slope in the high-voltage region as shown in Fig. 4. This suggested that the emission of electrons is cold cathode induced. The turn on field (defined as the point at which the Fowler–Nordheim plot deviates from its normal behavior) and the threshold field (defined as field at which a current of 1 mA is obtained) of samples varied from samples A to C because of different non-diamond contents, but were reduced
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(a)
301
(b)
(c) Fig. 2. Scanning electron micrographs of: (a) as-deposited sample A, (b) as-deposited sample B and (c) as-deposited sample C.
considerably after ion irradiation as shown in Table 2. The slope of high-voltage region of Fowler–Nordheim plot was lowered after ion irradiation. The lowering in the value of the slope of the Fowler–Nordheim plot of the irradiated samples as compared to the as-deposited samples depicts lowering of the work function after irradiation. A shallow slope corresponds to the ease of electron emission. The details of turn on field, threshold fields and slopes are given in Table 2. The current density of as-deposited sample A was 0.3 mA/cm2 at an applied field of 46.66 V/mm and was increased to 0.02 mA/cm2 for irradiated sample A for the same field. Asdeposited sample B gives a current density of 0.25 mA/cm2 at an electric field of 23.33 V/um, which was enhanced to 0.24 mA/cm2 after irradiation for the same applied field. The current density was increased from 11.2 mA/cm2 for as-deposited sample C to 0.08 mA/cm2 for irradiated sample C
for the same electric field of 22.23 V/mm. A positive slope region is also observed in all the plots of Fowler–Nordheim curves, which may be caused by leakage through the spacer. Kang et al. [23] also reported similar Fowler–Nordheim curve with distinctive positive and negative slope regimes. Zhu et al. [12] demonstrated a strong correlation between the emission properties and the defect densities in the diamond film. The results obtained by them suggest that the defects introduced by ion implantation increased the conductivity and lowered the work function of as-grown diamond surface. It has been suggested that the defects create additional sub-bands within the band gap of diamond and thus contribute to the electron field emission at low electric fields. In ion implantation (where the energies are Bfew keV), atomic displacements are the major effects introduced. However, in the case of high-energy ion irradiation, the ions passing through the solid cause
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3. Micro-Raman spectrum of as-deposited and ion-irradiated samples: (a) as-deposited sample A, (b) irradiated sample A, (c) as-deposited sample B, (d) irradiated sample B, (e) as-deposited sample C and (f) irradiated sample C.
electronic excitations (ionizations) of target atoms and displacements of ions and atoms are normally not observed. In an insulating medium columnar, defects are generally produced along the path of passing energetic ion [24]. It is argued that electronic relaxations are most efficient on atoms located at grain boundaries, defects and interstitials, which gain mobility as a result of irradiation [25]. In diamond films, in addition, highenergy ion irradiation leads to breaking of C–C bonds, C–H bonds and loss of hydrogen. Since hydrogen passivates the dangling bonds over the surface, bond breaking and loss of hydrogen leads to recreation of dangling bonds/vacancies in the film. Prolonged irradiation may lead to the
development of sp2 clusters and eventual graphitization. Micro-Raman results clearly indicated increase in non-diamond carbon in sp2 hybridization as Id =Ind decreases or ‘G’ band intensity increases after irradiation. Many researchers have studied the role of sp2 non-diamond components in affecting field emission characteristics. The some details of published work in this direction are given in Table 3. The increase in the amount of non-diamond carbon content and introduction of defects in the coating appears to be responsible for enhanced emission after irradiation. Our results also indicate similar trends, although exact role of defects on the field emission characteristics needs further examination.
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(e)
1000/V
(d)
1000/V
Log (I/V2)
1000/V
Log (I/V2)
(c)
(b)
Log (I/V2)
1000/V
Log (I/V2)
(a)
303
Log (I/V2)
Log (I/V2)
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1000/V
(f)
1000/V
Fig. 4. Fowler–Nordheim plot of as-deposited and ion-irradiated diamond thin films: (a) as-deposited sample A, (b) irradiated sample A, (c) as-deposited sample B, (d) irradiated sample B, (e) as-deposited sample C and (f) irradiated sample C.
