Origin of High-Conductivity Layer Near the Surface in As-Grown Diamond Films
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KOJI KAJIMURA * Electrotechnical Laboratory, 1-1-4, Umezono, Tsukuba, Ibaraki 305, Japan ABSTRACT In order to clarify the origin of p-type high-conductivity layers (HCL) near the surfaces of as-grown diamond films prepared by chemical vapor deposition, we have investigated the properties of Schottky junctions fabricated from HCL. The Schottky junctions between Al and HCL of undoped homoepitaxial films with step-flow growth showed high-rectification properties. The forward current-voltage characteristics of the junctions in the temperature range between 83 K and 400 K are found to be described by the thermionic-field emission theory. Analysis indicates that thin enough depletion layer is formed at the junction by a high concentration (~10' 8/cm3 ) of acceptors existing in HCL due to hydrogenation. The origin of HCL is the acceptors related to incorporated hydrogen near the surface. INTRODUCTION It has been known that as-grown diamond films prepared by chemical vapor deposition (CVD) have relatively high-conductivity layers (HCL) near the surfaces both with and without boron (B)-doping [1, 21. The HCL is of p-type conduction 13, 4], which disappears after oxidation and reappears by exposure to a hydrogen plasma. Using HCL, a semiconducting device such as a field effect transistor was actually fabricated [5]. However, the origin of HCL has not yet been clarified. Several works pointed out the relevance of hydrogenation to HCL [1-12]. Landstrass and Ravi [1] proposed that the origin of HCL is the deep level passivation by terminated hydrogen incorporated near the surface. Kawarada et al. 18,] suggested that negative charges in the surface states originated from hydrogen termination induce a hole accumulation layer due to
surface band bending. It has been reported that HCL is attributed to holes generated by additional acceptors arising from incorporated hydrogen [3, 9, 10]. Based on Hall measurements of HCL and secondary ion mass spectroscopy (SIMS), we supported the last model of the existence of acceptors related to incorporated hydrogen [4]. Recently, we have successfully grown undoped homoepitaxial diamond films with atomically flat region and good crystallinity I11]. The epitaxy has been attained by step-flow growth in which atomic precursors migrate on terraces and incorporate into atomic steps before they form isolated nuclei on the terraces. The current-voltage (l-V) characteristics of Schottky junctions between Al and HCL of the films showed excellent high-rectification properties with very low leak currents at reverse bias. In this study, we have investigated carrier-transport mechanism in the Schottky barrier by the temperature dependence of I-V characteristics of the good-quality junctions. We clarify the origin of HCL on the basis of the present experimental results, in addition to the results of Hall measurements and SIMS. EXPERIMENT Diamond films were deposited on synthetic Ib diamond (001) substrates using a CVD system made by Applied Science Technology, Inc. (ASTeX). The substrate was 4.0 X4.0 2 mm in
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