  Pentacene is known to grow on isotropic silicon oxide surfaces in a
substrate-induced phase with fiber textured crystallites. This growth study
reports on the growth of pentacene crystallites on uniaxially oriented
surfaces. Silica substrates have been treated by ion beam sputtering so that
ripples with a lateral corrugation length of 38 nm and a surface roughness of
1.3 nm are formed. Pentacene thin films with a nominal thickness in the range
from 20 nm up to 300 nm are deposited on top of the rippled surfaces. The films
are characterized by atomic force microscopy and grazing incidence X-ray
diffraction. Bi-axially oriented crystals are formed due to the grooves of the
substrate surface opening up the possibility of a defined in-plane alignment of
the crystals. In a first stage of thin film growth, the thin film phase (TFP)
of pentacene is formed, while in the later stage the bulk crystal structure (C,
Campbell phase) also appears. Due to the bi-axial alignment of the crystallites
the transition from the thin film phase to the bulk crystal structure can be
directly investigated. An epitaxial relationship with (120)TFP || (210)C and
[-210]TFP || [1-20]C is observed which can be explained by an adaption of the
herringbone layers of both crystal structures. This work reveals one possible
microscopic mechanism for the transition from the metastable substrate-induced
phase of pentacene to its equilibrium bulk structure.
