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| 2. |
- Berrier, Audrey, 1978-
(författare)
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InP-based photonic crystals : Processing, Material properties and Dispersion effects
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Photonic crystals (PhCs) are periodic dielectric structures that exhibit a photonic bandgap, i.e., a range of wavelength for which light propagation is forbidden. The special band structure related dispersion properties offer a realm of novel functionalities and interesting physical phenomena. PhCs have been manufactured using semiconductors and other material technologies. However, InP-based materials are the main choice for active devices at optical communication wavelengths. This thesis focuses on two-dimensional PhCs in the InP/GaInAsP/InP material system and addresses their fabrication technology and their physical properties covering both material issues and light propagation aspects. Ar/Cl2 chemically assisted ion beam etching was used to etch the photonic crystals. The etching characteristics including feature size dependent etching phenomena were experimentally determined and the underlying etching mechanisms are explained. For the etched PhC holes, aspect ratios around 20 were achieved, with a maximum etch depth of 5 microns for a hole diameter of 300 nm. Optical losses in photonic crystal devices were addressed both in terms of vertical confinement and hole shape and depth. The work also demonstrated that dry etching has a major impact on the properties of the photonic crystal material. The surface Fermi level at the etched hole sidewalls was found to be pinned at 0.12 eV below the conduction band minimum. This is shown to have important consequences on carrier transport. It is also found that, for an InGaAsP quantum well, the surface recombination velocity increases (non-linearly) by more than one order of magnitude as the etch duration is increased, providing evidence for accumulation of sidewall damage. A model based on sputtering theory is developed to qualitatively explain the development of damage. The physics of dispersive phenomena in PhC structures is investigated experimentally and theoretically. Negative refraction was experimentally demonstrated at optical wavelengths, and applied for light focusing. Fourier optics was used to experimentally explore the issue of coupling to Bloch modes inside the PhC slab and to experimentally determine the curvature of the band structure. Finally, dispersive phenomena were used in coupled-cavity waveguides to achieve a slow light regime with a group index of more than 180 and a group velocity dispersion up to 10^7 times that of a conventional fiber.
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| 3. |
- Elfström, Niklas, 1979-
(författare)
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Silicon Nanowires for Biomolecule Detection
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Starting from silicon on insulator (SOI) material, with a top silicon layer thickness of 100 nm, silicon nanowires were fabricated in a top down approach using electron beam (e-beam) lithography and subsequent eactive ion etching (RIE) and oxidation. Nanowires as narrow as 30 nm could be achieved. Further size reduction was done using electrochemical etching and/or oxidation. The nanowires were contacted creating drain, source and back gate contacts and characterized showing similar behavior as Schottky Barrier Metal Oxide Semiconductor Field Effect Transistors (SB-MOSFETs). As an alternative, by thinning the top silicon layer down nanoribbons, ~ 1 μm wide, with a thickness down to 45 nm could be produced using standard optical lithography showing similar behavior as the nanowires. The conduction mechanism for these devices is through electrons in an inversion current layer for positive back gate voltages and through holes in accumulation mode for negative back gate voltages. When the threshold voltage is extrapolated for the nanowires and the nanoribbons it scales with inverse width and thickness respectively, attributed to charged surface and/or interface states affecting more narrow/thinner devices essentially due to increased surface to volume ratio. Nanowires were functionalized with 3-aminopropyl triethoxysilane (APTES) molecules creating amino groups on the surface reactive to pH buffer solutions. By exposing the nanowires to buffer solutions of different pH value the conduction mechanism changed due to the surface becoming more or less negative. Threshold voltage shifts from pH = 3 to pH = 9 were seen to scale with inverse width again attributed to the larger surface to volume ratio for more narrow devices. Simulations confirm this behavior and further show that a charge change of a few elementary charges on the nanowire surface can alter the conductance significantly. Upon addition of the buffer solutions the channel is seen to be quenched for small drain bias attributed to negative surface charges screening the electron current. However, as the drain bias is increased the channel is restored. Computer simulations confirmed this behavior and further showed that the restoration of the channel was due to an avalanche process. A biomolecule detection experiment was set up using the specific binding of biotin to streptavidin. By functionalizing the nanoribbon with biotin molecules the current can be logged and as streptavidin molecules are added the current decreases (increases) if the nanoribbon is run in inversion (accumulation) mode due to the negative charge of the streptavidin molecule, delivered upon binding to biotin. A sensitivity significantly below the picomolar range was observed, corresponding to less than 20 streptavidin molecules attaching to the nanoribbon surface, assuming a homogeneous binding to the biotinylated surface. By decreasing the nanoribbon thickness the response was increased, a behavior attributed to the larger surface to volume ratio of these devices. The response was seen to be larger in the accumulation mode whereas close to the lower oxide in inversion mode. Computer simulations showed that this was due to the hole current running closer to the functionalized surface in accumulation mode and opposite in inversion mode. This was further investigated for different nanoribbon thicknesses and the response was shown to increase with inverse nanoribbon thickness again attributed to the larger surface to volume ratio. The nanoribbon has the advantage of simpler fabrication using standard optical lithography in comparison with e-beam lithography and it may provide a useful scheme for a practical biomolecule sensor.
