Hydrogen diffusion in polycrystalline boron doped and undoped diamond
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Hydrogen diffusion in polycrystalline boron doped and undoped diamond. D. Ballutaud1, A. Boutry-Forveille1, J.-M. Laroche1, N. Simon2, H. Girard2, M. Herlem2, and A. Etcheberry2 1 CNRS-LPSC, 1 place Aristide Briand, 92195 Meudon cedex, France. 2 Institut Lavoisier, Université de Versailles-St-Quentin en Yvelines, 78035 Versailles cedex, France. ABSTRACT Hydrogen (deuterium used as tracer) diffusion experiments were performed on undoped and boron doped diamond films ([B] = 1019 and 1020 cm-3) grown by plasma CVD or hotfilament assisted CVD. The samples were exposed either to a radiofrequency plasma or a microwave plasma at different temperatures between 400°C and 900°C. The deuterium profiles were analysed by secondary ion mass spectrometry (SIMS). The deuterium diffusion was explained mainly in term of trapping on intergranular defects. The passivation of boron acceptors, by B-D complex formation in the deuterium diffused superficial layers of the diamond films, was followed by electrochemical and mercury probe capacitance measurements. The results suggest a strong decrease of the free carrier density, which is in accordance with passivation of free carriers by deuterium trapping on dopant. INTRODUCTION Hydrogen is known to play an important role in the diamond film growth from a CH4/H2 precursor mixture. The diamond polycrystalline films are highly defective, and present a high density of grain boundaries or dislocations, and incorporated hydrogen in semiconductors is known to terminate dangling bonds and passivate both shallow and deep levels. Furthermore, the hydrogen present at the diamond surface induces a superficial highly p-type conductive layer, the origin of which is not yet completely understood [1]. In a previous paper [2], it has been shown that the hydrogen concentration in an asgrown CVD polycrystalline diamond film with 1 µm grain size is about 1019 cm-3. This paper deals with new data on the diffusion of hydrogen (deuterium used as tracer) in polycrystalline doped and undoped diamond films. The deuterium diffusion profiles are analysed, and capacitive measurements [3] are used to evidence the passivation of boron acceptors by borondeuterium complex formation. EXPERIMENTAL Boron ([B] = 2x1019 and 2x1020 cm-3) doped polycrystalline diamond films (respectively 0.3 and 0.2µm grain size) were deposited on monocrystalline silicon substrates in a hotfilament-assisted chemical vapour deposition reactor supplied with diboranne and methane in hydrogen and provided by CSEM [4]. Some undoped polycrystalline films grown by plasma CVD using a 0.5 % CH4/H2 mixture (950 W, 6000 Pa, 780° C) on a monocrystalline silicon substrate were also used throughout this study (1µm crystal size) [5]. The hydrogen
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concentrations in the original as-grown layers are respectively1020 cm-3 in the boron doped layers and 2x1019 cm-3 in the undoped layers [2]. The samples were submitted to hydrogen (deuterium used as tracer) remote radiofrequency (r.f.) plasma (power density between electrodes 0.4 W.cm-2) at 500°C, or
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