Surface conductivity induced by fullerenes on diamond: Passivation and thermal stability P. Strobel a, * , J. Ristein a , L. Ley a , K. Seppelt b , I.V. Goldt c , O. Boltalina d a Lehrstuhl fu ¨r Technische Physik, Universita ¨t Erlangen-Nu ¨rnberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany b Freie Universita ¨t Berlin, Germany c Moscow University, Russia d Colorado State University, USA Available online 2 December 2005 Abstract The surface conductivity of hydrogen terminated diamond under atmospheric conditions is a well known phenomenon. Inspired by the surface transfer doping model [F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85, (2000) 3472] we have recently investigated thin C 60 layers as an alternative to atmospheric adsorbates. We could indeed show that C 60 induces a sub-surface accumulation layer that results in a surface conductivity comparable to the air induced one [P. Strobel, M. Riedel, J. Ristein, L. Ley, Nature 430, (2004) 439]. In the present work we investigate fluorinated fullerenes, namely C 60 F 18 ,C 60 F 36 , and C 60 F 48 , as transfer dopants. The surface conductivity induced by fluorinated fullerenes increases with higher fluorination and achieves for C 60 F 48 a level that exceeds that observed under atmospheric conditions by up to a factor of three. Furthermore, we study the thermal stability of the fullerene layers on diamond and their stabilisation by passivation with different dielectric films (SiO, CaF 2 , and Si 3 N 4 ). In the case of fluorinated fullerenes we observe a significant improvement in thermal stability after passivation with dielectrics but the pristine conductivity level cannot be kept. A different kind of stabilisation is achieved for C 60 . After the simultaneous exposure to oxygen, light, and temperature of about 150 -C the surface conductivity induced by C 60 is stable up to 350 -C in vacuum with an undiminished doping efficiency. We ascribe this effect to an oxygen mediated polymerisation of the C 60 layer. D 2005 Elsevier B.V. All rights reserved. Keywords: p-type doping; Fullerenes; Passivation; Thermal stability 1. Introduction One of the great challenges in diamond research is its use in the field of device applications. For this, doping of the wide- band gap semiconductor diamond is of crucial importance. In addition to classical volume doping, the surface conductivity induced by air adsorbates has been utilized to build diamond based devices [3,4]. The mechanism underlying the surface conductivity is a sub-surface hole accumulation layer induced by an electron transfer from the valence band of hydrogen terminated diamond to the water layer on the surface [1]. The recent discovery that C 60 acts as a transfer dopant on hydrogen terminated diamond as well allows for a more productive search for further surface acceptors [2]. The relevant energy for transfer doping by molecular adsorbates is their electron affinity v. A hole accumulation of 10 12 ...10 13 cm À 3 which is necessary for a surface conductivity of the order of 10 À 5 S requires that v åI , where I = 4.2 eV is the ionization energy of hydrogen terminated diamond [5]. The electron affinity of isolated C 60 is 2.7 eV [6] which is far too small to induce a measurable charge transfer from diamond. However, this value increases to v = 4.1 eV for solid C 60 (fullerite) due to many body effects. Hence, several monolayers of C 60 on hydrogen terminated diamond lead to a surface conductivity comparable to the air induced one. As a result of the high electronegativity X of fluorine (X = 4) compared to X = 2.5 for carbon, the electron affinity of fluorinated fullerenes increases from 2.7 eV for C 60 up to 4.06 eV for C 60 F 48 , the most highly fluorinated species synthesized so far [7]. On account of their high electron affinities the fluorofullerenes are expected to exhibit higher doping effi- ciencies than C 60 , even for sub-monolayer coverages. 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.10.034 * Corresponding author. Tel.: +49 9131 85 28881; fax: +49 9131 85 27889. E-mail address: Paul.Strobel@physik.uni-erlangen.de (P. Strobel). Diamond & Related Materials 15 (2006) 720 – 724 www.elsevier.com/locate/diamond