Dual Beam Pulsed Laser Deposition Research Articles

Preparation of Ag-Cu nanoparticle film using a dual-beam pulsed laser deposition for power electronic packaging

Ag-Cu nanoparticles, integrating the advantages of Ag and Cu, are promising materials for power electronic packaging. In this work, a novel dual-beam pulsed laser deposition method was proposed to prepare an Ag-Cu nanoparticle film with various component ratios and used for die attach at low temperatures. The as-deposited Ag-Cu nanoparticle film was mainly composed of Ag-Cu solid solution, Ag element, and Cu element, and most of the nanoparticles were in the alloying state. The Ag-Cu sintered joint presented a dense microstructure with 10.8% porosity, and the shear strength of Ag-Cu sintered joints could reach 60 MPa at 250 °C. The sintered joint porosity increased as more Cu were added in the Ag-Cu nanoparticle film, resulting in a decrease in the interfacial connection ratio. The fracture mode of sintered joints gradually changed from the sintered layer to the mixed sintered layer and interface fracture. The dual-beam pulsed laser deposition method could guide in designing the component ratios of bimetallic nanoparticles.

Periodic DLC Interlayer-Functionalized Bi-Sb-Te-Based Nanostructures: A Novel Concept for Building Heterogenized Superarchitectures with Enhanced Thermoelectric Performance.

According to the innovative design concept of omnidirectional quasi-order hetero-nanocomposites proposed for potentially realizing high thermoelectric performance, a series of superarchitectures consisting of longitudinally periodic diamond-like carbon (DLC) interlayers in latitudinally well-aligned Bi-Sb-Te (BST)-based nanostructures were successfully demonstrated for the first time using dual-beam pulsed laser deposition. This work confirmed that the periodic appearance of DLC is a practical approach to instantly resetting the BST deposition into another new crystal growth cycle. The optimized Seebeck coefficient of ∼500 μV K-1 and the corresponding power factor of ∼40 μW cm-1 K-2 achieved are comparable to or higher than the reported values for BST or BST-based nanocomposites, which evidently originated from the periodically added DLC, as clarified in the Pisarenko plot. In addition, the DLC additives effectively reduce the thermal transport as qualitatively evidenced by micro-Raman characterizations.

Control of Lateral Composition Distribution in Graded Films of Soluble Solid Systems A1−xBx by Partitioned Dual-Beam Pulsed Laser Deposition

Lateral compositionally-graded thin films are powerful media for the observation of phase boundaries as well as for high-throughput materials exploration. We herein propose a method to prepare epitaxial lateral compositionally-graded films using a dual-beam pulsed laser deposition (PLD) method with two targets separated by a partition. Tuning the ambient pressure and the partition—substrate gap makes it possible to control of the gradient length of the deposits at the small sizes (≤ 10 mm) suitable for commercial oxide single crystal substrates. A simple Monte Carlo simulation qualitatively reproduced the characteristic features of the lateral thickness distribution. To demonstrate this method, we prepared (1−x)PbTiO3—xPbZrO3 and (1−x)LaMnO3—xLa0.6Sr0.4MnO3 films with lateral composition gradient widths of 10 and 1 mm, respectively, with the partitioned dual PLD.

Fabrication of a compound infrared microlens array with ultrashort focal length using femtosecond laser-assisted wet etching and dual-beam pulsed laser deposition.

We present the design, fabrication, and characterization of a compound infrared microlens array having an ultrashort focal length and a hyperbolic profile that comprises a PDMS microlens array and a GRIN lens. A concave microlens mold was first fabricated on a fused silica substrate using femtosecond laser irradiation followed by a wet etching process, and a standard replication process was employed to fabricate a PDMS convex lens array. To shorten the focal length further and cut off the visible spectrum of light, a graded DLC/silicon coating, which functioned as a GRIN lens and a visible light cut-off filter, was additionally deposited using dual-beam pulsed laser deposition. The lenslet diameter was 6 µm and the graded coating reduced the focal length from 4.5 to 2.9 µm.

