Epitaxial thin film growth refers to the process of depositing a crystalline film onto a substrate in a way that maintains a structural alignment between the film and the substrate. There are several epitaxial growth modes that describe the relationship between the film and substrate during the deposition process. The most common epitaxial growth modes are:
Layer-by-layer (Frank-van der Merwe): In this growth mode, atoms or molecules from the incoming film adsorb on the substrate surface and form a complete monolayer. The subsequent layer then grows on top of the first layer, resulting in a well-defined layer-by-layer growth. This mode is typically observed when the lattice mismatch between the film and substrate is small.
Island (Volmer-Weber): In the island growth mode, the incoming film atoms or molecules initially form small islands or clusters on the substrate surface. As the deposition continues, these islands grow in size and eventually coalesce to form a continuous film. Island growth occurs when the lattice mismatch between the film and substrate is large, leading to the formation of strained islands.
Step-flow (Bertaut): Step-flow growth occurs when the surface of the substrate contains steps or terraces. Atoms or molecules from the film adsorb preferentially at the step edges and migrate along the steps, filling the terraces as they go. This growth mode is commonly observed when the film and substrate have a small lattice mismatch and the substrate surface has well-defined steps.
Stranski-Krastanov: The Stranski-Krastanov growth mode is a combination of the island and layer-by-layer growth modes. Initially, small islands form on the substrate surface due to lattice mismatch. As the film thickness increases, these islands coalesce to form a continuous layer. This mode is observed when the lattice mismatch is moderate, leading to the formation of strained islands that eventually transform into a relaxed film.
It's important to note that the growth mode observed during epitaxial thin film growth can be influenced by factors such as temperature, deposition rate, surface preparation, and lattice mismatch between the film and substrate. Understanding and controlling the growth mode is crucial for achieving high-quality epitaxial thin films with desired properties.
