Low-temperature epitaxial growth of conductive LaNiO3 thin films by RF magnetron sputtering

Epitaxial growth of LaNiO₃ (LNO) thin films was successful on CeO₂/YSZ/Si(100), MgO(100) and SrTiO₃ (STO)(100) substrates by RF magnetron sputtering at 300 °C, although pulsed laser deposition requires 600 °C to prepare epitaxial LNO films according to the literature. Epitaxial LNO films deposited on CeO₂/YSZ/Si(100) and STO(100) had single orientation of LNO[100]//CeO₂[110]//YSZ[110]//Si[110]) and LNO[100]//STO[100], respectively. On the other hand, epitaxial LNO films deposited on MgO(100) had mixed orientations of LNO[100]//MgO[100] and LNO[100]//MgO[110]. The lattice parameter, composition and resistivity of the LNO thin films were strongly dependent on the substrate temperature. The minimum resistivity of LNO films was approximately 5×10⁻⁶ Ω m, which value almost agrees with the resistivity in the literature. It was found that the temperature to achieve minimum resistivity was 200 °C, irrespective of the type of substrate. The surface of the LNO films was smooth and flat.     

温度控制织构金属基带上YBCO外延薄膜生长及缺陷研究

采用直流溅射法在Y₂O₃/YSZ/CeO₂(YYC)缓冲层的织构NiW基带上, 通过基片温度调制YBa₂Cu₃O_7-δ(YBCO)外延薄膜生长。X射线衍射仪(XRD)表征显示, 基片温度强烈地影响YBCO薄膜的外延生长: 在较低的基片温度下薄膜趋于a轴取向生长, 随基片温度升高薄膜逐渐变为纯c轴取向生长。由于a轴晶粒引起的大角度晶界会阻碍超导电流在a-b面内的传输, 因此YBCO薄膜的微观结构和超导电性能随温度升高而得到改善, 但是随着基片温度继续升高, 基带的氧化程度加剧, YBCO与缓冲层间发生界面反应, 从而导致薄膜质量衰退。本     

高分子辅助沉积法制备LaNiO3外延导电薄膜

近年来LaNiO₃(LNO)作为铁电超晶格、超导异质结和催化剂材料引起了广泛关注。本研究采用简便、低成本的高分子辅助沉积法(Polymer Assisted Deposition, PAD), 在(001)取向的SrTiO₃(STO)单晶衬底上制备了导电性能优异的LNO外延薄膜, 并对其进行各种结构和电学表征。摇摆曲线半高宽为0.38°, 表明LNO薄膜结晶度良好。高分辨XRD的φ扫描进一步证实LNO薄膜在STO衬底上异质外延生长。原位变温XRD测试进一步表征了LNO薄膜的外延生长过程。结果表明, 聚合物分解之后金属阳离子在单晶基体上有序释放并外延结晶。XPS结果表明, 采用PAD制备的LaNiO₃薄膜不存在氧空位。薄膜表面光滑, 粗糙度为0.67 nm。在10~300 K温度区间内的变温电阻率表明LNO薄膜具有良好的导电性能。上述结果表明：PAD制备的LaNiO₃薄膜具有较好的综合性能, PAD在制备外延功能薄膜材料方面具有很大的潜力。     

Effects of p(O2) and p(CO2) on epitaxial growth of BaTiO3 thin films on MgO(100) substrates by using metal organic acid salts

BaTiO₃ thin films were prepared by using metal organic acid salts on MgO(100) substrates, which have large lattice-misfit with BaTiO₃. Amorphous films prefired at 470°C were crystallized to BaTiO₃ phase by heat treatment at higher temperature. Crystallinity and in-plane alignment of the prepared films were found to depend on the heat-treatment conditions. BaTiO₃ films with high crystallinity but poor (100)-orientation were obtained in air at higher than 1200°C. Whereas, (100)-oriented epitaxial BaTiO₃ film was fabricated by annealing at 900°C under low oxygen partial pressure (p(O₂)). Low carbon dioxide partial pressure (p(CO₂)) is also found to be essential for preparation of epitaxial BaTiO₃ films on MgO substrates by using metal organic acid salts.     

