Reduction in background carrier concentration has been investigated for 4H-SiC C-face epitaxial growth in order to be applied for ultra-high voltage power devices. Optimizing epitaxial growth parameters made it possible to achieve 7.6x1013 cm-3 as the background carrier concentration within a whole area of specular 3-inch wafers. In addition to the background carrier concentration reduction, epitaxial film thickness variation, surface defect density and the carrier lifetime have been confirmed to fulfill the requirements for the devices.

Research in the field of phase change materials (PCMs) requires that the temperature dependence of the thermal storage capacity be well known for the selection of PCMs as well as for simulation input. A differential scanning calorimeter (DSC) is often used, but it substantially misrepresents the true heat capacity in the vicinity of large-enthalpy phase transitions. Therefore, other suitable experimental techniques should be applied for the determination of the thermal storage capacity. Peltier-element-based adiabatic scanning calorimetry (pASC) measures the heat capacity and the enthalpy of PCMs in thermodynamic equilibrium, thus removing the rate dependence and deformation that are inherent to DSC. The technique is illustrated here by measurements on the pure alkane tricosane (C23), the commercial alkane mixture RT42 and its bound counterpart PX42.

We studied MOCVD processing for direct growth of BN on 2” sapphire substrates as a template for heterostructures with two dimensional (2D) and three dimensional (3D) materials. The combined experimental evidence points to three growth modes for BN: self-terminating, 3D random, and layer-by-layer, all of which are dependent on V/III ratio, temperature, pressure, and substrate surface modification via nitridation. At moderate temperature (950-1050°C), BN growth using high V/III ratio is self-terminating, resulting in c-oriented films aligned in-plane with respect to the orientation of the sapphire substrate. BN films grown under low V/III ratios are 3D, randomly oriented, and nano-crystalline. At higher temperature (1100°C), self-terminating growth transitions to a continuous layer-by-layer growth mode. When BN growth is self-terminating, films exhibit atomically smooth surface morphology and highly uniform thickness over a 2” sapphire wafer. Using these BN/sapphire templates we studied the growth of 2D and 2D/3D heterostructures. To study direct growth of 2D on 2D layered material we deposited graphene on BN in a continued process within the same MOCVD system. Furthermore, we explore the growth and nucleation of 3D materials (GaN and AlN) on BN. AlGaN/GaN based high electron mobility transistor (HEMT) structures grown on BN/sapphire exhibited two-dimensional electron gas characteristics at the AlGaN/GaN heterointerface, with room-temperature electron mobility and sheet electron density about 1900cm2/Vs and 1x1013cm-2, respectively.

The effects of TiCl4 post-treatment on the physicochemical properties of porous TiO2 (pTiO2) layers fabricated at 300 °C and 400 °C (denoted as pTiO2(300) and pTiO2(400), respectively) in CH3NH3PbI3 perovskite photovoltaic cells were investigated. Water contents (physisorbed water and water derived from surface hydroxyl groups) of pTiO2(300) and pTiO2(400) before and after TiCl4 post-treatment were measured by using temperature desorption spectroscopy (TDS). Moreover, structural analysis of the CH3NH3PbI3 perovskite part was performed by X-ray diffraction (XRD). In the case of pTiO2(300), the content of water was increased by the TiCl4 post-treatment due to the removal of residual organic compounds that existed before the treatment. It then caused a change in the surface activity of pTiO2(300) and enhancement of solar cell performance and photocurrent density, though suppression of CH3NH3PbI3 perovskite formation occurred. In comparison, contents of water were decreased for pTiO2(400), leading to enhancement of the conversion of PbI2 to CH3NH3PbI3 perovskite. As a result, there were significant increases in short circuit current density (Jscs) and PCEs. The results showed that TiCl4 post-treatment is an effective approach to prepare high-performance CH3NH3PbI3 perovskite solar cells without heat treatment at a very high temperature.

Resistive switching behaviors in Aloe vera films are being explored for nonvolatile memory applications. A simple structure in which the Aloe vera films sandwiched in between a top and bottom electrode are used. The switching behaviors of the devices in which the Aloe vera film is dried at different temperatures and the roles of top electrode materials (Al and Ag) are investigated. Current density–voltage measurements reveal that filamentary conduction is the dominant conduction process inducing resistive switching characteristics in Aloe vera films. Device with Al-top electrode requires a forming voltage higher than devices with Ag-top electrode, due to the tendency of oxide formation of these materials. The resistive switching behaviors are highly reproducible, as demonstrated by the data retention performance over an interval of 104 s and endurance capability of over 100 cycles.

