Combinatorial Pulsed Laser Deposition Research Articles

Demonstration of Two Multi‐Component Target Ablation Approaches and Their Application in Combinatorial Pulsed Laser Deposition

AbstractCombinatorial pulsed laser deposition (C‐PLD) based on segmented targets has led to new possibilities in the pace of discovery of novel advanced materials with significantly reduced deposition time and material consumption. However, fabrication of established segmented targets may be complex or not even possible for certain material combinations. In this article, two alternative C‐PLD techniques based on easy‐to‐fabricate segmented targets are presented. One approach uses two semi‐circular segments A and B with a systematic adjustable lateral shift of the target rotation axis from the target center. The other approach is based on a new target design defined by an ABA‐segmentation where B is a horizontal bar between two semi‐circular segments of A. Both approaches enable growth of discrete composition libraries. The concepts and important parameters are introduced and computer simulations as function of the geometric parameters are made to yield the expected thin film compositions. As proof‐of‐concept, the techniques are employed on the transparent, semiconducting ternary alloy zinc‐tin‐oxide. The simulations are in very good agreement with the experimental data. Physical properties of films grown by the demonstrated approaches are compared with those obtained by established PLD processes.

Mn Additive Improves Zr Grain Boundary Diffusion for Sintering of a Y-Doped BaZrO3 Proton Conductor.

Yttrium-doped barium zirconate (BZY) has garnered attention as a protonic conductor in intermediate-temperature electrolysis and fuel cells due to its high bulk proton conductivity and excellent chemical stability. However, the performance of BZY can be further enhanced by reducing the concentration and resistance of grain boundaries. In this study, we investigate the impact of manganese (Mn) additives on the sinterability and proton conductivity of Y-doped BaZrO3 (BZY). By employing a combinatorial pulsed laser deposition (PLD) technique, we synthesized BZY thin films with varying Mn concentrations and sintering temperatures. Our results revealed a significant enhancement in sinterability as Mn concentrations increased, leading to larger grain sizes and lower grain boundary concentrations. These improvements can be attributed to the elevated grain boundary diffusion of zirconium (Zr) cations, which enhances material densification. We also observed a reduction in Goldschmidt's tolerance factor with increased Mn substitution, which can improve proton transport. The high proton conduction of BZY with Mn additives in low-temperature and wet hydrogen environments makes it a promising candidate for protonic ceramic electrolysis cells and fuel cells. Our findings not only advance the understanding of Mn additives in BZY materials but also demonstrate a high-throughput combinatorial thin film approach to select additives for other perovskite materials with importance in mass and charge transport applications.

Accelerating the combinatorial optimization process for phosphor materials by Bayesian optimization

Abstract We investigated the acceleration of the combinatorial optimization process for phosphor materials using a machine learning method based on Bayesian optimization. Combinatorial pulsed laser deposition can be used to create a library of single-crystalline films with varying chemical compositions. However, the systematic evaluation of the target functional properties requires a long measurement time, impairing rapid material screening. In this study, Bayesian optimization was applied to sequential measurements of the photoluminescence (PL) properties of EuxY2−xO3 films to accelerate the combinatorial high-throughput evaluation. Although a conventional combinatorial PL evaluation of a binary composition-gradient film is composed of a sequential measurement of 80 points, the autonomous PL mapping technique based on Bayesian optimization drastically reduced the measurement points to only six points, demonstrating that the optimum chemical composition can be identified in a shorter experimental time.

