J. B. Boyce

High critical currents in strained epitaxial YBa2Cu3O7-δ on Si

Epitaxial YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) films were grown on Si (100) using an intermediate buffer layer of yttria-stabilized zirconia. Both layers are grown via an entirely {ital in} {ital situ} process by pulsed laser deposition. All films consist of {ital c}-axis oriented grains as measured by x-ray diffraction. Strain results from the large difference in thermal expansion coefficients between Si and YBCO. Thin ({lt}500 A) YBCO films are unrelaxed and under tensile strain with a distorted unit cell. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield for Ba as low as 12%. The normal-state resistivity is 280 {mu}{Omega} cm at 300 K; the critical temperature {ital T}{sub {ital c}} ({ital R}=0) is 86--88 K with a transition width ({Delta}{Tc}) of 1 K. Critical current densities of 2{times}10{sup 7} A/cm{sup 2} at 4.2 K and 2.2{times}10{sup 6} A/cm{sup 2} at 77 K have been achieved.Epitaxial YBa2Cu3O7−δ (YBCO) films were grown on Si (100) using an intermediate buffer layer of yttria‐stabilized zirconia. Both layers are grown via an entirely in situ process by pulsed laser deposition. All films consist of c‐axis oriented grains as measured by x‐ray diffraction. Strain results from the large difference in thermal expansion coefficients between Si and YBCO. Thin (<500 A) YBCO films are unrelaxed and under tensile strain with a distorted unit cell. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield for Ba as low as 12%. The normal‐state resistivity is 280 μΩ cm at 300 K; the critical temperature Tc (R=0) is 86–88 K with a transition width (ΔTc) of 1 K. Critical current densities of 2×107 A/cm2 at 4.2 K and 2.2×106 A/cm2 at 77 K have been achieved.

Raman spectroscopy of amorphous and microcrystalline silicon films deposited by low‐pressure chemical vapor deposition

In this work we used Raman spectroscopy to investigate the structural characteristics of as‐deposited amorphous and micro‐crystalline silicon films. For amorphous silicon films, the order (or disorder) of the silicon network was quantified using properties of the Raman spectra that were related to key deposition conditions. We found that a strong relationship exists between the structural order of the silicon matrix and the deposition temperature and deposition rate. A quantitative model was proposed relating the intensity ratio of transverse optical phonon peak to longitudinal optical phonon peak to the surface diffusion length, a parameter that was calculated from available data. It was found that optimization of the as‐deposited silicon microstructure is possible by selecting deposition conditions yielding peak–ratio values in the vicinity of 0.53. For as‐deposited micro‐crystalline silicon films, Raman spectroscopy was used to estimate the initial crystalline fraction of the film and monitor the cryst...

Capillary waves in pulsed excimer laser crystallized amorphous silicon

During short‐pulse laser crystallization of amorphous silicon on quartz, surface roughening occurs via the freezing of capillary waves excited in the silicon melt. The velocity and viscous damping of these capillary waves is computed and discussed. Volume change of the silicon during solidification appears to drive liquid silicon toward the last areas of solidification. Film thickness variation observed by transmission electron microscopy and atomic force microscopy shows increased film thickness at grain boundaries, and vertices of single pulse irradiated films. This effect is most pronounced within a narrow laser fluence regime wherein large lateral grain growth occurs. For 100 nm thick amorphous silicon films on quartz, this regime extends from approximately 520 to 560 mJ/cm2; standard deviation roughness can be as large as 40 nm. These effects have important implications for large area thin film transistor manufacturing.

Structural, optical, and spin properties of hydrogenated amorphous silicon‐germanium alloys

We report on a detailed study of structural and electronic properties of hydrogenated amorphous silicon‐germanium alloys deposited by rf glow discharge from SiH4 and GeH4 in a diode reactor. The chemical composition of the alloys is related to the deposition conditions, with special emphasis on preferential incorporation of Ge into the solid phase and on the role of inert dilutant gases. Hydrogen bonding in the alloys is investigated with nuclear magnetic resonance and vibrational (Raman and infrared) spectroscopy. The optical properties of a‐SiGe:H samples deposited under optimal conditions are analyzed with the help of subgap absorption measurements and band‐tail luminescence for the entire range of alloy composi‐tions. A large part of the article describes an investigation of the electron‐spin‐resonance response of undoped alloys. The spin density associated with dangling bond defects localized on Si and Ge atoms has been measured as a function of alloy composition for optimized material. In addition, ...

