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Some perspective concerning the capabilities and potential of epitaxial oxide films is gained by comparison with the field of semiconductor epitaxy. The specific epitaxial behavior of MgO, (ZrY)O2, and the layered cuprates is discussed. A suggestion is given for a method of searching for higher temperature superconductors by the use of epitaxial indusions in layered structures.
Here we review recent work on the preparation and characterization of magnetically ordered oxide Fe3O4/NiO superlattices. The materials were prepared by oxygen plasma-assisted molecular beam epitaxy. Their structural ordering was studied by x-ray, neutron, and RHEED electron diffraction techniques, and the superlattices are found to form as highly coherent strained-layer modulated single crystals. The magnetic ordering studies, using SQUID magnetometry, ferromagnetic resonance, and neutron diffraction, indicated strong interfacial coupling between the ferrimagnetic Fe3O4 layers and the antiferromagnetic NiO layers, with the magnetic ordering in each layer altered by the proximity to the magnetic moments in the adjacent layer. Strain and other layer-thickness effects are also evident in these magnetic layered structures. The special influence of interlayer coupling and strain on the Fe3O4 Verwey transition are discussed.
The growth of CoO and Fe3O4 single crystalline layers has been studied. CoO and Fe3O4 can be grown under similar growth conditions in a small pressure window. (001) oriented CoO/Fe3O4 multilayers were grown on MgO and MgAl2O4 substrates. On MgO substrates coherent heteroepitaxial growth was observed in the multilayers resulting in high crystalline quality multilayer structures with 0.018° FWHM of the multilayer rocking curve. The mean interface roughness is 4Å as determined by low angle X-ray reflection measurements and HRTEM observations.
Single crystal films of both rocksalt CoO and spinel Co3O4 were epitaxially grown with (001) orientations on MgO(001) substrates by pulsed laser deposition from CoO targets. The samples were characterized with low energy electron diffraction to determine the parallel orientation of the films with respect to the substrate, and x-ray diffraction to determine the perpendicular structure of the films. Both films were determined to have undergone tetragonal distortions, although to different degrees, due to the biaxial stress present in these epitaxial films.
Epitaxial magnetite (Fe3O4) thin films have been grown on MgO(001) substrates by pulsed laser deposition. The films have characteristics of the “Verwey transition”: the electric conductivity decreases by about one order of magnitude and the magnetization curve shows anomaly at the transition temperature, i.e. about 125 K. Effects of annealing the Fe3O4 thin films at various oxygen partial pressures have also been investigated. Phase identification was made using XRD techniques and infrared reflectivity measurements. The surface morphologies were studied by SEM and AFM. Under an oxidizing atmosphere, the Fe3O4 phase is transformed mainly into α-Fe2O3, and this transformation is accompanied by development of needle-like structures along <110> directions of MgO substrate. It is also found that electrical and magnetic properties of the iron oxide films are changed significantly by the annealing process.
We have made NiFe2O4 films by rf sputtering using the 90 °off-axis geometry. Films grown at 600°C on SrTiO3 and Y.15Zr.85O2 substrates are single crystals with (100) and (110) texture, respectively, but exhibit a very large and unexpected random anisotropy. A postdeposition air anneal at ˜1000°C has little effect on the crystallinity of the films but almost completely eliminates the random anisotropy; the remaining anisotropy is consistent with expected (bulk) values. A decrease of the saturation magnetization, indicating degradation, was observed for films on SrTiO3 (but not Y.15Zr.85O2) annealed at 1300°C.
Fe3O4 and NiFe2O4 films were deposited on <100> and <110> MgO substrates by dc magnetron reactive sputtering. X-ray diffraction studies indicate epitaxial films under an inplane tensile stress. Magnetization studies show that the moments of the films are unsaturated in 70 kOe applied fields and they approach bulk values only in extrapolation. Conversion electron Mössbauer spectroscopy (CEMS) studies indicate bulk parameters for the Fe3O4 films, but show deviations from bulk properties for NiFe2O4 films. CEMS studies further indicate a random moment distribution for all films which is an unexpected property. In-plane torque curves are discussed in terms of single crystal behavior. The effect of the inplane tensile stress is also indicated in the torque curves.
