Large-area Pulsed Laser Deposition Research Articles

Towards Scalable Large-Area Pulsed Laser Deposition.

One of the significant limitations of the pulsed laser deposition method in the mass-production-technologies of micro- and nanoelectronic and molecular device electronic fabrication is the issue of ensuring deposition of films with uniform thickness on substrates with large diameter (more than 100 mm) since the area of the laser spot (1–5 mm2) on the surface of the ablated target is incommensurably smaller than the substrate area. This paper reports the methodology that allows to calculate the distribution profile of the film thickness over the surface substrate with a large diameter, taking into account the construction and technological parameters of the pulsed laser deposition equipment. Experimental verification of the proposed methodology showed that the discrepancy with the experiment does not exceed 8%. The modeling of various technological parameters influence on the thickness uniformity has been carried out. Based on the modeling results, recommendations and parameters are proposed for manufacturing uniform thickness films. The results allow for increasing the film thickness uniformity with the thickness distribution < 5% accounts for ~ 31% of 300 mm diameter substrate.

Thin Film Barrier Layers with Increased Performance and Reduced Long-Term Degradation in SOFCs

Gadolinium-doped ceria (CGO) is placed between yttrium-stabilized zirconia (YSZ) electrolyte and strontium-based electrode as a barrier layer to avoid the formation of SrZrO3 insulating phase during solid oxide fuel cell operation. This phase is observed in as-fabricated state-of-the-art cells, which evidences cation inter-diffusion during the sintering process [1]. The impact of this element to the degradation of cells is still under study, in part due to the high complexity of long-term tests necessary for the appropriate understanding of the involved phenomena. Among the efforts for improving barrier layers, the use of vacuum techniques such as sputtering and Large Area Pulsed Laser Deposition (LA-PLD) have been explored, allowing the implementation of dense layers at lower processing temperature compared with traditional deposition techniques (i. e. screen printing). CGO barrier layer thin film deposition and annealing parameters were previously optimized in a leading to an important increase of power density compared to a state-of-the-art button cell at 750ºC using a dense layer of 2 µm. Remarkable results were also obtained when the PLD barrier layer was up-scaled to large area cells of 80 cm2 and tested in short-stack configuration for long-term operation [2].Here we present a thorough investigation of post mortem cells operated in short-stacks under realistic conditions during 15000h, comparing cells produced with state-of-the-art techniques with those produced by LA-PLD. At the light of the results, we provide tools for the improvement of the fabricated thin film layers, by finding the optimal combination between the protective role of the diffusion barrier layer, a good performance and the reduction of the PLD deposition time. Button cells with PLD barrier layer of 800 nm, 400 nm and 200 nm were fabricated and tested in SOFC mode at 750ºC. It is shown that the barrier layer thickness has no impact on the electrochemical performance and the durability which allows the use of thinner layers and a drastic reduction of the deposition time of the CGO barrier layer made by PLD.Summed to the increased performance, the durability of the layers is evaluated, subjecting the cell with the thinnest film to an aging process for 3,500 h. Post-mortem characterization of the barrier layers and interfaces was conducted by Scanning-electronic microscope and Electron Probe Micro Analysis with Wavelength Dispersive X-Ray (EPMA-WDX) to observe any microstructural changes as well as any changes in composition due to cation inter-diffusion.[1] Morales et al., Journal of Power Sources 344 (2017) 141-151.[2] Morales et al. ACS Applied Energy Materials 1 (2018) 1955-1964.

Enhanced Performance of Gadolinia-Doped Ceria Diffusion Barrier Layers Fabricated by Pulsed Laser Deposition for Large-Area Solid Oxide Fuel Cells

