Processing and Characterization of CVD c-BN Films and c-BN/Diamond Bilayers
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Processing and Characterization of CVD c-BN Films and c-BN/Diamond Bilayers Tien-Syh Yang, Jong-Bin Cheng, Ming-Show Wong Department of Materials Science and Engineering, National Dong Hwa University, Taiwan, ROC. ABSTRACT The paper reports the syntheses of c-BN films by the reaction of B2H6 and NH3 in the hydrogen and argon mixture using microwave plasma-assisted chemical vapor deposition (MPCVD). The effects of NH3/B2H6 ratios, hydrogen addition, pulsed-DC substrate-bias voltage and diamond as bufferlayer on the formation of c-BN were investigated. As-grown films are characterized by FTIR, grazing-incidence XRD, and SEM. The NH3/ B2H6 = 3 was selected to obtain a better crystallized BN films with moderate deposition rate. When the hydrogen content was at 85 vol.% in the gas mixture and the substrate bias at –200 V, the maximum c-BN content about 65 % was obtained. However, the film was easily cracked in a few hours. The film deposited at lower substrate biasing of –100 V could preserve for a few days without peeling off, although it had lower c-BN content about 40 %. Nanocrystalline diamond film, deposited from 1 vol.% CH4/Ar plasma, as bufferlayer can promote the heteroepitaxial growth of c-BN, resulting in 80 % c-BN content. The c-BN/diamond bilayer were adherent with a total thickness of 1.2 µm. INTRODUCTION Owing to the outstanding properties of diamond and cubic boron nitride (c-BN), many efforts have been focused on the preparation of thin films for both materials. c-BN has a high hardness next to diamond, and a higher thermal stability against oxygen and a better chemical inertness toward ferrous materials than diamond. Furthermore, c-BN is a III-V compound with a promising potential as a large wide band gap semiconductor (Eg= 6.4 eV) for electrical and optical application. The application as hard and protective coatings in high temperature, and high power semiconductor devices are envisioned for c-BN film [1,2]. Because the crystallinity of c-BN film is poor and non-cubic BN phases tend to form before c-BN is nucleated by various vapor deposition methods, electronic applications are not yet practical [3,4]. Many physical vapor deposition (PVD) in laboratory for preparing c-BN have been reported [5-7]. The formation of c-BN film is possible only within a well-defined window of substrate bias demanding to exceed a threshold value. However, higher ion bombardment can induce
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high compressive stress and resputtering, which prevent the growth of adherent c-BN films thicker than a few hundred nanometers. Therefore, a mild chemical vapor deposition (CVD) has been proposed to improve the shortcomings described above by chemical reactions and selective etching. However, CVD for c-BN film synthesis still has some disadvantages left overcome, including higher substrate temperature, more variables and possible contaminants, and toxicity of precursors. The reaction of B2H6 and NH3 [8-11] or B2H6 and N2 [12,13] in CVD system has been utilized to obtain c-BN films in literature. Kiel et al.[14] propose
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