Combinatorial Growth of Vertically Aligned Nanocomposite Thin Films for Accelerated Exploration in Composition Variation

Combinatorial growth is capable of creating a compositional gradient for thin film materials and thus has been adopted to explore composition variation mostly for metallic alloy thin films and some dopant concentrations for ceramic thin films. This study uses a combinatorial pulsed laser deposition method to successfully fabricate two‐phase oxide–oxide vertically aligned nanocomposite (VAN) thin films of La0.7Sr0.3MnO3 (LSMO)‐NiO with variable composition across the film area. The LSMO‐NiO compositional gradient across the film alters the two‐phase morphology of the VAN through varying nanopillar size and density. Additionally, the magnetic anisotropy and magnetoresistance properties of the nanocomposite thin films increase with increasing NiO composition. This demonstration of a combinatorial method for VAN growth can increase the efficiency of nanocomposite thin film research by allowing all possible compositions of thin film materials to be explored in a single deposition.

Combinatorial tuning of structure and optoelectronic properties of Zn-Ga-O thin films for deep ultraviolet photodetection

Ternary complex oxides exhibit strong compositional dependence on their structure and functionalities. While the profound impact of the cation stoichiometry on the physical properties of ZnGa2O4 is widely recognized, how it affects the ultraviolet photodetection behavior remains not clear. Here we apply a combinatorial pulsed laser deposition technique to establish graded composition in ZnxGa1-xO (0 ≤ x ≤ 1) across a 2-inch sapphire substrate. In combination with structural and optical characterizations, a systematic investigation of the compositional effect on the phase, crystallinity and bandgap for the ZnxGa1-xO system is performed, and their further impact on the photodetection performance is compared by fabricating multiple metal–semiconductor-metal (MSM) photodetectors based on the graded ZnxGa1-xO films. Decent photodetection behavior is seen for the photodetectors with x ≤ 0.33, above which the performance is largely limited by the conductive phase formation. An optimized compositional range of x = 0.18–0.27 is identified to deliver the best device performance, which includes a responsivity of 12.88 A/W and a photo-to-dark current ratio of 2.8 × 108. This slight zinc deficiency is considered beneficial for the formation of moderate level of oxygen vacancies or antisite defects that boosts carrier transport. This study sheds light on the composition-dependent evolution of structure and properties in ternary ultrawide-bandgap semiconductors for high-performance ultraviolet photodetection.

Structural Defect Effect on the Concentration Quenching of TbxY2–xO3 Phosphor Thin Films

This study investigates the structural defect effect on the luminescence properties of TbxY2–xO3 (x = 0–0.5) composition-gradient films using combinatorial pulsed laser deposition (PLD). TbxY2–xO3 films were grown on a SrTiO3 (001) substrate in an O2/He mixed atmosphere at PO2 = 1 × 10–4 Torr. A He buffer gas moderated the kinetic energy of the PLD plume and suppressed the resputtering of the film surface, resulting in the fabrication of defect-rich and poor TbxY2–xO3 films. The TbxY2–xO3 composition-gradient films exhibited varying green emissions from the Tb-doped Y2O3 films. The luminescence intensity of the defect-rich film was significantly weaker than that of the defect-poor film. Furthermore, concentration quenching of the luminescence occurred at high Tb concentrations. The analysis of the decay profiles of the luminescence as a function of the Tb concentration suggested two possible reasons for luminescence quenching: dynamic quenching induced by structural defects and static quenching induced by the shorter average distance of adjacent Tb ions. The investigation of the structural defect effect on the luminescence properties provides insights into the primary material parameters of phosphor materials and offers a pathway for enhancing their luminescence properties.

Improved methods for design of PLD and combinatorial PLD films

Pulsed laser deposition (PLD) is a powerful technique for prototyping thin film materials, both single component (single composition) films and films with a varying composition (e.g., lateral continuous compositional spread, CCS). In this work, we improve one of the simulation methods used to design the deposition of PLD films: We extend the mathematical model for the material spread on the substrate, T1(x,y), for each laser pulse hitting the target, and we use a more accurate method to determine T1(x,y) experimentally. The deposition of the material on the substrate is simulated by repetitively adding T1(x,y), from one or more targets, at the selected location on the substrate. Using the new model, a high agreement between the simulated and grown films’ thickness and composition across the substrate was obtained. The basis for the high agreement is the use of variable angle spectroscopic ellipsometry to carefully determine T1(x,y) by measuring at 794 locations on the 50.8 mm (2 in.) diameter substrates. Factors, such as variation in optical properties and porosity across the plume/calibration films, were considered in the determination of the thicknesses. As test cases, we simulated and deposited (single component) TiO2 thin films and (CCS) TiO2 films doped with Cr and N, deposited on 50.8 mm diameter Si wafers. The modeling and simulations are implemented in an open-source Python library, pyPLD.

