Epitaxial CeO2 Research Articles

Structure of Ultrathin Epitaxial CeO2 Films Grown on Si(111)

The structure of ultrathin epitaxial CeO2 films grown on Si(111) by molecular beam epitaxy was investigated using high-resolution Rutherford backscattering spectroscopy. The observed elemental depth profiles showed that there were oxygen defects in the CeO2 films and the amount of oxygen defects was about 10%. The strain distribution in the CeO2 film was measured using a channeling technique. It was found that the [100] axis of CeO2 was tilted toward the [211] direction by ∼0.7°. The observed tilt angle was more than twice larger than the expected tilt angle calculated from the lattice mismatch by assuming a pseudomorphic growth of CeO2. The present results suggest that the crystal structure of the CeO2 film was changed from a cubic structure due to the oxygen defects.

High temperature electrical conductivity of epitaxial Gd-doped CeO 2 thin films

Single-crystalline epitaxial Gd-doped CeO 2 (GCO, Gd 0.2Ce 0.8O 1.9) thin films were grown on (001) NdGaO 3 (NGO) and LaAlO 3 (LAO) substrates through pulsed-laser deposition. The electrical conductivity was investigated by AC impedance spectroscopy. The temperature dependence of the electrical conductivity gives an activation energy of 0.74 eV for GCO/NGO and a conductivity which is very close to that of the bulk polycrystalline material. For GCO films deposited on LAO, the conductivity of the substrate makes a significant contribution to the total measured conductivity.

Fabrication and characterization of YBCO thin film on CeO 2/ a-plane sapphire substrate

We have examined the fabrication conditions of epitaxial CeO 2 films on a-plane sapphire substrates, and also examined epitaxial growth of YBa 2Cu 3O 7− δ (YBCO) films on CeO 2/ a-plane sapphire substrate. The θ/2 θ XRD analysis of these composite films revealed that grains of the CeO 2 and YBCO on CeO 2/ a-plane sapphire substrate were perpendicular to the substrate. From the φ-scan XRD measurement, we found that the four 1 0 2 φ-scan peaks of the YBCO film were observed and the peak positions were shifted by 45° compared with those of the CeO 2 films. From the peak shifts we could estimate that the angle between the [1 0 0] axis of the CeO 2 and the [1 0 0] axis of the YBCO was 45°. From temperature dependence of resistivity of the YBCO/CeO 2/ a-plane sapphire substrates, we obtained a zero-resistance temperature ( T C) of 88.6 K. We found that their crystallinity and critical temperature ( T C) were quite similar to those of YBCO film on CeO 2/ r-plane sapphire substrate. At the moment, YBCO thin films on a-plane sapphire are inferior to YBCO thin film on r-plane sapphire in terms of surface resistance.

Reactive sputter deposition of epitaxial (001) CeO 2 on (001) Ge

The growth of epitaxial (001) CeO 2 on a (001) Ge surface using a reactive sputter deposition method is reported. Hydrogen gas (4%H 2/Ar sputtering gas) is introduced during film growth in order to reduce or eliminate the presence of the GeO 2 from the semiconductor surface during the initial nucleation of the metal oxide film. A metal cerium target was used as the cation source, with water vapor serving as the oxidizing species. Epitaxial films were sputter deposited at a substrate temperature of 550 °C in a H 2O vapor pressure of approximately 10 −3 Torr. The hydrogen partial pressure and substrate temperature were selected to be sufficiently high such that the germanium native oxides are thermodynamically unstable. The Gibbs free energy of CeO 2 is larger in magnitude than that of the Ge native oxides, making it more favorable for the metal oxide to reside at the interface in comparison to the native Ge oxides.

Proposal of general rule to prepare epitaxial ceramic thin films at low temperature from the point of crystal chemistry

We have succeeded to prepare epitaxial yttria stabilized zirconia (YSZ) thin films at room temperature (27 °C) by the use of nm thick YSZ buffer layer deposited at 800 °C on Si(0 0 1) substrate. Room temperature epitaxial growth was realized on 0.8 nm thick ultra thin YSZ buffer layer, however, to achieve high crystallinity, 6.7 nm thick was needed. On the 6.7 nm thick YSZ buffered Si(0 0 1) substrate, we have tried to prepare epitaxial oxide ceramic thin films. As the result, epitaxial CeO 2, In 2O 3, and MnZn-ferrite thin films were successfully deposited even at room temperature. The conditions of epitaxial growth at such low temperature was considered from the point of crystal chemistry. As the result, following conditions were proposed: (1) Materials with both small lattice mismatch and small electronegativity difference; or (2) Materials with small lattice mismatch and composed of single component. These proposed conditions will helpful to design epitaxial growth of oxide ceramic thin film at low temperature.