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| 4. |
- Fornara, Andrea, 1980-
(författare)
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Magnetic nanostructured materials for advanced bio-applications
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the recent years, nanostructured magnetic materials and their use in biomedical and biotechnological applications have received a lot of attention. In this thesis, we developed tailored magnetic nanoparticles for advanced bio-applications, such as direct detection of antibodies in biological samples and stimuli responsive drug delivery system. For sensitive and selective detection of biomolecules, thermally blocked iron oxide nanoparticles with specific magnetic properties are synthesized by thermal hydrolysis to achieve a narrow size distribution just above the superparamagnetic limit. The prepared nanoparticles were characterized and functionalized with biomolecules for use in a successful biosensor system. We have demonstrated the applicability of this type of nanoparticles for the detection of Brucella antibodies as model compound in serum samples and very low detection limits were achieved (0.05 mg/mL). The second part is concerning an in-depth investigation of the evolution of the thermally blocked magnetic nanoparticles. In this study, the formation of the nanoparticles at different stages during the synthesis was investigated by high resolution electron microscopy and correlated to their magnetic properties. At early stage of the high temperature synthesis, small nuclei of 3.5 nm are formed and the particles size increases successively until they reach a size of 17-20 nm. The small particles first exhibit superparamagnetic behavior at the early stage of synthesis and then transform to thermally blocked behavior as their size increases and passes the superparamagnetic limit. The last section of the Thesis is related to the development of novel drug delivery system based on magnetically controlled release rate. The system consists of hydrogel of Pluronic FP127 incorporating superparamagnetic iron oxide nanoparticles to form a ferrogel. The sol to gel formation of the hydrogel could be tailored to be solid at body temperature and thus have the ability to be injected inside living biological tissues. In order to evaluate the drug loading and release, the hydrophobic drug indomethacin was selected as a model compound. The drug could be loaded in the ferrogel owning to the oil in water micellar structure. We have studied the release rate from the ferrogel in the absence and presence of magnetic field. We have demonstrated that the drug release rate can be significantly enhanced by use of external magnetic field decreasing the half time of the release to more than 50% (from 3200 to 1500 min) upon the application of the external magnetic field. This makes the developed ferrogel a very promising drug delivery system that does not require surgical implant procedure for medical treatment and gives the possibility of enhancing the rate of release by external magnetic field.
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| 5. |
- Lee, Hyung-Seok, 1975-
(författare)
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Fabrication and Characterization of Silicon Carbide Power Bipolar Junction Transistors
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon carbide bipolar junction transistors (BJTs) are attractive power switching devices because of the unique material properties of SiC with high breakdown electric field, high thermal conductivity and high saturated drift velocity of electrons. The SiC BJT has potential for very low specific on-resistances and this together with high temperature operation makes it very suitable for applications with high power densities. For SiC BJTs the common emitter current gain (β), the specific on-resistance (RSP_ON), and the breakdown voltage are important to optimize for competition with silicon based power devices. In this thesis, power SiC BJTs with high current gain β ≈ 60 , low on-resistance RSP_ON ≈ 5 mΩcm2, and high breakdown voltage BVCEO ≈ 1200 V have been demonstrated. The 1200 V SiC BJT that has been demonstrated has about 80 % lower on-state power losses compared to a typical 1200 V Si IGBT chip. A continuous epitaxial growth of the base-emitter layers has been used to reduce interface defects and thus improve the current gain. A significant influence of surface recombination on the current gain was identified by comparing the experiments with device simulations. In order to reduce the surface recombination, different passivation layers were investigated in SiC BJTs, and thermal oxidation in N2O ambient was identified as an efficient passivation method to increase the current gain. To obtain a low contact resistance, especially to the p-type base contact, is one critical issue to fabricate SiC power BJTs with low on-resistance. Low temperature anneal (~ 800 oC) of a p-type Ni/Ti/Al contact on 4H-SiC has been demonstrated. The contact resistivity on the ion implanted base region of the BJT was 1.3 × 10-4 Ωcm2 after annealing. The Ni/Ti/Al p-type ohmic contact was adapted to 4H-SiC BJTs fabrication indicating that the base contact plays a role for achieving a low on-resistance of SiC BJTs. To achieve a high breakdown voltage, optimized junction termination is important in a power device. A guard ring assisted Junction Termination Extension (JTE) structure was used to improve the breakdown voltage of the SiC BJTs. The highest breakdown voltage of the fabricated SiC BJTs was obtained for devices with guard ring assisted JTE using the base contact implant step for a simultaneous formation of guard rings. As a new approach to fabricate SiC BJTs, epitaxial regrowth of an extrinsic base layer was demonstrated. SiC BJTs without any ion implantation were successfully demonstrated using epitaxial regrowth of a highly doped p-type region and an etched JTE using the epitaxial base. A maximum current gain of 42 was measured for a 1.8 mm × 1.8 mm BJT with a stable and reproducible open base breakdown voltage of 1800 V.