Diffusion barrier layers are typically introduced in solid oxide fuel cells (SOFCs) to avoid reaction between state-of-the-art cathode and electrolyte materials, La1–xSrxCo1–yFeyO3-δ and yttria-stabilized zirconia (YSZ), respectively. However, commonly used layers of gadolinia-doped ceria (CGO) introduce overpotentials that significantly reduce the cell performance. This performance decrease is mainly due to the low density achievable with traditional deposition techniques, such as screen printing, at acceptable fabrication temperatures. In this work, perfectly dense and reproducible barrier layers for state-of-the-art cells (∼80 cm2) were implemented, for the first time, using large-area pulsed laser deposition (LA-PLD). In order to minimize cation interdiffusion, the low-temperature deposited barrier layers were thermally stabilized in the range between 1100 and 1400 °C. Significant enhanced performance is reported for cells stabilized at 1150 °C showing excellent power densities of 1.25 W·cm–2 at 0.7 V and at a operation temperature of 750 °C. Improved cells were finally included in a stack and operated in realistic conditions for 4500 h revealing low degradation rates (0.5%/1000 h) comparable to reference cells. This approach opens new perspectives in manufacturing highly reproducible and stable barrier layers for a new generation of SOFCs.

A comparative study on macroscopic and nanoscale polarization mapping on large area PLD grown PZT thin films

By optimizing laser raster scanning conditions and controlling PbO evaporation, we have grown high quality large area PbZr.52Ti.48O3 (PZT) thin films using custom-built off-axis pulsed laser deposition (PLD) technique. The realization of high quality PZT thin films over a large area with uniform polarization is a challenging task due to the formation of non-ferroelectric or pyrochlore like PbTi3O7 phases at the processing temperature (>600°C). Our results show the polycrystalline nature of the sample with a uniform chemical surface composition. The macroscopic level of polarization mapping shows excellent saturated P-E loops with a narrow variation (6%) of the remnant polarization (2Pr) across the large area thin films. A comparative study of macroscopic and nanoscale level of mapping of polarization switching are performed, to demonstrate the existence of stoichiometry ferroelectric PZT thin films over a large area. The achievement of high quality large area PZT thin films with uniform polarization might be used for under-water SONAR devices and in power harvesting.

Wafer-scale growth of highly textured piezoelectric thin films by pulsed laser deposition for micro-scale sensors and actuators

Piezoelectric lead-zirconate-titanate (PZT) thin films were deposited on 4-inch (111)Pt/Ti/SiO2/Si(001) wafers using large-area pulsed laser deposition (PLD). This study was focused on the homogeneity in film thickness, microstructure, ferroelectric and piezoelectric properties of PZT thin films. The results indicated that the highly textured (001)-oriented PZT thin films with wafer-scale thickness homogeneity (990 nm ± 0.8%) were obtained. The films were fabricated into piezoelectric cantilevers through a MEMS microfabrication process. The measured longitudinal piezoelectric coefficient (d33f = 210 pm/V ± 1.6%) and piezoelectric transverse coefficient (e31f = -18.8 C/m2 ± 2.8%) were high and homogeneity across wafers. The high piezoelectric properties on Si wafers will extend industrial application of PZT thin films and further development of piezoMEMS.

Amorphous Boron Nitride: A Universal, Ultrathin Dielectric For 2D Nanoelectronics

Next‐generation nanoelectronics based on 2D materials ideally will require reliable, flexible, transparent, and versatile dielectrics for transistor gate barriers, environmental passivation layers, capacitor spacers, and other device elements. Ultrathin amorphous boron nitride of thicknesses from 2 to 17 nm, described in this work, may offer these attributes, as the material is demonstrated to be universal in structure and stoichiometric chemistry on numerous substrates including flexible polydimethylsiloxane, amorphous silicon dioxide, crystalline Al2O3, other 2D materials including graphene, 2D MoS2, and conducting metals and metal foils. The versatile, large area pulsed laser deposition growth technique is performed at temperatures less than 200 °C and without modifying processing conditions, allowing for seamless integration into 2D device architectures. A device‐scale dielectric constant of 5.9 ± 0.65 at 1 kHz, breakdown voltage of 9.8 ± 1.0 MV cm−1, and bandgap of 4.5 eV were measured for various thicknesses of the ultrathin a‐BN material, representing values higher than previously reported chemical vapor deposited h‐BN and nearing single crystal h‐BN.

