Epitaxial MgO thin films were grown on Si(001) by pulsed laser deposition. In spite of a large (−22.5%) lattice mismatch, epitaxy occurs with alignment of all crystallographic axes. Epitaxial quality and deposition rate are both sensitive to temperature and oxygen pressure. We believe this is the first demonstration of epitaxial MgO on Si. We employ MgO intermediate layers for superconducting epitaxial YBa2Cu3O7−δ/BaTiO3 thin films on Si with a critical current density of 6.7×105 A/cm2 at 77 K.
Ion milling of electrochemically etched tungsten tips is shown to improve the characteristics for scanning tunneling microscopy. The primary mechanism for the enhancement of tip reliability is identified to be the removal of a residual oxide. A greatly decreased radius of curvature is also achieved without significantly changing the macrostructural geometry of the tip.
Direct observations of early stages of molecular-beam epitaxial growth of GaAs on oriented and vicinal (100) Si surfaces are presented. Cross-sectional transmission electron microscopy and plan view scanning electron microscopy images directly reveal three-dimensional island growth for substrate temperatures above 300 °C. Island size, island spacing, surface morphology, and stacking fault defect spacing all increase with substrate temperature for fixed Ga and As fluxes. Below 300 °C, 7-nm-thick films are continuous and uniform. Films deposited on surfaces tilted from (100) coalesce anisotropically with respect to the tilt axis.
Thin films were deposited by pulsed uv-laser (ablation) deposition of Y1Ba2Cu3O7−x (YBCO), and composite zirconia and yttria targets onto silicon wafers. These films were analyzed to ascertain the chemical and physical structure of the film interfaces and further the development of Si substrates for superconducting YBCO films. Substrates were Si(100) with either a high-quality, thermal oxide (SiO2) film, or a spin-etch processed, oxide-free, hydrogen-terminated surface (Si:H). X-ray photoelectron spectroscopy (XPS) of Y, Ba, Cu, and Si core levels revealed adverse reactions for thin (nominally 2 nm) YBCO films deposited directly onto either substrate surface. The surfaces of thicker YBCO films (50–100 nm) and various oxide powders were compared with XPS results from these thin films. The thicker-film surfaces are similar to those of fractured bulk YBCO, while the thin YBCO films decomposed, as evidenced by changes in the Ba and Cu XPS. The Si XPS on these films showed the formation of metal-silicate compounds, even at deposition substrate temperatures of 550 °C, and silica (SiO2), especially for 670 °C deposition. A direct consequence of these reactions is that growth of high-quality epitaxial YBCO on Si will require the use of a buffer film. Yttria-stabilized zirconia (YSZ) shows considerable promise for use as a buffer, and XPS of thin films (4 and 8 nm thick) of ZrO2 on SiO2/Si and YSZ on Si:H substrates did not show any indication of decomposition, even at deposition temperatures near 800 °C. Transmission electron microscopy of cross-sectioned samples of YBCO/YSZ/Si showed that the lower YSZ interface is rough on the preoxidized (SiO2/Si) substrates but atomically sharp on the spin-etched Si wafers (Si:H). These sharp YSZ interfaces showed the presence of 3–5 nm of regrown oxide (SiOx ) next to the crystalline Si substrate. This regrown oxide was observed in samples deposited under a variety of conditions.
When two electrodes at different temperatures are placed m a reacting flame gas at atmospheric pressure, a potential dfference is estabhshed between them.A. von Engel and J. R. Cozens 1500.
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