Off-axis Pulsed Laser Deposition Research Articles

Characterization of polycrystalline BiFeO3 films prepared by magnetic-field-assisted 90° off-axis pulsed laser deposition

Polycrystalline BiFeO3 (BFO) films were prepared on a Pt/TiO2/SiO2/Si substate using magnetic field-assisted 90° off-axis pulsed laser deposition (PLD). We have successfully obtained polycrystalline BFO films owing to a high deposition rate derived by the confinement of the plume under a magnetic field. The obtained polycrystalline BFO films have a droplet-free surface morphology and a columnar-like microstructure. A RT ferroelectric hysteresis loop is obtained, and at the same time, the remanent polarization of 90 μC cm−2 and the reduced coercive field of 178 kV cm−1 are confirmed. Also, an evolution of the polarization switching is observed by the piezoresponse force microscopy. In this study, we provide a possible route to realize the polycrystalline film growth which has a good quality in a 90° off-axis deposition system using magnetic field-assisted PLD.

The effects of crystal orientation on the ferroelectric and optoelectronic properties of Pt/Na0.5Bi0.5TiO3/La0.5Sr0.5CoO3 heterostructures

Due to their excellent ferroelectric, semiconducting, and optoelectronic properties, perovskite ferroelectric thin films have attracted increasing attention in the photovoltaic field. Using La0.5Sr0.5CoO3 (LSCO) as the bottom electrode, flake, strip and pyramidal Na0.5Bi0.5TiO3 (NBT) films were prepared on (001), (110) and (111) SrTiO3 (STO) substrates by off-axis magnetron sputtering and pulsed laser deposition, respectively. XRD and Phi scan results show that both LSCO and NBT are perovskite structures and satisfy the epitaxial growth along the substrate STO. NBT orientation has a significant effect on the ferroelectric properties of Pt/NBT/LSCO heterostructures. The remnant polarization 2Pr values for NBT(111), NBT(110) and NBT(001) are 87.5, 62.6 and 28.5 μC/cm2, respectively, and thus 2Pr(111)>2Pr(110)>2Pr(001). NBT(111) shows the best ferroelectric properties (Pr=41.3 μC/cm2), excellent fatigue-resisting (up to 1010 cycles) and retention performance (duration 104s). The 2Pr-T association 2 Pr=41.72+16.05*T0.35 for NBT(111) was obtained according to the curve fitting, it is found that the NBT orientation has a significant effect on the PV effect for PT/NBT/LSCO photovoltaics and the open-circuit voltage ∆Voc. The short-circuit current ∆Jsc of polarization regulate intensity follows the laws of ∆Voc-111>∆Voc-110>∆Voc-001 and ∆Jsc-111>∆Jsc-110>∆Jsc-001, the Jsc for Pt/NBT(111)/LSCO heterostructures increases linearly with the illumination Ip increasing, while the Voc decreases linearly. The ferroelectro-polarized modulation PV mechanism was investigated by analyzing band structure of the Pt/NBT/LSCO heterostructures, which is attributed to the coupling effect of the ferroelectric depolarization field and the interfacial Schottky barrier.

Application of Pulsed Laser Deposition in the Preparation of a Promising MoSx/WSe2/C(В) Photocathode for Photo-Assisted Electrochemical Hydrogen Evolution.

We studied the possibility of using pulsed laser deposition (PLD) for the formation of a MoSx/WSe2 heterostructure on a dielectric substrate. The heterostructure can be employed for effective solar water splitting to produce hydrogen. The sapphire substrate with the conducting C(B) film (rear contact) helped increase the formation temperature of the WSe2 film to obtain the film consisting of 2H-WSe2 near-perfect nanocrystals. The WSe2 film was obtained by off-axis PLD in Ar gas. The laser plume from a WSe2 target was directed along the substrate surface. The preferential scattering of selenium on Ar molecules contributed to the effective saturation of the WSe2 film with chalcogen. Nano-structural WSe2 film were coated by reactive PLD with a nanofilm of catalytically active amorphous MoSx~4. It was established that the mutual arrangement of energy bands in the WSe2 and MoSx~4 films facilitated the separation of electrons and holes at the interface and electrons moved to the catalytically active MoSx~4. The current density during light-assisted hydrogen evolution was above ~3 mA/cm2 (at zero potential), whilst the onset potential reached 400 mV under irradiation with an intensity of 100 mW/cm2 in an acidic solution. Factors that may affect the HER performance of MoSx~4/WSe2/C(В) structure are discussed.

