Nb-doped SrTiO3 Research Articles

Oxygen Vacancy Compensation-Induced Analog Resistive Switching in the SrFeO3-δ/Nb:SrTiO3 Epitaxial Heterojunction for Noise-Tolerant High-Precision Image Recognition.

Neuromorphic computing, inspired by the brain's architecture, promises to surpass the limitations of von Neumann computing. In this paradigm, synaptic devices play a crucial role, with resistive switching memory (memristors) emerging as promising candidates due to their low power consumption and scalability advantages. This study focuses on the development of metal/oxide-semiconductor heterojunctions, which offer several technological advantages and have broad potential for applications in artificial neural synapses. However, constructing high-quality epitaxial interfaces between metal and oxide semiconductors and designing modifiable contact barriers are challenging. Herein, we construct high-quality epitaxial metal/semiconductor interfaces based on the metallicity of the perovskite phase SrFeO3-δ (PV-SFO) and a small Schottky barrier in contact with Nb-doped SrTiO3 (NSTO). X-ray diffraction patterns, reciprocal space mapping results, and cross-sectional transmission electron microscopy images reveal that the prepared PV-SFO film exhibits a perfect single-crystal structure and an excellent epitaxial interface with the NSTO (111) substrate. The corresponding memristor exhibits analog-type resistive-variable characteristics with an ON/OFF ratio of ∼1000, stable data retention after 10,000 s, and no noticeable fluctuation in resistance after 10,000 pulse cycles. Electron energy loss spectroscopy, first-principles calculations, and electrical measurements reveal that compensating or restoring oxygen vacancies at the NSTO surface decreases or increases the contact barrier between PV-SFO and NSTO, respectively, thereby gradually regulating the resistance value. Furthermore, high-quality epitaxial PV-SFO/NSTO devices achieve up to 98.21% recognition accuracy for handwriting recognition tasks using LeNet-5-based network structures and 92.21% accuracy for color images using visual geometry group (VGG) network structures. This work contributes to the advancement of interface-type memristors and provides valuable insights into enhancing synaptic functionality in neuromorphic computing systems.

Study of CO2 Adsorption Properties on the SrTiO3(001) Surface with Ambient Pressure XPS.

The adsorption properties of CO2 on the SrTiO3(001) surface were investigated using ambient pressure X-ray photoelectron spectroscopy under elevated pressure and temperature conditions. On the Nb-doped TiO2-enriched (1 × 1) SrTiO3 surface, CO2 adsorption, i.e., the formation of CO3 surface species, occurs first at the oxygen lattice site under 10-6 mbar CO2 at room temperature. The interaction of CO2 molecules with oxygen vacancies begins when the CO2 pressure increases to 0.25 mbar. The adsorbed CO3 species on the Nb-doped SrTiO3 surface increases continuously as the pressure increases but starts to leave the surface as the surface temperature increases, which occurs at approximately 373 K on the defect-free surface. On the undoped TiO2-enriched (1 × 1) SrTiO3 surface, CO2 adsorption also occurs first at the lattice oxygen sites. Both the doped and undoped SrTiO3 surfaces exhibit an enhancement of the CO3 species with the presence of oxygen vacancies, thus indicating the important role of oxygen vacancies in CO2 dissociation. When OH species are removed from the undoped SrTiO3 surface, the CO3 species begin to form under 10-6 mbar at 573 K, thus indicating the critical role of OH in preventing CO2 adsorption. The observed CO2 adsorption properties of the various SrTiO3 surfaces provide valuable information for designing SrTiO3-based CO2 catalysts.

Investigation of Grain Boundary Effects in Sm0.2Ce0.8O2-x Thin Film Memristors.

