AVS 63rd International Symposium & Exhibition
    Thin Film Tuesday Sessions
       Session TF+SA+MI-TuM

Paper TF+SA+MI-TuM11
Unusual Effects in Organic Thin Film Growth as Revealed by the Use of In Situ Real Time Synchrotron X-ray Techniques

Tuesday, November 8, 2016, 11:20 am, Room 102B

Session: Thin Films for Synchrotron and Magnetism Applications
Presenter: James Engstrom, Cornell University
Authors: R.K. Nahm, Cornell University
H.J. Bullen, Cornell University
T. Suh, Cornell University
J.R. Engstrom, Cornell University
Correspondent: Click to Email
The effects of molecular scale events on the growth of crystalline thin films has been long been an active area of research as the resulting thin film properties depend sensitively on the microstructure. The growth of high quality, smooth thin films in the vast majority of these systems is favored at low rates of growth and high substrate temperatures. Here we report on the growth of thin films of tetracene, an organic semiconductor that possesses one less aromatic ring as compared to the much more studied pentacene. We examine the growth on SiO2 using in situ real time X-ray synchrotron radiation and ex situ atomic force microscopy (AFM). First, using in situ X-ray reflectivity, we observe a transition from 3D island growth to 2D layer-by-layer growth as the growth rate of tetracene is increased on SiO2 at room temperature, Ts ~ 30 °C. This unusual phenomenon, 2D growth favored at higher rates, has not been observed in previous work with pentacene, despite the similarities between these two molecules. Results from AFM indicate that tetracene may tend to quickly traverse “upwards” on thin film features such as the edges of islands, making these thin films susceptible to reorganization during and/or after growth. Thus, at sufficiently high rates of growth the rate of admolecule attachment at the tetracene island/SiO2 substrate edges effectively outcompetes the rate of upward step-edge transport, and 2D growth results. We find additional evidence for this mechanism by examining the effect of substrate temperature. In particular we find that the transition to 2D growth occurs at a lower rate of growth at a substrate temperature of Ts ~ 0 °C, consistent with suppression in the rate of upward transport at lower values of Ts. Finally, we have also examined the evolution of the thin film crystal structure in real time using in situ grazing incidence X-ray diffraction. Many thin film organic semiconductors are known to crystallize in different structures, and tetracene is no exception. At room temperature we find that first a thin film phase grows, then, after a delay, a bulk phase begins to grow, while the growth of the thin film phase saturates. We also find that the film thickness at which X-ray scattering from the bulk phase becomes apparent is found to vary with deposition rate. These phenomena are also dependent on substrate temperature. For example, for the range of thin films thicknesses examined, we found no evidence for growth of the bulk phase at the lower substrate temperature, Ts ~ 0 °C. These results highlight the potential complexity of crystalline thin film growth in what appears to be a very simple system, tetracene on SiO2.