Single Crystal Oxide Substrates Research Articles

Temperature effect on surface structure of single crystal SrLaAlO4(001)

Development of next-generation electrochemical devices, such as solid oxide cells, requires control of the charge transfer processes across key interfaces. Structural strain at electrolyte:electrode interfaces could potentially alter the devices’ charge transport properties, therefore understanding the structural behavior of electrode surfaces under operating conditions is important. The functional oxide single crystal substrate SrLaAlO4 has been well-characterized with bulk structure studies; however, there are very few studies of SrLaAlO4 surface structures. Here, we present an investigation of the surface structure of SrLaAlO4 (001) substrates using surface x-ray diffraction under UHV conditions (10−10 Torr) with the substrate held at either room temperature or 650 °C. Best-fit models using a 1:1 ratio of Sr:La showed significant distortions to the surface AlO6 octahedra.

Epitaxial Growth of Aurivillius Bi3Fe2Mn2Ox Supercell Thin Films on Silicon

Nanoelectronic devices integrated with functional complex oxides have drawn much attention in recent years. However, due to material and processing compatibility issues, integrating functional complex oxides with Si-based devices is challenging, and success has been limited. As an example, the Bi3Fe2Mn2Ox (BFMO) supercell system, a single-phase layered oxide made by compositing BiFeO3 and BiMnO3, has been studied for much of the past decade for its unusual layered Aurivillius structure and magnetic and ferroelectric properties. However, most of the BFMO thin film growth has been demonstrated on single-crystal oxide substrates such as SrTiO3 and LaAlO3. In this work, we demonstrate that the BFMO layered supercell phase can be integrated on Si with high epitaxial quality using a buffer stack of TiN/SrTiO3/CeO2. Further understanding of the strain-controlled growth of the BFMO supercell phase has allowed such Si integration. Microstructure, magnetic, ferroelectric, and optical properties of the BFMO films on Si have been characterized and compared with those of BFMO on SrTiO3 single-crystal substrates, demonstrating comparable epitaxial quality and physical properties. Integrating multiferroic BFMO oxides on Si demonstrates the potential of layered supercell oxides in practical device applications such as ferroelectric field effect transistors.

Endotaxial α-Fe Nanoparticles in the High-Fluence Iron-Implanted Single-Crystal MgO

In this work, we report on the endotaxial growth of α-Fe nanoparticles in the near-surface layer under high-fluence iron ion implantation of the single-crystal magnesium oxide substrate. Comprehensive Mössbauer effect and magnetometry studies show that the implanted sample reveals a pronounced ferromagnetic response even at room temperature, and the α-Fe nanoparticles serve as its main source. The broad band at ~1000 Oe in the X-band magnetic resonance spectra originates from the α-Fe fraction. It manifests the properties of the easy-plane system with the four-fold in-plane anisotropy. The last indicates that the α-Fe nanoparticles are coherently incorporated into the host MgO matrix.

Si integration of La0.7Sr0.3MnO3:BiFeO3 nanocomposite thin films with strong exchange bias coupling

Integrating functional oxide-based nanocomposite thin films on Si wafer is a large step toward their applications in electronics and spintronics. Here, to overcome the large lattice mismatch and potential inter-diffusion between the complex oxides and Si, a set of buffer layers of SrRuO3 (SRO)/TiN have been applied. The La0.7Sr0.3MnO3:BiFeO3 system has been selected to grow on the SRO/TiN buffered Si, and the nanocomposite films exhibit highly textured growth along the c direction for both phases. The microstructure study shows a typical vertically aligned nanocomposite structure of the films, which is similar to the same films on single crystal oxide substrates. Furthermore, magnetic measurements indicate the strong ferromagnetic–antiferromagnetic coupling in the nanocomposite system, which refers to the large exchange bias value HEB of ∼1800 Oe in the L0.5B0.5 sample under out-of-plane magnetic field. This work provides an effective approach for the Si integration of oxide-based nanocomposite thin films.

