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Title: Strain relaxation mechanisms and stress-balancing of SiGe heterostructures
Author: Turner, Stephen George
ISNI:       0000 0001 3539 8993
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2008
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Until now, progress of the microelectronic industry has been maintained by scaling devices. Physical limits now necessitate alternative approaches such as enhancing material properties through strain engineering or increasing functionality through use of Si/SiGe heterostructures (e.g. optoelectronic integration), requiring control of strain, which may enhance electrical properties but can also lead to deleterious structural and electronic effects. Strain can be controlled via growth of stress-balanced structures on a fully strain relaxed virtual substrate (VS) which acts as a growth template for, and maintains strain in, the active layers, which need to be ordered and defect free. The crystalline quality of the VS therefore impacts directly upon the device. These basic elements are realised in a molecular beam epitaxy system using gaseous hydride precursors. Compositionally graded VS are used to investigate strain-relaxation processes. High resolution x-ray diffraction (XRD) allowed examination of lattice reciprocal space, where compositional and strain information is dispersed. Localised distortions of the lattice revealed the mosaic nature of the structure. Nomarski contrast microscopy and atomic force microscopy enabled quantitative analysis of surface defects and roughness, including characteristic crosshatch. VS grading rates, growth temperatures and precursor fluxes were varied to assess relaxation kinetics. The nucleation and propagation of strain-relieving dislocations were investigated through the growth of different VS structures on vicinal Si(001) substrates. XRD analysis showed tilting of the epitaxial layer with respect to the substrate. A correlation of tilt angle with offcut angle was observed, following energy barrier asymmetries on the available slip systems. Development of the tilt was seen to be a non-linear function of both Ge composition and grading rate, and was explained by interdislocation interactions. Stress-balancing was investigated by designing distributed Bragg reflector stacks. Structures without detrimental strain relief were achieved, with a reflectance in excess of those reported in the literature, demonstrating the applicability of the VS growth and stress-balance methods. The elastic properties of SiGe could also be probed via this approach, and small deviations from linear interpolation of bulk Si and Ge values were found, supported by theoretical modelling.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available