ELSEVIER Diamond and Related Materials 6 ( 1997 ~ 668-672 D|AMON@ AND RE T @ TER|AL$ Improvement of diamond nuclei orientation by double-step bias treatment in microwave plasma-assisted chemical vapor deposition using CzH 4 and CH 4 as carbon source Takeyasu Saito, Shigenori Tsuruga, Hideaki Maeda, Katsuki Kusakabe, Shigeharu Morooka * Department of Chemical Science and Technoh~gy, l(vushu Universio', 6-10-1, Hakozaki, Higashi-ku, Fukuoka 812-81, &q~an Abstract The effects of bias treatment conditions and carbon sources on the formation of diamond film oriented to an Si(100) substrate were investigated in detail by using microwave plasma-assisted chemical vapor deposition (MPCVD). In the [3-SIC formation stage using C2H 4 as the carbon source, the most definite mesh structure was formed by a bias treatment at - 130 V tbr 3 rain. The mesh structure became ambiguous at smaller bias voltages and was not observed at -50 V. In the subsequent diamond formation step, the optimum bias voltage was -40 V for C,H,~. Faceted diamond particles 5-50 nm in size were formed after 7 rain at a population density of 10.8-3)x 10t° cm--', and the fraction of oriented particles was 60%. The population density did not vary with the species of carbon source, but the fraction of oriented particles was larger with C.,H.~ than with CH4 under optimized reaction conditions. © 1997 Elsevier Science S.A. Keywords: Diamond nuclei; Double-step bias treatment; Microwave plasma-assisted chemical vapor deposition 1. Introduction Diamond has a variety of outstanding properties [1,2], and considerable efforts have been devoted to produce diamond films from the gas phase. When dia- mond is applied to electrical devices, defect-free films oriented on a substrate are required. To date, epitaxial growth of diamond on substrates such as Ni [3,4], Co [5], c-BN [6-10], BeO [11], graphite [12] and Pt [13] have been reported, but continuous epitaxial diamond films have been reported only on a c-BN substrate. Yugo et al. [14,15] found that a negative bias treatment enhanced the population density of diamond nuclei on a silicon substrate. The negative bias treatment was also effective in controlling the orientation of diamond nuclei. Stoner et al. [16], Wolter et al. [17] and Jiang et al. [18] reported the synthesis of a (100) oriented diamond (HOD) film on silicon or silicon carbide surfaces by microwave plasma-assisted chemical vapor deposition (MPCVD). Maeda et al. [19] also synthesized an HOD film on a carburized (100) silicon substrate by MPCVD and observed that a characteristic pattern referred to as "mesh structure" was formed on the Si substrate during * Corresponding author. Tel.:/fax: + 81 92 6423551. 0925-9635/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. Pll S0925-9635 (96) 00614-0 the bias treatment after the initial carburization. The lbrmation of the mesh structure appears to be intimately related to the orientation of diamond nuclei, and this orientation affe~.ts the quality of HOD thin film. The objective of the present study was to optimize reaction conditions by independently controlling the bias voltage in the initial B-SiC formation step and the subsequent diamond nucleation step. The carbon source was also changed and, as a result, high populations and fractions of oriented diamond nuclei were obtained. 2. Experimental Diamond deposition was carried out by MPCVD, details of which are described elsewhere [19]. A p-type silicon (100) wafer of approximately 5 mm wide and 10 mm long was used as the substrate unless otherwise mentioned. Native oxide on the substrate surface was removed by ultrasonic cleaning in acetone ar,d a buffered hydrofluoric acid. The substrate teml~erature was decided by the input microwave power, which was fixed at 600 W throughout the present study, and was mea- sured with a single color optical pyrometer. The carbon source was C2H4 or CH4 diluted in hydrogen. The bias