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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Strength and deformation of coherently strained superlattices

Author: P'NG, Mok Yew

Year: 2004

Supervisor(s): Andy Bushby

Deformation of small volumes and minimum volumes required for deformation are interesting and relevant for MEMs and nanotechnology. Electronic-grade single-crystal semiconductor structures provide a means to study this. Highly strained superlattices that are buried under lattice matched material gives us the ability to map out the initial volume of plastic deformation by spherical nanoindentation. We show that the initial deformation volume can be measured.

Strength of thin coherently strained InGaAs superlattices grown on thick InP substrates was measured in three-point bending at 500°C. We found the beams to be significantly strengthened by the presence of superlattices. The superlattices are found to display mechanical strength up to a hundred times greater than the strength of the bulk substrate material. This can be attributed only to the coherency strain in the superlattices.

Post-analysis of the bent beams were preformed using focused ion beam-transmission electron microscopy (FIB-TEM) to determine the mechanism of deformation. We found that dislocations are sufficiently mobile to create shear steps in the superlattices. FIB-TEM was also carried out on the area beneath nanoindented superlattices to reveal that, in contrast, twinning is the dominant deformation process.

Strain analysis of the bent superlattices was measured using high resolution X-ray diffraction. Results were inconclusive due to the limited amount of data. However, we can report that superlattices underwent relaxation; and superlattices with larger strains lose more strain when it is bent in compression. In contrast, larger strained superlattice retained most of its trains when bent in tension.

The size effect is measured using nickel foils in flexure. The relationship between yield strength and foil thickness has the same shape as Dunstan and Bushby’s (in press) mathematical prediction. However, the mathematical prediction is a factor of three too low compared to our results.