5182
J. Phys. Chem. B 2004, 108, 5182-5184
Electron Field Emission from Self-Organized Micro-Emitters of sp3-Bonded 5H Boron Nitride with Very High Current Density at Low Electric Field Shojiro Komatsu,*,† Akio Okudo,‡ Daisuke Kazami,‡ Dmitri Golberg,† Yubao Li,† Yusuke Moriyoshi,‡ Masaharu Shiratani,§ and Katsuyuki Okada† AdVanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan, College of Engineering, Department of Materials Science, Hosei UniVersity, 3-7-2 Kajino-machi, Koganei, Tokyo 184-8584, Japan, and Department of Electronics, Graduate School of Information Science and Electrical Engineering, Kyushu UniVersity, Hakozaki, Fukuoka 812-8581, Japan ReceiVed: February 13, 2004; In Final Form: March 22, 2004
Electron field emission with high current density ∼0.9 A/cm2 at a low electric field of 8.6 V/µm was achieved by self-organized cone-shaped emitters with dimensions on the order of ∼10 µm made of sp3-bonded 5H boron nitride, which was grown by plasma-assisted chemical vapor deposition with the assistance of 193 nm laser irradiation of the surface. The work function of this material proved to be ∼5 eV, whereas the geometrical field enhancement factor amounts to ∼106 cm-1. The known robustness of sp3-bonded BN with its excellent electron emission characteristics and the self-organization of emitter shaped structures may provide new applications for electron emitting devices.
Negative electron affinity (NEA) and geometrical enhancement factor are distinctly important to promote electron field emission (eFE), considering the possibilities to control them in the fabrication of electron emitting devices. NEA is observed in semiconductors when the vacuum level happens to lie below the conduction band minimum mainly due to the surface electronic dipole, as is known in doped diamond surfaces terminated with hydrogen.1,2 Wide band gap materials such as diamond, AlN, and BN are empirically known to show NEA.1,2 The geometrical enhancement factor β is defined as a constant characteristic to an electron field emitter in the FowlerNordheim equation.3,4 It is related to local electric field enhancement caused by protruded shapes, which is prominent in carbon nanotubes (CNT)5 and fabricated electron field emitters4 employing shapes with high aspect ratio. Here we report on self-organized electron emitters with high β, cone-shape structures and micrometer dimensions made of sp3-bonded 5H-BN.6,7 Generation of an emission density (Id) began at 90 mA/cm2 with a threshold of 6 V/µm, which is comparable to that of CNT, and the Id rapidly reached ∼0.9 A/cm2 with an applied electric field (E) of 8.6 V/µm; this value of Id was very high considering the low applied E in comparison with those of CNT. The samples were prepared by plasma-assisted chemical vapor deposition (CVD) with the assistance of 193 nm laser irradiation of the growing film surface as reported previously6 from the source gases flowing at a rate of 2.5 sccm diborane (B2H6), 10 sccm ammonia, (and 100sccm hydrogen) diluted in Ar (3SLM). A Si(100) disk of 2.5 cm in diameter and 0.5 mm thick was employed as the substrate. The crystal structure of the deposited film samples was determined to be sp3-bonded 5H-BN by X-ray diffraction.6 The eFE in a vacuum from the * To whom correspondence should be addressed. Phone: +81-298-604486. Fax: +81-298-52-7449. E-mail:
[email protected]. † National Institute for Materials Science. ‡ Hosei University. § Kyushu University.
samples as a function of E was measured by the procedure as reported previously.8 The probe electrode as an anode was a columnar type made of stainless steel with a diameter of 1.2 mm and a flat-shaped tip. The gap distance between the anode tip and the sample surface top was adjusted and measured with a micrometer, in which the zero point was determined by detecting a contact current. The macroscopic current density (Id) was estimated to be the observed current divided by the surface area of the anode tip. Work functions of the samples were measured by using a photoelectron spectrometer, the model AC-2 (RIKEN KEIKI, Japan), in which an “air-filled counter” enabled the measurements in air.9 By scanning electron microscopy (SEM), it was found that numerous cone-shaped entities, ranging from a few to 20 µm in length, had grown toward the incident laser light at a substrate temperature of 850 °C (Figure 1A,B). They proved to be made of fine crystallites of ∼10 nm from the broadness of X-ray diffraction peaks calculated by using the Scherer’s equation.6 These self-organized cone-shapes are naturally considered to play a role of electron emitters owing to the geometrical field enhancement effects in the vicinity of their tips. In Figure 1C, the electron field emissions (Id A/cm2) from the sample observed in Figure 1, parts A and B, which was deposited without hydrogen added, are plotted as a function of E. Two measurements were made for point A with gap distances between the top surface of the sample and the probe tip of 80 and 85 µm and three measurements for point B with gap distances, 80, 85, and 90 µm, respectively. The emissions are satisfactorily reproduced at both points A and B. The data A-80 µm has the emission threshold at 6 V/µm, and it reaches the Id of 0.9 A/cm2 at the E of 8.6 V/µm. The data taken at point A reaches a saturation current density at 0.9 A/cm2, which was limited by the capacity of the high-voltage power supply. The data taken at B has a larger emission threshold at 11 V/µm and a lower Id of 0.34 A/cm2, which is presumably due to the difference in the density of emitter cones between the points A and B. Since the emission curves themselves do not appear to
10.1021/jp0493475 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/06/2004
Letters
J. Phys. Chem. B, Vol. 108, No. 17, 2004 5183
Figure 1. Electron filed emission from self-organized cone-shaped micro emitters of sp3-bonded 5H-boron nitride. A. Scanning electron microscopy (SEM) image of the emitters with dimensions of ∼10 µM. B. The cone-shaped emitters are aligned toward the incident 193 nm laser light irradiated during film growth by plasma-assisted chemical vapor deposition from diborane and ammonia. C. Electron field emission from the sample as a function of the applied field. Two and three measurements for the sample points A and B were made, respectively, with different gap distances between the probe anode tip and the sample surface which served as the cathode. D. Fowler-Nordheim plots of the data corresponding to the C. The field enhancement factors β was evaluated by employing a representative work function of 5 eV in these plots.
