Properties Of SrTiO3 Thin Films Research Articles

Impact of Cationic Stoichiometry on Physical, Optical and Electrical Properties of SrTiO3 Thin Films Grown on (001)-Oriented Si Substrates.

In this study, we investigate the changes in the crystalline structure of MBE-deposited SrTiO3 layers on Si with different deviations from Sr/Ti stoichiometry as deposited but also after annealing at high temperatures (>600 °C). We show that as-grown 15 nm thick non-stochiometric SrTiO3 layers present surprisingly lower FWHM values of the (002) omega diffraction peak compared to fully stoichiometric layers of similar thickness. This can misleadingly point to superior crystalline quality of such non-stochiometric layers. However, thermal post-deposition anneals of these layers at temperatures up to 850 °C in oxygen show strong detrimental effects on the crystalline structure, surface and interface with the Si (001) substrate. On the contrary, the post-deposition anneals applied to the stoichiometric samples strongly improve the physical, optical and electrical properties of the epitaxial SrTiO3 thin films.

Exploring the electronic properties and oxygen vacancy formation in SrTiO3 under strain

SrTiO3 (STO) thin films are widely used as substrates in oxide-based devices, and although STO is one of the most studied materials, both experimentally and computationally in its bulk form, the potential for strain engineering of STO thin-films is largely ignored. In this work, we perform Density Functional Theory (DFT) calculations to investigate the tuning of the structural and electronic properties of STO thin films under uni- and biaxial strain. We find that for TiO2-terminated STO slab, the band gap changes due to strain are more significant than that of the SrO-terminated surface because the band gap is determined by the TiO2 layers closer to the surface. Furthermore, we also find that the formation energy of oxygen vacancies depends on the type of strain applied and the position of oxygen vacancies. These findings demonstrate the large and hitherto underexplored perspectives of using strain engineering to modulate the structural and electronic properties of perovskite film materials for a wide variety of energy and electronic applications.

Influence of Sr deficiency on structural and electrical properties of SrTiO3 thin films grown by metal\u2013organic vapor phase epitaxy

Homoepitaxial growth of SrTiO3 thin films on 0.5 wt% niobium doped SrTiO3 (100) substrates with high structural perfection was developed using liquid-delivery spin metal–organic vapor phase epitaxy (MOVPE). Exploiting the advantage of adjusting the partial pressures of the individual constituents independently, we tuned the Sr/Ti ratio of the gas phase for realizing, stoichiometric, as well as Sr deficient layers. Quantitative energy dispersive X-ray spectroscopy in a scanning transmission electron microscope confirm Sr deficiency of up to 20% in nominally off-stoichiometrically grown films. Our MOVPE process allows to grow such layers in phase pure state and without extended defect formation. Indications for oxygen deficiency could not be identified. Sr deficient layers exhibit an increased permittivity of ɛr = 202 and a larger vertical lattice parameter. Current–voltage characteristics (IVCs) of metal–oxide–semiconductor (Pt/SrTiO3/SrTiO3:Nb) structures reveal that Sr deficient SrTiO3 films show an intrinsic resistive switching with on–off ratios of three orders of magnitude at RT and seven orders of magnitude at 10 K. There is strong evidence that a large deviation from stoichiometry pronounces the resistive switching behavior. IVCs conducted at 10 K indicate a defect-based mechanism instead of mass transport by ion diffusion. This is supported by in-situ STEM investigations that show filaments to form at significant higher voltages than those were resistive switching is observed in our samples.

Microstructural and interface properties of Au/SrTiO3 (STO)/n-GaN heterojunction with an e-beam evaporated high-k STO interlayer

This paper reviews the microstructural and electrical properties of Au/n-GaN metal/semiconductor (MS) diode with an e-beam evaporated SrTiO3(STO) as an insulating layer between the Au and n-GaN substrate. The microstructural properties of STO thin films are assessed by employing XRD and TEM approaches and the results reveal that the STO thin films are formed on n-GaN surface. Then, the Au/STO/n-GaN heterojunction (HJ) type Schottky diode is fabricated and measured its electrical characteristics by current-voltage (I–V) and capacitance-voltage (C–V) techniques. The HJ exhibits lower reverse leakage current compared to the MS diode. Higher barrier height is achieved for the HJ (0.86 eV) than the MS diode (0.67 eV) with ideality factors of 1.86 and 1.21. This implies that the barrier height is modified by the STO insulating layer. Using Z(V,T)i - Vd and ΨS-V plots, the barrier heights of MS diode and HJ are also evaluated and compared with one another. The derived interface state density (NSS) of HJ is lower than the MS diode, demonstrating that the STO layer plays a substantial role in the reduced NSS. Results suggest that the STO thin film is a promising high-k material for the development of new electronic device applications.

