Kinetics-driven anisotropic growth of pentacene thin films

Abstract

The growth of nonepitaxial as well as epitaxial structures of (001)-oriented pentacene (${\mathrm{C}}_{22}{\mathrm{H}}_{14}$, Pn) thin films on silicon surfaces has been extensively studied in order to elucidate the intrinsic thin-film growth mechanism. The kinetically driven growth processes in pentacene films were found to be modified significantly by the anisotropy of the crystal structure. In situ real-time low-energy electron microscopy studies of diffusion-limited growth of Pn islands on $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ and $\ensuremath{\alpha}\ensuremath{\surd}3$-Bi-Si(111) surfaces reveal a definite anisotropy in their shapes. Although this anisotropy is associated with organic film crystal structure, it cannot be predicted directly from the equilibrium crystal shape. It has been found that under kinetic growth conditions the Pn islands are always elongated along the $\mathbit{b}$ axis of the in-plane unit cell, even though the step along the $\mathbit{a}$ axis has the lowest energy, regardless of crystalline polymorph or epitaxial relation, indicating that Pn thin-film growth has an intrinsic kinetic preference along the $\mathbit{b}$-axis direction. Utilizing this kinetic preference and constraining the direction of available flux on inert surfaces during Pn deposition enabled us to control the crystal orientation of Pn domains.