CeO2 Buffer Layers Research Articles

Orientation of YBa2Cu3O7−x films on unbuffered and CeO2-buffered yttria-stabilized zirconia substrates

YBa2Cu3O7−x (YBCO) films deposited by pulsed laser ablation on unbuffered and CeO2-buffered yttria-stabilized zirconia (YSZ) substrates were studied by x-ray diffraction and transmission electron microscopy to investigate film orientation. From φ scans it was determined that the unbuffered films possess two major in-plane orientation relationships with the substrate. Both have (001)YBCO∥(001)YSZ, with either [100]YBCO∥[100]YSZ or [110]YBCO∥[100]YSZ, a 0° or 45° orientation, respectively. As deposition temperature increases, satellite peaks that straddle the 0° or 45° orientations develop. The Σ boundary and near coincident site lattice descriptions are applied to the discussion of these misorientations. In general the CeO2-buffered YBCO films align with to the 45° orientation to the CeO2 buffer layer. Out-of-plane film orientation was investigated for both unbuffered and CeO2-buffered YBCO films and expressed as a ratio of the amount of c⊥ material to a⊥ material. Buffered films exhibited c⊥ material to a⊥ material ratios approximately twice those of unbuffered films. Transmission electron microscopy combined with the x-ray data was used to develop an explanation for the trends in the variation of the c⊥/a⊥ ratio with film deposition temperature.

Formation of Metal/Ferroelectric/Insulator/Semiconductor Structure with a CeO2 Buffer Layer

Experimental results derived from the capacitance vs voltage (C-V) characteristics of metal/ferroelectric/insulator/semiconductor (MFIS) structures are described. PbTiO3 ferroelectric films of 600 Å thickness were grown on CeO2/Si(100) substrate by digital chemical vapor deposition (digital CVD). As the buffer layer between ferroelectric and Si substrate, 150-Å-thick CeO2 intermediate epitaxial layers were grown on a (100) silicon substrate by vacuum evaporation. The density of surface states at the CeO2/Si(100) interface was estimated from the C-V characteristics of Al/ CeO2/Si(100) samples to be ∼1011/cm2 eV. Epitaxial CeO2 films on Si(100) would therefore be expected to function as the gate oxide for MFS-FET's. Furthermore, the MFIS structure has ferroelectric switching properties, as demonstrated by the roughly 1-V threshold hysleresis in C-V characteristics. This structure is thus a first step toward high-performance MFIS-FET's.

A system of epitaxial buffer layers on (100) SrTiO3 substrates for the preparation of bi-epitaxial grain boundaries in YBa2Cu3Ox for magnetometers

With a combination of special buffer layers, bi-epitaxial grain boundaries in YBa2Cu3Ox (YBCO) used as Josephson junctions can be fabricated. On SrTiO3 substrates MgO and CeO2 buffer layers were used. For the production of these grain boundaries the epitaxial growth of MgO, CeO2 and MgO/CeO2 on SrTiO3 was studied. The crystal quality of the buffer layers was optimized by variation of several deposition parameters. The crystallinity was examined by X-ray diffraction, X-ray pole figure measurement and RBS (Rutherford backscattering)-ion channelling. The fabrication of these bi-epitaxial grain boundaries requires an ion-etching process, which damages the substrate surface. Therefore, an annealing process was necessary after the etching step. The quality of the edge and of the layers after the annealing process was studied by scanning electron microscopy (SEM) and RBS-ion channelling. Such bi-epitaxial grain boundaries were used to realize SQUIDs (superconducting quantum interference devices).

High-quality YBa2Cu3O7 films on 4-in. Wafers of sapphire, gallium arsenide, and silicon

Our technique of reactive thermal co-evaporation has been extended to fabricate large films (up to 4 in.) of YBa2Cu3O7 with high quality. A rotating substrate holder is used to separate the deposition and oxidation processes. This allows free access of the metal vapors. As large substrate wafers we use Al2O3, Si, and GaAs with buffer layers of CeO2, YSZ, and MgO, respectively. On all substrates, the uniformity of thickness and composition was better than 2%. Inductively measuredTc andjc(77 K) were 87.5±0.2 K and >1×106 A/cm2, respectively, across the full wafer area. This holds also for GaAs substrates due to a new procedure of capping by Si3N4.

