Temperature dependence of transport anisotropy of planar-type graphite nanostructures fabricated by focused ion beam

We report in this paper on the observation of temperature-dependent anisotropic transport behavior for planar-type nanostructures (in-plane and out-of-plane) fabricated on a thin graphite layer using a three-dimensional focused-ion-beam (FIB) etching technique. The transport characteristics were studied for several in-plane areas with sizes of 6 x 6 microm2, 6 x 4 microm2 and 6 x 2 microm2 planar-type structures/patterns and out-of-plane structures with the dimensions of 2 x 1 x 0.3 microm3. Both in-plane (rho(a)) and out-of-plane (rho(c)) resistivities are measured for these structures and the ratio of resistivity anisotropy is determined. The observed values of anisotropy ratio rho(c)/rho(a) were approximately 12.5 at 300 K and approximately 54 at 25 K. The room temperature value of rho(c)/rho(a) varies by a few orders from the values of previously reported anisotropy results of bulk pyrolytic graphite. However, the value of resistivity anisotropy increases with decreasing temperature, which is an identical behavior to bulk pyrolytic graphite. From current (I)-voltage (V) characteristics, we observed an ohmic behavior at 300 K for both low- and high-current biasing. This behavior turns into nonlinear characteristics when the temperature goes down. As these fabricated structures consist of multiple elementary junctions along the c-axis, nonlinear I-V characteristics result. The impurity assisted interlayer hopping conduction and thermal excitation of carriers play a key role in this effect.