Dielectric Properties of Homoepitaxial SrTiO3SrTiO3SrTiO3 Thin Films Grown in the Step-Flow Mode

We have deposited SrTiO₃ thin films on Nb-doped SrTiO₃ substrates by pulsed laser deposition at temperatures of up to 1400°C. Reflection high energy electron diffraction was used to monitor the film growth mode at various temperatures and it was shown that growth proceeded in the step-flow mode at above 900°C. Capacitors were formed by evaporating platinum pads on the film surface and gold pads on the substrate. Films grown in the step-flow mode showed consistently higher dielectric constants below 200 K than films grown in the layer-by-layer mode. Films with the highest dielectric constant () were obtained using a stoichiometric ablation target at an oxygen pressure of 10SrTiO₃ ^–6SrTiO₃ Torr.     

Effects of oxidants on the deposition and dielectric properties of the SrTiO3 thin films prepared by liquid source metal-organic chemical vapor deposition (MOCVD)

Abstract

SrTiO₃ thin films are deposited by a liquid source metal-organic chemical vapor deposition (MOCVD). The effects of oxidants on the deposition characteristics and dielectric properties of the films are mainly tested. O₂, N₂O and O₂ + N₂O gases are used as the oxidants and the films with Ti-rich and Sr-rich compositions are obtained when O₂ and N₂O is used, respectively. Deposition of thin initial layer under O₂ atmosphere is very effective to obtain large dielectric constant of the SrTiO₃ thin film when the main layer is deposited under O₂ + N₂O atmosphere. The dielectric constants of 40 nm thick SrTiO₃ films with thin O₂, N₂O initial layers and without the initial layers are 235, 145 and 210, respectively.     

Strain tensor effects on SrTiO3 incipient ferroelectric phase transition

Structural distortion of ferroelectric thin films caused by film strain has a strong impact on the microwave dielectric properties. SrTiO₃ thin films epitaxially grown on (110) DyScO₃ substrates using molecular beam epitaxy (MBE) are extremely strained (i.e., ～1% in-plane tensional strain) from 3.905 Å of bulk SrTiO₃. The room temperature dielectric constant and its tuning of the films are observed to be 6000 and 75% with an electric field of 1 V/μm, respectively. The control of strain in SrTiO₃ provides a basis for room temperature tunable microwave applications by elevating its phase transition peak to room temperature. Also, a significant in-plane anisotropy in dielectric constant and tuning was observed in these SrTiO₃ films. The observed in-plane anisotropic dielectric properties have been interpreted based on the phenomenological thermodynamics of film strain.     

IMPEDANCE SPECTROSCOPY STUDY ON TRAP-CONTROLLED SPACE-CHARGE-LIMITED CONDUCTION OF Cr-DOPED SrTiO3 THIN FILMS

ABSTRACT

The electrical conduction properties of 0.2% Cr-doped SrTiO₃ thin film in metal–insulator–metal (MIM) structure was investigated by using dc and impedance spectroscopic measurements at room temperature. The dc measurement shows that the electronic conduction is the trap-controlled space-charge-limited current (SCLC) conduction. The impedance study shows that the electrical conduction is bulk-limited conduction. The behavior of the bulk resistance obtained in the equivalent circuit model from the impedance analysis is also consistent with the trap-controlled space-charge-limited conduction with exponential trap distribution.     

High Temperature Schottky Barrier on n-Type SrTiO3 and Its Sensitivity to Ambient Gases

Metal or oxide electrodes (Pt, Au, Ag, (La, Sr)CoO₃) were deposited on single crystals of 0.02 mol% Nb doped SrTiO₃ by pulsed laser deposition. Current-voltage and capacitance-voltage responses were measured using three-terminal electrode configuration. Under high oxygen partial pressures, clear rectification behaviors were observed. Diffusion model well explained the current vs. voltage relationship with ideality factors close to unity. The barrier height varied reversibly with oxygen partial pressure, and was almost independent of the electrode materials, which suggested that the Fermi level at the interface was pinned by the surface states. The origin of the surface states was discussed in terms of oxygen adsorption or oxidative formation of metal vacancies around the surface. Chemical interaction between the surface and oxygen and resulting cation rearrangement was concluded to play an important role from the long stabilization time on oxygen partial pressure change. The water vapor pressure dependence of the barrier height was also explained by competitive adsorption of oxygen and water.     

