title: Homoepitaxy of Crystalline Rubrene Thin Films.
abstract: The smooth surface of crystalline rubrene films formed through an abrupt heating process provides a valuable platform to study organic homoepitaxy. By varying growth rate and substrate temperature, we are able to manipulate the onset of a transition from layer-by-layer to island growth modes, while the crystalline thin films maintain a remarkably smooth surface (less than 2.3 nm root-mean-square roughness) even with thick (80 nm) adlayers. We also uncover evidence of point and line defect formation in these films, indicating that homoepitaxy under our conditions is not at equilibrium or strain-free. Point defects that are resolved as screw dislocations can be eliminated under closer-to-equilibrium conditions, whereas we are not able to eliminate the formation of line defects within our experimental constraints at adlayer thicknesses above ∼25 nm. We are, however, able to eliminate these line defects by growing on a bulk single crystal of rubrene, indicating that the line defects are a result of strain built into the thin film template. We utilize electron backscatter diffraction, which is a first for organics, to investigate the origin of these line defects and find that they preferentially occur parallel to the (002) plane, which is in agreement with expectations based on calculated surface energies of various rubrene crystal facets. By combining the benefits of crystallinity, low surface roughness, and thickness-tunability, this system provides an important study of attributes valuable to high-performance organic electronic devices.
authors.affiliation: Department of Electrical Engineering, Princeton University , Princeton, New Jersey 08544 United States.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Fusella
authors.affiliation: Institut für Angewandte Physik, Universität Tübingen , Auf der Morgenstelle 10, 72076 Tübingen, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Schreiber
authors.affiliation: Swagelok Center for Surface Analysis of Materials, Case Western Reserve University , Cleveland, Ohio 44106 United States.
authors.forename: Kevin
authors.initials: K
authors.lastname: Abbasi
authors.affiliation: Polymer Science and Engineering, University of Massachusetts , Amherst, Massachusetts 01003 United States.
authors.forename: Jae Joon
authors.initials: JJ
authors.lastname: Kim
authors.affiliation: Polymer Science and Engineering, University of Massachusetts , Amherst, Massachusetts 01003 United States.
authors.forename: Alejandro L
authors.initials: AL
authors.lastname: Briseno
authors.affiliation: Department of Electrical Engineering, Princeton University , Princeton, New Jersey 08544 United States.|Andlinger Center for Energy and the Environment, Princeton University , Princeton, New Jersey 08544 United States.
authors.forename: Barry P
authors.identifier: 0000-0003-4409-8751
authors.initials: BP
authors.lastname: Rand
keywords: Homoepitaxy; crystals; electron backscatter diffraction; line defect; roughness; rubrene; screw dislocation; thin films