Tin oxide-based thin films prepared by pulsed laser deposition for gas sensing

Thin tin dioxide films were prepared by large-area pulsed laser deposition, a ceramic target in oxygen atmosphere was used. The layers were structured on interdigitated electrode substrates using a two-side trenched silicon shadow mask. The influence of the oxygen background pressure during deposition on the electrical and morphological properties was investigated using four-point probe measurements, scanning electron microscopy, X-ray diffraction, and X-ray photo spectroscopy investigations. Pd was introduced into the films by DC magnetron sputtering. It was found that the resistive response to CO is increased for the Pd-doped samples. Cross-sensitivity to the oxidizing gas NO2 was investigated and the sensor showed high signals for this gas at lower temperatures.

Gas Sensing by SnO2 Thin Films Prepared by Large-area Pulsed Laser Deposition

We present resistive gas responses of tin oxide thin films prepared by large-area pulsed laser deposition. The influence of the oxygen background pressure during deposition on the sensor response to CO was investigated. Film properties such as resistance, morphology and gas sensing signal of samples deposited under different conditions were examined. Palladium was introduced by sputtering of thin layers and subsequent annealing to increase the stability and response to CO in humid background. The sensor's responses to 5-50 ppm CO and 0.5-1 ppm of NO2 were investigated at different operation temperatures and levels of relative humidity.

Pulsed laser deposition of functional oxides - towards a transparent electronics

Metal oxides, in particular with transition metals, show strong electronic correlations which determine a huge variety of electronic properties, together with other functionalities. For example, ZnO and Ga2O3 as wide-bandgap semiconductors have a high application potential as transparent functional layers in future oxide electronics [1-2]. Other oxides of current interest are ferrimagnetic spinels of the type MFe2O4 (M=Zn,Co,Ni), see K. Brachwitz et al. Appl. Phys. Lett. 102, 172104 (2013), or highly correlated iridate films, see M. Jenderka et al. Phys. Rev. B 88, 045111 (2013). Furthermore, combinations of ferroelectric and magnetic oxides in multiferroic composites and multilayers show promising magnetoelectric coupling. For the exploratory growth of the above mentioned novel oxides into nm-thin films, pulsed laser deposition (PLD) appears as the method of choice because of its extremely high flexibility in terms of material and growth conditions, high growth rate and excellent structural properties [3]. This talk highlights recent developments of new functional oxides using unique large-area PLD processes running for more than two decades in the lab of the author [3].

Fully Integrated Lambda Sensor Based on Micromachined Platforms and Ytria Stabilized Zirconia Thin Membranes for Oxygen Measurement

Abstract A cross plane potentiometric micro gas sensor has been developed. The device is based on a thin electrolyte membrane deposited on a silicon micromachined platform. This new fabrication route combines industrial clean room batch production processes with the use of nanometric thin films of Ytria Stabilized Zirconia as solid electrolyte. The membrane was deposited by means of large area pulsed laser deposition (PLD). The use of membranes of nanometric thickness (200 nm) can lead to a drastic reduction of the power consumption. On the one hand, the volume of the active part of the sensor is extremely reduced, implying a rapid heating with minimum power. On the other hand, the thin electrolyte has a lower resistance, making possible to reduce the operating temperature and increase the response velocity. This new paradigm opens the way for the fabrication of reliable, low cost, low consumption and rapid gas sensors.

Great enhancements in the thermoelectric power factor of BiSbTe nanostructured films with well-ordered interfaces

An innovative concept of twin-enhanced thermoelectricity was proposed to fundamentally resolve the high electrical resistance while not degrading the phonon scattering of the thermoelectric nanoassemblies. Under this frame, a variety of highly oriented and twinned bismuth antimony telluride (BixSb2-xTe3) nanocrystals were successfully fabricated by a large-area pulsed-laser deposition (PLD) technique on insulated silicon substrates at various deposition temperatures. The significant presence of the nonbasal- and basal-plane twins across the hexagonal BiSbTe nanocrystals, which were experimentally and systematically observed for the first time, evidently contributes to the unusually high electrical conductivity of ~2700 S cm(-1) and the power factor of ~25 μW cm(-1) K(-2) as well as the relatively low thermal conductivity of ~1.1 W m(-1) K(-1) found in these nanostructured films.

