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.     

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].     

Epitaxy and interfacial phase in thin films of lead scandium tantalate deposited by reactive sputtering on a platinum interlayer

Abstract

Lead scandium tantalate (PST) films with a lead-rich composition were deposited at 500°C by RF sputtering on sapphire substrates coated with Pt and were investigated by cross-sectional transmission electron microscopy. The films consisted almost entirely of the perovskite phase. The grains of perovskite PST had a crystallographic orientation almost parallel to that of the underlying Pt. Some elongated voids were observed. The presence of an interfacial phase, with interplanar spacings consistent with a pyrochlore phase, was demonstrated by electron diffraction and dark-field imaging. The crystallographic orientation of this phase is also nearly parallel to that of the Pt. Energy-dispersive X-ray microanalysis showed that the interfacial phase is lead-deficient. The interfacial phase is irregular in thickness. A reduction in the Pb content of the film leads to a thicker interfacial layer. The implications for the mechanism of growth of PST on Pt and for improvement of the film quality are discussed.     

Characterization of Crystalline Carbon Nitride Films Deposited on Si and Si3N4/Si Substrate by RF Magnetron Sputtering System with DC Bias

Crystalline carbon nitride films were deposited on Si and Si₃N₄/Si substrate by reactive RF magnetron sputtering system with chamber heating and DC bias. The deposited films showed -C₃N₄, -C₃N₄ and lonsdaleite phase by XRD, XPS and FTIR. The crystalline morphology was found to gave a hexagonal structure, which has theoretical unit cell of carbon nitride observed in SEM photographs. When nitrogen gas ratio is 70%, RF power is 300 W and DC bias is –80 V, the growth rate of carbon nitride film on Si₃N₄ substrate is 2.2 m/hr, which is a relatively high growth rate compared with those in previously reported papers. The deposited films have thermally stable properties in the range of 650_C to 1,400_C.     

Aging and Annealing Effects of ZnO Thin Films on GaAs Substrates Deposited by Pulsed Laser Deposition

Zinc oxide (ZnO) thin films were deposited on GaAs (100) substrates at different temperatures in the pulsed laser deposition (PLD) system. From the measurements of X-ray diffraction (XRD) at room temperature, 300–500°C were found to be good condition for the crystallization of the thin films. From the photoluminescence (PL) measurements, 500°C was found to be the optimized temperature for its optical property. Samples grown at 100, 200, 300, and 400°C showed near band-edge (NBE) emissions and deep-level emissions. The intensity of deep-level emissions decreased as time goes on, which is believed to originate from oxygen vacancies or zinc interstitials in thin films. While for the sample grown at 500°C, bright NBE emissions were observed at room temperature, and no deep-level emissions observed. This means that the high-optical-quality thin film was grown at 500°C. At the same time, annealing process of ZnO thin films grown at room temperature was carried out in PLD chamber. It was found that the annealing temperature of 600°C has strong effects on its PL. Aging and annealing effects in ZnO thin films grown on GaAs substrates by PLD were observed for the first time.     

Dielectric and Ferroelectric Properties of Ba(Zr0.35Ti0.65)O3 Thin Films Grown by a Sol-Gel Process

The Ba(Zr_0.35Ti_0.65)O₃ (BZT) thin films were deposited via sol-gel process on LaNiO₃-coated silicon substrates. XRD showed that the crystallinity of BZT film grown on LaNiO₃ coated silicon substrates is better than that of BZT film grown on Pt. Both films showed perovskite phase and polycrystalline structure. The temperature dependent dielectric measurements revealed that the thin films had the relaxor behavior and diffuse phase transition characteristics. The capacitor tuning was about 44% for each BZT film grown on LaNiO₃/Pt and Pt electrodes at 1 MHz. Especially, the values of dielectric loss at 1 MHz ranged from 0.02 to 0.009 in the bias range of 0 to 514 kV/cm, respectively. The leakage currents density of thin films grown on LaNiO₃/Pt and Pt electrodes at 300 kV/cm was about 8.5 × 10^–7 and 1.1 × 10^–5 A/cm², respectively. This work demonstrates a potential use of BZT films for application in tunable microwave devices.     

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.     