偏轴磁控溅射法外延BiFeO3薄膜的介电性能与阻变效应

利用偏轴射频磁控溅射法, 在(001) SrTiO₃(STO)单晶基片上制备了Pt/BiFeO₃/La_0.5Sr_0.5CoO₃/STO (Pt/BFO/ LSCO/STO)异质结电容器。研究了BiFeO₃薄膜的结构和物理性能。原子力显微镜(AFM)和X射线衍射(XRD)分析表明: BFO薄膜结晶质量良好, 且为单相(00l)外延钙钛矿结构。介电性能测试结果发现: 在5 V驱动电压下, Pt/BFO/LSCO电容器呈现饱和的蝶形回线, 调谐率和介电损耗分别为14.1%和0.19。此外, 阻变机制研究表明: 在0→5→0 V正向电压和0→-5→0 V负向电压下, 阻变均为高阻向低阻转变规律, 呈现为铁电二极管的阻变开关行为。通过I-V曲线拟合, 得到0→5→0→-5 V时阻变机制为空间电荷限制电流陷阱能级的填充和脱陷, 而-5→0 V时符合界面限制的F-N隧穿机制。     

Deposition of CeO2 films including areas with the different orientation and sharp border between them

Epitaxial films from one material, with sharp borders between contacting regions having different film orientation are grown on one surface of the substrate for the first time. The main reason for the deposition of thin ceria layers with mixed (001) and (111) orientations on a (1 02) sapphire substrate is determined. We suggest that this is related to the availability of surface defects which, in thin near-surface layers, deviate from stoichiometric composition. This in turn is connected with the loss of oxygen.

A technique for influencing CeO₂ film orientation is demonstrated. This involves specific preliminary processing of the substrate, and the selection of oxygen partial pressure during the deposition process.

High quality thin (30–50 nm) “protective” (001) CeO₂ epitaxial layers are prepared on (1 02) Al₂O₃. Structures comprising two epitaxial protective CeO₂ layers, orientations (001) and (111), are made on the base of (0001) and (1 02) sapphire substrates. The interface between the epitaxial layers is <1 000 nm.

Preliminary results using this method are described, and the possibility of creating a “bi-epitaxial” transition in thin YBa₂Cu₃O_7−x layers is explored.     

Heteroepitaxial growth of tungsten oxide films on sapphire for chemical gas sensors

The performance of chemiresistive gas sensors made from semiconducting metal oxide films is influenced by film stoichiometry, crystallographic structure, surface morphology and defect structure. To obtain well-defined microstructures, heteroepitaxial WO₃ films were grown on r-cut and c-cut single crystal sapphire substrates using rf magnetron Ar/O₂ reactive sputtering of a W target. On r-cut sapphire, an epitaxial tetragonal WO₃ phase is produced at a 450°C deposition temperature whereas 650°C growth stabilizes an epitaxial monoclinic WO₃ phase. On c-cut sapphire, a metastable hexagonal WO₃ phase is formed. RHEED and X-ray diffraction indicate that the films have a ‘polycrystalline epitaxial structure’ in which several grains are present, each having the same crystallographic orientation. STM analysis of the film surfaces reveals morphological features that appear to be derived from the substrate symmetries. The monoclinic phase has a step/terrace growth structure, has the smallest mosaic spread in XRD rocking curves and exhibits the highest degree of reproducibility suggesting that it is the best suited for sensor applications. Measurements of film conductivity versus temperature indicate that the charge transport mechanisms are also dependent on the crystallographic phase and microstructure of the WO₃ films.     

Growth and structure of epitaxial CeO2 films on yttria-stabilized ZrO2

Thirty to a hundred-nm thick epitaxial CeO₂ layers are grown on YSZ (100), (110) and (111) surfaces of yttria-stabilized ZrO₂ (YSZ) by electron beam evaporation of Ce in oxygen at reduced pressure. Their growth, structure and thermal stability are studied with several bulk and surface sensitive techniques including Rutherford backscattering spectrometry, cross-sectional high resolution electron microscopy, low energy electron diffraction and low energy reflection electron microscopy. Excellent epitaxy is obtained on all YSZ surfaces at a growth temperature of 750 K. The surfaces of films grown on (111)-oriented substrates are flat, whereas those on the other substrates are faceted into small (111) planes. The grain sizes in the films are in the 10 nm range and smaller.     

