Strain engineering on silicon/germanium nanoscale heterostructures using molecular dynamics

Yumi Park, Purdue University

Abstract

Nanoscale architectures provide additional variables to engineer electronic/mechanical properties of material systems due to their high surface volume ratio and physics that arise from their extremely small size. To date, the device performance of microelectronics has been improved largely by miniaturization. However, with feature sizes below 100 nm, the fundamental challenges demand development of new architectures, new materials, and strain engineering. Strain engineering has been one of the most widely used techniques to achieve desired electronic properties of materials. For example, uniaxial compression and tension are desirable for high speed p-and n-MOSFET, respectively. However, accurate experimental characterization of strain in nanomaterials remains challenges such as resolving strain components and quantifying strain gradient which can affect electronic properties. Molecular dynamics (MD) describe materials with atomic resolution and it can provide invaluable information and insight into nanoscale strain engineering. MD simulations are used to study strain relaxation in Si/Ge heteroepitaxial structures of interest to nanoelectonic applications. Nanopatterning is considered as an avenue for strain engineering to achieve uniaxial strain state from epitaxially integrated Si/Ge heterostructures. Using MD, it is studied how size affect strain relaxation on strained Si/Ge/Si nanobars representing the structures obtained by patterning the films in nanoscale. The MD results demonstrate that Ge with a roughly square cross section has a uniaxial strain state desirable for hole mobility enhancement. Also, process-induced strain relaxation on Si/Ge heterostructures is discussed. The simulations suggest that, by engineering the aspect ratio of Si/Ge nanolaminates, local amorphization followed by recrystallization can be used for either preserving the engineered strain or achieving the desired strain state in crystalline region, showing a possibility as a new avenue for strain engineering. Finally, MD is used to explore surface roughness effect on the coherency limit of Si-core/Ge-shell nanowires. Rough surface in the nanowires with larger core size promotes defect nucleation. After coherency loss, transverse strains along on the plane normal to the wire direction are relaxed regardless of core size and surface roughness. However, the axial strain relaxations is only observed in the nanowires with the highest surface roughness. The defects types responsible for the strain relaxation are also analyzed. In summary, the MD simulation provides a lot of abilities to study materials behavior in nanoscale. Also, atomic level analysis such as local strain calculation and defects analysis help us to have physical insight of nanostructures to achieve the desired materials properties leading to maximized device performance.

Degree

Ph.D.

Advisors

Strachan, Purdue University.

Subject Area

Materials science

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