S. Y. Hou

Low interface state density oxide‐GaAs structures fabricated by in situ molecular beam epitaxy

Several oxide‐GaAs heterostructures were fabricated using in situ multiple‐chamber molecular beam epitaxy. The oxides include SiO2, MgO, and Ga2O3(Gd2O3), all evaporated by an electron beam method. The SiO2 and Ga2O3(Gd2O3) films are amorphous while the MgO films are crystalline and part of the films are epitaxially grown on GaAs(100). Among these heterostructures, the Ga2O3(Gd2O3)–GaAs shows a photoluminescence intensity comparable to that of Al0.45Ga0.55As–GaAs, and forms accumulation and inversion layers as measured from capacitance voltage measurement in quasistatic and high frequency modes.

Structure and properties of epitaxial Ba0.5Sr0.5TiO3/SrRuO3/ZrO2 heterostructure on Si grown by off‐axis sputtering

We report the growth and characterization of epitaxial Ba0.5Sr0.5TiO3/SrRuO3/ZrO2 on Si for potential charge storage applications. Both Ba0.5Sr0.5TiO3 (BST) and SrRuO3 (SRO) are grown (110)‐oriented on yttrium‐stabilized ZrO2 (YSZ) (100)‐buffered Si. These films show a high degree of crystallinity with minimal interdiffusion at the interfaces as evidenced from x‐ray diffraction, Rutherford backscattering, and transmission electron microscopy. Studies on the in‐plane crystallographic relations between the layers revealed an interesting rectangle‐on‐cube epitaxy between BST/SRO and YSZ. The dielectric constant and loss tangent of the BST dielectric layer are 360 and 0.01 at 10 kHz, respectively. The leakage current density is lower than 4×10−7 A/cm2 at 1 V. A strong frequency dependence on both dielectric constant and loss tangent is observed in 1–10 MHz frequency range. This is attributed to the effect of a series resistance in the measurement loop, which is likely related to the bottom SrRuO3 electrode.

Transparent conducting thin films of GaInO3

GaInO3 is recently identified transparent conducting material which is structurally and chemically distinct from indium tin oxide [R. J. Cava, J. M. Phillips, J. Kwo, G. A. Thomas, R. B. van Dover, S. A. Carter, J. J. Krajewski, W. F. Peck, Jr., J. H. Marshall, and D. H. Rapkine, Appl. Phys. Lett. 64, 2071 (1994)]. We have used both dc reactive sputtering in the on‐ and off‐axis geometries and pulsed laser deposition to grow films of this material. Layers of pure GaInO3 as well as those partially substituted with Ge for Ga or Sn for In have been studied. Both growth techniques are capable of producing films with conductivity ∼400 (Ω cm)−1 and transmission as high as 90% throughout the visible spectrum for ∼1‐μm‐thick films. The growth techniques differ in the morphology of the films produced as well as in the degree of dopant incorporation that can be achieved. A post‐growth anneal in H2 can help produce an optimized oxygen content and a reduction of resistivity. Hall measurements indicate a carrier conce...

HETEROEPITAXIAL GROWTH OF BA1-XSRXTIO3/YBA2CU3O7-X BY PLASMA-ENHANCED METALORGANIC CHEMICAL VAPOR DEPOSITION

Epitaxial Ba1−xSrxTiO3(BST)/YBa2Cu3O7−x heterostructures with superior electrical and dielectric properties have been fabricated by plasma‐enhanced metalorganic chemical vapor deposition (PE‐MOCVD). Data of x‐ray diffraction and high resolution transmission electron microscopy showed that 〈100〉 oriented Ba1−xSrxTiO3 layers were epitaxially deposited on epitaxial (001) YBa2Cu3O7−x layers. The leakage current density through the Ba1−xSrxTiO3 films was about 10−7 A/cm2 at 2 V (about 2×105 V/cm) operation. Moreover, the results of capacitance‐temperature measurements showed that the PE‐MOCVD Ba1−xSrxTiO3 films had Curie temperatures of about 30 °C and a peak dielectric constant of 600 at zero bias voltage. The Rutherford backscattering spectrometry and x‐ray diffraction results showed that the BST film composition was controlled between Ba0.75Sr0.25TiO3 and Ba0.8Sr0.2TiO3. The structural and electrical properties of the Ba1−xSrxTiO3/YBa2Cu3O7−x heterostructure indicated that conductive oxide materials with cl...

