- Email: [email protected]
Surface engineering of diamond tips for improved field electron emission
Diamond and Related Materials 10 (2001) 2178–2183
Surface engineering of diamond tips for improved field electron emission A.V. Karabutova,*, V.G. Ralchenkoa, I.I. Vlasova, R.A. Khmelnitskyb, M.A. Negodaevb, V.P. Varninc, I.G. Teremetskayac a
General Physics Institute, Vavilova str. 38, Moscow 117942, Russia P.N. Lebedev Physical Institute, Leninsky pr. 53, Moscow 117942, Russia c Institute of Physical Chemistry, Leninsky pr. 31, Moscow 117915, Russia
b
Received 4 September 2000; accepted 7 June 2001
Abstract Results are reported on the study of surface engineering of diamond microtips for improved field electron emission. Twodimensional (2D) arrays of diamond pyramids were prepared using a molding technique. As-grown diamond pyramids of 3 and 9 mm in size with sharp apexes were insulating and showed a relatively poor field electron emission. Several ways are examined to provide an electrical conductivity of diamond in order to supply electrons for the emission: diamond boron doping, partial graphitizationyamorphization by nitrogen ion implantation andyor high temperature annealing, formation of built-in conductive metal layers. A perceptible improvement of surface electrical conductivity and reduction of field electron emission thresholds down to 10 Vymm was observed. For explanation of the results it was enough to take into account only the geometric field enhancement on pyramids apexes. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Field electron emission; Diamond film; Patterning; Diamond doping; Ion implantation
1. Introduction Diamond sharp microstructures such as pyramids, tips, etc., are of interest for field electron emission applications due to unique diamond properties and additional geometric field enhancement w1–6x. On the other hand, there are a lot of reports describing excellent field emission properties of flat diamond structures (films) without any special relief formation w7–11x. In this case the low-field emission properties are often explained using negative electron affinity (NEA) w12x. The question still being asked is whether NEA plays an important role in the diamond field emission, or could it be explained using geometric field enhancement only? Here we report on engineering of two-dimensional (2D) arrays of diamond pyramids for improved field electron emission using a molding technique. Since high quality diamond is an insulator, several ways have been * Corresponding author. Tel.: q7-951328229; fax: q7-951357672. E-mail address: [email protected] (A.V. Karabutov).
considered to provide an electrical conductivity of diamond in order to supply electrons for the field electron emission: diamond doping, partial graphitizationyamorphization, or formation of built-in conductive layers. 2. Experimental The molding technique is based on diamond growth on a patterned substrate, the replica of those patterns being formed on the nucleation side of the diamond film. Typically the main steps of free-standing diamond film fabrication process include: (i) patterning of Si substrates by dry or wet etching; (ii) diamond growth; and (iii) substrate removal (see, for example w3,13x). Inverted pyramids in (100) Si substrates were prepared using a standard photolithography patterning with SiO2 mask followed by anisotropic etching in 3% KOH alkali. Diamond films were produced in a microwave plasma CVD reactor ASTeX PDS19 (2.45 GHz, 5 kW) using CH4 yH2 reaction mixtures w14x, and hot-filament CVD in CH4 yH2 gas mixtures w15x. Prior to deposition the
0925-9635/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 5 0 1 - 5
A.V. Karabutov et al. / Diamond and Related Materials 10 (2001) 2178–2183
substrates were seeded with 5-nm diamond particles in ultrasonic bath with diamond suspension in alcohol. This step greatly enhancing the nucleation density is important for precise replication of the pyramids by growing diamond. The diamond film was deposited under the following conditions: microwave power 5 kW, 3% CH4 concentration in H2, pressure 90 torr, substrate temperature 7208C. Then the Si substrate was chemically etched in HF–HNO3 acid to leave a free-standing film of approximately 100-mm thickness. Thus, 2D pyramid arrays with base length of one pyramid of 3 and 9 mm and period of 4.5 and 12 mm, respectively, have been formed. The pyramids have low aspect ratio (height-tobase length) hyas0.7. MPECVD diamond pyramids were insulating and showed a relatively poor field electron emission. Since the virgin pyramid surface was etched in acid (during the Si substrate removal) it can be hydrogen-terminated and show some traces of NEA and surface conductivity. Nevertheless, we did not find signs of noticeable surface conductivity for the virgin pyramid surface. Four ways to improve the electrical conductivity have been applied: 1. Annealing in high vacuum at temperatures of 400– 15508C. 2. Nitrogen ion bombardment at different doses. 3. A thin Ti layer buried into diamond at submicron depth. 4. Boron doped thin film on insulating diamond (bilayered pyramids). The bombardment with Nq ions was performed at an energy of 15.5 keV and two different doses (1=1018 and 2=1018 cmy2) to form doped andyor modified amorphous conductive surface layer. The estimated thickness of this modified layer is approximately 20 nm according to calculation of Nq ion projection range. In addition the ion sputtering at these conditions could be expected to sharpen the pyramid apexes w2,4,16x resulting in further field enhancement. The samples were then annealed in steps at temperatures between 400 and 15508C in vacuum (10y5 torr) in a graphite wall furnace for 1 h at each temperature step. We observed a darkening of the originally translucent sample after annealing at 1450 and 15508C, while at lower temperatures (F12008C) this was not noticed. Another attempt to improve surface electroconductivity of the diamond pyramids was to create a buried thin metal (Ti) layer at submicron depth, as shown in Fig. 1a. At first, ultrathin (200 nm) diamond film was grown on Si template. Then a submicron Ti layer was deposited using a sputtering technique. Finally, a thick diamond film was grown to provide the mechanical strength. The electroconductivity of the surface layer in this case increased by many orders of magnitude compared to ‘normal’ diamond pyramids.