Using facing target sputtering, randomly oriented crystalline barium ferrite(BaFe12O19, BaM) has been deposited onto a Ultra Densified Amorphous Carbon® (UDAC) substrate, producing high quality films in-situ at a substrate temperature of 6400°C without any post-deposition annealing. Cross section transmission electron microscopy (TEM) reveals that the films have columnar structure. A ˜100Å thick interdiffusion layer between BaM and silicon nitride underlayer was observed. Films grown at low oxygen partial pressure have lower saturation magnetization (Ms), that may be caused by the formation of some amorphous phase at the grain boundaries as noticed by plan-view TEM. The existence of the Fe2O3 phase in the BaM was also revealed by electron diffraction.
Epitaxial yttrium-iron-garnet (YIG) films, bismuth-iron-garnet (BIG) films, and YIG/BIG heterostructures have been grown on  oriented single crystalline gadolinium-gallium-garnet (GGG) substrates by pulsed laser deposition (PLD), using a KrF excimer laser system. The films under study were grown over a range of temperatures from 600°C to 800°C and at 100 mTorr oxygen partial pressure. The effects of oxygen partial pressure during cooling on the structure, composition and magnetic properties of the films were investigated, employing X-ray diffraction, Rutherford back scattering spectroscopy coupled with He ion channeling, and vibration sample magnetometry. All specimens under study indicated that, independent of the film-substrate mismatch, the grown films were single crystalline in the  orientation. Preliminary studies on the effects of cooling oxygen partial pressure on the film structure indicate an increase in lattice distortion in the direction normal to the film surface with decreasing pressure. The magnetic properties of the films are comparable to the YIG bulk properties, and all films indicated in plane preferential magnetization, independent of cooling conditions.
A review of the growth of oriented oxides on Si and Ill-V semiconductors provides insight into some of the common themes of oxide/semiconductor epitaxy. The nature and success of the epitaxy can be attributed to four primary factors: (i) semiconductor surface preparation; (ii) oxide/semiconductor reaction thermodynamics; (iii) surface and interfacial polarity; and (iv) structural matching (lattice matching, thermal expansion, and symmetry). Semiconductor surface preparation governs the initial stages of epitaxy for systems such as MgO/GaAs and In2O3/InAs. In these cases, the epitaxial development depends on the presence or absence of a native oxide layer prior to growth. Chemical reaction can also influence the epitaxial process, as is illustrated in the growth of gadolinium oxide on Si. In general, the initial stages of epitaxy reflect a thermodynamic competition between the formation of the desired oxide phase, oxidation of the semiconductor, and formation of intermediate phases such as silicides and silicates. An analysis of possible reactions is presented for selected binary and ternary oxides with Si and GaAs. Surface and interfacial energy can also play an important role in determining the morphology and orientation of oxides having polar low-index faces, as illustrated in the growth of fluorite and related bixbyite oxides such as CeO2, In2O3 and Y2O3. The epitaxial relationships between the oxide and semiconductor may be rationalized in terms of either direct lattice matching or higher order epitaxy.
Yttria (Y2O3) films have been used for a variety of applications including MIS diodes, capacitors, insulators in transistor gates and electro-luminescent devices, and as optical AR coatings. Several conventional Physical Vapor Deposition (PVD) techniques have been used to deposit Y2O3 films such as evaporationl,2,3,4, ion beam, RF and DC magnetron sputtering5,6,7,8. Here we report on the growth of Y2O3 films using excimer-based Pulsed-Laser Deposition (PLD). This study was motivated by the need to deposit high-quality yttria films onto the backsides of CdTe and CdZnTe substrates to act as an Infra-Red (IR) Anti-Reflective (AR) coating. AR coatings can theoretically increase the efficiency of back-side illuminated HgCdTe photo-voltaic IR detectors from about 65% to 80% over the 3 to 5 μm and 8 to 12 μm bands. For detector fabrication Y2O3 is deposited onto the substrate backside, after HgCdTe diodes have been formed. Low deposition temperatures (T< 75°C) are thus essential in order to reduce Hg diffusion and minimize the creation of vacancies in the HgCdTe lattice. Unfortunately, conventional PVD techniques inadvertently heat uncooled substrates to temperatures above 100°C, even at modest deposition rates (1 Å /sec). Effective cooling of the CdTe or CdZnTe is not practical since these substrates are very fragile and cannot be clamped with significant pressure to cooled backing plates. Furthermore, the fact that the HgCdTe diode array has already been formed on the front-side of the substrate precludes the use of thermal grease or epoxy to improve contact with a water cooled stage. PLD on the other hand, has the ability to deposit Y2O3 fims at relatively high deposition rates when compared to sputtering, with only a small rise in substrate temperature.