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| 6. |
- Li, Shanghua, 1981-
(författare)
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Fabrication of Nanostructured Materials for Energy Applications
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- World energy crisis has triggered more attention to energy saving and energy conversion systems with high efficiency. There is a growing awareness that nanoscience and nanotechnology can have a profound impact on energy generation, conversion, and recovery. Nanotechnology-based solutions are being developed for a wide range of energy problems such as, solar electricity, hydrogen generation and storage, batteries, fuel cells, heat pumps and thermoelectrics. This thesis deals with the design and fabrication of novel functional materials/architectures for energy-related applications. The study includes two parts: Nanostructured thermoelectric (TE) materials for energy conversion and nanostructured metallic surfaces for energy heat transfer. In the first part, the focus is given to the fabrication of novel nanostructured TE materials and architectures. TE materials are very important functional materials that can convert heat to electrical energy and vice versa. Recently, nanostructuring TE materials showed very promising potential to improve their TE figure of merit which opens a new venue for the TE world. As a result, some advanced nanostructured TE architectures are proposed as the state-of-the-art TE materials/structures. Among these advanced TE architectures, bismuth telluride nanowires/thick films and skutterudite nanocomposites with nanoinclusions have been successfully fabricated and some of their advantageous TE performance has been demonstrated. For example, an improvement of 11% on the figure of merit, ZT, was achieved in the CoSb3 nanocomposite with 5 mole% ZrO2 as nanoinclusion. Comprehensive physico-chemical characterization techniques have been used for the synthesized TE materials. The potential-Seebeck microprobe, 4-point probe and laser flash apparatus have been used for the measurement of TE parameters on the TE materials. In the second part of the thesis, we developed a nanostructured macro-porous (NMp) surface for enhancing heat transfer in boiling process. Enhanced surfaces for boiling improve the energy efficiency of heat pumping equipment such as air conditioners, refrigerators, etc. Conventional techniques currently used for fabricating enhanced surfaces are often based on the use of complicated mechanical machine tools and require a large consumption of materials and give only limited enhancement of the boiling heat transfer. In this thesis, we present a new approach to fabricate enhanced surfaces by using electrodeposition under specific conditions forming in-situ dynamic gas bubble templates. As a result, the NMp metallic surface layer comprising of dendritically ordered copper branches is obtained. Since the structure is formed during the evolution of the dynamic bubbles, it is ideal for the bubble generation applications such as boiling. The efficiency of the NMp surfaces for boiling heat transfer was evaluated in pool boiling experiments. At the heat flux of 1 W/cm2, the heat transfer coefficient for the NMp surface is found to be more than 17 times higher than the reference surface. It's estimated that such an effective boiling surface would improve the energy efficiency of many heat pumping machines with 10 - 30 %. The extraordinary enhancement of boiling performance is explained by the structure characteristics, which assist in enhancing nucleation of the gas bubbles, subsequent coalescence, and facilitated departure from the surfaces.
