William R. Wampler

Incorporation of nitrogen in nitride-arsenides: Origin of improved luminescence efficiency after anneal

A key to the utilization of nitride-arsenides for long wavelength optoelectronic devices is obtaining low defect materials with long nonradiative lifetimes. Currently, these materials must be annealed to obtain device quality material. The likely defect responsible for the low luminescence efficiency is associated with excess nitrogen. Photoluminescence and capacitance–voltage measurements indicate the presence of a trap associated with excess nitrogen which decreases in concentration upon anneal. Our films are grown by elemental source molecular beam epitaxy and the background impurity concentration is low, thus we have investigated the role of crystalline defects. High resolution x-ray diffraction showed improved crystal quality after anneal. We observed that the lattice parameter does not decrease linearly with nitrogen concentration for levels of nitrogen above 2.9 mol % GaN. The fact that Vegard’s law is not observed, despite theoretical calculations that it should, indicates that nitrogen incorporat...

Far SOL transport and main wall plasma interaction in DIII-D

Far Scrape-Off Layer (SOL) and near-wall plasma parameters in DIII-D depend strongly on the discharge parameters and confinement regime. In L-mode discharges cross-field transport increases with the average discharge density and flattens far SOL profiles, thus increasing plasma contact with the low field side (LFS) main chamber wall. In H-mode between edge localized modes (ELMs) the plasma?wall contact is weaker than in L-mode. During ELM fluxes of particles and heat to the LFS wall increase transiently above the L-mode values. Depending on the discharge conditions, ELMs are responsible for 30?90% of the net ion flux to the outboard chamber wall. ELMs in high density discharges feature intermittent transport events similar to those observed in L-mode and attributed to blobs of dense hot plasma formed inside the separatrix and propagating radially outwards. Though the blobs decay with radius, some of them survive long enough to reach the outer wall and possibly cause sputtering. In lower density H-modes, ELMs can feature blobs of pedestal density propagating all the way to the outer wall.

Equilibrium state of hydrogen in gallium nitride: Theory and experiment

Formation energies and vibration frequencies for H in wurtzite GaN were calculated from density-functional theory and used to predict equilibrium state occupancies and solid solubilities at elevated temperatures for p-type, intrinsic, and n-type material. The solubility of deuterium (D) was measured in p-type, Mg-doped GaN at 600, 700, and 800 °C as a function of D2 pressure and compared with theory. Agreement was obtained by reducing the H formation energies 0.22 eV from ab initio theoretical values. The predicted stretch-mode frequency for H bound to the Mg acceptor lies 5% above an observed infrared absorption attributed to this complex. More limited solubility measurements were carried out for nominally undoped material rendered n-type by donors provisionally identified as O impurities, and results agree well with theory after the aforementioned adjustment of formation energies. It is concluded that currently recognized H states and physical processes can account for the equilibrium, elevated-temperat...

Ion beam studies of hydrogen in metals

Abstract Methods based on ion implantation and nuclear reaction analysis have been developed and used to investigate fundamental aspects of the behavior of hydrogen isotopes in metals. The binding enthalpy of deuterium at irradiation defects, helium bubbles, deuterium bubbles and metal-oxide interfaces was measured for aluminum, iron, nickel, copper, palladium, austenitic stainless steel, Inconel and amorphous Fe 40 Ni 40 P 14 B 6 . The binding enthalpies determined for the pure metals are in excellent agreement with mechanistic calculations based on effective medium theory and other information. Surface-limited release of deuterium from iron, stainless steel and Inconel was measured as a function of temperature and the state of surface oxidation. The release rate was accurately proportional to the square of the deuterium concentration in solution, permitting the results to be expressed in terms of a surface recombination coefficient. This quantity was up to four orders of magnitude greater for an ion-sputtered surface than for a surface with electropolish oxide. The diffusion coefficient and solid solubility of tritium in stainless steel were measured for the first time at the ice point, thereby extending downward by three orders of magnitude the diffusivities available from conventional permeation experiments. Deuterium concentration profiles resulting from electrochemical charging of Incology 903 were measured as a function of charging current, thereby providing a direct systematic calibration of such charging in an austenitic material where conventional permeation measurements are precluded by the small hydrogen diffusion rate.

