Surfaces structure of bulk and thin film ferroelectrics

Abstract

Based on the experimental results of thickness dependences of breakdown voltage and dielectric permittivity for BaTiO₃ family ceramics, (Pb, La)TiO₃ thin films and commercial multilayer capacitors, surface layer structures are discussed. Surface layers inside ferroelectric materials are consisting of non-ferroelectric Kanzig layer with low dielectric permittivity and with higher concentrations of impurities due to the inter diffusion between substrates and bulk, internal stresses induced by lattice mismatch, cubic-tetragonal phase transition, electric field induced anisotropies and internal bias field (space charge field). The Voltage - Current characteristics (V-I curve), D-E loops of ferroelectric materials show asymmetric behaviors. Saturation phenomena of V-I curves are observed only ferroelectric temperature region. The breakdown voltage almost depends on the non-ferroelectric Kanzig layer and the dielectric permittivity depends on the volume fraction of non-ferroelectric parts. Log-log plots of dielectric permittivity and breakdown voltage suggest that the thickness of non-ferroelectric Känzig layer should be at least more than 10～20 nm and in the grain boundary thickness of bulk ferroelectric materials should be more, especially in the liquid phase sintering, the thickness of grain boundary is a order of 0.1～0.2 μm.     

Thin film bulk acoustic wave piezoelectric resonators with circular ring driving electrodes for mass sensing

We study thickness-extensional vibration frequencies and modes in a thin ZnO or AlN film on a silicon layer with a circular ring driving electrode. It is shown that the vibration tends to be trapped in the annularly electroded area and decays away from the electrode edges. The convexity of the vibration distribution near the film center can be adjusted through design, and can be made nearly uniform for mass sensor application. The theoretical analysis presented can be used to optimize the design of the device.     

Flexible film bulk acoustic resonators and filter-like structure made directly on polymer substrates

This paper reports a novel solid-mounted flexible film bulk acoustic resonator (FBAR) directly on polymer substrate with the merits of flexibility and no need for back etching process. (0002) oriented ZnO films deposited by sputtering, and aluminum and gold as the electrodes were used to fabricate the FBARs. Results show that the devices with aluminum electrodes have a resonant frequency at 1.2 GHz with an effective electromechanical coupling coefficient of ～ 4.7%, a filter-like architecture improved the quality factor of FBAR from 19.6 to 164.2, and the devices remained working after bending at a large angle for hundreds of times.     

Fabrication of piezoelectric MEMS devices-from thin film to bulk PZT wafer

Integrating patterned functional piezoelectric layer onto silicon substrate is a key technique challenge in fabrication of piezoelectric Micro Electro Mechanical System (pMEMS) devices. Different device applications have different requirements on the thickness and in-plane geometry of the piezoelectric layers and thus have their own processing difficulties. In this paper, the techniques of integrating piezoelectric function into pMEMS has been discussed together with some diaphragm-based pMEMS devices which have relatively lenient requirement on patterning of the piezoelectric layers. Sol-gel thin film can meet the requirement of most of the sensor applications. The composite thick film is one of the promising solutions for thick film devices due to its good processing compatibility. Si/Pb(Zr _x Ti_1 − x )O₃ wafer bonding technique makes it possible to thin down the ceramic wafer to less than 10 μm by using chemical mechanical polishing, which, therefore, provide us another approach to integrate thick piezoelectric layer on silicon to cover the need of most of the thick film devices. Directly make double side aligned electrode patterns on bulk piezoelectric wafer/plate by using photolithography opens up a new area of pMEMS. The advantage of using bulk piezoelectric wafer/plate in pMEMS is that we can select commercial available ceramics or single crystals with excellent piezoelectric properties and thus ensure the overall performance of the devices.     

Selective growth of ZnO nanorods by patterning of sol-gel-derived thin film

An effective method of fabricating micro and sub micro patterned zinc oxide nanorods without a transient metal catalytic assistants via hydrothermal synthesis using micro molding technique was presented. Micro molding of sol-gel-derived precursor layer was clearly accomplised by conformal contact of elastomeric molds with various kinds of patterns. The selective growth of the nanorods over a large area of the pre-patterned seed layer was observed with a pattern feasibility of 250 nm. The orientation of the nanorods was normal to the seed layer while characteristic sizes of the rod didn’t show any dependence on the diameter of the seed particles. Presented synthetic process can provide simpler way of fabricating an array of semiconductor one-dimensional nanostructures.     

