Quantitative modeling ofin situx-ray reflectivity during organic molecule thin film growth

Abstract

Synchrotron-based x-ray reflectivity is increasingly employed as an in situ probe of surface morphology during thin film growth, but complete interpretation of the results requires modeling the growth process. Many models have been developed and employed for this purpose, yet no detailed, comparative studies of their scope and accuracy exists in the literature. Using experimental data obtained from hyperthermal deposition of pentane and diindenoperylene (DIP) on SiO2, we compare and contrast three such models, both with each other and with detailed characterization of the surface morphology using ex-situ atomic force microscopy (AFM). These two systems each exhibit particular phenomena of broader interest: pentacene/SiO2 exhibits a rapid transition from rough to smooth growth. DIP/SiO2, under the conditions employed here, exhibits growth rate acceleration due to a different sticking probability between the substrate and film. In general, independent of which model is used, we find good agreement between the surface morphology obtained from fits to the in situx-ray data with the actual morphology at early times. This agreement deteriorates at later time, once the root-mean squared (rms) film roughness exceeds about 1 ML. A second observation is that, because layer coverages are under-determined by the evolution of a single point on the reflectivity curve, we find that the best fits to reflectivity data -corresponding to the lowest values of χ 2 ν -do not necessarily yield the best agreement between simulated and measured surface morphologies. Instead, it appears critical that the model reproduce all local extrema in the data. In addition to showing that layer morphologies can be extracted from a minimal set of data, the methodology established here provides a basis for improving models of multilayer growth by comparison to real systems.

Identification of an organic semiconductor superlattice structure of pentacene and perfluoro-pentacene through resonant and non-resonant X-ray scattering

“…Elongated islands resemble the shape of islands in PFP films, 20,29 but there are also more compact islands as in PEN thin films. 16,30 On top of some islands, molecular step edges with a height of ∼15 Å can be discerned, which points to upright standing PEN and PFP molecules.…”

“…PFP only grows in a layer by layer fashion for 1 -2 MLs 20 and PEN for less than 4 MLs on top of silica. 30 Beyond this thickness, surface roughening leads to holes in PEN layers that get filled with PFP and vice versa, reducing the structural definition of the superlattice. Using molecules that exhibit a more pronounced layer-by-layer growth such as DIP, PTCDI-C8 or C60 would enable an extended superlattice with more periods.…”

Real-time x-ray scattering study of the initial growth of organic crystals on polymer brushes

“…12,17,20,26 We found that the pentacene grown on the polymer brush layers underwent significant incomplete condensation, where desorption of pentacene molecules from the substrate surface becomes important. The incomplete condensation was more severe at higher substrate temperatures (T ∼ T g,surf ) than at lower substrate temperatures (T < T g,surf ).…”

“…X-ray scattering measurements enable real-time structural monitoring during the earlystage growth of a small molecule organic layer. X-ray scattering in the anti-Bragg configuration 12,[16][17][18][19][20][21][22] or energydispersive x-ray reflectivity [23][24][25] methods are useful tools for characterizing the coverage of individual small molecule organic thin film layers in real-time. Real-time x-ray scattering methods have been applied to organic films deposited onto bare SiO 2 or SAM-treated SiO 2 surfaces.…”

Monitoring the thin film formation during sputter deposition of vanadium carbide

“…In situ X-ray scattering techniques fulfill these conditions. For epitaxial systems, measurements at fixed points of a crystal truncation rod have been successfully performed by Braun et al (2003), Krause et al (2004), Jenichen et al (2007), Woll et al (2011) and Chinta & Headrick (2014). However, this method is not applicable for polycrystalline coatings.…”