Polarity inverted gallium nitride for photonic crystal biosensors.

Photonic crystals are periodic nanostructures used to control the propagation of electromagnetic waves. Depending on geometry and refractive index contrast between adjacent regions, periodic variation of the refractive index can result in a photonic band gap or non-allowed set of frequencies that cannot propagate through the crystal. Defects can be introduced at photonic crystal lattice sites, resulting in localized modes that lie within the photonic gap. Such defect cavities can be tuned to resonant frequencies of a defect mode and whole planes or lines of defects can be fabricated in photonic crystals resulting in optical confinement of defect modes. These properties of photonic crystals make them useful in a wide variety of applications such as chemical and biological sensors, high Q lasers, and optical wave guiding. With its transparency in the visible wavelength regime of the electromagnetic spectrum, GaN is a candidate for photonic crystal structures with photonic band gaps corresponding to visible wavelengths. GaN is a wide, direct band gap semiconductor which exists primarily in the wurtzite crystal structure. The wurtzite crystal structure lacks inversion symmetry, resulting in two distinct crystal polarities or crystal growth directions, the Ga-polar or [0001] and N-polar or [0001¯]. Through choice of substrate or growth conditions, GaN can be grown with either polarity. An unusual, but potentially useful, result is that by generation of near-monolayer surface coverage of Mg, the crystal polarity can be inverted during growth from gallium polar to nitrogen polar without introducing any additional defects at the domain boundary. Subsequent patterning and etching of the inversion layer, followed by re-growth, results in periodically poled GaN. This changes the nonlinear optical response of the material and such a structure can be used in a variety of applications. This study uses a subsequent highly anisotropic wet etch of polarity inverted GaN to selectively etch N-polar regions, where Ga-polar regions remain unaffected without introducing any additional structural damage. This wet etching technique for fabricating nanostructures has potential advantages over other dry etching techniques, such as inductively coupled plasma and reactive ion etching. The specific aim of this work is to develop the knowledge and techniques to allow fabrication of GaN photonic crystals via wet etching of periodically poled GaN. Growth conditions for polarity inversion by Mg doping during Molecular Beam Epitaxy growth of GaN, as well as process development for fabrication of photonic crystal structures on both the micron and nanometer scales are investigated. This study also involves theoretical modeling using MIT photonic bands software to determine photonic crystal geometries for the fabrication of GaN photonic crystals with photonic band gaps in the visible as well as the infrared wavelength regimes for future optical characterization. This work is part of a larger collaborative effort at West Virginia University for the design, fabrication, and testing of a flow-though, resonant florescence based GaN photonic crystal biosensor

Development of nano-patterned sapphire substrates for deposition of AlGaInN semiconductors by molecular beam epitaxy

