Thermally Induced Nano-Structural and Optical Changes of nc-Si:H Deposited by Hot-Wire CVD

We report on the thermally induced changes of the nano-structural and optical properties of hydrogenated nanocrystalline silicon in the temperature range 200–700 °C. The as-deposited sample has a high crystalline volume fraction of 53% with an average crystallite size of ~3.9 nm, where 66% of the total hydrogen is bonded as ≡Si–H monohydrides on the nano-crystallite surface. A growth in the native crystallite size and crystalline volume fraction occurs at annealing temperatures ≥400 °C, where hydrogen is initially removed from the crystallite grain boundaries followed by its removal from the amorphous network. The nucleation of smaller nano-crystallites at higher temperatures accounts for the enhanced porous structure and the increase in the optical band gap and average gap

Thermal annealing of protocrystalline a-Si:H

It proves difficult to obtain a set of protocrystalline silicon materials with different characteristics from the same deposition chamber to study the exact nature of these transition region materials. Hot-wire deposited protocrystalline silicon was thus isochronically annealed at different temperatures to investigate the bonded hydrogen configurations and structural disorder. Modeling of optical reflection and transmission spectra with Scout® yielded the optical parameters and infrared spectroscopy confirms that bonded hydrogen remains in the material, with the exception of a longer anneal of six hours at 520 °C. Sub bandgap absorption as inferred from photothermal deflection spectroscopy was related to these characteristics

Synthesis of nanocrystalline silicon thin films using the increase of the deposition pressure in the hot-wire chemical vapour deposition technique

Nanostructured thin silicon-based films have been deposited using the hot-wire chemical vapour deposition (HWCVD) technique at the University of the Western Cape. A variety of techniques including optical and infrared spectroscopy, Raman scattering spectroscopy, X-rays diffraction (XRD) and transmission electron microscopy (TEM) have been used for characterisation of the films. The electrical measurements show that the films have good values of photoresponse, and the photocurrent remains stable after several hours of light soaking. This contribution will discuss the characteristics of the hydrogenated nanocrystalline silicon thin films deposited using increased process chamber pressure at a fixed hydrogen dilution ratio in monosilane gas

Employing the effective medium approximation to model the optical properties of crystallized a-Si:H obtained by MIC

Metal induced crystallization of hydrogenated amorphous silicon has been the subject of intense scrutiny in recent years. In this contribution we report on the metal-mediated-thermally induced changes of the structural and optical properties of hydrogenated amorphous silicon deposited by hot-wire CVD, where aluminium and nickel were used to induce crystallization. The metal-coated amorphous silicon was subjected to a thermal annealing regime of between 150 and 520°C. The structural measurements, obtained by Raman spectroscopy, show partial crystallization occurring at 350 °C. At the higher annealing temperatures of 450°C and 520°C complete crystallization occurs. Reflection and transmission measurements in the UV-visible range were then used to extract the optical properties. By adopting the effective medium approximation a single optical model could be constructed that could successfully model material that was in different structural phases, irrespective of metal contamination. Changes in the absorption of the material in various stages of transition were confirmed with a directly measured absorption technique, and the modelled absorption closely followed the same trends

Sum-frequency spectroscopy and imaging of aligned helical polypeptides

The sum-frequency spectroscopy signatures of NH-(amide A) and C = O (amide I) groups, the amide segments in all proteins, are measured in thin films that consist of an ensemble of right-handed, helical poly-gamma-benzyl-L-glutamate (PBLG) macromolecules that are endgrafted and self-organized into a monomolecular film with a large degree of unidirectional order. Distinct sum-frequency spectral signatures associated with the amide A and the amide I bands are observed because of a strong noncentro-symmetry produced by intra- and intermolecular forces. Hydrogen bonding self-organizes amino and acidic groups within the molecular helical scaffold. In an endgrafted thin film, repulsive electrostatic forces between PBLG macromolecules stabilize the organization between molecules. The average orientation of the PBLG chain was measured. Imaging scans using sum-frequency generation, complemented by atomic force microscopy, were used to investigate the uniformity of orientation of the PBLG chains

Optical Dispersion Properties of Tricyanovinylaniline Polymer Thin Films for Ultrashort Optical Pulse Diagnostics

We have investigated a series of tricyanovinylaniline (TCV) polymer thin films for their use in ultrashort optical pulse (USP) diagnostics of femto second Ti:Sapphire lasers. These thin films are ideally suited for USP diagnostics since they eliminate the angle tuning associated with birefringent phase-matched crystals, minimize pulse distortion introduced by group velocity dispersion, and exhibit excellent photochemical stability. The linear optical dispersion of these polymers can be tailored over a wide range for efficient and distortionless frequency conversion. Coherence lengths between 420 nm and 54 microns at a wavelength λ = 800 nm have been found for the two extreme cases of dispersion in these materials. Film thicknesses of at least two microns are tolerable without introducing any significant pulse distortion at the same wavelength (λ = 800 nm)

Optical Dispersion Properties of Tricyanovinylaniline Polymer Thin Films for Ultrashort Optical Pulse Diagnostics

We have investigated a series of tricyanovinylaniline (TCV) polymer thin films for their use in ultrashort optical pulse (USP) diagnostics of femto second Ti:Sapphire lasers. These thin films are ideally suited for USP diagnostics since they eliminate the angle tuning associated with birefringent phase-matched crystals, minimize pulse distortion introduced by group velocity dispersion, and exhibit excellent photochemical stability. The linear optical dispersion of these polymers can be tailored over a wide range for efficient and distortionless frequency conversion. Coherence lengths between 420 nm and 54 microns at a wavelength λ = 800 nm have been found for the two extreme cases of dispersion in these materials. Film thicknesses of at least two microns are tolerable without introducing any significant pulse distortion at the same wavelength (λ = 800 nm)