Substrate SrTiO3 Research Articles

The effects of crystal orientation on the ferroelectric and optoelectronic properties of Pt/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 heterostructures

Due to their excellent ferroelectric, semiconducting, and optoelectronic properties, perovskite ferroelectric thin films have attracted increasing attention in the photovoltaic field. Using La0.5Sr0.5CoO3 (LSCO) as the bottom electrode, flake, strip and pyramidal Na0.5Bi0.5TiO3 (NBT) films were prepared on (001), (110) and (111) SrTiO3 (STO) substrates by off-axis magnetron sputtering and pulsed laser deposition, respectively. XRD and Phi scan results show that both LSCO and NBT are perovskite structures and satisfy the epitaxial growth along the substrate STO. NBT orientation has a significant effect on the ferroelectric properties of Pt/NBT/LSCO heterostructures. The remnant polarization 2Pr values for NBT(111), NBT(110) and NBT(001) are 87.5, 62.6 and 28.5 μC/cm2, respectively, and thus 2Pr(111)>2Pr(110)>2Pr(001). NBT(111) shows the best ferroelectric properties (Pr=41.3 μC/cm2), excellent fatigue-resisting (up to 1010 cycles) and retention performance (duration 104s). The 2Pr-T association 2 Pr=41.72+16.05*T0.35 for NBT(111) was obtained according to the curve fitting, it is found that the NBT orientation has a significant effect on the PV effect for PT/NBT/LSCO photovoltaics and the open-circuit voltage ∆Voc. The short-circuit current ∆Jsc of polarization regulate intensity follows the laws of ∆Voc-111>∆Voc-110>∆Voc-001 and ∆Jsc-111>∆Jsc-110>∆Jsc-001, the Jsc for Pt/NBT(111)/LSCO heterostructures increases linearly with the illumination Ip increasing, while the Voc decreases linearly. The ferroelectro-polarized modulation PV mechanism was investigated by analyzing band structure of the Pt/NBT/LSCO heterostructures, which is attributed to the coupling effect of the ferroelectric depolarization field and the interfacial Schottky barrier.

Competing conduction mechanisms for two-dimensional electron gas at LaTiO3/SrTiO3 heterointerfaces

In pursuit of understanding the mechanism of two-dimensional electron gas (2DEG), artificial heterostructures based on SrTiO3 have sparked a wealth of exciting experimental and theoretical results. However, to date, the physical origin of this phenomenon still remains controversial. In our endeavor to unravel the mystery, we have synthesized a series of LaTiO3+δ/SrTiO3 films by controlling growth temperature and oxygen pressure, respectively. X-ray absorption results identify the transition of Ti ions from Ti3+ to Ti4+. It is proved that electrical transport properties in the LaTiO3/SrTiO3 film are dominated by oxygen vacancies in SrTiO3. In addition, the metallic behavior observed in the amorphous LaTiO3/SrTiO3 film suggests contributions from growth-induced donor defects to 2DEG. Our work presents direct evidence of competing conduction mechanisms for the observed 2DEG at the LaTiO3/SrTiO3 interface and highlights the important role of oxygen diffusion from the substrate SrTiO3. Meanwhile, we emphasize the significance of LaTiO3 as an oxygen scale meter to investigate interfacial properties of oxide heterostructures.

Navigating the magnetic contribution of the commonly used single-crystal substrates SrTiO3 (100) and LaAlO3 (100) in weak magnetic thin films

In recent years, thin films of weak ferromagnetic materials have been in huge demand; however, probing their magnetic characteristics has been difficult due to contributions from underlying substrates. In the present study, we have analyzed the magnetic properties of the commonly used single-crystal SrTiO3 (100) and LaAlO3 (100) substrates and performed a time-dependent annealing protocol in vacuum and ambient oxygen pressure to mitigate the intrinsic weak ferromagnetic contributions from these substrates arising due to the presence of disorder or defects such as vacancies in the pristine substrates. It is shown that after proper air annealing, the substrate magnetic background becomes diamagnetic. When such air-annealed diamagnetic substrates are used for the deposition of low-thickness films carrying low magnetic moments such as SrRuO3 and SrMnO3 thin films, their magnetic transitions are explicitly observed. The proposed annealing protocols help to improve the signal from weak magnetic samples. This allows us to analyze the film’s magnetic properties without worrying about the contribution from the substrate.