Thin layers of Y2O3 have been prepared on silicon (100) by an activated reactive evaporation process involving evaporation of metal Y in an atomic oxygen plasma. The presence of the oxygen plasma was found to be crucial for the formation of homogeneous Y2O3 films on Si. The formation of Y2O3 films on Si (100) at different substrate temperatures was investigated. X-ray diffraction analysis showed that Y2O3 films formed between 300 °C and 650 °C were (111) textured while Y2O3 prepared at lower substrate temperatures (80 °C) exhibited mixed orientations. Rutherford backscattering spectrometry indicated that films were stoichiometric. No pronounced channeling was observed in films grown at 350 °C, suggesting polycrystalline film structures. Atomic force microscopy revealed very smooth surface morphologies with average surface roughness < 20 Å for films 700 Å thick deposited at 350 °C. Secondary ion mass spectroscopy indicated the abundance of intermediate layers in the film-substrate interface.
The substrate off-orientation effect is systematically studied on epitaxial CeO2(110) layers on Si(100), and the crystalline quality is significantly improved by enhancement of domains whose 〈110〉 is perpendicular to the offset-direction (Si〈110〉). AFM measurements indicate that the CeO2 layer surface consists of stripe-shaped facets and that their size is typically 100˜200 nm long, 20 nm wide and ∼3 nm high for a 150 nm-thick layer. RHEED and XTEM reveal that CeO2〈110〉 axis is inclined from wafer normal by the off-angle. The step arrangement at Si surface observed by XTEM relates closely to the inclination of the facets. It is found that off-orientation (≥∼,2.5°) leads to single crystal layer formation. RBS analyses verify that the crystalline quality is significantly improved, especially at the surface. The best result is obtained at the off-angle of 2.5°.
Epitaxial growth of CeO2 was obtained on the Si(111) surface by laser ablation in UHV atmosphere. However, a dual amorphous layer formed at the interface, yielding a CeO2/α:-CeOx/α-SiO2/Si(111) structure. This structure is speculated to be caused by a reaction occurring between Ce oxide and Si. Post annealing in O2 ambient caused the regrowth of CeO2, eliminated the α-CeOx layer, and increased the thickness of the SiO2 layer. The new CeO2/SiO2/Si(111) structure shows improved breakdown voltage and fewer interfacial states as observed by C-V and I-V measurements. The SiO2 is expected to tie surface states with Si, whereas the single crystal CeO2 will allow the epitaxial growth of lattice-matched Si on this insulating film. The effect of growth conditions and O2 annealing on both the structural and the electrical properties of this epitaxial oxide will be presented.
Crystalline strontium titanate (SrTiO3:STO) films were grown on Si(111) and Si(100) substrates using thin SrF2 and CaF2 buffer layers by two-step growth method. In all cases, fluoride buffer layers were effective in growing STO films on Si substrates, which is probably due to that fluoride buffer layers have excellent crystallinity and they can prevent formation of amorphous SiO2 layers on Si substrates at the initial stage of the STO deposition. It was found from X-ray diffraction and pole-figure measurements that (110)-oriented STO crystallites with three different positions to the substrate were grown on Si(111) substrates for both SrF2 and CaF2 buffer layers. In constrast, (100)-oriented STO films with 12-fold symmetry were grown on a SrF2/Si(100), and mixed (110)- and (100)-oriented STO crystallites were grown on a CaF2/Si(100) structure. It was concluded from these results that better crystallinity of STO films can be obtained on the SrF2 buffer layer in case of Si(111) and on the CaF2 buffer layer in case of Si(100). It was also found from I-V and C-V analyses that the STO films have good insulating and dielectric characteristics. For a SrTiO3 film on SrF2/Si(111) structure, the best values of breakdown field (at l.μA/cm2), resistivity (at IMV/cm) and dielectric constant were 2.3MV/cm, 8.2 × 1012 Ωcm and 72, respectively.
