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In order to create more sensitive and accurate gas sensors, we have studied the interactions of gas mixtures on metal oxide nanoparticle decorated porous silicon interfaces. The nanoparticles control the magnitude and direction of electron transduction from the interaction of analyte gases to an extrinsic porous silicon semiconductor. These interactions can be predicted by the Inverse Hard Soft Acid Base (IHSAB) principle. Moreover, the metal oxide nanoparticles can be functionalized with nitrogen and sulfur, modifying the oxide’s band structure. These modifications are demonstrated to change analyte interactions in line with the IHSAB concept and allow for light enhanced sensors. Further we looked at how the analyte gases interact with other analyte gases on the surface of these sensors. Studying these systems does two things, first the research will lead to cheaper, more accurate gas sensors, and second it helps explore the role of nanoparticles in modifying the interactions between bulk materials (porous silicon) and molecules (analyte gases).

by Cheung Wing-yiu. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves [170-185]). / ACKNOWLEDGMENT --- p.I / abstract --- p.II / contents --- p.IV / figure captions --- p.C-1 / table captions --- p.C-10 / photo captions --- p.C-11 / Chapter CHAPTER 1 --- INTRODUCTION --- p.1 / Chapter 1.1 --- Novel Silicon-Based Materials Structures - Background and Perspectives --- p.2 / Chapter 1.2 --- Light Emission from Porous Silicon --- p.3 / Chapter 1.2.1 --- Quantum size effect --- p.7 / Chapter 1.2.2 --- Chemical luminescence model --- p.9 / Chapter 1.3 --- Germanium Silicon Alloy --- p.11 / Chapter 1.3.1 --- Formation of germanium silicon alloy by ion implantation --- p.16 / Chapter 1.4 --- Scope of this Work --- p.19 / Chapter CHAPTER 2 --- EXPERIMENTAL METHODS --- p.20 / Chapter 2.1 --- Preparation of Porous Silicon Layers --- p.20 / Chapter 2.1.1 --- Anodization --- p.21 / Chapter 2.1.2 --- Post - anodization treatments --- p.25 / Chapter 2.2 --- Preparation of Germanium Silicon Alloy --- p.27 / Chapter 2.2.1 --- Ion implantation --- p.27 / Chapter 2.2.2 --- Thermal treatment --- p.27 / Chapter 2.3 --- Characterization Methods --- p.28 / Chapter 2.3.1 --- Microscopy studies --- p.28 / Chapter 2.3.2 --- Structural studies --- p.30 / Chapter 2.3.3 --- Compositional studies --- p.31 / Chapter 2.3.4 --- Electron spin resonance --- p.32 / Chapter 2.3.5 --- Optical methods --- p.36 / Chapter 2.3.6 --- Electrical measurements --- p.38 / Chapter 2.3.6.1 --- Spreading resistance profiling --- p.38 / Chapter 2.3.6.2 --- Other electrical measurements --- p.40 / Chapter CHAPTER 3 --- POROUS SILICON - RESULTS --- p.41 / Chapter 3.1 --- General observation of on the Appearance of Samples --- p.41 / Chapter 3.2 --- Formation Current Voltage Characteristics --- p.41 / Chapter 3.3 --- Surface Morphology --- p.52 / Chapter 3.4 --- Electron Spin Resonance --- p.56 / Chapter 3.5 --- Composition Characteristics --- p.68 / Chapter 3.6 --- Optical Characteristics --- p.72 / Chapter 3.6.1 --- Infra-red transmittance studies --- p.72 / Chapter 3.6.2 --- Photoluminescence --- p.74 / Chapter 3.7 --- Electrical Properties --- p.82 / Chapter CHAPTER 4 --- POROUS SILICON - DISCUSSION --- p.84 / Chapter 4.1 --- Formation Properties --- p.84 / Chapter 4.2 --- Structural Properties --- p.87 / Chapter 4.3 --- Paramagnetic Centres in Porous Silicon --- p.88 / Chapter 4.4 --- Compositional Properties --- p.93 / Chapter 4.5 --- Photoluminescence --- p.95 / Chapter 4.6 --- Electrical Properties --- p.105 / Chapter 4.7 --- Summary --- p.106 / Chapter CHAPTER 5 --- GERMANIUM SILICON ALLOY - RESULTS --- p.108 / Chapter 5.1 --- Structural Characteristics --- p.108 / Chapter 5.1.1 --- Defect structure --- p.109 / Chapter 5.1.2 --- Crystal structure --- p.115 / Chapter 5.2 --- Optical Characteristics --- p.127 / Chapter 5.3 --- Electrical characteristics --- p.129 / Chapter 5.3.1 --- Spreading resistance profiling --- p.129 / Chapter 5.3.2 --- Other electrical measurements --- p.138 / Chapter CHAPTER 6 --- GERMANIUM SILICON ALLOY - DISCUSSION --- p.142 / Chapter 6.1 --- Structure Analysis --- p.142 / Chapter 6.2 --- Optical Properties --- p.146 / Chapter 6.3 --- Electrical Properties --- p.147 / Chapter 6.4 --- Summary --- p.150 / Chapter CHAPTER 7 --- CONCLUSIONS --- p.152 / Chapter 7.1 --- Porous Silicon --- p.152 / Chapter 7.2 --- Germanium Silicon Alloys --- p.154 / Chapter CHAPTER 8 --- FURTHER WORK --- p.156 / Chapter 8.1 --- Porous Silicon --- p.156 / Chapter 8.2 --- Germanium Silicon Alloys --- p.156 / APPENDIX / Chapter I --- SPECTRA OF GERMANIUM SILICON ALLOY --- p.A1 / Chapter 1.1 --- Rutherford Backscattering Spectra --- p.A2 / Chapter 1.2 --- Spreading Resistance Depth Profile --- p.A8 / Chapter II --- PUBLICATIONS --- p.A14 / BIBLIOGRAPHY --- p.A15