Polar phonons in polycrystalline SrTiO3 thin films: The role of the substrate

Recently we have demonstrated that out-of-plane dielectric properties of polycrystalline SrTiO3 (ST) films depend on the strain/stress induced by the substrates due to thermal expansion misfit. Here, we present a complementary infra-red (IR) spectroscopy investigation of the role of Al2O3 and MgO substrates on the lattice dynamics and in-plane dielectric properties of ST thin films also prepared by sol-gel. We verify that ST films deposited on Al2O3 substrates, being under a tensile stress, present the soft mode behaviour similar to that of ST single crystals, deviating just below 150 K. In contrast, ST films deposited on MgO substrates, being under compressive stress, have higher soft mode frequency from 10 to 300 K. It is thus proved that for polycrystalline ST films the intrinsic in-plane relative permittivity may be tuned through an induced stress state.

Surface effects on the photoconducting properties of SrTiO3 thin films

We report a study of the photoconducting properties of semiconducting SrTiO3 thin films. The photoconducting spectrum shows a pronounced rise around 3.2 eV with a typical indirect gap dependence, involving a transversal optical phonon of 25 meV. While these features remain unaltered under the influence of an applied electric field in ambient conditions, in a vacuum the rest of the spectrum does not, shifting to lower energies for higher electric fields. Time dependent photoconductivity response while illumination is applied confirms the loss of efficiency of the 3.7 eV transition. At low-temperatures, the photoconducting spectrum at low-electric fields has striking similarities to the ones at room-temperature for high-electric fields. This ability to control the photoconducting response through external parameters is explained considering a model of a downward band bending generated by oxygen vacancies at the surface in concomitant with recent findings at the surface of SrTiO3.

Improving the structural quality and electrical resistance of SrTiO3 thin films on Si (001) via a two-step anneal

We report on the optimization of structural and electrical properties of SrTiO3 thin films grown on Si (001) by hybrid molecular beam epitaxy. Using a dual buffer layer template, 46-nm-thick films grown at high temperatures (850 °C) resulted in a layer-by-layer growth mode and a good crystalline quality with rocking curve full width at half maximum (FWHM) of the 002 SrTiO3 peak of nearly 0.6°, which was reduced to 0.4° by increasing the film thickness to 120 nm. A high temperature post-deposition anneal was employed to further reduce the rocking curve FWHM down to 0.2° while preserving a smooth film surface morphology. The low sheet resistance of as-grown and post-growth annealed samples was increased by five orders of magnitude exceeding 107 Ω/◻ using a lower temperature anneal in dry air. This two-step annealing method provides an easy and effective way to improve the crystalline quality of SrTiO3 thin films on Si, providing a path towards the development of electrically insulating, wafer scale virtual perovskite substrates.

Effect of Al2O3 insertion on the electrical properties of SrTiO3 thin films: A comparison between Al2O3-doped SrTiO3 and SrTiO3/Al2O3/SrTiO3 sandwich structure

The effect of Al2O3 insertion on the electrical properties of SrTiO3 films is systemically investigated in metal–insulator–metal (MIM) capacitor because SrTiO3 films with a high dielectric constant generally suffer from high leakage current problem caused by grain boundaries and a narrow band gap. To find an effective Al2O3 insertion method, Al2O3 is inserted into SrTiO3 thin films by two different ways. The first method is doping of Al2O3 in SrTiO3 thin films and the second method is sandwiching a nanometer-thick Al2O3 layer between SrTiO3 thin films. With respect to leakage blocking properties, the leakage current of Al2O3-doped SrTiO3 films is effectively reduced when the SrTiO3 film becomes amorphous by doping. In case of the SrTiO3/Al2O3/SrTiO3 structure, an Al2O3 layer with a thickness of more than 1.19nm effectively acts as a leakage current blocking layer without SrTiO3 amorphization. Moreover, the degradation of the dielectric properties of Al2O3-doped SrTiO3 films is more severe, caused by structural degradation, than of SrTiO3/Al2O3/SrTiO3 structured films. Therefore, compared with Al2O3-doped SrTiO3, a more than two times higher value (∼45) of the dielectric constant can be obtained in the SrTiO3/Al2O3/SrTiO3 structured films with a similar leakage current density of 10−7A/cm2.