Stabilizing Metastable Polymorphs of van der Waals Solid MoS2 on Single Crystal Oxide Substrates: Exploring the Possible Role of Surface Chemistry and Structure

In this work we address the question of growth of thin films of a soft van der Waals solid (2D MoS2) on a hard, crystalline metal oxide substrates. Thus, MoS2 thin films are grown on different single-crystal metal oxide substrates (SrLaAlO4, c-Al2O3, SrTiO3, LaAlO3) by pulsed laser deposition (PLD) and characterized by different techniques to confirm and quantify their phase constitution (2H, 1T′). We observe that, on the SrLaAlO4 (001) substrate, a mixed 1T′+2H phase of MoS2 grows with a dominant (∼75%) 1T′ phase, while on c-Al2O3 (0001) pure 2H phase grows. On the SrTiO3 and LaAlO3 substrates also the mixed phase grows, but with a much lesser component of the 1T′. Higher 1T′ contribution in SLAO and STO points to the chemical role of strontium in the early growth. It is also noted that, with increased film thickness on SrLaAlO4, the contribution of the pure 2H phase is enhanced, indicating the pivotal role of lattice strain in stabilizing the initial layer(s). The mixed 1T′+2H phase in the ultrathin film shows significantly lower room temperature resistivity (∼2 mΩ cm) with respect to that of the pure 2H phase (∼14 mΩ cm). The substrate selective polymorphism with distinct electronic features could invite multiple application interests.

Control of Lateral Composition Distribution in Graded Films of Soluble Solid Systems A1−xBx by Partitioned Dual-Beam Pulsed Laser Deposition

Lateral compositionally-graded thin films are powerful media for the observation of phase boundaries as well as for high-throughput materials exploration. We herein propose a method to prepare epitaxial lateral compositionally-graded films using a dual-beam pulsed laser deposition (PLD) method with two targets separated by a partition. Tuning the ambient pressure and the partition—substrate gap makes it possible to control of the gradient length of the deposits at the small sizes (≤ 10 mm) suitable for commercial oxide single crystal substrates. A simple Monte Carlo simulation qualitatively reproduced the characteristic features of the lateral thickness distribution. To demonstrate this method, we prepared (1−x)PbTiO3—xPbZrO3 and (1−x)LaMnO3—xLa0.6Sr0.4MnO3 films with lateral composition gradient widths of 10 and 1 mm, respectively, with the partitioned dual PLD.

Bimodal size distribution of dewetted gold nanoparticles with regrown oxide bases

Gold nanoparticles, supported on oxides, have received considerable attention for their wide range of applications. Using dewetted gold particles on single-crystal oxide substrates as model systems, the unusual particle size distribution and relevant mechanism are investigated. Gold particles usually exhibit unimodal size distributions; however, when the growth of oxide bases is detected underneath these gold particles, particles have an unusual bimodal size distributions. A simple model, considering the effect of growing oxide bases on gold adatom migration, is proposed to explain this bimodal distribution. These results demonstrate the possibility of controlling the migration of surface adatoms and subsequent particle size at high temperatures.

Superconducting Iron Chalcogenide Thin Films Integrated on Flexible Mica Substrates

With the goal of integrating superconducting iron chalcogenides with future flexible electronics, superconducting FeSe0.1Te0.9 (FST) thin films have been directly deposited on flexible Mica substrates without any buffer layer by a pulsed laser deposition method. Microstructural characterization and X-ray diffraction results show excellent film quality with mostly c -axis growth on Mica substrates. Superconducting properties (such as superconducting transition temperature $T_{c}$ and upper critical field $H_{c2}$ ) have been measured to be comparable to that of the films on single crystal oxide substrates. This paper demonstrates the feasibility of integrating superconducting iron chalcogenide (FST) thin films with future flexible electronics for superconductor-based devices.

