Pyrolytic Graphites: Their Description as Semimetallic Molecular Solids

Abstract

This paper is concerned with an investigation of the electrical, the galvanomagnetic, and the thermoelectric properties of pyrolytic graphites whose morphological features are conditioned by the deposition temperature, the heat treatment, and the doping level. (1) Basal-plane magnetoresistance and c-direction specific resistance of deposits prepared at temperatures ranging from 1900° to 2500°C point to a remarkable improvement of the crystallites' alignment with rising deposition temperature. In both crystallographic directions the Seebeck coefficient closely follows semi-empirical predictions based on the two-dimensional model of the π-electron bands. The Fermi level of a standard deposit (2100°C) is at 0.025 eV below the valence-band edge and thus indicates that crystal defects trap about ${\text{7.5×10}}^{18}\hspace{0.17em}\mathrm{electrons}/{\mathrm{cm}}^3$ at room temperature; this figure is in accord with a Hall coefficient of $\text{0.33\hspace{0.17em}}{\mathrm{cm}}^3/\mathrm{C}$ . The average in-plane mobility $\text{(930\hspace{0.17em}}{\mathrm{cm}}^2/\mathrm{V-sec})$ corresponds to a mean free path of the order of the crystallite diameter (250 Å). (2) Post-deposition treatment at temperatures above 2500°C results in (a) a rapid drop of the room-temperature basal-plane resistivity down to 50 μΩ-cm or less, (b) a Hall effect conversion from p to n type in the early stages of graphitization, and (c) a trend toward negative Seebeck coefficients in the layer planes. In conjunction with low-field magnetoresistance measurements these results can be described in terms of semimetallic concepts, the simultaneous presence of holes and electrons with equal concentrations (${\text{6×10}}^{18}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ at room temperature) stemming from a slight band overlap. Average mobilities imply that the carrier behavior approaches single-crystal characteristics (${\text{\hspace{0.17em}≈\hspace{0.17em}10}}^4\hspace{0.17em}{\mathrm{cm}}^2/\mathrm{V-sec}$ at room temperature) after heat treatment above 3000°C. Normal to the layers, the specific resistance always exceeds 0.1 Ω-cm, which points to a molecular conduction process in this direction. (3) An incorporation of boron into the carbon-hexagon networks lowers the electrical resistance of graphite particularly in the c direction (twenty-fold decrease at a composition of 0.6 at.%B); concurrently the two temperature coefficients become approximately equal to zero. In the rigid-lattice approximation band-population figures derived from the resistivity temperature dependence reflect the Hall coefficient behavior, the peak occurring at an equivalent boron content of 0.04%. The ionization efficiency is of the order of 50% with a Fermi level depressed by more than 0.1 eV. Thermoelectric power measurements in the c direction accord with the view that charge transport across the layer planes involves most of the excess holes, and reveal that boron enhances the Seebeck anisotropy of graphite.