4. Conclusions Polycrystalline diamond thin films have been deposited by a hot filament chemical vapor deposition technique. The as deposited samples were subjected to high-energy ion irradiation using 100 MeV Au7+ ions. The irradiation-induced changes are studied in terms of structure and properties. High-energy ion irradiation leads to creation of defects and non-diamond in the coat-
ing. Electronic energy loss of the bombarding ions is responsible for the microstructural changes observed. The field emission studies were carried out for the as-deposited and ion-irradiated samples. High-energy ion irradiation leads to the lowering of turn on and threshold fields required for emission. The introduction of defects and nondiamond carbon in sp2 hybridization appears to be responsible for the lowering of electric fields required for emission.
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Table 3 Role of sp2 clusters and sp2/sp3 ratio on field emission characteristics published in the literature Nature of the Film
Role of sp2 clusters and sp3/sp2 ratio on field emission
Reference no.
CVD Diamond
1. Lowering of work function due to sp2 defect-induced band 2. Increase in field enhancement due to sp2 diamond–sp2 microstructures 3. Decrease in turn on field and increase in emission current with increase in sp2 content sp2/sp3 high enough for good conduction paths leading to enhanced field emission Localized sp2 sites cause conductive paths leading to field emission enhancement sp2 graphitic inclusions cause lowering of work function and threshold field Large grain boundary density and sp2 bonded carbon cause decrease in threshold field sp2 carbon act as source of emission sites resulting in low turn on field at optimum sp2/sp3 ratio Diamond sp2-bonded carbon grain boundaries act as emission sites and cause enhancement in field emission Lowering of turn on field via sp2 carbon inclusions
[26]
DLC DLC CVD Diamond Nano-Diamond DLC CVD CVD Diamond
Acknowledgements The authors are grateful to Mr. Sharad Gupta and Dr. Asima Pradhan of Physics Department, Indian Institute of Technology, Kanpur, India, for recording the Raman Spectra. The financial assistance to Pratishtha T. Pandey by IIT Delhi is gratefully acknowledged.
References [1] Himpsel FJ, Knapp JA, Van Vechten JA, Eastman DE. Phys Rev B 1979;20:624. [2] Liu J, Zhirnov VV, Wojak GJ, Myers AF, Choi WB, Hren JJ, Wolter SD, McClure MT, Stoner BR, Glass JT. Appl Phys Lett 1994;65:2842. [3] Givargove EI, Zhirnov VV, Stepanova AN, Plekhanov PS, Kozlov RI. Appl Surf Sci 1996;94/95:117. [4] Zhirnov VV, Choi WB, Cuomo JJ, Hren JJ. Appl Surf Sci 1996;94/95:123. [5] Gupta S, Weiss BL, Weiner BR, Morell G. J Appl Phys 2001;89:5671. [6] Wu K, Wang XR, Liu S, Wang EG. J Appl Phys 2001;90:4810. [7] Okano K, Yamada T, Ishihara H, Koizumi S, Itoh J. Appl Phys Lett 1997;70:2201. [8] Nagao M, Kondo T, Gotoh Y, Tsuji H, Ishikawa J, Miyata K, Kobashi K. Appl Phys Lett 1997;71:2807. [9] Andrienko I, Cimmino A, Hoxley D, Prawer S, Kalish R. Appl Phys Lett 2000;77:1221. [10] Sugino T, Kuriyama K, Kimura C, Yokota Y, Kawasaki S, Shirafugi J. J Appl Phys 1999;86:4635.