Imposed quasi-layer-by-layer homoepitaxial growth of SrTiO 3 films by large area pulsed laser deposition

The homoepitaxial growth of SrTiO 3 (STO) films was investigated by a large-area pulsed laser deposition (PLD), which was in-situ monitored by a high pressure reflective high energy electron diffraction. By combining a conventionally continuous film deposition with a followed interval relaxation, a persistent layer-by-layer (LBL) film growth of more than 100 unit cells STO films was achieved. This interrupted PLD technique could realize persistent LBL film growth at any laser frequency between 1 and 10 Hz and provides an effective way to fabricate high quality complex oxide films on unit cell scale.

Linearity and temperature dependence of large-area processed high-q barium strontium titanate thin-film varactors [Correspondence

Ba(0.6)Sr(0.4)TiO(3) (BST) thin-films with large dielectric tunability as high as 4:1 were obtained using a large-area pulsed laser deposition process, with low loss-tangents below 0.01 at zero-bias and 10 GHz. This paper summarizes experimental results obtained on large-area processed BST thin films on 100-mm-diameter sapphire substrates characterized using a varactor shunt switch test structure. Varactors with 0.25-mumthick BST films exhibited large dielectric tunability, the relative dielectric permittivity at zero bias of 990 tuned to 250 at an electric field of 320 kV/cm. The leakage current through the BST film was below 2 nA up to 6 V dc bias. The quality factor (Q) exceeded 300 at relatively low 6 V dc bias for the BST varactors at 1 GHz. These results confirm that large-area processed BST thin films are ready to compete with semiconductor varactors for commercial applications at RF, microwave, and millimeterwave frequencies.

THICKNESS DEPENDENT PROPERTIES OF BARIUM STRONTIUM TITANATE THIN-FILM VARACTORS

ABSTRACT Ba0.6Sr0.4TiO3 (BST) thin-films with large dielectric tunability as high as 5.8:1 were obtained using a large area pulsed laser deposition process, with low loss-tangents below 0.01 at zero-bias and 10 GHz. This paper summarizes experimental results obtained on large area processed BST thin film varactors. Varactors with 0.6 μm thick BST films exhibited large dielectric tunability, the relative dielectric permittivity at zero bias of 1280 tuned to 220 at 20 V, at an electric field of 333 kV/cm. The leakage current through the BST film was below 1 nA up to 10 V dc bias. The Quality factor (Q) exceeded 500 at relatively low 10 V dc bias for both 0.6 and 1 μm thick BST varactors.

Improved photoluminescence of vertically aligned ZnO nanorods grown onBaSrTiO3 by pulsed laser deposition

ZnO nanorods were grown on a variety of substrates such as Si,SiO2/Si and sapphire in a large-area pulsed laser deposition chamber designed for sensor device fabrication.Processing conditions were optimized to grow ZnO nanorods with or without seed layers. Au, Cr andBaSrTiO3 (BST) seed layers were investigated to compare their effects on the diameter andorientation of ZnO nanorods. ZnO nanorods were observed to align better when grown onsapphire, Cr or BST seed layers as compared to Au or Si layers. The highest qualitynanorods were those grown on BST seed layers, as shown by 4 K photoluminescencedonor-bound-exciton linewidths of only 0.5 meV.

Enhanced random pinning with oxygen annealing in YBCO films prepared by large-areapulsed laser deposition

We investigated the effect of oxygen annealing on the magnetic-field angular-dependent critical currentdensity (Jc (θ))in YBa2Cu3O7−x films deposited by large-area pulsed laser deposition. In an as-grown film,Jc (θ) exhibits a large maximum centered at , and a small one centered at . After oxygen annealing at 425 °C, the peak grew and surpassed the peak. We analyzed the Jc (θ) curves using the anisotropic scaling theory, and the result shows that mostly the peak due to random pinning increased with oxygen annealing. It is shown that thepinning center due to random defects is more sensitive to the oxygen content of the film.