Characterization of VO2/ferroelectric thin film heterostructures deposited on various complex oxide single crystal substrates

Oxide multilayer heteroepitaxy combining Mott-insulator vanadium dioxide (VO2) films and functional conducting/ferroelectric/dielectric films opens new opportunities in creating functional devices with applicability in the field of nonvolatile memories for neuromorphic devices. The growth of high quality VO2 films is challenging due to the necessity of precise control of the vanadium cation valence state. In this study, the authors report on electrical and structural properties of VO2 thin films deposited on various single crystal oxide substrates commonly used in oxide electronics and on PbZrxTi(1−x)O3/SrRuO3 ferroelectric heterostructures deposited on SrTiO3 and GaScO3 single crystal substrates. The optimized VO2 films exhibit a metal-to-insulator phase transition on all applied substrate/film combinations.

Experimental realization of atomically flat and AlO2 -terminated LaAlO3 (001) substrate surfaces

Oxide single-crystal substrates with atomically smooth and chemically uniform surfaces are indispensable for constructing high-quality epitaxial heterostructures and sharp heterointerfaces. In this paper, the authors develop a simple and efficient recipe to optimize the surface structure in LaAlO${}_{3}$ (001) single crystal, a widely used substrate for growing perovskite oxide heterostructures. The authors combine thermal annealing and subsequent deionized water leaching processes to treat the LaAlO${}_{3}$ (001) surface. Thanks to the distinct solubility between AlO${}_{2}$ and LaO surface layers, the treated substrate exhibits an atomically flat and uniformly AlO${}_{2}$-terminated surface. This method circumvents the high-temperature instability of LaAlO3 (001) surface due to the intrinsic surface polarity.

Epitaxial integration of high-mobility La-doped BaSnO3 thin films with silicon

La-doped BaSnO3 has been epitaxially integrated with (001) Si using an SrTiO3 buffer layer via molecular-beam epitaxy (MBE). A 254 nm thick undoped BaSnO3 buffer layer was grown to enhance the mobility of the overlying La-doped BaSnO3 layer. The x-ray diffraction rocking curve of the BaSnO3 002 peak has a full width at half maximum of 0.02°. At room temperature, the resistivity of the La-doped BaSnO3 film is 3.6 × 10−4 Ω cm and the mobility is 128 cm2 V−1 s−1 at a carrier concentration of 1.4 × 1020 cm−3. These values compare favorably to those of La-doped BaSnO3 films grown by all techniques other than MBE on single-crystal oxide substrates. Our work opens an exciting arena for integrating hyper-functional oxide electronics that make use of high-mobility oxide films with the workhorse of the semiconductor industry, silicon.

Multifunctional La0.67Sr0.33MnO3 (LSMO) Thin Films Integrated on Mica Substrates toward Flexible Spintronics and Electronics.

Integrating oxide thin films on flexible substrates is a critical step toward future applications of multifunctional oxides for flexible electronics and spintronic devices. As a demonstration, multifunctional La0.67Sr0.33MnO3 (LSMO) thin films have been deposited on flexible mica substrates. The crystallinity and microstructure of the films have been characterized to show the good epitaxial quality of the films. The LSMO thin films on mica present excellent ferromagnetic and magnetoresistance properties (such as saturation magnetization Ms of 125-400 emu/cm3 at 10 K and a high MR value of ∼45% at 5 K under 1 T for the 50 mTorr deposited sample), which is even better than the ones on conventional rigid single-crystal oxide substrates. More interestingly, no deterioration of the properties is observed under mechanically bending condition, which demonstrates the good mechanical stretchability and property stability of the LSMO thin films on mica. The demonstration of functional oxides integrated on flexible mica substrates paves a route toward future flexible spintronics and electronics.

Photosensitization of Single-Crystal Oxide Substrates with Quantum Confined Semiconductors

Dye-sensitized solar cells have been studied for many years as a potential inexpensive and scalable alternative to silicon solar cells. They have recently expanded their list of photosensitizers to include quantum dots. In recent years, there has been substantial progress in the field of quantum dot solar cells, with certified efficiencies now reaching 13.4%. Fundamental studies on nanomaterial/semiconductor electrode coupling have led to a deeper understanding of photoinduced electron-transfer processes that are important for both of these devices. This Feature Article will highlight the use of a model system, nanomaterials sensitizing single-crystal oxide substrates, that is useful for investigating how changes in nanomaterial shape, dimensionality, size, and local environment affect the photoinduced charge separation efficiency.

