D. S. Ginley

Passivation of grain boundaries in polycrystalline silicon

Preferential diffusion of various gases down the grain boundaries in polycrystalline silicon is shown to promote significant changes in the density of defect states in these regions. A plasma of monatomic hydrogen provides a significant reduction in both the state density and the accompanying grain‐boundary potential barrier while plasmas of oxygen, nitrogen, and sulfur hexafluoride are shown to increase this density of states. Boundaries passivated with hydrogen have as much as a factor of 1000 larger transconductance after treatment. Hydrogenated barriers are stable over long periods at 375 °C and essentially indefinitely at 23 °C. The results have important implications for the development of low‐cost thin‐film silicon photovoltaic devices.

The photoelectrolysis of water using iron titanate anodes

Single‐crystal and polycrystalline samples of the iron titanates FeTiO3, Fe2TiO4, and Fe2TiO5 have been studied as anodes for the photoelectrolysis of water. Band gaps and flatbands for the various compositions have been determined and aging studies have been conducted which indicate electrochemical leaching of iron atoms from the surface.

Stoichiometry of bulk superconducting La2CuO4+δ: A superconducting superoxide?

Abstract A variety of analytical tools have been combined to show that bulk superconducting La2CuO4+δ has a La to Cu ratio of 2.00 (±0.02) and an oxygen stoichiometry of 4.13 (±0.1). The combination of these analytical results and magnetization data with a model leads us to suggest that the excess charged oxygen is introduced as O-2.

Structural and compositional characterization of polycrystals and single crystals in the Bi- and Tl-superconductor systems: Crystal structure of TlCaBa2Cu2O7

Abstract The growth of mm-sized crystals in the Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O superconductor systems shows the common presence of polycrystals, which can be described as syntactic intergrowth crystals, as well as normal single crystals. Three distinct phases with superconducting transitions at 103 K, 106–108 K and 112–114 K have been grown in the Tl-Ca-Ba-Ca-O system. The newly identified 103 K phase is TlCaBa 2 Cu 2 O 7 which is structurally similar to YBa 2 Cu 3 O 7 but without the presence of chains. Transmission electron diffraction and electron microprobe analysis on several polycrystals show that they are composed of interlayers of these distinct phases grown epitaxially along the c -axis. Crystals have been grown for two phases in the Bi-Ca-Sr-Cu-O system that exhibit transition temperatures at 80 K and at 92 K. Oxygen annealing substantially improves the superconducting properties of most of the crystals.

Studies of the hydrogen passivation of silicon grain boundaries

A model is presented which predicts the current flow through and around grain boundaries which have been treated with atomic hydrogen to reduce their trap‐state densities. Measurements on hydrogenated silicon grain boundaries are shown to be in agreement with this model and quantitative estimates of hydrogen penetration depth are made. The dependence of this depth on sample temperature, surface preparation, hydrogen pressure, and geometry are systematically investigated. Maximum penetration is achieved in high‐pressure discharges for sample temperatures between 350 and 400 °C. The condition of the surface of the polycrystalline silicon is shown to be critically important for the in‐diffusion process.

Sequential electron beam evaporated films of Tl2CaBa2Cu2Oy with zero resistance at 97 K

Thin unoriented polycrystalline films in the Tl‐Ca‐Ba‐Cu‐O system were prepared by sequential electron beam evaporation of multiple Tl, Ca, Ba, and Cu layers and a two‐stage anneal under controlled Tl and oxygen overpressures. These films show zero resistance as high as 97 K and transport critical current densities at 76 K up to 110 000 A/cm2 with typical values of 50 000 A/cm2. The critical currents exhibit a modest magnetic field dependence. Meissner effect data show an onset at 110 K and a superconducting fraction of 75%. Compositional and structural studies indicate that the films, although polyphase, are predominantly Tl2Ca1Ba2Cu2Oy. There is no evidence of other superconducting phases.

Hydrogen sensing with palladium-coated optical fibers

Palladium‐coated, single‐mode optical fibers have been used to detect hydrogen in a concentration range 20 parts per billion to 2% in 1 atm of nitrogen. When the coated fibers are exposed to hydrogen, a hydride is formed with an expanded lattice constant, which stretches the optical fiber. This change in optical path length is measured using a Mach–Zehnder interferometer. A simple one‐dimensional model is presented and shown to accurately reproduce the fiber response. Using electrodeposited palladium films, a factor of 7 enhancement in the hydrogen solubility is observed, as well as a deeply bound site which is saturated in the parts per million concentration range. The data suggest that this site may be due to hydrogen binding to a free surface of the palladium.

Improvement of polycrystalline silicon solar cells with grain‐boundary hydrogenation techniques

Electron‐beam‐induced‐current– and dark‐current–voltage measurements have been made on p/n photovoltaic cells fabricated from polycrystalline silicon. These data have demonstrated that grain‐boundary hydrogenation greatly reduces grain‐boundary minority carrier recombination and improves diode current‐voltage characteristics.

Morphology control and high critical currents in superconducting thin films in the Tl-Ca-Ba-Cu-O system

Abstract Superconducting polycrystalline thin films in the Tl-Ca-Ba-Cu-O system have been prepared by electron beam evaporation followed by appropriate sintering and annealing. The configuration employed (free or confined surface) to sinter the films determines the morphology varying from random to highly oriented, respectively. These films are predominantly the Tl2Ca2Ba2Cu3O10 phase, but some contain up to 50 at % Tl2CaBa2Cu2O8, demonstrating a toleration of variable stoichiometry. The key to obtaining high quality materials with short air sintering times is precise control of the Tl and oxygen partial pressures. Transition temperatures to 110 K and transport critical current densities to 600 000 A/cm2 at 76 K have been attained. The best films (room temperature resistance p 1 mΩcm show a definite correlation between critical current and p, consistent with the presence of weak links.

Confocal resonators for measuring the surface resistance of high‐temperature superconducting films

A quasioptical technique of measuring superconductor surface resistance using a confocal resonator has been developed. The method has advantages of nondestructive analysis, high sensitivity, easy extension to higher frequencies, convenient experimental setup, and flexibility in sample size. Tl‐Ca‐Ba‐Cu‐O high‐temperature superconducting films have been measured with this technique and the measured surface resistances were less than 0.01 Ω at 36.135 GHz and 77 K. The measurements have been performed from 29 to 39 GHz, and all films showed roughly a quadratic dependence of surface resistance with frequency.