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Field emission from cleaved diamond fibers
Thin Solid Films 308–309 (1997) 204–208
Field emission from cleaved diamond fibers J.W. Glesener*, H.B. Lin, A.A. Morrish Naval Research Laboratory, Washington, DC 20375, USA
Abstract Diamond fibers 100–200 mm in diameter were investigated as electron emitters. Uniform, straight boron doped fibers were grown on tungsten and sapphire fiber cores. The field emitting surface was created by mechanically cleaving the polycrystalline fiber perpendicular to the fiber axis, a procedure in which it was assumed the necessary morphology would be present for field emission. Other than cleaving, no other special surface or bulk treatments were employed. Voltage dependence and noise power spectral density (PSD) measurements were carried out on the field emission current. A subset of the diamond coated sapphire fibers exhibited electron emission at a factor of 5 reduction in the applied field as compared to the tungsten core fibers. Within experimental error, the PSD exhibited a f − 1.5 dependence on frequency. 1997 Elsevier Science S.A. Keywords: Field emission; Cleaved diamond fibers; Power spectral density
1. Introduction Diamond has been viewed as a potential field emitting material due to in part the chemical inertness of the surface, the low sputtering coefficient of carbon [1] and a work function for boron doped material of 4.15 eV [2]. Previous work has shown that electron emission from polycrystalline diamond films suffers from areal non-uniformity across the film at a fixed applied field [3]. Several authors have attempted to modify the surface morphology of diamond in an effort to enhance the uniformity of the emission characteristics [4,5]. The motivation to fabricate arrays of sharp tips is consistent with the understanding that in boron doped diamond, field emission is from the valence band [6,7] and therefore high fields are a condition for electron emission. Alternately, using ion beam processing methods to produce arrays of sharp tips may alter the electronic characteristics of the diamond surface [8]. Previous work has shown that carbon based fibers have promise as cold cathode emitters [9,10]. Diamond is not only grown as films but also as long thin fibers [11,12]. Field emitters using these fibers can be created by cleaving. These types of emitters offer the potential advantage of a compact diamond based cold electron source. Presented in
* Corresponding author.
0040-6090/97/$17.00 1997 Elsevier Science S.A. All rights reserved PII S0040-6090 (97 )0 0413-6
this paper is this alternative method of producing field emitting sites using diamond fibers in which the emitting surface is not exposed to the processing required for fabricating arrays of emitter tips. Arrays of emitters may be achieved by just bundling the fibers. Enhancing the understanding of field emission in diamond can be undertaken by using either surface or electrical experimental methods. Surface based techniques can yield accurate information concerning an area surrounding an emission point, unfortunately spatially resolving the specific emission site is non-trivial. Although the field emission current is strongly affected by the condition of the emitting site, interpretation of such measurements can be obscure. Multiple electrical measurements were performed in order investigate the nature of the emission sites.
2. Experimental The two types of fibers reported on in this paper were grown using filament assisted chemical vapor deposition using a methane/hydrogen ratio of 1.2/120 at a process pressure of 15 Torr. Diborane was used as the dopant source and the boron/carbon ratio during growth was fixed at 40 p.p.m. Pre-growth preparation consisted of oil coating the 20-mm tungsten cores and abrading the growth surface of the sapphire fiber with a 0.1-mm diamond paste. During the growth phase, the fiber substrate was suspended approxi-
J.W. Glesener et al. / Thin Solid Films 308–309 (1997) 204–208
205
Fig. 1. Raman spectrum of metal core diamond fiber. Excitation wavelength was 514 nm.
