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.     

Electrical and optical properties of microwave dielectric thin films prepared by pulsed laser deposition

Abstract

Bi₂(Zn_1/3Nb_2/3)₂O₇, BiZN, materials possess high dielectric constant and low loss factor in microwave frequency region. They have good potential for device application, especially in the form of thin films. However, the microwave dielectric properties of a thin film are very difficult to be accurately measured. Evaluation on the dielectric behavior of the films through the performance of the microstrip line devices made of these films involves metallic conduction and stray field losses. A novel measuring technique, which can directly evaluate the microwave dielectric properties of a thin film is thus urgently needed.

In this paper, BiZN thin films were grown on [100] MgO single crystal substrates using pulsed laser deposition process. The high-frequency dielectric properties of thus obtained thin films were determined using optical transmission spectroscopy (OTS). The [100] preferentially oriented films with pyrochlore structure can be obtained for the thin films deposited at 400–600°C substrate temperature under 0.1 mbar oxygen pressure. OTS measurements reveal that the index of refraction (n=1.95–2.35) and absorption coefficient (k=0.28x10^?4-2.25 × 10^?4 nm^?1) of the films vary insignificantly with the crystallinity of the BiZN films.     

MOCVD of Pb(Zr x Ti1-x )O3 thin films on MgTiO3/Si substrates and their electrical properties

Abstract

Pb(Zh _x , Ti_1-x )O₃(PZT) thin films were deposited on Si substrates using MgTiO₃ as the buffer layer and the electrical properties of those MFIS structures were investigated. PZT and MgTiO₃ films were made by MOCVD using ultrasonic spraying technique. Perovskite PZT films have been succesfully made at the substrate temperature of 550 to 600°C only when using MgTiO₃ buffer layer. AES depth profile analysis and RBS analysis revealed that there is no remarkable interdiffusion and no formation of reaction layer between PZT and MgTiO₃ and/or between MgTiO₃ and Si substrate. The capacitance-voltage (C-V) curves of the MFIS structure which were made with PZT and MgTiO₃buffer layer have shown the hysteresis resulted from the ferroelectric switching of the PZT films.     

Growth and electrical properties of (211) BaTiO3 thin films on Pt-coated Si(100) substrates

Abstract

Barium titanate (BaTiO₃) thin films with high (211) orientation have been prepared on Pt(111)/Si(100) substrates by R. F. magnetron sputtering at a substrate temperature between 550°C and 580°C in an Ar/O₂ atmosphere. The I-V curve of a thin film capacitor (Ag-BaTiO_3-Pt) has been measured and the C-V curves are obtained for frequencies between 100Hz and 1MHz. Neither the I-V curve nor the C-V curves are symmetrical and a very large change in the slope of all curves is found to occur at ～+0.5v.     

Pulsed laser deposition of single phase LiNbO3 thin film waveguides

We performed a parametric study to suppress secondary phases in lithium niobate thin films by using pulsed laser deposition (PLD). By reducing the growth rate and changing the ambient gas pressure respectively, we found that the main parameter suppressing secondary phases is controlling the plume strength. For investigating the relation between film phases and the plume strength, extensive parametric studies were performed by changing the oxygen ambient pressure, the target to substrate distance, and also the Li content in the target. The surface morphologies of single phase films obtained with different deposition parameters are compared and deposition parameters to achieve lower loss single-phase films are discussed.     

Growth and microstructures of sputtered ferroelectric PbTiO3 thin films

Abstract

PbTiO₃ thin films, 5–200 nm in thickness, were epitaxially growth on miscut (001) SrTiO₃ substrates by planar magnetron sputtering for understanding of film growth mechanism and their ferroelectricity. The surface of the miscut substrates with miscut angle of 1.7 degree contains periodic step lines and terraces; the step height is 0.4 nm and terrace width is 14 nm. The surface structures of PbTiO₃ films comprised periodic striped patterns which was reflected in the initial surface of the substrate. It was found that under a stoichiometric film composition the film growth was governed by Frank-van der Merwe type and resultant epitaxial films showed extremely smooth surface. Deposition on a miscut substrate under a stoichiometric condition is essential to making the uniform ferroelectric thin films.     