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| 7. |
- Marcks von Würtemberg, Rickard, 1973-
(författare)
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Design and fabrication of long wavelength vertical cavity lasers on GaAs substrates
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Vertical cavity surface emitting lasers (VCSELs) are today a commodity on the short wavelength laser market due to the ease with which they are manufactured. Much effort has in the last decade been directed towards making long wavelength VCSELs as successful in the marketplace. This has not been achieved due to the much more difficult fabrication technologies needed for realising high performance long wavelength VCSELs. At one point, GaInNAs quantum wells gain regions grown on GaAs substrates seemed to be the solution as it enabled all-epitaxial VCSELs that could make use of high contrast AlGaAs-based distributed Bragg reflectors (DBRs) as mirrors and lateral selective oxidation for optical and electrical confinement, thereby mimicking the successful design of short wavelength VCSELs. Although very good device results were achieved, reproducible and reliable epitaxial growth of GaInNAs quantum wells proved difficult and the technology has not made its way into high-volume production. Other approaches to the manufacturing and material problems have been to combine mature InP-based gain regions with high contrast AlGaAs-based DBRs by wafer fusion or with high contrast dielectric DBRs. Commonly, a patterned tunnel junction provides the electrical confinement in these VCSELs. Excellent performance has been achieved in this way but the fabrication process is difficult. In this work, we have employed high strain InGaAs quantum wells along with large detuning between the gain peak and the emission wavelength to realize GaAs-based long wavelength VCSELs. All-epitaxial VCSELs with AlGaAs-based DBRs and lateral oxidation confinement were fabricated and evaluated. The efficiency of these VCSELs was limited due to the optical absorption in the doped DBRs. To improve the efficiency and manufacturability, two novel optical and electrical confinement schemes based on epitaxial regrowth of current blocking layers were developed. The first scheme is based on a single regrowth step and requires very precise processing. This scheme was therefore not developed beyond the first generation but single mode power of 0.3 mW at low temperature, -10ºC, was achieved. The second scheme is based on two epitaxial regrowth steps and does not require as precise processing. Several generations of this design were manufactured and resulted in record high power of 8 mW at low temperature, 5ºC, and more than 3 mW at high temperature, 85ºC. Single mode power was more modest with 1.5 mW at low temperature and 0.8 mW at high temperature, comparable to the performance of the single mode lateral oxidation confined VCSELs. The reason for the modest single mode power was found to be a non-optimal cavity shape after the second regrowth that leads to poor lateral overlap between the gain in the quantum wells and the intensity of the optical field.
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| 8. |
- Olsson, Fredrik, 1975-
(författare)
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Selective Epitaxy of Indium Phosphide and Heteroepitaxy of Indium Phosphide on Silicon for Monolithic Integration
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A densely and monolithically integrated photonic chip on indium phosphide is greatly in need for data transmission but the present day’s level of integration in InP is very low. Silicon enjoys a unique position among all the semiconductors in its level of integration. But it suffers from its slow signal transmission between the circuit boards and between the chips as it uses conventional electronic wire connections. This being the bottle-neck that hinders enhanced transmission speed, optical-interconnects in silicon have been the dream for several years. Suffering from its inherent deficient optical properties, silicon is not supposed to offer this feasibility in the near future. Hence, integration of direct bandgap materials, such as indium phosphide on silicon, is one of the viable alternatives. This thesis addresses these two issues, namely monolithic integration on indium phosphide and monolithic integration of indium phosphide on silicon. To this end, we use two techniques, namely selective epitaxy and heteroepitaxy by employing hydride vapor phase epitaxy method. The first part deals with the exploitation of selective epitaxy for fabricating a discrete and an integrated chip based on InP. The former is a multi-quantum well buried heterostructure laser emitting at 1.55 µm that makes use of AlGaInAs and InGaAsP as the barrier and well, respectively. We demonstrate that even though it contains Al in the active region, semi-insulating InP:Fe can be regrown. The lasers demonstrate threshold as low as 115A/cm2/quantum well, an external quantum efficiency of 45% and a characteristic temperature of 78 K, all at 20 oC. Concerning the integrated device, we demonstrate complex and densely packed buried arrayed waveguide (AWG) structures found in advanced systems-on-the-chip for optical code-division multiple-access (O-CDMA). We present a case of an error-free 10 Gb/s encoding and decoding operation from an eight-channel AWGs with 180 GHz channel spacing. Selective epitaxial growth aspects specific to these complicated structures are also described and guidance on design implementation of these AWGs is given. Mass transport studies on these AWGs are also presented. The second part deals with various studies on and relevant to epitaxial lateral overgrowth (ELOG) of high quality InP on silicon. (i) ELOG often encounters cases where most part of the surface is covered by mask. From the modeling on large mask area effects, their impact on the transport and kinetic properties has been established. (ii) It is known that ELOG causes strain in the materials. From synchrotron X-ray measurements, strain is shown to have large effect on the mask edges and the underlying substrate. (iii) The combination of strain and the influence of image forces when reducing the opening dimensions in ELOG has been modeled. It is found to be very beneficial to reduce openings down to ~100 nm where effective filtering of dislocations is predicted to take place even in vicinity of the openings. We call it nano-ELOG. (iv) By combining the modeling results of nano-ELOG and of a pre-study of ELOG on pure InP, a novel net pattern design is invented and experimented for nano-ELOG of InP on Si. PL measurements together with transmission electron microscopy observations indicate beneficial effects of small size openings (200 nm) compared to 1000 nm openings. (v) ELOG of InP on silicon-on-insulators together with a multi-quantum well structure grown on it has been demonstrated for the first time. This is particularly interesting for integrating silicon/silicon dioxide waveguides with InP.
