Fabrication of highly oriented rubrene thin films by the use of atomically finished substrate and pentacene buffer layer

We report the remarkable effects of physical and chemical treatments of substrate surface on the physical vapor deposition of rubrene thin films. Highly c-axis oriented rubrene thin films were fabricated by combinatorial molecular beam epitaxy on atomically flat alpha-Al2O3 (0001) substrates, the surface of which was partially modified with pentacene buffer film. Rubrene thin films grown at room temperature on a sapphire substrate without pentacene buffer layer did not exhibit any X-ray diffraction pattern, whereas films deposited on a pentacene buffer layer exhibited peaks of c-axis orientation. Atomic force microscope images of the crystalline films show the steps of 1.3 nm height which correspond to the half c-axis length of the rubrene crystal. Preliminary, p-type operation was observed in bottom-gate field effect transistors using this rubrene film deposited on a pentacene buffer.