Capacitance and Transient Photocapacitance Studies of Tetrahedral Amorphous Carbon
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CAPACITANCE AND TRANSIENT PHOTOCAPACITANCE STUDIES OF TETRAHEDRAL AMORPHOUS CARBON KIMON C. PALINGINIS*, A. ILIE**, W.I. MILNE** and J. DAVID COHEN* * Department of Physics, University of Oregon, Eugene, OR 97403 U.S.A. ** Engineering Department, University of Cambridge CB2 1PZ, U.K. ABSTRACT We have applied junction capacitance and transient photocapacitance measurements to undoped tetrahedral amorphous carbon (ta-C)/silicon carbide (SiC) heterostructures to deduce defect densities and defect distributions in ta-C. The junction capacitance measurements show two thermally activated processes. One can be related to the activation of carriers out of defects at the ta-C/SiC interface while the other one with an activation energy of 0.36eV is an intrinsic property of the ta-C. The defect density at the ta-C/SiC interface is estimated to be roughly 9 ± 2 x 109 cm-2. The transient photocapacitance measurements have allowed us to observe the broader band tail of ta-C, giving a value (Urbach energy) of 230meV. INTRODUCTION With optimum growth conditions, thin tetrahedral amorphous carbon (ta-C) films prepared by the filtered cathodic arc system (FCVA) method have fractions of sp3 bonds up to 80% [1]. However, the electronic properties are greatly influenced by the residual sp2 bonds, which, in particular lead to an optical band gap of 2 to 2.5eV [2]. Undoped ta-C is a p-type semiconductor with dark conductivities in the range of 10-7 to 10-8 Ω-1cm-1 [3]. Films can be n-type doped by the incorporation of nitrogen or phosphorous [4] with decreases in the resistivity up to five orders of magnitude. Particular properties of ta-C, such as its significant electron emission at a low electric field threshold as well as its chemical inertness and high hardness [5] have made it an interesting material for applications such as flat-panel displays or large area coatings. However, due to difficulties in applying standard spectroscopic measurements to such thin films (< 1000Å), detailed knowledge about its electronic structure is still strongly debated. Recently we have succeeded in applying transient junction capacitance with other junction capacitance techniques to study ta-C/c-Si and ta-C:H/c-Si heterostructures [6,7]. However, because these thin ta-C films were grown on a substrate with a much smaller energy gap, this precluded being able to observe the optical transitions in the ta-C. In this paper we report new results of transient photocapacitance measurements on a ta-C/SiC heterostructure. The high band gap of SiC (~ 3eV) provides a sufficiently wide energy window to observe optical transitions directly in the bulk of the ta-C film. In this study the sample consisted of a ta-C film of thickness less than 600Å deposited on a n-type doped silicon carbide substrate. Before we applied the photocapacitance technique, we characterized the ta-C/SiC heterostructure using admittance spectroscopy. We found that carriers in the bulk of the ta-C are thermally activated with an energy of 0.36eV. We also deduced the density of defects at the ta
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