[27] [28] [29] [30] [31] [32] [33]
[11] Geis MW, Twichell JC, Macualay J, Okano K. Appl Phys Lett 1995;67:1328. [12] Zhu W, Kochanski GP, Lin S, Seibles L. J Appl Phys 1995;78:2707. [13] Dilawar N, Sah S, Mehta BR, Vankar VD, Awasthi DK, Mehta GK. Vacuum 1996;47:1269. [14] Fowler RH, Nordheim LW. Proc Roy Soc London 1928;Ser A119:173. [15] Knight D, White W. J Mater Res 1989;4:385. [16] Regueiro MN, Abello L, Lucazeau G, Hodeau J-L. Phys Rev B 1992;46(15):9903. [17] Ager III JW, Veirs DK, Rosenblat GM. Phys Rev B 1991;43:6491. [18] Hyer RC, Green M, Sharma SC. Phys Rev B 1993;49:14573. [19] Faniculli M, Moustakes TD. Phys Rev B 1993;48:14982. [20] Jamieson DN, Prawer S, Nugert KW, Dooley SP. Nucl Instrum Methods B 1995;106:641. [21] Wong MS, Karions FG, Cartz L. Surf Coat Technol 1990;43–44:63. [22] Maeta H, Haruna K, Bang L, Ono F. Nucl Instrum Methods B 1993;80/81:1477. [23] Kang WP, Davidson JL, Howell M. J Vac Sci Technol 1996;14:2066. [24] Erchak DP, Gerfand RN, Penia NM, Stelmakh VF, Tolstykh VP, Ulyashien AG, Variencho VS, Zaitsev AM. Phys Stat Sol A 1990;121:63. [25] Belykh TA, Gorodischensky AL, Kazak LA, Semyanikov VE, Urmanov AR. Nucl Instrum Methods B 1990;51:242. [26] Kang WP. Wisitsora-at A, Davidson JL, Kerns DV, Li Q, Xu JF, Kim CK. J Vac Sci Technol B 1998;16(2):684. [27] Kleps I, Nicolaesu D, Stamatin I, Correia A, Zlakin GA. Appl Surf Sci 1999;146:152. [28] Groning O, Kuttel OM, Groning P, Schlapbach L. Appl Surf Sci 1997;111:135.
ARTICLE IN PRESS P.T. Pandey et al. / Vacuum 72 (2004) 297–305 [29] Yang S-H, Yokoyama M. Jpn J Appl Phys 1999;38: 209. [30] Wang SG, Zhang Q, Yoon SF, Ahn J, Zhou Q, Wang Q, Yang DJ, Li JQ, Shanyong SZ. Surf Coat Technol 2003;167:143.
305
[31] Umehara Y, Murai S, Koide Y, Murakami M. Diam Relat Technol 2002;11:1429. [32] Wang C, Garcia A, Ingram DC, Lake M, Kordesch ME. Electron Lett 1991;27:1459. [33] Peng XL. Thin Solid Films 2000;372:292.
Vacuum 72 (2004) 297–305
Field emission characteristics of high-energy ion-irradiated polycrystalline diamond thin films P.T. Pandeya, G.L. Sharmaa, D.K. Awasthib, V.D. Vankara,* a
Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110 016, India b Nuclear Science Centre, New Delhi-110 067, India Received 2 March 2003; received in revised form 18 July 2003; accepted 3 August 2003
Abstract Diamond thin films have been synthesized by hot-filament chemical vapor deposition process using a mixture of methane and hydrogen gases. The samples were subjected to very high-energy ion irradiation (100 MeV Au7+ ions). The field emission characteristics of ion-irradiated samples have been studied. High emission currents and low turn-on and threshold fields were obtained for ion-irradiated samples. The as-deposited and the ion-irradiated samples have been characterized by X-ray Diffraction, Scanning Electron Microscopy and Micro-Raman Spectroscopy techniques and the resulting changes are correlated with field emission results. r 2003 Elsevier Ltd. All rights reserved. Keywords: Diamond; Chemical vapor deposition; Field emission; Ion irradiation
1. Introduction Diamond and diamond like carbon thin films have become the most probable candidates for field emission devices due to their negative electron affinity [1], high mechanical stability, high thermal conductivity (2600 W/mK), large intrinsic breakdown field (107 V/cm) and robust mechanical and chemical properties. Diamond-based cold cathodes have durability for high-temperature applications and high-voltage operation due to large breakdown field. Impressive emission currents are obtainable under low-voltage operation from the *Corresponding author. Tel.: +91-11-2686-1329; fax: +9111-2658-1114. E-mail address: [email protected] (V.D. Vankar).