Diffusion, release, and uptake of hydrogen in magnesium-doped gallium nitride: Theory and experiment

The diffusion and release of H and its uptake from the gas phase are modeled for Mg-doped, wurtzite GaN using formation energies and vibration frequencies from the density-function theory. Comparison is made with rates of deuterium release and uptake measured by nuclear-reaction analysis of deuterium concentration. Good agreement is found when account is taken of a surface permeation barrier.

Trapping and surface recombination of ion‐implanted deuterium in stainless steel

Nuclear‐reaction analysis was used to investigate the behavior of deuterium (D) in three austenitic stainless steels following ion implantation with D and He. The D was bound to irradiation defects with a binding enthalpy of 0.23±0.08 eV relative to solution sites. Helium bubbles of diameter ∼1 nm trapped D with a binding enthalpy of 0.42±0.08 eV, presumably through a chemisorptionlike process at the bubble walls. Surface‐controlled release of D to the gas phase was characterized over the temperature range 425–575 K, and the rate was found to be proportional to the square of the solution concentration within the matrix. Electropolish oxide reduced the surface release rate by as much as 1000 times from that of an ion‐sputtered surface.

Solute binding at void surfaces in silicon and germanium

The strongly exothermic reactions of H and Cu with internal surfaces in Si and Ge were examined in experiments employing ion implantation, ion-beam analysis, transmission electron microscopy, and infrared spectroscopy. The dissociation energy of the Si-H surface bond was determined to be 2.6[plus minus]0.1 eV, making the monohydride more stable than the molecular H[sub 2], whose dissociation energy per atom is 2.26 eV. Initial experiments indicate a dissociation energy for the Ge-H surface bond of [approx]1.9 eV. Copper is bound to the Si surface with an energy of 2.2[plus minus]0.2 eV relative to the solution state, as compared to a reported binding energy of 1.5 eV for Cu in the precipitated Cu[sub 3]Si phase. 5 figs, 14 refs.

Molybdenum erosion measurements in Alcator C-Mod

Erosion of molybdenum was measured on a set of 21 tiles after a run campaign of 1090 shots in the Alcator C-Mod tokamak. The net erosion of molybdenum was determined from changes in the depth of a thin chromium marker layer measured by Rutherford backscattering. Net Mo erosion was found to be approximately 150 nm near the outer divertor strikepoint and much less everywhere else. Gross erosion rates by sputtering were estimated using ion energies and fluxes obtained from Langmuir probe measurements of edge-plasma conditions. Predicted net erosion using calculated gross erosion with prompt redeposition agrees with measured net erosion within a factor of three. Sputtering by impurities, mainly boron, dominates erosion.

Chemical and electrical properties of cavities in silicon and germanium

Cavities were formed in Si and Ge by He ion implantation and annealing, and resultant chemical and electrical properties were investigated. The dissociation energies for Si-H and Ge-H surface monohydride bonds were determined, showing that H chemisorption on Si is energetically stable with respect H{sub 2} gas whereas H chemisorption on Ge is not. Cavity walls in Si were found to trap transition metals strongly, suggesting application to impurity gettering in devices. Measurement and modeling of cavity electrical properties elucidated surface electronic states and indicated a potential for controlled electrical isolation in devices. 35 refs.

The use of silicon carbide as a tritium permeation barrier

Abstract Vapor deposited silicon carbide has a very low tritium diffusivity and solubility. When these characteristics are combined with its very good thermal properties, silicon carbide appears to be an excellent tritium permeation barrier for use in fusion reactors and other applications. Calculations were performed to compare the tritium retention in bare graphite and graphite coated with a thin layer of silicon carbide. The coated graphite retained several orders of magnitude less tritium than the bare material. Similar calculations showed a beryllium/copper duplex structure coated with silicon carbide to allow several orders of magnitude less tritium permeation than an uncoated structure.