Formation of carbon cluster on the surface of diamond films for improving electron field emission properties

Physica B 323 (2002) 161–164

Formation of carbon cluster on the surface of diamond films

for improving electron field emission properties

Yu-Chen Chang

, Jeroge C. Tu

, Cheng-Tzu Kuo

, Chien-Yi Wang

, I-Nan Lin

*

a

Materials Science Center, Department of Materials Science and Engineering, National Chiao-Tung University, 300 Hsin-Chu, Taiwan, ROC

b

Institute of Materials Research and Development, Chun-Shan Institute of Science and Technology, 325, Taiwan, ROC

c

Materials Science Center, Department of Materials Science and Engineering, National Tsing-Hua University, 300 Hsin-Chu, Taiwan, ROC

Abstract

The electron field emission properties of the diamond films were significantly improved via the formation of nano-sized carbon clusters on their surface. The microstructures and Raman spectroscopy are markedly altered when the diamond films were coated with a thin layer of Fe/Co films (o10 nm) and then post-annealed at high enough temperature (>8001C). The turn-on field was decreased from 14.8 to 7.6 V/mm and the electron field emission current density was increased more than 2 order of magnitude, from 20 to 2400 mA/cm2, due to post-annealing process. r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Carbon nanotubes; Diamond films; Electron field emission

1. Introduction

Diamond films possess negative electron affinity (NEA) characteristics [1] and are considered to be highly promising for applications in electron field emission devices, such that the related emission properties have been widely investigated [1–3]. However, the electron field emission properties of boron- and nitrogen-doped diamond films [4–9] are still much inferior to that of carbon nano-tubes. In this paper, a novel process was adopted to form nano-tube like carbon clusters on the surface of diamond films, so as to improve their electron field emission properties.

2. Experimental

Diamond films (B1 mm) were grown by a microwave plasma enhanced chemical vapor de-position process. The CH4/H2gases with flow rate

of 18 sccm/300 sccm (B70 Torr) were excited by 2500 W microwave power. In addition, 1 sccm B(OCH3)3 and 3 sccm (NH3)2CO were

incorpo-rated to grow boron/nitrogen co-doped diamond films on silicon substrates. To modify the surface characteristics of the diamond films, a thin layer of Fe0.8Co0.2 alloy (o10 nm) was deposited on

diamond films using a DC sputtering process, followed by post-annealing in a reducing atmo-sphere (N2/3% H2) at 500–9501C for 1 h.

The morphology and structure of the diamond films were examined using SEM and Raman spectroscopy (Renishaw), respectively.

*Corresponding author. Tel.: 5742574; fax: +886-3-571-6977.

E-mail address:[email protected] (I.-N. Lin).

0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 8 8 8 - 8

The electron field emission properties of these diamond films were measured using a parallel setup, using 100 mm glass beads as spacer. The current–voltage (I–V) characteristics of the dia-mond films were measured by Keithley 237 electrometers. The current–voltage (I–V) proper-ties of the diamond films were analyzed using Fowler–Nordheim model [10].

3. Results and discussion

The as-deposited diamond films contain faceted diamond grains about 0.5–0.8 mm in size (Fig. 1a), possessing unsatisfactory electron field emission properties. It requires ðE0Þ ¼ 14:8 V/mm to turn on the electron field emission, achieving only ðJeÞ ¼ 20 mA/cm2 at 21.6 V/mm applied field (Fig. 2a). Raman spectrum of these films, as shown in Fig. 3a, is predominated by D-band resonance (1335 cm1) and G-band resonance (1680 cm1) peaks, the characteristics of the strained diamond grains.

The surface morphology changes markedly when the Fe/Co-coated (B10 nm) diamond films were post-annealed in reducing atmosphere at a temperature higher than 5001C. The tiny clusters (o100 nm) presence on the diamond grains for 5001C post-annealed films, indicating that interac-tion has occurred between the Fe/Co thin layer and the diamonds. Fig. 2b shows that the turn-on field has been lowered to ðE0Þ₅₀₀¼ 10:8 V/mm and the electron field emission current density has been increased to ðJeÞ500¼ 100 mA/cm

2

at 21.6 V/mm applied field. Raman spectroscopy in Fig. 3b cannot resolve the nature of the reacted layer.