Mg substitution effect on the electron affinity of ZnO films

We investigated the effect of Mg doping on the electron affinity of ZnO thin films. MgxZn1−xO (x = 0–0.29) composition-gradient films were deposited on an α-Al2O3(001) substrate using combinatorial pulsed laser deposition. The combinatorial high-throughput analysis of the optical transmittance systematically revealed that the bandgap of ZnO films was tunable between 3.3 and 4.0 eV by doping the ZnO thin films with Mg2+ ions. To investigate the electronic structure, photoelectron yield spectroscopy measurements were performed on the MgxZn1−xO composition-gradient films. The ionization potential, which denotes the distance between the valence band maximum and vacuum level, was independent of the Mg content in the ZnO films. By comparing with the optical bandgap results, the electron affinity was tunable from 4.1 to 3.5 eV by the Mg content in the MgxZn1−xO films.

Electrical property modulation of Au/Ba0.6Sr0.4TiO3/La0.7Sr0.3MnO3 structure by continuous composition spread Mn doping

Composition spread Mn-doped Ba0.6Sr0.4Ti1-xMnxO3 (BSTMx, 0 ≤x ≤ 0.05) thin films were prepared on La0.7Sr0.3MnO3/SrTiO3 (LSMO/STO) layer by combinatorial pulsed laser deposition. When x is less than 3.3%, the BSTMx films are single phase according to X-ray diffraction data, and the Au/BSTMx/LSMO structure presents asymmetric transport properties. The asymmetry of the transport property gets weak with increasing Mn doping concentration. The reverse-bias current density of the Au/BSTMx/LSMO structure remains at a low value of 10−7 A/cm2, and the forward-bias current density reduces rapidly with the ascending doping content (x ≤ 1.8%). Further increasing the Mn content, the differences between the positive and reverse current densities become smaller, and almost vanish when the Mn content is greater than 3.8%, indicating the disappearance of the asymmetry of the transport property. The effects of the LSMO electrode, Au/BSTMx Schottky junction, BSTMx thin film, and BSTMx/LSMO interface on the whole electrical transport properties are discussed by virtue of the experimental and fitted impedance spectrums. Our results are helpful for insight into the doping mechanisms in perovskite oxides.

Evidence of Ba-rich surface segregation in Ba1-xSrxTiO3 and Ba-rich surfactant in SrTiO3/ Ba1-xSrxTiO3 stacks grown by combinatorial pulsed laser deposition

The interface of a La0.7Sr0.3MnO3/SrTiO3 bilayer was modulated by introducing 3 unit cells of Ba1-xSrxTiO3 using Combinatorial Pulsed Laser Deposition. A wide range of chemical compositions was studied within the same sample, with BSTx stoichiometry variable from 0.5 to 1 along Y-axis, while the SrTiO3 overlayer thickness was modified along the X direction [Fig. 1(a)]. We performed high-resolution, laboratory-based angle-resolved XPS studies of the BSTx film surface providing information on the thickness and composition of the surface and sub-surface layers. Based on the attenuation of the La 3d corelevel photoemission signal from the La0.7Sr0.3MnO3 bottom layer, the BST layer is 1.2 nm thick. XPS Ba 3d5/2 core-level spectra were acquired at positions corresponding to different nominal Ba/Sr stoichiometry. In all measurements, the Ba 3d5/2 core-level spectra can be represented by two main components, i.e. one component at higher binding energy (BE = 780.54 eV) corresponding to surface contribution and the other one at lower binding energy (BE = 778.92 eV) corresponding to sub-surface contribution (Figs. 2 and 3). Going from normal to 60° emission angle and using a 3-unit cell thick film model, the surface to sub-surface intensity ratio clearly evolves providing evidence of a Ba-rich surfactant. The surfactant effect is more significant for lower nominal Ba stoichiometry.