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Fig. 1. Preparation processes of diamond pyramids with buried Ti layer (a) and with top B-doped layer (b).
Boron doped diamond pyramids (3-mm base length) were produced as shown schematically in Fig. 1b. First, the B-doped 1.5-mm-thick diamond film was grown by hot filament CVD in methane–acetone–hydrogen–trimethylborate gas mixtures at Ts8208C w15x, and then it was covered by a thick undoped MPECVD diamond. Boron concentration in the top layer of the pyramids was approximately 1020 cmy3. Fig. 2 shows arrays of as-grown undoped pyramids of 9-mm base length, and B-doped pyramids of 3-mm base length. The pyramids have sharp apexes with curvature radius less than 50 nm as estimated from SEM images taken at higher magnification. Due to high nucleation density of diamond during the initial growth stage the tip surface consists of fine-grained material that is favorable for enhanced field emission w17x. For the structural characterization MicroRaman spectra were obtained with an ISAyJobin Yvon instrument (S3000 model) using the 514.5-nm line of Arq ion laser with the beam spot of 2 mm. The spectra were recorded on a CCD detector with 1.8 cmy1 spectral resolutions. Field electron emission characteristics of the 2D pyramid arrays were measured in a high vacuum of 10y7 torr using a local probe technique (for details see w10,11x). A tungsten probe with the tip radius of 20 mm was positioned 20–100 mm above the pyramids apexes, so that at least 10 pyramids could contribute to the field emission current. The electrical contact to the sample was made at three areas: one on the back (growth side) of the free-standing film, and two on the emitting surface at a distance of approximately 1 mm from the probe position. High d.c. voltage up to 8 kV was cyclically applied between the probe and the sample, and the emission from local area at currents of 10 pA to 0.1 mA was recorded to obtain I–V plots. The measurements were performed at least at four probe positions over the sample surface and the results were averaged. Another
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Fig. 3. I–V plots for 9-mm diamond pyramids before and after vacuum annealing at 600 and 15508C taken in contact mode for measurements of electrical conductivity.
Fig. 2. SEM images of 9-mm diamond pyramids annealed at 15508C (a), and 3-mm pyramids with top B-doped layer (b).
a limiting influence of the electrical resistance of the samples: the voltage drop on the sample during the emission measurements could be more than 1 kV. After the vacuum annealing at 6008C the electrical conductivity of the sample increased by three orders of magnitude (Fig. 3), being accompanied by the decrease of the field emission threshold down to 10 Vymm (Fig. 4). However, the annealing to temperatures higher than 6008C does not change the emission threshold in spite of further rising of the conductivity. Apparently, a value of the surface conductivity achieved by annealing at Ts6008C is quite enough to realize the effective field
probe with bigger flat apex of 0.5 mm2 was also used for a comparison of the emission results. For electrical resistance evaluation of the pyramids the local probe was placed in a contact on the pyramid surface, and the I–V plots were recorded. Another electrical contact to the film to measure the conductivity (‘basis’ contact) was applied at the same sample side (surface) using two gold probes. This contact was also used to supply electrons for the emission because a ‘cross-film’ (from the back side to the pyramid side) conductivity for all the samples studied was very poor and did not allow measuring the field emission. This conductivity study showed almost the same plots at different surface sites, so it can be definitely associated with the resistivity of the pyramids to the field emission current. 3. Results and discussion The electron emission of as-grown diamond pyramids is relatively poor with a threshold field of approximately 30 Vymm. Analysis of these emission I–V plots shows
Fig. 4. Dependence of the emission threshold field on the annealing temperature for 9-mm as-grown and ion bombarded (with dose of 2=1018 cmy2) diamond pyramids. Emission plot for the film annealed at 14508C is shown on insert.