Thesis (M.Sc.Eng.)This research addressed the design and fabrication of nano-patterned sapphire substrates (NPSS) as well as the growth by molecular-beam epitaxy (MBE) on such substrates of AlGaN and InGaN multiple quantum wells (MQWs). In recent years a number of LED manufacturers are developing nitride LED devices emitting in the visible part of the electromagnetic spectrum on micron-patterned sapphire substrate (MPSS). These devices are reported to have lower threading dislocation densities, resulting in improvement of the LED internal quantum efficiency (IQE). Furthermore, the LED devices fabricated on MPSS were also found to have improved light extraction efficiency (LEE), due to light scattering by the patterned substrate. My research focuses on the development of nano-patterned sapphire substrate aiming to improve the performance of LEDs grown by MBE and emitting at the deep ultraviolet region of the electromagnetic spectrum. In order to optimize the nano-patterning of the sapphire substrates for maximum light-extraction, the Finite-Difference Time-Domain (FDTD) simulation method was employed. The LEE enhancement was calculated as a function of the diameter, height and perion of the pattern. The calculations were performed only at a single wavelength, corresponding to the maximum of the emitted LED spectrum, which was taken to be 280 nm. These calculations have shown that the best sapphire substrate patterning strategy for this wavelength is the cone shape pattern in hexagonal array structure. Based on limited number of calculations I found that the optimum period, diameter and height of this cone shaped pattern are 400nm 375nm and 375nm respectively. Experimentally, nano patterned substrates were fabricated through natural and nano-imprint lithography. In natural lithography the first step for the definition of the nano-pattern consists of coating the sapphire substrate with photoresist (PMMA) followed by depositing a monolayer of polystyrene nanospheres, 400nm in diameter, using the Langmuir–Blodgett method. These spheres assemble on the substrate and form a closed packed hexagonal array pattern. Following this step the size of the spheres was slightly reduced using reactive-ion etching (RIE) in oxygen plasma. This was followed by the deposition a chromium film, lift-off to remove the polystyrene spheres and RIE to remove the PMMA from the footprints of the spheres. The substrate was then coated with a nickel or chromium films followed by another lift-off which defines the final mask for the formation of cone shaped features by RIE in a CHF3 plasma. An alternative method for pattern definition was the nanoimprint lithography; the stamp for this method (2 mm2 in size) was formed on Silicon substrate using e-beam lithography. NPSS with high quality pillar shape was also fabricated by this method, however, this method can produce only small size patterns. AlGaN films and GaN/InGaN MQWs were deposited on the NPSS by MBE, and characterized by Scanning electron microscopy and photoluminescence and cathodoluminescence measurements. The cathodoluminescence and photoluminescence spectra show that films grown on NPSS has much stronger luminescence than the films grown on flat sapphire substrate, consistent with enhanced light extraction efficiency

AlN ja Sc0.2Al0.8N ohutkalvojen märkäkemiallinen etsaus

Aluminium nitride is a piezoelectric material commonly used in piezoelectric microelectromechanical systems (MEMS) in the form of thin films deposited by sputtering. AlN-based devices are found in wireless electronics in the form of acoustic filters, but they also have prospective applications in a wide variety of sensor systems. To enhance the piezoelectric properties of AlN, some of the Al can be replaced with scandium, which is required for next-generation devices. However, addition of Sc makes both the deposition and patterning of the film more difficult. This work focuses on patterning of AlN and Sc0.2Al0.8N thin films with wet etching. Both materials are etched anisotropically, which in theory enables etching the materials with little deviation from the mask dimensions. However, in practise, undercutting at the mask edges occurs easily making the structures narrower compared to the etch mask. This work investigates and compares the mechanisms and etch rates of AlN and Sc0.2Al0.8N. Tetramethyl ammonium hydroxide was mostly used for etching, but also H3PO4 and H2SO4 were tested. Addition of 20 atom-% Sc lowered the etch rate of the material and resulted in more undercutting. The causes behind mask undercutting were examined by using 11 differently deposited etch masks, and the undercutting was minimized by optimizing the mask deposition, using thermal annealing, and optimizing the etching temperature. Finally, the work identifies and discusses the relevant factors in depositing and patterning the AlN, ScxAl1-xN and mask films

Growth, processing and chracterization of gallium nitride based coaxial LEDs grown by MOVPE

Gallium nitride (GaN) based coaxial (core-shell type) light emitting diodes (LEDs) offer a wide range of advantages. The active region of these LEDs is located on non-polar, {1-100} m-plane GaN sidewalls, which helps eliminate the quantum confined Stark effect (QCSE) and improve the radiative recombination efficiency of LEDs. The recent evolution of a catalyst free, scalable, repeatable and industrially viable device quality GaN nanowire and nanowall metal organic vapor phase epitaxy (MOVPE) growth process has enhanced the possibility of these LEDs going into production from laboratory. Previous work has shown that these nanowires exhibited an intense photoluminescence (PL), in spite of their large surface-area to volume ratio, and lasing was observed when these nanowires were optically pumped at high intensity. In this dissertation, it is shown that as long as the GaN three dimensional (3D) structures have their critical dimension below a micron, the threading defect (TD) density along the c- direction approaches zero. A TD that enters into this structure bends towards the surface vii ({1-100} m-plane side wall) in its vicinity, thereby reducing its dislocation line energy. The possibility of growing zero defect GaN templates is extremely important in the breakdown voltage improvement, the reverse bias leakage current reduction and efficiency droop reduction. This growth method has also been extended to device quality micron sized features, thereby presenting us with opportunity to study and explore LEDs of different sizes and shapes. In addition to the microstructure growth, two different repeatable approaches have been identified and demonstrated for the microelectronic processing of these micron-sized LEDs. Despite being far from perfect, the characterization results obtained from these LEDs have been encouraging. The technological challenges associated with the fabrication of the coaxial LEDs are also discussed in this dissertation