Electromechanical properties of PMN-PT thin films prepared by pulsed laser deposition technique

Abstract

Ferroelectric relaxor thin films of the composition 0.9Pb(Mg_1/3Nb_2/3)O₃-0.1PbTiO₃ (PMN-PT) were fabricated by pulsed laser deposition technique on Pt/Ti/SiO₂/Si(100) substrates using a La_0.5Sr_0.5CoO₃ film as a template layer. The films were polycrystalline with random orientation of the grains. Electromechanical properties of the films were evaluated by a sensitive interferometric technique as a function of the dc and ac electric fields and frequency. Experimental results show that the effective piezoelectric coefficient d₃₃ can be tuned over a wide range of magnitudes by varying dc bias fields. A small hysteresis and a low remanent d₃₃ were observed, which favor the use of PMN-PT films in electric field-controlled micromechanical devices. The maximum d₃₃ ranged from 50 to 100 pm/V, depending on the thickness of the films and frequency. The piezoelectric properties are explained based on the expression for the electrostriction biased by polarization. At low electric field, strain is proportional to the square of electric field, while at higher fields it can be better described as the polarization squared. The asymmetry of the strain response in PMN-PT films is attributed to the polarization offset appearing due to the different electrode materials.     

Influence of deposition parameters on physico-chemical and optical properties of sputtered PbTiO3 thin films

Abstract

Stoichiometric thin films of lead titanate (PbTiO₃) have been grown “in situ” without postannealing on (0001) sapphire substrates by rf magnetron sputtering technique. X-ray diffraction scans have revealed that as-grown films consist of the perovskite phase and are polycrystalline with a high (111) orientation. The structure, the microstructure and the optical properties have been studied as a function of the process parameters i.e., substrate temperature, gas pressure and the target composition, in particular the lead content. We report the dependence of the deposition conditions on the optical constants. The optimum experimental conditions (100m Torr and 600°C) to produce thin films with high transparency and refractive indices (n = 2.61 at 633 nm), similar to values for bulk materials, are given in this paper.     

Optical properties of BaTiO3 thin films: Influence of oxygen pressure utilized during pulsed laser deposition

The crystallographic properties of BaTiO₃ thin films, grown by pulsed laser deposition on MgO substrates, were found to be strongly influenced by the oxygen pressure used during growth. Low pressure grown films were c-oriented while increasing oxygen pressure produced films with preferred a-orientation. The crystal reorientation resulted in the shift of optical birefringence from +0.04 to −0.025 with low levels of birefringence in films possessing low tetragonal distortion. Mach-Zehnder electro-optic waveguide modulators were fabricated to characterize the electro-optic properties of the deposited films and to evaluate the suitability of these films for planar optical applications. An effective electro-optic coefficient of 23 pm/V was obtained for a c-axis oriented film near the crystal c→a reorientation point.     

Microwave dielectric properties of B2O3-added Li2MgTiO4 ceramics for LTCC applications

Li₂MgTiO₄ (LMT) ceramics which are synthesized using a conventional solid-state reaction route. The LMT ceramic sintered at 1250°C for 4 h had good microwave dielectric properties. However, this sintering temperature is too high to meet the requirement of low-temperature co-fired ceramics (LTCC). In this study, the effects of B₂O₃ additives and sintering temperature on the microstructure and microwave dielectric properties of LMT ceramics were investigated. The B₂O₃ additive forms a liquid phase during sintering, which decreases the sintering temperature from 1250°C to 925°C. The LMT ceramic with 8 wt% B₂O₃ sintered at 925°C for 4 h was found to exhibit optimum microwave dielectric properties: dielectric constant 15.16, quality factor 64,164 GHz, and temperature coefficient of resonant frequency -28.07 ppm/°C. Moreover, co-firing of the LMT ceramic with 8 wt% B₂O₃ and 20 wt% Ag powder demonstrated good chemical compatibility. Therefore, the LMT ceramics with 8 wt% B₂O₃ sintered at 925°C for 4 h is suitable for LTCC applications.     