Strain states and relaxation for alpha-(Al_xGa_{1-x})_2O_3 thin films on prismatic planes of alpha-Al_2O_3 in the full composition range: Fundamental difference of a- and m-epitaxial planes in the manifestation of shear strain and lattice tilt

Pseudomorphic and relaxed alpha-(Al_xGa_{1-x})_2O_3 thin films are grown by combinatorial pulsed laser deposition in the entire composition range on prismatic a- and m-plane alpha-Al_2O_3 substrates. Pseudomorphic growth on m-plane sapphire has been achieved for x ge 0.45. A distinct difference between the a- and m-epitaxial plane is observed in reciprocal space map measurements being in agreement with continuum elasticity theory for rhombohedral heterostructures. While pseudomorphic layers on m-plane sapphire show a pronounced shear strain e'_5 along the c-axis direction, relaxed layers exhibit a global lattice tilt in the same direction. Both effects are not present on the a-epitaxial plane. Out-of-plane lattice constants as well as e'_5 are modeled as function of x employing elasticity theory, confirming theoretical values of the elastic stiffness tensor for alpha-Ga_2O_3, especially the non-zero value of the C_{14} component. Possible pyramidal slip systems for strain relaxation in c-axis direction are examined to explain and numerically model the difference in lattice tilt for the two substrate orientations.Graphic abstract

Tunability Investigation in the BaTiO3-CaTiO3-BaZrO3 Phase Diagram Using a Refined Combinatorial Thin Film Approach

Tunable ferroelectric capacitors, which exhibit a decrease in the dielectric permittivity under an electric field, are widely used in electronics for RF tunable applications. Current devices use barium strontium titanate (BST) as the tunable dielectric, but new applications call for tunable materials with specific performance improvements. It is then of crucial importance to dispose of a large panel of electrically characterized materials to identify the most suited compound for a given set of device specifications. Here, we report on the dielectric tuning properties of Ba1−xCaxTi1−yZryO3 (BCTZ) thin films libraries (0 ≤ x ≤ 30% and 0 ≤ y ≤ 28.5%) synthesized by combinatorial pulsed laser deposition (CPLD). An original CPLD approach allowing reliable and statistical ternary phase diagrams exploration is reported. The effects of Ca and Zr content on tunability, breakdown voltage and dielectric losses are explicated and shown to be beneficial up to a certain amount. Compounds close to (Ba0.84Ca0.16)(Ti0.8Zr0.2)O3 exhibit the highest figures of merit, while a zone with compositions around (Ba0.91Ca0.09)(Ti0.81Zr0.19)O3 show the best compromise between tuning ratio and figure of merit. These results highlight the potential of BCTZ thin films for electrically tunable applications.

Combinatorial screening of the crystal structure in Ba–Sr–Mn–Ce perovskite oxides with ABO3stoichiometry

High throughput experimental screening using combinatorial pulsed laser deposition identifies BaCexMn1−xO3–Sr1−xCexMnO3pseudo-binary oxide alloys with tetragonal perovskite structure and broad composition–temperature range of stability.

