Reactions at the interfaces of thin films of Y-Ba-Cu- and Zr-oxides with Si substrates

Abstract

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.