Quantitative analysis of lattice strains and crystal polarity in semiconductor nanomaterials using convergent beam electron dffraction

In this thesis, different Convergent Beam Electron Diffraction (CBED) methods were developed and verified for the investigation of local lattice strains between ≈ 10nm wide Si1-xGex-Si Strained Layer Superlattice (SLS) layers and crystal polarity of novel CdS nanostructures. As part of strain calculations, the shifts & splitting of Higher Order Laue Zone (HOLZ) lines were quantified for the deformation along the electron beam direction and the asymmetric crystal growth of CdS structures was studied based on the analytical intensity calculations performed using double diffraction phenomenon and many-beam intensity profiles plotted using JEMS® software. The lattice strain evaluation methodology involves extraction of HOLZ lines using Hough transformation and quantifying the shifts in HOLZ line positions for a unique set of lattice parameters/strain variations. Due to the appearance of split HOLZ lines at the outer interfaces of the SLS structures, the quantification method adopted for HOLZ lines shifts cannot be applied for this particular case. It is shown that the splitting of HOLZ lines are related to the surface relaxation occurred during the Transmission Electron Microscope (TEM) specimen preparation and using the kinematical simulation, the experimental magnitude of the split HOLZ lines is well reproduced and subsequently, the specimen deformation is determined. The quantification methodologies of shift and split HOLZ lines are validated by comparing the determined profiles to Finite Element simulations that were modelled by considering the surface relaxations in TEM thin foils. Based on the appearance of the experimental CBED patterns, it is shown that even at relatively thicker regions of TEM specimen, there exists a impact of surface relaxation on the SLS structure and is demonstrated that the best achievable accuracy of the evaluated lattice parameters are in the range of 2 – 4 x 10-4 nm. Based on a simple method that requires no specimen tilt from its exact zone axis orientation, the crystal polarity of CdS nanostructures is experimentally determined. The characterization technique is mainly dependent upon the multiple scattering among the zone axis reflections which has caused the asymmetric intensity distribution in 0002 beams. It is shown that the considered method is viable for a crystal thickness of 16nm which is less than one extinction distance of ±0002 reflections.