The interaction between the Fe/Co layer and diamonds increases with post-annealing tempera-ture. So do the electron field emission properties of the films. Fig. 2c indicates that post-annealing at 8001C results in the most significant improve-ment on the materials’ electron field emission properties. The turn-on field is further lowered to ðE0Þ800¼ 7:6 V/mm and the electron field emission capacity has been increased more than 2 order of magnitude, i.e., ðJeÞ₈₀₀¼ 2400 mA/cm2

at 21.6 V/ mm applied field, for the 8001C post-annealed diamond films. SEM micrograph in Fig. 1b and the inset reveals the presence of nano-sized carbon clusters on these samples. Raman spectrum shown in Fig. 2c indicates clearly the presence of G0

1¼ 1586 cm

1 _{resonance peak, and the higher}

order resonance peaks at around G0₂¼ 2708 cm1, the characteristics of graphitic materials, which supports the assumption that interacted layer is nano-sized carbon clusters. Similar interaction occurs when the Fe/Co-coated diamond films were post-annealed at 9501C, but the Fe/Co-to-dia-mond interaction was much more rigorous such that the carbon soots were resulted and the electron field emission properties of the films are degraded markedly (Fig. 2d).

It should be noted that the presence of Fe/Co layer on diamond surface is of prime importance to induce the formation of nano-sized carbon clusters. The mechanism for the formation of these carbon clusters due to post-annealing process is not clear yet. The most probable process is the dissolution and re-precipitation of carbon species in diamonds into the Fe/Co layer during annealing.

Fig. 1. SEM micrograph of (a) as-deposited diamond films, and (b) Fe/Co-coated and 8001C post-annealed diamond films.

Y.-C. Chang et al. / Physica B 323 (2002) 161–164 162

4. Conclusion

Electron field emission properties of the dia-mond films were markedly improved, when they

were coated with ultra-thin Fe/Co alloy (o10 nm) and post-annealed at high enough temperature (>8001C). Such a phenomenon is ascribed to the formation of nano-sized carbon clusters on their

0 5 10 15 20 25 0 500 1000 1500 2000 2500 E0(V/µm) φe(eV) (a)RB1N3 14.8 0.164 (b)Fe+Co/RB1N3 5000_{C 10.8 0.121} (c)Fe+Co/RB1N3 8000 C 7.6 0.086 (d)Fe+Co/RB1N3 9500_{C 8 0.096} Current Density ( µ A/cm 2 ) Applied Field ( V/µm ) 0.0 0.5 1.0 1.5 2.0 2.5 -16 -14 -12 -10 -8 -6 -4 -2 0 2 ln(J/E 2) 1/E

Fig. 2. Electron field emission properties and the corresponding Fowler–Nordheim plot of (a) as-deposited diamond films, and those of Fe/Co-coated and post-annealed diamond films: the post-annealing temperatures are (b) 5001C, (c) 8001C and (d) 9501C.

1000 1500 2000 2500 3000 3500 4000 4500 13361609 1333 1609 13521586 2708 1358 1584 2712 (a) RB1N3 (b) 5000_C (c) 8000_C Intensity (A.U.) Raman shift (cm-1) (d) 9500_C

Fig. 3. Raman spectra of (a) as-deposited diamond films, and those of Fe/Co-coated and annealed diamond films: the post-annealing temperatures are (b) 5001C, (c) 8001C and (d) 9501C.

surface. The electron emission current density for the diamond films increased more than 2 order of magnitude, achieving Je¼ 2:4 mA/cm2

.

Acknowledgements

The authors gratefully acknowledge the finan-cial support of National Science Council though the project No. NSC-90-2218-E-003-002.

References

[1] C.A. Spindt, I. Brodie, L. Humphrey, E.R. Westerberg, J. Appl. Phys. 47 (1976) 5248.

[2] G.G.P. Van Gorkom, A.M.E. Hoeberechts, J. Vac. Sci. Technol. B 4 (1986) 108.

[3] F.J. Himpsel, J.A. Knapp, J.A. Van Vechten, Phys. Rev. 20 (1979) 624.

[4] W. Zhu, G.P. Kochanski, A.E. White, Appl. Phys. Lett. 68 (1995) 1157.

[5] T.K. Ku, S.H. Chen, H.C. Cheng, IEEE Elec. Device Lett. 17 (1996) 208.

[6] J.W. Glesener, A.A. Morrish, Appl. Phys. Lett. 69 (1996) 785.

[7] K. Okano, S. Koizumi, S.R.P. Silva, G. Amaratunga, Nature 381 (1996) 140.

[8] K. Okano, K.K. Gleason, Electron. Lett. 31 (1) (1995) 74.

[9] M.W. Geis, J.C. Twichell, T.M. Lyszczarz, Appl. Phys. Lett. 68 (1996) 2294.

[10] A. Vander Ziel, Solid State Physical Electronics, Prentice-Hall, Englewood Cliffs, NJ, 1968, p. 144.

Y.-C. Chang et al. / Physica B 323 (2002) 161–164 164