Optoelectronic properties of nano-structured silicon carbide prepared by anodic electrochemical etching

Abstract

Silicon carbide (SiC) nanostructures are appealing as non-toxic, water-stable and oxidation resistant nanomaterials. Owing to these unique properties, 3-dimensionally confined SiC nanostructures, namely SiC quantum dots (QDs) have found applications in bioimaging of living cells. Photoluminescence (PL) investigations however revealed that across the polytypes: 3C-, 4H- and 6H-SiC, excitation wavelength dependent PL is observed for larger sizes but deviate for sizes smaller than approximately 3 nm, thus exhibiting a dual-feature in the PL spectra. Additionally, nanostructures of varying polytypes and bandgaps exhibit strikingly similar PL emission centred at approximately 450 nm. At this wavelength, 3C-SiC emission is above bulk bandgap as expected of quantum size effects, but for 4H-SiC and 6H-SiC the emissions are below bandgap. 4H-SiC is a suitable polytype to study these effects. In this thesis, the hypotheses that mixed phases of polytypes or surface related defects obscuring the quantum confinement of 4H-SiC based nanostructure were investigated. Density functional theory (DFT) calculations within the Ab initio Modelling Programme (AIMPRO) were performed on OH-, F- and H-terminated 4H-SiC QDs with diameters in the range of 10 to 20 A° . The chosen surface terminations relate to the HF/ethanol electrolyte used in preparation of SiC QDs and the choice of size coincide with where deviation was observed in experiments. It was found that the absorption onset energies deviated from quantum confinement with -OH and -F terminations, but conform to the prediction when terminated with -H. The weak size-dependent absorption onsets for -OH and -F is due to surface states arising from lone pair orbitals that are spatially localised to the quantum dot surface where these terminations reside. On the other hand -H termination show strong size-dependent absorption onsets due to delocalisation of the electron wavefunction towards the quantum dot core assisting quantum confinement. It is predicted that the surface related states dominate up to sizes 25 and 27 °A for -F and -OH terminations respectively. As a result, the recombination mechanism would involve the interplay between quantum confinement and surface states affecting the resultant energy gap and the resulting PL. The PL would exhibit

a dual-feature: excitation-wavelength independence for small sizes and excitationwavelength dependence for diameters larger than 3 nm as observed in the experiments. Mesoporous 4H-SiC was fabricated by anodic electrochemical etching in ethanoic HF electrolyte. The porous SiC suspended in ethanol exhibited three PL bands, those at wavelengths of 303 nm and 345 nm were rarely reported, above bulk bandgap and indicative of quantum confinement. The usually observed emission at 455 nm was below bulk bandgap. Dual-feature and below bandgap PL observed for wavelengths around 450 nm indicate that mesoporous 4H-SiC exhibited optical properties dictated by both quantum confinement (red-shifting with longer excitation wavelengths) and surface states (below bandgap). X-ray photoelectron spectroscopy provided evidence of -F, C=O and -COOH surface terminations that may contribute to these surface states. Raman scattering data exhibited a red-shift of 12.2 cm􀀀1 and broadening in the lower frequency side of the longitudinal optical (LO) mode peak indicative of carrier depletion, surface phonons or phonon confinement as dimensions were reduced. The following ultrasonication process produced dimensions ranging from 16.9 5.5 down to 2.9 1.0 nm. The data from high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) showed lattice spacing of 0.267 nm and peaks corresponding to the 4H-SiC polytype. No evidence of polytypic transformation from 4H-SiC to 3C-SiC resulting from ultrasonication was found in this work. Instead high crystallinity of 4H-SiC lattice was retained which suggested that the obscured quantum confinement may arise from surface effects rather than mixed polytypes. Thermal oxidation and subsequent HF dip of mesoporous 4H-SiC resulting in pore wall thinning and surface removal was undertaken. Cross sectional SEM analysis showed reductions in average pore wall thickness to (20.5 2.8) nm, (18.2 2.9) nm, (17.0 1.8) nm and (15.9 1.4) nm for 1, 3, 6 and 9 hours of oxidation respectively. Following ultrasonication, the PL and PL excitation (PLE) characterisation showed absorption/emission band centred at 290/325 nm which were above bandgap. The below bandgap emission centred at 455 nm was removed and is a significant finding. Surface removal by thermal oxidation and HF dip resulted in suppressed below bandgap PL but retained the above bandgap PL. The evidence strongly indicate that the dual-feature PL and below bandgap emission in 4H-SiC seen in experiments are surface related rather than due to polytypic transformation during ultrasonication