Fabrication of functionally-graded yttria-stabilized zirconia coatings by 355 nm picosecond dual-beam pulsed laser deposition

This article presents the design, fabrication, and characterization of functionally-graded yttria-stabilized zirconia (YSZ) coatings by using a novel dual-beam pulsed laser deposition method based on a 355 nm ps laser. Graded coatings were fabricated by mixing YSZ and the substrate material (stainless steel). The two materials were ablated and deposited on a substrate inside a vacuum chamber, in a way that the mixture composition gradually changes from stainless steel to YSZ, along the coating's growth direction, following a predesigned linear composition profile. The Rockwell indentation method was employed for the adhesion strength analysis based on the interfacial crack radius (c) and indentation load (P). When a load P is applied on the sample surface, a crack with a radius c will be formed. Hence, by using more than one loads and measuring the generated crack diameters, dP/dc can be estimated, which is considered as a critical parameter and is proportional to the coating's adhesion strength. The graded coating was found to provide improved adhesion strength (dP/dc = 9.41 N/μm) over pure YSZ coatings (dP/dc = 8.35 N/μm and 8.43 N/μm). In this study, the thermal barrier effects of the coatings were examined by measuring the thermal conductivity values of the coatings using a high-power multi-mode fiber laser as an impulsive heat source and an infrared camera, and the deposited YSZ coatings showed a low thermal conductivity (∼1.14 W/m·K).

Amorphous carbon nanocomposite films doped by titanium: Surface and sub-surface composition and bonding

Hydrogen free Ti-doped amorphous carbon layers were prepared by dual beam pulsed laser deposition using two excimer lasers. The air-exposed surfaces were analyzed by high-energy resolved and angular-resolved core-level photoelectron spectroscopy, and were then step-by-step sputtered with an argon gas cluster ion beam (ArGCIB), which is known to be a very gentle technique with respect to changes in surface chemistry. The results show that the top surface of the sample and its sub-surface region differ in composition and in bonding. The top surface is enriched by oxygen-bearing species. Carbon-bearing species located on the surface are mostly in sp3 hybridization. Titanium carbide clusters, TiC, are not directly exposed at the surface. They are embedded in a carbon network with dominating C sp2 hybridization. Their interface is formed by a distinct carbon-rich titanium carbide with stoichiometry close to TiC3. The surface damage induced by ArGCIB was found to be minimal, verifiably affecting carbon atoms in the carbon network.

Pulsed Laser Deposition of BiCl3 Doped Bi2S3/Cu Nanoparticle Composite Films for Thermoelectric Applications

Thermoelectric (TE) materials capable of directly converting heat into electricity have become a highly potential category in energy materials and are expected to tackle the energy issue of the future. Among a variety of TE materials, lead telluride (PbTe) has been recognized as an outstanding mid-temperature TE material with an applicable TE figure of merit (ZT) of ~1.2. However, due to the toxicity of both Pb and Te, development of alternative mid-temperature TE materials is urgently required. Bismuth sulfide (Bi2S3) is specially selected in the present study as the most promising candidate for alternating PbTe mainly because of its potentially high Seebeck coefficient (102−103 μVK-1). Unfortunately, the generally poor electrical conductivity (<102 S/cm) and the resulting low ZT (~0.2) of Bi2S3 severely restrict its applicability. In this work, by controlling the substrate temperature, the volatility of sulfur would lead the compositional deviation from the chemical stoichiometry of Bi2S3 and the electrical conductivity of the sulfur-vacancy introduced Bi2S3-x would thus be improved. Similarly, BiCl3 was also considered and applied as a donor dopant to improve the electrical conductivity. Furthermore, a series of novel BiCl3 doped Bi2S3-Cu nanoparticles layer-by-layer composite films were designed and fabricated through the newly built dual-beam pulsed laser deposition for improving the electric conductivity without seriously sacrificing the Seebeck coefficient. The prepared highly crystalline intrinsic Bi2S3 films on insulated SiO2/Si substrates exhibit a very high electric conductivity of 638 Scm-1 evidently due to the simultaneously enhanced carrier concentration and mobility, and the Seebeck coefficient of - 453 μVK-1. The resulting power factor is 137 μWcm-1K-1 which is four times higher than the reported highest value (29 μWcm-1K-1) from the chemically prepared Bi2S3 so far. The introduction of BiCl3 into Bi2S3 indeed leads to an enhancement of the electrical conductivity from 170 Scm-1 (before BiCl3 doping) to 450 Scm-1 whereas the Seebeck coefficient decreases from -522 μVK-1 to -221 μVK-1. On the sake of enhancing the electricity conductivity and Seebeck coefficient simultaneously, the Cu nanoparticles were introduced into the BiCl3 doped Bi2S3 film. The heterostructure of Cl doped Bi2S3/Cu played an important role in obtaining the high electrical conductivity of 715 Scm-1, Seebeck coefficient of -413 μVK-1 and the power factor of 122 μWcm-1K-1 which is twice higher than those of the intrinsic Bi2S3 film (49 μWcm-1K-1).