diamond surface. The traditional field emitters were made up of high work function materials such as tungsten, molybdenum or silicon tips. The field strength required in metallic tips are >1000 V/mm and for Si tips B100 V/mm. Silicon tips are prone to oxidation and to damage by particle bombardment. Enhanced field emission has been demonstrated by diamond-coated Mo tips and Si tips [2–4]. Coatings of polycrystalline diamond have been used in tip and planar configuration, resulting in considerable lowering of applied voltage required for emission. Low threshold fields have been obtained from nanocrystalline diamond thin films [5,6]. Various other treatments such as doping with nitrogen [7], boron [8,9] and phosphorous [10], exposure to O2 plasma and Cs deposition [11], etc., lower the field required for emission to various extents. Effect of
0042-207X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.08.007
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ion implantation [12] using keV energy has been found to reduce the field strength and this has been associated to defects produced in the film. The nature of defects produced by high-energy ions (100 MeV) is totally different and must effect the field emission characteristics. This kind of study has not been reported in the literature. In this communication, we report the effect of very highenergy ion irradiation (100 MeV Au7+ ions) on electron field emission from polycrystalline CVD diamond thin films. The effect of high-energy ion irradiation is to produce defects due to inelastic scattering and is expected to modify the surface/ interface in a much different way as compared to other techniques especially ion implantation. And since surface/interface plays a vital role in the field emission process, this study is extremely important.
2. Experimental details The details of the deposition set up used to grow diamond films studied in this work are described elsewhere [13]. Briefly, diamond films were deposited on silicon substrates by hot-filament chemical vapor deposition process. The reactant gases were a mixture of H2 and 10% CH4+90% Ar. A tungsten filament was employed which was heated to 2200 C. The substrate temperature was measured by a chromel–alumel thermocouple, which was maintained around 750 C. The deposition pressure was 4000 N/m2. The substrates were pretreated by scratching with 0.5 mm diamond powder followed by degreasing and cleaning in trichloroethylene and propanol. The high-energy ion irradiation was performed by subjecting the diamond samples to 100 MeV Au7+ ions from (at Nuclear Science Centre, New Delhi, India) 15 MV Pelletron accelerator. The beam current was maintained at 2 pnA (particle nanoampere, which is defined as the number of particles in a current of 1 nA per second). The samples were subjected to a dose approximately 9.5 1012 ions/cm2. High-energy heavy ions were chosen due to higher electronic stopping power. The 100 MeV Au7+ have electronic losses of the ( while the nuclear stopping is order of 1667 eV/A
( The irradiation-induced effects are 2.44 eV/A. expected to be due to electronic stopping only. The damage effects of nuclear stopping are negligible. This is the major difference when high-energy ions are used as compared to the effects due to low-energy ions of keV range. The projected range of ions as calculated by ‘‘TRIM’’ (transport of ions in matter) analysis was about 10.36 mm, which is higher than the film thickness. Electron field emission measurements were performed using a specially fabricated diode assembly in a vacuum system maintained at 106 Torr. The cathode and anode were separated by 30 mm thick spacer. The anode was a copper plate, which was biased positively, and the current was measured using Keithley electrometer. The I– V measurements were analyzed using Fowler– Nordheim model [14] which follows the relation I ¼ aV 2 expðbf3=2 =bV Þ; where a and b are constants, I is the emission current, V is the applied voltage to the material, f is the work function of the cathode material and b is the field enhancement factor. The negative slope of the Fowler–Nordheim plot is proportional to f3=2 =b: The as-deposited and irradiated samples were characterized by X-ray Diffraction, Scanning Electron Microscopy and Micro-Raman Spectroscopy and the resulting changes observed were correlated with its field emission property. The XRD diffractograms were recorded using CuKa radiation. (Giegerflex D/MAX-RB 300 from Rigaku Corporation, Japan). The surface morphological studies of diamond thin films were carried out using a scanning electron microscope (Cambridge Instruments, Model S320). A 514.6 nm line of 15 W Argon ion laser (model 2030, Spectra Physics) was used to record the micro-Raman spectra at room temperature.