Cerium-based materials (CeO2-x) are of significant interest in the development of vacancy-modulated resistive switching (RS) memory devices. However, the influence of grain boundaries on the performance of memristors is very limited. To fill this gap, this study explores the influence of grain boundaries in cerium-based thin film resistive random-access memory (RRAM) devices. Sm0.2Ce0.8O2-x (SDC20) thin films were deposited on (100)-oriented Nb-doped SrTiO3 (NSTO) and (110)-oriented NSTO substrates using pulsed laser deposition (PLD). Devices constructed with a Pt/SDC20/NSTO structure exhibited reversible and stable bipolar resistive switching (RS) behavior. The differences in conduction mechanisms between single-crystal and polycrystalline devices were confirmed, with single-crystal devices displaying a larger resistance window and higher stability. Combining the results of XPS and I-V curve fitting, it was confirmed that defects near the grain boundaries in the SDC-based memristors capture electrons, thereby affecting the overall performance of the RRAM devices.

Optimizing surface treatment for Nb-doped SrTiO3 substrates: effects on structural and chemical properties

Optimizing surface treatment for Nb-doped SrTiO3 substrates: effects on structural and chemical properties

Colossal dielectric behavior of Nb-doped SrTiO3 single crystal

Crystals are usually used to explore the origin of material properties due to the fact that they have no grain boundaries and no pores. Although doped SrTiO3 crystal have remarkable dielectric properties, the origin of this improvement is still not fully understood. Herein, Nb5+ has been selected as a donor ion to study the dielectric properties of SrTiO3 due to its radius close to Ti4+. It has been found that the dielectric permittivity of the Nb-doped crystal is greater than 94,500 and the dielectric loss is maintained below 0.002 in the frequency range from 20 Hz to 0.2 MHz. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy indicate that the dielectric properties of doped crystals were improved due to the existence of oxygen vacancies and there was no effect of interface polarization. It was beneficial for the Nb-doped crystal to be annealed in a N2 atmosphere resulting in oxygen loss more oxygen vacancies, and a higher concentration of Ti3+. The electron can be bound by VO•• and Ti4+ and form the defect dipole [Ti4+∙e − VO•• − Ti4+∙e], which showed a higher permittivity and lower dielectric loss. In summary, the origin and optimization of dielectric properties of SrTiO3-based materials can be deeply understood when from doped SrTiO3 crystals.

Enhancing mid-infrared surface phonon polariton modes: optimization of structural parameters in Nb-doped SrTiO3 with hole array structures for advanced infrared sensors

This study utilized the finite difference time domain method to investigate the mid infrared surface phonon polaritons and localized surface phonon resonances in undoped and niobium (Nb)-doped SrTiO3 (STO) with planar and holes array structures. Research has shown that Nb-doped STO operates in the Reststrahlen band of 8.06–18.48 µm, providing a wider spectral response than undoped STO (12.58–18.26 µm) and effectively covering the atmospheric window of long wave infrared. This indicates that the increase in virtual permittivity has the least impact on spectral broadening, indicating that the new infrared sensor technology has broad prospects. The optimization of structural parameters, including the period, filling factor, and depth of STO holes array, as well as the response to changes in incident light angle, is crucial for guiding the design of high-performance optoelectronic devices. In addition, this study explored the excitation of four resonant modes within a holes array and analyzed their relationship with array parameters to enhance the design of optoelectronic applications.

Resistive switching and synaptic simulation behaviors of epitaxial ZnO/Nb-doped SrTiO3 heterojunction controlled by the substrate orientation

Zinc oxide (ZnO) films were grown epitaxially on Nb-doped SrTiO3 (NSTO) substrates with different orientations using laser pulse deposition to create the (112¯0)ZnO/(100)NSTO, (0002)ZnO/(110)NSTO and (0002)ZnO/(111)NSTO heterojunctions. Epitaxial ZnO films on the (100) NSTO had two orthorgonal domains, resembling the fourfold symmetry of the (100) NSTO substrate. This good matching between the grown ZnO film and NSTO substrate resulted in a lower growth rate, smoother and denser surface, and a higher density of interface defect states. The interface state density was approximately 4.4 × 1012 eV−1cm−2, which was notably higher than that of the other two heterojunctions. Electrical transport studies revealed that ZnO/(100)NSTO heterojunction demonstrated bipolar resistive switching and negative differential resistance (NDR) due to a higher density of donor interface states from contributions of oxygen vacancies, while both the ZnO/(110)NSTO and ZnO/(111)NSTO heterojunction exhibited typical rectifying behavior. The preliminary investigation into synaptic behaviors suggested that (112¯0) ZnO/(100)NSTO heterojunction holds significant potential in brain-inspired neuromorphic computing systems.

