EFFECT OF ANNEALING ON THE ELECTRICAL PROPERTIES OF SrTiO3 THIN FILMS PRODUCED BY ION BEAM SPUTTERING

ABSTRACT

Thin film capacitors with SrTiO₃ (STO) as dielectric and Pt as electrode material have been prepared by ion beam sputtering. The as-deposited film is amorphous and exhibits a crystallization temperature around 321°C as proved by X-ray diffraction. The effect of post annealing on the crystalline quality of the films was systematically studied by x-ray diffraction and Atomic Force microscopy (AFM). The temperature and frequency dependent dielectric properties were measured from 30°C to 200°C and 0.01 Hz to 10⁵ Hz, respectively. The influence of the microstructure of SrTiO₃ thin films on their electrical properties was investigated through an extensive characterization. The electrical properties of SrTiO₃ films appear to be strongly depending on the annealing temperatures. The capacitance voltage (C-V) characteristics reveal an improvement of capacitance density with increasing the annealing temperature.     

SrTiO3 thin films by mocvd for 1 gbit DRAM application

Abstract

Three important aspects of the preparation of SrTiO₃ thin films by MOCVD are discussed in detail in view of the application of these films as the capacitor dielectric of Gbit-scale DRAMs: CVD reactions in the Sr(DPM)₂-Ti(i-OC₃H₇)₄-O₂ system, step coverage and relations between microstructure and electrical properties. The effect of the substrate temperature on the Sr and Ti deposition rates was first investigated for thermal and ECR CVD SrTiO₃ films. SrO and TiO₂ deposition by thermal CVD above 550°C were found to be controlled by the surface reaction and gas transport, respectively, whereas both SrO and TiO₂ deposition are controlled by gas transport for ECR CVD at 450 to 600°C. The influence of the Sr and Ti deposition regimes on the step coverage of SrO, TiO₂ and SrTiO₃ were then assessed. SrO films prepared by thermal CVD at 600°C exhibited the best step coverage, indicating that a relation exists between reaction controlled deposition and good step coverage. The effect of the film composition and film thickness on the microstructure of SrTiO₃ thin films were finally investigated and correlations were made to other analyzed physical and electrical properties. Polycrystalline perovskite phase SrTiO₃ films were obtained for a composition 0.7 ≤ Sr/Ti ≤ 1.2. The best crystallinity, maximum permittivity and maximum refractive index were obtained for Sr/Ti = 0.95. Titanium rich films are thought to be composed of a mixture of a titanium rich amorphous phase and crystalline SrTiO₃, and strontium rich films are believed top correspond to a (SrTiO₃)_m (SrO)_n structure. The dielectric constant slowly decreased as the film thickness was reduced. The sharp decrease observed near 400–500 Å could be due to the existence of some perturbed layer at the interface with one or both of the electrodes     

Preparation of ferroelectric thin films for Si-based devices

Abstract

Lead titanate (PbTiO₃) thin films have been prepared on titanium dioxide coated silicon wafers by chemical vapor deposition (CVD). The pure PbTiO₃ thin films were deposited by controlling the experimental conditions. The gas phase reaction of TiO₂ occurred by exceeding the critical value of titanium input fraction at constant oxygen partial pressure. Strontium titanate (SrTiO₃) thin films have been prepared on p-type silicon wafers by radio frequency (RF) magnetron sputtering. The SrTiO₃ thin film was polycrystalline and the Sr/Ti ratio of this film was 0.91. The SrTiO₃ thin films contain three regions, an external surface layer, a main layer and an interfacial layer. The stoichiometric SrTiO₃ thin film was obtained by using the SrO excess target. The SrTiO₃ film annealed at 600[ddot]C has an ideal capacitance-voltage (C-V) curve and maximum effective dielectric constant.     

Structure and properties of sol-gel processed (Pb,La)TiO3 ferroelectric thin films

Abstract

The influence of heat treatment and substrate materials on PLT thin films by sol-gel processing has been researched. Epitaxial growth PLT thin films with perovskite-type structure on sapphire, SrTiO₃ and MgO single crystal substrates have been prepared. The epitaxial relations are (100)PLT28/(100)SrTiO₃, (111)PLT28//(0001)sapphire and (100)PLT14//(100)MgO. The PLT polycrystal thin films with perovskite-type structure on Si single crystal and quartz glass substrates have been prepared. The remanent polarization Pr and the coercive field Ec of PLT15 ceramic thin films are 7.7 μc/cm² and 34 kv/cm at 4 KHz respectively. The optical transmittance of PLT28 ceramic thin films within the wavelength range of λ = 500–1000 nm is approximately 80%.     