3. Results and discussion A number of samples deposited under different deposition conditions were studied for structure and field emission properties. Representative data for three sets of samples (called A, B and C) is presented. The details of the deposition parameters are listed in Table 1. The variation in the
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Table 1 Deposition details of the samples used in the present study Sample name
CH4/H2 ratio (sccm)
Working pressure (N/m2)
Substrate temperature ( C)
A B C
2.5:145 2.5:135 2.5:115
4000 4000 4000
750 750 750
deposition condition is seen to reflect in crystalline quality of the coating. The entire data for all the studies carried out is presented in Table 2. 3.1. Structural observations The X-ray diffraction pattern of one of the asdeposited sample is shown in Fig. 1. All samples show peaks at 2y ¼ 43:9 and 75.3 corresponding to diamond (1 1 1) and (2 2 0) planes, respectively. The X-ray diffraction pattern of ion-irradiated samples showed similar peaks. This suggested that no phase change or any visible major microstructural variation was introduced because of ion irradiation. The X-ray diffraction patterns of all samples (both as-deposited and ion irradiated) showed similar peaks, hence they are not shown here. No amorphous phase was detected within the limits of the experiment. Fig. 2(a–c) show the SEM micrographs of samples A–C. The sample A shows faceted, dense and uniform growth of diamond particles. The SEM micrograph of as-deposited sample B is shown in Fig. 2(b). The sample B also shows faceted growth of diamond particles. The sample C was grown with more CH4/H2 ratio and shows a slight deterioration in the morphology as compared to samples A and B. The surface morphology of any of these samples does not show remarkable change in the microstructure after irradiation. Hence, they are not shown here. 3.2. Micro-Raman analysis The Raman spectra of as-deposited samples A–C are shown in Fig. 3. In all these figures main features corresponding to cubic diamond phase and ‘G’ band of amorphous carbon are observable. The ‘G’ band peak is attributed to highly
disordered carbon atoms (non-diamond carbon) in sp2 hybridization [15]. The details of the voigt analysis i.e. peak position, peak line width and Id =Ind ratio are listed in Table 2. The post-irradiation Raman spectra of all the samples studied also shows a cubic diamond peak and ‘G’ band peak of amorphous carbon (Fig. 3). The ion–irradiated Raman spectrum of samples B and C show additional bands at 1440 and 1447 cm1, respectively. This band is attributed to amorphous carbon phase [16]. Absence of this band in ion-irradiated sample A is due to less nondiamond content in the as-deposited sample A as it was grown with lowest CH4/H2 ratio. Sample A showed minimum radiation damage. The sample C shows more radiation damage as compared to samples A and B. This may be because of more non-diamond content in sample C. An increase in the line width after high-energy ion irradiation reflects the introduction of defects in the coating. A decrease in the value of Id =Ind is also observed after irradiation. This showed an increase in the relative abundance of amorphous carbon phases present in the coating after highenergy ion irradiation. Ager et al. [17] have argued that in natural diamonds, the Raman line broadening is related to the decay of optical phonon created in the Raman process, into two acoustic phonons with opposite wave vectors. In CVD grown polycrystalline diamond films, the phonon may also decay at defect sites and grain boundaries, leading to the broadening of the line widths. Hence one would expect the line widths to increase with increasing defect density. Hyer et al. [18] and Faniculli and coworkers [19] also attributed broadening of Raman lines to reduction of phonon lifetime due to production of point defects. Jamieson and coworkers [20] formulated a linear correlation
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60
D (220)
D (111)
INTENSITY (a.u.)
