BSTO Thin Films Research Articles

Space Resolved Optical Emission Spectroscopy During Deposition of Ba<sub>x</sub>Sr<sub>1-x</sub>TiO<sub>3</sub> Thin Films by Double Hollow Cathode Plasma Jet System

Pulse modulated double hollow cathode RF plasma jet system with two separate independent nozzles made of BaTiO3 (BTO) and SrTiO3 (STO) was used for deposition of BSTO thin films on Si and on multi-layer Si/SiO2/TiO2/Pt substrates. Dielectric properties of BSTO layers strongly depend on ratio composition expressed by parameter x = Ba/(Ba+Sr) and on accuracy in presence of other elements. Space resolved optical emission spectroscopy (OES) was used mainly for monitoring of concentration of particles sputtered from the hollow cathode and for feedback correction of power supplied in both nozzles because applied power was responsible for sputtering speed of Ba and Sr particles. Main attention was focused on relation between ratio of spectral intensity of Ba, Ba+, Sr and Sr+ lines close to substrate and ratio of Ba and Sr concentration in the deposited film. 2D map of emission lines intensity distribution for Ba, Ba+, Sr, Sr+, Ti, Ar, and Ar+ for double hollow cathode plasma jet system with BTO and STO nozzles was created. OES was also used for observing of excess of Ti particles in final layer with negative effect on layer properties and for measurement of rotational temperature of OH radicals. Preliminary results of all these optical measurements are published in this paper. Deposited thin films were analyzed by X-ray diffraction, which confirmed presence of BSTO and STO perovskite phase in the films, by atomic force microscopy (AFM), by electron microprobe and by micro-Raman scattering measurement.

A DIELECTRIC PROPERTY ANALYSIS OF FERROELECTRIC THIN FILM USING TERAHERTZ TIME-DOMAIN SPECTROSCOPY

ABSTRACT This paper presents complex dielectric properties of ferroelectric BSTO thin film, deposited on MgO substrate by pulsed laser deposition, in the frequency range of 0.5 ∼ 3.0 THz using terahertz time-domain spectroscopy (THz-TDS). In order to extract complex dielectric properties of the thin film, multiple reflections within the thin film were considered and an error function between the calculated and measured data was introduced. The real part of the dielectric constant of the BSTO thin film was less than 170 and the dielectric loss tangent of the BSTO thin film varied between 0.9 and 3.0. All values of an error function were less than 7 × 10− 4 in the measured frequency ranges. The analysis method using THz-TDS enables us to find out complex dielectric properties of ferroelectric thin film exactly.

Effects of deposition temperature on the crystallinity of Ba 0.5Sr 0.5TiO 3 epitaxial thin films integrated on Si substrates by pulsed laser deposition method

Epitaxial Ba 0.5Sr 0.5TiO 3 (BSTO) thin films were grown on TiN buffered Si (0 0 1) substrates by PLD method and the effects of deposition temperature on their crystallinity and microstructure were studied. BSTO thin films were prepared with substrate temperature ranging from 350 to 650 °C. The BSTO films grown at below 400 °C showed amorphous phase and the film grown at 450 °C showed mixed phase of crystalline and amorphous, where crystalline phase was observed only at the top surface portion of the film. The BSTO films with fully crystalline phase were obtained in the samples deposited at above 500 °C. The (0 0 l) preferred orientation and the crystallinity of the BSTO films were improved with increasing the temperature. The dielectric constant, measured at 100 kHz and at room temperature, of the BSTO film grown at 650 °C was measured to be as high as 1129.

Thickness dependent ferroelectric properties of BSTO thin films deposited by RF magnetron sputtering

The thickness dependence of ferroelectric permittivity of (Ba, Sr)TiO3 has been investigated. The BST films could be obtained to have a simple cubic perovskite structure, space group Pm3m, and practically c-axis epitaxial structure deposited at 800∘C. Through post-annealing process, we have improved the dielectric properties; dielectric permittivity, dielectric loss, and tunability. The change in dielectric properties before and after annealing is attributed to the change in film strain and the contraction in film lattice. As the thickness of BST films increases from 55 nm to 350 nm, the dielectric constant of BST films increases from about 100 to above 670 due to the reduction of interfacial dead layers with low dielectric constant between films and top electrodes. The dielectric loss of BST thin films decreased as the thickness increases. The existence of interfacial dead layers in a thinner film had a larger effect on the effective dielectric constant than tensile strain between the BST films and MgO substrate.