Thin metal films on compliant polymer substrates are of major interest for flexible electronic technologies. The suitability of a film system for flexible applications is based on the electro-mechanical performance of the metal film/polymer substrate couple. This study demonstrates how a 10 nm Cr interlayer deteriorates the electro-mechanical performance of 50 nm Au films on polyimide substrates by inducing the formation of cracks in the ductile layer. Combined in-situ measurements of the film lattice strains with x-ray diffraction and electrical resistance with four point probe of the Au-Cr and Au layers during uniaxial straining confirmed different electro-mechanical behaviours. For Au films with a Cr interlayer the film stress decreases rapidly as cracking initiates and reaches a plateau as the saturation crack spacing is reached. Crack formation and stress drop correspond to a rapid increase in the film resistance. Without the interlayer the Au film stress reaches a maximum around 2% engineering strain and remains constant throughout the experiment. The film resistance is unaffected by the applied elongation up to a maximum strain of 15%, giving no sign of cracking in the metal layer. The outstanding electro-mechanical performance of the gold film indicates that adhesion layers, like Cr, may not be necessary to improve the performance of ductile films on polymers.

We demonstrate a stabilized liquid crystal photoalignment using a surface-localized polymer method. A protection layer is developed on a photoalignment layer (PAL) through phase separation of a mixture composed of reactive monomers (RMs) and liquid crystals (LCs). The RM is polymerized on the PAL which enhances its stability against heat and light with short wavelength. The effects of the concentration and molecular structure of RMs on the electro-optical response and surface anchoring of photoaligned LC device are studied. The concentration of RMs affects the effective cell gap of the LC device. The rigid core length of the RM structure modulates the surface anchoring strength of the alignment layer. Both effective cell gap and surface anchoring strength are key elements for the enhanced dynamic response of LC devices.

Summer research placements are an effective training and research tool. Over three years, our group has hosted nine pre-university students over periods of four to six weeks. Apart from student training and skills acquisition, the placements have produced several peer-reviewed technical publications. Our approach relies on careful pre-planning of activities, frequent student interaction, coupled with independent and group learning. We explore the advantages and disadvantages of this manner of running summer placements.

Understanding particle adhesion is vital to any industry where particulate systems are involved. There are multiple factors that affect the strength of the adhesion force, including the physical properties of the interacting materials and the system conditions. Surface roughness on the particles and the surfaces to which they adhere, including roughness at the nanoscale, is critically important to the adhesion force. The focus of this work is on the capillary force that dominates the adhesion whenever condensed moisture is present. Theoretical capillary forces were calculated for smooth particles adhered to smooth and rough surfaces. Simulations of the classical centrifuge technique used to describe particle adhesion to surfaces were performed based on these forces. A model was developed to describe the adhesion of the particles to the rough surface in terms of the adhesion to a smooth surface and an ‘effective’ contact angle distribution.

In the oil and gas industry, acoustic transducers have been found to provide valuable geological sonic information such as compressional wave velocity, shear wave velocity, and rock formation slowness. These data can be used to indicate lithology, determine porosity, detect over-pressured formation zones, and check well to well correlation. One category of such acoustic transducers is equipped with piezoelectric elements. Conventional piezoelectric transducers are packaged by epoxy resin. Because of the liquid nature of uncured epoxy resin, it is difficult to position the piezoelectric elements accurately. The introduction of polyether ether ketone (PEEK) as the packaging material solved this issue. Due to the ease of machining on solid form, architectures of the composite acoustic transducers can be devised with great flexibility and creativity. These designs can be modeled with finite element methods (FEM) and the best design for the oil drilling application can be finalized and fabricated.

COMSOL Multiphysics® solves problems in a programming environment that integrates relevant physics. In this case, it includes electrical circuit, solid mechanics, acoustics, and piezoelectricity. Here a compete model and procedure to study the performance of an architected composite acoustic transducer is provided. The displacement analysis gives insights into the resonance modes of the piezoelectric elements. The acoustics analysis gives the necessary information on the acoustic performance of the transducers, such as acoustic pressure spatial distribution, acoustic pressure frequency response, transmitting voltage response, and directivity. These are important criteria to judge the effectiveness of an architected transducer.

