High-Temperature Stable Operation of Nanoribbon Field-Effect Transistors

We experimentally demonstrated that nanoribbon field-effect transistors can be used for stable high-temperature applications. The on-current level of the nanoribbon FETs decreases at elevated temperatures due to the degradation of the electron mobility. We propose two methods of compensating for the variation of the current level with the temperature in the range of 25–150°C, involving the application of a suitable (1) positive or (2) negative substrate bias. These two methods were compared by two-dimensional numerical simulations. Although both approaches show constant on-state current saturation characteristics over the proposed temperature range, the latter shows an improvement in the off-state control of up to five orders of magnitude (−5.2 × 10−6)

Effect of Alcohol Structure on the Kinetics of Etherification and Dehydration over Tungstated Zirconia

Linear and branched ether molecules have attracted recent interest as diesel additives and lubricants that can be produced from biomass-derived alcohols. In this study, tungstated zirconia was identified as a selective and green solid acid catalyst for the direct etherification of primary alcohols in the liquid phase, achieving ether selectivities of >94 % for C6 -C12 linear alcohol coupling at 393 K. The length of linear primary alcohols (C6 -C12 ) was shown to have a negligible effect on apparent activation energies for etherification and dehydration, demonstrating the possibility to produce both symmetrical and asymmetrical linear ethers. Reactions over a series of C6 alcohols with varying methyl branch positions indicated that substituted alcohols (2°, 3°) and alcohols with branches on the β-carbon readily undergo dehydration, but alcohols with branches at least three carbons away from the -OH group are highly selective to ether. A novel model compound, 4-hexyl-1dodecanol, was synthesized and tested to further demonstrate this structure-activity relationship. Trends in the effects of alcohol structure on selectivity were consistent with previously proposed mechanisms for etherification and dehydration, and help to define possible pathways to selectively form ethers from biomass-derived alcohols

GaAsSb-based heterojunction tunnel diodes for tandem solar cell interconnects

We report a new approach to tunnel junctions that employs a pseudomorphic GaAsSb layer to obtain a band alignment at a InGaAs or InAlAs p-n junction favorable for forward bias tunneling. Since the majority of the band offset between GaAsSb and InGaAs or InAlAs is in the valence band, when an GaAsSb layer is placed at an InGaAs or InAlAs p-n junction the tunneling distance is reduced and the tunneling current is increased. For all doping levels studied, the presence of the GaAsSb-layer enhanced the forward tunneling characteristics. In fact, in a InGaAs/GaAsSb tunnel diode a peak tunneling current sufficient for a 1000 sun intercell interconnect was achieved with p = 1.5{times}l0{sup 18} cm{sup -3} while a similarly doped all-InGaAs diode was rectifying. This approach affords a new degree of freedom in designing tunnel junctions for tandem solar cell interconnects. Previously only doping levels could be varied to control the tunneling properties. Our approach relaxes the doping requirements by employing a GaAsSb-based heterojunction

The Effect of Hydrogen Carrier Gas on the Morphological Evolution and Material Properties of GaN on Sapphire

In-situ optical reflectance is used to monitor the morphological evolution of the two-step GaN growth on sapphire. The amount of H{sub 2} carrier gas used in the growth is observed to strongly influence the morphological evolution of the low temperature buffer layer and the subsequent high temperature nucleation behavior, which in turn affects the structural and electrical properties of the GaN epitaxial films. The optical reflectance transients correlate with the sizes and distributions of nuclei as observed by AFM

Integration of diffractive lenses with addressable vertical-cavity laser arrays

An optical interconnection system is being developed to provide vertical, digital data channels for stacked multichip modules. A key component of the system is an array of individually addressable vertical-cavity surface-emitting lasers with diffractive lenses integrated into the substrate to control beam divergence and direction. The lenses were fabricated by direct-write e-beam lithography and reactive ion beam etching into the GaAs substrate. Preliminary device performance data and the design and fabrication issues are discussed