Orientation selective epitaxial growth of CeO2(100) and CeO2(110) layers on Si(100) substrates

It is found that epitaxial CeO2 layers with (100) or (110) orientation can be selectively grown on Si(100) substrates by controlling substrate bias in reactive dc magnetron sputtering. Adopting a two step growth method; ultrathin metallic Ce layer deposition at room temperature followed by a silicidation process at 800 °C, and subsequent reactive sputtering in an Ar/O2 mixture environment, the CeO2(100) layer is grown on practical Si(100) surfaces prepared by the usual wet cleaning method.

MOD approach for the growth of epitaxial CeO2 buffer layers on biaxially textured Ni–W substrates for YBCO coated conductors

We have grown epitaxial CeO2 buffer layers on biaxially textured Ni–W substrates for YBCO coated conductors using a newly developed metal organic decomposition (MOD) approach. Precursor solution of 0.25 M concentration was spin coated on short samples of Ni–3 at%W (Ni–W) substrates and heat-treated at 1100 °C in a gas mixture of Ar–4%H2 for 15 min. Detailed x-ray studies indicate that CeO2 films have good out-of-plane and in-plane textures with full-width-half-maximum values of 5.8° and 7.5°, respectively. High temperature in situ XRD studies show that the nucleation of CeO2 films starts at 600 °C and the growth completes within 5 min when heated at 1100 °C. SEM and AFM investigations of CeO2 films reveal a fairly dense microstructure without cracks and porosity. Highly textured YSZ barrier layers and CeO2 cap layers were deposited on MOD CeO2-buffered Ni–W substrates using rf-magnetron sputtering. Pulsed laser deposition (PLD) was used to grow YBCO films on these substrates. A critical current, Jc, of about 1.5 MA cm−2 at 77 K and self-field was obtained on YBCO (PLD)/CeO2 (sputtered)/YSZ (sputtered)/CeO2 (spin-coated)/Ni–W.

Growth of epitaxial CeO2 thin films on r-cut sapphire by molecular beam epitaxy

Abstract Epitaxial (0 0 l ) CeO 2 films were grown on r -cut sapphire by molecular beam epitaxy. The CeO 2 films grown on as supplied r -Al 2 O 3 substrate constituted of (1 1 1) oriented CeO 2 films. We have found that annealing of r -Al 2 O 3 substrates at 1050 °C for 10 h dramatically enhances the epitaxial growth of (0 0 l ) oriented CeO 2 films on r -Al 2 O 3 substrate. The use of RF activated oxygen source for oxidation during growth further enhances the growth of (0 0 l ) oriented CeO 2 films on r -Al 2 O 3 . High quality epitaxial (0 0 l ) oriented CeO 2 films can be grown on annealed r -Al 2 O 3 substrates by employing lower deposition rate ( 2 ceramic sources. The CeO 2 films were characterized by 2 θ – θ , ω -scan and φ -scan measurements. The CeO 2 films grown on annealed r -Al 2 O 3 substrates showed high crystallinity with rocking curve width of 0.07° for (0 0 2) reflection of CeO 2 and had excellent in-plane epitaxy. These CeO 2 films can be used as buffer layers for the growth of high quality high temperature superconductor films on sapphire substrates.

Preparation of YBCO films on CeO 2-buffered MgO substrates by coating pyrolysis

Epitaxial CeO 2 (35 nm) buffer layers were deposited on MgO(0 0 1) single crystal substrates by an electron beam gun for preparation of YBCO films by coating pyrolysis (CP). When the substrate temperature was higher than 660 °C, the CeO 2 layer exhibited complete (0 0 1) orientation as judged from the θ–2 θ X-ray diffraction pattern. Atomic force microscopic observations demonstrated that the surface of the CeO 2 layer was very smooth; the average surface roughness being as small as 1 nm. Subsequently, YBCO films were prepared by the CP process using a metal acetylacetonate-based solution on the CeO 2-buffered MgO substrates. Both the YBCO film and CeO 2 buffer layer showed high crystallinity and fourfold symmetry in XRD θ–2 θ and φ-scans, respectively. An average inductive- J c value was 0.2 MA/cm 2 at 77 K.