Temperature stability of ZnO thin film SAW device on fused quartz

Surface acoustic wave (SAW) filters for low-frequency (38-65 MHz) applications have been developed using a radio frequency (RF)-magnetron-sputtered ZnO film on fused-quartz substrates. SAW propagation characteristics such as electromechanical coupling coefficient (K/sup 2/), SAW phase velocity (v), insertion loss, and temperature coefficient of delay (TCD) have been measured. The intergidital transducer (IDT)/ZnO/fused-quartz device structure yields almost zero TCD (1 ppm/spl middot//spl deg/C/sup -1/) with 0.316 /spl lambda/ thick ZnO layer (for the device operating at 60 MHz). Alternately, an overlayer of positive TCD material (ZnO itself) has also been deposited on the IDT/ZnO(<0.316 /spl lambda/)/fused-quartz device at a low substrate temperature to reduce the TCD. A modified layered structure consisting of ZnO/IDT/ZnO/fused quartz yields almost zero TCD (-3 ppm/spl middot//spl deg/C/sup -1/) with a 5.3-/spl mu/m-thick ZnO overlayer and a 8.1-/spl mu/m-thick (0.183 /spl lambda/) ZnO bottom layer. Experimentally obtained SAW propagation characteristics have been compared with the theoretical results.     

Sapphire orientation dependence of the microstructure of ZnO thin film during annealing

The sapphire orientation dependence of the microstructure of ZnO thin films has been studied in real-time synchrotron X-ray scattering experiments. The ZnO films with a 2400-Å-thick were grown on sapphire (001) and sapphire (110) substrates at room temperature by radio frequency magnetron sputtering. The as-deposited ZnO film on sapphire (001) has the only (002) crystal grains, while that on sapphire (110) has not only (002) crystal grains but (100) and (101) additional grains. The ZnO films were changed into fully epitaxial ZnO (002) grains both on sapphire (001) and sapphire (110) substrates with increasing the annealing temperature to 600^°C. The epitaxial relationships of the ZnO grains were summarized as ZnO (00l)[100]//sapphire (00l)[110] and ZnO (00l)[110]//sapphire (110)[001].     

Optical and electrical properties of ZnO thin film containing nano-sized Ag particles

Low-temperature crystallized ZnO thin film was achieved by sol–gel process using zinc acetate dihydrate and 2-methoxyethanol as starting precursor and solvent, respectively. Ag nanoparticles were prepared with uniform size at 4.4 nm by spontaneous reduction method of Ag 2-ethylhexanoate in dimethyl sulfoxide (DMSO). The optical and electrical properties of ZnO thin films containing various contents of Ag-nanoparticles were monitored. Light scattering and charge emission and scattering behaviors of Ag nanoparticles in ZnO film were found. The incorporation of Ag nanoparticles into Al-doped ZnO film was also investigated. The optical transmittance was not degraded but the increase of electrical sheet resistance was found. The effect of Al-dopant on the transmittance and electrical sheet resistance of ZnO film was found too great to distinguish the positive effect of the incorporation of Ag nanoparticles into Al-doped ZnO thin films.     

A study of flexible piezoelectric generators by sputtering ZnO thin film on PET substrate

Abstract

Flexible technology has recently received much attention in the field of flexible sensors, wearable electronic devices, flexible transparent displays, and energy harvesters. Zinc oxide (ZnO) thin film is the preferred material for use in flexible devices due to its environmental friendliness, high electrical performance and low synthesis temperature. In addition, ZnO possesses a non-centrosymmetric crystal structure, causing a piezoelectric effect in the material. This work presents the fabrication of flexible piezoelectric generators using the deposition of ZnO on a PET substrate using sputtering techniques. The fabricated flexible generators are capable of generating an output power of 14 µW through an optimal resistive load of 750 kΩ. An output voltage of 2.00 Vp and a current of 150 μA measured across a 750 kΩ resistor were subsequently obtained.     