Comprehensive study of Raman optical response of typical substrates for thin-film growth under 633 nm and 785 nm laser excitation.

Raman spectroscopy is one of the most efficient and non-destructive techniques for characterizing materials. However, it is challenging to analyze thin films using Raman spectroscopy since the substrates beneath the thin film often obscure its optical response. Here, we evaluate the suitability of fourteen commonly employed single-crystal substrates for Raman spectroscopy of thin films using 633 nm and 785 nm laser excitation systems. We determine the optimal wavenumber ranges for thin-film characterization by identifying the most prominent Raman peaks and their relative intensities for each substrate and across substrates. In addition, we compare the intensity of background signals across substrates, which is essential for establishing their applicability for Raman detection in thin films. The substrates LaAlO3 and Al2O3 have the largest free spectral range for both laser systems, while Al2O3 has the lowest background levels, according to our findings. In contrast, the substrates SrTiO3 and Nb:SrTiO3 have the narrowest free spectral range, while GdScO3, NGO and MgO have the highest background levels, making them unsuitable for optical investigations.

Control of grain density by varying lattice mismatch in FePt-C film for heat assisted magnetic recording

Controlling the number density of grains in magnetic recording media is critical for improving the areal density of the recording. Herein, we report the dependence of grain density and microstructure of FePt-based granular films for heat-assisted magnetic recording media on the lattice mismatch between the FePt films and substrates SrTiO3, MgAl2O4, MgO and TiO2. The grain density and grain size of the FePt films increased and decreased, respectively, with the increasing lattice mismatch between the films and substrates. The optimum lattice mismatch for the FePt-based granular films was found to be 8%, which corresponds to the MgO substrate. FePt films grown on TiO2 substrate, which had the largest lattice mismatch of 16%, showed poor morphology and low grain density. The results imply that an underlayer with which a film develops a suitable mismatch plays a key role in controlling the microstructure and magnetic properties of the film.

Investigation on Controllable Orientation Growth of BiFeO3 Thin Films Deposited by DC Magnetron Sputtering

Nanocrystalline BiFeO3 thin films were synthesized on single crystal SrTiO3(100) substrate by the magnetron sputtering method at a mixture of Ar/O2 gas discharge, structure and morphology were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The miscut angle of 4° and 8° for substrate and the effect of deposition temperature of 650 °C, 700 °C, 750 °C and 800 °C were discussed, respectively. The other two different crystal orientations substrates of SrTiO3(110) and SrTiO3(111), different miscut angles substrate SrTiO3(100) were selected to characterize orientation growth of BiFeO3 thin films. The results showed single phase nanocrystalline BiFeO3 thin films were synthesized at 700 °C–800 °C. The best crystallinity of BiFeO3 thin films was obtained at 750 °C with the grain size of 50 nm approximately. Via optimizing controllable orientation growth through varing the miscut angle, the orientation of the film sample can be regulated by the crystal orientation of different substrates, further proved that possessed different electric domain directions with change of miscut angle for substrate.

Polar-discontinuity-induced stability of spontaneous polarization in ultrathin perovskite films