Conception et fabrication de FinFET GaN verticaux de puissance normalement bloqués

Abstract: The tremendous demands for high-performance systems driven by economic constraints forced the semiconductor industry to considerably scale the device's dimensions to compensate for the relatively modest Silicon physical properties. Those limitations pave the way for III-V semiconductors, which are excellent alternatives to Silicon and can be declined in many compositions. For example, Gallium Nitride (GaN) has been considered a fabulous competitor to facilitate the semiconductor industry's horizon beyond the performance limitations of Silicon due to its high mobility, wide bandgap, and high thermal conductivity properties for T>300K (Bulk GaN). It promises to trim the losses in power conversion circuits and drive a 10 % reduction in power consumption. Both lateral and vertical structures have been considered for GaN power devices. The AlGaN/GaN HEMT device's immense potential comes from the high density, high mobility electron gas formed at its heterojunction. The device is vulnerable to reliability issues resulting from the frequent exposure to high electric field collapse, temperature, and stress conditions, thus limiting its performance and reliability. Contrariwise, the vertical GaN power devices have attracted much attention because of the potential to reach high voltage and current levels without enlarging the chip's size. Furthermore, such vertical devices show superior thermal performance to their lateral counterparts. Meanwhile, Vertical GaN devices have the challenges of high leakage current and the breakdown occurring at the corners of the channel. Another challenge associated with Normally off devices is the lack of an optimized method for eliminating the magnesium diffusion from the p-GaN layer. This thesis has two strategic objectives; Firstly, a Normally-OFF GaN Power FinFET has been designed and optimized to overcome the vertical GaN FinFET challenges. It was done by optimizing the performance parameters such as threshold voltage VTH, high breakdown VBR, and the specific ON-state-resistance RON. Accordingly, the impact of both structural and physical parameters should be incorporated to have an exact optimization process. Afterward, the identification and optimization of a low-cost and high-quality fabrication process for the proposed structure underlined this thesis as the second objective.Les énormes demandes de systèmes à hautes performances motivées par des contraintes économiques ont forcé l'industrie des semi-conducteurs à réduire considérablement les dimensions des dispositifs pour compenser les propriétés physiques relativement modestes du silicium. Ces limitations ouvrent la voie aux semi-conducteurs III-V, qui sont d'excellentes alternatives au silicium et peuvent être déclinés dans de nombreuses compositions. Par exemple, le nitrure de gallium (GaN) a été considéré comme un concurrent fabuleux pour faciliter l'horizon de l'industrie des semi-conducteurs au-delà des limitations de performances du silicium en raison de sa grande mobilité, de sa large bande interdite et de ses propriétés de conductivité thermique élevées pour T>300K (Bulk GaN). Il promet de réduire les pertes dans les circuits de conversion de puissance et de réduire de 10 % la consommation d'énergie. À l'heure actuelle, les structures latérales et verticales ont été considérées pour les dispositifs de puissance en GaN. L'immense potentiel du dispositif HEMT AlGaN/GaN provient du gaz d'électrons à haute densité et à haute mobilité formé au niveau de son hétérojonction. Le dispositif est vulnérable aux problèmes de fiabilité résultant de l'exposition fréquente à des conditions d'effondrement de champ électrique, de température et de contrainte élevés, limitant ainsi ses performances et sa fiabilité. En revanche, les dispositifs de puissance verticaux en GaN ont attiré beaucoup d'attention en raison de leur capacité à atteindre des niveaux de tension et de courant élevés sans augmenter la taille de la puce. De plus, ces dispositifs verticaux présentent des performances thermiques supérieures à leurs homologues latéraux. Par ailleurs, les dispositifs GaN verticaux sont confrontés aux défis d'un courant de fuite élevée et de claquage se produisant aux coins du canal. Un autre défi associé aux dispositifs normalement bloqués est l'absence d'une méthode optimisée pour éliminer la diffusion de magnésium de la couche p-GaN. Cette thèse a deux objectifs stratégiques ; premièrement, un dispositif de puissance FinFET GaN normalement bloqué a été conçu et optimisé pour surmonter les défis du FinFET vertical en GaN. Cela a été fait en optimisant les paramètres de performance tels que la tension de seuil VTH, la tension de claquage VBR et la résistance spécifique à l'état passant RON. En conséquence, l'impact des paramètres structurels et physiques doit être incorporé pour avoir un processus d'optimisation précis. Par la suite, l'identification et l'optimisation d'un processus de fabrication à faible coût et de haute qualité pour la structure proposée à souligner cette thèse comme deuxième objectif