10B-based films grown by pulsed laser deposition for neutron conversion applications

Solid-state neutron detectors exploit nuclear interactions producing energetic charged particles within a converter layer embedding nuclei with high neutron capture cross section and with thickness compatible with the charged particle range. Recently, boron-10 (10B) is being considered as a valid alternative to the expensive and decreasingly available 3He gas thanks to its large neutron absorption cross section and high-energy reaction products. Minimized amounts of impurities, films with optimal and well-controlled thickness, good uniformity over large areas and good adhesion to the substrates are essential to achieve efficient neutron detection performance. In this study, we present well-adherent 1-µm-thick 10B-enriched boron coatings, deposited over a large area by off-axis pulsed laser deposition (PLD) using a nanosecond Nd-YAG laser beam operating at 1064 nm and high fluence (~ 10 J/cm2), onto 1-mm-thick Al substrate as well as smooth and rough C substrates. By combining plasma plume divergence and peaked profile under off-axis deposition geometry, uniform10B films were obtained over an area of a 3.4 × 3.4 cm2. We discuss the morphological characteristics of our deposits as related to the mechanisms of nanosecond laser ablation and present energy dispersive spectroscopy (EDS) elemental analysis of the film, paying attention to the different surface features. Negligible presence of O, N and C contaminants was achieved by controlled vacuum conditions. Moreover, we present encouraging neutron detection performances of our film deposited onto Al.

Comparative Study of the Structure, Composition, and Electrocatalytic Performance of Hydrogen Evolution in MoSx~2+δ/Mo and MoSx~3+δ Films Obtained by Pulsed Laser Deposition.

Systematic and in-depth studies of the structure, composition, and efficiency of hydrogen evolution reactions (HERs) in MoSx films, obtained by means of on- and off-axis pulsed laser deposition (PLD) from a MoS2 target, have been performed. The use of on-axis PLD (a standard configuration of PLD) in a buffer of Ar gas, with an optimal pressure, has allowed for the formation of porous hybrid films that consist of Mo particles which support a thin MoSx~2+δ (δ of ~0.7) film. The HER performance of MoSx~2+δ/Mo films increases with increased loading and reaches the highest value at a loading of ~240 μg/cm2. For off-axis PLD, the substrate was located along the axis of expansion of the laser plume and the film was formed via the deposition of the atomic component of the plume, which was scattered in Ar molecules. This made it possible to obtain homogeneous MoSx~3+δ (δ~0.8–1.1) films. The HER performances of these films reached saturation at a loading value of ~163 μg/cm2. The MoSx~3+δ films possessed higher catalytic activities in terms of the turnover frequency of their HERs. However, to achieve the current density of 10 mA/cm2, the lowest over voltages were −162 mV and −150 mV for the films obtained by off- and on-axis PLD, respectively. Measurements of electrochemical characteristics indicated that the differences in the achievable HER performances of these films could be caused by their unique morphological properties.

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.

Soft x-ray ptychography studies of nanoscale magnetic and structural correlations in thin SmCo5 films

High spatial resolution magnetic x-ray spectromicroscopy at x-ray photon energies near the cobalt L3 resonance was applied to probe an amorphous 50 nm thin SmCo5 film prepared by off-axis pulsed laser deposition onto an x-ray transparent 200 nm thin Si3N4 membrane. Alternating gradient magnetometry shows a strong in-plane anisotropy and an only weak perpendicular magnetic anisotropy, which is confirmed by magnetic transmission soft x-ray microscopy images showing over a field of view of 10 μm a primarily stripe-like domain pattern but with local labyrinth-like domains. Soft x-ray ptychography in amplitude and phase contrast was used to identify and characterize local magnetic and structural features over a field of view of 1 μm with a spatial resolution of about 10 nm. There, the magnetic labyrinth domain patterns are accompanied by nanoscale structural inclusions that are primarily located in close proximity to the magnetic domain walls. Our analysis suggests that these inclusions are nanocrystalline Sm2Co17 phases with nominally in-plane magnetic anisotropy.

