Epitaxial Growth Of Oxide Films Research Articles

Investigation of epitaxial growth of La2Zr2O7 thin films with different carbonaceous phases

During the metal–organic deposition (MOD) process, the carbon-rich impurities remaining in a film are harmful to the epitaxial growth of the oxide film. Thus, it is of great importance to investigate epitaxial growth of oxide films with different carbonaceous phases. To control the carbonaceous phases in the as-fabricated LZO film, different dopant concentrations and annealing atmospheres have been suggested during the MOD process. A characterization of specimens was performed using X-ray diffraction θ–2θ scans, out-of-plane and in-plane scans, X-ray photoelectron spectroscopy, electron backscattering diffraction, and atomic force microscopy. It was found that a small number of carbonaceous phases, regardless of their type, has no obvious negative influence on the growth orientation of the film. Moreover, because some bridge-pillar spots in the graphene layer enable the epitaxial growth of oxide film, the texture degree of LZO film doped with a high concentration of reduced graphene oxide (RGO) is even sharpened. With increasing doping RGO concentration, the surface roughness of the LZO film increases, which indicates an exaggerated impact of the surface defects on the metallic substrate on the grain size of the LZO film doped with RGO. Different oxygen partial pressures in the annealing atmosphere determine the type of carbonaceous phase in the as-prepared oxide film. The carbonate species formed in the film under high oxygen partial pressure have a negative influence on the texture degree and the surface flatness of LZO film, while low oxygen partial pressure favors the reduction of the residual carbon in the film. Therefore, oxide films with high performance can be achieved by controlling the presence of the positive carbonaceous phases in the film during the introduction of an appropriate artificial doping source and the selection of a suitable annealing atmosphere.

Strain Engineering of Ferroelectric Domains in KxNa1−xNbO3 Epitaxial Layers

The application of lattice strain through epitaxial growth of oxide films on lattice mismatched perovskite-like substrates strongly influences the structural properties of ferroelectric domains and their corresponding piezoelectric behavior. The formation of different ferroelectric phases can be understood by a strain-phase diagram, which is calculated within the framework of the Landau-Ginzburg-Devonshire theory. In this paper, we illustrate the opportunity of ferroelectric domain engineering in the KxNa1-xNbO3 lead-free material system. In particular, the following examples are discussed in detail: (i) Different substrates (NdGaO3, SrTiO3, DyScO3, TbScO3, GdScO3) are used to systematically tune the incorporated epitaxial strain from compressive to tensile. This can be exploited to adjust the NaNbO3 thin film surface orientation, and, concomitantly, the vector of electrical polarization, which rotates from mainly vertical to exclusive in-plane orientation. (ii) In ferroelectric NaNbO3 thin films grown on rare-earth scandate substrates, highly regular stripe domain patterns are observed. By using different film thicknesses, these can be tailored with regard to domain periodicity and vertical polarization component. (iii) A featured potassium concentration of x = 0.9 of KxNa1 xNbO3 thin films grown on (110) NdScO3 substrates favors the coexistence of two equivalent, monoclinic, but differently oriented ferroelectric phases. A complicated herringbone domain pattern is experimentally observed which consists of alternating MC and a1a2 domains. The coexistence of different types of ferroelectric domains leads to polarization discontinuities at the domain walls, potentially enabling high piezoelectric responses. In each of these examples, the experimental results are in excellent agreement with predictions based on the linear elasticity theory.

Homogeneity and variation of donor doping in Verneuil-grown SrTiO3:Nb single crystals.

The homogeneity of Verneuil-grown SrTiO3:Nb crystals was investigated. Due to the fast crystal growth process, inhomogeneities in the donor dopant distribution and variation in the dislocation density are expected to occur. In fact, for some crystals optical studies show variations in the density of Ti3+ states on the microscale and a cluster-like surface conductivity was reported in tip-induced resistive switching studies. However, our investigations by TEM, EDX mapping, and 3D atom probe reveal that the Nb donors are distributed in a statistically random manner, indicating that there is clearly no inhomogeneity on the macro-, micro-, and nanoscale in high quality Verneuil-grown crystals. In consequence, the electronic transport in the bulk of donor-doped crystals is homogeneous and it is not significantly channelled by extended defects such as dislocations which justifies using this material, for example, as electronically conducting substrate for epitaxial oxide film growth.

