Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

SiC nanowires (NWs) designed from selected bulk polytypes are investigated using quantum mechanics-based simulations (both ab initio and semiempirical methods), and their structure–property relationships examined vis-à-vis their size, shape, and orientation. It is found that 2H–SiC NWs of hexagonal morphology oriented along the <0001> direction are the most stable ones compared to NWs designed using other bulk polytypes (e.g., 3C, 4H, or 6H) and other morphologies (round, square, rhombus, etc.) for diameters with 1 nm < d < 14 nm. Based on the electronic density calculations, it is found that 2H–SiC <0001> NWs exhibit semiconductor-like characteristics (akin to their bulk counterparts), even when their diameters approach 1 nm. On the other hand, SiC NWs designed from 3C, 4H, and 6H bulk polytypes, regardless of their morphology, exhibit gapless features for diameters less than 3.5 nm. Undoubtedly, these novel properties of SiC NWs can be exploited in the fabrication of nanoscale devices.

InTaO4 is an efficient visible-light photocatalyst, which used to be synthesized by solid-state fusion at over 1100 °C. However, irregular morphology and severe agglomeration of particles were acquired due to nonuniform fusion of solid precursors. In this study, InTaO4 was synthesized by two sol-gel routes, the thermal hydrolysis and esterification methods. The precursors were indium (III) nitrate pentahydrate [In(NO3)3] and tantalum(V) butoxide [Ta(OC4H9)5] dissolved in solutions. The InTaO4 powders with a uniform grain size of 17.7 nm were successfully synthesized using the esterification method at a calcination temperature of 950 °C. A uniform InTaO4 thin film nearly 40 nm thick formed on an optical fiber at 1100 °C using the sol prepared by the esterification method. For the first time, InTaO4 was evaluated by the photocatalytic activity of CO2 photo reduction, which was conducted in aqueous solution under visible light irradiation. Cocatalyst NiO was loaded on the surface of InTaO4 to further enhance the methanol yield. The methanol yields of NiO/InTaO4 by esterification method were significantly higher than those by solid-state fusion. The esterification method provided homogeneous mixing of Ta(OC4H9)5 and In(NO3)3, resulting in nano-sized InTaO4 with uniform crystallinity and superior photocatalytic activity.

Titania and copper-loaded titania were synthesized by an improved sol-gel method using a homogeneous hydrolysis technique. Unlike the conventional sol-gel procedure that added water directly, the esterification of anhydrous butanol and glacial acetic acid provided the hydrolyzing water. In addition, acetic acid also served as a chelating ligand to stabilize the hydrolysis-condensation process and minimize the agglomeration of titania. Fourier transform infrared spectra confirmed the presence of bidental ligand. The sol was dried, then calcined at 500°C to remove organics and transformed to anatase titania. Transmission electron microscopy revealed that the titania particles were uniform, and the particle size ranged from 10 to 25 nm. The band gaps of TiO2 and Cu/TiO2 ranged from 3.01 to 3.17 eV based on the diffusive reflective ultraviolet-visible spectrometry. X-ray photoelectron spectroscopy analysis showed a positive shift of binding energy of Ti2p3/2 and a negative shift of Cu 2p3/2 in Cu/TiO2. The redistribution of electric charge is due to the Schottky barrier of Cu and TiO2.

In this work, power and reliability performance of pseudomorphic AIGaAs/InGaAs HEMT's are investigated by 2-D device simulation, spatially-resolved electro-luminescence, light emission spectra analysis, and gate current instabilities. A two-dimensional device simulation was used to exploit the off/on state breakdown origins in the power PHEMT's and to explore the physical mechanisms responsible for light emission in both conditions. A correlation between simulated results and light emission spectra highlights the breakdown origins in PHEMT's.

PHEMT's subjected to off-state breakdown stress and on-state hot carrier stress show changes in device characteristics. While gate leakage current, i.e. a surface leakage component associated with the surface passivation layer is reduced by these stresses, a reduction in drain current, transconductance degradation, and an increase in the impact ionization generated gate current are also observed.

Further improvement in off/on state breakdown voltages and device reliability calls for device structure optimization for lower electric field design, surface passivation treatment for lower surface leakage current, and Schottky barrier enhancement for lower gate current.

The reliability of high performance carbon and beryllium-doped heterojunction bipolar transistors (HBT’s) is investigated using a pulsed mode current stress. After the current stress, the collector current reduction (measured at a fixed Vbe) and the inverted collector current ratio change (measured at two different reverse Vbe biases) are used as measures of HBT degradation due to dopant acceptors migration. For carbon and beryllium doped HBT’s, degradation was found to be only significant beyond a threshold current density (Jth), showing that the carbon doped devices have a higher Jth than that of the Be-doped HBT's. Beyond the Jth, the device degradation is insensitive to the stress current density but depends on the total stress charge. As a result, the device degradation mechanisms of electric field assisted dopant and carrier recombination enhanced dopant migration can be distinguished under pulsed mode current stress.

Pseudomorphic HEMT (PHEMT) devices have demonstrated superior performance at microwave and millimeter wave frequency ranges. They exhibit multi-functional characteristics such as high power, high efficiency and low noise over a broad frequency range (C-band through W-band). Because of their important, broad range of applications, the microwave industry has recently begun developing the PHEMT manufacturing technology in order to cope with the increasing demand for PHEMT insertion into microwave products. In this paper we will discuss the advantages of using PHEMT devices for microwave and millimeter wave applications and provide an overview of the published state-ofthe-art performance of PHEMT devices and monolithic microwave integrated circuits (MMICs) at microwave and millimeter wave frequencies. We will also present recent progress in Hughes PHEMT technology development including device epitaxial design and process enhancements. Finally, we will present recent data on PHEMT reliability.

Schottky barrier structures often exhibit forward bias current voltage characteristics possessing a marked deviation from ideal behavior. In many cases, this deviation can be attributed to the influence of an appreciable series resistance on the diode characteristic making the determination of the actual Schottky barrier height from this characteristic difficult. In a study of rare earth silicides on Si, the present authors have successfully employed an alternative technique which overcomes this series resistance effect. This technique is derived from the conventional methods for obtaining barrier height and series resistance values in photovoltaic devices, although not necessarily requiring transparent metallizations. Photogenerated current from the device periphery is utilized here in the determination of the barrier height. The measurement of both open circuit voltage and the short circuit current at varying illumination intensities allows the junction characteristic to be determined essentially free from series resistance effects. A comparison will be made between the values of the barrier height derived from conventional J-V and photoresponse measurements, with photovoltaic measurements made on structures possessing either a semi-transparent or opaque metallization.