A.V. Karabutov et al. / Diamond and Related Materials 10 (2001) 2178–2183
Fig. 5. Raman spectra of 9-mm diamond pyramids before (lower spectrum) and after annealing at 15508C (upper spectrum). For upper spectrum a device aperture was increased up to 10 cmy1 for more efficient signal collection.
emission from the pyramids due to geometric field enhancement. Very similar behavior was observed for nitrogen ionimplanted pyramids (Fig. 4). Before annealing the ion bombardment enhanced the emission and the conductivity (more for higher ion dose), and the threshold field decreased to 20 Vymm. Further emission improvement was achieved by the annealing, and again, a saturation is observed at T)6008C. This improvement can be ascribed to some degree of graphitization of fine grained diamond (or amorphous carbon layer) at elevated temperatures accompanied with the increase in its electrical conductivity as was also observed by other authors w18x. MicroRaman spectra taken from the as-grown pyramid surface showed a narrow diamond peak at 1332 cmy1 (FWHMs3.2 cmy1) without any trace of amorphous carbon or graphitic inclusions (Fig. 5). The intensity of the diamond peak for non-annealed ionimplanted pyramids was 10 times weaker than for asgrown one. It is probably due to formation of absorbing disordered layer on the pyramid surface, being eyevisible as some darkening, but this layer didn’t exhibit any new features in Raman spectra probably because of its very small thickness. After the annealing at 1200 and 14508C the intensities of the Raman diamond peak for implanted and non-implanted samples became practically of the same (low) intensity, and both samples became gray in color, however, no sign of amorphous carbon or graphitic inclusions appeared in the spectra. Finally, after annealing at 15508C a weak nanocrystalline graphite signal (D- and G-broad bands) was recorded on pyramids for both samples (stronger for implanted
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sample), as shown in Fig. 5. The diamond peak was broadened, strongly reducing in intensity, while no changes in Raman diamond peak at the coarse-grained growth side were found. Since there are no traces of graphite on the growth side of the diamond film, it can be concluded that the graphitization occurs preferably on the fine-grained ‘substrate’ side (pyramid surface). It was reported w19x that the hydrogen evolution in polycrystalline CVD diamond at T)13008C leads to formation of a graphite-like phase at the grain boundaries. The increase of the pyramid surface conductivity with B-doping or incorporation of Ti sublayer reduces the emission onset field to 18 Vymm and 10 Vymm, respectively (Fig. 6). It seems that B-doped pyramids emit at higher fields compared to 9-mm pyramids due to their smaller height (decreased geometric field enhancement). Thus, using different types of pyramid surface structure modification we didn’t see a marked difference for these samples in optimized emission characteristics. On the other hand, we found a marked difference in the surface electric conductivity for the samples produced with similar fine-grained surface structure, but with different accompaniments such as doping, annealing and buried Ti sublayer. The similarity of the emission results for the samples with different surface structure led us to suppose that the emission is determined in this case mainly by geometric field enhancement (accompanied with almost the same value of the work function of approx. 5 eV), if the surface electrical conductivity is high enough to supply electrons. To calculate an effective value of the field enhancement factor for the 9-mm pyramids the emission meas-
Fig. 6. Field electron emission plots for as-grown 9-mm undoped, 3mm boron-doped, and 9-mm with buried Ti-sublayer diamond pyramids (field increasing and decreasing plots).
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Fig. 7. Field electron emission per single 9-mm diamond pyramid annealed at 14508C calculated from I–V plots measured with 20-mm microprobe (solid symbols) and 0.5-mm2 flat probe (open symbols). Fowler–Nordheim plot is shown on insert.