Kinetics of formation and growth of epitaxial SrTiO3 films of single-crystal (001) SrTiO3 supports

The aim of this study was to quantitatively estimate the kinetics of the formation and growth of oxide SrTiO3 (STO) films using the method of the in situ reflection high-energy electron diffraction (RHEED) and compare the obtained results with the known growth models and theoretical estimates. The kinetics of the relaxation and crystallization of particles is studied under pulsed laser deposition (PLD) from oxide targets onto (001) STO supports or onto the surface of STO film growth at 650–800°C. Deposition frequencies of 0.1–10 Hz typical of PLD were used. The surface morphology and film structure was studied ex situ using the methods of AFM and X-ray-structural analysis. It was found that the time of relaxation of deposited particles is within the range of 2–20 s, which greatly exceeds or is comparable to the relative pulse duration. It was experimentally shown that structural distortions in epitaxial films for temperatures of ≤900°C are mainly due to the high rate of deposition and limited surface mobility of particles. The effect of structural relaxation in films is observed after the end of deposition; the time constant of bulk structural relaxation is ∼10 − ∼102 s or more. The obtained kinetic parameters of the formation of an oxide structure may be useful for the development of crystallization theory, as well as to optimize the conditions of epitaxial oxide film growth.

A NEW KINETIC MONTE CARLO METHOD FOR THE THIN FILM GROWTH OF PEROVSKITES

We have developed a novel kinetic Monte Carlo method, which simulates the epitaxial growth of oxide films with perovskite structure. First results are presented for two cases of adsorption dynamics for one of the metals — adsorption of single Ti atoms and adsorption of larger particles TiO2 and TiO . For both types of adsorption a two-by-two layer growth was observed. The simulations have been performed for different ratios between the flux of metals and the flux of oxygen.

Growth and characterization of ZnO film on Si(1 1 1) substrate by helicon wave plasma-assisted evaporation

We investigated ZnO film on Si(1 1 1) substrates grown by high density helicon wave oxygen plasma (>10 11/cm 2) assisted evaporation, which is a potentially promising approach for growing epitaxial oxide semiconductor films without causing surface damage. Adequate addition of Ar into oxygen plasma enhances production of excited atomic oxygen utilized for epitaxial oxide film growth. ZnO films could not be directly deposited on Si(1 1 1) substrate at temperatures greater than 600°C due to large thermal expansion coefficient difference and lattice mismatching. In this work, however, ZnO films were deposited on Si substrate up to 780°C after introducing low temperature (LT) ZnO buffer layers. In addition, adopting LT ZnO buffer layers turns out to significantly improve both crystal quality and optical properties. Structural and optical properties were characterized with the aid of X-ray diffraction (XRD), photoluminescence (PL), and field emission scanning electron microscopy (FE-SEM). As the growth temperature increases, the FWHM value of the XRD θ-rocking curve decreases significantly. The result of PL spectra for ZnO film deposited in the temperature 600–680°C range shows strong and sharp excitonic emission with very weak deep-level emission. In contrast, beyond these temperatures, relatively strong deep-level emission peak appears at 1.9 eV and the FWHM value of near-band-edge emission peak broadens. FE-SEM shows typical ZnO hexagonal columns but as growth temperature increases, each column collapsed, which influenced optical emission property.

2D and 3D X-Ray Structural Microscopy Using Submicron-Resolution Laue Microdiffraction

ABSTRACTWe have developed a scanning, polychromatic x-ray microscopy technique with submicron spatial resolution at the Advanced Photon Source. In this technique, white undulator radiation is focused to submicron diameter using elliptical mirrors. Laue diffraction patterns scattered from the sample are collected with an area detector and then analyzed to obtain the local crystal structure, lattice orientation, and strain tensor. These new microdiffraction capabilities have enabled both 2D and 3D structural studies of materials on mesoscopic length-scales of tenths-to-hundreds of microns. For thin samples such as deposited films, 2D structural maps are obtained by step-scanning the area of interest. For example, 2D x-ray microscopy has been applied in studies of the epitaxial growth of oxide films. For bulk samples, a 3D differential-aperture x-ray microscopy technique has been developed that yields the full diffraction information from each submicron volume element. The capabilities of 3D x-ray microscopy are demonstrated here with measurements of grain orientations and grain boundary motion in polycrystalline aluminum during 3D thermal grain growth. X-ray microscopy provides the needed, direct link between the experimentally measured 3D microstructural evolution and the results of theory and modeling of materials processes on mesoscopic length scales.

