Growth and Field Emission Properties of Boron Nitride Island Films by Low-energy Ion-assisted Deposition
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Growth and Field Emission Properties of Boron Nitride Island Films by Low-energy Ion-assisted Deposition K. Teii1, J. H. C. Yang1, R. Yamao1, and S. Matsumoto2 1
Department of Applied Science for Electronics and Materials, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
2
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-004, Japan and Ceramic Forum Co. Ltd., 1-6-6 Taitoh, Taitoh-ku, Tokyo 110-0016, Japan.
ABSTRACT We report the growth and field emission properties of boron nitride (BN) island films by chemical vapor deposition in inductively coupled plasma. Fine-grained island films with large surface roughness can be grown for initial sp2-bonded BN and subsequent cubic BN (cBN) phases by using low-energy (~20 eV) ion bombardment. Ultraviolet photoelectron spectroscopy indicates that the electron affinity is as low as 0.3 eV for both sp2-bonded BN and cBN phases. The evolution of cBN islands reduces the turn-on field down to around 9 V/µm and increases the current density up to 10-4 A/cm2. The surface potential barrier height is estimated to be about 3.4 eV for emission from the Fermi level. INTRODUCTION Field emission cold cathodes are of great technological importance for development of high-efficiency vacuum microelectronic devices such as point electron sources, vacuum diodes, and ultrathin flat panel displays. Among the main factors which determine the emission performance are the turn-on field, current density, and durability of the emission. There are mostly three types of promising emitter materials, tip-shaped metals and Si, carbon nanotubes, and semiconductors. Wide band-gap semiconductors have gained a heightened interest because diamond [1], aluminum nitride [2], and hexagonal and cubic boron nitride [3,4] (hBN and cBN) can feature negative electron affinity (NEA). hBN is the stable phase under standard and lowpressure conditions, while cBN is the metastable phase. In particular, cBN is believed to have the highest potential for field emission due to the widest band-gap and the largest band bending [5]. The emission properties of cBN particles electrophoretically coated on tip-shaped Mo have been studied in detail [6]. However, there are only a few reports working on the emission properties of vapor-deposited cBN films [7,8]. In the rare work, the emission performance of cBN films is far from awesome. This could be due to the following reasons. First, low-pressure deposition of cBN films seems to need an impingement of energetic ions (typically above 50 eV) on the growing surfaces. The resulting films usually have very smooth surfaces with root-mean-square (rms) roughness of around 1 nm presumably due to ion impact-induced downhill currents [9]. This suggests that the enhancement of the local electric field is hardly expected. Second, the
formation of a cBN layer usually follows an initial sp2-bonded BN layer consisting typically of turbostratic and amorphous phases, and the top cBN surface is not always phase
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