Mounded Growth of Organic Semiconductor Thin Films: Desorption and Next-Layer Nucleation

Abstract

The mechanisms governing thin film growth play a crucial role in determining the final properties of thin films. Advanced techniques, such as in situ coherent x-ray scattering, offer valuable insights into the surface dynamics associated with these growth mechanisms. A well-ordered, smooth thin film is highly desirable due to its superior physical properties. However, molecular diffusion on the film\u27s surface is hindered by the Ehrlich-Schwoebel barrier, which inhibits the downward movement of molecules across step edges. This leads to the formation of mounds that steepen as the film grows due to restricted inter-layer transport. This dissertation presents in situ x-ray photon correlation spectroscopy (XPCS) measurements of diindenoperylene (DIP) vapor deposition on thermally oxidized silicon surfaces across a temperature range of 40 $^{\circ}$C to 120 $^{\circ}$C. Our observations indicate that DIP forms a nearly complete two-dimensional first layer before transitioning into mounded growth during subsequent deposition stages. Within these mounds, local step flow was observed, revealing terrace-length-dependent behavior in the step edge dynamics. This terrace-length dependence led to unstable growth, characterized by rapid roughening ($\beta$ \u3e 0.5) and a deviation from a symmetric error-function-like height profile. At higher temperatures, we observed that grooves between the mounds tended to heal , resulting in nearly flat, poly-crystalline films. A numerical analysis using a (1 + 1) - dimensional model suggests that terrace-length-dependent desorption of ad-molecules plays a key role in influencing step dynamics and morphology evolution. Desorption, the process in which surface atoms (adatoms) gain enough energy to escape into the vacuum, is found to suppress growth rates during thin film formation. Additionally, desorption time scales, when shorter than the time required for diffusion to defect sites, step edges, or kinks, significantly influence surface morphology during crystal growth. New layer nucleation, a critical process in mounded thin film growth, is examined using a lab-developed Kinetic Monte Carlo (KMC) growth simulation program. Our results suggest that larger nucleation critical sizes at higher temperatures contribute to the transition from mounded to flat films. By comparing experimental data with KMC simulation results and theoretical models, we estimate key parameters such as the additional Ehrlich-Schwoebel energy barrier ($\Delta E_{s}$) and the molecular thermal attempt rate ($v_{0}$). These findings enhance our understanding of the mechanisms underlying mounded growth and offer potential insights for improving thin film growth in materials beyond DIP, paving the way for more controlled and high-quality film production