(310f) Nanostructure Pattern Formation on Epitaxial Semiconductor Films Grown on Pit-Patterned Substrates

Semiconductor nanostructures such as quantum dots (QDs) and nanorings enable numerous technological applications in electronic and optoelectronic device technologies and data storage systems. The Stranski-Krastanow (SK) growth instability is a common approach to trigger the formation of such nanostructures on surfaces of thin films deposited epitaxially on thick semiconductor substrates due to the induced biaxial stress in the film as a result of the lattice mismatch between the deposited film and substrate materials. However, the QDs forming through SK growth instabilities are randomly arranged on the film surface, and their size and distribution are not uniform, while uniform positioning and ordering of quantum dots is required for device fabrication purposes. Recently, numerous strategies have been studied for guiding the growth of QDs that are uniformly arranged and consistently sized. Experimental studies have shown that, among such strategies, depositing thin films epitaxially on properly engineered pit-patterned substrate surfaces is a promising, effective approach toward the assembly of ordered nanostructures.

Here, we report results for the surface morphological evolution of coherently strained Ge thin films grown epitaxially on pit-patterned Si{100} substrates. Our analysis is based on self-consistent dynamical simulations according to an atomistically-informed, 3D continuum-scale epitaxial film surface evolution model that has been validated experimentally by comparisons of its predictions with experimental observations on Ge/Si and InAs/GaAs heteroepitaxial systems employing pit-patterned substrates. We discuss the design of patterns of two pit geometries, namely, inverted truncated conical pits and pyramidal pits, and the effects on the resulting film surface nanopattern of varying the relevant geometrical design parameters including film thickness, pit-pattern period, pit depth, pit opening size, and pit wall inclination. For conical pits, we find that varying the pit opening diameter and the pit wall slope leads to formation of complex nanostructures inside the pits of a regular pit pattern on the film surface, which include QDs, as well as single nanorings and multiple concentric nanorings that may or may not surround a central QD inside each pit. For pyramidal pits, we show that varying the pit opening length and width and the pit wall inclination can cause the formation of nanostructures inside regularly arranged pits on the film surface that include equi-spaced smaller pits or rectangular arrays of multiple QDs, also known as quantum dot molecules. We have also shown that using pits with different pit wall inclinations along the two principal pit directions leads to the formation of linear arrays of multiple QDs inside elongated pits on the deposited film surface, in agreement with a recent experimental study. Our simulation results, over the entire parameter range examined, are explained fully by a nonlinear morphological stability theory.

Furthermore, we have studied the effect of pit-pit interactions on the epitaxial film surface for films grown on pit-patterned substrates. We find that the pit-pit interaction energetics follows a power law, with the interaction energy decreasing with increasing pit separation distance (i.e., pit-pattern period), and with the interaction strength increasing with increasing pit size. We find that the substantial increase in the strain energy density of the epitaxial thin film with decreasing pit pattern period affects qualitatively the epitaxial film surface morphology, leading to the formation of nanostructure patterns on the rim of each surface pit as well as on the film surface between neighboring pits. Detailed characterization of the resulting film surface nanostructure patterns is carried out for pits with steep walls over a range of pit depths and pit opening sizes, and further engineering strategies for the design of ordered nanostructures on epitaxial semiconductor film surfaces are proposed.