10.10 8.33
0.31
20.83 11.90
1.81 16.00 13.33
2.63 33.33 18.31
4.80 40.13 26.67
5.60 61.72 40.16
Slope Threshold field (V/mm) Turn on field (V/mm)
2.18
3.95
2.68
5.85
8.34
15.40 44.27 20.19 120.23 17.70 64.10 28.76 70.19 139.08 20.27 106.48 33.42 179.77 73.30 Highly faceted morphology
Highly faceted morphology
Faceted morphology
Faceted morphology
Deteriorated morphology
Deteriorated morphology
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
Diamond (1 1 1) & (2 2 0) peaks
A (as-deposited)
A (irradiated)
B (as-deposited)
B (irradiated)
C (as-deposited)
C (irradiated)
1333 1574 1335 1562 1334 1562 1330 1440 1569 1334 1551 1336 1447 1566
3.30
Id =Ind Line width (cm1) SEM
Peak position (cm1)
20
80
2θ (DEG.)
X-ray diffraction
Field emission studies Raman analysis Structural observations Sample name
Table 2 Summary of the various studies used for the present work
1.33
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Fig. 1. X-ray diffraction pattern of a representative diamond thin film.
between increased defect density due to highenergy ion irradiation and the Raman line width. The decrease in the value of Id =Ind indicates the onset of amorphization due to breaking of bonds by high-energy ion irradiation. Wong et al. [21] have also observed that diamond powders irradiated with MeV ions were a mixture of diamond and non-diamond carbon phases. Maeta et al. [22] have suggested that most of the radiation damage in their ion-irradiated diamonds occurred in the form of interstitials dislocation loops and vacancies. Our micro-Raman results indicate the formation of non-diamond structure as well introduction of defects in diamond crystallites as a result of high-energy irradiation, which is consistent with the reported data. 3.3. Field emission characteristics Field emission studies were carried out of asdeposited and the ion-irradiated samples. Fowler– Nordheim plot of all the samples studied showed a straight line of negative slope in the high-voltage region as shown in Fig. 4. This suggested that the emission of electrons is cold cathode induced. The turn on field (defined as the point at which the Fowler–Nordheim plot deviates from its normal behavior) and the threshold field (defined as field at which a current of 1 mA is obtained) of samples varied from samples A to C because of different non-diamond contents, but were reduced
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(b)
(c) Fig. 2. Scanning electron micrographs of: (a) as-deposited sample A, (b) as-deposited sample B and (c) as-deposited sample C.
considerably after ion irradiation as shown in Table 2. The slope of high-voltage region of Fowler–Nordheim plot was lowered after ion irradiation. The lowering in the value of the slope of the Fowler–Nordheim plot of the irradiated samples as compared to the as-deposited samples depicts lowering of the work function after irradiation. A shallow slope corresponds to the ease of electron emission. The details of turn on field, threshold fields and slopes are given in Table 2. The current density of as-deposited sample A was 0.3 mA/cm2 at an applied field of 46.66 V/mm and was increased to 0.02 mA/cm2 for irradiated sample A for the same field. Asdeposited sample B gives a current density of 0.25 mA/cm2 at an electric field of 23.33 V/um, which was enhanced to 0.24 mA/cm2 after irradiation for the same applied field. The current density was increased from 11.2 mA/cm2 for as-deposited sample C to 0.08 mA/cm2 for irradiated sample C
for the same electric field of 22.23 V/mm. A positive slope region is also observed in all the plots of Fowler–Nordheim curves, which may be caused by leakage through the spacer. Kang et al. [23] also reported similar Fowler–Nordheim curve with distinctive positive and negative slope regimes. Zhu et al. [12] demonstrated a strong correlation between the emission properties and the defect densities in the diamond film. The results obtained by them suggest that the defects introduced by ion implantation increased the conductivity and lowered the work function of as-grown diamond surface. It has been suggested that the defects create additional sub-bands within the band gap of diamond and thus contribute to the electron field emission at low electric fields. In ion implantation (where the energies are Bfew keV), atomic displacements are the major effects introduced. However, in the case of high-energy ion irradiation, the ions passing through the solid cause
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3. Micro-Raman spectrum of as-deposited and ion-irradiated samples: (a) as-deposited sample A, (b) irradiated sample A, (c) as-deposited sample B, (d) irradiated sample B, (e) as-deposited sample C and (f) irradiated sample C.