mately 3–4 mm above a heating element at a nominal temperature of 900°C and approximately 1 cm below a multistrand tantalum filament at a temperature of 2150°C as indicated by an infrared pyrometer. Following completion of the growth, the fibers were allowed to cool down in a hydrogen ambient. The Raman spectrum taken of a diamond coated tungsten fiber is shown in Fig. 1 and the Raman spectrum of a diamond coated sapphire fiber is shown in Fig. 2. Both spectra show a distinct diamond peak at 1330 cm−1 and a low luminescence background. The interpretation of the Raman spectra for both types of fibers confirms the presence of diamond and a lack of a graphitic component. Further details regarding the growth system and procedures may be found in Ref. [11]. After completion of the growth cycle, fibers 7.5-cm long were withdrawn from the growth system and samples 1 cm in length were produced by cleaving in air. Diamond surfaces cleaved in air have been shown to react with oxygen
Fig. 2. Raman spectrum of diamond coated sapphire fiber. Excitation wavelength was 514 nm.
Fig. 3. SEM photograph of cleaved end of a diamond fiber with tungsten core.
and are expected to have a sizable amount of chemisorbed oxygen on the surface [13]. Photographs from a scanning electron microscope (SEM) show surfaces typical of the fibers employed. Fig. 3 shows a cleaved tungsten core fiber and Fig. 4 shows a sapphire fiber core surrounded by a diamond overcoat. Prior to insertion
Fig. 4. SEM photograph of cleaved end of diamond coated sapphire fiber.
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J.W. Glesener et al. / Thin Solid Films 308–309 (1997) 204–208
into the UHV characterization chamber, an aluminum foil tab was attached to the fiber to act as a holder/electrical contact. A flat edged cylindrical metal probe 4 mm in diameter was positioned above the fiber using a three-axis vacuum manipulator feedthrough, with each axis accurate to within ±2 mm. The height above the sample was determined by defining zero separation as the point where the probe and sample were in electrical contact. Before the current-voltage (I-V) measurements were carried out, the diamond emitters were conditioned by operating them at the highest voltage employed in the measurement for at least 1 h. The current-voltage measurements were carried out using a Keithley electrometer. For each measurement, the current was allowed to settle for 10 s and then was averaged 20 times. The observed data is shown in Fig. 5 for a probe-fiber distance of 300 mm. A least squares fit of the data to the Fowler-Nordheim equation was carried out. From the Fowler-Nordheim [14] equation, the relationship between the field enhancement factor b and the work function f is: f3 2 V b = (6:52 × 10 ) b (cm − eV 3 2 ) =
7
=
(1)
where b is the fitted slope from Fig. 5. b is defined in the equation E = bV, where E is the electric field and V is the applied voltage. A graph of bd versus f is shown in Fig. 6, for the distance d = 300 mm. I-V measurements were also performed on the diamond coated sapphire fibers. To initiate electron emission an approximately 100% increase in the initial operating voltage was required. A subset of these fibers (50%) exhibited anomalously large electron emission at low fields. The I-V curve for these fibers is shown in Fig. 7. The noise properties of the metal fibers were also characterized. Fig. 8 shows the power spectral density (PSD) measured on two fibers. Theoretical work has shown that the PSD of the field emission current is propor-
Fig. 5. Fowler-Nordheim plot of the I-V characteristics for fiber no. 9 at a probe-sample distance of 300 mm. The emission current was normalized by the fiber area.
Fig. 6. Unitless field enhancement factor bd versus work function for a probe sample distance of d = 300 mm.
tional to f − 1.5 where f is the frequency [15] in the high frequency limit. The theoretical underpinnings of such a relationship were derived assuming fluctuations in the surface absorbate density perturbed the work function of the bulk material. This effect has been reported by other authors (see references in [16]. A point to make concerning the observation of a f − 1.5 relationship is that if electron emission were from many sites, a PSD closer to 1/f would result because of averaging.
