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theoretical and experimental analysis of silicon nanocrystallites embedded in an amorphous silicon matrix

Abdelaal, Reem (2008) theoretical and experimental analysis of silicon nanocrystallites embedded in an amorphous silicon matrix. PhD thesis, Murdoch University.

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Silicon is one of the world’s most plentiful elements and has found many uses in the electronics industry. Thin films with silicon nanostructure are promising materials for a wide range of applications such as photovoltaic devices. In this thesis theoretical and experimental studies have been carried out on silicon nanocrystallites embedded in an amorphous tissue. Samples with average crystallite sizes (L) in the range (5nm < L ≤16nm) were deposited using hot wire chemical vapor deposition (HWCVD) technique. The samples were classified according to their Raman line shape and average crystallite sizes as: protocrystalline silicon (pc-Si), nanocrystalline silicon (nc-Si) and microcrystalline silicon (μc-Si). Theoretical analysis and experimental techniques have been applied to study the structure of the samples.

The phenomenological phonon confinement (PC) model was applied to the Raman spectra to obtain the average crystallite sizes of all samples. Raman spectroscopy was also used to identify the phases of the materials. A peak fitting procedure was applied to decouple the Raman spectra and obtain the crystalline volume fraction (Xc). Field Emission Scanning Electron Microscope (FESEM) was employed to investigate the nanostructure and the morphology of the films. Ultraviolet-visible spectroscopy was used to measure the effective thickness of the samples.

Treatments and techniques have been applied either to the substrates or the samples as means to control and tailor the properties of the films we deposited and to produce a wider range of sample morphologies and crystallite sizes. These consisted of; grooving substrates by mechanical means, using inductively coupled plasma (ICP) isotropic etching technique and developing an anisotropic alkaline etching technique.

The PC model was used to evaluate the crystallite sizes from the frequency downshift and the peak broadening of the Raman line. To confirm the validity of the model, a comparison was obtained between the modelling prediction and the crystallite sizes as measured from the FESEM images. In this study, the model worked very well in predicting the average crystallite sizes of pc-Si and nc-Si samples. For μc-Si, the experimental data and the theoretical model deviate more evidently.

Grooved substrates possess better crystallization and induce more compact crystal growth than normal substrates. Although, grooving has no significant effect on the crystallite sizes it enhances the coalescence of nanocrystallites and results in larger grain sizes in pc Si and nc-Si samples.

ICP and alkaline etching have been applied, to all types of silicon samples deposited. ICP etching is an isotropic; it etched the three phases of silicon films (i.e., the crystalline grains, the amorphous tissue and the grain boundaries) at the same rate. ICP etching study helped us to predict and draw the nanostructure of the three types of silicon films we are studying. Alkaline solution is a strongly anisotropic etchant. It etched the amorphous tissue at the highest rate and the grain boundaries at a slower rate. Etching the amorphous tissue has enhanced the crystallinity of the silicon samples by increasing the crystalline volume fractions (Xc) while etching the grain boundaries has enhanced the average crystallite sizes.

By applying the PC model and studying the FESEM images of both the grooved and the etched samples, a good agreement was found between the theoretical predictions and the experimental measurements of crystallite sizes.

Theoretical and experimental analyses conducted in this thesis have also clarified the characteristics and the origin of the intermediate phase in pc-Si, nc-Si and μc-Si materials.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Engineering and Energy
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Cornish, John
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