Enhanced energy storage performance in Bi4Ti3O12 thin films with oxygen depletion layers

The imprint effect in ferroelectric materials can significantly enhance the performance of energy storage devices. Bi4Ti3O12 (BTO) and oxygen-deficient Bi4Ti3O11.2 (DBTO) thin films were deposited on single-crystal Nb-doped SrTiO3 substrates using pulsed laser deposition. In stark contrast, multilayer DBTO/BTO thin films incorporating an oxygen-deficient layer were also fabricated, with the DBTO layers having thicknesses of 5 nm and 10 nm, respectively. Both BTO and DBTO thin films exhibited epitaxial growth and c-axis-oriented crystallinity. Regarding ferroelectric properties, DBTO/BTO capacitors with an oxygen-deficient layer demonstrated a pronounced imprint effect in their ferroelectric hysteresis loops, compared to the BTO and DBTO capacitors. This effect was particularly noticeable in the DBTO/BTO capacitor with a 10 nm oxygen-deficient layer, as opposed to the thin film with a 5 nm oxygen-deficient layer. We suggest that the imprint effect can be attributed to localized electric field variations and imbalances in charge transport at the interface between the oxygen-depleted layer and the remaining thin film. Consequently, the imprint effect observed in the DBTO/BTO capacitors contributed to an enhancement in energy storage density and efficiency, compared to BTO and DBTO capacitors. This study demonstrates the potential of multilayer DBTO/BTO thin films in advancing capacitive energy storage systems.

Conducting atomic force microscopy studies on domain wall currents of Bi5Ti3FeO15 nanodots fabricated by anodic aluminum oxide nanotemplate and sol-gel process.

We investigated the local current characteristics of Bi5Ti3FeO15 (BTFO) nanodots on Nb-doped SrTiO3 substrates affected by their ferroelectric domain structures and domain walls. The BTFO nanodots with a diameter of about 50 nm were fabricated by anodic aluminum oxide nanotemplates and a BTFO sol-gel process. Based on a piezoresponse force microscope, it was confirmed that domain walls were formed in the ferroelectric domain structures of the epitaxial BTFO nanodots. Current changes due to ferroelectric tunneling junctions according to ferroelectric polarizations in epitaxial BTFO nanodots were confirmed by conduction atomic force microscopy. In particular, the domain walls formed in the epitaxial BTFO nanodots formed high currents compared to the currents in ferroelectric tunneling junctions due to polarizations. RESEARCH HIGHLIGHTS: Ferroelectric Bi5Ti3FeO15 nanodots with a diameter of 50 nm. Ferroelectric domain structures observed with piezoresponse force microscopy. High domain wall currents observed at domain boundaries observed with conducting atomic force microscopy.

Nanomechanical-ferroelastics behavior, and the low-temperature ferroelectric manifestation of BiMnO3 thin films

Ferroelastic-nanomechanical behavior of BiMnO3 thin films on (100) Nb-doped SrTiO3 substrate is studied at the nanoscale, for the first time using the theories of depth sensing indentation and finite element analysis. This was to understand the redirection of polarization in ferroelastic behavior using piezoelectric direct effect and applying a controlled local force by nanoindentation. The values of the nanomechanical properties Young’s modulus (E), hardness (H), and Stiffness (S) were measured to be E = 142 ± 3 GPa, H = 8 ± 0.2 GPa, and S = 44072 ± 45 N m−1 respectively. Using the experimental mechanical properties, a finite element analysis was carried out to understand the relationship between the irreversible work versus depth curve and plastic-deformation evolution of the thin film-interface-substrate system, associated with the pop-in observed. Von Mises stress maps are presented to clearly illustrate the deformations, mechanical failures, and influences between the BiMnO3 film and the SrTiO3 Substrate. The ferroelastic range for BiMnO3 material was observed between 0 to 12 nm, and the yield strength was Y = 2.6 ± 0.7 GPa. The ferroelectric properties through hysteresis curves were evaluated at two different temperatures of 200 K and 300 K in order to observe the effect of polarization with temperature. Providing better polarization performance at lower temperatures; P s = 9.14 ± 0.01 ( μ C cm–2), P r = 2.23 ± 0.02 ( μ C cm–2) and H c = 0.77 kV cm−1.