urements were made using a tungsten probe with flat apex of 0.5 mm2 area. This probe can collect electrons from approximately 3500 pyramids. There is good agreement in emission plots recalculated per single pyramid, as measured using arrays of 10 or 3500 pyramids (see steep branches of the plots in Fig. 7). The difference in ‘saturation’ branches in Fig. 7 originates from electroconductivity limitations: they demonstrate ohmic I–V behavior being redrawn in linear scale (Fig. 4, insert), resistivity being higher for higher total current through the surface. Note, that the resistance calculated from emission ‘saturation’ branch is approximately 1.6 GV per pyramid for the sample annealed at 14508C. The emission curve is well fitted by a Fowler– Nordheim plot over four orders of magnitude (Fig. 7, insert), and the field enhancement factor calculated assuming the work function value of 5 eV is ms650. For dense arrays of conductive pyramids the field enhancement factor cannot be calculated simply by dividing the pyramid height by tip radius, that is m/h yr (see, for example w20x). A more realistic calculation of m gives a value of tip radius of approximately 4 nm
for the 9-mm pyramids. Unfortunately, the SEM apparatus used was not able to resolve such small features. However, the presence of nanometric protrusions (comparable with size of seeded diamond nanoparticles) on the pyramid apex is not excluded w4,5,21x. Another possibility to have such sharp emitters is the presence of narrow conducting channels in the insulating diamond matrix on the pyramid top (for example, grain boundaries in the fine-grained diamond). Since we used 4–5nm diamond powders for seeding of the growth side of the diamond pyramids, a typical size of surface nanoprotrusions (or grain boundaries) due to this fact is in good agreement with that calculated from the emission plots the value of 4 nm. Note also, that the local current density flowing through such small protrusions in our emission experiments can be estimated to be more than 106 Aycm2! This very high current density needs an effective heat sink such as diamond. The emission with average current per pyramid of 1 mA was quite stable for at least 24 h without any current degradation (Fig. 8). Furthermore, a value of intersection of the Fowler– Nordheim plot in Fig. 7 allows us to evaluate an effective area of the emitting surface for 3500 pyramid arrays as 25 000 nm2. Compared with 4 nm average apex radius, a percentage of mostly effective emitting pyramids can be evaluated as 15–25%. This means that the density of emitting sites (‘brightest’ pyramids) is of 105 cmy2. The pyramids designed can be used for effective field electron emitters, as well as for fabrication of conductive tips for microsensors and cantilevers for scanning probe microscopy w22x. 4. Conclusion 2D arrays of diamond pyramids of 3 and 9 mm in size were prepared using a molding technique. Four ways were examined to provide an electrical conductivity of the pyramids in order to improve the field electron emission: boron doping, partial graphitizationyamorphization by nitrogen ion implantation andyor high temperature annealing, and formation of a built-in conductive metal (Ti) layer. All the methods applied allow achieving effective emission at threshold fields of as low as 7 Vymm due to increasing surface electrical conductivity. The emission is ascribed to geometric field enhancement only, and the emitter radius of the pyramid apex of approximately 4 nm with the density of emission sites as of 105 cmy2 can be evaluated. Acknowledgements
Fig. 8. Long-term stability of the field emission current at high current density for the 9-mm diamond pyramids annealed at 15508C as measured with 20-mm microprobe.
The authors wish to thank S. Voronina for assistance in diamond deposition, S. Lavrischev for SEM images, V. Frolov for help in Si substrate patterning, and P. Belobrov for stimulating discussions.
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References w1x K. Okano, K. Hoshina, M. Ida, S. Koizumi, T. Inuzuka, Appl. Phys. Lett. 64 (1994) 2742. w2x T. Asano, Y. Oobuchi, S. Katsumata, J. Vac. Sci. Technol. B 13 (1995) 431. w3x W.P. Kang, J.L. Davidson, M. Howell et al., J. Vac. Sci. Technol B 14 (1996) 2068. w4x C. Nutzenadel, O.M. Kuttel, O. Groning, L. Schlapbach, Appl. Phys. Lett. 68 (1996) 2662. w5x E.I. Givargizov, V.V. Zhirnov, A.N. Stepanova, P.S. Plekhanov, R.I. Kozlov, Appl. Surf. Sci. 94y95 (1996) 117. w6x V.V. Zhirnov, G.J. Wojak, W.B. Choi, J.J. Cuomo, J.J. Hren, J. Vac. Sci. Technol. B 15 (1997) 1733. w7x C. Wang, A. Garcia, D.C. Ingram, M. Lake, M.E. Kordesch, Electron. Lett. 27 (1991) 1459. w8x M.W. Geis, N.N. Efremov, J.D. Woodhouse, M.D. McAleese, M. Marchywka, D.G. Socker, J.F. Hochedez, IEEE Electron Device Lett. 12 (1991) 456. w9x M.W. Geis, J.C. Twichell, T.M. Lyszczarz, J. Vac. Sci. Technol. B 14 (1996) 2060. w10x A.V. Karabutov, V.I. Konov, S.M. Pimenov et al., J. Phys. IV (1996) C5–113. w11x A.V. Karabutov, V.I. Konov, S.M. Pimenov et al., J. Wide Bandgap Mater. 7 (1) (1999) 68.