3. Conclusions Previous work has catalogued a variety of material bulk conditions which can enhance electron emission from diamond, specifically diamond films with a substantial graphitic component [17], while in this present work field emission has been observed in diamond with a low graphitic component. The field emission characteristics of the metal core boron doped fibers resemble previously reported results from polycrystalline diamond [7]. Specifically, data from Fig. 6 indicates that field emission occurs from a site with a high field enhancement factor. This implies electron emission from the valence band. This in conjunction with the possibility of absorbate motion, illustrate a potential design trade-off. Although sharper edges would yield a lower electron emission field threshold, the fact that absorbates would be more weakly bound to such sites would imply an increase in the AC part of the field emission current1. The field emission properties of two different types of diamond fibers were investigated. The cleaved, unprocessed 1 From reference [13] the noise PSD is proportional to D, where D is the absorbate surface diffusion coefficient. D is dependent on a e –E/kT where E is the absorbate binding energy and k is Boltzmann’s constant.
J.W. Glesener et al. / Thin Solid Films 308–309 (1997) 204–208
207
Fig. 7. I-V curves for diamond coated sapphire fibers. The emission current was normalized by the diamond cross sectional area.
surfaces and high quality of the diamond indicate that probably for this class of emitters, sharp edges are a necessary condition for electron emission. In contrast to reports in the
literature for low quality diamond, decent electron emission was observed from diamond with a low graphitic background. The noise PSD measurements suggest within
Fig. 8. Power spectral density of the field emission current for two tungsten core fibers. Fiber no. 4 had a f − 1.57 dependence on frequency and fiber no. 9 had a f − 1.66 dependence on frequency. Data was taken using an HP 3562A.
208
J.W. Glesener et al. / Thin Solid Films 308–309 (1997) 204–208
experimental error that surface absorbate fluctuations are present and appear responsible for the AC component of the field emission current.
References [1] P. Sigmund, Phys. Rev., 184 (1969) 383. [2] W.A. Mackie, J.E. Plumlee and A.E. Bell, J. Vac. Sci. Technol. B, 14 (1996) 2041. [3] C. Wang, A. Garcia, D.C. Ingram, M. Lake and M.E. Kordesch, Electron. Lett., 27 (1991) 1459. [4] K. Okano, K. Hoshina and M. Iida, Appl. Phys. Lett., 64 (1994) 2742. [5] W.P. Kang, J.L. Davidson, Q. Li, J.F. Xu, D.L. Kinser and D.V. Kerns, in Application of Diamond Films and Related Materials: Third International Conference, NIST Special Publication 885, p. 37. [6] C. Bandis and B.B. Pate, Appl. Phys. Lett., 69 (1996) 366.
[7] J.W. Glesener and A.A. Morrish, Appl. Phys. Lett., 69 (1996) 785. [8] L.J. Huang, I. Bello, W.M. Lau, S.T. Lee, P.A. Stevens and B.D. DeVries, J. Appl. Phys., 76 (1994) 7483. [9] F.S. Baker, A.R. Osborn and J. Williams, J. Phys. D: Appl. Phys., 7 (1974) 2105. [10] A.G. Chakhovskoi, E.P. Sheshin, A.S. Kupryashkin and V.A. Seliverstov, J. Vac. Sci. Technol. B, 11 (1993) 511. [11] A.A. Morrish, J.W. Glesener, M. Fehrenbacher, P.E. Pehrsson, B. Maruyama and P.N. Natishan, Diamond Relat. Mater., 3 (1994) 173. [12] A.A. Morrish, P.N. Natishan, B. Maruyama and P.E. Pehrsson, US Patent No. 5 374 414. [13] R.G. Cavell, S.P. Kowalczyk, L. Ley, R.A. Pollak, B. Mills, D.A. Shirley and W. Perry, Phys. Rev. B, 7 (1973) 5313. [14] C.A. Spindt, I. Brodie, L. Humphrey and E.R. Westerburg, J. Appl. Phys., 47 (1976) 5248. [15] M.A. Gesley and L.W. Swanson, Phys. Rev. B, 32 (1985) 7703. [16] C. Kleint, Surf. Sci., 200 (1988) 472. [17] W. Zhu, G.P. Kochanski, S. Jin and L. Seibles, J. Vac. Sci. Technol. B, 14 (1996) 2011.