Texture in atomic layer deposited Hf0.5Zr0.5O2 ferroelectric thin films

HfO2-based ferroelectric materials have excellent CMOS compatibility and polarization even in ultrathin films, which is regarded as promising candidate material for memory devices and related electronic devices. However, as the ferroelectric layer thickness scaling down, the consistency of device performance is inevitably affected by the complex polyphase and polycrystalline nature of HfO2-based ferroelectric thin films. Therefore, obtaining and controlling the texture of HfO2-based ferroelectric thin films is extremely important for regulating polarization orientation and improving ferroelectric polarization. In this work, 10 nm Hf0.5Zr0.5O2 (HZO) films were prepared by atomic layer deposition (ALD) on Nb-doped SrTiO3 (NSTO) single crystal substrates. The texture and orientation of HZO films were analyzed through the state-of-the-art spherical aberration corrected transmission electron microscope (Cs-TEM) technique. The domain matching epitaxy between HZO films and NSTO substrate helps eliminate lattice mismatch and residual misfit strain, and ultimately leads to the formation of <112> texture in HZO ferroelectric films. The obtained results represent lattice and interface play an extremely important role in regulating the ferroelectricity of HfO2-based films, which provides a way to design ultra-thin HfO2-based materials with excellent ferroelectric polarization by controlling the crystal structure and orientation.

Optimization of thermoelectric properties in La–Nb-doped bulk SrTiO3 synthesized by HPHT method

SrTiO3 (STO) is considered a promising high-temperature N-type thermoelectric material. However, its intrinsic insulating properties and high thermal conductivity limit the dimensionless quality factor (zT) due to its simple crystal structure and the existence of light elements. Along these lines, in this work, the optimization of the thermoelectric properties of STO-based materials was achieved by using the high temperature and high pressure (HPHT) method and adjusting the doping ratios of the La and Nb heavy elements. To enhance the doping effect, oxygen vacancies need to be formed within the sample. For this reason, titanium powder was added to the sample. Due to the confined environment induced by the high pressure, which can be attributed to the application of the HPHT method and the strong oxygen trapping ability of Ti at high temperatures, a reducing environment was attained for the samples, the latter, promoted the formation of oxygen vacancies in the synthesized samples, and semiconducting properties were achieved. The thermoelectric properties and microscopic morphology of the synthesized samples with different doping ratios of La and Nb were also thoroughly investigated. Finally, the maximum zT value obtained for the samples was 0.28@973 K. A fast, simple, and effective method for the preparation of oxygen-deficient packet-crystalline oxide materials was developed in this work, which paves the way for significantly improving their thermoelectric properties.

Colossal permittivity and ultralow dielectric loss in Nb-doped SrTiO 3 ceramics

Colossal permittivity and ultralow dielectric loss in Nb-doped SrTiO ₃ ceramics

In-gap states and local structures around substitutional defects in La- or Nb-doped n-type SrTiO3