Grain-Boundary Effect on the Curie-Weiss Law of Ferroelectric Ceramics and Polycrystalline Thin Films: Calculation by the Method of Effective Medium

The influence of grain boundaries on the dielectric properties of ferroelectric ceramics and polycrystalline thin films is described theoretically by the method of effective medium. Grain boundaries are modeled by low-permittivity (dead) layers, which do not exhibit ferroelectric instability. The effective permittivity of a polycrystalline material is calculated in the paraelectric regime above the transition temperature. The calculations are based on the solution of electrostatic problem for a spherical dielectric inclusion separated from the surrounding dissimilar matrix by a low-permittivity interface layer. For isotropic bulk ceramics, an analytic expression is derived for the effective permittivity as a function of the grain size, dead-layer thickness, and its permittivity. Temperature dependence of the aggregate dielectric response is calculated for BaTiO₃ (BT) ceramics of different grain sizes and found to be in good agreement with measurements. It is shown that grain boundaries not only renormalize the Curie-Weiss temperature and constant, but may also cause deviations from the Curie-Weiss law. For BT polycrystalline thin films grown on dissimilar substrates, numerical calculations of the effective dielectric constants are performed, taking into account both the grain-boundary and substrate effects on the film anisotropic dielectric response. Theoretical predictions are compared with the grain size dependence of the permittivity of BT films grown on Pt-coated Si.     

Effects of Heating Process on Crystalline Orientation and Electrical Properties of (Bi,La)4Ti3O12 Thin Films Derived by Chemical Solution Deposition Method

(Bi,La)₄Ti₃O₁₂ (BLT) thin films were prepared on Pt-coated Si (Pt/Ti/SiO₂/Si(100)) substrates by chemical solution deposition method. The effect of heating process on structural and electrical properties of BLT thin films was investigated. The c-axis preferentially oriented films were obtained by lower drying temperature with heating from bottom by a hot plate or IR lamp heater. On the other hand, randomly oriented films were obtained by higher drying temperature with heating from top of films by IR lamp heater or with heating substrate all around by tube type furnace. The orientation of BLT thin films deposited on Pt-coated substrates can be controlled by heating process.     

Fabrication and Characterization of Ba(Ti,Zr)O3 Thin Films Through the Chemical Solution Deposition Process

Lead- and bismuth-free Ba(Ti_{1 ? x} Zr _x )O₃ (BTZ) thin films were fabricated on Pt(111)/Ti/SiO₂/Si(100) substrates by the chemical solution deposition (CSD) process. The single phase BTZ thin films were obtained at 650°C by conventional process and the control of lattice parameter a was possible by Zr substitution. As the D-E hysteresis loops and J-V characteristics depended on the precipitates on film surface, the fabrication process was reexamined by 2-step sintering process. Consequently the decreasing of first sintering time was able to prevent the precipitates, and the larger grain of about 40–50 nm were obtained by additional sintering for 2 hour.     

Effect of Oxygen Plasma on Growth, Structure and Ferroelectric Properties of SrBi2Ta2O9 Thin Films Formed by Pulsed Laser Ablation Technique

Growth of SrBi₂Ta₂O₉ (SBT) thin films has been carried out in the presence of O₂-plasma created by applying a potential at an auxiliary ring electrode placed near the substrate. Effect of plasma excitation potential and polarity, especially negative polarity, on the formation of a proper SBT phase at 700°C and in modifying crystallite orientation and microstructure of SBT films over (1 1 1) oriented Pt film coated over TiO₂/SiO₂/Si(1 0 0) substrates has been demonstrated. Preferred c-axis orientation of SBT films changes to (a–b) orientation with decrease in plasma excitation potential from –700 to –350 V and eliminates secondary Bi₂Pt phase formation even at 600°C Microstructural study show a 2-dimensional large flat c-oriented crystallites formed at –700 V change to small crystallites in conformity with the changed aspect ratio for crystallites in (a–b) plane parallel to film plane. Spectroscopic ellipsometric results are in agreement with the microstructural data. These affects are attributed to O₂-ion bombardment during film growth which reduces nucleation barrier for growth of crystallites in (a–b) plane. O₂-plasma sustains the cationic species formed by laser ablation, which along with O ₂ ⁺ ions, provide necessary activation energy and enhance the oxidation rates required for SBT phase formation even at 700°C. SBT films grown in O₂-plasma show enhancement in remnant polarization value from 1.2 to 6.6 C/cm² and display ferroelectric properties superior to those formed without plasma. Further O₂-plasma eliminates post deposition annealing step for observance of enhanced polarization values. This study shows O₂-plasma excitation potential could be exploited as a new process parameter in laser ablation growth of ferroelectric oxide thin films.