Studies on inclined nuclei as a cause of crystallinity deterioration in epitaxial CeO 2(1 1 0) layers on Si(1 0 0) substrates

In order to get insight into the origin of deterioration of epitaxial layers on Si substrates, crystallographic aspects of CeO 2(1 1 0) layers grown on Si(1 0 0) substrates are studied using atomic force microscopy, X-ray diffraction and cross-sectional transmission electron microscopy in correlation with crystalline quality of the layer determined by reflection high-energy electron diffraction. It is found that surface morphology changes with crystallinity of the CeO 2 layers. Single crystalline CeO 2(1 1 0) layers with excellent crystallinity have a nanometer-scale-periodically corrugated surface structure, which consists of (1 1 1)-facets. As the crystalline quality of the CeO 2 layers becomes worse, the number of irregular shaped hillocks increases, which correspond to distorted (1 1 0)-grains. It is found that crystalline quality is not uniform but different crystallinity regions are distributed from place to place and their population ratio changes according to the degree of crystalline quality. Direct observation of the irregular shaped hillocks is carried out from cross-sectional lattice images of high-resolution transmission electron microscopy. It is found that the irregular shaped hillock is inclined by ∼5°.

Heteroepitaxial Growth of Nanostructured Cerium Dioxide Thin Films by MOCVD on a (001) TiO2 Substrate

The deposition of epitaxial CeO2 nanostructured thin films on rutile (TiO2) substrates by metal−organic chemical vapor deposition (MOCVD) has been carried out in a wide temperature range. Films have been grown on (001)TiO2 from the Ce(III) 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato diglyme adduct (Ce(hfa)3·diglyme). The X-ray diffraction patterns of all samples grown in the 450−750 °C deposition temperature range point to the formation of 〈100〉-oriented CeO2 films, whereas at higher deposition temperatures (850−1050 °C) all the deposited CeO2 films show 〈111〉 texture. fwhm rocking curves values clearly indicate that the texturing of CeO2 crystallites is greatly improved upon increasing the deposition temperature in both cases. Typical pole figure patterns demonstrate a cube-on-cube epitaxial growth in the case of 〈100〉-oriented CeO2 films, whereas 〈111〉-oriented CeO2 films show different in-plane alignments of the crystallites. A possible explanation for textural changes is proposed.

Redox properties of water on the oxidized and reduced surfaces of CeO 2[formula omitted

We present X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) results probing the surface chemistry of water on the oxidized and reduced surfaces of a 500 Å epitaxial CeO 2(1 1 1) film grown on yttria-stabilized ZrO 2(1 1 1). Oxidation with O 2 at 773 K under UHV conditions was sufficient to generate XPS spectra reflective of fully oxidized CeO 2(1 1 1). Surface reduction was carried out by annealing in UHV between 773 and 973 K, and the level of reduction was quantified using changes in the Ce3d 3/2 4f 0 photoemission peak at 917 eV which results primarily from Ce 4+ sites. As expected, the level of surface reduction (generation of Ce 3+ sites) increased with increasing temperature. These Ce 3+ sites were primarily in the first layer based on the fact that exposure of the film to O 2 at RT resulted in nearly complete conversion of Ce 3+ to Ce 4+. Annealing at 773 K led to a surface in which ≈40% of the surface Ce 4+ sites were reduced to Ce 3+, whereas annealing at higher temperatures led to more substantial reduction of the first layer along with some subsurface reduction that was not reoxidized by RT exposure to O 2. Comparisons with results in the literature for reduction of single crystal CeO 2(1 1 1) surfaces suggest that the volume-to-surface ratio of ceria samples influences, in part, the reduction conditions that result in detectable levels of surface Ce 3+ sites. In other words, the annealing temperatures required to achieve a specific extent of surface reduction likely depends on the thickness of the sample. Water TPD studies on the oxidized CeO 2(1 1 1) surface reveal strong coverage dependence that destabilizes high coverages of water relative to low coverages. The presence of surface reduction (on the order 30% oxygen vacancy sites) removes much of the coverage dependent behavior. TPD uptake measurements, H 2 TPD spectra and XPS spectra in the Ce3d core level and Ce4f valence band (VB) regions all indicate that little or no irreversible water decomposition or Ce 3+ oxidation was observed for water on this reduced surface. In contrast, exposure of water at 650 K resulted in additional surface reduction above that observed from annealing at 650 K in the absence of water. This is attributed to a redistribution of oxygen vacancies from the bulk to the surface as a result of high temperature water treatment. Because water oxidation of Ce 3+ surface sites has been observed for reduced ceria powders, but was not observed on the reduced CeO 2(1 1 1) surfaces studied here, we propose that the reduced (1 1 1) surface is more resistant than non-(1 1 1) terminations to being oxidized by water.

