D. B. Fenner

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.

Epitaxial yttria‐stabilized zirconia on hydrogen‐terminated Si by pulsed laser deposition

Epitaxial yttria‐stabilized zirconia films were grown on Si (100) and Si (111) by pulsed laser deposition. Rutherford backscattering spectroscopy indicates a high degree of crystalline perfection with a channeling minimum yield of 5.3%. A necessary predeposition process is removal of native silicon oxide from the Si prior to film growth. This is done outside the deposition chamber at 23 °C using a wet‐chemical hydrogen‐termination procedure. Epitaxial YBa2Cu3O7−δ films have been grown on these films.

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

Growth of epitaxial PrO2 thin films on hydrogen terminated Si (111) by pulsed laser deposition

A new epitaxial oxide, PrO2, has been grown on Si (111) by pulsed laser deposition. X‐ray diffraction shows that films are oriented with the PrO2[111] direction parallel to the substrate [111]. The full width at half maximum for the omega rocking curve on the PrO2 (222) peak is as low as 0.75°, while phi scans indicate in‐plane epitaxial alignment to better than one degree. In the best quality films, epitaxy is almost pure type‐b epitaxy which is characteristic of epitaxial CaF2 on Si. To achieve epitaxy, it is essential to remove the native silicon oxide from the substrate prior to film growth. This is done at room temperature using a wet‐chemical hydrogen‐termination procedure.

Reaction patterning of YBa2Cu3O7−δ thin films on Si

A novel technique exploiting the severe chemical reaction between Si and YBa2Cu3O7−δ (YBCO) has been developed for patterning epitaxial YBCO films in situ. Patterning is achieved by etching features in epitaxial YSZ on Si(100), and then depositing a final layer of YBCO; the material which grows on the exposed Si is insulating. Linewidths down to 3 μm have been demonstrated with a zero resistance critical temperature (Tc) of 86 K and a transport critical current density of 1.6×106 A/cm2 at 77 K. 45° and low‐angle twist grain boundaries occur under some circumstances but can be eliminated by regrowing 20 A of homoepitaxial YSZ on the surface prior to YBCO growth. Si diffusion in insulating portions has been characterized by x‐ray photoemission spectroscopy, indicating vertical diffusion through the film.

Epitaxial YBa2Cu3O7−y bolometers on micromachined windows in silicon wafers

Epitaxial YBCO thin‐film bolometers have been successfully fabricated on thin Si(100) substrates. Substrates included prethinned wafers ranging from 400 μm down to 4 μm thick, and a window, 0.75 μm thick, micromachined into a 400‐μm wafer. As the Si is made thinner, the speed and responsivity both improve considerably. A 500‐μs rise time was achieved on the micromachined window bolometer (0.75‐μm‐thick Si) under chopped infrared illumination. Calculations of heat flow in Si windows are in excellent agreement with the observed window‐bolometer response waveform.

OBSERVATION OF THE VORTEX-PINNING RESONANCE IN YBA2CU3O7

Abstract A resonance has been observed in the mixed state of YBa2Cu3O7 thin films at h ω ≅ 3 meV in the (hole) cyclotron resonance active mode of circular polarization. This chiral response is inconsistent with the conventional models of vortex dynamics. Based on a clean-limit theory recently developed by T. Hsu [Physica C 213 (1993) 305] which successfully describes these experiments we identify the resonance as a hybridized cyclotron-pinning resonance.

In-Situ Growth Of Superconducting YBa2Cu3Oy Films By Pulsed Laser Deposition

YBa2Cu3Oy thin films have been deposited in-situ on several substrate materials using pulsed excimer laser deposition. On the substrates, SrTiO3, MgO, LaA103, and yttrium-stabilized zirconia (YSZ), excellent films were obtained. These films had high superconducting transition temperatures (91K) with narrow transition widths (≈0.5K), metallic conductivity in the normal state, low room-temperature resistivity ( ~250 µΩ-cm), high critical currents (~3x107 A/cm2 at 4.2K), c-axis orientation perpendicular to the plane of the film, and epitaxial alignment with the substrate. On the more technologically relevant substrates of A12O3 and Si, less optimal results were obtained. The transition temperatures were high (86-88K) and metallic conductivity way obtained in the normal state. However, the room-temperature and microwave surface resistivities were higher and the critical currents were lower than for the above benchmark substrates. These diminished transport properties correlate with the imperfect alignment and epitaxy of the YBCO and substrate. For A12O3 substrates, a narrow substrate-temperature window was found for the best in-situ YBCO films. The poorer transport properties correlate with the lack of registry of the YBCO a-b plane with the sapphire r-plane. For Si substrates, a buffer layer is required due to high reactivity even at substrate temperatures as low as 550 C. YSZ provides a good buffer, and our best results were obtained on clean, hydrogen-terminated surfaces rather than oxidized Si. The amount of Y2O3 in ZrO2 was varied, and the best films were obtained with x near 0.1 where (ZrO2)1-x(Y2O3)x is cubic. Epitaxial alignment of the YBCO with the Si was achieved, but there was a substantial spread in orientations, accounting for the diminished transport properties.

Fe Substituted, Laser Ablated YBa 2 Cu 3 O 7 films using Off-Stoichiometric Targets

We present data on thin films of YBa 2 Cu 3 O 7 (YBCO) and YBa 2 (Cu 1−x Fe x ) 3 O 7 prepared by laser ablation using a sequence of targets, including Cu-deficient YBa 2 Cu 2.4 O 7 , CuO, and (CuO) 1−y (FeO) y . This technique achieves mixing on an atomic scale. We find that stochiometric films, made using a combination of the Cudeficient target and CuO, have a sharp transition (width c . We have achieved substitution of Fe on the Cu sites, using a sequence of the three targets for a range of Fe concentrations. T c decreases nearly linearly with concentration and T c → 0 near 15% Fe. The T c suppression for the films is slightly less than the corresponding value obtained for bulk samples.

In-Situ Growth Of Superconducting YBa 2 Cu 3 O y Films By Pulsed Laser Deposition

YBa2Cu3Oy thin films have been deposited in-situ on several substrate materials using pulsed excimer laser deposition. On the substrates, SrTiO3, MgO, LaA103, and yttrium-stabilized zirconia (YSZ), excellent films were obtained. These films had high superconducting transition temperatures (91K) with narrow transition widths (≈0.5K), metallic conductivity in the normal state, low room-temperature resistivity ( ~250 µΩ-cm), high critical currents (~3x107 A/cm2 at 4.2K), c-axis orientation perpendicular to the plane of the film, and epitaxial alignment with the substrate. On the more technologically relevant substrates of A12O3 and Si, less optimal results were obtained. The transition temperatures were high (86-88K) and metallic conductivity way obtained in the normal state. However, the room-temperature and microwave surface resistivities were higher and the critical currents were lower than for the above benchmark substrates. These diminished transport properties correlate with the imperfect alignment and epitaxy of the YBCO and substrate. For A12O3 substrates, a narrow substrate-temperature window was found for the best in-situ YBCO films. The poorer transport properties correlate with the lack of registry of the YBCO a-b plane with the sapphire r-plane. For Si substrates, a buffer layer is required due to high reactivity even at substrate temperatures as low as 550 C. YSZ provides a good buffer, and our best results were obtained on clean, hydrogen-terminated surfaces rather than oxidized Si. The amount of Y2O3 in ZrO2 was varied, and the best films were obtained with x near 0.1 where (ZrO2)1-x(Y2O3)x is cubic. Epitaxial alignment of the YBCO with the Si was achieved, but there was a substantial spread in orientations, accounting for the diminished transport properties.