In Situ Real-Time Studies Of Organic Semiconductor Thin Film Growth

Abstract

This thesis discusses the thin film deposition of small molecule organic semiconductors. Small molecule organics are attracting significant interest primarily due to their ability to form well ordered thin films at low temperatures with reasonable electronic properties. Potential applications of organic based electronics include thin film transistors, display technologies, flexible integrated circuits and photovoltaics. The growth and morphology of these organic thin films is very sensitive to the nature (chemical and physical) of the underlying substrate. A significant challenge in fabricating organic thin film devices with superior electrical characteristics is that of controlling and more importantly understanding the properties at the interface between the organic semiconducting layer and the underlying substrate. In this thesis, the use of supersonic molecular beams as a means to deposit organic semiconductor thin films is discussed in conjunction with in situ real-time synchrotron scattering and ex situ atomic force microscopy as thin film characterization techniques. This thesis discusses the effects of the incident kinetic energy of the small molecule organic and the nature of dielectric (clean silicon dioxide, SiO2; or SiO2 modified with self-assembled monolayers, SAMs, of varying thickness and chemical functionality; or SiO2 modified with polymers of varying surface energy) on the fundamental thin film processes occurring at the organic semiconductor/substrate interface. These thin film processes include adsorption, nucleation and diffusion, and the filling up of individual monolayers during thin film growth. Experiments have provided significant insight into these fundamental thin film processes. The results indicate that the probability of adsorption is a strong function of the incident kinetic energy of the organic molecule and thickness of the underlying SAM. The submonolayer island shape and island density is also a strong function of the underlying substrate with the later implying a change in the diffusivity of the organic with the identity of the substrate. Finally, the results suggest that multilayer thin film morphology such as feature/grain size and the thin film roughness is also a function of the underlying substrate.