Feasibility of multiwavelength Raman spectroscopy was studied as a potentialin-line monitoring technique for grain size distribution in channel poly-Si usedin three dimensional stacked NAND (3D NAND) Flash memory devices. Variouschannel poly-Si materials in 3D-NAND Flash memory devices, converted fromchemical vapor deposition (CVD) grown a-Si, were characterized usingnon-contact, multiwavelength Raman spectroscopy and high resolutioncross-sectional transmission electron microscopy (HRXTEM). The Ramancharacterization results were compared with HRXTEM images. The correlationbetween the grain size distribution characterized by multiwavelength Ramanspectroscopy and “on current” (ION) of 3D NANDFlash memory devices was investigated. Good correlation between these techniqueswas seen. Multiwavelength Raman spectroscopy is very promising as anon-destructive in-line monitoring technique for grain size distribution inchannel poly-Si used in 3D NAND Flash memory devices.

Black phosphorus (black-P) research has been the most absent for 100 years since its date of first synthesis in 1914 among all the polymorphs of phosphorus. However, recently it has been re-examined due to its specific puckered single layer geometry. Few or single atomic layer forms of black-P can be isolated using techniques such as micromechanical or liquid exfoliation. However, the exfoliation techniques limit the use of black-P, hence a method of black-P layer deposition onto a substrate is needed. Few or atomic layer deposition of black-P leads to substrate-material interactions and a possible appearance of a band gap opening at the K point. Hence, a series of experiments were conducted in order to grow black-P on different substrates. With the incorporation of doping elements, there was substantial modification in the vibrational and electrical properties. It was observed that sulfur and boron doped films exhibit improved electrical and electronic properties as compared to pristine black-P. Density functional theory predicts significant changes in the band structure and density of states as a consequence of doping. The effects of doping was also reflected in Raman A1g mode. The shift in peak position was also found to be consistent with molar mass of dopants.

A-D-A architectured quinoxaline and bezodithiophene based small molecule exhibiting low band gap was designed and synthesized for bulk heterojunction solar cell application. Two dimension conjugation to the molecule was imparted by using alkylated aromatic units. This effective extension of conjugation broadens the absorption spectra. Optical and electrochemical properties suggest that the synthesized molecule has a low band-gap and well matching frontier molecular orbital energy levels with that of PCBM. A photovoltaic device using this A-D-A molecule as the active layer was fabricated to evaluate the optoelectronics properties.

Cellulose is an abundant green polymer, which can be obtained in a variety ofnanoscale structures broadly grouped as nano/microfibrils (CNF/MFC), bacterialcelluloses (BC) or nano/microcrystals (CNC/CMC). There is increasing interest ofnanocelluloses by the research and industrial communities due to increasingavailable materials (facilities than can produce ton per day), impressivestrength properties, low density, renewability and biodegradability. However,one problem is the lack of knowledge on the nanomechanical properties ofcellulose nanofibrils, which creates barriers for the scientists and producersto optimize and predict behavior of the final product.

In this research, the behavior of thin filmed (t≤100 μm)cellulose nanofibrils’, located on aluminum pin stubs, under nanocompression loads were investigated using an Asylum Research MFP-3D Atomic ForceMicroscope equipped with a nanoindenter. Unloading curves were analyzed usingOliver-Pharr. As a result of 58 successful nanoindents, the average modulusvalue was estimated as 16.6 GPa with the reduced modulus value of 18.2 GPa. TheCNF Modulus values varied between 12.4 GPa – 22.8 GPa with 16.9%coefficient of variation (COV) while the reduced modulus ranged from 13.7 GPa to24.9 GPa with a 16.2 % COV.

This research provides practical knowledge for producers of nanocellulose,researchers and applications developers who focus on nanocellulose reinforcedcomposite materials.

Iron oxide nanotubes (NTs) and zinc doped iron oxide NTs were prepared by sol-gel method. Obtained NTs have spinel structure represented by ZnδFe3-δO4. Iron oxide NTs indicated a spectral sensitivity maximum at the wavelength of 500-550 nm and a long tail toward red light. These features were explained by a typical bandgap of iron oxide NTs to be ∼2.3 eV and bandgap narrowing for a small diameter NT. Organometal perovskite solar cell was constructed by using these NTs and evaluated by measuring the spectral sensitivity between the wavelength of 390 and 710 nm. For the present perovskite solar cell, drastic increase of the spectral sensitivity was obtained when we adjusted penetration depth of organometal perovskite into the porous layer of NTs. However, deep penetration worsened the spectral sensitivity. A similar trend was obtained for the cell using Zn doped iron oxide NTs, but the magnitude of incident power conversion efficiency (IPCE) became almost a half. This was explained by worsening the electrical conductivity, even the bandgap became narrower by Zn dope.