Very Smooth FeSb2Te and Ce0.1Fe0.7Co3.3Sb12 Layers Prepared by Modified PLD

We report on the preparation of thermoelectric layers of FeSb2Te and Ce0.1Fe0.7Co3.3Sb12 on silicon (100) and fused silica substrates via a combination of pulsed laser deposition (PLD) and rapid thermal annealing methods. A wide range of deposition conditions were tested including on- and off-axis approaches and variation of the annealing temperature profile. Wavelength dispersive x-ray spectroscopy was used to determine stoichiometry. An optical microscope, mechanical profilometer, and atomic force microscopy served to map layer topology. For the FeSb2Te layers, Sa was between 1.4 nm and 6 nm, with Sq ranging from 2.1 nm to 7.8 nm. For Ce0.1Fe0.7Co3.3Sb12 layers, Sa was from 1.3 nm to 4.2 nm and Sq between 1.7 nm and 6.2 nm. Crystalline structure was determined by x-ray diffraction. The best layers (in terms of smooth surface and crystalline structure) prepared using a modified off-axis PLD arrangement were then characterized for thermoelectric properties. The smooth, stoichiometric and crystalline layers of both types were prepared at an annealing temperature of 423 K for 10 min. All experiments were conducted with the aim of finding the deposition condition providing smooth and crystalline layers. These conditions should be used for the deposition of multilayered systems composed of the FeSb2Te and Ce0.1Fe0.7Co3.3Sb12 layers with a period of about 1 nm, where there could be layer roughness comparable to the period.

Thermoelectric Simple and Multilayers Prepared by Laser

Thermoelectric layers as Bi2Te3, Yb0.19Co4Sb12, FeSb2Te, Ce0.1Fe0.7Co3.3Sb12 and FeSb2Te/Ce0.1Fe0.7Co3.3Sb12 multilayers were prepared by Pulsed Laser Deposition (PLD). Smooth, nano-crystalline, stoichiometric layers were synthetized in a classical PLD arrangement or in a special off-axis PLD arrangement, followed by Rapid thermal annealing. Results of physical characterizations such as morphology—Atomic Force Microscope, Scanning Electron Microscope, composition-Energy Dispersive X-ray analysis, crystallinity—X-ray Diffraction, separation of multilayers—Secondary of ion beam mass spectroscopy SIMS and study of thermoelectric properties such as the thermoelectric figure of merit ZT, in-plane electrical resistivity, Seebeck coefficient and thermal conductivity are presented. For thermal conductivity measurement a newly developed Atomic force thermal microscope (AFMTh) was tested. Results obtained on the single layers compared to multi-layered structures are discussed.

Material optimization via combinatorial deposition and analysis for thermoelectric thin films

This work presents a custom, high-throughput combinatorial approach for the optimization of thermoelectric thin films consisting of materials with complex chemistry and structures (e.g., the layered misfit cobaltite, Ca3FexCo4−xO9). Combinatorial thin films with graded compositions are produced on 100mm Si wafers from multiple target materials using pulsed laser deposition. Film thickness and composition are mapped as a function of wafer location. Crystal structures are determined using x–y mapping XRD analysis with specially designed algorithms for automated peak location and analysis. Thermoelectric properties, specifically the Seebeck coefficient and the electrical resistivity, are screened using a custom designed automated probe system. By combining the rapid synthesis of many compositions and structures simultaneously using combinatorial deposition and automated analytical tools capable of spatial mapping, trends in material performance are shown to be quickly obtained primarily due to the elimination of one-at-a-time synthesis and analysis. The possible approaches for such complex multivalent combinatorial optimization of thin films are identified and discussed.For the Ca3FexCo4−xO9 system presented, variations to the thermoelectric power factor are dominated by changes in the electrical resistivity. Enhancements to the Seebeck coefficient are observed due to the incorporation of Fe into the Ca3FexCo4−xO9 structure; however, this improvement is overshadowed by increases in the electrical resistivity due to variations in film thickness and the presence of secondary phases (Co3O4 and Ca2Fe2O5) which result from increasing Fe content and off-axis pulsed laser deposition.