Integration of MgO on Si(001) Using SrO and SrTiO3 Buffer Layers by Molecular Beam Epitaxy

Epitaxial MgO was deposited onto Si(001) substrates by molecular beam epitaxy using elemental metallic sources and molecular oxygen at temperatures from 150 to 400C. To facilitate epitaxy through misfit strain relaxation, epitaxial MgO layers were grown on SrO and SrTiO₃ buffer layers deposited on Si(001) substrates. The structure of the epitaxial layers was determined by X-ray diffraction, reflection high-energy electron diffraction and transmission electron microscopy. The observed orientation for the MgO/SrO/Si multilayer is cube-on-cube. The X-ray rocking curve full width half maximum of the MgO on SrO buffer layers was 2.2. SrTiO₃ buffer layers grown by recrystallization were epitaxial and exhibited improved morphology relative to those grown at a fixed growth temperature. X-ray analysis of a 5.2 nm recrystallized SrTiO₃ film indicates a fully relaxed and phase pure film. The observed orientation of MgO using SrTiO₃ buffer layers is MgO[100]SrTiO₃[100]Si[110].     

Mixed Electronic-Ionic Conduction in Ru-Doped SrTiO3 at High Temperature

The electronic and ionic conduction behavior of Ru-doped SrTiO₃ at high temperature was investigated. The conductivity increased significantly with increasing Ru content. SrTi_0.80Ru_0.20O_3– exhibits fairly high conductivities, e.g., 3 S cm^–1 at 1000°C, and 2 S cm^–1 at 600°C. The conductivity had only a slight dependence on the partial pressure of oxygen over a wide range and was largely attributed to n-type electronic conduction. Ru-doped SrTiO₃ showed mixed oxide-ionic and electronic conduction under reducing atmospheres. The mechanism of the electronic and ionic conduction is discussed.     

Effects of molten salt synthesis (MSS) parameters on the morphology of Sr3Ti2O7 and SrTiO3 seed crystals

A two-step molten salt synthesis process was utilized to fabricate Sr₃Ti₂O₇ and SrTiO_3. High aspect ratio SrTiO₃ seed crystals were developed by optimizing processing conditions such as temperature, salt-to-oxide ratio, and flux type in a systematic fashion. Sr₃Ti₂O₇ seeds were synthesized at temperatures ranging from 1050–1350°C, using salt-to-oxide ratios of 3:1, 1:1, and 1:3, and various salt types, including NaCl, KCl, and a 1:1 combination of NaCl and KCl. Sr₃Ti₂O₇ seeds synthesized at 1250°C with a 1:1 salt-to-oxide ratio in 100% NaCl salt resulted in a majority of higher aspect ratio platelets and elongated platelets as opposed to lower aspect ratio cubic-like and tetragonal-like morphologies. The seeds were 10–40 μm in length with aspect ratios of highly elongated platelets as high as 25:1. A second MSS step was used to synthesize SrTiO₃ seeds of the proper composition by TiO₂ addition to the Sr₃Ti₂O₇ seeds and heat treatment at 1100°C. These studies showed that highly anisotropic SrTiO₃ seeds could be produced at 1250°C using a 1:1 salt-to-oxide ratio in 100% NaCl flux. XRD studies of the resulting SrTiO₃ seeds revealed that the increase in aspect ratio for these particular seeds also resulted in the enhancement of (200) peaks, which are of major interest for texturing of PMN-PT.     

High temperature conductivity behavior of doped SrTiO3 thin films

Abstract

In order to investigate the dominant charge transport mechanisms of doped SrTiO₃ thin films, high temperature measurements were performed under varying oxygen partial pressures. To meet specific demands of SrTiO₃ thin films, a common 4-point measuring setup was improved profoundly by full triaxial shielding and the use of a solid state oxygen pump (made of YSZ). This allowed a precise analysis in the temperature range from 700°C to 1000°C and at oxygen partial pressures (pO₂) between 10^?20 bar and 1 bar. The conduction behavior of (doped) SrTiO₃ thin films, as a function of pO₂, revealed characteristics that substantially differ from those of bulk ceramics and cannot be explained by point defect chemistry. Additionally, segregation effects have been observed which lead to a restructuring of the film's morphology to a significant extent.     

RHEED Intensity Oscillations for the Stoichiometric Growth of SrTiO3 Thin Films by Reactive Molecular Beam Epitaxy

The growth of high quality multicomponent oxide thin films by reactive molecular beam epitaxy (MBE) requires precise composition control. We report the use of in situ reflection high-energy electron diffraction (RHEED) for the stoichiometric deposition of SrTiO₃ (1 0 0) from independent strontium and titanium sources. By monitoring changes in the RHEED intensity oscillations as monolayer doses of strontium and titanium are sequentially deposited, the Sr:Ti ratio can be adjusted to within 1% of stoichiometry. Furthermore, the presence of a beat frequency in the intensity oscillation envelope allows the adjustment of the strontium and titanium fluxes so that a full monolayer of coverage is obtained with each shuttered dose of strontium or titanium. RHEED oscillations have also been employed to determine the doping concentration in barium- and lanthanum-doped SrTiO₃ films.     