Characteristics of thin film supercapacitor with ruthenium oxide electrode and Ta2O5+x solid oxide thin film electrolyte

We report findings related to a solid-state thin film supercapacitors (TFSCs) fabricated with ruthenium oxide electrodes and hydrogen doped tantalum oxide electrolyte in order to investigate a feasibility for solid oxide thin film electrolyte in all solid state micropower sources with hydrogen conducting electrolyte. The TFSCs in this study has a cell structure of RuO₂/Ta₂O₅/RuO₂/Pt. Radio frequency, off-axis r.f. sputtering deposition of a Ta₂O₅ thin film was performed on the bottom amorphous RuO₂ electrode which was deposited by an on-axis direct current reactive sputtering, within hydrogen atmosphere to enhance the concentration of mobile proton (H⁺) ions. Scanning electron microscopy (SEM) measurements reveal that the RuO₂/Ta₂O₅/RuO₂ hetero-interfaces have no inter-diffusion problems. Room temperature charge–discharge measurements with constant current clearly reveal typical supercapacitor behavior. Also, owing to the fast diffusion of H⁺ ions within the Ta₂O₅ lattice without any structural deterioration, the capacitance per volume of RuO₂/Ta₂O₅/RuO₂/Pt TFSCs was maintained to be constant during above 800 cycles. This result indicated that the solid oxide thin film has possibility as the solid proton conducting electrolyte for all solid state micropower sources.     

The annealing effect on properties of ZnO thin film transistors with Ti/Pt source-drain contact

ZnO-based thin film transistors (TFTs) with Ti/Pt contacts were fabricated on SiO₂/Si substrates. The as-deposited ZnO TFT did not work well as a TFT device but the annealed ZnO TFT showed acceptable characteristics with a mobility (μ_sat), threshold voltage (V_th), on/off ratio and subthreshold swing (SS) of 0.8 cm²/V.s, 2.5 V, over 10⁶ and 0.84 V/dec, respectively. Complete oxygen loss was observed in ZnO after annealing at 300°C under a N₂ atmosphere. The annealing process altered the crystallinity, density and composition of the ZnO active layers due to the formation of oxygen vacancies as shallow donors. This process is expected to play an important role in controlling the TFT performance of ZnO. In addition, it is expected to form the basis of the future electronic devices applications, such as transparent displays and active matrix organic lighting emitted displays (AMOLED).     

Characteristics on electrical resistance change of Ag doped chalcogenide thin film application for programmable metallization cell

Programmable Metallization Cell (PMC) Random Access Memory is based on the electrochemical growth and removal of nanoscale metallic pathways in thin films of solid electrolyte. This paper investigates the resistance change characteristics with applied voltage bias on Ag/Ge–Se and Ag/As–Se chlacogenide thin film structure and describes the electrical characteristics of PMC-RAM made from these materials following annealing at 150 °C. As a result, it was obtained that R _reverse/R _forward ratio which represent by reversible resistance change was about 10⁶, which ratio value can be get a high sensing margin when PMC-RAM detect the data. PMC-RAM technology promises to be non-volatile, low current and potentially, low cost for the next generation of nonvolatile memory application such as RFID chip to replace EEPROM.     

Epitaxially grown ferroelectric thin film capacitors for sensing applications

Abstract

Pyroelectric infrared detectors based on La-modified PbTiO₃ (PLT) thin films have been fabricated by RF magnetron sputtering and micromachining technology. The detectors form PB_1?xLa_xTi_1?x/4O₃ (x = 0.05) thin film ferroelectric capacitors epitaxially grown in-situ by RF magnetron sputtering on Pt/ MgO(100) substrate. The sputtered PLT thin film exhibits highly c-axis oriented crystal structure (90%) that poling treatment for sensing applications is not required. The c-axis orientation ratio a of deposited PLT thin film strongly depends on the morphology of Pt layer, which in turn varies with the thickness of Pt layer on MgO substrate. We have successfully grown highly c-axis oriented PLT film on Pt electrode with a conductive percolating network structure. Micromachining technology is used to lower the thermal mass of the detector by coating Polyimide on top of the sensing elements to support the fragile structure and by selectively etching the backside of the MgO substrate to reduce the heat loss. The sensing element exhibited a low noise equivalent power (NEP) of 1.7 × 10^?10 W and a very high detectivity D? value of 8.5 × 10⁸ cmVHz/W at room temperature. The high performance for pyroelectric infrared sensing is primarily due to the highly c-axis oriented PLT thin film and its minimized thermal mass.     