Growth and characterization of Ba2YCu3O7−δ films in reduced oxygen partial pressures using the BaF2 post-annealing process

Epitaxial films of Ba 2 YCu 3 O 7−δ (BYCO), as thin as 250 A and with J c s approaching those for the best in situ grown films, can be formed by coevaporating BaF 2 , Y, and Cu followed by a two-stage anneal. These results extend the work of R. Feenstra et al. [J. Appl. Phys. 69 , 6569 (1991)] for film thicknesses >2000 A. This involves using low oxygen partial pressure [ p (O 2 ) = 4.3 Torr] during the high temperature portion of the anneal, which we vary from T a = 600 to 950 °C. The BYCO melt line is seen to be the upper limit for T a . The use of lower p (O 2 ) shifts the window for stable BYCO film growth to lower temperature. The lower growth temperature required for the low p (O 2 ) process allows the formation of smooth films with greater microstructural disorder than for films grown in p (O 2 ) = 740 Torr at higher T a , resulting in higher J c values by a factor of four. The relationship between the T a required to grow films with the strongest pinning force and p (O 2 ) is log [ p (O 2 )] ∝ T a −1 , independent of growth method ( in situ or ex situ ) over a range of five orders of magnitude in p (O 2 ).

Tc enhancement by low energy electron irradiation and the influence of chain disorder on resistivity and Hall coefficient in YBa2Cu3O7 thin films

Abstract The effect of electron irradiation with incident energy ( E ≤ 40 keV) lower than the threshold for displacement of in-plane atoms on T c , resistivity and Hall coefficient has been studied on fully oxygenated YBa 2 Cu 3 O 7−δ thin. films. In contrast to the known T c suppression at high-energy particle irradiation, an increase in T c has been found as well as a factor of two increase in both the T -linear resistivity slope and the Hall coefficient as a function of radiation fluence. It has been shown that the changes observed in transport properties can be explained as being caused by disruption of the conductivity of CuO chains by radiation-induced chain oxygen defects.

Epitaxial growth of SrTiO3/YBa2Cu3O7−x heterostructures by plasma‐enhanced metalorganic chemical vapor deposition

We report heteroepitaxial growth of SrTiO3 on YBa2Cu3O7−x/LaAlO3 substrates by plasma‐enhanced metalorganic chemical vapor deposition. X‐ray diffraction results indicated that SrTiO3 films were epitaxially grown on a (001) YBa2Cu3O7−x surface with [100] orientation perpendicular to the surface. The film composition, with Sr/Ti molar ratio in the range of 0.9 to 1.1, was determined by Rutherford backscattering spectrometry and energy dispersive spectroscopy. The thickness of the SrTiO3 films is 0.1–0.2 μm. The epitaxial growth was further evidenced by high‐resolution transmission electron microscopy and selected area diffraction. Atomically abrupt SrTiO3/YBa2Cu3O7−x interface and epitaxial growth with [100]SrTiO3∥[001]YBa2Cu3O7−x were observed in this study. The superconducting transition temperature of the bottom YBa2Cu3O7−x layer, as measured by ac susceptometer, did not significantly degrade after the growth of overlayer SrTiO3. The capacitance‐voltage measurements showed that the dielectric constant of...

Use of carbon films for passivation and environmental protection of superconducting YBa2Cu3O7−x

Electronic device applications of high‐temperature superconducting cuprates require the production of patterned thin films that are stable and inert in various processing and operating environments; however, the superconducting rare‐earth barium cuprates have been shown to decompose in water and to degrade in superconducting properties in a humid atmosphere. The thermodynamics that forms the driving force for the degradation is reviewed. Protective coatings or removable protective films can realize many applications and facilitate processing of superconducting devices. Diamondlike carbon films as well as amorphous carbon films are investigated as possible protective layers for YBa2Cu3O7−x (YBCO). Amorphous carbon (a‐carbon) films deposited by evaporation are shown to protect superconducting YBCO films from degradation by humidity. The YBCO films with a‐carbon coatings have been shown to have critical current densities four orders of magnitude higher than the uncoated YBCO films after 2 h at a stressing of...

Nanometer‐scale patterning of high‐Tc superconductors for Josephson junction‐based digital circuits

A straightforward method for nanometer‐scale patterning of high‐Tc superconductor thin films is discussed. The technique combines direct‐write electron beam lithography with well‐controlled aqueous etches and is applied to the fabrication of Josephson junction nanobridges in high‐quality, epitaxial thin‐film YBa2Cu3O7. We present the results of our studies of the dimensions, yield, uniformity, and mechanism of the junctions along with the performance of a representative digital circuit based on these junctions. Direct current junction parameter statistics measured at 77 K show critical currents of 27.5 μA±13% for a sample set of 220 junctions. The Josephson behavior of the nanobridge is believed to arise from the aggregation of oxygen vacancies in the nanometer‐scale bridge.

Precipitates in YBa2Cu3O7-δ thin films annealed at low oxygen partial pressure

We have studied the precipitates in YBa2Cu3O7−δ(YBCO) thin films grown by the BaF2 process in pO2=4 Torr and 700 °C. While stoichiometric films result in BaCuO2 surface precipitates, we have found Y2Cu2O5 precipitates embedded in the matrix of the same film. Off stoichiometric films with Ba/Y<1.5 have a precipitate‐free surface but with higher abundance of Y2Cu2O5 in the film matrix. The estimated densities of the two precipitates favor a stoichiometric YBCO film matrix. This behavior is not explainable in terms of phase equilibria and is attributed to kinetic effects. The electrical properties of the films degrade as the Ba/Y ratio deviates from 2.00.