Heated Atomic Force Microscope Cantilevers For Polymer Based Additive Nanomanufacturing

This dissertation demonstrates the design, simulation, fabrication and characterization processes of a novel heated atomic force microscope cantilever for polymer based additive nanomanufacturing. Fabrication and integration of heterogeneous nanostructures is an essential task for manufacturing next generation organic electronic devices. Current state-of-the-art in heated tip additive manufacturing has a limited write time and cannot accurately control polymer deposition rate. The new design presented here includes two embedded joule heaters connected by a microchannel, where thermocapillary forces induced by the temperature gradient between heaters can deliver about 40 ng of polymer to the tip. The heated tip design presented here was informed by multiphysics finite element analysis to optimize the thermo-mechanical and thermo-fluidic performance of the device. Computational fluid dynamics simulations of molten polymer flowing in the microchannel shows the velocity of the leading edge depends significantly on the imposed temperature gradient. Thus, the cantilever tip can be inked, cleaned, and re-inked by controlling the temperature of the integrated heaters. Following design optimization, this work details the step-by-step microfabrication processes for manufacturing the heated cantilevers. Electrical and thermal characterizations are performed to evaluate the temperature response and electrical resistance of the fabricated cantilevers, and is compared to the developed models. Preliminary results show a maximum temperature of 500 °C before thermal runaway occurs in the fabricated cantilevers, with temperature gradients as large as 2.0E6 C/m. Investigation of solid-liquid interactions at the nanoscale is crucially important to understand the mechanism of polymer spreading along the cantilever microchannel and tip. A new AFM-based measurement technique for dynamic measurement of polymer nanodroplet spreading at elevated temperatures is developed. The experimental setup is used to measure the spreading dynamics of polystyrene droplets with 2 µm diameters at 115-175 °C on flat surfaces. Custom image processing algorithms determine the droplet height, radius, volume, and contact angle of each AFM image over time to calculate the droplet spreading dynamics. The new cantilever design and the AFM-based spreading measurement technique presented here, provide a framework to make better tools for wafer scale heterogeneous polymer nanostructure fabrication with high throughput, multiple feature registration, and high spatial resolution

An Integrated Gas Sensing System Based on Surface-Functionalized Gallium Nitride Nanowires with Embedded Micro-Heaters