Infrared emission (IRE) spectra were obtained from two borophosphosilicate glass (BPSG) thin-film sample sets. The first set consisted of 21 films deposited on undoped silicon wafers, and the second set consisted of 9 films deposited on patterned and doped (product) wafers. The IRE data were empirically modeled using partial least-squares calibration to simultaneously quantify four BPSG thin-film properties. The standard errors of the determinations when modeling the 21 monitor wafers were < 0.1 wt% for boron and phosphorus content, 36 Å for film thickness, and 1.9°C for temperature. The standard errors of the determinations based on the product wafers were 0.13 wt% each for B and P content, 120 Å for film thickness, and 5.9°C for temperature.
Metastable solid-solutions in the MgO-CaO system grow readily on MgO at 300°C by molecular beam epitaxy. We observe RHEED oscillations indicating a layer-by-layer growth mode; in-plane orientation can be described by the Matthews theory of island rotations. Although some films start to unmix at 500°C, others have been observed to be stable up to 900°C. The Mgl-xCaxO solid solutions grow despite a larger miscibility gap in this system than in any system for which epitaxial solid solutions have been grown. We describe attempts to use these materials as adjustable-lattice constant epitaxial building blocks
Molecular layer epitaxy and the topmost surface structure in laser MBE of perovskite and other related oxides were investigated by in situ RHEED, in situ angle-resolved XPS (ARXPS) and ex situ co-axial ISS (CAICISS) analyses. Two-dimensional epitaxial growths of SrTiO3 (001) and BaCuO2(001) on SrTiO3(001) substrate were achieved by optimizing the growth conditions. Periodicities of RHEED oscillation corresponded well to the growth of a repeating unit cell (molecular layer). ARXPS and CAICISS spectra revealed that (Sr-0) atomic plane came at the top of SrTiO3(001) film growing on SrTiO3 (001) substrate with (Ti-O) atomic plane on the top. This indicates the structural conversion of the topmost atomic layer from TiO2 to SrO occurred during the SrTiO3 homoepitaxial growth.
The macroscopic properties of many materials are controlled by the structure and chemistry at grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted directly without the need for preconceived atomic structure models (1). Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale (2). The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface (3). In this paper we use the complimentary techniques of high resolution Zcontrast imaging and PEELS to investigate the atomic structure and chemistry of a 25° symmetric tilt boundary in a bicrystal of the electroceramic SrTiO3.
Both lanthanum aluminate (LaAIO 3) and spinel (MgAl2O 4) epitaxial thin films have been deposited on either planar and stepped (100) SrTiO3 single crystal substrates by pyrolysis of mixed nitrate precursors. The precursors pyrolyze initially into amorphous films. Nucleation of lanthanum aluminate and spinel occurs at the filnVsubstrate interface at higher temperature. Crystallization of LaAlO3 on SrTiO3 substrates occurs at approximately 650°C, whereas nucleation occurs at approximately 800'C without lattice-matched substrates. Similarly, latticematched substrates reduce the crystallization temperature of spinel to below 700°C. The epitaxial film grows at the expense of the amorphous film after the initial nucleation at the interface. The rapid growth and volume change due to the crystallization leave behind an epitaxial film with nanoporosity of 15 to 30 nm. Nevertheless, the surfaces of these films have roughness of only 6–9 Å. Ba2Ycu3O7-x films derived from metalorganic deposition of metal trifluoroacetate precursors was deposited on these epitaxial LaAlO3 films on both planar and stepped SrTiO3 substrates. The resultant YBCO films on LaAlO3 film on planar SrTiO3 substrate have critical current densities of > 2 × 106 A/cm2 at 77K and zero field.