High Temperature Thermoelectric Properties of Strontium Titanate Thin Films with Oxygen Vacancy and Niobium Doping

We report the evolution of high temperature thermoelectric properties of SrTiO3 thin films doped with Nb and oxygen vacancies. Structure-property relations in this important thermoelectric oxide are elucidated and the variation of transport properties with dopant concentrations is discussed. Oxygen vacancies are incorporated during growth or annealing in Ar/H2 above 800 K. An increase in lattice constant due to the inclusion of Nb and oxygen vacancies is found to result in an increase in carrier density and electrical conductivity with simultaneous decrease in carrier effective mass and Seebeck coefficient. The lattice thermal conductivity at 300 K is found to be 2.22 W m(-1) K(-1), and the estimated figure of merit is 0.29 at 1000 K.

Correlation between growth kinetics and nanoscale resistive switching properties of SrTiO3 thin films

We deliberately fabricated SrTiO3 thin films deviating from ideal stoichiometry and from two-dimensional layer-by-layer growth mode, in order to study the impact of well pronounced defect arrangements on the nanoscale electrical properties. By combining transmission electron microscopy with conductive-tip atomic force microscopy we succeeded to elucidate the microstructure of thin films grown by pulsed laser deposition under kinetically limited growth conditions and to correlate it with the local electrical properties. SrTiO3 thin films, grown in a layer-by-layer growth mode, exhibit a defect structure and conductivity pattern close to single crystals, containing irregularly distributed, resistive switching spots. In contrast to this, Ti-rich films exhibit short-range-ordered, well-conducting resistive switching units. For Ti-rich films grown in a kinetically more restricted island growth mode, we succeeded to identify defective island boundaries with the location of tip-induced resistive switching. The observed nanoscale switching behavior is consistent with a voltage driven oxygen vacancy movement that induces a local redox-based metal-to-insulator transition. Switching occurs preferentially in defect-rich regions, that exhibit a high concentration of oxygen vacancies and might act as easy-diffusion-channels.

Improved Growth Behaviors and Electrical Properties of SrTiO3 Thin Films Using the Optimized Seed Layer Deposited by Atomic Layer Deposition

SrTiO3 (STO) thin films were deposited by an atomic layer deposition at a higher temperature (370 oC) in order to improve the crystallinity and density of the film using Ti(O-iPr)2(thd)2 and Sr(thd)2 as Ti-, and Sr-precursors, respectively, and H2O as the oxidant. With the help of the crystallized seed-layer, in-situ crystallized STO film could be deposited even at the as-deposited state with the excellent electrical performances. The process conditions including the annealing temperature and the thickness of the seed-layer were crucial for achieving the high dielectric constant. From a planar capacitor structure consisting of Pt/20-nm-thick STO/Ru(bottom) Tox of 0.72 nm (dielectric constant of 108) and a low leakage current density (~ 10-7 A/cm2 at 0.8 V) were achieved. The Schottky conduction mechanism mainly contributed to the electrical conduction process in the low electric field region (< ~ 0.3 MV/cm) with a Schottky barrier height of 0.47 eV between Ru and STO. The Fowler-Nordheim tunneling was appeared as the primary conduction mechanism in the high electric field region (0.3~0.5 MV/cm).

Dielectric Properties of SrTiO3 Thin Films on SrRuO3 Seed Prepared by Plasma-enhanced Atomic Layer Deposition

The dielectric properties of SrTiO3 thin films deposited on SrRuO3 seed layer was investigated. The SrTiO3 thin films were deposited by plasma enhanced atomic layer deposition using titanium tetra-isopropoxide and bis(dipivaloylmethanato) strontium as precursors and oxygen as an oxidant. The SrRuO3 seed layer was formed through deposition of ultra-thin SrO on RuO2 substrate and a post-annealing in O2 ambient. As the results of introducing SrRuO3 seed layer, the dielectric constants of SrTiO3 thin films were increased and the thickness dependency of dielectric properties was reduced compared with the SrTiO3 thin films deposited on Ru directly or seed formed on Ru substrate.