Relationship between epitaxial deposition and growth modes of CeO 2 films

Epitaxial CeO 2 films were deposited on (0 0 1) yttrium stabilised zirconia single crystal substrates by pulsed laser deposition in oxygen. As analysed by atomic force microscopy, the films deposited at different temperatures were laid down according to different growth modes, which were correlated with different epitaxial qualities and surface morphologies. As film thickness increases, the growth modes of CeO 2 films develop in different ways at different deposition temperatures. For the optimum epitaxial temperature of 790 °C, CeO 2 films have a layer-by-layer growth mode up to a thickness of 100 nm. In this case, the resultant film has very low roughness and round shaped grains. Epitaxial CeO 2 films grown at 775 and 805 °C (at the two ends of the epitaxial temperature range) are of a lower quality. Their growth modes are initially layer-by-layer (up to a thickness of 10 nm), but then change to island growth mode (by a thickness of 100 nm). Imperfectly c-axis oriented CeO 2 films grown at 750 °C have grains that are approximately rectangular in shape. Its growth mode is typical island growth throughout.

Synthesis and characterization of volatile, fluorine-free beta-ketoiminate lanthanide MOCVD precursors and their implementation in low-temperature growth of epitaxial CeO(2) buffer layers for superconducting electronics.

A new class of volatile, low-melting, fluorine-free lanthanide metal-organic chemical vapor deposition (MOCVD) precursors has been developed. The neutral, monomeric Ce, Nd, Gd, and Er complexes are coordinatively saturated by a versatile, multidentate ether-functionalized beta-ketoiminato ligand series, the melting point and volatility characteristics of which can be tuned by altering the alkyl substituents on the keto, imino, and ether sites of the ligand. Direct comparison with conventional lanthanide beta-diketonate complexes reveals that the present precursor class is a superior choice for lanthanide oxide MOCVD. Epitaxial CeO(2) buffer layer films can be grown on (001) YSZ substrates by MOCVD at significantly lower temperatures (450-650 degrees C) than previously possible by using one of the newly developed cerium beta-ketoiminate precursors. Films deposited at 540 degrees C have good out-of-plane (Deltaomega = 0.85 degrees ) and in-plane (Deltaphi = 1.65 degrees ) alignment and smooth surfaces (rms roughness approximately 4.3 A). The film growth rate decreases and the films tend to be smoother as the deposition temperature is increased. High-quality yttrium barium copper oxide (YBCO) films grown on these CeO(2) buffer layers by pulsed organometallic molecular beam epitaxy exhibit very good electrical transport properties (T(c) = 86.5 K, J(c) = 1.08 x 10(6) A/cm(2) at 77.4 K).

Room-temperature epitaxial growth of indium tin oxide thin films on Si substrates with an epitaxial CeO 2 ultrathin buffer

Room-temperature epitaxy of indium tin oxide (ITO) thin films was achieved on Si(111) substrates with an epitaxial CeO 2 ultrathin buffer using a pulsed laser deposition technique. The epitaxial CeO 2 buffer layer was also grown at room temperature. Reflection high-energy electron diffraction and pole figure X-ray diffraction analyses confirmed the formation of a double heteroepitaxial structure of ITO(111)/CeO 2(111)/Si(111) with the epitaxial relationship of [−110] ITO//[−110] CeO2//[1–10] Si. The junction of [ITO: 100 nm thick/CeO 2: 3 nm thick/ p-Si(111)] fabricated at room temperature exhibited solar cell properties.