Field emission from cleaved diamond fibers J.W. Glesener*, H.B. Lin, A.A. Morrish Naval Research Laboratory, Washington, DC 20375, USA
Abstract Diamond fibers 100–200 mm in diameter were investigated as electron emitters. Uniform, straight boron doped fibers were grown on tungsten and sapphire fiber cores. The field emitting surface was created by mechanically cleaving the polycrystalline fiber perpendicular to the fiber axis, a procedure in which it was assumed the necessary morphology would be present for field emission. Other than cleaving, no other special surface or bulk treatments were employed. Voltage dependence and noise power spectral density (PSD) measurements were carried out on the field emission current. A subset of the diamond coated sapphire fibers exhibited electron emission at a factor of 5 reduction in the applied field as compared to the tungsten core fibers. Within experimental error, the PSD exhibited a f − 1.5 dependence on frequency. 1997 Elsevier Science S.A. Keywords: Field emission; Cleaved diamond fibers; Power spectral density
1. Introduction Diamond has been viewed as a potential field emitting material due to in part the chemical inertness of the surface, the low sputtering coefficient of carbon [1] and a work function for boron doped material of 4.15 eV [2]. Previous work has shown that electron emission from polycrystalline diamond films suffers from areal non-uniformity across the film at a fixed applied field [3]. Several authors have attempted to modify the surface morphology of diamond in an effort to enhance the uniformity of the emission characteristics [4,5]. The motivation to fabricate arrays of sharp tips is consistent with the understanding that in boron doped diamond, field emission is from the valence band [6,7] and therefore high fields are a condition for electron emission. Alternately, using ion beam processing methods to produce arrays of sharp tips may alter the electronic characteristics of the diamond surface [8]. Previous work has shown that carbon based fibers have promise as cold cathode emitters [9,10]. Diamond is not only grown as films but also as long thin fibers [11,12]. Field emitters using these fibers can be created by cleaving. These types of emitters offer the potential advantage of a compact diamond based cold electron source. Presented in
* Corresponding author.
0040-6090/97/$17.00 1997 Elsevier Science S.A. All rights reserved PII S0040-6090 (97 )0 0413-6
this paper is this alternative method of producing field emitting sites using diamond fibers in which the emitting surface is not exposed to the processing required for fabricating arrays of emitter tips. Arrays of emitters may be achieved by just bundling the fibers. Enhancing the understanding of field emission in diamond can be undertaken by using either surface or electrical experimental methods. Surface based techniques can yield accurate information concerning an area surrounding an emission point, unfortunately spatially resolving the specific emission site is non-trivial. Although the field emission current is strongly affected by the condition of the emitting site, interpretation of such measurements can be obscure. Multiple electrical measurements were performed in order investigate the nature of the emission sites.
2. Experimental The two types of fibers reported on in this paper were grown using filament assisted chemical vapor deposition using a methane/hydrogen ratio of 1.2/120 at a process pressure of 15 Torr. Diborane was used as the dopant source and the boron/carbon ratio during growth was fixed at 40 p.p.m. Pre-growth preparation consisted of oil coating the 20-mm tungsten cores and abrading the growth surface of the sapphire fiber with a 0.1-mm diamond paste. During the growth phase, the fiber substrate was suspended approxi-
J.W. Glesener et al. / Thin Solid Films 308–309 (1997) 204–208
205
Fig. 1. Raman spectrum of metal core diamond fiber. Excitation wavelength was 514 nm.