Fabrication and Optical Properties of Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/Thin Films on Si Substrates Using the PLD Method

Epitaxial 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) thin films with electro-optic effects were fabricated on (La_0.5Sr_0.5)CoO₃(LSCO)/CeO₂/YSZ-buffered Si(001) substrates using double-pulse excitation pulsed laser deposition (PLD) method with a mask placed between the target and the substrate. Epitaxial growth of PMN-PT thin films was undertaken using the two-step growth method of PMN-PT film. The PMN-PT seed layer was deposited at 500degC on the LSCO/CeO₂/YSZ/Si, which temperature was the same as that used for LSCO deposition. The PMN-PT thin films were deposited on the PMN-PT seed layer at 600degC, which enables growth of high-crystallinity PMN-PT films with smooth surfaces. We obtained optimum fabrication conditions of PMN-PT film with micrometer-order thickness. Resultant films showed high crystallinity with full width at half maximum (FWHM)=0.73 deg and 1.6 mum thickness. Electro-optic properties and the refractive index value were measured at 633 nm wavelength using the prism coupling method. The obtained refractive index was 2.59. The electro-optic coefficients r₁₃ and r₃₃ were determined by applying the electrical field between a semitransparent, thin top electrode of Pt and a bottom LSCO electrode. The electro-optic coefficient was r₁₃=17 pm/V at transverse electric field (TE) mode and r₃₃=55 pm/V at transverse magnetic field (TM) mode.     

Analysis of SAW properties of epitaxial ZnO films grown on R-Al2O3 substrates

ZnO thin films with a high piezoelectric coupling coefficient are widely used for high frequency and low loss surface acoustic wave (SAW) devices when the film is deposited on top of a high acoustic velocity substrate, such as diamond or sapphire. The performance of these devices is critically dependent on the quality of the ZnO films as well as of the interface between ZnO and the substrate. In this paper, we report the studies on piezoelectric properties of epitaxial (112¯0) ZnO thin films grown on R-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) technique. The c-axis of the ZnO film is in-plane. The ZnO/R-Al₂O₃ interface is atomically sharp. SAW delay lines, aligned parallel to the c-axis, were used to characterize the surface wave velocity, coupling coefficient, and temperature coefficient of frequency as functions of film thickness to wavelength ratio (h/λ). The acoustic wave properties of the material system were calculated using Adler's matrix method, and the devices were simulated using the quasi-static approximation based on Green's function analysis     

A three-step process for epitaxial growth of (111)-oriented C60 films on alkali–halide substrates

Epitaxial (111)-oriented C₆₀ films have been grown on alkali–halide substrates, KCl (100), KBr (100) and NaCl (100) by a three-step process: (1), substrate surface cleaning by high temperature heating; (2), initial deposition with a low deposition rate to grow two or three monolayers (ML); and (3), deposition with a high deposition rate to grow a film with expected thickness. It was found that (111)-oriented epitaxial C₆₀ films could be grown at low temperatures in a wide temperature range, from 40 to 120°C. By this three-step process, we can also grow epitaxial C₆₀ films at deposition rates as high as 35 Å/min.     

Study of microstructure of epitaxial fullerenes films

A comprehensive study was made on the structure of epitaxial thin films of C₆₀ and C₇₀ by means of transmission electron microscopy. Both the films show similar face-centered cubic structure and are epitaxial on (001) mica with close-packed plane parallel to the substrate surface. Two main kinds of defects-stacking faults and twins-were observed and are discussed. The effect of the remaining C₇₀ impurity on the crystal orientation of C₆₀ films was studied by comparing different samples made from high-purity fullerene and C₆₀/C₇₀ mixtures. The results show that there is a higher density of planar defects in the films containing larger amounts of impurities: moreover, some faint anomalous reflections located at so-called 2a₀ fcc reciprocal lattice points were also detected, probably as a result of C₇₀ contamination. Finally, it is found that stacking disorders can be easily increased by keeping the high-quality pure C₆₀ film in air at room temperature for a few weeks, implying the instability of the crystal orientation of the epitaxial fullerene films.     