Titanium as a substrate for thin film PZT deposition

Abstract

During high temperature processing an interface layer is formed between the PZT and Ti substrate by the oxidation of the substrate. The effects on the measured electrical properties of PZT are investigated using dielectric, IV and CV analysis.     

Novel 3D PZT thin film structure for micromechanics

In this work, fabrication and properties of 3-dimensional structures coated with piezoelectric Pb(Zr,Ti)O₃ (PZT) thin films have been studied in order to improve the piezoelectric coupling into the third dimension. Calotte layers have been chosen as demonstration devices. The base diameters range from 40 to 120 μm, the height varies between 10 to 40 μm. A dynamic, in-situ co-sputtering process allowing for in-situ growth was applied. Micromoulds were formed by wet etching in silicon. The etchant was a HNA solution (HF, HNO₃, CH₃COOH) on a silicon dioxide mask. Calottes were obtained with the desired geometry and smooth surface state after few minutes etching time, and the use of chemical mechanical polishing (CMP). After deposition of the PZT membrane, deep silicon dry etching was then used to liberate the calotte layer. The dielectric constant and loss tangent of the calotte capacitors amounted to 830 and 5%, respectively (10 kHz). The fundamental resonance frequencies varied between 2.5 and 16.5 MHz, and were found to be inversely proportional to the base area of the calotte, the proportionality factor being 0.08 Hz m².     

用于CIGS太阳电池的AZO特性研究

采用射频磁控溅射方法制备掺铝氧化锌(AZO)薄膜,研究了气体流量对薄膜晶体质量及光电性能的影响。测试结果表明,当氩气流量为70 m L/min时制备的薄膜结晶较好,方块电阻为8.25Ω/□,电阻率为4.46×10-4Ω·cm,玻璃基底上可见光范围内的平均透过率为83.56%,综合性能最优,可用于CIGS薄膜太阳电池。采用光谱椭偏仪研究薄膜的光学特性,通过建模计算得到350~800 nm波段内AZO薄膜的光学常数,并通过作图法计算得到薄膜的光学禁带宽度为3.38 e V。     

薄膜二次锂离子电池正极研究进展

薄膜二次锂离子电池是锂离子电池发展的最新领域,正极材料的薄膜化是薄膜二次锂离子电池的重要部分.综述了近年来发展的一些薄膜正极的制备方法,包括溶胶-凝肢法(Sol-gel)、化学沉积法(CVD)、激光高温灼烧法(LA)、脉冲激光沉积法(PLD)、射频磁控溅射法(RMP),对各种方法的优缺点进行了比较,并对正极薄膜制备的发展方向进行了展望.     

Ferroelectric thin film research in france

Abstract

After a review of the research made on thin film deposition in France, this work presents the comparison between two chemical deposition methods for PZT films: sol-gel method based on low molecular weight precursors and solvent, and MOD (Metallo-Organic Deposition) process based on a hydrophobic solvent. The influence of the viscosity of the solvent and of the solubility of water is shown with regard to stability and drying properties. The final electrical properties of the ferroelectric films are also compared such as the hysteresis loop which characterizes the most important properties for memory application: coercive voltage and rectangularity.     

Degradations in PZT thin film capacitors

Abstract

Degradations in ferroelectric capacitors are categorized as DC and AC types. In memory applications, AC type degradations appears to be more serious than those of DC type. Breakdown, for example, occurs earlier under AC operation than DC. AC type degradations were reviewed and induction period was also discussed extensively. An induction mechanism was proposed based on domain rearrangement under AC conditions.

Curve fitting was demonstrated according to induction equation. It is suggested that the fatigue equation should be modified when induction period is considered.     

Simulation of top-contact pentacene thin film transistor

In this paper we present both finite element based and analytic model simulations of pentacene based organic thin film transistor. The finite element type simulation is done using Silvaco’s Atlas simulator and the analytic model simulation is performed using a Matlab code based on the standard transistor equations. Both the Atlas and Matlab simulations agree approximately with the published experimental result. The results of the simulations show a current ratio of 2.11×10⁶ for Atlas and 1.8×10⁷ for Matlab simulations. The threshold voltage extracted from the finite element type simulation is 1.1 V which is in good agreement with 1.2 V used for Matlab simulation. However, the contact resistance shows a quite significant variation between the two simulation mechanisms. The finite element type simulation gives a contact resistance of 10 kΩ whereas the Matlab simulation predicts a contact resistance 1.9 kΩ.