Growth and characterization of non-c-axis-oriented SrBi2Ta2O9 epitaxial thin films on Si(100) substrates with SrRuO3 bottom electrodes

Abstract

We have successfully grown non-c-axis-oriented epitaxial ferroelectric SrBi₂Ta₂O₉ (SBT) films with (116) and (103) orientations on Si(100) substrates using epitaxial (110)- and (111)-oriented SrRuO₃ (SRO) bottom electrodes, respectively. The SRO orientations have been induced by coating the Si(100) substrates with epitaxial YSZ(100) and MgO(111)/ YSZ(100) buffer layers, respectively. All films were sequentially grown by pulsed laser deposition. Specific in-plane orientations of the epitaxial SBT films were found, which are in turn determined by specific in-plane orientations of the epitaxial SRO bottom electrodes. These include a diagonal rectangle-on-cube epitaxy of SRO(110) on YSZ(100) and a triangle-on-triangle epitaxy of SRO(111) on MgO(111).     

Micro-structure and ferroelectric properties of BiFeO3 thin films formed on Pt-coated r-plane sapphire substrates

Magnetoelectric BiFeO₃ (BFO) materials exhibit ferroelectric and ferromagnetic properties simultaneously, therefore they have a potential to be applied in magnetic as well as ferroelectric devices. BFO thin films were formed by depositing sol-gel solutions on Pt-coated r-plane sapphire dielectric substrates. We did not observe any secondary phase such as Bi₂Fe₄O₉ on the r-plane sapphire substrates, which is generally observed on Si substrates. We observed small ferroelectric grains of about 0.1 μm on Pt/sapphire structures. The leakage current density in BFO films was found to be decreased dramatically after optimizing process conditions of stoichiometric BFO chemical solution. The leakage current densities were in the range of 10^{− 7} A/cm² at room temperature and 10^{− 9} A/cm² at 80 K under 0.4 MV/cm applied electric field. The main reason for low leakage current is considered to be reduction of oxygen vacancies due to the presence of exclusive Fe^{3 +} valance state in the films. An applied electric field higher than 0.5 MV/cm was required to pole the BFO films, which made it difficult to obtain the saturated polarization at room temperature. We could measure the saturated remanent polarization in the BFO films at 80 K and the obtained remanent polarization was 100 μC/cm².     

GHZ polarization dynamics in ferroelectric thin films

Abstract

We present results from experiments which measure the local dielectric response of ferroelectric thin films driven by microwave-frequency electric fields. The repetition rate of a mode-locked Ti:Sapphire laser is used to generate a microwave drive signal that is phase-locked to an optical probe pulse and applied to the ferroelectric thin film. The induced polarization change in the ferroelectric film is measured stroboscopically via the electro-optic effect. Polarization images are acquired by scanning the laser beam across the sample in a confocal geometry. Time resolution is achieved by changing the delay between the electrical pump and the optical probe. Initial results show large local phase shifts in the ferroelectric response of closely separated regions of a Ba_0.5Sr_0.5TiO₃ thin film. This new experimental technique may help to understand the physical mechanisms of dielectric loss in these materials.     

化学水浴法制备ZnS薄膜

采用了化学水浴法制备异质结薄膜太阳电池中的窗口层ZnS薄膜.尝试了酸性溶液制备ZnS薄膜,讨论了溶液成分、水浴时间和温度对薄膜成分和形貌的影响.研究结果表明在75℃,6 h条件下沉积得到的薄膜最平整,成分最符合化学计量比,退火后薄膜主要为纤锌矿结构的ZnS相.     