Dual laser deposition of Ti:DLC composite for implants

Ti-doped hydrogen free diamond-like carbon (DLC) layers of dopation up to ~25 at.% were prepared by dual beam pulsed laser deposition (PLD) using two excimer lasers. The arrangement allows continuous fine tuning of dopant concentration on a large scale and deposition flexibility. The layers were prepared on Si(1 0 0) and Ti6Al4V substrates at room temperature. The surface morphology, mechanical properties, bonds, composition, morphology of human osteoblast-like Saos-2 cells, their metabolic activity and production of osteocalcin, a marker of osteogenic cells’ differentiation were tested. The films’ composition changed after x-ray photoelectron spectroscopy (XPS) surface cleaning by argon clusters. Adhesion moved with Ti dopation from 4 N (DLC film) to 11 N (25 at.% of Ti in DLC). Creation of TiC was observed for higher Ti dopation. The contact angle and surface free energy stayed unchanged for higher Ti dopation. Saos-2 cells had the highest metabolic activity/viability on DLC with 10 at.% of Ti and on control polystyrene dishes on days 1 and 3. The Ti dopation improved the formation of vinculin-containing focal adhesion plaques in Saos-2 cells. Immunofluorescence staining revealed similar production of osteocalcin in cells on all tested samples.

Pulsed laser deposition of refractive-index-graded broadband antireflection coatings for silicon solar cells

This paper reports the design, fabrication and characterization of broadband antireflection coatings for silicon-based solar cells, using a graded-index technique and a novel dual-beam pulsed laser deposition method. A graded refractive index was realized by mixing two different optical materials, silicon and soda-lime glass, in a way that the composition of the mixture changes gradually from silicon (that of the solar cell) to glass (that of the outside protective glass) along the coating thickness direction following pre-designed refractive-index profiles. In this study, two different refractive-index profiles were fabricated, linear and Southwell profiles, on silicon substrates. In order to emulate a protective cover glass for solar-cells, coatings with an additional 400nm-thick outer glass layer were also fabricated. The antireflection coatings showed reflectance of 2.2–5.5% (mostly around 4%) for a wavelength range of 400–1000nm, and for the Southwell profile with a top glass layer, the reflectance was found to be between 2.2% and 4%, which is believed to be close to the theoretically best antireflection performance of a solar cell with a cover glass. Finite-difference time-domain simulations were also performed and the numerical results were in line with the measurement results. Because these coatings are based on silicon and glass and can be fabricated by the one-step pulsed laser deposition procedure, it can simplify the solar-cell structure considerably. The fabricated antireflection coatings may be also used for many other optical applications, such as thin-film optical attenuator and some optical sensors, and the presented pulsed laser deposition method is capable of fabricating a variety of high-performance optical coatings, which cannot be fabricated by other methods.

Epitaxial composition-graded perovskite films grown by a dual-beam pulsed laser deposition method

We prepared SrTiO3 (STO) to Ba0.6Sr0.4TiO3 (BST06) out-of-plane composition-graded films on STO (100) substrates by means of a dual-beam dual-target pulsed laser deposition technique. In the deposition system, a sliding mirror divides one KrF excimer laser beam into two, realizing the dual-beam of controlled intensity ratio. X-ray diffraction reciprocal space mapping has revealed that the graded films deposited under oxygen pressure at or lower than 1×10−3mbar were coherently strained with the same in-plane lattice parameter as the substrate. Their composition gradient along the growth direction was confirmed by Rutherford backscattering analysis to be uniform. We deposited BST06 top layers of various thickness on epitaxial composition-graded (ECG) buffer layers and examined their coherency and crystallinity. In comparison with the cases of STO homoepitaxial buffer layers, ECG buffer layers achieved better crystallinity of top BST06 layers, suggesting that the crystallinity of a heteroepitaxially-grown film is affected not only by the in-plane lattice matching but also by the out-of-plane lattice continuity with the substrate. ECG films that bridge compositions of substrate and top layer materials can be useful buffer layers for epitaxial growth of lattice-mismatched oxide films.

