Influence of methane concentration on the electric transport properties in heavily boron-doped nanocrystalline CVD diamo
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Influence of methane concentration on the electric transport properties in heavily borondoped nanocrystalline CVD diamond films Stoffel D. Janssens1, Paulius Pobedinskas1, Vladimíra Petráková2, Miloš Nesládek1,3, Ken Haenen1,3, and Patrick Wagner1,3 1 Hasselt University, Institute for Materials Research (IMO), Diepenbeek, Belgium 2 Academy of Sciences of the Czech Republic, v.v.i., Institute of Physics, Prague, Czech Republic 3 IMEC vzw, Division IMOMEC, Diepenbeek, Belgium ABSTRACT A study is presented on the morphology and electric properties of heavily boron-doped nanocrystalline diamond (B:NCD) thin films (у150snm) grown with two different C/H-ratios (1% and 5%) and a fixed 5000sppm B/C-ratio in gas phase on fused silica substrates. AFM measurements confirm that a higher C/H-ratio leads to smaller grains and more grain boundaries. Electric transport measurements reveal a higher resistivity and a lower mobility as function of the C/H-ratio for all temperatures measured. The resistivity of the 1% sample is almost not temperature dependent while the 5% sample is much more temperature dependent. The electric transport properties of the grain boundaries, more present in the 5% sample, can be responsible for the difference in transport properties of both samples. The active boron concentration, calculated from the electric transport measurements, is remarkably higher for the 5% sample which indicates there is more boron incorporation for higher C/H-ratios. Although both samples are disordered metals, the 1% sample with the least grain boundaries tends more to the behavior of a highly doped single crystalline diamond film, which behaves like a real metal when heavily boron-doped.
INTRODUCTION Charge transport in boron-doped nanocrystalline diamond (B:NCD) has attracted much attention due to its versatile properties leading to numerous fundamental and application driven investigations. Recent experiments carried out on B:NCD doped with various concentrations of boron show a very rich transport behaviour in which the boron concentration and the presence of grain boundaries play an essential role [1,2]. When the carrier concentration exceeds the value of 1021scm-3, superconducting properties can be detected in which the granular nature of the material has proven to be important but not fully understood [3-5]. It is generally accepted that an increase in the amount of methane leads to faster growth and an enhanced incorporation of defects, including wanted dopant atoms like boron. Nevertheless, it is extremely difficult to rigorously confirm the presence of such an effect in transport data, as one has to take into account several coupled effects, such as grain size and grain boundary density [6]. In this work the influence of the C/H-ratio on B:NCD in the disordered metal regime [7], grown by microwave plasma enhanced chemical vapor deposition (MWPECVD) is investigated. The samples in this study have a boron concentration in the order of 2 ͼ 1021scm-3. To compare
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the morphology, structure and electric properties, at
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