A reduced growth rate for plasma-assisted molecular beam epitaxy of GaN often limits growth to temperatures less than 750 °C. The growth rate reduction can be significantly larger than expected based on thermal decomposition. Conditions producing a flux consisting predominantly of either atomic nitrogen or nitrogen metastables have been established using various radio-frequency sources. The use of atomic nitrogen, possibly coupled with the presence of low-energy ions, is associated with the premature decrease in growth rate. When the active nitrogen flux consists primarily of nitrogen metastables, the temperature dependence of the decrease is more consistent with decomposition rates. A significant improvement in electrical properties is observed for growth with molecular nitrogen metastables.

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High temperature limitations for GaN growth by rf-plasma assisted molecular beam epitaxy: Effects of active nitrogen species, surface polarity, hydrogen, and excess Ga-overpressure J.The relation of active nitrogen species to high-temperature limitations for (0001) GaN growth by radio-frequencyplasma-assisted molecular beam epitaxy A reduced growth rate for plasma-assisted molecular beam epitaxy GaN growth often limits growth to temperatures less than 750°C. The growth rate reduction is significantly larger than expected based on thermal decomposition. Characterization of various rf plasma source configurations indicated that a flux consisting predominantly of either atomic nitrogen or nitrogen metastables can be produced. The use of atomic nitrogen, possibly coupled with the presence of low energy ions, is associated with the premature decrease in growth rate. When the active nitrogen flux consists primarily of nitrogen metastables, the temperature dependence of the decrease is more consistent with decomposition rates. A significant improvement in electrical properties is observed for growth with molecular nitrogen metastables. In addition, atomic hydrogen stabilizes the growing surface of (0001 គ ) GaN.

Experimental studies of C2 H4, C3 H8, and CH4 reactions on the Si(111) surface and C2 H4 reaction on the Si(100) surface have been performed for surface temperatures in the range of 1062–1495 K. These studies used x-ray photoelectron spectroscopy and related ultrahigh vacuum procedures to identify the reaction products, characterize the solid-state transport mechanisms, determine the surface nucleation mechanisms and growth kinetics, and assess the effects of surface orientation. The reaction product was found to be essentially carbidic throughout the course of the reaction, although the surface layer may contain partially hydrogenated C adspecies. The dominant transport process was shown to be Si out-diffusion rather than C in-diffusion. The Si adspecies produced by out-diffusion react at the gas-surface interface with C adspecies to form the SiC film via a two-dimensional nucleation and layer-by-layer growth mechanism. The reaction efficiency for C2 H4 on the Si(111) and Si(100) surfaces was shown to be ∼10−3. The reaction efficiency for C3 H8 and CH4 on the Si(111) surface was shown to be ∼10−5. For growth temperatures above 1395 K, Si diffusion limitations and sublimation from the SiC surface were found to limit the availability of Si for the SiC growth process. In the absence of Si adspecies, the adlayer formed by the reaction of C2 H4 on SiC appeared to passivate the surface with respect to further C2 H4 reaction. When combined with previously reported modeling studies of the associated gas phase chemistry, these results provide the basis for a mechanistic model of the β-SiC chemical vapor deposition process.

In the energy industry, there is a great need for novel low-cost gas-sensing solutions. This is particularly true for shale gas operations where there is a need to monitor both performance and compliance with environmental regulations. Specifically, there is a need to monitor the integrity of well casings as oil and gas producers try to understand and mitigate environmental issues, as well as avoid unfair claims against the industry. To address this need, we report studies on the additive fabrication and characterization of a graphene-based gas sensor through multilayer direct ink writing of graphene-based inks. An evaporation-assisted solvent exchange method allows tunability of graphene concentration while the addition of ethyl cellulose (EC) allows tuning of rheological properties in printable ink formulations. Robotically controlled direct ink writing enables the deposition of films with arbitrary size and shape. Printed films incorporated into sensor packages exhibit voltage dependent sensitivity to chemical effects of CH4 and H2 in an Ar environment. Surface analysis of the printed sensors suggests disordered layering and orientation of the graphene flakes because of distributed nondecomposed residues of EC from film processing. Capitalizing on the EC residues to form 3D scaffolding enables the spatial arrangement of graphene flakes. The disordered arrangement of flakes resulting from their interaction with the EC residue scaffolding contributes to increased surface area availability for gas sensing.