In the last few decades, significant improvements have been made in gas sensor technologies. Metal-oxide sensors have been used for low-cost detection of combustible and toxic gases. However, hurdles relating to sensitivity, stability and selectivity still remain. Recently, nanotechnology has helped tremendously through the introduction of nano-engineered materials like nanowires and nanoclusters. Nanowire sensors have much better sensitivity as compared with thin-film devices due to the larger detecting surface-to-volume ratio. But clearly, improvements are still needed. For real-world applications, selectivity between different classes of compounds, such as combustible and toxic gases, is highly desirable. An ideal chemical sensor should distinguish between the individual analytes from a single class of compounds. For example, in detection of benzene or toluene, a good sensor will not be disturbed by other aromatic compounds present in the environment. This is a huge challenge for semiconductor based metal-oxide sensors, such as TiO2, SnO2, Fe2O3 and ZnO, which have inherent non-selective surface adsorption sites. Recently, a new class of nanowire-nanocluster (NWNC) based gas sensors has gained interest. This type of sensor represents a new method of functionalizing the surface for selective adsorption and detection. The adjustable sensitivity can be achieved by tuning the density, size or composition of the nanoparticles that decorate the nanowires. These advantages make the NWNC sensors a good alternative to conventional thin-film sensors. So far, research into NWNC sensors has demonstrated the potential in sensing many important classes of compounds. However, most of these NWNC devices require elevated working temperatures. They also have long response/recovery times and must function in an inert atmosphere. All these limitation will be the obstacles in real-world usage for domestic, environmental or industrial applications. And finally, the sensors thus developed must be manufacturable. That is, they must be batch fabricated with high yield. To remedy these problems, my thesis was divided into the following tasks, 1. Develop dry etching techniques to fabricate horizontally aligned GaN nanowires (NW), combining these techniques with wet etching treatment for surface damages removal. I call this a “top-down approach” using a subtractive process that fabricates NWs from thin-films and adding sensitive nanocrystals after the initial NW definition. This is to be compared to the additive “bottom-up” nanowire growth by MBE/HVPE/Sol-gel, in which NWs are grown, harvested from the growth surface and subsequently re-attached to a new surface. The top-down approach enhances the yield and homogeneity of the NW and it is mass-production oriented. 2. Study the metal-oxide nanoclusters (NCs) deposition method by physical vapor deposition (PVD) and rapid thermal annealing (RTA) for TiO2, SnO2, WO3, Fe2O3, etc. Develop the metal nanoparticle deposition method by PVD for Au, Ag, Pt, Pd, etc. 3. Study the crystalline phases and gas adsorption sites formed by the method and establish a database connecting metal-oxide bonding sites with different target chemicals. 4. Utilize Si doped n-type and unintentionally doped GaN nanowires functionalized with different metal-oxide and metal-oxide/metal composite nanoclusters to create a series of highly selective and sensitive gas sensing nanostructure devices. 5. Develop a low-cost micro-heater (MH) for local high temperature generation with low power consumption. This allows the rapid chemical desorption cycles as we anticipate frequently re-use or reset of the sensor. It also enables the use of these NWs in high temperature sensor applications. 6. Integrate the NW, NCs and MH into one working sensor, and integrate multiple types of gas sensors on a single chip. The chip can simultaneously sense many types of gases without interference. In this study, the potential of multicomponent NWNC based sensors for developing the next-generation of ultra-sensitive and highly selective chemical sensors was explored. We have achieved uA and nA levels of baseline detector current and we have shown that low UV illumination enhances sensitivity for some cases. These sensors have low power consumption making them suitable for portable devices

Fabrication Techniques for III-V Micro-Opto-Electro-Mechanical Systems

This thesis studies selective etching techniques for the development of AlxGa1-xAs micro-opto-electro-mechanical systems (MOEMS). New MEMS technology based on materials such as AlxGa1-xAs enables the development of micro-systems with embedded active micro-optical devices. Tunable micro-lasers and optical switching based on MOEMS technology will improve future wavelength division multiplexing (WDM) systems. WDM vastly increases the speed of military communications and sensor data processing. From my designs, structures are prepared by molecular beam epitaxy. I design a mask set for studies of crystal plane selectivity. I perform a series of experiments on the selective removal of GaAs and AlAs. I convert AlAs and Al0.98Ga0.02As layers within the test structures to AlOx and Al0.98Ga0.02Ox and perform selective etching experiments on these sacrificial oxide layers. The etchants and materials studied showed high selectivity for removal of all materials studied. Results suggest that any of these material layers are useful as sacrificial layers for general MOEMS technology. I design, fabricate, and characterize prototype III-V MOEMS. Using AlOx sacrificial layers, I investigate a new technique for transplanting microcavity light-emitting devices. I successfully transplant arrays of light-emitting diodes. Finally, I discuss ideas on how this work forms the basis for nano-electro-mechanical systems (NEMS) fabrication in III-V materials