Investigation of YBa2Cu3O7−δ growth mode effect on the dielectric properties of BaxSr1−xTiO3 for BaxSr1−xTiO3/YBa2Cu3O7−δ bilayered thin films epitaxially grown on vicinal LaAlO3 substrates

Epitaxial bilayered thin films of BaxSr1−xTiO3 (BSTO, x = 0.1 and 0.5)/YBa2Cu3O7−δ (YBCO), with different YBCO film thicknesses, have been grown in situ by pulsed-laser deposition on 1.2° vicinal LaAlO3 substrates. The critical thickness for the transformation of the growth mode in YBCO between two-dimensional (2D) step flow and three-dimensional island is experimentally confirmed to be around 180 nm. As the YBCO-film thickness increases from 145 to 250 nm, the tremendous degradation of the film quality, due to the growth-mode change in YBCO, leads to drastic deterioration in the dielectric properties of BSTO thin films, with severe decrease in the figure of merit factor from 148 to 10 in Ba0.1Sr0.9TiO3/YBCO (at 77 K and 100 kHz) and from 56 to 6 in Ba0.5Sr0.5TiO3/YBCO (at 300 K and 100 kHz). As a result, the dielectric properties of BSTO thin films could be significantly enhanced by strictly tailoring the growth mode in YBCO as a 2D step flow for this kind of BSTO/YBCO bilayers.

INFLUENCE OF HYDROGEN ON ELECTRICAL PROPERTIES OF Pt/(Ba,Sr)TiO3/Pt THIN FILM CAPACITORS

ABSTRACT The influence of hydrogen on the electrical properties of Pt/(Ba,Sr)TiO3/Pt thin film capacitors have been studied by secondary ion mass spectroscopy (SIMS) and thermal desorption spectroscopy (TDS) methods. From the analyses, it is suggested that in a BSTO thin film grown by RF sputtering, there exist two types of hydrogen with different energy states. One stays thermally stable in BSTO even at 600°C, and the other is thermally unstable and is released from BSTO above 450°C. When the Pt/BSTO/Pt capacitors were exposed to hydrogen-containing plasma, a pronounced hydrogen accumulation was observed at the Pt/BSTO interface in the SIMS profiles. The concentration of accumulated hydrogen at the interface is identical to that of the thermally unstable hydrogen that is released above 450°C. It is experimentally confirmed that thermally unstable hydrogen degrades the electrical properties of Pt/BSTO/Pt thin film capacitors, and its degradation mechanism is proposed.

Large dielectric tunability and microwave properties of Mn-doped (Ba,Sr)TiO3 thin films

Ferroelectric Ba0.6Sr0.4TiO3 thin films with 2% Mn additional doping were grown on (001) MgO by pulsed laser deposition. The microstructural studies from x-ray diffraction and transmission electron microscopy indicate that the films are highly epitaxial with c-axis oriented and atomic sharp interface. Dielectric property measurements at 1 MHz and room temperature reveal that the as-grown films have outstanding dielectric properties with large tunability of 80% at 40KV∕cm, very large dielectric constant value of 3800, and extra low dielectric loss of only 0.001. The high frequency (10–30 GHz) dielectric measurements demonstrate that the films are excellent in both dielectric property and very low dielectric insertion loss. Compared with the pure BSTO films or traditional doping, the additional doping of Mn in BSTO thin films can significantly improve the dielectric property of the as-grown films.

BaxSr1 − xTiO3 Thin Films Deposited by RF Hollow Cathode Plasma Jet Technique

The RF modulated plasma jet system was used for the deposition of Ba x Sr1− x TiO3 (BST), and SrTiO3 (STO) thin films on Si and multi-layer system Si/SiO2/TiO2/Pt substrates. BSTO and STO thin films were analyzed using X-ray diffraction. During deposition, the substrate was heated at 500°C. Emission spectroscopy of the plasma was used to control the concentration of sputtered particles from the hollow cathode, mainly Sr and Ba atoms. XRD diffraction confirmed the presence of BSTO and STO perovskite phase in the films. The optical band gap obtained from spectroscopic ellipsometry was correlated with composition.