Comparison of PbI2 and HgI2 for direct detection active matrix x-ray image sensors

The factors determining the x-ray sensitivity of HgI2 and PbI2 as direct detector materials for large area matrix addressed x-ray image sensors are described, along with a model to explain their different properties. The imaging studies are made on test arrays with 512×512 pixels of size 100 μm. The x-ray sensitivity and spatial resolution are reported, along with measurements of the various mechanisms that influence the sensitivity, such as charge collection, x-ray absorption, fill factor, and image lag. The spatial resolution of PbI2 decreases with increasing film thickness, but this effect is not observed in HgI2. The x-ray response data are used to compare the sensitivity to the theoretical values for the ionization energy and to identify the various loss mechanisms. We find that the sensitivity of HgI2 can be explained by a few small and well characterized loss factors. This material exhibits good spatial resolution, high fill factor, and high charge collection. PbI2 films exhibit lower sensitivity, ...

LASER DEHYDROGENATION/CRYSTALLIZATION OF PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITED AMORPHOUS SILICON FOR HYBRID THIN FILM TRANSISTORS

A low temperature process for laser dehydrogenation and crystallization of hydrogenated amorphous silicon (a‐Si:H) has been developed. This process removes hydrogen by laser irradiations at three energy steps. Studies of hydrogen out‐diffusion and microstructure show that hydrogen out‐diffusion depends strongly on film structure and the laser energy density. Both high quality and low leakage bottom gate polycrystalline silicon and a‐Si:H thin film transistors were monolithically fabricated on the same Corning 7059 glass substrate with a maximum process temperature of only 350 °C.

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

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 compo...

Properties of epitaxial YBa2Cu3O7 thin films on Al2O3 {1̄012}

Epitaxial YBa2Cu3O7 films were grown on Al2O3 {1012} by a laser ablation technique. X‐ray diffraction shows that films are epitaxial with the c axis perpendicular to the substrate surface and ‘‘123’’ [110] aligned with sapphire [1011], although the full width at half maximum of the rocking curve is larger than those of epitaxial films on SrTiO3. Typical Tc’s are between 85 and 88 K with transition widths between 0.5 and 3 K. The normal‐state resistivity is 270 μΩ cm at 300 K and extrapolates to zero at zero temperature while the magnetization Jc is as high as 5×106 A/cm2 at 4.2 K. High‐frequency loss measurements show that 2000‐A‐thick epitaxial films on Al2O3 {1012} have a surface impedance about 1 mΩ at 13 GHz at 4.2 K.

Low temperature crystallization of amorphous silicon using an excimer laser

Low temperature processing is a prerequisite for compatible technologies involving combined a-Si and poly-silicon devices or for fabricating these devices on glass substrates. This paper describes excimer-laser-induced crystallization of thin amorphous silicon films deposited by plasma CVD (a-Si:H) and LPCVD (a-Si). The intense, pulsed UV produced by the laser is highly absorbed by the thin amorphous material, but the average temperature is compatible with low temperature processing. The process produces crystallites whose structure and electrical characteristics vary according to starting material and laser scan parameters. The crystallized films have been principally characterized using x-ray diffraction, TEM, and transport measurements. The results indicate that crystallites nucleate in the surface region and are randomly oriented. The degree of crystallization near the surface increases as the doping level and/or deposited laser energy density is increased. The crystallite size increases with a power law dependence on deposited energy, while the conductivity increases exponentially above threshold for unintentionally doped PECVD films. The magnitude of the Hall mobility of the highly crystallized samples is increased by two orders of magnitude over that of the amorphous starting material.

Buffer layers for high-quality epitaxial YBCO films on Si

Efforts aimed at producing device-quality YBa/sub 2/Cu/sub 3/O/sub 7- delta / (YBCO) films on Si, which have resulted in films with properties comparable to what can be achieved with conventional oxide substrates such as SrTiO/sub 3/, are described. It is reported how epitaxial YBCO films were grown on Si