Epitaxial CeO2 buffer layers for YBa2Cu3O7−δ films on sapphire

Epitaxial CeO2 buffer layers and YBa2Cu3O7−δ thin films have been grown in situ on (11̄02) sapphire by electron beam evaporation. Buffer layers of only 20 nm thickness inhibit interdiffusion between YBa2Cu3O7−δ and Al2O3 as determined by depth profiling using x-ray photoelectron spectroscopy. The layers show smooth surfaces and narrow interfaces. High lattice perfection of the CeO2 layer has been shown by x-ray diffraction. Laue oscillations up to ninth order have been observed in thin CeO2 buffer layers on sapphire. We found only one epitaxial orientation with YBa2Cu3O7−δ (001) ∥ CeO2 (001) ∥ Al2O3 (11̄02) and YBa2Cu3O7−δ [110] ∥ CeO2 [100] ∥ Al2O3 [112̄0]. YBa2Cu3O7−δ films grown on these buffer layers reveal Tc=88±0.5 K, ρ(300 K)=380 μΩ cm, and jc(77 K, 0 T)=1.3×106 A/cm2.

Solid source MOCVD for the epitaxial growth of thin oxide films

A novel solid source metalorganic chemical vapor deposition (MOCVD) process has been used to grow highly textured thin films of CeO2 and MgO, and highly oriented thin films of Sr1-xBaxNb206 (SBN). Both (100)-oriented and (lll)-oriented CeO2 films were reproducibly grown on (1102)-oriented Al203 (r-plane sapphire) substrates. Higher substrate temperatures and higher growth rates were found to favor the (111) orientation. The epitaxial (100) CeO2 films, 150 A thick, had very smooth surfaces, with an RMS surface roughness of 7 A, as measured by atomic force microscopy. MgO films with (100) orientation were readily grown on both r-plane sapphire and (100) SrTiO3 substrates at temperatures below 600°C. SBN films, 1.6 μm thick, were also grown on r-plane sapphire substrates at 700-800°C using a CeO2 buffer layer and all tetramethylheptanedionate (Sr, Ba, and Nb) metalorganic sources.

Superconducting epitaxial Tl2Ba2CaCu2O8 films on sapphire with cerium oxide buffer layers

Epitaxial Tl2Ba2CaCu2O8 films were prepared on (11̄02)Al2O3 with (100) CeO2 buffer layers. The Tl2Ba2CaCu2O8 was epitaxially oriented with its c axis perpendicular to the substrate surface and [100] Tl2Ba2CaCu2O8 aligned with [100] CeO2 and [22̄01] Al2O3. The films exhibited Tc of 97–98 K as measured both inductively and by ac magnetic susceptibility. Direct transport critical current density Jc at 75 K was 2.8×105 A/cm2.

Microstructure of biepitaxial grain boundary junctions in YBa2Cu3O7

The microstructure of photolithographically defined 45° biepitaxial grain boundary junctions in YBa2Cu3O7 (YBCO) have been investigated by transmission electron microscopy. The junctions have been formed by growing YBCO and a buffer layer on a patterned MgO seed layer previously grown on r-plane sapphire or SrTiO3 coated (through the use of suitable buffer layers) (100) yttrium-stabilized-zirconia substrates. SrTiO3 or CeO2 buffer layers grown on these patterned MgO substrates reproduced the pattern with two variants crystallographically rotated 45° with respect to each other about the surface normal. The morphology of the YBa2Cu3O7 grown on these differently oriented patterned regions show marked differences; in particular, the ion milling damage to the sapphire surface during patterning causes the formation of low angle grain boundaries. The locations of the weak link grain boundaries with respect to the patterning in the MgO seed layer have been identified using electron diffraction.

A-axis oriented YBa2Cu3O7−x thin films on Si with CeO2 buffer layers

We have grown a-axis oriented YBa2Cu3O7−x (YBCO) thin films on Si(100) substrates with (110) oriented insulating buffer layers of cerium dioxide (CeO2) using the pulsed laser deposition technique. The films are highly oriented and textured as determined by θ–2θ x-ray diffraction, x-ray pole-figure scan, scanning electron microscopy, Rutherford backscattering spectroscopy, and ion channeling. No diffusion at the interface has been found at growth temperatures up to 760 °C, indicating the CeO2 is a chemically stable and structurally compatible intermediate material for the growth of YBCO on Si. A zero resistance superconducting transition temperature of 87 K and a critical-current density (Jc) of 1.5×105 A/cm2 at 75 K have been measured; Jc obtained represents the highest value for the a-axis oriented YBCO films.

Epitaxial Y1Ba2Cu3O7 thin films on CeO2 buffer layers on sapphire substrates

Pulsed laser deposition has been used to deposit Y1Ba2Cu3O7 layer on CeO2 buffer layers on (11_02) sapphire. Both layers are epitaxial with the 〈110〉 direction of the CeO2 layer aligned with the 〈2_021〉 direction of the sapphire substrate. The c-axis Y1Ba2Cu3O7 layer has its 〈100〉 direction alligned with the 〈110〉 direction of the CeO2. Cross-sectional transmission electron microscopy shows the epitaxy to be coherent and the interfaces to be abrupt at an atomic level. The best films have a critical current of 9 × 106 A/cm2 at 4.2 K and lower microwave surface resistance than copper at 77 K and at a frequency of 31 GHz.