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w12x P.K. Baumann, R.J. Nemanich, Diamond Relat. Mater. 7 (1998) 612. w13x V. Ralchenko, L. Schirone, G. Sotgiu et al., Proc. of the 6th Int. Symp. on Diamond Materials, The Electrochemical Soc., Pennington, USA, Vol. 99–32 (2000) 72. w14x V.G. Ralchenko, A.A. Smolin, V.I. Konov et al., Diamond Relat. Mater. 6 (1997) 417. w15x A.Ya. Sakharova, Yu.V. Pleskov, F. Di Quatro, S. Piazza, C. Sunseri, I.G. Teremetskaya, V.P. Varnin, J. Electrochem. Soc. 142 (1995) 2704. w16x A.N. Stepanova, V.V. Zhirnov, L.V. Bormatova, E.I. Givargizov, E.S. Mashkova, V.A. Molchanov, J. Phys. IV (1996) C5–103. w17x W. Zhu, G.P. Kochanski, S. Jin, L. Seibles, J. Appl. Phys. 78 (1995) 2707. w18x C. Lui, D. Shu, T.M. Kuzay, L. Wen, C.A. Melendres, J. Vac. Sci. Technol. A 15 (1997) 1200. w19x A. Khomich, V. Ralchenko, L. Nistor, I. Vlasov, R. Khmelnitskii, Phys. Stat. Solidi (a) 181 (2000) 37. w20x I.N. Slivkov, Electroisolation and Vacuum Discharge, Energoatomizdat, Moscow, 1972, p. 19 (in Russian). w21x W.P. Kang, J.L. Davidson, A. Wisitsora-at, D.V. Kerns, S.E. Kerns, Paper presented at the 13th International vacuum microelectronics conference, 14–17 August 2000, Guangzhou, China, Abstract book, pp. 92–95. w22x Ph. Niedermann, W. Hanni, N. Blanc, R. Christoph, J. Burger, J. Vac. Sci. Technol. A 14 (1996) 1233.
Surface engineering of diamond tips for improved field electron emission A.V. Karabutova,*, V.G. Ralchenkoa, I.I. Vlasova, R.A. Khmelnitskyb, M.A. Negodaevb, V.P. Varninc, I.G. Teremetskayac a
General Physics Institute, Vavilova str. 38, Moscow 117942, Russia P.N. Lebedev Physical Institute, Leninsky pr. 53, Moscow 117942, Russia c Institute of Physical Chemistry, Leninsky pr. 31, Moscow 117915, Russia
b
Received 4 September 2000; accepted 7 June 2001
Abstract Results are reported on the study of surface engineering of diamond microtips for improved field electron emission. Twodimensional (2D) arrays of diamond pyramids were prepared using a molding technique. As-grown diamond pyramids of 3 and 9 mm in size with sharp apexes were insulating and showed a relatively poor field electron emission. Several ways are examined to provide an electrical conductivity of diamond in order to supply electrons for the emission: diamond boron doping, partial graphitizationyamorphization by nitrogen ion implantation andyor high temperature annealing, formation of built-in conductive metal layers. A perceptible improvement of surface electrical conductivity and reduction of field electron emission thresholds down to 10 Vymm was observed. For explanation of the results it was enough to take into account only the geometric field enhancement on pyramids apexes. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Field electron emission; Diamond film; Patterning; Diamond doping; Ion implantation
1. Introduction Diamond sharp microstructures such as pyramids, tips, etc., are of interest for field electron emission applications due to unique diamond properties and additional geometric field enhancement w1–6x. On the other hand, there are a lot of reports describing excellent field emission properties of flat diamond structures (films) without any special relief formation w7–11x. In this case the low-field emission properties are often explained using negative electron affinity (NEA) w12x. The question still being asked is whether NEA plays an important role in the diamond field emission, or could it be explained using geometric field enhancement only? Here we report on engineering of two-dimensional (2D) arrays of diamond pyramids for improved field electron emission using a molding technique. Since high quality diamond is an insulator, several ways have been * Corresponding author. Tel.: q7-951328229; fax: q7-951357672. E-mail address: [email protected] (A.V. Karabutov).
