We have developed a process for growing as-deposited Y1Ba2Cu3O7−x (YBCO) thin films on R-plane sapphire substrates with an intermediate layer of epitaxial MgO. The orientation of the layers has YBCO (001) parallel to MgO (100) which is parallel to the substrate normal. These films are superconducting by 88.5 K and exhibit Jc=1×106 A/cm2 at 77 K and 2.5×107 A/cm2 at 4.2 K. The MgO layers may be grown at temperatures as low as 370 °C and are very stable in air. Little or no diffusion occurs between the substrate and the two layers as measured by Auger profiling.
Radiative efficiency, band gap narrowing, and band filling are studied in Si-doped GaN films as a function of carrier concentration ͑n͒, using room and low temperature cathodoluminescence ͑CL͒. Using the Kane model, a band gap narrowing ⌬E g of −͑3.6Ϯ 0.6͒ ϫ 10 −8 and −͑2.6Ϯ 0.6͒ ϫ 10 −8 n 1/3 eV n 1/3 is obtained for epitaxially strained and relaxed material, respectively. Band-edge CL time response and absolute external photon yield are measured. The internal radiation efficiency is deduced. Its monotonic increase as n increases is explained by the increase in the spontaneous radiative rate with a radiative free carrier band-to-band recombination coefficient B = ͑1.2Ϯ 0.3͒ ϫ 10 −11 cm 3 s −1 .
We have developed an electron beam evaporator designed to deal with the special requirements of thin films of oxide superconductors. The growing surface is sprayed with plasma-excited oxygen while the sources and rate monitors operate in a low background pressure. This allows us to reproducibly grow films of Y-Ba-Cu-O on Si, Al2O3, and SrTiO3 substrates at a substrate temperature of 540 °C which are superconducting without the need for annealing. The resistive transitions of most films show an onset of 90 K and zero resistance by 68 K. X-ray diffraction indicates a preferred orientation for growth on most Si and SrTiO3 substrates. A preliminary measurement yields critical currents of at least 104 A/cm2 at 4.2 K for a film on silicon.
We have made YBa2Cu3O7−δ (YBCO) thin film dc superconducting quantum interference devices (SQUIDs) on sapphire substrates that operate above 77 K. The YBCO thin films are free of weak links and have critical current densities (Jc) of 106 A/cm2 at 77 K. Across the step edge, typical Jc ranges from 105 to 103 A/cm2 at 77 K and increases linearly with decreasing temperature. Typically, we obtain IcRn products near 30 μV at 77 K. We have operated the dc SQUIDs between 4.2 and 77 K and observed voltage modulation in rough agreement with the expected values. The flux noise is typically 1×10−10 Φ20/Hz, with a corresponding noise energy, SE, of 1.3×10−29 J/Hz at 77 K and 1 kHz.
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