Superconducting FeSe0.1Te0.9 thin films integrated on Si-based substrates

With the goal of integrating superconducting iron chalcogenides with Si-based electronics, superconducting FeSe0.1Te0.9 thin films were directly deposited on Si and SiOx/Si substrates without any buffer layer by a pulsed laser deposition (PLD) method. Microstructural characterization showed excellent film quality with mostly c-axis growth on both types of substrates. Superconducting properties (such as superconducting transition temperature Tc and upper critical field Hc2) were measured to be comparable to that of the films on single crystal oxide substrates. The work demonstrates the feasibility of integrating superconducting iron chalcogenide (FeSe0.1Te0.9) thin films with Si-based microelectronics.

Optical NIR-VIS-VUV constants of advanced substrates for thin-film devices

The optical properties of several commonly used single-crystal oxide substrates were explored by spectroscopic ellipsometry over a wide spectral range from 0.74 eV to 8.8 eV. The crystals examined are (100) SrTiO3, 0.7 % wt Nb-doped (100) SrTiO3,(100) (LaAlO3)0.29(SrAl0.5Ta0.5O3)0.7, (011) DyScO3, (100) MgAl2O4, (100) MgO, and (100) LaAlO3, all of which enable epitaxial growth of numerous perovskite-type and other optical thin films. An analytic form for the complex dielectric function was derived from ellipsometric data through a physically consistent modeling process. The obtained dielectric spectra were further utilized to calculate the complex index of refraction and absorption coefficient for each substrate material. The absorption spectra and optical band gap were analyzed using Tauc plots. The parameters for reconstructing the dielectric functions are given in detail, allowing for extensive applications of the results of this work.

Ultrastrong Terahertz Emission from InN Nanopyramids on Single Crystal ZnO Substrates

The creation of high efficiency and room temperature terahertz (THz) emitters has long been expected in both scientific and industrial communities. Despite the recent progress in THz source such as quantum cascade lasers, high efficiency THz emitters capable of operating at room temperature are still elusive. Indium nitride (InN), a narrow bandgap semiconductor, has emerged as a promising THz emitter due to its unique electronic properties. However, the efficiency of InN THz emitters reported up to now is still far from theoretically predicted because of inadequately engineered electrical conduction and radiative coupling. In this study, the authors report a novel, high performance THz emitting structure consisting of nanoengineered InN micro/nanopyramid arrays on a single crystal zinc oxide (ZnO) substrate. With improved electronic conduction from Zn diffusion induced doping and enhanced radiation coupling benefiting from uniquely structured geometry, the InN nanopyramids yielded THz emission intensity is close to an order of magnitude stronger than that of p‐type indium arsenide (InAs). These findings prove that InN is a promising THz material and of wide importance in material science, optical engineering sectors, etc.

Multifunctional epitaxial systems on silicon substrates

Multifunctional heterostructures can exhibit a wide range of functional properties, including colossal magneto-resistance, magnetocaloric, and multiferroic behavior, and can display interesting physical phenomena including spin and charge ordering and strong spin-orbit coupling. However, putting this functionality to work remains a challenge. To date, most of the work reported in the literature has dealt with heterostructures deposited onto closely lattice matched insulating substrates such as DyScO3, SrTiO3 (STO), or STO buffered Si(100) using concepts of lattice matching epitaxy (LME). However, strain in heterostructures grown by LME is typically not fully relaxed and the layers contain detrimental defects such as threading dislocations that can significantly degrade the physical properties of the films and adversely affect the device characteristics. In addition, most of the substrates are incompatible with existing CMOS-based technology, where Si (100) substrates dominate. This review discusses recent advances in the integration of multifunctional oxide and non-oxide materials onto silicon substrates. An alternative thin film growth approach, called “domain matching epitaxy,” is presented which identifies approaches for minimizing lattice strain and unwanted defects in large misfit systems (7%–25% and higher). This approach broadly allows for the integration of multifunctional materials onto silicon substrates, such that sensing, computation, and response functions can be combined to produce next generation “smart” devices. In general, pulsed laser deposition has been used to epitaxially grow these materials, although the concepts developed here can be extended to other deposition techniques, as well. It will be shown that TiN and yttria-stabilized zirconia template layers provide promising platforms for the integration of new functionality into silicon-based computer chips. This review paper reports on a number of thin-film heterostructure systems that span a variety of ferroelectric, multiferroic, magnetic, photocatalytic, and smart materials. Their properties have been extensively investigated and their functionality found to be comparable to films grown on single-crystal oxide substrates previously reported by researchers in this field. In addition, this review explores the utility of using laser processing to introduce stable defects in a controlled way and induce magnetism and engineer the optical and electrical properties of nonmagnetic oxides such as BaTiO3, VO2, NiO, and TiO2 as an alternative for incorporating additional magnetic and conducting layers into the structure. These significant materials advancements herald a flurry of exciting new advances in CMOS-compatible multifunctional devices.