show the tendency of saturation for the data taken at A, we may expect higher current density by using a sufficiently large power-supply. This high Id at the relatively low applied field, ∼0.9 A/cm2 at 8.6 V/µm, seems to be comparable or even superior to those known already of CNT; for instance, bundled CNT, which contains 70 volume % of single wall CNT deposited on a Si substrate by laser ablation, achieved Id of 0.5 A/cm2 at the applied E of 16 V/µm (ref 10). Well-aligned regular arrays of CNT have shown much lower Id values reaching 10 mA/cm2 (ref 11,12). It should be noted that the high current density of ∼1 A/cm2 was achieved also13 by BN coated CuLi; however, in this case, the field intensity was much higher, 150 V/µm. Fowler-Nordheim (FN) plots14 of the data corresponding to Figure 1C are shown in Figure 1D, which are fitted well by straight lines and are indicative of the FN type field emissions by tunneling. The geometrical enhancement factors β are also plotted. To calculate the β from the FN plots by using the FN equation, we employed a representative work function for our sample of 5 eV that will be discussed later. In this study, the β were ∼106 cm-1, which is comparable to the value of 7.86 × 106 cm-1 for CNT lateral field emitters using multiwalled CNTs with a length of 5 µm and 50-100 nm in diameter.4 The observed β in the samples showed a scatter ranging from 1.44 × 106 to 2.21 × 106 cm-1 depending on the gap distance and the surface point. One of the possible factors impacting β here is the field shielding effect15 due to the surface distribution of the cones, which was not uniform, although it was viewed within the coarse resolution of the probe tip of 1.2 mm in diameter over the sample surface area of 4 × 4 mm2. This indicates there is room for improvement in β and accordingly in the emission by optimizing this factor. Emission of photoelectrons as a function of the incident photon energy is plotted in Figure 2A, where the emission yield
is in the unit of (counts per second)1/3 in accordance with the customary work function plot for semiconductors.16 The work function φ, determined as the photon energy at which the photoelectron begins to emit, was estimated by a least-squares fittings using straight lines. We measured the sample for the Figure 1, where no hydrogen gas was added for the deposition, as well as another sample deposited with hydrogen gas added. These data required two straight lines to obtain satisfactory fittings encompassing both the low and high energy regions, where the gradients were obviously different. For both samples we obtained a unique set of φ1 of 4.99 eV with a low emission yield in the lower energy region and φ2 of 5.28 eV with a high emission yield in the higher energy region. The stability of the emission current at an applied voltage of 959 V, although tentatively tested for 1000 s, was shown in Figure 2B. No evidence of decreasing emission current was observed indicating high stability. This sample with excellent emission proved to be sparsely populated by ∼10 µm cones (Figure 1B). Densely populated samples as shown in Figure 2C have shown poor emission so far. This is in agreement with the case in CNT or carbon nanofibers, where densely populated ones have lower emitting characteristics than sparsely populated ones, owing to the field shielding effects.5,15 The cone-shaped emitters observed here are considered to result from the sort of photoinduced surface growth reactions that promote structures aligned in the direction of the incident light, as seen in the laser-assisted plasma CVD of boron films previously.17,18 On the other hand, it was theoretically suggested that 193 nm laser light should depassivate the (100) nitrogen surface of cBN that is passivated by hydrogen during CVD due to the strong surface N-H bonds, and that the depassivation should enhance the growth of sp3-bonded cubic BN.19 We expect a similar effect to take place here, too.