electronic excitations (ionizations) of target atoms and displacements of ions and atoms are normally not observed. In an insulating medium columnar, defects are generally produced along the path of passing energetic ion [24]. It is argued that electronic relaxations are most efficient on atoms located at grain boundaries, defects and interstitials, which gain mobility as a result of irradiation [25]. In diamond films, in addition, highenergy ion irradiation leads to breaking of C–C bonds, C–H bonds and loss of hydrogen. Since hydrogen passivates the dangling bonds over the surface, bond breaking and loss of hydrogen leads to recreation of dangling bonds/vacancies in the film. Prolonged irradiation may lead to the
development of sp2 clusters and eventual graphitization. Micro-Raman results clearly indicated increase in non-diamond carbon in sp2 hybridization as Id =Ind decreases or ‘G’ band intensity increases after irradiation. Many researchers have studied the role of sp2 non-diamond components in affecting field emission characteristics. The some details of published work in this direction are given in Table 3. The increase in the amount of non-diamond carbon content and introduction of defects in the coating appears to be responsible for enhanced emission after irradiation. Our results also indicate similar trends, although exact role of defects on the field emission characteristics needs further examination.
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1000/V
(d)
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Log (I/V2)
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Log (I/V2)
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Log (I/V2)
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Log (I/V2)
(a)
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Log (I/V2)
Log (I/V2)
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(f)
1000/V
Fig. 4. Fowler–Nordheim plot of as-deposited and ion-irradiated diamond thin films: (a) as-deposited sample A, (b) irradiated sample A, (c) as-deposited sample B, (d) irradiated sample B, (e) as-deposited sample C and (f) irradiated sample C.
4. Conclusions Polycrystalline diamond thin films have been deposited by a hot filament chemical vapor deposition technique. The as deposited samples were subjected to high-energy ion irradiation using 100 MeV Au7+ ions. The irradiation-induced changes are studied in terms of structure and properties. High-energy ion irradiation leads to creation of defects and non-diamond in the coat-
ing. Electronic energy loss of the bombarding ions is responsible for the microstructural changes observed. The field emission studies were carried out for the as-deposited and ion-irradiated samples. High-energy ion irradiation leads to the lowering of turn on and threshold fields required for emission. The introduction of defects and nondiamond carbon in sp2 hybridization appears to be responsible for the lowering of electric fields required for emission.
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Table 3 Role of sp2 clusters and sp2/sp3 ratio on field emission characteristics published in the literature Nature of the Film
Role of sp2 clusters and sp3/sp2 ratio on field emission
Reference no.
CVD Diamond
1. Lowering of work function due to sp2 defect-induced band 2. Increase in field enhancement due to sp2 diamond–sp2 microstructures 3. Decrease in turn on field and increase in emission current with increase in sp2 content sp2/sp3 high enough for good conduction paths leading to enhanced field emission Localized sp2 sites cause conductive paths leading to field emission enhancement sp2 graphitic inclusions cause lowering of work function and threshold field Large grain boundary density and sp2 bonded carbon cause decrease in threshold field sp2 carbon act as source of emission sites resulting in low turn on field at optimum sp2/sp3 ratio Diamond sp2-bonded carbon grain boundaries act as emission sites and cause enhancement in field emission Lowering of turn on field via sp2 carbon inclusions
[26]
DLC DLC CVD Diamond Nano-Diamond DLC CVD CVD Diamond
Acknowledgements The authors are grateful to Mr. Sharad Gupta and Dr. Asima Pradhan of Physics Department, Indian Institute of Technology, Kanpur, India, for recording the Raman Spectra. The financial assistance to Pratishtha T. Pandey by IIT Delhi is gratefully acknowledged.