Comparative studies on Zn0.95Co0.05O thin films on C- and R-sapphire substrates

Zn0.95Co0.05O precipitate-free single crystal thin films were synthesized by a dual beam pulsed laser deposition method. The films form a wurtzite structure whose hexagonal axis is perpendicular or parallel to the plane of the surface depending on the C-plane (0001) or R-plane (112̄0) sapphire substrate. Based on the results of high-resolution transmission electron microscopy and x-ray diffraction, C-plane films show larger lattice mismatch. The films exhibit magnetic and semiconductor properties at room temperature. The coercivity of the film is about 8000 A/m at room temperature. They are soft magnetic materials with small remanent squareness S for both crystal orientations. There is no evidence to show that the anisotropy is fixed to the hexagonal axis (C-axis) for the wurtzite structure.

Analysis of magnetic mechanisms of 3d-doped ZnO diluted magnetic semiconductors by an abnormal peak on M-T curve

The crystallographic structures and magnetic properties of a Zn0.95Co0.05O thin film deposited on a C-sapphire substrate using a dual-beam pulsed laser deposition method were characterized. It was shown from crystallographic analysis that the film belongs to the wurtzite structure with the C-axis aligned with that of the substrate. Magnetic hysteresis loops were observed till up to room temperature. A small peak around 55 K was noticed on the magnetization vs. temperature curve. The corresponding temperature of the small peak is close to that of ‘the abnormal peak’ reported by X.M. Zhang et al. From the results obtained, no correlation was found between the abnormal peak and the quantum effects. The magnetic behaviors in the Zn0.95Co0.05O film cannot be explained by the ferromagnetism in diluted magnetic semiconductors. The magnetic mechanisms in ZnO-based diluted magnetic semiconductors are also discussed.

Synthesis and characterization of ferromagnetic cobalt-doped tin dioxide thin films

Co-doped SnO 2 thin films are grown on sapphire (0001) substrates at 600 °C by the technique of dual-beam pulsed laser deposition. The prepared films show preferred orientation in the [100] direction of the rutile structure of SnO 2. Nonequilibrium film growth process results in doping Co into SnO 2 much above the thermal equilibrium limit. A Film with 3% of Co is ferromagnetic at room temperature with a remanent magnetization of ∼ 26% and a coercivity of ∼ 9.0 mT. As Co doping content x increases, the optical band gap absorption edge ( E 0) of the Co-doped SnO 2 thin films initially shows a redshift at low x up to x = 0.12 and then increases at the higher x, which are attributed to the sp–d exchange interactions and alloying effects, respectively.

Growth and characterization of dual-beam pulsed-laser-deposited Zn1−xCoxO thin films

Dual beam pulsed laser deposition was used to synthesize Zn1−xCoxO thin films with a large range of Co concentrations. In this paper, the influence of growth temperature on the crystal structures, magnetic properties, and semiconductor properties of these films was studied. It was found that the growth temperature has a significant effect on dopant concentration, as well as the structure and properties of the films. The optimum growth temperature is around 650°C. The thin films were of single crystal especially at low Co concentrations. Epitaxial growth of the thin films was observed. No precipitates were observed at low Co concentrations. Magnetic hysteresis loops were observed at room temperature, indicating that magnetism can be realized with Co doped into ZnO. However, the improvement was limited. The Hall resistivity tended to increase, while carrier density tended to decrease with Co concentration until a certain Co concentration (10%) is achieved. When the growth temperature was greater than 650°C, the Hall resistivity decreased, while carrier density increased. The Hall mobility increased with growth temperature. The possible reasons of the observed magnetism and semiconductor properties were also discussed.

Room temperature diluted magnetic semiconductor synthesized by dual beam laser deposition

Diluted magnetic semiconductor Co-doped ZnO film has been synthesized by a dual beam pulsed laser deposition method. The magnetic, electrical, and optical properties of the Zn0.985Co0.015O0.67 film are studied in this paper. The film shows ferromagnetic behavior with a coercivity of about 300 Oe at room temperature, and semiconductor behavior with carrier concentration of 2.2×1018 cm-3, and a resistivity of 102 mΩ cm. Structural investigations indicated that the film has similar lattice constants to that of ZnO. It is shown that the film exhibits excellent optical properties with a band gap energy of 3.31 eV, which is close to that of ZnO. The origins of the magnetism are also discussed.