Structural and Elastic Properties of α‐(AlxGa1−x)2O3 Thin Films on (11.0) Al2O3 Substrates for the Entire Composition Range

Structural properties of rhombohedral α‐(AlxGa1−x)2O3 thin films grown by two combinatorial pulsed laser deposition (PLD) techniques are investigated for the entire composition range. One α‐(AlxGa1−x)2O3 thin film is deposited on a 2 inch in diameter large a‐plane sapphire substrate using the continuous composition spread (CCS) PLD technique to fabricate a thin film with varying Al content ranging between x = 0.13 and x = 0.84. Laterally homogeneous α‐(AlxGa1−x)2O3 thin films exhibiting discrete Al contents are fabricated using radially segmented PLD targets on (11.0) Al2O3. Independent of the PLD technique, for x ≈ 0.55, a change from relaxed to pseudomorphic growth is observed as confirmed by the evolution of in‐ and out‐of‐plane lattice constants. The crystal structure is studied depending on the cation composition by X‐ray diffraction confirming the fabrication of epitaxial, corundum‐structured thin films.

Bandgap tuning and optimization of green-emitting Zn2SnO4-Mg2SnO4:Mn2+ using combinatorial pulsed laser deposition

We investigated a compositional spread Mn2+-doped Zn2SnO4-Mg2SnO4 (Mn-ZMTO) film prepared by combinatorial pulsed laser deposition. The composition of the prepared film gradually changed from Zn2SnO4 (ZTO) to Mg2SnO4 (MTO). Moreover, the lattice constant decreased with an increase in MTO composition. Under ultraviolet irradiation, the film exhibited bright green luminescence attributable to the transition from 4T1 to 6A1 of Mn2+. This is the report of the observation of green luminescence from Mn-ZMTO on the ZTO-rich side. In addition, the most intense luminescence was located at the center of the sample. The emission intensity and the most intense position were changed by irradiation at various excitation wavelengths. This was clarified by the positional dependence of PLE spectra. The bandgap of the prepared film, which corresponded to the PLE peak wavelength, was larger than that of the bulk material and increased with an increase in Mg concentration. Since emission cannot usually be observed from Mn-doped ZTO, we suggest that the luminescence from the prepared film was caused by an increase in the bandgap attributable to the 1-dimentional thin film size effect along thickness direction. Thus, using the thin film technique we revealed that bandgap tuning is important in phosphor material.

Solubility limit and material properties of a κ-(AlxGa1−x)2O3 thin film with a lateral cation gradient on (00.1)Al2O3 by tin-assisted PLD

A ternary, orthorhombic κ-(AlxGa1−x)2O3 thin film was synthesized by combinatorial pulsed laser deposition on a 2 in. in diameter c-sapphire substrate with a composition gradient. Structural, morphological, and optical properties were studied as a function of the alloy composition. The thin film crystallized in the orthorhombic polymorph for Al contents of 0.07 ≤ x ≤ 0.46, enabling bandgap engineering from 5.03 eV to 5.85 eV. The direct optical bandgap and the c-lattice constant, as well, show a linear dependence on the cation composition. XRD measurements, especially 2θ-ω- and ϕ-scans, revealed the growth of κ-(AlxGa1−x)2O3 in [001]-direction and in three rotational domains. The surface morphology was investigated by atomic force microscopy and reveals root mean square surface roughnesses below 1 nm. Furthermore, the dielectric function (DF) and the refractive index, determined by spectroscopic ellipsometry, were investigated in dependence on the Al content. Certain features of the DF show a blue shift with increasing Al concentration.

Effects of Sm1+xBa2-xCu3Oy Films with Non-Stoichiometric Composition Fabricated by Combinatorial Pulsed Laser Deposition method on the Superconducting Properties

Sm1+xBa2-xCu3Oy, (Sm-rich) had higher critical current density (Jc) in magnetic fields and higher irreversible fields than stoichiometric SmBa2Cu3Oy (SmBCO) thin films. In this study, to clarify the features of artificial pinning centers (APCs) doped Sm-rich films, we prepared BaHfO3 (BHO)-doped Sm1+xBa2-xCu3Oy films with x = 0.00 ∼ 0.20 using combinatorial pulsed laser deposition (C-PLD) method on a metallic substrate and we evaluate surface morphrogies, crystal orientations and superconducting properties of the films. As a result, when x increased, the terrace width also increased. From the measurement of Jc in magnetic fields, the BHO-doped Sm-rich films showed Jc plateaus around 1 ∼ 3 T. From the angulaer dependence of Jc, the BHO-doped Sm-rich films showed Jc peak where B is parallel to c-axis. From several measurements, we discuss effects of Sm1+xBa2-xCu3Oy films with and without BHO on the superconducting properties.