The critical thickness for ferroelectricity arising from the depolarization field seriously hinders the progress of device miniaturization. Herein, we demonstrate that the polar-discontinuity-induced electronic reconstruction is capable to screen the depolarization field and stabilizes the polarization state in ultrathin ferroelectric films. In this work, the asymmetric Ba(Mg1/3Nb2/3)O3 (BMN) was inserted between the ferroelectric PbTiO3 (PTO) and the substrate SrTiO3 (STO) to create the polar discontinuity. First-principles calculations show that the two-dimensional electron gas (2DEG) and the two-dimensional hole gas (2DHG) are generated in the heterostructure PbTiO3/Ba(Mg1/3Nb2/3)O3/SrTiO3 (PTO/BMN/STO), which will screen the depolarization field. Consequently, the polarization value of PTO still maintains at 20 μC/cm2 (~25%) even when its thickness is only 1.5 unit cells. Moreover, the positions of the 2DEG and 2DHG, as well as the orientation of the spontaneous polarization, could be tuned by changing the configuration of BMN. The universality of this strategy in stabilizing spontaneous polarization is also confirmed in BaTiO3/Ba(Mg1/3Nb2/3)O3/SrTiO3 (BTO/BMN/STO) case. This work sheds a light on how to miniaturize the device with stable spontaneous polarization.

From LaAlO3/SrTiO3 to LaAlO3/KNbO3: Improving the transport properties of two-dimensional electronic gas in created +1/+1 interfaces

The discovery of two-dimensional electron gases at LaAlO3/SrTiO3 heterointerface makes it a promising candidate for new generation of nano-devices. Further enhance of interfacial transport properties is the main challenge. To increase the limit of interfacial carrier densities of two-dimensional electron gas (2DEG) of well-studied LaAlO3/SrTiO3 heterostructure from 0.5 to 1 e− per unit cell, we replaced substrate SrTiO3 by KNbO3 to acquire the +1/+1 interface with one extra electron donor and less localized valence electrons. First-principles calculations are used to study (LaBO3)m/(KNbO3)6.5 (B = Al, Ga; m = 2–8) heterostructures. The studied heterostructures all show n-type metallic state without critical thickness, and the distribution of charge densities of different monolayers can be modulated by the polarization effect of KNbO3. The interfacial carrier density of studied heterostructures perform one order of magnitude larger than LaAlO3/SrTiO3 because of the increased intrinsic carrier limit. With the increase of LaBO3 (B = Al, Ga) layer thickness, the interfacial carrier density rises up to 1.26 × 1014 cm−2, and the electron effective masses decrease monotonically, resulting in higher carrier mobility and around 6 times larger electrical conductivity. This work proved an effective approach for tuned interfacial 2DEG and future device engineering.

First-principles prediction of a rising star of solar energy material: SrTcO3.

SrTcO3 as a new star of solar energy material is investigated in terms of its band gap evolution with biaxial strain from first-principles calculations. Compared to the theoretical equilibrium lattice constant a(b) of bulk SrTcO3, a set of lattice constants with a deviation of -8.75% to +3.35% are considered to include the strain effect. Since the in-plane lattice constant of SrTcO3 is larger than that of the commonly used substrate SrTiO3(STO)/La0.3Sr0.7Al0.35Ta0.35O9 (LSAT)/NdGaO3(NGO)/LaAlO3(LAO), we mainly focus on the modulation of compressive strain. It is found that the band gap decreases with increasing compressive/tensile strain. When the compressive strain reaches 8.75%, the band gap drops to zero and an insulator-metal phase transition appears. Particularly, upon a compressive strain of 1.3%/2.2%/2.4%/4.1%, which can be realized by growing SrTcO3 on substrate STO/LSAT/NGO/LAO, the band gap becomes 1.56/1.47/1.43/1.12 eV, which falls in the range for efficient solar cell materials. Our work suggests that SrTcO3 is a good candidate for a new solar energy material.

Tunable enhancement of ferroelectric properties in BaxSr1−xTiO3/La0.7Sr0.3MgO3 heterostructures observed by means of second harmonic generation technique

This work is devoted to research an influence of Ba cation concentration on polarization properties of ferroelectric layer which is included in bilayer structures as functional stratum. We have created bilayer nanocomposites consisting of a ferroelectric Ba(x)Sr(1−x)TiO3 (BST) layer and a ferromagnetic La0.7Sr0.3MnO3 (LSMO) layer with various concentrations of Ba. The epitaxial heterostructure BST/LSMO was fabricated using metal-organic aerosol deposition technique (MAD) on a single-crystal substrate SrTiO3 (001). The structural parameters of the BST/LSMO were defined by X-ray diffraction analysis. The in-plane ferroelectric polarization, measured by means of optical second harmonic generation (SHG) method, was shown to be strongly dependent on the Ba concentration. We consider this effect as structural distortion at room temperatures which close to the structural phase transition and altering the Ba concentration affects the vicinity to the phase transition point.