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| 9. |
- Ricciardi, Sébastien
(författare)
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Polymeric Microcavities for Dye Lasers and Wavefront Shapers
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Over the last few years, the available computing power allows us to have a deeper insight into photonics components than we ever had before. In this thesis we use the finite element method (FEM) to explore the behavior of the waves in 2D planar microcavities. We demonstrate the tunability of the cavity over a wide range of frequencies taking into account both the thermo-mechanical and the thermo-optical effect. Geometry and material choices are done so that the latter is predominant. We also demonstrate an odd mode disappearing phenomenon reported here for the first time as far as we know. Using this knowledge, we design two structures with these remarkable properties. One of the devices will be used as micro-sized solid-state dye laser with Rhodamine 6G as the active medium and SU-8 polymer as a cavity material in sizes that have never been reached before. This opens new opportunities not only for future implementation for “labs-on-a-chip” (LOC) but also for a higher integration density of optical communication systems. The second device is a wavefront shaper creating plane waves from a point source performing the functions of beam shaper and beam splitter with plane wave as the output result. After an introduction to FEM and comparison with a rival algorithm, some issues related to FEM in electromagnetic simulation are resolved and explained. Finally, some fabrication techniques with feature sizes <100 nm, such as electron beam lithography (EBL) and nano-imprint lithography (NIL), are described and compared with other lithographic techniques.
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| 10. |
- Shi, Yaocheng, 1981-
(författare)
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Design, Simulation and Characterization of Some Planar Lightwave Circuits
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Optical devices based on planar lightwave circuit (PLC) technology have the advantages of small size, high reliability, possibility for large scale production, and potential integration with electronics. These devices are widely employed in optical telecommunications, sensing, data storage, imaging, and signal processing. This thesis focuses on some selected PLC based devices, such as power splitters, demultiplexers, triplexers and polarization beam splitters. First, the basic principle of the waveguides and the simulation methods for PLC devices are discussed. A novel effective index method is introduced to reduce a two-dimensional structure to a one-dimensional one, and can be implemented for arbitrarily shaped waveguides. Numerical methods, such as finite-difference mode solver, beam propagation method, finite-difference time-domain method are introduced to analysis the mode profile of the waveguides, and the propagation properties of light in PLC devices. Multimode interference (MMI) couplers are widely used in many PLCs, such as power splitters, ring lasers, optical switches, and wavelength division multiplexers/demultiplexers. In this work, concepts for improving the self-imaging quality of MMI couplers are analyzed and new designs are proposed. A significant improvement in performance together with compact sizes were obtained with taper sections at the input/output of MMI couplers based on SOI, and deeply etched ridges in MMI couplers based on SiO2. A polarization insensitive dual wavelength demultiplexer based on sandwiched MMI waveguides was presented. Novel devices including triplexers and polarization beam splitters were realized by using photonic crystal (PhC) structures. Two stages of directional couplers based on PhC waveguides are cascaded to form an ultracompact triplexer. The special decoupling property of the PhC waveguide based directional coupler was utilized in the design. A novel polarization beam splitter was realized by combining a MMI coupler and a PhC which works as a polarization sensitive reflector. Finally, fabrication and optical characterization of an ultra-compact directional coupler and PhC structures in InP are presented. In a single etching step, by using the lag-effect in inductively coupled plasma reactive ion etching, a compact directional coupler (55 μm) is demonstrated. Carrier life times in PhC structures etched by chemically assisted ion beam etching were investigated, for emitter and switching applications.
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