  A method has been developed to inject mobile charges at the surface of
organic molecular crystals, and the DC transport of field-induced holes has
been measured at the surface of pentacene single crystals. To minimize damage
to the soft and fragile surface, the crystals are attached to a pre-fabricated
substrate which incorporates a gate dielectric (SiO_2) and four probe pads. The
surface mobility of the pentacene crystals ranges from 0.1 to 0.5 cm^2/Vs and
is nearly temperature-independent above ~150 K, while it becomes thermally
activated at lower temperatures when the induced charges become localized.
Ruling out the influence of electric contacts and crystal grain boundaries, the
results contribute to the microscopic understanding of trapping and detrapping
mechanisms in organic molecular crystals.