Structural and Optical Properties of Co-Doped ZnO Thin Films

Received 20 August 2004; accepted 29 September 2004 Co-doped zinc oxide thin films (Zn1−xCoxO) have been deposited on c-plane sapphire substrates by dual-beam pulsed laser deposition. The films have lattice parameters similar to that of ZnO, and the lattice parameters are closely distributed. The films grew along a preferred direction, following the epitaxial relationship Zn1−xCoxO (0001)//substrate (0001). Excitonic emission was suppressedat higher Co-dopant concentration in ZnO because of increase in the distortion of host lattice and defects. When more Zn is replaced by Co, more impurity levels are developed within the bandgap, and more defect are generated. Under our experimental conditions, the bandgap of the films tends to increase with increasing dopant concentration.

Effect of Ga Re-evaporation on AlxGa1-xN Thin Film Growth by Dual-Beam Pulsed Laser Deposition Method in N2 Ambient

Ternary compound AlxGa1-xN thin films were grown on sapphire substrates by the dual-beam pulsed laser deposition (PLD) method in N2 atmosphere. Due to Ga re-evaporation, a compositional change of the film developed with change of the growth temperature. The activation energy of Ga re-evaporation was estimated to be 1–2 eV which was about 1 eV lower than that so far obtained for molecular-beam-epitaxy.

Enhanced magnetoresistance in Ag-doped granular La2/3Sr1/3MnO3 thin films prepared by dual-beam pulsed-laser deposition

In this article, we report our investigations on the doping effect metallic Ag on the intergrain magnetoresistance phenomenon. La2/3Sr1/3MnO3 (LSMO) thin films with Ag doping were prepared on (001) LaAlO3 at different substrate temperatures (Ts) by the dual-beam pulsed-laser deposition technique. Grown at Ts=750 °C, the films are perfectly epitaxial with their c axis perpendicular to the film surface. Ag dopant cannot substitute into the LSMO lattice, thus showing no obvious effect on the magnetotransport properties of the film, though it did impair the film in-plane epitaxy and improve the intergrain diffusion of the lattice atoms. However, grown at lower Ts(400 °C), the films are granular with c-axis texture. The Ag dopant exists at GBs and helps to increase the local Mn spin disorder at GBs and the phase interfaces, thus enhancing the eMR value by a factor of 2 compared with the undoped film. Experiment data also suggest that the transport mechanism underlying the Ag-doping enhanced eMR is spin-dependent scattering.

The growth mode and microstructure of Ag-doped YBa 2Cu 3O 7−δ thin films prepared by dual beam pulsed-laser deposition

Ag-doped c-axis YBa 2Cu 3O 7−δ thin films were fabricated by using dual-beam pulsed-laser deposition. When a small amount of silver was doped in the film grown at 700 °C, the J c value was obviously enhanced. The temperature dependence of J c was found proportional to ( 1- T/T c0 ) 3 2 at T close to T c). In the films grown at 730 °C, we observed long bar-like structures with lengths in tens of μm, oriented along or at 45° to the a/ b axes of the film. The bars consisted mainly a-axis YBCO grains and a mixture of polycrystalline and amorphous oxides. Defects on substrate surface and growth temperature were found dominating in formation of the bars, which could be attributed to a Ag-assisted diffusion mechanism during the deposition process.

The distribution of Ag in Ag-doped YBa2Cu3O7- thin film prepared by dual-beampulsed-laser deposition

<P>The Ag distribution in Ag-doped YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-<IMG SRC="common/delta.gif" ALT="delta" ALIGN="TOP"/></SUB> (YBCO) thin films fabricated by dual-beam pulsed-laser deposition on SrTiO<SUB>3</SUB> (100) substrates has been studied by Auger electron spectroscopy, microproton-induced x-ray emission, atomic force microscopy and scanning electron microscopy. All the results consistently show that Ag aggregated in the bar-like structures observed in the film. These bars are aligned along the <I>a</I>-<I>b</I>-axis or at 45° to the <I>a</I>-<I>b</I>-axis of the YBCO thin film. The main body of the long bars aligned with the <I>a</I>-<I>b</I>-axes of the film was found to be a combination of metallic Ag with other precipitates of YBCO film that may grow from the substrate surface to the YBCO film surface. There were other precipitates aggregating as well at the surface of these bars, such as oxides of Cu and Ba. The short bars that aligned along 45° to the <I>a</I>-<I>b</I>-axes of the film were found to be deficient in Ag but rich in Cu, Ba and O, which could be oxide precipitates of YBCO. The growth mechanisms of the two types of bars seem quite different.</P>