A. R. Kortan

Totally relaxed GexSi1-x layers with low threading dislocation densities grown on Si substrates

We have grown compositionally graded GexSi1−x layers on Si at 900 °C with both molecular beam epitaxy and rapid thermal chemical vapor deposition techniques. Triple‐crystal x‐ray diffraction reveals that for 0.10<x<0.53, the layers are totally relaxed. GexSi1−x cap layers grown on these graded layers are threading‐dislocation‐free when examined with conventional plan‐view and cross‐sectional transmission electron microscopy. Electron beam induced current images were used to count the low threading dislocation densities, which were 4×105±5×104 cm−2 and 3×106±2×106 cm−2 Eq. 2×106 cm−2 for x=0.23 and x=0.50, respectively. Photoluminescence spectra from the cap layers are identical to photoluminescence from bulk GexSi1−x.

Deposition and characterization of fullerene films

Thermal sublimation of pure C60 and C70 has been used for depositing well‐characterized fullerene films on a variety of substrates. Film purity is determined by infrared absorption spectra and the extent of crystallinity of the face‐centered cubic structure by x rays. Thickness‐dependent optical and electrical measurements reveal uniform films over the thickness range 200–1000 A. We obtain optical absorption coefficients having values between those of Si and Ge and a relative permittivity having a value close to that of amorphous SiO2.

In situ epitaxial growth of Y1Ba2Cu3O7−x films by molecular beam epitaxy with an activated oxygen source

Highly oriented, epitaxial Y1Ba2Cu3O7−x thin films were prepared on MgO(100) by molecular beam epitaxy at a substrate temperature of 550–600u2009°C. The in situ growth was achieved by incorporating reactive oxygen species produced by a remote microwave plasma in a flow‐tube reactor. The epitaxial (001) orientation is demonstrated by x‐ray diffraction and ion channeling. In situ reflection high‐energy electron diffraction showed that a layer‐by‐layer growth has produced a well‐ordered, atomically smooth surface in the as‐grown tetragonal phase of an oxygen stoichiometry of 6.2–6.3. A 500u2009°C anneal in 1 atm of O2 converted the oxygen content to 6.7 to 6.8. Typical superconducting transport properties of an Y1Ba2Cu3O7−x film 1000 A thick are ρ(300 K)=325 μΩu2009cm, ρ(300 K)/ρ(100 K)=2.4, Tc(onset)=92 K, and Tc(R=0)=82 K. The transport Jc at 75 K is 1×105 A/cm2, and increases to 1×106 A/cm2 at 70 K.

Reactivity of C60 in pure oxygen

In pure oxygen at moderate temperatures of 200u2009°C, an fcc C60 transforms into amorphous carbon‐oxygen compounds and the icosahedral C60 molecular structure is destroyed. The maximum oxygen uptake of pure C60, O/C60, is 12. Isothermal TGA transformation curves are sigmoid‐shaped with the kinetic exponent n∼5/2 which conforms with a two‐dimensional nucleation and growth mode. The heat of formation for the carbon‐oxygen compounds is 90 kcal/mol O, and the formation energy for the reaction: 60C (graphite)→C60 molecule is estimated to be ∼600 kcal/mol.

Strontium doped fullerite compounds

Abstract We report synthesis, structures and properties of strontium intercalated fullerite, SrxC60 compounds. This is identified as the only fulleride system where fcc and bcc phases compete in the same compositional range. In the vicinity of x=3, a bcc A15 phase coexists with a fcc phase with lattice constants 11.140 and 14.144 A, respectively. For Sr6C60, an Im 3 bcc phase with ao=10.975 A is observed. This phase becomes superconducting at T=4 K. These structural and electronic properties, being intermediate to grossly different calcium and barium intercalated fullerides, provide a unifying picture of all alkaline-earth intercalated fullerides.

Superconducting Tl‐Ba‐Ca‐Cu‐O films by sputtering

Superconducting Tl‐Ba‐Ca‐Cu‐O thin films 0.2–1.0 μm thick have been prepared on MgO(100) and SrTiO3(100) substrates by dc diode sputtering using a single oxide target. Films containing primarily the Tl2Ba2Ca1Cu2O8 phase were obtained with a Tc (R=0) at 102 K and a transport Jc of 104 A/cm2 at 90 K. For the nearly pure phase Tl2Ba2Ca2Cu3O10 films, the Tc’s (R=0) are higher at 116 K and the transport Jc’s at 100 K are of 105 A/cm2. Both types of films show a strong preferred orientation with the c axis perpendicular to the film plane. The rocking curve of the Tl2Ba2Ca2Cu3O10 films is 0.32° wide and the typical grain size is over 10 μm.

Structure and stability of metastable α-Sn

We report for the first time the growth of high quality films of metastable α‐Sn. The structural properties of the molecular beam epitaxy grown metastable α‐Sn are investigated by means of in situ reflection high‐energy electron diffraction (RHEED) analysis, scanning electron microscopy including electron channeling patterns, and high‐resolution x‐ray scattering techniques. Scanning electron microscopy reveals a growth morphology of smooth and uniform surfaces. RHEED patterns yield a highly streaked (2×1) surface reconstruction suggesting a layer‐by‐layer growth mechanism. Triple‐axis x‐ray diffractometry was employed to determine structural parameters and the strain distribution. In‐plane rocking scans of the (400) reflection indicate a half width of 3 arcsec for the heterostructure α‐Sn/InSb. Out‐of‐plane scans reveal a tetragonal distortion perpendicular to the film plane, contributing a net strain of ∼0.28%. These results are in quantitative agreement with values calculated using simple elastic theory.

Epitaxial growth of metastable SnGe alloys

We have grown homogeneous alloys of Sn1−xGex with 0.01<x<0.1 by molecular beam epitaxy on 〈100〉 InSb substrates. The Sn‐Ge system is immiscible due to differing equilibrium crystal structures at the growth temperature. By stabilizing the diamond cubic α‐Sn with the InSb template, we have created a metastable miscibility region for alloying Sn and Ge. Pure α‐Sn grown on 〈100〉 InSb shows a tetragonal expansion perpendicular to the substrate because of the slightly larger lattice parameter of α‐Sn. As the smaller Ge atom is added, the strain converts from compressive to tension resulting in an effective 1.66% tetragonal contraction in the growth direction for Sn0.92Ge0.08.

Growth of epitaxial YbBa2Cu3O7 superconductor by liquid‐gas‐solidification processing

Superconductive films of YbBa2Cu3O7 have been epitaxially grown on SrTiO3 substrate by a novel liquid‐gas‐solidification process. A layer of metallic YbBa2Cu3 melt was coated on the substrate and in situ oxidized. The oxide films grown on the SrTiO3u2009(100) substrate are epitaxially oriented in structure with the c axis normal to the plane film. Values of Tc (R=0) of 82 K with a transition width of 1 K have been achieved. The critical current density Jc is typically 105 A/cm2 at 50 K and 104 A/cm2 at 77 K.

In-situ growth of Y1Ba2Cu3O7-x films by molecular beam epitaxy with an activated oxygen source

Abstract Epitaxial, (001) oriented Y 1 Ba 2 Cu 3 O 7-x thin films were grown on MgO(100) by molecular beam epitaxy at 650°C. The activated oxygen species were produced by a remote microwave-excited discharge in a flow tube. Remarkable improvements of superconducting properties were demonstrated due to enhancements of the flow speed of O 2 stream. The as-prepared films 500A thick show reproducibly R=0 transitions at 89K, and a ϱ(100K) of 70 μΩ-cm. Ultrathin films 90A thick still exhibit metallic-like resistivity, and a T c (R=0) of 70K.