Improved ferroelectricity of strained SrTiO3 thin films on sapphire

Magnetron sputtered and laser deposited SrTiO₃ thin films are deposited on CeO₂ buffered sapphire substrates. Their structural properties are investigated and correlated to the dielectric properties of the SrTiO₃ films. It is shown, that the biaxial compressive strain imposed by the substrate on the ferroelectric films leads to a considerable increase of the permittivity and tunability of SrTiO₃ thin films in technically relevant temperature regimes. Generally, the permittivity and tunability decreases with increasing strain. However, the ferroelectric phase transition of the SrTiO₃ films is shifted to higher temperatures compared to that of single crystalline SrTiO₃. As a consequence, the permittivity of the films is larger than that of undistorted SrTiO₃ single crystals for small strain (Δa/a < 0.005) and temperatures above the Curie temperature. Furthermore, a linear dependence of the loss tangent and the tunability on the permittivity is observed, which indicates, that all three properties are affected by the same mechanism that itself is affected by the lattice strain.     

Preparation of Textured Growth Pb(Zr,Ti)O3 Thin Films on Si Substrate Using SrTiO3 as Buffer Layers

PZT thin films are deposited on SiO₂/Si substrate by metallo-organic decomposition (MOD) process, using SrTiO₃ (STO) as buffer layer for textured growth. The STO layers deposited on SiO₂/Si substrate by pulsed laser deposition process show (100)/(200) preferred orientation, whereas the STO buffer layer deposited on silica substrate using spin-coating technique show random orientation behavior. The use of STO as buffer layers enhanced the crystallization and the preferred orientations of the PZT films. The PZT on STO buffered SiO₂/Si substrates thus obtained possess high refractive index, (n)_PZT/STO = 2.1159, and are of good enough quality for optical waveguide applications.     

Electron emission of silicon field emitter arrays coated with N-doped SrTiO3 film

Nitrogen doped SrTiO₃ (STO) thin films have been fabricated on Si field emitter arrays (FEAs) using reactive RF magnetron sputtering in Ar-N₂ mixture ambient for electron emission applications. The nitrogen incorporation in STO films was revealed both in Fourier transform infrared (FTIR) spectroscopy and in Auger electron spectroscopy (AES). Low dose incorporation of nitrogen in STO films shows enhanced crystallinity, whereas the overdosed films show the degraded perovskite structure. The results demonstrate that the threshold emission field is lowered tremendously from 36.24 V/μm for uncoated FEAs to 17.37 V/μm for 30-nm-thick STO coated FEAs deposited in 50% N₂ ambient. The enhanced electron emission characteristics are highly correlated with the nitrogen incorporation in STO and film thickness. The substitution of nitrogen for oxygen may result in the band-gap narrowing in STO with enhanced electron emission. The thickness dependence might be related to the formation of space-charge-induced band-bending interlayer at STO/Si interface.     

Growth and characterization of epitaxial SrBi2Ta2O9 films on (110) SrTiO3 substrates

Abstract

SrBi₂Ta₂O₉ (SBT) is an attractive material for nonvolatile ferroelectric memory applications. In this paper we report on the deposition of highly epitaxial and smooth SrBi₂Ta₂O₉ films on (110) SrTiO₃substrates. The films were grown by pulsed laser deposition at temperatures ranging from 600 to 800°C and at various laser fluences from a Bi-excess SBT target. The background oxygen pressure was maintained at 28 Pa during the film deposition. Structural characterization of the films was performed by x-ray diffraction. Atomic force microscopy was used to investigate morphology and growth of the films. The films grew with preferred (115) or (116) orientation. The roughness was of the order of unit cell height. The films display a growth pattern resulting in corrugated film morphology.     

Control of the Grain Boundary Electrical Features of the Semiconducting SrTiO3 Ceramics Synthesized from Surface-Coated Powders

Polycrystalline SrTi_0.99Nb_0.01O₃ (STNO) ceramics were synthesized by hot-press sintering Na-coated semiconducting STNO powders. The chemical and electrical characteristics of the grain boundary of the ceramics were investigated, and their relations were discussed in terms of process parameters. The diffusion coefficients as well as the activation energy of the Na ions near the grain boundary were obtained at particular heat-treatment conditions; it was demonstrated that these values can be used to design specific electrical features of the grain boundary. A systematic variation in the electrical characteristics of the grain boundary with process parameters was observed; it indicates that this synthesis method can be used for fine control over the chemical and electrical properties of the semiconducting ceramics.     