High-Throughput Experimental Study of Wurtzite Mn1-x Zn x O Alloys for Water Splitting Applications.

We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn1–xZnxO wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn1–xZnxO thin films were synthesized using combinatorial pulsed laser deposition, permitting for characterization of a wide range of compositions with x varying from 0 to 1. The solubility limit of ZnO in MnO was determined using the disappearing phase method from X-ray diffraction and X-ray fluorescence data and found to increase with decreasing substrate temperature due to kinetic limitations of the thin-film growth at relatively low temperature. Optical measurements indicate the strong reduction of the optical band gap down to 2.1 eV at x = 0.5 associated with the rock salt-to-wurtzite structural transition in Mn1–xZnxO alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn1–xZnxO compositions above x = 0.4. The wurtzite Mn1–xZnxO samples with the 0.4 < x < 0.6 range were studied as anodes for photoelectrochemical water splitting, with a maximum current density of 340 μA cm–2 for 673 nm-thick films. These Mn1–xZnxO films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn1–xZnxO materials with Ga dramatically increases the electrical conductivity of Mn1–xZnxO up to ∼1.9 S/cm for x = 0.48, but these doped samples are not active in water splitting. Mott–Schottky and UPS/XPS measurements show that the presence of dopant atoms reduces the space charge region and increases the number of mid-gap surface states. Overall, this study demonstrates that Mn1–xZnxO alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties.

Magnetic and Photoluminescent Coupling in SrTi0.87Fe0.13O3-δ/ZnO Vertical Nanocomposite Films.

Self-assembled growth of SrTi0.87Fe0.13O3-δ (STF)/ZnO vertical nanocomposite films by combinatorial pulsed laser deposition is described. The nanocomposite films form vertically aligned columnar epitaxial nanostructures on SrTiO3 substrates, in which the STF shows room-temperature magnetism. The magnetic properties are discussed in terms of strain states, oxygen vacancies, and microstructures. The nanocomposites exhibit magneto-photoluminescent coupling behavior that the near-band-edge emission of ZnO is shifted as a function of magnetic field.

Optimized silicon reinforcement of carbon coatings by pulsed laser technique for superior functional biomedical surfaces fabrication

We report on the fabrication of silicon-reinforced carbon (C:Si) structures by combinatorial pulsed laser deposition to search for the best design for a new generation of multi-functional coated implants. The synthesized films were characterized from the morphological, structural, compositional, mechanical and microbiological points of view. Scanning electron microscopy revealed the presence, on top of the deposited layers, of spheroid particulates with sizes in the micron range. No micro-cracks or delaminations were observed. Energy dispersive x-ray spectroscopy and grazing incidence x-ray diffraction pointed to the existence of a C to Si compositional gradient from one end of the film to the other. Raman investigation revealed a relatively high sp3 hybridization of up to 80% at 40–48 mm apart from the edge with higher C content. Si addition was demonstrated to significantly increase C:Si film bonding to the substrate, with values above the ISO threshold for coatings to be used in high-loading biomedical applications. Surface energy studies pointed to an increase in the hydrophilic character of the deposited structures along with Si content up to 52 mN m−1. In certain cases, the Si-reinforced C coatings elicited an antimicrobial biofilm action. The presence of Si was proven to be benign to HEp-2 cells of human origin, without interfering with their cellular cycle. On this basis, reliable C:Si structures with good adherence to the substrate and high efficiency against microbial biofilms can be developed for implant coatings and other advanced medical devices.