Characterization of Tetrahedrally Bonded Amorphous Carbon Via Capacitance Techniques
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ABSTRACT We report the results of junction capacitance measurements on thin tetrahedral amorphous carbon (ta-C) films to deduce their defect densities. We find defect densities in the range 3 - 7 x 1017 cm 3 in the undoped ta-C films, and roughly an order of magnitude larger in the nitrogen doped (n-type) films. In some cases fairly uniform defect profiles were obtained covering a thickness of a couple of hundreds angstroems. We also observed a thermal activation process of carriers from defect states at the ta-C/c-Si interface with an activation energy in the range of 0.4eV to 0.5eV. INTRODUCTION Tetrahedral amorphous carbon (ta-C) films have attracted a great deal of interest in recent years. Thin ta-C films deposited from a filtered cathodic vacuum arc system (FCVA) [1] have been found to have high chemical inertness and exhibit electron emission at low electric field threshold [2]. Accordingly, this material is interesting for large area field emission displays. Depending on ion energies and growth temperatures during the FCVA deposition, ta-C2 3 contains up to 80% sp bonds [3], leading to a tetrahedral bonding structure. The residual sp optical band gap, bonds control the electronic and transport properties and in particular the which is about 2 - 2.5eV. Undoped ta-C exhibits a resistivity typically 107 - 108 Qcm, it is p_ type [4] and it has been suggested that the Fermi level lies about 0.3 - 0.4eV above the valence band. The usefulness of ta-C has been greatly enhanced by the possibility of n-type doping by nitrogen or phosphorous. Trace levels of such doping increasesthe resistance to 1010 iacm, and higher doping decreases the resistivity of the n-type ta-C to 103 i"cm. Nitrogen doping of up to 1% leaves the energy of the optical band gap unchanged [5]. However, knowledge about the electronic properties of ta-C is still greatly lacking, in part due to difficulties in applying most standard spectroscopic methods to such very thin films (
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Finally, we applied an alternative method
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to estimate the density of defect states in the ta-C film. As discussed previously we
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were able to calculate the variation in depletion region width in the c-Si with dc bias in the low temperature Distance from front barrier (pam) regime. Since we also know the donor Fig. 2. Drive level density vs spatial variation. (a) In density in the n-type c-Si, we know the total space charge in the depleted region. an undoped ta-C film for two different temperatures at 10kHz. (b) In a n-type doped ta-C film at 360K Using charge neutrality we can therefore and 10kHz. In both cases the front barrier was set deduce the density of defect states on the ta-C side. This approach was found to under reverse bias. yield a defect density which is one order of magnitude smaller than that obtained using the DLCP method. We attribute this discrepancy to the fact that such a calculation ignores any significant charge contained in interface states at the c-Si/ta-C heterojunction. This can result in a dramatic underestimate of the defect dens
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