立方相Y掺杂ZrO2纳米晶薄膜的制备与辐照效应研究

采用溶胶-凝胶结合旋涂法在单晶Si衬底上制备了立方相Y掺杂ZrO₂纳米晶薄膜(YSZ), 并分析了制备工艺参数对YSZ成膜的影响。采用光学显微镜、扫描电子显微镜、X射线衍射和透射电镜等手段对样品进行了表征和分析。结果表明, 加入PVA作为分散剂、采用分级干燥工艺以及提高匀胶转速可大大提高YSZ薄膜的成膜质量, 制备的YSZ薄膜表面十分平整, 没有出现裂纹。YSZ薄膜为立方相结构, 没有出现其它相。薄膜由平均晶粒尺寸为9.4 nm的纳米晶组成, 薄膜的厚度约为60 nm。在室温条件下, 低剂量的Xe离子辐照YSZ薄膜后出现微裂纹, 而当辐照剂量比较高时, 由于热峰效应, 辐照引起的微裂纹逐渐发生愈合。并且, 随着辐照剂量的增加, YSZ薄膜的平均晶粒尺寸增大。     

Semiconducting and structural properties of CrSi2 A-type epitaxial films on Si(111)

Chromium disilicide (CrSi₂) films 1 000 Å thick have been prepared by molecular beam epitaxy on CrSi₂ templates grown on Si(111) substrate. The effect of the substrate temperature on the structural, electrical and optical properties of CrSi₂ films has been studied by transmission and scanning electron microscopies, optical microscopy, electrical resistivity and Hall effect measurements and infrared optical spectrometry. The optimal temperature for the formation of the epitaxial A-type CrSi₂ film have been found to be about 750°C. The electrical measurement have shown that the epitaxial A-type CrSi₂ film is p-type semiconductor having a hole concentration of 1 × 10¹⁷cm⁻³ and Hall mobility of 2 980 cm² V⁻¹ s⁻¹ at room temperature. Optical absorption coefficient data have indicated a minimum, direct energy gap of 0.34 eV. The temperature dependence of the Hall mobility (μ) in the temperature range of T = 180–500 K can be expressed as μ = 7.8 × 10¹⁰T⁻³cm²V⁻¹s⁻¹.     

Epitaxy of atomically flat CaF2 films on Si(1 1 1) substrates

The growth of CaF₂ films with a thickness of approximately 3–4 nm on well-oriented Si(1 1 1) substrates by molecular beam epitaxy at temperatures between 410 and 560 °C were investigated by ex vacuo atomic force microscopy. Layer-by-layer growth producing atomically flat CaF₂ surfaces has been observed in a very narrow growth temperature window between approximately 430 and 470 °C. Perfect triangular shaped islands of one CaF₂ layer height are found on the surface with all corners aligned with the Si directions, indicating a pure B-stacking of the CaF₂ film. Surprisingly, also the substrate steps have been overgrown without visible defects. Below 410 °C, two different island orientations revealed a mixture of A- and B-stacking areas in the films. Above 520 °C non-wetting of the CaF interface layer leads to epitaxial films with a rough surface morphology.     

Inhomogeneous electronic properties of epitaxial La2/3Ca1/3MnO3 thin films

In this paper, we have reported on epitaxial fully strained La_2/3Ca_1/3MnO₃ films grown on single-crystalline SrTiO₃ substrates, with thicknesses ranging from 6 to 108 nm. ⁵⁵Mn NMR experiments, combined with magnetization data, show that the films have non-homogeneous magnetoelectronic properties and that non-ferromagnetic insulating regions are present close to the interface with the SrTiO₃. This observation may account for the repeatedly described disruption of the performance of manganite-based tunneling devices with increasing temperature and may help to better identify the possible causes of the modification of the La_2/3Ca_1/3MnO₃/SrTiO₃ interface properties.     

Epitaxial growth of RuO2 thin films by metal-organic chemical vapor deposition

Conductive RuO₂ thin films were epitaxially grown on LaAlO₃(100) and MgO(100) substrates by metal-organic chemical vapor deposition (MOCVD). The deposited RuO₂ films were crack-free, and well adhered to the substrates. The RuO₂ film is (200) oriented on LaAlO₃ (100) substrates at deposition temperature of 600°C and (110) oriented on MgO(100) substrates at deposition temperature of 350°C and above. The epitaxial growth of RuO₂ on MgO and LaAlO₃ is demonstrated by strong in-plane orientation of thin films with respect to the major axes of the substrates. The RuO₂ films on MgO(100) contain two variants and form an orientation relationship with MgO given by RuO₂(110)//MgO(100) and RuO₂[001]//MgO[011]. The RuO₂ films on LaAlO₃(100), on the other hand, contain four variants and form an orientation relationship with LaAlO₃ given by RuO₂(200)//LaAlO₃(100) and RuO₂[011]//LaAlO₃[011]. Electrical measurements on the RuO₂ thin films deposited at 600°C show room-temperature resistivities of 40 and 50 μΩ cm for the films deposited on the MgO and LaAlO₃ substrates, respectively.     