Barrier mechanism of Pt/Ta and Pt/Ti layers for SrTiO3 thin film capacitors on Si

Abstract

The barrier effect of Pt/Ta and Pt/Ti has been investigated, when used as bottom electrodes for SrTiO₃ thin film capacitors on Si. The Pt/Ta/Si stacks were more stable than the Pt/Ti/Si, both in vacuum and in oxygen annealing. Though the Pt/Ta bilayer was suitable for the SrTiO₃ deposition at 400[ddot]C, its resistivity became slightly higher after the deposition at 600[ddot]C, due to Ta layer oxidation during the SrTiO₃ deposition. This would result in a contact resistance problem for high density dynamic random access memory application.     

The Applicability of Sr-deficient <Emphasis Type="Italic">n</Emphasis>-type SrTiO<Subscript>3</Subscript> for SOFC Anodes

Here we discuss the effect of preparation conditions on structural stability and electrical properties of Sr-deficient n-type SrTiO₃. In particular, an explanation of a wide scatter of conductivity values in Y- and Nb-doped SrTiO₃. reported in the literature is proposed, based on the existing defect chemistry model of n-doped SrTiO₃. It was confirmed that when sintered in air, Sr-deficient SrTiO₃ doped with Nb and/or Y, remains single phase until the solubility limit (e.g., 30% for Nb or 4% for Y). However, when sintered at low po₂, the material transforms from a vacancy compensated to an electronically compensated compound with a strontium deficient second phase. Measured at 800°C in low po₂, the maximum conductivity of these multi-phase compounds was 340 S/cm and 100 S/cm for the Nb-doped and Y-doped sample, respectively. However, the conductivity dropped dramatically to less than 10 S/cm when samples of the same compositions were sintered in air, again measured in reducing atmosphere.     

Growth of BaTiO3/SrTiO3 superlattices by injection MOCVD

Abstract

BaTiO₃ (BTO) and SrTiO₃ (STO) and Ba_xSr_1-xTiO₃ (x=0–1) (BST) thin films have been epitaxially grown on LaAlO₃ and SrTiO₃:Nb at a substrate temperature of 800°C using a new liquid source delivery technique called injection MOCVD. A X-ray study evidenced FWHMs of 0.16° and 0.45° for SrTiO₃ and BaTiO₃ respectively.

In a next step the feasibility of BaTiO₃/SrTiO₃ superlattices was studied. The multilayers obtained were epitaxially grown on LaAlO₃ as well as on SrTiO₃:Nb. The structural properties were studied using X-ray diffraction as well as XPS, proving the low interface roughness of 1nm. The XPS study also confirmed the absence of carbon contamination in the film.     

Single-Crystalline KNbO3Thin Film Grown by Liquid Phase Epitaxy

The effects of melt composition on the transmittance of single-crystalline KNbO₃ films grown on SrTiO₃ substrates by liquid phase epitaxy (LPE) technique were investigated. The growth morphology and transmittance of the KNbO₃ films strongly depended on the extra K₂CO₃ self-flux content in K₂CO₃ (K₂O)-Nb₂O₅ system. With increasing K₂CO₃ content close to the eutectic composition (66 mol%) in the phase diagram, the transmittance of KNbO₃ film/SrTiO₃ substrate increased up to 70% in the visible wavelength range. Atomic force microscopy (AFM) clearly showed that the parallel flat steps formed toward the dipping direction and their interval decreased with increasing the K₂CO₃ content.     

Initial Stage Nucleation and Growth of Epitaxial SrRuO3 Thin Films on (0 0 1) SrTiO3 Substrates

We have investigated the initial stage nucleation and growth of epitaxial SrRuO₃ thin films grown on both polished (as received) and buffered HF (BHF) etched single crystal (0 0 1) SrTiO₃ substrates by 90° off-axis sputtering. Atomic force microscopy indicates a dramatic difference in the initial stage growth of SrRuO₃ films on the two substrates. The films on polished substrates nucleate as rectangular islands, which merge together to form a continuous film as the thickness increases. Complete coverage is obtained at film thickness of 20 nm. In contrast, the film on BHF etched substrate nucleates as finger-shaped islands at the step sites and continues to grow by adatom diffusion to the step sites. Complete coverage is obtained at a film thickness of 10 nm. This difference in the initial stage nucleation is attributed to the difference in surface morphology and termination layer of the two substrates. However, the thicker films on both as received and BHF etched substrates have identical surface morphologies. Such studies on the initial stage nucleation will also help us understanding the growth kinetics and development of surface morphology and interfaces in multilayered perovskite thin film heterostructures and devices.