Vacuum microelectronic (VME) devices are of interest for the development of flat-screen displays and microwave devices. In many cases, their operation depends on the field emission of electrons from micron-sized cathodes (semiconductor or metal), into a vacuum. Major challenges to be met before these devices can be fully exploited include obtaining - low operating voltages, high maximum emission currents, uniform emission characteristics, and long-term emission stability. The research in this thesis concerns the production of silicon field emitters and the improvement of their emission properties by the process of anodisation. Anodisation was carried out for short times, in order to form a very thin layer of porous silicon (PS) at the surface of both p and p⁺-type silicon emitters. The aim in doing this was to form a high density of asperities over the surface of the emitters. It was the intention that these asperities, rather than the "macroscopic" apex of the emitter, would control emission. This was the first work of its kind to be carried out. Transmission electron microscopy was used to characterise the morphology of p and p⁺-type silicon emitters before and after anodisation. Both the structure and arrangement of the surface fibrils, the thickness of the PS layers at the apex and nature of PS cross-sections were studied. The morphology was correlated to subsequent field emission measurements. Field emission characteristics, before and after anodisation, were obtained using a scanning electron microscope adapted for field emission measurements, and a field emission microscope. Extensive measurements showed that, following anodisation, there was substantial improvement in emission behaviour. After anodisation, the following was found to be true: i) The starting voltage was reduced by up to 50% (with p<sup>+</sup -type PS emitters exhibiting a greater reduction in starting voltage than p-type PS emitters). ii) Number of emitting tips per array was increased. iii) Higher maximum currents (up to 3 times higher) were obtained before tips underwent destruction. iv) The resistive effect of the PS layer at the apex was important in determining the maximum current obtained from a tip. In addition, both field emission and field ion microscopy were used to identify the emission source following anodisation. It was shown that individual fibrils on the emission surface caused an increase in field enhancement over a flat plane, leading to emission at lower voltage. Overall, porous silicon appears to be a very promising material for field emission displays.

The objective of this work is to develop essential building blocks for the lab-on-a-chip optical sensing systems with high performance. In this study, the silicon-on-insulator (SOI) platform is chosen because of its compatibility with the mature microelectronics industry for the great potential in terms of powerful data processing and massive production. Despite the impressing progress in optical sensors based on the silicon photonic technologies, two constant challenges are larger sensitivity and better selectivity. To address the first issue, we incorporate porous materials to the silicon photonics platform. Two porous materials are investigated: porous silicon and porous titania. The demonstrated travelling-wave resonators with the magnesiothermically reacted porous silicon cladding have shown significant enhancement in the sensitivity. The process is then further optimized by replacing the thermal oxide with a flowable oxide for the magnesiothermic reduction. A different approach of making porous silicon using porous anodized alumina membrane leads to better flexibility in controlling the pore size and porosity. Porous titania is successfully integrated with silicon nitride resonators. To improve the selectivity, an array of integrated optical sensors are coated with different polymers, such that each incoming gas analyte has its own signature in the collective response matrix. A multiplexed gas sensor with four polymers has been demonstrated. It also includes on chip references compensating for the adverse environmental effects. On chip spectral analysis is also very critical for lab-on-a-chip sensing systems. For that matter, based on an array of microdonut resonators, we demonstrate an 81 channel microspectrometer. The demonstrated spectrometer leads to a high spectral resolution of 0.6 nm, and a large operating bandwidth of ~ 50 nm.