5184 J. Phys. Chem. B, Vol. 108, No. 17, 2004
Letters Known robustness of sp3-bonded BN itself, the excellent electron emission properties, and the self-organization of emitter shapes may provide new applications in vacuum microelectronic devices. Acknowledgment. We are grateful to Messrs. S. Miyata, Y. Nakajima, and D. Yamashita in Riken Keiki, Japan, for the photoelectron spectroscopy. We also thank Dr. J. Cross in Fujitsu Co., Japan, for his help in improving the English in our manuscript. References and Notes
Figure 2. Work functions, emission stability and densely packed emitters. A. Photoelectron spectrum of two samples. Closed squares represent the emission from the sample of Figure 1, which was deposited without hydrogen gas added in the chemical vapor deposition, while the open squares represent the emission from the sample deposited with hydrogen gas added. Emission yield was plotted in units of (counts per second)1/3 in accordance with the customary work function plot for semiconductors. We obtained a unique set of work functions of Φ1 ) 4.99 eV for the lower energy region and that for higher energy region of Φ2 ) 5.28 eV for both samples. B. The stability of the emission tested for 1000 s at an applied voltage of 959 V. C. An example of densely packed emitters.
NEA has been found also in BN.20,21 Since the chemisorbed hydrogen is suggested to be important for NEA also in sp3bonded BN (cBN),22 stronger bonding of the hydrogen with surface nitrogen in comparison with that of surface hydrogen in diamond19,23 is expected to possibly construct more robust electron filed emitters. Although the possibility of NEA in our case remains to be investigated, it is likely that the very stable surface dipole layer formed by the N-H bonds has produced a surface state favorable for the electron field emission here.
(1) Himpsel, F. J.; Knapp, J. A.; Van Vechten; Eastman, D. E. Phys. ReV. B 1979, 20, 624. (2) Nemanich, R. J.; Baumann, P. K.; Benjamins, M. C.; King, S. W.; van der Weide; Davis, R. F. Dia., Relat. Mater. 1996, 5, 790. (3) Bozeman, S. P.; Baumann, P. K.; Ward, B. L.; Powers, M. J.; Cuomo, J. J.; Nemanich, R. J.; Dreifus, D. L. Dia., Relat. Mater. 1996, 5, 802. (4) Lee, S.-B.; The, A. S.; Teo, K. B. K.; Chhowalla, M.; Hasko, D. G.; Milne, W. I.; Amaratunga, G. A. J.; Ahmed, H. Nonotehnology 2003, 14, 192. (5) Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; Pirio, G.; Legagneux, P.; Wyczisk, F.; Pribat, D.; Hasko, D. G. Appl. Phys. Lett. 2002, 80, 2011. (6) Komatsu, S.; Okada, K.; Shimizu, Y.; Moriyoshi, Y. J. Phys. Chem. B 1999, 103, 3289. (7) Komatsu, S.; Kurashima, K.; Kanda, H.; Okada, K.; Moriyoshi, Y.; Shimizu, Y.; Shiratani, M.; Nakano, T.; Samukawa, S. Appl. Phys. Lett. 2002, 81, 4547. (8) Li, Y. B.; Band, Y.; Golberg, D. Appl. Phys. Lett. 2003, 82, 1962. (9) Kirihara, H.; Uda, M. ReV. Sci. Instrum. 1981, 52, 68. (10) Zhu, W.; Bower, C.; Zhou, O.; Kochanski, G.; Jin, S. Appl. Phys. Lett. 1999, 75, 873. (11) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (12) Murakami, H.; Hirakawa, M.; Tanaka, C.; Yamakawa, H. Appl. Phys. Lett. 2000, 76, 1776-1778. (13) Badi, N.; Tempez, A.; Starikov, D.; Bensaoula, A.; Ageev, V. P.; Karabutov, A.; Ugarov, M. V.; Frolov, V.; Loubnin, E.; Waters, K.; Shultz, A. J. Vac. Sci. Technol. A 1999, 17, 1191. (14) Gomer, R. Field Emission and Field Ionization; Harvard University Press: Cambridge, MA, 1961. (15) Nilsson, L.; Groening, O.; Emmenegger, C.; Kuettel, O.; Schaller, E.; Schlapbach, L.; Kind, H.; Bonard, J.-M.; Kern, K. Appl. Phys. Lett. 2000, 76, 2071. (16) Ballantyne, J. M. Phys. ReV. B 1972, 6, 1436. (17) Komatsu, S.; Kasamatsu, M.; Yamada, K.; Moriyoshi, Y. Appl. Phys. Lett. 1991, 59, 608. (18) Komatsu, S.; Kasamatsu, M.; Yamada, K.; Moriyoshi, Y. J. Appl. Phys. 1992, 71, 5654. (19) Komatsu, S. J. Mater. Res. 1997, 12, 1675. (20) Powers, M. J.; Benjamin, M. C.; Porter, L. M.; Nemanich, R. J.; Davis, R. F.; Cuomo, J. J.; Doll, G. L.; Harris, S. J. Appl. Phys. Lett. 1995, 67, 3912. (21) Pryor, R. W. Appl. Phys. Lett. 1996, 68, 1802. (22) Loh, K. P.; Nishitani-Gamo, M.; Sakaguchi, I.; Taniguchi, T.; Ando, T. Appl. Phys. Lett. 1998, 72, 3023. (23) Komatsu, S.; Yarbrough, W.; Moriyoshi, Y. J. Appl. Phys. 1997, 81, 7798.
