A. T. Fiory

Pseudomorphic growth of GexSi1−x on silicon by molecular beam epitaxy

GexSi1−x layers are grown on Si substrates over the full range of alloy compositions at temperatures from 400–750 °C by means of molecular beam epitaxy. At a given growth temperature films grow in a smooth, two‐dimensional manner up to a critical germanium fraction xc. Beyond xc growth is rough. xc increases from 0.1 at 750 °C to 1.0 at ∼550 °C. Rutherford ion backscattering measurements indicate good crystallinity over a wide range of growth conditions. Transmission electron microscopy reveals that in thin films, the lattice mismatch between the GexSi1−x and Si layers can be accommodated by lattice distortion rather than by misfit dislocation formation. This pseudomorphic growth condition can persist to alloy thicknesses as large as l/4 μm.

Preparation and substrate reactions of superconducting Y‐Ba‐Cu‐O films

Multiple metal‐target dc magnetron sputter deposition of a metallic YBa2Cu3 alloy in pure Ar followed by ex situ oxygen annealing was used to prepare superconducting films on various substrates. This work particularly examines film‐substrate reactions which are degrading to superconductivity. Better superconductors were obtained using predeposited buffer layers, notably on cubic zirconia and MgO substrates covered with Ag and Nb. Best films have Tc=80 K, metallic resistivities with resistance ratio≳2, and critical current density ≲10 kA cm−2 at 4.2 K. Normal‐state resistivity measurements were made for O2 furnace temperatures up to 850 °C. Procedures for lithographically patterning these films are also described.

Stability of semiconductor strained‐layer superlattices

The criteria for strained‐layer growth of semiconductor superlattices are discussed. It is shown that to avoid misfit dislocations, careful attention has to be paid to the composition, geometry, and dimensions of the quantum well structure. Two critical thicknesses are found to apply to strained‐layer superlattice growth, one relating to the thickness of individual layers and one relating to the overall thickness of the superlattice. We derive the relationship between these two critical values. Experimental studies of the GexSi1−x/Si on Si and GexSi1−x/Ge on Ge systems using electron microscopy, Raman scattering, and ion channeling show that exceeding the critical superlattice thickness results in a network of dislocations between the substrate and first superlattice layer.

Commensurate and incommensurate structures in molecular beam epitaxially grown GexSi1−xfilms on Si(100)

The transition between commensurate and incommensurate growth of GexSi1−x alloys on Si is observed directly by means of ion channeling and x‐ray diffraction measurements. Molecular beam epitaxial films of thickness h up to 2500 A thick show commensurate epitaxy for x≲50% and h≲hc, a critical thickness dependent upon x. The observed values of hc are discussed in terms of a model invoking the maximum theoretical interfacial shear strength and a barrier to misfit dislocation formation.

Penetration depths of high Tc films measured by two‐coil mutual inductances

A fundamental parameter of sample quality in epitaxial films of high Tc oxides is the effective penetration depth λ∥ of the superconducting sheet. A contactless audio‐frequency method is described, in which an epitaxial YBa2Cu3O7 film is sandwiched between sets of stacked coils, and the procedure is given for computing the complex sheet impedance and hence λ∥ from the change in mutual inductance produced by screening currents in the film. Temperature dependence of the complex impedance is presented.

Anisotropic critical current density in superconducting Bi2Sr2CaCu2O8 crystals

Critical current densities and resistivities were measured in a single crystal of the high Tc superconductor Bi2Sr2CaCu2O8 within the ab basal plane (∥) and along the c direction (⊥). A large anisotropy in critical current density is found, Jc∥/Jc⊥≊103, in quantitative agreement with the large normal‐state resistivity anisotropy near Tc. The data provide strong evidence for a two‐dimensional layered structure of metallic planes separated by semi‐insulating barriers.

Ripple Pyrometry for Rapid Thermal Annealing

Ripple pyrometry was proposed by Accufiber as a method for measuring wafer temperature in lamp-heated ovens. The technique obtains the wafer temperature from the wafer emittance radiation by simultaneously determining the effective emittance of the wafer and suppressing the interference from reflected lamp radiation. This paper reviews the technique for rapid thermal annealing and presents experimental results on silicon wafers instrumented with thermocouples and on processing blanket-film monitor wafers.

Oxygen‐rich polycrystalline magnesium oxide—A high quality thin‐film dielectric

Sputtering of a magnesium target with a beam of argon ions in the presence of a partial pressure of reactive oxygen gas has been found to yield smooth MgOx thin‐film dielectrics with low electrical loss, good mechanical stability, and excellent reproducibility. The films consist of polycrystalline MgO with a grain size ≲50 A and an OH‐containing component. The thickness dependence of the areal capacitance is discussed in the context of a two‐layer model in which this oxygen‐rich phase (x≂1.4) overlays a thin (≲40 A) stoichiometric phase (x≂1.0). Observation of superconducting tunneling characteristics in trilayer Au‐MgOx‐Pb structures confirms pinhole‐free coverage which may have possible application as artificial tunnel barriers.

Optical Fiber Pyrometry with in-Situ Detection of Wafer Radiance and Emittance—Accufiber's Ripple Method

A two-channel optical-fiber pyrometry technique for simultaneous measurement of thermal radiance and emittance was used to measure and control the temperature of silicon wafers heated by quartz-halogen-tungsten lamps in various rapid-thermal processors, including an A.G. Associates Heatpulse. The ripple method dynamically determines emittances by sensing time-dependent components in radiation from the lamps and radiation reflected by the wafer surface. Wafers were coated with films of various textures and patterns to test the technique over a range of surface emittances spanning about an order of magnitude. Temperature accuracies of at least ±5°C are achievable from 650°C to 1050°C, and above, for pyrometry at lμm wavelength.

Transport, tunneling X-ray, and penetration depth studies of superconducting Bi2+xSr2−yCuO6±δ crystals

Abstract In crystals of the single-layer cuprate Bi 2+x Sr 2−y CuO 6±δ , x ∼ 0.1, y ∼ 0.1 (BSCO), the ab-plane resistivity is linear in temperature and the ϱ c /ϱ ab resistivity anisotropy is 10 5 . The tunneling conductance is close to being a linear function of the bias voltage. Screening measurements show evidence of non-uniform superconductivity. X-ray diffraction indicates superlattice structure approaching commensurate periodicity.