Uniform texture in meter-long YBa 2Cu 3O 7 tape

A reel-to-reel tape handler mounted on a four-circle diffractometer is used to provide a characterization by X-ray diffraction of the entire 1 m length of a YBa 2Cu 3O 7/CeO 2/Y x Zr 1− x O 2/CeO 2/Pd/Ni (YBCO/CeO 2/YSZ/CeO 2/Pd/Ni) tape. Cube-textured Ni was formed by rolling and annealing; epitaxial CeO 2/YSZ/CeO 2/Pd buffer layers were deposited by reactive sputtering; YBCO was converted ex situ from Y, BaF 2, and Cu codeposited by e-beam evaporation. Rocking curve FWHM (mean±standard deviation) for 95 segments of 1 cm length are: YBCO(0 0 5)=6.2±0.5° , YSZ(0 0 2)=10.4±0.4° , and Ni(0 0 2)=7.6±0.3° . φ scan FWHM are: YBCO(1 1 3)=9.6±0.4° , YSZ(1 1 1)=13.0±0.4° , and Ni(1 1 1)=10.6±0.4° . Greater than 95% of the tape at each point is cube textured from Ni to YBCO. The critical current density J c is 0.36±0.04 MA/cm 2 at 77 K and is inversely correlated with the rocking curve FWHM. Calculations suggest that J c might be increased by a factor of 2.1 by producing a sharper texture and that the uniformity of the texture will support scaling to kilometer lengths.

Electrical Properties of Single Crystalline CeO2 High-k Gate Dielectrics Directly Grown on Si (111)

We have grown epitaxial single crystalline CeO2 directly on Si (111) by molecular beam epitaxy. Metal-insulator-semiconductor (MIS) capacitors were fabricated to evaluate electrical properties of CeO2 films. By eliminating an interfacial layer, equivalent oxide thickness (EOT) as small as 0.38 nm was obtained for a CeO2 physical thickness of 5 nm. The dielectric constant (ε) was calculated to be ε∼52, which is two times larger than the value reported for bulk (polycrystalline) CeO2 (ε∼26). Enhancement of ε is probably attributable to the anisotropy of the single crystalline CeO2 and/or the lattice distortion of CeO2 directly grown on Si. A leakage current density of ∼ 1 A/cm2 was obtained at an equivalent electrical field of 5 MV/cm for the CeO2 MIS capacitors, which is much smaller than the expected value for SiO2 capacitors with the same EOT. It is revealed that single crystalline CeO2 is promising as an alternative gate dielectric.

Hydrogen-assisted pulsed-laser deposition of epitaxial CeO2 films on (001)InP

We report on the growth of epitaxial CeO2 on (001) InP using hydrogen-assisted pulsed-laser deposition. Epitaxy is achieved via laser ablation of a CeO2 target in the presence of molecular hydrogen that is introduced during nucleation to reduce native In2O3 from the InP surface. X-ray diffraction scans confirm a cube-on-cube epitaxial relationship between the oxide film and the InP substrate. Rapid heating to the deposition temperature proved important in avoiding significant decomposition of the InP surface prior to film growth. This result should enable the integration of electronic oxide functionality with InP-based semiconductor technologies, and provide a means to explore InP metal–oxide–semiconductor field-effect transistor structures.

Crystal-orientation controlled epitaxial CeO 2 dielectric thin films on Si(100) substrates using pulsed laser deposition

Cerium oxide (CeO 2) is the potential dielectric candidate for the Si-based devices due to its stable chemical properties, high dielectric constant and good lattice match with silicon. In this paper, the impacts of process conditions on the structural and electrical characteristics of CeO 2 thin films deposited on Si(100) substrates have been studied. Epitaxial CeO 2 thin films with different crystal orientations on Si(100) substrates have been grown by pulsed laser deposition (PLD). The structural and electrical characteristics of CeO 2 thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), and high frequency capacitance–voltage measurements (C–V).

Characteristics of ferroelectric Pb(Zr,Ti)O3 films epitaxially grown on CeO2(111)/Si(111) substrates

The sol–gel derived epitaxial growth of ferroelectric Pb(Zr,Ti)O3 (PZT) films on epitaxial CeO2(111)/Si(111) substrates has been demonstrated. We found that the growth of the PZT(111) film on the substrate is dominated by the cube-on-cube type relationship, where the areal mismatch between the PZT(111) and CeO2(111) plane was as small as 0.72%. The CeO2(111) layer was effective in suppressing the interdiffusion between the PZT film and Si. Typical memory window of the Al/PZT(111)/CeO2(111)/Si(111) structure was 1.54 V, which was determined by the polarization reversal of the PZT(111) layer. The leakage current density and the resistivity measured at 5 V were 2.08×10−7 A/cm2 and 1.41×1012 Ω cm, respectively. Therefore, the epitaxial PZT(111)/CeO2(111)/Si(111) structure suggests a great potential for application to a metal-ferroelectric-insulator-semiconductor (MFIS) memory device.