considered to provide an electrical conductivity of diamond in order to supply electrons for the field electron emission: diamond doping, partial graphitizationyamorphization, or formation of built-in conductive layers. 2. Experimental The molding technique is based on diamond growth on a patterned substrate, the replica of those patterns being formed on the nucleation side of the diamond film. Typically the main steps of free-standing diamond film fabrication process include: (i) patterning of Si substrates by dry or wet etching; (ii) diamond growth; and (iii) substrate removal (see, for example w3,13x). Inverted pyramids in (100) Si substrates were prepared using a standard photolithography patterning with SiO2 mask followed by anisotropic etching in 3% KOH alkali. Diamond films were produced in a microwave plasma CVD reactor ASTeX PDS19 (2.45 GHz, 5 kW) using CH4 yH2 reaction mixtures w14x, and hot-filament CVD in CH4 yH2 gas mixtures w15x. Prior to deposition the
0925-9635/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 5 0 1 - 5
A.V. Karabutov et al. / Diamond and Related Materials 10 (2001) 2178–2183
substrates were seeded with 5-nm diamond particles in ultrasonic bath with diamond suspension in alcohol. This step greatly enhancing the nucleation density is important for precise replication of the pyramids by growing diamond. The diamond film was deposited under the following conditions: microwave power 5 kW, 3% CH4 concentration in H2, pressure 90 torr, substrate temperature 7208C. Then the Si substrate was chemically etched in HF–HNO3 acid to leave a free-standing film of approximately 100-mm thickness. Thus, 2D pyramid arrays with base length of one pyramid of 3 and 9 mm and period of 4.5 and 12 mm, respectively, have been formed. The pyramids have low aspect ratio (height-tobase length) hyas0.7. MPECVD diamond pyramids were insulating and showed a relatively poor field electron emission. Since the virgin pyramid surface was etched in acid (during the Si substrate removal) it can be hydrogen-terminated and show some traces of NEA and surface conductivity. Nevertheless, we did not find signs of noticeable surface conductivity for the virgin pyramid surface. Four ways to improve the electrical conductivity have been applied: 1. Annealing in high vacuum at temperatures of 400– 15508C. 2. Nitrogen ion bombardment at different doses. 3. A thin Ti layer buried into diamond at submicron depth. 4. Boron doped thin film on insulating diamond (bilayered pyramids). The bombardment with Nq ions was performed at an energy of 15.5 keV and two different doses (1=1018 and 2=1018 cmy2) to form doped andyor modified amorphous conductive surface layer. The estimated thickness of this modified layer is approximately 20 nm according to calculation of Nq ion projection range. In addition the ion sputtering at these conditions could be expected to sharpen the pyramid apexes w2,4,16x resulting in further field enhancement. The samples were then annealed in steps at temperatures between 400 and 15508C in vacuum (10y5 torr) in a graphite wall furnace for 1 h at each temperature step. We observed a darkening of the originally translucent sample after annealing at 1450 and 15508C, while at lower temperatures (F12008C) this was not noticed. Another attempt to improve surface electroconductivity of the diamond pyramids was to create a buried thin metal (Ti) layer at submicron depth, as shown in Fig. 1a. At first, ultrathin (200 nm) diamond film was grown on Si template. Then a submicron Ti layer was deposited using a sputtering technique. Finally, a thick diamond film was grown to provide the mechanical strength. The electroconductivity of the surface layer in this case increased by many orders of magnitude compared to ‘normal’ diamond pyramids.
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Fig. 1. Preparation processes of diamond pyramids with buried Ti layer (a) and with top B-doped layer (b).
Boron doped diamond pyramids (3-mm base length) were produced as shown schematically in Fig. 1b. First, the B-doped 1.5-mm-thick diamond film was grown by hot filament CVD in methane–acetone–hydrogen–trimethylborate gas mixtures at Ts8208C w15x, and then it was covered by a thick undoped MPECVD diamond. Boron concentration in the top layer of the pyramids was approximately 1020 cmy3. Fig. 2 shows arrays of as-grown undoped pyramids of 9-mm base length, and B-doped pyramids of 3-mm base length. The pyramids have sharp apexes with curvature radius less than 50 nm as estimated from SEM images taken at higher magnification. Due to high nucleation density of diamond during the initial growth stage the tip surface consists of fine-grained material that is favorable for enhanced field emission w17x. For the structural characterization MicroRaman spectra were obtained with an ISAyJobin Yvon instrument (S3000 model) using the 514.5-nm line of Arq ion laser with the beam spot of 2 mm. The spectra were recorded on a CCD detector with 1.8 cmy1 spectral resolutions. Field electron emission characteristics of the 2D pyramid arrays were measured in a high vacuum of 10y7 torr using a local probe technique (for details see w10,11x). A tungsten probe with the tip radius of 20 mm was positioned 20–100 mm above the pyramids apexes, so that at least 10 pyramids could contribute to the field emission current. The electrical contact to the sample was made at three areas: one on the back (growth side) of the free-standing film, and two on the emitting surface at a distance of approximately 1 mm from the probe position. High d.c. voltage up to 8 kV was cyclically applied between the probe and the sample, and the emission from local area at currents of 10 pA to 0.1 mA was recorded to obtain I–V plots. The measurements were performed at least at four probe positions over the sample surface and the results were averaged. Another
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Fig. 3. I–V plots for 9-mm diamond pyramids before and after vacuum annealing at 600 and 15508C taken in contact mode for measurements of electrical conductivity.
