Attempts to p-Dope Ultrananocrystalline Diamond Films in a Hot Filament Reactor
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0956-J09-31
Attempts to p-Dope Ultrananocrystalline Diamond Films in a Hot Filament Reactor Paul William May and Matthew Hannaway School of Chemistry, University of Bristol, Cantock's Close, BRISTOL, BS8 1TS, United Kingdom
ABSTRACT Ultrananocrystalline diamond (UNCD) films have been deposited using hot filament chemical vapour deposition using Ar/CH4/H2 gas mixtures plus additions of B2H6 in an attempt to make p-type semiconducting films. With increasing additions of B2H6 from 0 to 40,000 ppm with respect to C, the film growth rate was found to decrease substantially, whilst the individual grain sizes increased from nm to µm. With 40,000 ppm of B2H6, crystals of boric oxide were found on the substrate surface, which slowly hydrolysed to boric acid on exposure to air. These results are rationalised using a model for UNCD growth based on competition for surface radical sites between CH3 and C atoms. INTRODUCTION Recently, so-called ultrananocrystalline diamond (UNCD) films have become a topic of great interest, since they offer the possibility of making smooth, hard coatings at relatively low deposition temperatures, which can be patterned to nm resolution [1,2]. These differ from nanocrystalline diamond (NCD) films [3], since they have much smaller grain sizes (~2-5 nm), and have little or no graphitic impurities at the grain boundaries. Most reports of the deposition of these films describe using a microwave (MW) plasma chemical vapour deposition (CVD) reactor and gas mixture of 1%CH4 in Ar, usually with addition of 1-3% H2 [1]. Addition of N2 to the gas feed mixture has a profound impact on the conductivity, field emission, and electrochemical behaviour. It is believed that the nitrogen preferentially accumulates in the grain boundaries, since both the grain size and grain-boundary widths increase with the addition of N2, but the overall bonding structure in both regions remains mostly unchanged [4]. This is not doping in the conventional sense, and conductivity is related to midgap states in the diamond film. Nevertheless these ‘nitrogen-doped’ UNCD films exhibit overall n-type semiconducting behaviour which shows great promise for electronic devices [5]. Standard microcrystalline diamond (MCD) [6] and nanocrystalline diamond (NCD) [7] films can be readily doped with boron to give p-type conductivity. This gives an activation energy of 0.37 eV, and conductivity can be controlled from intrinsic to near metallic. This is achieved by addition to the gas mixture of boron (usually in the form of gaseous B2H6 or B(CH3)3) during CVD. However, to date, surprisingly, there are no reports in the literature of ptype UNCD being deposited by a similar method, despite the obvious benefits that this would bring in terms of potential UNCD p-n devices.
We have previously reported the use of Ar/CH4/H2 gas mixtures to deposit UNCD in a hot filament (HF) reactor [8,9,10], with the compositional diagram for mixtures of Ar, CH4 and H2 being mapped out corresponding to the type of film grown. The gas mixture required for UNCD depos
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