mately 3–4 mm above a heating element at a nominal temperature of 900°C and approximately 1 cm below a multistrand tantalum filament at a temperature of 2150°C as indicated by an infrared pyrometer. Following completion of the growth, the fibers were allowed to cool down in a hydrogen ambient. The Raman spectrum taken of a diamond coated tungsten fiber is shown in Fig. 1 and the Raman spectrum of a diamond coated sapphire fiber is shown in Fig. 2. Both spectra show a distinct diamond peak at 1330 cm−1 and a low luminescence background. The interpretation of the Raman spectra for both types of fibers confirms the presence of diamond and a lack of a graphitic component. Further details regarding the growth system and procedures may be found in Ref. [11]. After completion of the growth cycle, fibers 7.5-cm long were withdrawn from the growth system and samples 1 cm in length were produced by cleaving in air. Diamond surfaces cleaved in air have been shown to react with oxygen
Fig. 2. Raman spectrum of diamond coated sapphire fiber. Excitation wavelength was 514 nm.
Fig. 3. SEM photograph of cleaved end of a diamond fiber with tungsten core.
and are expected to have a sizable amount of chemisorbed oxygen on the surface [13]. Photographs from a scanning electron microscope (SEM) show surfaces typical of the fibers employed. Fig. 3 shows a cleaved tungsten core fiber and Fig. 4 shows a sapphire fiber core surrounded by a diamond overcoat. Prior to insertion
Fig. 4. SEM photograph of cleaved end of diamond coated sapphire fiber.
206
J.W. Glesener et al. / Thin Solid Films 308–309 (1997) 204–208
into the UHV characterization chamber, an aluminum foil tab was attached to the fiber to act as a holder/electrical contact. A flat edged cylindrical metal probe 4 mm in diameter was positioned above the fiber using a three-axis vacuum manipulator feedthrough, with each axis accurate to within ±2 mm. The height above the sample was determined by defining zero separation as the point where the probe and sample were in electrical contact. Before the current-voltage (I-V) measurements were carried out, the diamond emitters were conditioned by operating them at the highest voltage employed in the measurement for at least 1 h. The current-voltage measurements were carried out using a Keithley electrometer. For each measurement, the current was allowed to settle for 10 s and then was averaged 20 times. The observed data is shown in Fig. 5 for a probe-fiber distance of 300 mm. A least squares fit of the data to the Fowler-Nordheim equation was carried out. From the Fowler-Nordheim [14] equation, the relationship between the field enhancement factor b and the work function f is: f3 2 V b = (6:52 × 10 ) b (cm − eV 3 2 ) =
7
=
(1)
where b is the fitted slope from Fig. 5. b is defined in the equation E = bV, where E is the electric field and V is the applied voltage. A graph of bd versus f is shown in Fig. 6, for the distance d = 300 mm. I-V measurements were also performed on the diamond coated sapphire fibers. To initiate electron emission an approximately 100% increase in the initial operating voltage was required. A subset of these fibers (50%) exhibited anomalously large electron emission at low fields. The I-V curve for these fibers is shown in Fig. 7. The noise properties of the metal fibers were also characterized. Fig. 8 shows the power spectral density (PSD) measured on two fibers. Theoretical work has shown that the PSD of the field emission current is propor-
Fig. 5. Fowler-Nordheim plot of the I-V characteristics for fiber no. 9 at a probe-sample distance of 300 mm. The emission current was normalized by the fiber area.
Fig. 6. Unitless field enhancement factor bd versus work function for a probe sample distance of d = 300 mm.
tional to f − 1.5 where f is the frequency [15] in the high frequency limit. The theoretical underpinnings of such a relationship were derived assuming fluctuations in the surface absorbate density perturbed the work function of the bulk material. This effect has been reported by other authors (see references in [16]. A point to make concerning the observation of a f − 1.5 relationship is that if electron emission were from many sites, a PSD closer to 1/f would result because of averaging.
