The surface conductivity of (111)- and (100)-oriented hydrogen-terminated diamonds was investigated at low temperatures for different carrier densities. The carrier density was controlled in a wide range in an electric doublelayer transistor configuration using ionic liquids. As the carrier density was increased, the temperature dependences of sheet resistance and mobility changed from semiconducting to metallic ones: the sheet resistance and mobility for the (111) surface were nearly independent of temperature for a sheet carrier density of ̃4 × 1013 cm-2, indicating metallic carrier transport. It was also found that the interface capacitance, determined from the gate voltage dependence of the Hall carrier density, depended significantly on the crystal orientation.
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