Origin of the thickness dependence of critical current densities in YBCO films prepared by pulsed laser deposition

Introducing porosity allows us to increase film thickness of the YBa2Cu3O7-x (YBCO) films on sapphire substrates without microcracking. However, we have observed that porous films possess relatively low values of Jc, especially in thick film region (> 500 nm). In order to find out the cause of the degradation of Jc, we examined the depth profiles of the magnetic-field angular dependence of critical current density Jc (t,θ) and other properties for YBCO films (t ≤ 1550 nm). The YBCO films were deposited by a large-area pulsed laser deposition system on CeO2-buffered r-cut sapphire. Depth profiles were obtained for YBCO films successively milled with an Ar ion beam irradiation. In the as-grown film, the Jc (θ) exhibits a broad Jc peaks at H ∥ c, and smaller one centered at H ∥ ab. After the milling, the Jc(H ∥ ab) / Jc (H ∥ c) ratio increases with decreasing film thickness. The YBCO (005) peak of x-ray diffraction shifts towards higher angle with decreasing film thickness. The depth profiles reveal that the oxygen deficit is one of the causes of the decrease of the Jc of the large-area PLD YBCO/CeO2/Al2O3 films with increasing film thickness.

Deposition of La 0.8Sr 0.2Cr 0.97V 0.03O 3 and MnCr 2O 4 thin films on ferritic alloy for solid oxide fuel cell application

Single layer dense films of La 0.8Sr 0.2Cr 0.97V 0.03O 3 (LSC) and MnCr 2O 4 with a thickness of 500 nm were deposited on a commercially available ferritic alloy (Crofer 22APU) by large-area Pulsed Laser Deposition. The deposited samples were subsequently oxidized at 1173 K for 500 h in humidified air. The effects of the deposited thin films on the growth rate and the morphology of the oxide were investigated by weight gain measurements as well as by scanning electron microscopy. The growth of the oxide scales was reduced by both coatings, and most effectively by the LSC coating. The overall oxidation resistance and oxidation growth mechanism of the coated samples are discussed.

Large-area pulsed-laser deposition of dielectric and ferroelectric thin films

The authors have developed a large-area pulsed-laser deposition (PLD) technique, suited in particular for depositing high-k dielectric and ferroelectric thin films of high quality on Si wafers of 8in. diameter. By using this technique, 42- and 6-nm-thick HfO2 films were deposited on 8 and 6in. Si wafers at room temperature, respectively. The standard deviations of thickness were 1.3% and 1.6% of their respective means. Cross-sectional transmission-electron microscopy pictures of the 6-nm-thick HfO2 film on Si showed that an interfacial layer between HfO2 and Si was thinner than 1nm. In order to measure the electrical properties, Pt top electrodes were deposited on the HfO2 film and the Al bottom electrode was deposited on the Si. Capacitance-voltage measurements of the Pt∕HfO2∕Si showed that the standard deviation of capacitances was as small as 1.9%. A ferroelectric SrBi2Ta2O9 (SBT) film was also deposited on a 4in. Si wafer at 400°C using the large-area PLD technique. The standard deviation of the SBT film thickness was 1.2% of 94nm, the average thickness. The large-area PLD technique the authors developed can meet the industrial needs for high and uniform quality over large-area high-k dielectric and/or ferroelectric thin films for large-scale integrated circuits.

Comparative studies of nanostructural and morphological evolution ofCeO2 thin films induced by high-temperature annealing

High-temperature annealing performed at∼1000 °C orhigher in an oxidizing atmosphere is a simple technique to significantly modify the as-grown nanostructuresof CeO2 films, which are typically granular with high surface roughness.We evaluated the effect of high-temperature annealing on variousCeO2 films deposited by three deposition methods, namely standard pulsed laser deposition (PLD),large-area pulsed laser deposition, and electron-beam evaporation. High-temperature annealingpromotes high epitaxy and crystallinity, as well as stabilization of the flat (001) surfaces ofCeO2. This is most commonly observed on the annealedCeO2 film grown by standard PLD, which is usually atomically smooth and pore-free. In contrast,CeO2 films grown by large-area PLD and electron-beam evaporation are characterized bysimilarly flat surfaces but are discontinuous due to the presence of pores. Depending on thelength of the annealing time, the pores range from small-sized nanopores with widths of∼30–100 nm anddepths of ∼3–5 nm, up to relatively large-sized pores with widths of∼100–500 nm anddepths of ∼10–20 nm. The differences in the nanostructural and morphological evolution of theCeO2 films are further correlated to their as-grown properties, including grain size, orientation,and possibly the initial porosity of the film.