3. Conclusions Previous work has catalogued a variety of material bulk conditions which can enhance electron emission from diamond, specifically diamond films with a substantial graphitic component [17], while in this present work field emission has been observed in diamond with a low graphitic component. The field emission characteristics of the metal core boron doped fibers resemble previously reported results from polycrystalline diamond [7]. Specifically, data from Fig. 6 indicates that field emission occurs from a site with a high field enhancement factor. This implies electron emission from the valence band. This in conjunction with the possibility of absorbate motion, illustrate a potential design trade-off. Although sharper edges would yield a lower electron emission field threshold, the fact that absorbates would be more weakly bound to such sites would imply an increase in the AC part of the field emission current1. The field emission properties of two different types of diamond fibers were investigated. The cleaved, unprocessed 1 From reference [13] the noise PSD is proportional to D, where D is the absorbate surface diffusion coefficient. D is dependent on a e –E/kT where E is the absorbate binding energy and k is Boltzmann’s constant.
J.W. Glesener et al. / Thin Solid Films 308–309 (1997) 204–208
207
Fig. 7. I-V curves for diamond coated sapphire fibers. The emission current was normalized by the diamond cross sectional area.
surfaces and high quality of the diamond indicate that probably for this class of emitters, sharp edges are a necessary condition for electron emission. In contrast to reports in the
literature for low quality diamond, decent electron emission was observed from diamond with a low graphitic background. The noise PSD measurements suggest within
Fig. 8. Power spectral density of the field emission current for two tungsten core fibers. Fiber no. 4 had a f − 1.57 dependence on frequency and fiber no. 9 had a f − 1.66 dependence on frequency. Data was taken using an HP 3562A.
208
J.W. Glesener et al. / Thin Solid Films 308–309 (1997) 204–208
experimental error that surface absorbate fluctuations are present and appear responsible for the AC component of the field emission current.
References [1] P. Sigmund, Phys. Rev., 184 (1969) 383. [2] W.A. Mackie, J.E. Plumlee and A.E. Bell, J. Vac. Sci. Technol. B, 14 (1996) 2041. [3] C. Wang, A. Garcia, D.C. Ingram, M. Lake and M.E. Kordesch, Electron. Lett., 27 (1991) 1459. [4] K. Okano, K. Hoshina and M. Iida, Appl. Phys. Lett., 64 (1994) 2742. [5] W.P. Kang, J.L. Davidson, Q. Li, J.F. Xu, D.L. Kinser and D.V. Kerns, in Application of Diamond Films and Related Materials: Third International Conference, NIST Special Publication 885, p. 37. [6] C. Bandis and B.B. Pate, Appl. Phys. Lett., 69 (1996) 366.
[7] J.W. Glesener and A.A. Morrish, Appl. Phys. Lett., 69 (1996) 785. [8] L.J. Huang, I. Bello, W.M. Lau, S.T. Lee, P.A. Stevens and B.D. DeVries, J. Appl. Phys., 76 (1994) 7483. [9] F.S. Baker, A.R. Osborn and J. Williams, J. Phys. D: Appl. Phys., 7 (1974) 2105. [10] A.G. Chakhovskoi, E.P. Sheshin, A.S. Kupryashkin and V.A. Seliverstov, J. Vac. Sci. Technol. B, 11 (1993) 511. [11] A.A. Morrish, J.W. Glesener, M. Fehrenbacher, P.E. Pehrsson, B. Maruyama and P.N. Natishan, Diamond Relat. Mater., 3 (1994) 173. [12] A.A. Morrish, P.N. Natishan, B. Maruyama and P.E. Pehrsson, US Patent No. 5 374 414. [13] R.G. Cavell, S.P. Kowalczyk, L. Ley, R.A. Pollak, B. Mills, D.A. Shirley and W. Perry, Phys. Rev. B, 7 (1973) 5313. [14] C.A. Spindt, I. Brodie, L. Humphrey and E.R. Westerburg, J. Appl. Phys., 47 (1976) 5248. [15] M.A. Gesley and L.W. Swanson, Phys. Rev. B, 32 (1985) 7703. [16] C. Kleint, Surf. Sci., 200 (1988) 472. [17] W. Zhu, G.P. Kochanski, S. Jin and L. Seibles, J. Vac. Sci. Technol. B, 14 (1996) 2011.