Physical properties of transparent perovskite oxides (Ba,La)SnO3with high electrical mobility at room temperature

Transparent electronic materials are increasingly in demand for a variety of optoelectronic applications. BaSnO3 is a semiconducting oxide with a large band gap of more than 3.1 eV. Recently, we discovered that La doped BaSnO3 exhibits unusually high electrical mobility of 320 cm^2(Vs)^-1 at room temperature and superior thermal stability at high temperatures [H. J. Kim et al. Appl. Phys. Express. 5, 061102 (2012)]. Following that work, we report various physical properties of (Ba,La)SnO3 single crystals and films including temperature-dependent transport and phonon properties, optical properties and first-principles calculations. We find that almost doping-independent mobility of 200-300 cm^2(Vs)^-1 is realized in the single crystals in a broad doping range from 1.0x10^19 to 4.0x10^20 cm^-3. Moreover, the conductivity of ~10^4 ohm^-1cm^-1 reached at the latter carrier density is comparable to the highest value. We attribute the high mobility to several physical properties of (Ba,La)SnO3: a small effective mass coming from the ideal Sn-O-Sn bonding, small disorder effects due to the doping away from the SnO2 conduction channel, and reduced carrier scattering due to the high dielectric constant. The observation of a reduced mobility of ~70 cm^2(Vs)^-1 in the film is mainly attributed to additional carrier-scatterings which are presumably created by the lattice mismatch between the substrate SrTiO3 and (Ba,La)SnO3. The main optical gap of (Ba,La)SnO3 single crystals remained at about 3.33 eV and the in-gap states only slightly increased, thus maintaining optical transparency in the visible region. Based on these, we suggest that the doped BaSnO3 system holds great potential for realizing all perovskite-based, transparent high-frequency high-power functional devices as well as highly mobile two-dimensional electron gas via interface control of heterostructured films.

Synthesis, Structures, and Physical Properties of Yttrium-Doped Strontium Manganese Oxide Films

AbstractCubic strontium manganese oxide is an end-member of the colossal magnetoresistive (CMR) family of manganese-based perovskites, Ln1-xAExMnO3. Because normal synthesis conditions lead to the 4-H hexagonal polymorph, high-pressure conditions are typically used to obtain the cubic perovskite polymorph. In this work, we describe the synthesis and structural/physical characterization of the cubic perovskite form of the high-alkaline-earth containing phases of Y1-xSrxMnO3 (x ≥ 0.7) as epitaxial thin films. Thin films of various stoichiometries were grown on single-crystal perovskite substrates SrTiO3, NdGaO3, and LaAlO3 using pulsed laser deposition. After optimizing deposition conditions, the perovskite polymorph is obtained using PLD at 800°C and 10-100 mTorr O2 for x=1, 0.9, 0.8, and 0.7, as demonstrated by x-ray diffraction. Epitaxial growth was determined to be cube-on-cube. Electrical property measurements demonstrated insulating behavior and no metal-insulator transition or magnetoresistive behavior, similar to related stable compounds.

Crystalline orientations of Tl2Ba2Ca2Cu3Ox grains on MgO, SrTiO3, and LaAlO3 substrates

Crystalline orientations of Tl2Ba2Ca2Cu3Ox grains in magnetron sputtered films on MgO (001), SrTiO3 (001), and LaAlO3 (001) substrates were investigated by scanning electron microscopy. In contrast to the nearly single crystalline films on the lattice matched substrates SrTiO3 and LaAlO3, films on the MgO (001) substrate, being polycrystalline in nature, exhibit several preferred in-plane grain orientations. These orientations agree well with a simplified theory of near-coincidence site lattices between Tl2Ba2Ca2Cu3Ox and MgO.