Hot holes in naphthalene: High, electric-field-dependent mobilities

Intrinsic time-of-flight hole mobilities (${\mu }^{+}$) were obtained in naphthalene single crystals down to 4.2 K. Between 300 and 150 K the tensor component ${\mu }_{\mathrm{aa}}^{+}$ increases with decreasing temperature T, obeying a μ∝$T^n$ dependence, with n=-2.9. At low temperature the hole transport becomes nonlinear (sub-Ohmic) with the hole velocity tending to saturate with increasing electric field, E, at about 2×${10}^6$ cm/s. The highest experimental ${\mu }_{\mathrm{aa}}^{+}$ (obtained at the lowest E which allowed the observation of a distinct hole transit pulse) was 400 ${\mathrm{cm}}^2$/V s at 10 K and 3 kV/cm. It will be shown that the low-temperature results can be understood in terms of a standard band-model description, whereas the continuation of the experimental temperature-dependence law (μ∝$T^n$) (for both holes and electrons) into the high-temperature regime remains a problem for future theoretical work. Electron transits were obtained down to 22.5 K [${\mu }_{\mathrm{aa}}^{\mathrm{-}}$ (22.5 K) = 24.5 ${\mathrm{cm}}^2$/V s]. No field dependence of the electron mobility was detected.