First-principles calculations with DFT+U method were conducted on the local structures and the in-gap electronic states of n-type SrTiO3, whose electron doping was realized by substitution of Sr by La or Ti by Nb. For both La-doped and Nb-doped SrTiO3, cubic structures were proved to be stable e3ven when tetragonal structures were assumed, as suggested also in a reported experimental study. In La-doped SrTiO3, O atoms neighboring the substitutional La were symmetrically attracted toward the dopants. Sharp electron band observed at the top of the bandgap, which gave n-type conductivity and resulted from the intersection of Fermi energy level at the conduction band bottom with La-doping, was composed of 3d orbitals in Ti. The La-doping transformed some of the p electrons in O into the d electrons in Ti in the sharp band, and these electrons at Fermi level were expected to act as n-type carriers. In Nb-doped SrTiO3, on the other hand, Ti atoms at the first, second and third nearest neighbors of the Nb, which were in the same Ti-O plane surrounding the dopants, were repelled away from the Nb dopants to be isotropic local expansion, while O atoms neighboring Nb were attracted toward the dopants in the expanded local structure. Sharp band of the in-gap states just below the Fermi energy of the Nb-doped SrTiO3 was not only of the 3d orbitals in the first nearest-neighbor Ti atoms, but also of those in the second and third nearest-neighbor Ti atoms of the dopants. The degree of Fermi level intrusion into conduction band and atomic rearrangements near Nb substitutions were milder than those of La substitutions. As replacement of Sr atoms by La not contained in Ti-O network does not disturb the electron transport within Ti-O conduction planes, Nb-doping in Ti site does not interrupt the n-type carrier transportation even though Nb atoms are in Ti-O network, because of the formation of conduction path which bypasses the unit cell containing the dopants.

Photonic Physical Reservoir Computing with Tunable Relaxation Time Constant.

Recent years have witnessed a rising demand for edge computing, and there is a need for methods to decrease the computational cost while maintaining a high learning performance when processing information at arbitrary edges. Reservoir computing using physical dynamics has attracted significant attention. However, currently, the timescale of the input signals that can be processed by physical reservoirs is limited by the transient characteristics inherent to the selected physical system. This study used an Sn-doped In2 O3 /Nb-doped SrTiO3 junction to fabricate a memristor that could respond to both electrical and optical stimuli. The results show that the timescale of the transient current response of the device could be controlled over several orders of magnitude simply by applying a small voltage. The computational performance of the device as a physical reservoir is evaluated in an image classification task, demonstrating that the learning accuracy could be optimized by tuning the device to exhibit appropriate transient characteristics according to the timescale of the input signals. These results are expected to provide deeper insights into the photoconductive properties of strontium titanate, as well as support the physical implementation of computingsystems.

Leakage current characteristics affected by crystallinity and domain wall current of epitaxial Bi5Ti3FeO15 thin films

We investigated leakage current characteristics affected by crystallinity and domain wall currents of epitaxial Bi5Ti3FeO15 (BTFO) thin films on Nb-doped SrTiO3 substrates. Highly a-oriented BTFO thin films, highly c-oriented BTFO thin films, and BTFO thin films with a mixture of a-oriented and c-oriented crystallinity were prepared by controlling the substrate temperature and the pulsed laser deposition deposition rate. Highly c-oriented BTFO thin films exhibited the best leakage current characteristics because the Bi2O2 layers were placed perpendicular to the c-axis to reduce leakage currents. The BTFO thin films with a mixture of a-oriented and c-oriented crystallinity showed larger leakage currents compared to highly c-oriented BTFO thin films. The current domains of the BTFO thin films corresponding to the ferroelectric domain structures were observed by a conducting atomic force microscope, and it was observed that leakage currents were formed around the domain walls. In particular, the largest leakage currents are formed at the boundaries of c-oriented domains and a-oriented domains, and these domain boundaries confirmed that the BTFO thin films with a mixture of a-oriented and c-oriented crystallinity were responsible for the largest leakage currents.

Full polarization reversal at room temperature in unsubstituted AlN

Room temperature ferroelectricity in unsubstituted AlN films is studied to examine the role of cation substitution into wurtzite materials. AlN and (Al0.7Sc0.3)N films deposited on (111) 0.5 wt. % Nb-doped SrTiO3 have a (0001)-orientation with different in-plane lattice alignments with respect to those of the substrate, depending on the composition and the deposition temperature. The AlN films deposited at 450 °C showed complete ferroelectric switching above 140 °C but local polarization switching at room temperature because a dielectric breakdown occurred before complete switching, while full polarization reversal was observed at all measurement temperatures for (Al0.7Sc0.3)N. Low-temperature deposition, such as at 250 °C, significantly enhanced the dielectric breakdown field and also increased leakage current. As a result, sufficient polarization switching at room temperature was observed in the AlN film deposited at 250 °C. Positive-up/negative-down pulse measurements showed remanent polarization of 150 μC/cm2 and a coercive field of 8.3 MV/cm, in agreement with the theoretical value and temperature dependence observed for the AlN film deposited at 450 °C. The observed coercive field value lies on the line composed of the previously reported data in Sc concentration dependence. This tendency implies that the reduction in the coercive field is primarily attributable to the alteration of crystal lattice anisotropy caused by Sc.