Fig. 2. SEM images of 9-mm diamond pyramids annealed at 15508C (a), and 3-mm pyramids with top B-doped layer (b).
a limiting influence of the electrical resistance of the samples: the voltage drop on the sample during the emission measurements could be more than 1 kV. After the vacuum annealing at 6008C the electrical conductivity of the sample increased by three orders of magnitude (Fig. 3), being accompanied by the decrease of the field emission threshold down to 10 Vymm (Fig. 4). However, the annealing to temperatures higher than 6008C does not change the emission threshold in spite of further rising of the conductivity. Apparently, a value of the surface conductivity achieved by annealing at Ts6008C is quite enough to realize the effective field
probe with bigger flat apex of 0.5 mm2 was also used for a comparison of the emission results. For electrical resistance evaluation of the pyramids the local probe was placed in a contact on the pyramid surface, and the I–V plots were recorded. Another electrical contact to the film to measure the conductivity (‘basis’ contact) was applied at the same sample side (surface) using two gold probes. This contact was also used to supply electrons for the emission because a ‘cross-film’ (from the back side to the pyramid side) conductivity for all the samples studied was very poor and did not allow measuring the field emission. This conductivity study showed almost the same plots at different surface sites, so it can be definitely associated with the resistivity of the pyramids to the field emission current. 3. Results and discussion The electron emission of as-grown diamond pyramids is relatively poor with a threshold field of approximately 30 Vymm. Analysis of these emission I–V plots shows
Fig. 4. Dependence of the emission threshold field on the annealing temperature for 9-mm as-grown and ion bombarded (with dose of 2=1018 cmy2) diamond pyramids. Emission plot for the film annealed at 14508C is shown on insert.
A.V. Karabutov et al. / Diamond and Related Materials 10 (2001) 2178–2183
Fig. 5. Raman spectra of 9-mm diamond pyramids before (lower spectrum) and after annealing at 15508C (upper spectrum). For upper spectrum a device aperture was increased up to 10 cmy1 for more efficient signal collection.
emission from the pyramids due to geometric field enhancement. Very similar behavior was observed for nitrogen ionimplanted pyramids (Fig. 4). Before annealing the ion bombardment enhanced the emission and the conductivity (more for higher ion dose), and the threshold field decreased to 20 Vymm. Further emission improvement was achieved by the annealing, and again, a saturation is observed at T)6008C. This improvement can be ascribed to some degree of graphitization of fine grained diamond (or amorphous carbon layer) at elevated temperatures accompanied with the increase in its electrical conductivity as was also observed by other authors w18x. MicroRaman spectra taken from the as-grown pyramid surface showed a narrow diamond peak at 1332 cmy1 (FWHMs3.2 cmy1) without any trace of amorphous carbon or graphitic inclusions (Fig. 5). The intensity of the diamond peak for non-annealed ionimplanted pyramids was 10 times weaker than for asgrown one. It is probably due to formation of absorbing disordered layer on the pyramid surface, being eyevisible as some darkening, but this layer didn’t exhibit any new features in Raman spectra probably because of its very small thickness. After the annealing at 1200 and 14508C the intensities of the Raman diamond peak for implanted and non-implanted samples became practically of the same (low) intensity, and both samples became gray in color, however, no sign of amorphous carbon or graphitic inclusions appeared in the spectra. Finally, after annealing at 15508C a weak nanocrystalline graphite signal (D- and G-broad bands) was recorded on pyramids for both samples (stronger for implanted
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sample), as shown in Fig. 5. The diamond peak was broadened, strongly reducing in intensity, while no changes in Raman diamond peak at the coarse-grained growth side were found. Since there are no traces of graphite on the growth side of the diamond film, it can be concluded that the graphitization occurs preferably on the fine-grained ‘substrate’ side (pyramid surface). It was reported w19x that the hydrogen evolution in polycrystalline CVD diamond at T)13008C leads to formation of a graphite-like phase at the grain boundaries. The increase of the pyramid surface conductivity with B-doping or incorporation of Ti sublayer reduces the emission onset field to 18 Vymm and 10 Vymm, respectively (Fig. 6). It seems that B-doped pyramids emit at higher fields compared to 9-mm pyramids due to their smaller height (decreased geometric field enhancement). Thus, using different types of pyramid surface structure modification we didn’t see a marked difference for these samples in optimized emission characteristics. On the other hand, we found a marked difference in the surface electric conductivity for the samples produced with similar fine-grained surface structure, but with different accompaniments such as doping, annealing and buried Ti sublayer. The similarity of the emission results for the samples with different surface structure led us to suppose that the emission is determined in this case mainly by geometric field enhancement (accompanied with almost the same value of the work function of approx. 5 eV), if the surface electrical conductivity is high enough to supply electrons. To calculate an effective value of the field enhancement factor for the 9-mm pyramids the emission meas-
Fig. 6. Field electron emission plots for as-grown 9-mm undoped, 3mm boron-doped, and 9-mm with buried Ti-sublayer diamond pyramids (field increasing and decreasing plots).
