(496g) Nanostructure Pattern Formation in Epitaxially Grown Strained Semiconductor Thin Films As an Outcome of a Nonlinear Surface Morphological Instability

Semiconductor nanostructures such as quantum dots (QDs) and nanorings enable numerous technological applications in electronic and optoelectronic device technologies and data storage systems. A common approach for the formation of such nanostructures on surfaces of thin films deposited epitaxially on thick semiconductor substrates is through the Stranski-Krastanow growth instability that is triggered due to the induced biaxial stress in the film as a result of the lattice mismatch between the deposited film and substrate materials. However, quantum dots formed as a result of Stranski-Krastanow growth instabilities nucleate randomly on the epitaxial 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. 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 the development of an atomistically informed, three-dimensional continuum-scale kinetic model for monitoring the surface morphological evolution of coherently strained epitaxial thin films, which has been validated experimentally by comparisons of its predictions with experimental observations on Ge/Si{100} and InAs/GaAs heteroepitaxial systems employing pit-patterned substrates. We discuss the design of patterns of two pit geometries, namely, inverted truncated conical 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 dimensions, 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 also find 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. The results of our computational analysis are supported by a nonlinear morphological stability theory for the observed nanopattern formation on the film surface as the outcome of a “tip-splitting” instability that accompanies the Stranski−Krastanow instability during epitaxial growth. Our simulation results agree very well with the predictions of the nonlinear tip-splitting instability theory, which provides a comprehensive interpretation and explanation for the findings of the numerical simulations.

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, which are also explained by the nonlinear tip-splitting instability theory. Our findings have important implications for designing optimal semiconductor surface patterns toward enabling future nanofabrication technologies by exploiting nonlinear surface morphological instabilities, and further engineering strategies for the design of ordered nanostructures on epitaxial semiconductor film surfaces are proposed.