Fibrous structures on diamond and carbon surfaces formed by hydrogen plasma under direct-current bias and field electron
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Kazuhiro Yamamoto, Yoshinori Koga, and Shuzo Fujiwara National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan
Yasuhito Gotoh, Hironori Nakahara, Hiroshi Tsuji, and Junzo Ishikawa Department of Electronic Science and Engineering, Graduate School of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
Franz A. Ko¨ck and Robert J. Nemanich Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202 (Received 15 April 2002; accepted 24 October 2002)
Polycrystalline diamond films, single crystal bulk diamonds, and diamond powder were treated in microwave plasma of hydrogen at 1.6 torr under a negative direct-current bias of −150 to −300 V without metal catalyst. It was found that fibrous structures, uniformly elongated along the direction normal to the specimen surface, were formed on the diamond surfaces. Similar experiments for glasslike carbon resulted in conical structures with frizzy fibers at the tops. Transmission electron microscopy measurements indicated that the fibers formed on diamond consisted of randomly oriented diamond nanocrystals with diameters of less than 10 nm, while the conical structures formed on glasslike carbon consisted of graphite nanocrystals. Field emission measurements of the fibrous specimens exhibited better emission efficiency than untreated ones. The field emission electron microscopy of the fibrous glasslike carbon showed a presence of discrete electron emission sites at a density of approximately 10,000 sites/cm2.
I. INTRODUCTION
Formation of fibrous structures on material surfaces is of practical importance for such applications as field emission of electrons, where sharp tips are effective to reduce the threshold electric field,1–3 and chemical electrodes, including chemical and biosensors,4,5 where large surface areas are preferable to achieve better performance. For diamond, a high density of needlelike structures is often formed by oxygen-plasma etching,6 where a presence of dislocations or unintentional impurity particles at the initial surface masks and prevents local etching of the diamond surface. It has been, however, found by Stoner et al.7 that diamond films deposited on Si substrates can also be etched by microwave plasma of hydrogen, if a negative direct current (dc) bias is applied to the specimen against a counter electrode placed above the plasma. The etching conditions were {gas pressure P, bias voltage Vb, microwave power Pm, specimen temperature Ts} ⳱ {15 torr, − 250 V, 800 W, 450 °C} J. Mater. Res., Vol. 18, No. 2, Feb 2003
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(1 torr ⳱ 133.3 Pa), where a modified stainless-steeltype microwave plasma generator (ASTeX-type, Seki Technotron Corp., Tokyo, Japan) was used. In the experiment, the entire diamond film was etched down to the Si substrate with an estimated etching rate of 艌6 m/h. Jiang et al.8,9 performed similar experiments using (100)oriented polycrystalline diamond fil
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