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Fig. 7. Field electron emission per single 9-mm diamond pyramid annealed at 14508C calculated from I–V plots measured with 20-mm microprobe (solid symbols) and 0.5-mm2 flat probe (open symbols). Fowler–Nordheim plot is shown on insert.
urements were made using a tungsten probe with flat apex of 0.5 mm2 area. This probe can collect electrons from approximately 3500 pyramids. There is good agreement in emission plots recalculated per single pyramid, as measured using arrays of 10 or 3500 pyramids (see steep branches of the plots in Fig. 7). The difference in ‘saturation’ branches in Fig. 7 originates from electroconductivity limitations: they demonstrate ohmic I–V behavior being redrawn in linear scale (Fig. 4, insert), resistivity being higher for higher total current through the surface. Note, that the resistance calculated from emission ‘saturation’ branch is approximately 1.6 GV per pyramid for the sample annealed at 14508C. The emission curve is well fitted by a Fowler– Nordheim plot over four orders of magnitude (Fig. 7, insert), and the field enhancement factor calculated assuming the work function value of 5 eV is ms650. For dense arrays of conductive pyramids the field enhancement factor cannot be calculated simply by dividing the pyramid height by tip radius, that is m/h yr (see, for example w20x). A more realistic calculation of m gives a value of tip radius of approximately 4 nm
for the 9-mm pyramids. Unfortunately, the SEM apparatus used was not able to resolve such small features. However, the presence of nanometric protrusions (comparable with size of seeded diamond nanoparticles) on the pyramid apex is not excluded w4,5,21x. Another possibility to have such sharp emitters is the presence of narrow conducting channels in the insulating diamond matrix on the pyramid top (for example, grain boundaries in the fine-grained diamond). Since we used 4–5nm diamond powders for seeding of the growth side of the diamond pyramids, a typical size of surface nanoprotrusions (or grain boundaries) due to this fact is in good agreement with that calculated from the emission plots the value of 4 nm. Note also, that the local current density flowing through such small protrusions in our emission experiments can be estimated to be more than 106 Aycm2! This very high current density needs an effective heat sink such as diamond. The emission with average current per pyramid of 1 mA was quite stable for at least 24 h without any current degradation (Fig. 8). Furthermore, a value of intersection of the Fowler– Nordheim plot in Fig. 7 allows us to evaluate an effective area of the emitting surface for 3500 pyramid arrays as 25 000 nm2. Compared with 4 nm average apex radius, a percentage of mostly effective emitting pyramids can be evaluated as 15–25%. This means that the density of emitting sites (‘brightest’ pyramids) is of 105 cmy2. The pyramids designed can be used for effective field electron emitters, as well as for fabrication of conductive tips for microsensors and cantilevers for scanning probe microscopy w22x. 4. Conclusion 2D arrays of diamond pyramids of 3 and 9 mm in size were prepared using a molding technique. Four ways were examined to provide an electrical conductivity of the pyramids in order to improve the field electron emission: boron doping, partial graphitizationyamorphization by nitrogen ion implantation andyor high temperature annealing, and formation of a built-in conductive metal (Ti) layer. All the methods applied allow achieving effective emission at threshold fields of as low as 7 Vymm due to increasing surface electrical conductivity. The emission is ascribed to geometric field enhancement only, and the emitter radius of the pyramid apex of approximately 4 nm with the density of emission sites as of 105 cmy2 can be evaluated. Acknowledgements
Fig. 8. Long-term stability of the field emission current at high current density for the 9-mm diamond pyramids annealed at 15508C as measured with 20-mm microprobe.
The authors wish to thank S. Voronina for assistance in diamond deposition, S. Lavrischev for SEM images, V. Frolov for help in Si substrate patterning, and P. Belobrov for stimulating discussions.
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