Porous silicon (PSi) photonic crystals have aroused research interest as label-free chemical and biological sensing transducers owing to the ease of fabrication, high quality optics and a sensitive optical response to changes in efractive index. A major impediment to using PSi materials as sensors is the relative instability of the silicon surface to oxidation in ambient air and aqueous environments. This thesis reports methods for derivatising PSi towards realisation of 1-D silicon-based photonic materials for applications in biology and medicine. Narrow-linewidth rugate filters, a class of photonic crystal, are fabricated on silicon to display a high reflectivity resonant line in the reflectance spectrum. The position of the resonance is sensitive to changes in refractive index, thus allowing quantification of infiltrating biological species. The efficacy of rugate filters as biosensing transducers requires 1) protection from aqueous degradation, 2) resistance to non-specific adsorption and 3) distal reactivity for coupling of biorecognition molecules. Two chemical strategies based on hydrosilylation of functional alkenes are compared for stabilising the PSi structure against oxidation whilst resisting non-specific adsorption of biomolecules. Immobilisation of peptides to the organic layers is demonstrated for optical detection of protease enzymes. Introduction of protease results in cleavage of the immobilised peptides within the rugate filters, detected by an optical blue-shift to shorter wavelengths. To increase the sensitivity to proteolysis, covalent mmobilisation of biopolymers is evaluated using gelatin as a model substrate. Digestion of gelatin is detected down to 37 attomoles of protease. Furthermore, the surface chemistry allows specific capture of live cells and incubation with stimulated macrophages in tissue culture results in optical detection of released gelatinase enzymes. The generality of the surface chemistry allows for a range of other biological applications to be investigated. An alternative biorecognition interface, hybrid lipid bilayer membranes, containing specific recognition elements for cholera toxin allows optical detection of affinity capture and concentration within the PSi. In addition, the suitability of chemically modified photonic crystals as reservoirs for mass spectrometry is evaluated towards biomolecule quantification after optical detection. A robust and flexible surface chemistry on PSi photonic crystals is critical to performance in a range of biological assays and a necessary requirement for wide-scale employment.

published_or_final_version / Chemistry / Master / Master of Philosophy

Porous silicon (PSi) photonic crystals have aroused research interest as label-free chemical and biological sensing transducers owing to the ease of fabrication, high quality optics and a sensitive optical response to changes in efractive index. A major impediment to using PSi materials as sensors is the relative instability of the silicon surface to oxidation in ambient air and aqueous environments. This thesis reports methods for derivatising PSi towards realisation of 1-D silicon-based photonic materials for applications in biology and medicine. Narrow-linewidth rugate filters, a class of photonic crystal, are fabricated on silicon to display a high reflectivity resonant line in the reflectance spectrum. The position of the resonance is sensitive to changes in refractive index, thus allowing quantification of infiltrating biological species. The efficacy of rugate filters as biosensing transducers requires 1) protection from aqueous degradation, 2) resistance to non-specific adsorption and 3) distal reactivity for coupling of biorecognition molecules. Two chemical strategies based on hydrosilylation of functional alkenes are compared for stabilising the PSi structure against oxidation whilst resisting non-specific adsorption of biomolecules. Immobilisation of peptides to the organic layers is demonstrated for optical detection of protease enzymes. Introduction of protease results in cleavage of the immobilised peptides within the rugate filters, detected by an optical blue-shift to shorter wavelengths. To increase the sensitivity to proteolysis, covalent mmobilisation of biopolymers is evaluated using gelatin as a model substrate. Digestion of gelatin is detected down to 37 attomoles of protease. Furthermore, the surface chemistry allows specific capture of live cells and incubation with stimulated macrophages in tissue culture results in optical detection of released gelatinase enzymes. The generality of the surface chemistry allows for a range of other biological applications to be investigated. An alternative biorecognition interface, hybrid lipid bilayer membranes, containing specific recognition elements for cholera toxin allows optical detection of affinity capture and concentration within the PSi. In addition, the suitability of chemically modified photonic crystals as reservoirs for mass spectrometry is evaluated towards biomolecule quantification after optical detection. A robust and flexible surface chemistry on PSi photonic crystals is critical to performance in a range of biological assays and a necessary requirement for wide-scale employment.