Epitaxial growth of oxide films (La–Ca–Mn–O and Y–Ba–Cu–O) by the facing-target sputtering technique

A facing-target sputtering system is described in detail. The plasma characteristics were studied with Cu targets. A step I– V curve was observed with decreasing pressure. A flat area of about 1×1 cm 2 at the center exists with maximum deposition rate. The growth rate varied with changing sputtering power almost linearly. La 2/3Ca 1/3MnO 3 (LCMO) and YBa 2Cu 4O 8 (YBCO) films were grown on SrTiO 3 (1 0 0) and LaAlO 3 (1 0 0) substrates. Increasing the substrate temperature T s can significantly improve the superconductivity of YBCO. Although the lattice mismatch of YBCO–STO is smaller than that of YBCO–LAO, the properties of YBCO/STO films are not better than that of YBCO/LAO films. Because of larger lattice mismatch, LCMO/LAO film has lower T p∼120 K with a wider transition than LCMO/STO film. X-ray diffraction confirmed the c-axis orientation of LCMO/YBCO/LCMO/STO sandwich. The temperature dependence of the resistance indicated the coexistence of superconductivity and ferromagnetism.

Heteroepitaxial growth of ZrO2−CeO2thin films on Si (001) substrates

We have studied Zr1−XCeXO2 film growth on (001) Si by Pulsed Laser Deposition using sputtering of metallic alloy targets and sintered ceramic targets. The conditions of the epitaxial growth have been found and optimized. The epitaxial oxide film growth (001) [100]||(001) [100] Si was obtained for a range of CeO2 content in ZrO2: from 4.5 up to 14% mol in ZrO2. The oxide film structure corresponds to a tetragonal phase with strong preference for c-axis orientation normal to the growth surface. The results obtained by RHEED, XRD and AFM methods have confirmed the high quality of heteroepitaxial Zr1−XCeXO2 layers, and the difference in crystallinities for the films grown from metallic alloy targets and ceramic targets was evaluated. The XRD results show the absence of any reflection distinct from (00l) and (l00) for films grown from alloy targets, and, in contrast with this, the film structure contains some random oriented inclusions in the case of oxide target deposition. Use of metallic alloy Zr-12% Ce targets and low oxygen pressure during deposition provide the best film quality with the minimum of surface microrelief (Rrms < 0.3 nm for 1 × 1 µm2 surface area was achieved).

Properties of Epitaxial SrTiO3 Thin Films Grown on Silicon by Molecular Beam Epitaxy

ABSTRACTSingle crystalline perovskite oxides such as SrTiO3 (STO) are highly desirable for future generation ULSI applications. Over the past three decades, development of crystalline oxides on silicon has been a great technological challenge as an amorphous silicon oxide layer forms readily on the Si surface when exposed to oxygen preventing the intended oxide heteroepitaxy on Si substrate. Recently, we have successfully grown epitaxial STO thin films on Si(001) surface by using molecular beam epitaxy (MBE) method. Properties of the STO films on Si have been characterized using a variety of techniques including in-situ reflection high energy electron diffraction (RHEED), ex-situ X-ray diffraction (XRD), spectroscopic ellipsometry (SE), Auger electron spectroscopy (AES) and atomic force microscopy (AFM). The STO films grown on Si(001) substrate show bright and streaky RHEED patterns indicating coherent two-dimensional epitaxial oxide film growth with its unit cell rotated 450 with respect to the underlying Si unit cell. RHEED and XRD data confirm the single crystalline nature and (001) orientation of the STO films. An X-ray pole figure indicates the in-plane orientation relationship as STO[100]//Si[110] and STO(001)// Si(001). The STO surface is atomically smooth with AFM rms roughness of 1.2 AÅ. The leakage current density is measured to be in the low 10−9 A/cm2 range at 1 V, after a brief post-growth anneal in O2. An interface state density Dit = 4.6 × 1011 eV−1 cm−2 is inferred from the high-frequency and quasi-static C-V characteristics. The effective oxide thickness for a 200 Å STO film is around 30 Å and is not sensitive to post-growth anneal in O2 at 500-700°C. These STO films are also robust against forming gas anneal. Finally, STO MOSFET structures have been fabricated and tested. An extrinsic carrier mobility value of 66 cm2 V−11 s−1 is obtained for an STO PMOS device with a 2 μm effective gate length.