Single-crystal oxide substrate dependent electrical properties of sputtered BiFeO3 thin films

A series of BiFeO3 thin films were prepared on SrRuO3-buffered (100) YAlO3, (La, Sr)(Al, Ta)O3, SrTiO3 and LaAlO3 single-crystal oxide substrates via radio-frequency magnetron sputtering. An excellent (00l)-oriented growth of BiFeO3 films with a dominant rhombohedral-like phase was identified by XRD analyses. The electrical properties, e.g. dielectric, ferroelectric and leakage current characteristics, of BiFeO3 films grown on these single-crystal substrates showed significant differences. It was found that the BiFeO3 film deposited on LaAlO3 substrate exhibited the best overall electrical properties among the four films. It showed low loss tangent (tanδ<0.05) and leakage current density (J<8×10−4A/cm2), as well as a remnant polarization of 2Pr~150μC/cm2 and a reduced coercive field of 2Ec~340kV/cm.

Comparison between the ferroelectric/electric properties of the PbZr0.52Ti0.48O3 films grown on Si (100) and on STO (100) substrates

Ferroelectric/electric properties of PbZr0.52Ti0.48O3 (PZT) thin films grown by pulsed laser deposition (PLD) on two different substrates, Si (001) and SrTiO3 (STO) (001), were comparatively analyzed. The structural characterization has revealed the epitaxial relationship between the grown layers and the two types of substrates, with larger density of structural defects for the films deposited on Si (001) with buffer STO layer. The ferroelectric/electric properties are also different, with lower remnant polarization (about half of the value obtained on STO substrate), higher dielectric constant (about two times larger), and lower leakage current (about two orders of magnitude lower) for the PZT films deposited on Si (001) compared to those deposited on (001) STO substrates. Nevertheless, the results show that the use of a STO buffer layer on Si can be a solution to obtain good quality PZT capacitor structures without using expensive single-crystal oxide substrates. In this way, applications based on PZT capacitors (e.g. non-volatile memories, pyroelectric detectors, light switches, etc.) would be more easily integrated directly on Si wafers.

Magnetic Analysis of Polar and Nonpolar Oxide Substrates

The magnetism of different polar and nonpolar cuts of single-crystal oxide substrates has been analysed. In addition to their intrinsic diamagnetism, some crystals exhibit a Curie-law susceptibility, which is associated with 3d impurities at the 1 ppm level, except for MgO where it is ~30 ppm. Others exhibit an anhysteretic ferromagnetic-like signal typical of d 0 magnetism. The magnitude of the saturation moment depends critically on the type of surface in the case of the perovskite oxide substrates-SrTiO 3 , LaAlO 3 , and LSAT. The moment on (111) SrTiO 3 reaches a maximum of 80 μB nm -2 on annealing in vacuum in the region of 650°C but no moment (<;2 μ B nm -2 ) is detectable on (100) SrTiO 3 . The moments on the two polar cuts of LaAlO 3 are 40-70 μB nm -2 . No correlation was found between the magnetism and the polar nature of the cut in non-perovskite oxide substrates-MgO, MgAl 2 O 4 , TiO 2 , and Al 2 O 3 .