Electrostatic force microscopy studies of boron-doped diamond films
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O.A. Williams Institute for Materials Research, Universiteit Hasselt, BE-3590 Diepenbeek, Belgium
E. Bohannan Department of Chemistry, University of Missouri, Rolla, Missouri 65409 (Received 12 December 2006; accepted 6 March 2007)
Much has been learned from electrochemical properties of boron-doped diamond (BDD) thin films synthesized using microwave plasma-assisted chemical vapor deposition about the factors influencing electrochemical activity, but some characteristics are still not entirely understood, such as its electrical conductivity in relation with microscale structure. Therefore, to effectively utilize these materials, understanding both the microscopic structure and physical (electrical, in particular) properties becomes indispensable. In addition to topography using atomic force microscopy, electrostatic force microscopy (EFM) in phase mode measuring the long-range electrostatic force gradients, helps to map the electrical conductivity heterogeneity of boron-doped micro-/nanocrystalline diamond surfaces. The mapping of electrical conductivity on boron doping and bias voltage is investigated. Experimental results showed that the BDD films’ surfaces were partially rougher with contrast of conductive regions (areas much less than 1 m2 in diameter), which were uniformly distributed. Usually, the EFM signal is a convolution of topography and electrostatic force, and the phase contrast was increased with boron doping. At the highest boron doping level, the conductive regions exhibited quasi-metallic electrical properties. Moreover, the presence of a “positive–negative–positive” phase shift along the line section indicates the presence of “insulating–conducting–insulating” phases, although qualitative. Furthermore, the electrical properties, such as capacitance and dielectric constants at operating frequency, were quantitatively evaluated through modeling the bias-dependent phase measurements using simple and approximate geometries. It was found that decreasing grain size (or increasing the boron concentration) lowers the dielectric constant, which is attributed to the change in the crystal field caused by surface bond contraction of the nanosized crystallites. These findings are complemented and validated with scanning electron microscopy, x-ray diffraction, and “visible” Raman spectroscopy revealing their morphology, structure, and carbon-bonding configuration (sp3 versus sp2), respectively. These results are significant in the development of electrochemical nano-/microelectrodes and diamond-based electronics.
I. INTRODUCTION
Diamond is a promising wide band-gap semiconductor material with a large potential offering excitement and interest due to its unique blend of superlative physical
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Address all correspondence to this author. e-mail: [email protected], [email protected] DOI: 10.1557/JMR.2007.0229 3014 J. Mater. Res., Vol. 22, No. 11, Nov 2007 http://journals.cambridge.org Downloaded: 14 Mar 2015
(electronic, optical, mechanical, and chemical) properties.1 Diamond thin films (DTF) in the fo
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