Effect of seed layer on electrical properties of Pb(Zr,Ti)O3 films grown by metalorganic chemical vapor deposition

Abstract

Pb(Zr_xTi_1?x)O₃ (PZT) ferroelectric thin films were prepared by metalorganic chemical vapor deposition (MOCVD) on Pt/Ti/SiO₂/Si substrate. Very thin PZT films, which were deposited at a lower temperature and post-annealed at higher temperature for crystallization, were used as a seed layer. PZT films grown on the seed layer exhibited superior characteristics in the crystalline structure and electrical properties, compared to those deposited without seed layer. Depending on the deposition conditions of PZT seed layer, a wide variation of surface morphology and stoichiometry was found between samples, whereas chemical composition was found to be very similar.     

Influence of the growth temperature on the epitaxial quality of PbTiO3 films deposited in-situ by sputtering

Abstract

Thin films of lead titanate were prepared in-situ on SrTiO₃ substrates using radio-frequency magnetron sputter deposition. The epitaxial quality of the films has been studied as a function of the substrate temperature. Stoichiometric films have been obtained in the temperature range [550°C, 600°C]. The films deposited in the equilibrium zone have a high degree of c-axis oriented epitaxial crystalline structure as shown by X-ray diffraction in the 2θ/θ, θ, and ? scans configuration as well as by electron channeling pattern. The optimum conditions for growing epitaxial PbTiO₃ layers were determined. The crystallinity of films deposited at 550°C is suprior to those deposited at 600°C. The PbTiO₃ films grown at 550°C have a rocking curve full width at half maximum (FWHM) of 0.2°; Normaski optical and atomic force microscopy show that the surface is apparently free of grain boundaries and very smooth. The refractive index of these films has been evaluated from transmission spectra; it is very close to the bulk material value.     

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.     

Epitaxial structure SrBi2Ta2O9<116> /SrTiO3<011> /Ce0.12Zr0.88O2<001> /Si<001> for ferroelectric-gate FET memory

Abstract

Epitaxial thin film growth of SrBi₂Ta₂O₉/SrTiO₃/Ce_0.12Zr_0.88O₂ on Si was studied, and this epitaxial layer structure was applied to fabrication of ferroelectric-gate field effect transistors (FETs). The films were prepared by a pulsed laser deposition technique and epitaxial growth was identified by x-ray diffraction. The devices exhibited excellent electrical performances: Capacitance-voltage characteristic of a metal-ferroelectric-insulator-semiconductor (MFIS) diode showed a retention longer than 10 days and I_d-V_g characteristic of an MFIS-FET showed 1 day retention. It is proved that the crystalline quality of ferroelectric thin films is of great importance to develop integrated devices with high performance.     

Structural and dielectric properties of BaTiO3SrTiO3-multilayers deposited by PLD

Abstract

Dielectric superlattices are characterized by the structural variability of the film periodicity additional to the stochiometry. They have interesting dielectric properties different from the solid solutions of the same overall composition.

In this paper the structural and dielectric properties of BaTiO₃/SrTiO₃-superlattices prepared by pulsed laser deposition on single crystalline substrates are characterized in comparison to a (Ba_0.5Sr_0.5)TiO₃ solid solution. The films were characterized using X-ray diffraction, SEM and dielectric measurements.

The BaTiO₃/SrTiO₃-superlattices have a different frequency and a weaker bias and temperature dependence of the dielectric properties than the solid solution. The mechanical stresses caused by the lattice mismatch between the different layers could be one reason for this behaviour.     

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.     

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.     

Shift of Phase Transition Temperature in Strontium Titanate Thin Films

The results of structural and electrical characterizations of SrTiO₃ thin films deposited onto MgO and LaAlO₃ substrates by pulsed laser deposition technique are presented. The influence of substrate and annealing procedure on the crystalline structure and dielectric properties of these ferroelectric thin films are investigated. The obtained experimental data are analyzed in terms of the Landau theory taking into account the room-temperature lattice mismatch of the ferroelectric and substrates as well as the difference in their thermal expansion. It is shown that the behavior of the SrTiO₃ thin films could not be attributed to the effect of the film/substrate mechanical coupling. As a possible nature of the observed behavior one can consider the non-stoichiometry of the film composition caused by the chemical contact of the film with the substrate and by the annealing.     

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.     

