Electron Microscopy and Density Functional Theory Investigations of Degenerate Boron Doped Silicon

Abstract

This thesis investigates the impact of degenerate boron doping on vibrational and scattering properties of crystalline silicon using a combined approach involving electron microscopy, multislice simulations, and density functional theory (DFT). Momentum-resolved and momentum-integrated electron energy loss spectroscopy measurements revealed broad, high-energy optical phonon modes centred around 132 meV, features absent in intrinsic silicon. DFT simulations attributed these signatures to localised
vibrational modes arising from neutral interstitial boron clusters, particularly compact configurations such as three boron neutral interstitial clusters (Configuration 1 and 2), which produced non-dispersive modes in agreement with experimental data. Selected area diffraction patterns (SADPs), supported by multislice simulations, were used to probe diffuse scattering behaviour in doped silicon. At low scattering angles, enhanced Kikuchi lines and increased background intensity were observed without Bragg peak broadening, consistent with localised strain from boron incorporation. Simulations confirmed that both substitutional and interstitial defects contributed to this behaviour. At high scattering angles, an anomalous
increase in diffuse intensity was observed in doped silicon. This enhancement, absent in frozen phonon simulations, was attributed to boron-induced localised phonon modes. Cryogenic experiments confirmed this interpretation. The final study provides a foundational step in bridging theoretical and experimental thermal diffuse scattering
(TDS). First-order TDS profiles computed from DFT were fitted to experimental data, and empirical corrections using Gaussian and exponential models significantly improved agreement. These findings demonstrate the feasibility of integrating phonon theory with experimental electron scattering in doped silicon.