Alkoxide route to La0.5Sr0.5CoO3 epitaxial thin films on SrTiO3

An all alkoxide based sol–gel route was investigated for preparation of epitaxial La_0.5Sr_0.5CoO₃ (LSCO) films on 100 SrTiO₃ (STO) substrates. Films with 20–30 to 80–100 nm thickness were prepared by spin-coating 0.2–0.6 M (metal) solutions on the STO substrates and heat treatment to 800 °C at 2 °C min^− 1, 30 min, in air. The films were epitaxial with a cube-on-cube alignment and the LSCO cell was strained to match the STO substrate of 3.905 Å closely; a and b = 3.894 Å and 3.897 Å for the 20–30 and 80–100 nm films, respectively. The c-axis was compressed to 3.789 Å and 3.782 Å for the 20–30 and 80–100 nm films, respectively, which resulted in an almost unchanged cell volume as compared to polycrystalline film and nano-phase powders prepared in the same way. The SEM study showed mainly very smooth, featureless surfaces, but also some defects. A film prepared in the same way on an -Al₂O₃ substrate was dense and polycrystalline with crystallite sizes in the range 10–50 nm and gave cubic cell dimensions of a_c = 3.825 Å. The conductivity of the ca 30–40 nm thick polycrystalline film was 1.7 mΩcm, while the epitaxial 80–100 nm film had a conductivity of around 1.9 mΩcm.     

Epitaxial and polycrystalline BaFe12O19 thin films grown by chemical vapour deposition

Epitaxial and polycrystalline barium hexaferrite BaFe₁₂O₁₉ thin films were prepared by metalorganic chemical vapour deposition (MOCVD). Films were grown by a liquid MOCVD technique which aim is to control precisely the precursor vapour pressures. Two kinds of substrates were used: sapphire (001) and silicon thermally oxidized. On Si/SiO₂ films are polycrystalline and the magnetization is isotropic. On Al₂O₃ (001), structural studies reveal the films to be predominantly single phase, well crystallized without annealing procedure and with the c-axis perpendicular to the film plane; epitaxial relationships between the film and the substrate were determined. The magnetic parameters, deduced from vibrating sample magnetometer measurements, show a high dependence of the magnetization with the orientation of the field with respect to the surface of the film.     

Thickness dependence of structural and ferroelectric properties of sol-gel Pb(Zr0.56Ti0.44)0.90(Mg1/3Nb2/3)0.10O3 films

Sol-gel Pb(Zr_0.56Ti_0.44)_0.90(Mg_1/3Nb_2/3)_0.10O₃ (PZT-PMN) films were prepared onto the Ti/Pt/Ti bottom electrode by multilayer spin coating. The film thickness ranged from 0.22 to 0.88 μm. The Pt top electrodes were deposited on the PZT-PMN films by DC sputtering. The structural and ferroelectric properties of PZT-PMN films were investigated as a function of film thickness by X-ray diffraction (XRD), scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and by measuring the relative permittivity. The film retains the tetragonal perovskite structure with the [111] and [100] preferred orientations perpendicular to the film surface independent of film thickness. The [100] texture increases with increasing film thickness although the [111] texture is always predominant. The film consists of columnar grains. The average grain size is nearly independent of film thickness. The surface layer containing fine grains about 30 nm in diameter is induced on the top of the film. As the film thickness exceeds 0.44 μm, the number of the fine grains decreases remarkably. The crystalline interface layer about 10 nm thick is formed between the film and the bottom electrode. This interface layer is composed of Pt, Pb, Zr, Ti and O, while it is rich in Ti and deficient in Pb and O as compared with the inside of the film. The measured relative permittivity of the film increases with increasing film thickness, following the low permittivity interface model. On the basis of this model, the relative permittivity is estimated to be 3200 for the intrinsic PZT-PMN film, 750 for the surface layer and 50 for the interface layer.