Epitaxial behavior and interface structures of BSTO thin films

Abstract

Ferroelectric Ba_(1-x)Sr_xTiO₃ (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 χ_min 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.     

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.     

Improvement of the electrical properties of PZT thin films using TiO2 buffer layer

Abstract

Thin TiO₂ layers were sputter-deposited on Pt/Ti/SiO₂/Si wafers, as buffer layers for PZT thin film capacitors. It was found that TiO2 buffers of less than 4-nm-thickness could assist in obtaining highly uniform PZT thin films with no second phase. The leakage current behaviors of the PZT based capacitor are improved, while retaining the ferroelectric properties of PZT thin films such as remanent polarization and coercive field. In addition, the uniform distribution of oxygen in PZT on TiO₂/Pt indicates that the TiO₂ buffer layer act as a barrier for lead-platinum reaction, as well as for oxygen diffusion.     

Ferroelectric electrode interactions in BaTiO3 and PZT thin films

Abstract

We have studied ferroelectric thin films deposited by sol-gel processing onto non-noble metal substrates. The results obtained indicate that by careful control of processing conditions, ferroelectric materials can be deposited onto these substrates, although interdiffusion between the metal and the ferroelectric can significantly degrade the ferroelectric properties of the materials. The effects of this interdiffusion are demonstrated and possible implications of ferroelectric-electrode interactions for fatigue in ferroelectric materials are discussed.     

Synthesis and properties of ferroelectric Si-doped (Bi, Nd)4Ti3O12 thin films by chemical solution deposition

Ferroelectric Si-doped (Bi,Nd)₄Ti₃O₁₂ thin films have been prepared on Pt/TiO_x/SiO₂/Si substrates through metal-organic compounds by the chemical solution deposition. The Bi_3.25Nd_0.75Ti_2.9Si_0.1O₁₂ (BNTS) precursor films were found to crystallize into the Bi-layered perovskite Bi₄Ti₃O₁₂ single-phase above 600^∘C. The synthesized BNTS films revealed a random orientation having a strong 117 reflection. The BNTS thin films prepared between 600^∘C and 700^∘C showed well-saturated P-E hysteresis loops with P _r of 13–14 μ C/cm² and E _c of 100–110 kV/cm at an applied voltage of 5 V. The surface roughness of the BNTS thin films was improved by Si doping compared with that of undoped Bi_3.35Nd_0.75Ti₃O₁₂ films.     

Crystallographic and dielectric properties of highly oriented BaTiO3 films: Influence of oxygen pressure utilized during pulsed laser deposition

The crystal structure of BaTiO₃ thin films grown by pulsed laser deposition on MgO substrates was found to be strongly influenced by the oxygen pressure used during growth. Low pressures produced epitaxial films with highly strained out-of-plane lattice parameter c compared to in-plane parameter a, while increasing oxygen pressure resulted in the ratio c/a < 1 with a concomitant increase in polycrystallinity. The dielectric properties varied with changing crystal structure reaching a maximum permittivity value in films near the minimum point of tetragonal distortion and exhibiting relaxor-like behavior in c/a < 1 films close to this point. Hysteresis observed in dielectric tuning loops pointed to the presence of the ferroelectric phase in all films at room temperature. As a result of high electric field poling treatment at 300 °C, the tunability generally increased and initially symmetric tuning curves became asymmetric. The tuning curve in c/a < 1 samples became nearly linear, supporting the premise of polarization reorientation with changing deposition condition. Phase transitions to the paraelectric phase were highly suppressed and shifted upwards in temperature from the bulk transition temperature of 130 to ∼250 °C in strongly c-oriented films. Moderate shifts in oxygen working pressure were demonstrated to produce films with very different properties thereby offering convenient means for strain engineering and control of preferred crystal orientation and polarization direction of highly oriented BTO films.