Low-temperature magnetoresistance study of electrical transport in N- and B-doped ultrananocrystalline and nanocrystalline diamond films M. Nesladek a, * , D. Tromson a , P. Bergonzo a , P. Hubik b , J.J. Mares b , J. Kristofik b , D. Kindl b , O.A. Williams c,1 , D. Gruen d a CEA-LIST (Recherche Technologique), CEA-Saclay, 91191 Gif sur Yvette, France b Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka ´ 10, 162 53 Prague 6, Czech Republic c Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA d Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Available online 6 March 2006 Abstract In this paper, we discuss the transport mechanism in nitrogen-doped ultrananocrystalline (N-UNCD) and B-doped nanocrystalline (B-NCD) diamond thin films, which have recently attracted significant attention due to possible applications in electronics and bioelectronics. We present clear evidence that the transport in UNCD films at LHeT has low-dimensional quantum character and can be explained by a weak localisation (WL) model. Our model explains the negative magnetoresistance, observed in these films for the first time, and confirms the WL phenomena. For comparison, we have prepared thin B-NCD films, doped by B using TMB. Films with thickness of about 150 nm deposited on glass wafers are fully transparent and highly conductive. B-concentrations close to the Metal Insulating Transition (MIT) are confirmed by Raman measurements. We discuss the positive magnetoresistance data observed also for the first time in B-NCD films and compare the transport mechanism with UNCD films. D 2006 Published by Elsevier B.V. Keywords: U-NCD; NCD; Doping; Low-temperature transport; Weak localisation 1. Introduction Nanocrystalline (NCD) and ultrananocrystalline diamond (UNCD) thin films, prepared by microwave PE-CVD (plasma- enhanced chemical-vapor deposition) [1–4], are new promis- ing thin films, interesting for several applications in bioelec- tronics [5,6]. Such applications often require highly conductive films, as, for example, for bio-electrochemical detection. However, the conduction mechanism in such kind of diamond films is currently not well understood. In contrast to single crystal diamond, UNCD can be doped relatively easily by adding nitrogen during the PE-CVD film growth, generating n-type conduction. For example, by means of an admixture of 20% of nitrogen (N 2 ) into the gas phase, high electric conductivity (¨ 1.4  10 4 Sm  1 ) of the resulting UNCD layer may be achieved [2,3]. The segregation of a nitrogen-containing non-diamond phase component at the grain boundaries [6] is generally considered as the reason for high conductivity in UNCD films; however, the exact transport mechanism is as yet unresolved. Apparent controversy between relatively high mobility of in N-UNCD, typically 2–5 cm 2 V  1 s  1 , and a transport which is non-metallic (e.g., observed is decreasing conductivity with decreasing temperature) on one side and also not thermally activated as expected for doped semiconductor on the other side, was discussed in Ref. [3]. Recently, on the basis of the evaluation of the temperature dependences of the conductivity, the mechanism controlling the electron transport has been studied with a suggestion of hopping via strongly localized states [7]. These open questions concerning the transport in UNCD, together with the fact that some thin diamond films reveal superconductivity [8,9], is a sufficient motivation for the study into the low-temperature transport (i.e., quantum) phenomena in the N-UNCD material. At the same time, we were interested in preparing boron- doped nanocrystalline diamond (B-NCD) as a p-type counter- part to n-type N-UNCD, which can be especially interesting for electrochemical applications due to its large potential window. 0925-9635/$ - see front matter D 2006 Published by Elsevier B.V. doi:10.1016/j.diamond.2005.11.001 * Corresponding author. E-mail address: milos.nesladek@cea.de (M. Nesladek). 1 Current address: Institute for Materials Research, Wetenchapspark 1, B-3590 Diepenbeek, Belgium. Diamond & Related Materials 15 (2006) 607 – 613 www.elsevier.com/locate/diamond