Dielectric Properties of SrTiO3 Thin Films on SrRuO3 Seed Prepared by Plasma-enhanced Atomic Layer Deposition

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Effect of Sr–Ruthenate Seed Layer on Dielectric Properties of SrTiO3 Thin Films Prepared by Plasma-Enhanced Atomic Layer Deposition

The effect of a strontium ruthenate seed layer on the dielectric properties of thin films was investigated. The thin films were deposited by plasma-enhanced atomic layer deposition using titanium tetra-isopropoxide and bis(dipivaloylmethanato) strontium as precursors and oxygen as an oxidant. A strontium ruthenate seed layer was formed through the deposition of ultrathin SrO and a postannealing process. The thin films deposited on a seed layer, prepared by deposition of SrO and postannealing prior to deposition, showed considerably enhanced dielectric properties in comparison to films deposited on Ru directly; that was attributed to enhancement of the films’ crystallinites and a reduction of low- interfacial layers. For optimization of seed-layer formation, the dependency of the inserted SrO layer thickness on the dielectric properties of the films was investigated for a SrO thickness range of . The thick thin film on the seed layer formed by deposition of a thick SrO layer and postannealing exhibited a dielectric constant of about 75 [equivalent oxide thickness ], which meets the requirement of dynamic random access memory technology.

Enhanced electrical properties of SrTiO3 thin films grown by atomic layer deposition at high temperature for dynamic random access memory applications

Sr Ti O 3 (STO) thin films were deposited at 370°C by atomic layer deposition using H2O as the oxidant, and Ti(O–iPr)2(thd)2 and Sr(thd)2 as Ti, and Sr precursors, respectively. Denser STO films were produced at this deposition temperature. The saturated growth rate was 0.15Å∕cycle. The adoption of a thin crystallized seed layer resulted in crystallized perovskite STO films at the as-deposited state without higher temperature post-annealing. A tox of 0.72nm (dielectric constant of 108) and a low leakage current density (∼10−7A∕cm2 at 0.8V) were obtained from a planar capacitor structure consisting of Pt∕20-nm-thick STO/Ru (bottom).

Increment of dielectric properties of SrTiO3 thin films by SrO interlayer on Ru bottom electrodes

Sr Ti O 3 thin films were deposited on Ru using plasma-enhanced atomic layer deposition with and without a SrO interlayer. When the SrTiO3 films were deposited on Ru directly, the dielectric constants of the films decreased abruptly from 65 to 16 as the thickness fell below 20nm. This change was related to film crystallinity. Conversely, when a seed layer was prepared by depositing 2.7nm SrO and postannealing before SrTiO3 deposition, the crystallinity of the SrTiO3 films was enhanced and the thickness dependency of the dielectric constant was reduced. As a result, the dielectric constant of 10nm SrTiO3 films was improved from 16 to 50.

Preparation and Photoelectrochemical Properties of SrTiO&lt;sub&gt;3&lt;/sub&gt; Thin Films on Nickel Substrate by Hydrothermal Method

Well-crystallized nanopolycrystalline thin films of cubic strontium titanate, SrTiO3, have been prepared on the TiNi-deposited Ni substrate in Sr(OH)2 aqueous solutions at temperatures between 130 and 200°C under saturated vapor pressures. The microstructure and morphology of the films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The photoelectrochemical properties of SrTiO3 thin films on TiNi-deposited Ni substrate were investigated on a three-electrode assembly. The grain sizes increased from 30 nm to 300 nm and the relative crystal intensity in XRD pattern increased with the reaction temperature. Its photocurrent increasesdfirst with the grain size, then decreased. When the grain size of the film is about 200 nm, the photocurrent is highest.