Ferroelectric films at high microwave power of ka-band frequency

Abstract

This paper presents the experimental and modeling results on microwave (～37 GHz) investigations of SrTiO₃ and (Ba,Sr)TiO₃ ferroelectric films at high levels of microwave power. The ferroelectric film planar varactors were incorporated into the fin-line. The threshold microwave power leading to the appreciable variation in the microwave response of the tunable structure was determined. Thermal conditions of the varactors under test were analyzed. Thermal time constant and overheating of the ferroelectric films were estimated. The dependence of the varactor overheating on geometry was studied. Design optimization possibilities for the varactors in high microwave power applications are discussed.     

MOCVD PBZRxti1−xo3 thin films on platinized silicon wafers and srtio3 crystals

Abstract

PbZr_xTi_1-xO₃ (PZT) thin films were grown on 6” platinized silicon substrates (Pt / Si) and SrTiO₃ (STO) crystals by Metal-Organic Chemical Vapor Deposition (MOCVD) as a function of the Zr / (Zr+Ti) ratio in the gas phase. Morphology, optical properties, and crystal structure were investigated by scanning electron microscopy, atomic force microscopy, ellipsometry, and X-ray diffraction. The morphology, structure, and optical properties of the polycrystalline and epitaxial films were compared. The determination of the refractive index by ellipsometry (from 550 nm to 2000 nm) was not sensible for the films grown on (Pt / Si) but successful for the films grown on SrTiO₃.     

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.     

Electrical characteristics of high dielectric constant BCTZ thin films

Abstract

High dielectric constant Ba_0.96Ca_0.04Ti_0.84Zr_0.16O₃ (BCTZ) thin films were deposited on Pt/Ti/SiO₂/Si substrates by spin on metal-organic decomposition (MOD) technique. Undoped and 0.4% Mg-doped BCTZ thin films were annealed in the temperature range from 600 to 900 °C for 1 hour in oxygen environment. The crystal structure of BCTZ thin films was analyzed by X-ray diffraction. The electrical properties of BCTZ thin films were investigated by capacitance—voltage (C—V) characteristics. Also, the electrical properties of these films were compared in conjunction with 0.4% Mg doping effect of BCTZ thin films for possible high dielectric constant material applications.     

Aspects of the Electrocaloric Behavior of Ferroelectric Thin Films: A Review of the Predictions of the Landau-Ginzburg Theory

A thermodynamic model is used to investigate the electrocaloric response of thin film perovskite ferroelectrics under the influence of differing electrical, thermal and mechanical boundary conditions including bias and driving field, temperature, lateral clamping and misfit strain. A comparison of the electrothermal properties of ferroelectric solid solutions comprised of BaTiO₃, PbTiO₃ and/or SrTiO₃ illustrates the influence of composition on electrocaloric properties. Computations made for (001) textured polycrystalline BaTiO₃ films on IC-friendly substrates quantify the effects of thermal stresses. The combined results provide insights concerning how the deposition temperature, substrate material and composition can be adjusted to obtain desired electrocaloric response.     

Electrical characterizations of MgTiO3 thin films grown on Si

Abstract

We have analyzed MgTiO₃ thin films grown on the Si substrate with/without SiO₂ using pulsed laser deposition (PLD). We find that MgTiO₃ thin films start to crystallize at 600°C, causing electrical instabilities in the MIS capacitors above this temperature. Detailed analysis by XRD technique reveals that structural differences of MgTiO₃ thin films were not obvious below 600°C, whereas the electrical characteristics changes as a function of deposition temperature and the presence of thermally grown SiO₂. We observe that the decrease of deposition temperature results in the increase of leakage current and anomalous positive charge (APC) density. These drawbacks were effectively suppressed by growing 100A SiO₂ layer on the Si substrate prior to the deposition of MgTiO₃ thin films.     

Performance of ferroelectric based tunable capacitors as a function of electrode geometry

Abstract

High quality Ba_0.4Sr_0.6TiO₃ and SrTiO₃ films were grown by Pulsed Laser Deposition on single crystal LaAlO₃ and MgO substrates. Temperature dependencies of the dielectric constant and loss tangent of the films were studied using planar interdigitated test capacitors with various electrode geometries. The temperature where the maximum or “peak” capacitance occurs (T_p and referred to as the “peak temperature”) is found to depend on the electrode geometry in these films. As much as 40 K difference in T_p was observed between the STO test capacitors with 5 μm and 40 μm gaps between electrodes. Interface built-in electric field and metal-ferroelectric thermal mismatch strain are considered as possible explanation of the effect of electrode geometry on peak temperature of the capacitors.