Charter D. Stinespring

Fluorination of diamond (100) by atomic and molecular beams

Diamond (100) substrates have been fluorinated under ultrahigh vacuum conditions with both atomic and molecular fluorine. X‐ray photoelectron spectra of the resulting samples indicate that atomic fluorine, F, reacts efficiently at 300 K producing a saturation coverage of about three quarters of a monolayer (one monolayer ≂1.6×1015 cm−2) after 40 monolayers exposure. The carbon fluoride adlayer is thermally stable to 700 K but slowly desorbs at temperatures above this. In contrast, molecular fluorine, F2, reacts quite slowly; a saturation coverage of less than one fifth of a monolayer is achieved after several hundred monolayer exposure to F2 at temperatures from 300 to 700 K. Diamond surfaces saturated with fluorine atoms showed no loss of fluorine after sequential exposure to beams of H2 and O2 at temperatures between 300 and 700 K.

The relation of active nitrogen species to high-temperature limitations for (0001̄) GaN growth by radio-frequency-plasma-assisted molecular beam epitaxy

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.

Direct liquefaction of coal using ferric-sulfide-based, mixed-metal catalysts containing Mg or Mo

Direct liquefaction of coal was studied using ferric-sulfide-based mixed-metal catalysts containing magnesium or molybdenum as the second metal. The catalysts were mostly impregnated in situ on the coal, although physical mixtures of catalyst and coal were also used in some runs for comparison. The liquefaction was performed at 350–440°C under a hydrogen pressure of 6.9 MPa (cold). Tetralin and phenanthrene were used as solvents. The catalytic effects became more evident with phenanthrene as solvent. The activities of impregnated catalysts were 5–8% higher than those of the physical mixtures of catalyst and coal. The addition of magnesium was found to be not particularly beneficial to the activity and selectivity of the catalyst. The addition of molybdenum increased the catalyst activity by up to 8 wt%, resulting in conversions of >90 wt% at 400°C. The yield of the oil fraction also increased considerably in the presence of molybdenum, especially at 400 and 440°C. The activity of the catalyst decreased by ∼5% when it was exposed to air.

Hydrogen ion interactions with silicon carbide and the nucleation of diamond thin films

Ultrahigh‐vacuum surface studies of hydrogen ion interactions with silicon carbide thin films were performed to provide new insights into the mechanisms of diamond thin‐film nucleation. These experiments were carried out at room temperature using hydrogen ions with energies of 10, 100, 500, and 2000 eV. In situ analyses using Auger electron spectroscopy indicated that silicon atoms were removed from the surface and near‐surface layers of the film, and the resulting carbon‐rich layers were converted to a mixture of sp2 and sp3 carbon. The relative amounts of sp2 and sp3 species formed were strongly dependent upon ion energy. The highest concentration of sp3 carbon was obtained using 500 eV ions. Theoretical considerations suggest this behavior was the result of both chemical and energy transfer effects.

Novel surface chemical synthesis route for large area graphene-on-insulator films

The feasibility of a halogen-based surface chemical route to the synthesis of large area graphene-on-insulator films is reported. Both CF4- and Cl2-based plasmas have been used to etch 6H-SiC (0001) surfaces, which were then annealed at 970 °C. These surfaces were characterized using x-ray photoelectron spectroscopy, reflection high energy electron diffraction, atomic force microscopy, and Raman spectroscopy. It was shown that the etching process leads to selective removal of silicon from the SiC matrix to produce carbon rich surface layers. When annealed, these layers reconstruct to form a graphene film. Electrical measurements indicated the resistivity and carrier density of these films are similar to those of few layer graphene.

Electrical properties of strained nano-thin 3C–SiC/Si heterostructures

The effects of strain on the conduction mechanism in heterostructures consisting of strained nano-thin 3C–SiC films on Si are reported. These films exhibit significantly different electrical behaviours than the bulk material. Strained nano-thin 3C–SiC films were grown on n-type Si substrates by gas source molecular beam epitaxy. Reflection high-energy electron diffraction patterns show that these films are about 3% strained relative to the SiC lattice constant. In order to investigate the electrical properties of thin film structures, Al, Cr and Pt contacts to a nano-thin film 3C–SiC were deposited and characterized. The I–V measurements of the strained nano-thin films demonstrate back-to-back Schottky diode characteristics and the band offsets due to the biaxial tensile strain introduced within the 3C–SiC films were calculated and simulated. Based on the experimental and simulation results, an empirical model for the current transport in the heterostructures based on strained nano-thin films has been proposed. It was found that due to the band alignment of this structure, current is constrained at the surface which allows use of nano-thin films as surface sensors.

Evidence for surfactant mediated nucleation and growth of diamond

Auger electron spectroscopy has been used as an in situ diagnostic in ultrahigh vacuum studies of diamond nucleation and growth on silicon. It is demonstrated that sp3-bonded carbon can be formed under ultrahigh vacuum conditions in the absence of excess hydrogen using a flux of C2H4 molecules. For the conditions reported here, a silicon adlayer is always present on the surface of the growing thin film. The presence of this adlayer suggests that the formation of sp3-bonded carbon occurs by a surfactant mediated process. Specifically, silicon is thought to maintain the template for sp3-carbon growth by minimizing the surface free energy for this structure. Further, the silicon adlayer is thought to enhance the growth kinetics of sp3 carbon by providing a more reactive growth surface than a hydrogen-terminated carbon surface.

Surface Studies Relevant to the Initial Stages of Diamond Nucleation

Studies of diamond heteroepitaxy on silicon indicate that C-C surface species act as nucleation precursors. We have investigated the conversion of the Si(100) 2×1 surface to SiC using C 2 H 4 to obtain an understanding of how C-C species may be formed and to determine the effect of an O-adlayer on enhancing or selecting the reaction channel which leads to these species. Under appropriate conditions, the interaction between C 2 H 4 and the clean silicon surface yields both SiC and C-C species. The presence of an O-adlayer significantly reduces the activity of silicon and enhances the formation of sp 2 and sp 3 C-C species. These results provide key insights into diamond nucleation conditions in conventional growth processes.

Laser Induced Surface Chemical Epitaxy

Studies of the thermal and photon-induced surface chemistry of dimethyl cadmium (DMCd) and dimethyl tellurium (DMTe) on GaAs(100) substrates under ultrahigh vacuum conditions have been performed for substrate temperatures in the range of 123 K to 473 K. Results indicate that extremely efficient conversion of admixtures of DMTe and DMCd to CdTe can be obtained using low power (5 - 10 mJ cm-2) 193 nm laser pulses at substrate temperatures of 123 K. Subsequent annealing at 473 K produces an epitaxial film.

Method for rapid determination of ion gauge sensitivity factors

In ultrahigh vacuum thin film growth processes using gas phase growth precursors, the pressure of the gas at or near the substrate is a critical parameter since it is directly related to the collision frequency of the precursor with the substrate and ultimately to the growth rate. These pressures are usually measured using a nude Bayrd-Alpert-type ion gauges, which are generally calibrated for nitrogen. Consequently, it is necessary to know the ion gauge sensitivity factor that relates the measured pressure to the actual pressure of the growth precursor. The purpose of this article is to describe a simple method to obtain such sensitivity factors. This method uses a simple gas manifold comprised of equipment commonly found in laboratory settings where ultrahigh vacuum work is performed. Results are reported for dimethyl silane, monomethyl silane, methane, and hydrogen. The gauge sensitivity factors for the latter two gases are known and, therefore, provide a basis for validating the method.