The solar absorptance property of W/WAlN/WAlON/Al2O3-based coatings, deposited by DC/RF magnetron sputtering on stainless steel substrate was studied by measuring the reflectance spectra in the wavelength range of 250 - 2500 nm. The effect of thermal annealing on the optical properties of the solar selective absorber coatings was investigated. Annealing the coatings at 450°C for 150 hrs in air did not show any significant change in the spectral properties of the absorber coating indicating the excellent thermal stability of the coating. The W layer acts as infrared reflective layer and diffusion barrier on stainless steel substrate. The top Al2O3 layer serves as dense shield to protect the under layers from oxidation in air. In summary, the present study indicates the potential application of W/WAlN/WAlON/Al2O3-based selective coatings in high temperature photo thermal conversion systems.

Diatoms are found in nearly every aqueous environment and play a vital part of the global primary production system contributing with up to 25 % and are efficient light harvesting organisms. Unique to diatoms are the hard cell wall, called the frustule surrounding the single cell. The frustule is made from bio-synthesized silicate, perforated by wavelength sized features where the morphology of the nano-structured “greenhouse” is species dependent. Diatoms would therefore make for one of the most interesting “green” resources since it has not only potential as a biomass production system but also for nano-structured inorganic material. To understand the biological significance and to integrate diatomic frustules as active material in devices a fundamental understanding of how light interacts with the frustule is needed. In this study we focus on centric diatoms, i.e. having rotational symmetry where morphological parameters vary between the different investigated species. We report how light interacts with the frustule in the wavelength range from UV-A (320-380 nm) to NIR (900 nm). High resolution spectroscopy and CCD images are used to identify photoluminescence (PL) and variations in the transmitted light caused by the nano-structured frustule. Furthermore we show, by placing the frustule on a quartz half sphere how light transmission is a function of the angle of incidence and wavelength.

We present a review of our recent studies of Bias Temperature Instability (BTI) in Metal-Oxide-Semiconductor Field-Effect-Transistors (MOSFETs) fabricated with different material systems, highlighting the reliability opportunities and challenges of each novel device family. We discuss first the intrinsic reliability improvement offered by SiGe and Ge p-channel technologies, if a Si cap is used to passivate the channel, in order to fabricate a standard SiO2/HfO2 gate stack. We focus on SiGe gate stack optimizations for maximum BTI reliability, and on a simple physics-based model able to reproduce the experimental trends. This model framework is then used to understand the suboptimal BTI reliability and excessive time-dependent variability induced by oxide defect charging in different high-mobility channel gate stacks, such as Ge/GeOx/high-k and InGaAs/high-k. Finally we discuss how to pursue a reduction of charge trapping in alternative material systems in order to boost the device reliability and minimize time-dependent variability.

Flexible SAW devices based on ZnO piezoelectric thin film deposited on ultra-thin flexible glass were fabricated and their performances as a strain sensor have been investigated. The XRD and AFM characterizations showed that the ZnO layers have good crystal quality and smooth surface. The flexible SAW devices show excellent strain sensitivity which increases from ∼87 to ∼137 Hz/με with the increasing ZnO thickness, and the sensors can withstand strains up to ∼3000 με, 4∼6 times larger than those of SAW strain sensors on rigid substrates. The sensors exhibited remarkable stability up to hundreds of times bending under large strains. The effects of environmental variables (temperature, humidity, UV light) on the sensor performance have been investigated. The temperature has a significant effect on the performance of the SAW strain sensor, while humidity and light have limited effect.

The dynamic evolution and interaction of defects under the conditions of shock loading in nanocrystalline Al with an average grain size of 20 nm is investigated using molecular dynamics simulations for an impact velocity of 1 km/s. Four stages of defect evolution are identified during shock deformation and failure that correspond to the initial shock compression (I), the propagation of the compression wave (II), the propagation and interaction of the reflected tensile waves (III), and the nucleation, growth, and coalescence of voids (IV). The results suggest that the spall strength of the nanocrystalline Al system is attributed to a high density of Shockley partials and a slightly lower density of twinning partials (twins) in the material experiencing the peak tensile pressures.