Quaterrylene thin films were grown on a SiO2 surface at an ultralow flux rate using a vacuum deposition technique with a hot wall cell, and their detailed growth process was investigated. We discuss the influence of growth parameters such as substrate temperature, flux rate of molecules, and film thickness. Atomic force microscopy (AFM) observations revealed the presence of two different phases: one with a layered structure and the other with a fibrous structure. X-ray diffractometry (XRD) clarified the orientation of the molecules in each phase, which were lying down in the fibrous structure and standing up in the layered one. The fibrous structure appeared on the surface of the underlying layered structure at low substrate temperature and high flux rate, showing that the phase was formed under nonequilibrium conditions. On the other hand, the layered structure with an upright orientation grew consistently under equilibrium conditions of high substrate temperature and low flux rate. Next, the initial growth process was evaluated under optimized conditions. The films were found to evolve following a Stranski-Krastanov (S-K) mode. First, the films were grown two-dimensionally with a standing-up orientation up to 4 monolayers (ML) followed by three-dimensional (3D) island growth. This result showed 2D growth to be enhanced by ultraslow deposition. XRD measurement demonstrated that the c-lattice constant expanded in the 2D growth region but relaxed as film thickness increased up to 4 ML, eventually coinciding with that of the bulk crystal in the 3D growth region. These results indicate that molecules are subjected to compressive stress in the lateral direction in the 2D growth region, whereas the crystal lattice relaxes as the growth mode changes from the 2D growth to the 3D growth. We concluded that relaxation of the crystal lattice was the origin of the transformation of growth mode from 2D growth to 3D growth.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films