References [1] Himpsel FJ, Knapp JA, Van Vechten JA, Eastman DE. Phys Rev B 1979;20:624. [2] Liu J, Zhirnov VV, Wojak GJ, Myers AF, Choi WB, Hren JJ, Wolter SD, McClure MT, Stoner BR, Glass JT. Appl Phys Lett 1994;65:2842. [3] Givargove EI, Zhirnov VV, Stepanova AN, Plekhanov PS, Kozlov RI. Appl Surf Sci 1996;94/95:117. [4] Zhirnov VV, Choi WB, Cuomo JJ, Hren JJ. Appl Surf Sci 1996;94/95:123. [5] Gupta S, Weiss BL, Weiner BR, Morell G. J Appl Phys 2001;89:5671. [6] Wu K, Wang XR, Liu S, Wang EG. J Appl Phys 2001;90:4810. [7] Okano K, Yamada T, Ishihara H, Koizumi S, Itoh J. Appl Phys Lett 1997;70:2201. [8] Nagao M, Kondo T, Gotoh Y, Tsuji H, Ishikawa J, Miyata K, Kobashi K. Appl Phys Lett 1997;71:2807. [9] Andrienko I, Cimmino A, Hoxley D, Prawer S, Kalish R. Appl Phys Lett 2000;77:1221. [10] Sugino T, Kuriyama K, Kimura C, Yokota Y, Kawasaki S, Shirafugi J. J Appl Phys 1999;86:4635.
[27] [28] [29] [30] [31] [32] [33]
[11] Geis MW, Twichell JC, Macualay J, Okano K. Appl Phys Lett 1995;67:1328. [12] Zhu W, Kochanski GP, Lin S, Seibles L. J Appl Phys 1995;78:2707. [13] Dilawar N, Sah S, Mehta BR, Vankar VD, Awasthi DK, Mehta GK. Vacuum 1996;47:1269. [14] Fowler RH, Nordheim LW. Proc Roy Soc London 1928;Ser A119:173. [15] Knight D, White W. J Mater Res 1989;4:385. [16] Regueiro MN, Abello L, Lucazeau G, Hodeau J-L. Phys Rev B 1992;46(15):9903. [17] Ager III JW, Veirs DK, Rosenblat GM. Phys Rev B 1991;43:6491. [18] Hyer RC, Green M, Sharma SC. Phys Rev B 1993;49:14573. [19] Faniculli M, Moustakes TD. Phys Rev B 1993;48:14982. [20] Jamieson DN, Prawer S, Nugert KW, Dooley SP. Nucl Instrum Methods B 1995;106:641. [21] Wong MS, Karions FG, Cartz L. Surf Coat Technol 1990;43–44:63. [22] Maeta H, Haruna K, Bang L, Ono F. Nucl Instrum Methods B 1993;80/81:1477. [23] Kang WP, Davidson JL, Howell M. J Vac Sci Technol 1996;14:2066. [24] Erchak DP, Gerfand RN, Penia NM, Stelmakh VF, Tolstykh VP, Ulyashien AG, Variencho VS, Zaitsev AM. Phys Stat Sol A 1990;121:63. [25] Belykh TA, Gorodischensky AL, Kazak LA, Semyanikov VE, Urmanov AR. Nucl Instrum Methods B 1990;51:242. [26] Kang WP. Wisitsora-at A, Davidson JL, Kerns DV, Li Q, Xu JF, Kim CK. J Vac Sci Technol B 1998;16(2):684. [27] Kleps I, Nicolaesu D, Stamatin I, Correia A, Zlakin GA. Appl Surf Sci 1999;146:152. [28] Groning O, Kuttel OM, Groning P, Schlapbach L. Appl Surf Sci 1997;111:135.
ARTICLE IN PRESS P.T. Pandey et al. / Vacuum 72 (2004) 297–305 [29] Yang S-H, Yokoyama M. Jpn J Appl Phys 1999;38: 209. [30] Wang SG, Zhang Q, Yoon SF, Ahn J, Zhou Q, Wang Q, Yang DJ, Li JQ, Shanyong SZ. Surf Coat Technol 2003;167:143.
305
[31] Umehara Y, Murai S, Koide Y, Murakami M. Diam Relat Technol 2002;11:1429. [32] Wang C, Garcia A, Ingram DC, Lake M, Kordesch ME. Electron Lett 1991;27:1459. [33] Peng XL. Thin Solid Films 2000;372:292.