Self-assembled monolayers and chemical derivatization of Ba 0.5Sr 0.5TiO 3 thin films: Applications in phase shifter devices

Thin films of barium strontium titanate (Ba 1− x Sr x TiO 3 (BSTO)) have been used in coupled microstrip phase shifters (CMPS) for possible insertion in satellite and wireless communication platforms primarily because of their high dielectric constant, low loss, large tunability, and good structural stability. In an attempt to improve the figure of merit K (phase shift °/dB of loss) of phase shifters, modification of the metal/BSTO interface of these devices has been done through surface modification of the BSTO layer using a self-assembled monolayer approach. The impact of this nanotechnology promises to reduce RF losses by improving the quality of the metal/BSTO interface. In this study, compounds such as 3-mercaptopropyltrimethoxysilane (MPS), 16-mercaptohexadecanois acid (MHDA) and 3-mercaptopropionic acid (MPA) were used to form the self-assembled monolayers on the BSTO surface. As a result of the previous modification, chemical derivatization of the self-assembled monolayers was done in order to increase the chain length. Chemical derivatization was done using 3-aminopropyltrimethoxysilane (APS) and 16-mercaptohexadecanoic acid. Surface chemical analysis was done to reveal the composition of the derivatization via X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared (FT-IR). Low and high frequencies measurements of phase shifters were done in order measure the performance of these devices for insertion in antennas. X-ray photoelectron spectroscopy characterization of modified BSTO thin films with MPS showed a binding energy peak at 162.9 eV, indicative of a possible S O interaction: sulfur of the mercapto compound, MPS, used to modify the surface with the oxygen site of the BSTO thin film. This interaction is at higher binding energies compared with the thiolate interaction. This behavior is observed with the other mercapto compounds such as: MHDA and MPA. An FT-IR analysis present a band at 780 cm −1, which is characteristic of an O S C stretching and reveals the modification of the BSTO thin film by the coupling of the O of the BSTO with the S of the mercapto compound. All the modification using mercapto compounds is through sulfur to the BSTO thin film. MHDA SAM on BSTO thin film was chemically derivatized using APS shown by XPS and FT-IR. The SAMs modified phase shifters showed an improvement in performance with respect to those phase shifters fabricated with standard methods.

Preparation and characterization of ferroelectric/superconductor heterostructures on vicinal LaAlO 3 substrates

Ba 0.1Sr 0.9TiO 3/YBa 2Cu 3O 7− δ heterostructure films have been fabricated by a pulsed-laser deposition method on vicinal LaAlO 3 substrates. The effect of the growth mode of YBCO thin films on the dielectric properties of BSTO thin films was studied. The BSTO thin films grown epitaxially on YBCO thin films in step-flow mode showed high dielectric constant and large tunability. The dielectric constant and loss tangent measured were 1100 and 0.005, respectively, and a tunability of 55% was achieved at ±50 V/μm electric field at 77 K and 1MHz for a BSTO thin film grown on 120 nm YBCO thin film.

Tunable microwave filters using ferroelectric materials

A new design approach for a microstrip tunable filter on a two-layered substrate with BSTO thin films is suggested. The coupling factor of a ferroelectric capacitor connected to the microstrip resonators of the filter is considered to make possible the design of the filter with constant pass band width while tuning. Results of the filter characteristic simulation and measurement are presented. Tunable coplanar resonator on single crystal STO disk is also considered.

Hydrogen induced tunnel emission in Pt/(BaxSr1−x)Ti1+yO3+z/Pt thin film capacitors

The leakage current density–applied field (J−EA) characteristics of (BaxSr1−x)Ti1+yO3+z (BSTO) thin film capacitors with Pt electrodes that have been annealed in forming gas (95% Ar 5% H2 or D2) were investigated over the temperature range from −60 to +60 °C. Forming gas annealing significantly increased the leakage current density. The J–EA characteristics exhibited features that could not be fully explained by either a simple thermionic emission or tunneling (Fowler–Nordeim) formalism. Using the general charge transport theory of Murphy and Good, we show that the J–EA characteristics can be successfully interpreted in terms of tunneling of electrons through the interfacial Schottky barrier with the peak in energy distribution of the incident carriers strongly dependent on applied field. At high applied fields the energy distribution of incident carriers is peaked near the Fermi level in the electron injecting metal electrode at all temperatures considered in this study, leading to almost temperature independent J–EA characteristics. At lower applied fields the peak in energy distribution shifts towards the conduction band edge where thermally assisted tunneling occurs and a more pronounced temperature dependence of the current density is observed. Good agreement between experiment and theory is demonstrated for a reasonable parameter set for BSTO thin films strongly suggesting that the high leakage current density often observed after forming gas annealing results from tunneling of electrons through the interfacial Schottky barrier.

