(262b) Driven Morphological Evolution of Strained Thin Film Surfaces and Two-Dimensional Materials: Morphological Stability and Pattern Formation

Mass transport in solid materials driven by externally applied fields, such as stresses, electric fields, and temperature gradients, can cause morphological instabilities, leading to failure of materials used in electronic and optoelectronic devices. However, properly controlled applied fields can also stabilize planar surface morphology, reduce surface roughness, and drive the formation of intriguing nanoscale morphological features, providing a path toward precise nanopatterning for the development of electronic and photonic materials with optimal functionality. In this presentation, we report our recent work on driven morphological evolution of epitaxial thin film surfaces and two-dimensional materials.

Thin film surface roughness is responsible for various materials reliability problems in microelectronics and nanofabrication technologies, which requires the development of surface roughness reduction strategies. Toward this end, we have conducted a modeling and simulation study that established the electrical surface treatment of conducting thin films as a viable physical processing strategy for surface roughness reduction. We have developed a continuum model of surface morphological evolution that accounts for the residual stress in the film, surface diffusional anisotropy and film texture, the film’s wetting of the layer that is deposited on, and surface electromigration. Supported by linear stability theory, self-consistent dynamical simulations based on the model have demonstrated that the action over several hours of a sufficiently strong and properly directed electric field on a conducting thin film can reduce its nanoscale surface roughness and lead to an atomically smooth planar film surface. The modeling predictions are in agreement with experimental measurements on copper thin films deposited on silicon nitride layers.

In addition, we have developed a theory for the experimentally observed formation of multiple quantum dots (QDs) in strained-layer heteroepitaxy based on surface morphological stability analysis of a coherently strained epitaxial thin film on a crystalline substrate. Using a fully nonlinear model of surface morphological evolution that accounts for a wetting potential contribution to the epitaxial film’s free energy as well as surface diffusional anisotropy, we have demonstrated the formation of multiple QD patterns in self-consistent dynamical simulations of the evolution of the epitaxial film surface perturbed from its planar state. The simulation predictions are supported by weakly nonlinear analysis of the epitaxial film surface morphological stability. We have found that, in addition to the Stranski-Krastanow instability, long-wavelength perturbations from the planar film surface morphology can trigger a nonlinear instability, resulting in the splitting of a single QD into multiple QDs of smaller sizes, and predicted the critical wavelength of the film surface perturbation for the onset of the nonlinear tip-splitting instability. The theory provides a fundamental interpretation for the observations of “QD pairs” or “double QDs” and other multiple QDs reported in experimental studies of epitaxial growth of semiconductor strained layers, such as the multiple QD patterns on the surface of an epitaxial film grown on a patterned substrate. The theory also sets the stage for precise engineering of tunable-size nanoscale surface features in strained-layer heteroepitaxy by exploiting film surface nonlinear, pattern forming phenomena. We have also used this model for detailed simulation of the kinetics of nanoring formation from QDs in coherently strained epitaxial films upon their thermal annealing and to conduct a systematic exploration of the resulting nanoring structures upon variation of the processing conditions.

Moreover, we have conducted a systematic analysis of pore-edge interactions in graphene nanoribbons (GNRs) using first-principles density functional theory (DFT) calculations and molecular-statics computations based on reliable interatomic potentials, which has revealed strongly attractive interactions for nanopores in the vicinity of GNR edges. These attractive interactions provide the thermodynamic driving force for nanopore migration toward the GNR edge, leading to its coalescence with the GNR edge through a sequence of carbon ring reconstructions. We have studied nanopore dynamics near GNR edges in detail through molecular-dynamics (MD) simulations at high temperature. We have constructed the optimal kinetic pathways of the mechanisms mediating the coalescence of the nanopore and the GNR edge, as identified by the MD simulations, using climbing-image nudged elastic band calculations. The post-coalescence morphological evolution of an armchair GNR edge leads to the formation of a V-shaped edge pattern consisting of zigzag linear segments (facets). DFT calculations of the electronic band structure of such patterned GNRs show that the zigzag segments formed at the armchair edges can be used to tune the bandgap of the GNR. The bandgap of the patterned GNRs exhibits a linear dependence on the density of the zigzag edge atoms, which is controlled by the size and concentration of the pores introduced in the defect-engineered GNR.