Epitaxial MgO(100) Film Grown on a NiO(100) Single Crystal Surface, Studied by XPS and UPS

MgO and NiO have the rocksalt structure with a less than 1% lattice mismatch. Therefore, they are excellent materials for studies of epitaxial oxide film growth. This article reports comparison XPS and UPS spectra from both a clean stoichiometric NiO(100) single crystal substrate and a 5 nm thick MgO(100) epitaxially grown film, measured using a double-pass CMA. The NiO(100) surface was prepared by cleaving a NiO single crystal along the (100) plane in air, inserting it in UHV, and treating the surface with an Ar+ bombardment and oxidation treatment. The MgO film was grown by thermally evaporating Mg on the NiO(100) substrate at 523 K at an O2 partial pressure of 7 × 10−5 Pa. Epitaxial (100) structure of the MgO film was confirmed by LEED and in situ RHEED. Steady state sample charging was evident in UPS and XPS spectra from both the NiO surface and the MgO film. The charging was minimized by acquiring spectra at 523 K, rather than 298 K, due to the increased bulk conductivity of NiO. To correct for charging, all XPS spectra reported here are charge referenced to the measured Ni 2p3/2 of NiO, which is assigned a binding energy of 854.2 eV. The UPS spectra are referenced to the top of the valence band.

Crystal engineering of high-T c and related oxide films for future electronics

Advanced technology and future prospect of oxide-based electronic materials are described with a focus on the significance of atomically controlled epitaxy of high-T c superconductors and related oxide films. Problems in suitably forming the oxides whose power is potentially superior to silicon’s are discussed to stimulate technology development for engineering oxide film growth on an atomic scale. Our experimental results on controlled epitaxial growth of oxide films are presented with respect to pulsed laser deposition of YBa2Cu3O7 −δ films as well as laser MBE growth of SrTiO3 homoepitaxy and CeO2 heteroepitaxy on Si substrates.

Synthesis of epitaxial Pt on (100)Si using TiN buffer layer by pulsed laser deposition

High-quality epitaxial Pt films were deposited by pulsed laser deposition on (100)Si using TiN as a buffer layer. The films were (100) oriented normal to the substrate surface with a high degree of in-plane orientation with respect to the major axes of the substrate and buffer layer. An ion beam minimum channeling yield of 39% was obtained for the Pt films, indicating high crystallinity. High-resolution transmission electron microscopy results showed that interfaces between substrate/film and film/film were quite smooth and no perceptible interdiffusion was observed. The epitaxial TiN layer effectively acts as a barrier to impede metal-substrate reaction and helps in good adhesion of the Pt films on (100)Si. This structure is suitable for epitaxial growth of oxide films on Si with an underlying conductive electrode.

Controlled formation of oxide materials by laser molecular beam epitaxy

Ever since our first application of laser MBE, i.e. pulsed-laser deposition in ultrahigh vacuum to the growth of oxide films, this method, especially in combination with in situ RHEED and XPS, has been verified to be quite useful for the unit cell layer-by-layer epitaxial growth of oxide films. The concept and experimental results of laser MBE growth of oxide films and heterostructures are presented in respect of typical perovskite oxides and infinite-layer alkaline earth cuprates growths on SrTiO 3 substrate as well as of CeO 2 growth on Si substrate. Atomic scale characterization of surface and interface states of the films has been conducted to discuss the problems and practical applicability of laser MBE growth of oxide films.

Multilayer structure for epitaxial growth of oxide films on Si with an underlying electrode

Multilayers of CaF2 , Pd, and Pt were epitaxially grown on HF treated (100)Si by sequential deposition of the desired materials in ultrahigh vacuum. This structure is suitable for epitaxial growth of oxide films on Si with an underlying electrode. The epitaxial layer CaF2 effectively acts as a barrier to impede metal-substrate reaction, and the metal bilayer Pd/Pt forms an epitaxial electrode with good adhesion to the underlying substrate and high resistance to oxidation. KNbO3 deposited on this multilayer structure showed epitaxial growth in a mixed (001)/(110) orientation.