Effect of oxygen vacancies on nonlinear dielectric properties of SrTiO3 thin films

Ferroelectrics (FE) are a class of nonlinear dielectrics which exhibit an electric field dependent dielectric constant. High temperature superconductors (HTSC) offer lower microwave surface resistance than conventional materials. There have been significant interest and effort in combining the low microwave losses of HTSC with the voltage control of FE dielectric constant er to implement high quality voltage tunable microwave devices, such as phase shifters, filters, delay lines, and tunable oscillators [1–4]. In such devices, it is desirable to have a large capacitance change ratio [tunability = (Cmax–Cmin)/Cmax] under a certain electric field range accompanied by a small dielectric loss. SrTiO3 (STO) thin films which posses a transition peak located at 60–70 K is one of the most interesting FE materials due to its high nonlinearity and reasonably low dielectric loss. A main focus on research for effective utilization of STO thin films is the development of materials with simultaneously optimized permittivity (absolute magnitude as well as its electric field dependence.) and low dielectric loss (tand). However, the tunability and dielectric loss in STO thin films have not been comparable to that of bulk materials. Comparing with STO single crystal, most STO thin films showed low dielectric constant, low tunability and large dielectric loss. Many reasons have been suggested to the degradation in dielectric properties of STO thin films. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), the dead layer near interface, and the local polar regions near charged defects like oxygen vacancies are expected to degrade the dielectric properties of STO thin films [5–7]. Several researchers have investigated the effect of oxygen vacancies on dielectric properties of heteroepitaxially grown STO thin films [8, 9]. It is worthwhile to note that the study of oxygen vacancies on dielectric properties of hetero-epitaxial STO thin films is complicated because of the additional effect of film stress due to film-substrate crystal lattice mismatch and difference in thermal expansion coefficients. There have been no systematic studies on the influence of oxygen vacancies on dielectric properties of STO thin films. In this study, STO thin films were homoepitaxially grown on Nb doped STO (Nb:STO) substrate. No additional stress is caused by lattice constant mismatch and thermal expansion coefficient differences between films and substrate in the homo-epitaxial STO thin films. The effect of oxygen vacancies on dielectric properties of STO thin films are presented. STO thin films were deposited on Nb:STO substrates by pulsed laser deposition (PLD). KrF excimer laser radiation (Lamda Physik Compex 201) with repetition rate of 5 Hz was focused on a rotating target set in the chamber through a quartz window. The incident angle of laser beam to the target surface was fixed to be 45 . The output laser pulse energy was kept as 150 mJ which yielded a growth rate of 0.02 nm per pulse. The laser energy density was about 2 J/cm. X. Z. Liu B. W. Tao (&) Y. R. Li The National Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science & Technology of China, Jianshe Road, Chengdu 610054, China e-mail: bwtao@uestc.edu.cn

Effects of substrate on the dielectric and tunable properties of epitaxial SrTiO3 thin films

Tunable dielectric thin films of SrTiO3 (STO) were prepared on different single-crystalline substrates, including insulating LaAlO3, conductive Nb-doped STO (NSTO), and superconducting YBa2Cu3O7−δ. Substrate effects including morphology, orientation, and lattice mismatch induced strains were investigated. We found that a change of substrate used for STO thin films can strongly affect the dielectric properties of STO thin films in terms of dielectric constant, loss tangent, and tunability. Effects of substrate properties on the temperature dependence of dielectric constant and loss tangent were investigated. At low temperatures, STO thin films under minimal strain yield high dielectric constant and low loss tangent while the thin films under either tensile or compressive strain exhibit the reduced dielectric constant and high loss. The tunability of about 77% in STO∕NSTO system, close to the value found in STO single crystal, was observed at 10K. Physical origin of observed phenomena was discussed.

Optical properties of SrTiO3 thin films deposited by radio-frequency magnetron sputtering at various substrate temperatures

SrTiO 3 thin films were deposited on vitreous silica substrates at various substrate temperatures (300–700°C) by rf magnetron sputtering technique. The transition from amorphous phase to polycrystalline phase for the films occurred at the substrate temperatures of 300–400°C. Their optical properties were investigated by transmittance measurements. The fitting method was used to calculate the refractive index and the film thickness from the transparent region of the transmittance spectra. The refractive index increased and the film thickness decreased with the substrate temperatures increasing. The dispersion of the refractive index was studied by considering a single electronic oscillator model. The band gaps of the films were estimated from Tauc’s law and showed a decreasing tendency to that of the bulk SrTiO3 with the substrate temperatures increasing. These results provide some useful references for the potential application of SrTiO3 films in integrated optics devices.