(596h) Formation of Quantum Dots on Epitaxial Semiconductor Films Grown on Pit-Patterned Substrates

Semiconductor quantum dots enable a broad range of applications in electronic and optoelectronic device technologies. Quantum dots formed as a result of Stranski-Krastanow growth instabilities nucleate randomly on an epitaxial grown film surface and lack uniformity in their size and arrangement, which is undesirable for applications where uniform positioning and ordering of quantum dots is required. To self-assemble uniformly arranged and consistently sized quantum dots, numerous recent studies have explored strategies for guiding quantum dot formation in epitaxial films by depositing them on substrates with a modified morphology. Following such a strategy, recent experimental studies have demonstrated the formation of ordered patterns of Ge quantum dots on surfaces of Ge thin films grown epitaxially on pit-patterned Si{100} substrates.

Here, we report the development of an atomistically informed, three-dimensional continuum-scale kinetic model for monitoring the surface morphological evolution of coherently strained heteroepitaxial thin films that captures the morphological response of Ge thin films grown epitaxially on pit-patterned Si{100} substrates. The model accounts for curvature-driven atomic diffusion on the film surface, biaxial strain in the film due to its lattice mismatch with the substrate, and the wetting potential between the film and the substrate. Self-consistent dynamical simulations based on our model show formation of complex nanostructures on the epitaxial film surface, including nanorings at the rims of pits, a single quantum dot at the center of a pit, as well as multiple quantum dots inside pits with rectangular openings, consistent with experimentally observed nanostructures. Our simulation results reproduce the variation in the formed nanostructural features observed experimentally by properly varying the pit size and geometry and, therefore, validate our epitaxial film surface evolution model.

Another recent experimental study has reported the formation of linear arrays of multiple quantum dots inside elongated pits on the deposited film surface, with unequal pit wall inclinations along the two principal pit directions. The experiments have also shown that the number of quantum dots forming inside the pits increases with increasing length of the longer side of the pit. Based on self-consistent numerical simulations according to our validated film surface evolution model, we demonstrate that the number and features of quantum dots growing inside each pit can be controlled by properly designing the pits on the substrate surface through tuning of the pit geometrical parameters, especially the pit opening dimensions and pit wall inclinations. In agreement with the experimental observations, we find that, for sufficiently steep pit wall slopes, the number of quantum dots forming inside each pit can be tuned by varying the length of the longer side of the elongated pit. The simulation results are explained fully by a nonlinear morphological stability theory.