Heteroepitaxial Nanomaterials

The department is currently extending its research areas to nanoscale characterization and analysis on heteroepitaxial semiconductor materials led by the newly appointed assistant professor Dr. Chi-Chin Wu. Dr. Wu’s recent research activities focus on nanoscale analysis for heteroepitaxial semiconductor thin films such as Si-Ge alloy on Si substrates utilizing different characterization skills and instruments such as simulations/modeling, chemical analysis, and microscopic experiments.  

Heteroepitaxial thin films are important for the fabrication of advanced devices and as model systems for understanding fundamental physical and chemical phenomena. Depending on the growth conditions, film thickness, and extent of lattice mismatch at the film/substrate interface (determined by the film composition), during growth the film will undergo different mechanisms to help release the stored elastic strain energy. These mechanisms include surface roughening, interdiffusion, and/or formations of misfit dislocations and have been a centerpiece of intensive advanced research activities worldwide. They are schematically shown in the figures below.

Surface roughening	Dislocation formation	Inter-diffusion

The developing research schemes here at VMI will surround quantitative measurements and characterization of these mechanisms for different types of structures. These are a few examples of microscopic images with Si0.7Ge0.3 thin films (30~50 nm thickness).

Plan view TEM image showing surface undulations	A cross-sectional view of surface undulation under
Plan view TEM image showing straight dislocation lines	AFM topography showing surface undulations
Plan view TEM image showing the very tip of a misfit dislocation line

The goal of simulations/modeling in this lab is twofold: to quantify local phenomena (such as stresses and dislocation shapes) in a thin structure after growth and to identify the dynamic structural evolutions during growth. In the past, we have explored the finite element method and numerical calculations. For the near future, we would like to explore atomistic simulations via Monte Carlo.

Due to her diverse background but recent focus on nanoscale research, Dr. Wu’s future research activities will be of a cross-disciplinary nature important in advancing the innovations and applications of nanomaterials. In addition to thin films, she is also interested in exploring other nanostructures such as nanowires, nanotubes, and nanoporous materials. In this lab, students can expect to learn fundamental but hands-on nanomaterials characterization skills and techniques including simulations, specimen preparations for microscopic experiments such as TEM and AFM, and operational principles for these microscopies.