Low-temperature synthesis of Nb-doped and Mn-doped SrTiO3 with easily controllable particle size

Heterovalently doped perovskite-type oxides are used in various electronic applications. Herein, we propose a low-temperature synthesis method for Nb- and Mn-doped SrTiO3 particles with a wide range of particle sizes that is conducted at temperatures close to room temperature (RT). The cation-doped SrTiO3 particles were synthesized under diverse solution conditions by varying the aging temperature, aging period, viscosity of solvents, and OH− concentration using facile and low-cost methods. The particle sizes were simply controlled by changing the ratio of H2O/ethanol solvents owing to their difference in viscosity at low temperatures. The uniformity of the particle sizes was significantly improved via a two-step aging process, that is, heating at a low temperature followed by RT aging. Additionally, the OH− concentration in aqueous solution had considerable impact on the rate of crystal growth, affecting particle size as well as morphology. This study demonstrates the feasibility for the low-temperature synthesis of cation-doped perovskite-type materials.

Effect of heat treatment on resistive switching memory characteristics of NiO nanodots of tens of nanometers shattered by AFM tips

The characteristics of resistive random access memory (RRAM) at the nanometer scale are increasingly important for RRAM cells with further miniaturization. We report the RRAM characteristics of NiO nanodots in the order of tens of nanometers. NiO nanodots of approximately 70 nm were created on a single-crystal Nb-doped SrTiO3 (Nb:STO) substrate by dip-pen nanolithography. For comparison, a NiO thin film was also deposited on the Nb:STO substrate by spin coating. To reduce the size of NiO nanodots, they were shattered by an atomic force microscope tip, obtaining nanodots of approximately 10 nm. Using a conductive atomic force microscope, we characterized the current according to the applied electric field of the NiO nanodots on the Nb:STO substrate with a capacitor configured of Pt/NiO/Nb:STO substrate. When compared with the NiO thin film, the forming voltage of the NiO nanodots decreased. In addition, the shattered NiO nanodots exhibited unipolar switching characteristics, and the forming and set/reset voltages decreased with the reduction in nanodot size. Furthermore, the NiO nanodots exhibited a substantially reduced forming and set/reset voltages after heat treatment in vacuum. We suggest that the oxygen-depleted layer formed on the surface of NiO nanodots by heat treatment plays a crucial role in the reduction of the forming and operation voltages.

Band-engineered LaFeO3–LaNiO3 thin film interfaces for electrocatalysis of water

Iron and nickel-based perovskite oxides have proven promising for the oxygen evolution reaction (OER) in alkaline environments, as their catalytic overpotentials rival precious metal catalysts when the band alignment is tuned through substitutional doping or alloying. Here, we report the engineering of band alignment in LaFeO3/LaNiO3 (LFO/LNO) heterostructures via interfacial doping that yields greatly enhanced catalytic performance. The 0.2 eV offset (VBO) between the Fermi level in metallic LNO and the valence band in semiconducting LFO that we predict using density functional theory makes LFO a p-type semiconductor, resulting in significantly lower barriers for hole transport through LFO compared to the intrinsic material. Experimental band alignment measured with in situ x-ray photoelectron spectroscopy of epitaxial LFO/LNO heterostructures confirms these predictions, producing a measured VBO of 0.3(1) eV. Furthermore, OER catalytic measurements on these samples in the alkaline solution show an increase in catalytic current density by a factor of ∼275 compared to LFO grown on n-type Nb-doped SrTiO3. These results demonstrate the power of tuning band alignments through interfacial band engineering for improved catalytic performance of oxides.