Three dimensional (BA, SR) TIO3 stack capacitors for dram application

Three dimensional integration of BSTO thin films for Gigabit DRAM application is performed by a MOCVD BST process combined with a high temperature barrier stack of TaSiN/Ir/IrO2 with SiO2 sidewall protection. The SiO2 layer is formed by a new low temperature HDP process. This barrier stack enables the use of a new two step high temperature MOCVD BST process, which results in high quality BST films. First, a very thin BST seed layer is deposited at 450°C. The second layer is formed at 640°C. Using these two key processes, we have established a baseline for BST capacitors on 0.2μm feature size. 256k capacitor arrays show capacitance up to 18.1 fF/cell with excellent leakage current around 1 fA/cell (DC, 1V). The capacitance is scaling with different capacitor arrays (4k to 256k).

Epitaxial behavior and interface structures of BSTO thin films

Ferroelectric Ba(1-x)SrxTiO3 (x = 0.5 and 0.25) thin films were grown on (001) LaAlO3 by using pulsed laser ablation. Extensive x-ray diffraction and selected area electron diffraction reveal that the as-grown films were (001) oriented with a good in plane relationship of <100>BSTO // <100>LAO. Rutherford Backscattering Spectroscopy ion-channeling studies suggested that the films had excellent epitaxial quality and crystallinity with an ion beam minimum yield χmin of only 2.6%. Atomically sharp interfaces were seen by cross-sectional high-resolution electron microscopy, indicated that the density of misfit dislocations was consistent with the lattice mismatch from the theoretical calculation.

Microstructure properties of Ba 0.5Sr 0.5TiO 3 thin films on Si with conductive SrRuO 3 bottom electrodes

Quaternary Ba 0.5Sr 0.5TiO 3 (BSTO) thin films were deposited on (100) Si by pulsed laser deposition with conductive SrRuO 3 (SRO) bottom electrodes. The growth of highly (001)-oriented SRO film on (100) Si was accomplished by using a bilayer buffer CeO 2 and yttria-stabilized zirconia (YSZ). The BSTO films deposited on SRO on Si using such a buffer system were well aligned in the plane with respect to the major axes of the substrate confirmed by X-ray diffraction. The crystalline nature of the BSTO films on Si was further illustrated by Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy. The BSTO thin films exhibited a dielectric constant above 340 at 10 kHz tested from a capacitor configuration of Au/BSTO/SRO. © 1997 Elsevier Science S.A.

Layered Structures HTSC/Ferroelectric, Prepared by Sputtering

We have investigated some formation features of thin film structures PZT/YBCO and YBCO/BSTO. PZT films have been synthesized by reactive ion beam sputtering of a multi-elemental metal target at room temperature and ex-situ annealing at 700-750°C. BSTO and YBCO films have been deposited in-situ by magnetron sputtering of stoichiometric ceramic targets in an Ar + 50% O 2 gas mixture (pure oxygen for BSTO) at temperature of 700°C. Si (100) buffered with TiN to prevent the diffusion in YBCO has been used as a substrate in the case PZT / YBCO. The layers YBCO / BSTO have been deposited on MgO (100). The substrates have been chosen according to prospective applications of PZT / YBCO as non-volatile memory cells integrated with existing CMOS transistor circuitry, and of YBCO/BSTO as voltage-tunable microwave devices Films with composition Pb(Zr 0.44 Ti 0.56 )O 3 have been obtained by sputtering of a target with a ratio of 13:36:51 for Pb, Sr and Ti, respectively. The measured dielectric constant at 1 MHz in the structures Au / PZT / YBCO / TiN / Si has been 540 and the -emnant polarization and coercive field values have been 5.4 μC/cm 2 and 25 kV/cm, respectively. Planar capacitors with a dielectric of BSTO thin film and YBCO electrodes have been produced and investigated as voltage-controlled devices at 1 MHz. A controllability of e(0V) /e(50V) = 1.6 has been demonstrated at 77 K. The dielectric hysteresis has been compared in capacitors with YBCO electrodes and with metal ones. It has been shown that the boundary phenomena at the interface of the epitaxially grown layers differ from that in the ferroelectric / metal structures and the hysteresis effect diminishes significantly,