The characteristics of dc glow discharge and its effects on enhancement of diamond nucleation in HF-CVD system
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ABSTRACT We have investigated enhancement of diamond nucleation due to electrode biasing and compared these results with those obtained using substrate biasing. The electrode biasing results in near-surface dc glow discharge with characteristic negative glows and dark regions on both sides of the electrode (cathode). We have studied systematically diamond nucleation density as a function of substrate location with respect to the electrode and gas pressure. Using scanning electron microscopy and Raman spectroscopy, we have determined that a significant enhancement up to 109 cm2 in diamond nucleation is achieved when the substrate is located in the negative glow region of the near-surface dc glow discharge. It is also found that gas pressure enhances nucleation site density but the quality of diamond decreases. The optimization of these parameters in obtaining high-quality diamond films is discussed. I. INTRODUCTION Nucleation of diamond on non-diamond substrates during chemical vapor deposition is usually a difficult step. A number of nucleation enhancement methods have been employed to increase nucleation site density (NSD) and obtain continuous diamond films' 5. These include ex situ and in situ substrate pretreatment methods. The most common ex situ pretreatment involves mechanical scratching with diamond paste or powder 6' 7 , diamondlike carbon and carbide buffer layers 7' 8 and laser-assisted nucleation 7. The most common in situ method for nucleation enhancement is substrate biasing, usually a negative voltage is applied to the substrate during a short period prior to normal growth. The negative substrate biasing works well for microwave chemical vapor deposition (MWCVD) 9 -4 The similar BEN (Bias Enhanced Nucleation) step during hot-filament chemical vapor deposition (HFCVD) has not shown consistent results. Researchers have obtained enhanced nucleation under HFCVD using negative, positive 15 as well as ac16 substrate biasing conditions. Using a negative biasing in HFCVD, Zhu et al. 17 reported an increase in nucleation density from I05 to 109 cm"2 on polished silicon substrates. The enhancement in the nucleation density was achieved whenever a dc plasma was formed near the substrate due to biasing. Gouzman et al. 8 reported a nucleation of 108 cm 2 via dc glow discharge pretreatment on mirror polished Si(100) without additional gas activation by hot filament. Electron bombardment that assists gas phase 20 dissociation was proposed for enhanced nucleation19. A comparative study by Sanchez, et al. showed that electrode biasing resulted in formation of smaller amount of nondiamond carbon compared to substrate biasing because in the latter ion bombardment leads to formation of nondiamond carbon. In this investigation, we have performed comparative studies on enhancement of diamond nucleation due to substrate and electrode biasing, using our new experimental configuration in . On leave from Institute of Physics, Chinese Academy of Sciences. Present address: Department of Mechanical Engineering, North Carolina A&
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