Deposition and characterization of multilayer DLC:Mo thin films grown on silicon substrate by off-axis pulsed laser deposition technique

Off-axis pulsed laser deposition technique was used to deposit a diamond-like carbon (DLC) thin film and three multilayer DLC:Mo thin films of various compositions on p-type Si (111) substrate at room temperature keeping the DLC content constant. Excimer laser was used to ablate graphite and Mo targets in a vacuum of ∼10−5mbar, whereas the p-type Si (111) substrate was placed downside the target at a distance of 1cm in off-axis PLD geometry. Morphological, structural, optical, and electrical characteristics of the deposited thin films were then investigated. SEM images of the thin films show that surface roughness increases as Mo content in the film increases. XRD patterns of DLC:Mo thin films rule out any carbide formation. Both crystallite size of Mo particles and film thickness increase whereas microstrain decreases with the increase in Mo content. Moreover, both refractive index and indirect optical band gap decrease exponentially while Urbach energy increases linearly with the increase in Mo content. Electrical resistivity of the thin films has been found to follow an exponential decay with the increase in Mo content in multilayer DLC:Mo thin films.

In-situ x-ray diffraction study of the growth of highly strained epitaxial BaTiO3 thin films

In-situ synchrotron x-ray diffraction was performed during the growth of BaTiO3 thin films on SrTiO3 substrates using both off-axis RF magnetron sputtering and pulsed laser deposition techniques. It was found that the films were ferroelectric during the growth process, and the presence or absence of a bottom SrRuO3 electrode played an important role in the growth of the films. Pulsed laser deposited films on SrRuO3 displayed an anomalously high tetragonality and unit volume, which may be connected to the previously predicted negative pressure phase of BaTiO3.

Synthesis of heteroepytaxial 3C-SiC by means of PLD

Thin films of silicon carbide (SiC) have been prepared by means of pulsed laser deposition (PLD) on sapphire (Al2O3) and Si(100) substrates with a Nd-YAG laser 1064 nm. We achieved the growth of cubic silicon carbide (3C-SiC) films at the temperatures of 650°C from a SiC target in vacuum. The as-deposited films are morphologically and structurally characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The use of off-axis PLD method placing the sample at 90° with respect to the target leads to a good quality smooth film.

Fabrication of ZnO nanorods using metal nanoparticles as growth nuclei

Abstract ZnO nanorods were produced by pulsed laser deposition (PLD). Drops of nanoparticle colloid (gold or silver) were placed on silica substrates to form growth nuclei. All nanoparticles were monocrystalline, with well-defined crystal surfaces and a negative electrical charge. The ZnO nanorods were produced in an off-axis PLD configuration at oxygen pressure of 5 Pa. The growth of the nanorods started from the nanoparticles in different directions, as one nanoparticle could become a nucleus for more than one nanorod. The low substrate temperature used indicates the absence of a catalyst during the growth of the nanorods. The diameters of the fabricated 1-D ZnO nanostructures were in the range of 50–120 nm and their length was determined by the deposition time.

Epitaxial growth, structure, and intermixing at theLaAlO3/SrTiO3interface as the film stoichiometry is varied

${\mathrm{LaAlO}}_{3}$ epitaxial films with La:Al cation ratios ranging from 0.9 to 1.2 were grown on ${\mathrm{TiO}}_{2}$-terminated ${\mathrm{SrTiO}}_{3}$ (001) substrates by off-axis pulsed laser deposition. Although all films are epitaxial, rocking curve measurements show that the crystallographic quality degrades with increasing La:Al ratio. Films with La:Al ratios of 0.9, 1.0, and 1.1 were coherently strained to the substrate. However, the out-of-plane lattice parameter increases over this range, revealing a decrease in film tetragonality. Although all film surfaces exhibit hydroxylation, the extent of hydroxylation is greater for the La-rich films. Rutherford backscattering spectrometry reveals that La from the film diffuses deeply into the ${\mathrm{SrTiO}}_{3}$ substrate and secondary-ion-mass spectroscopy shows unambiguous Sr outdiffusion into the films.

Enhanced photoluminescence of rare-earth doped films prepared by off-axis pulsed laser deposition

Alternate pulsed laser deposition from the host (Al 2O 3) and dopant (Er, Yb) targets has been used to prepare artificially nanostructured films in which the rare earth ion–ion separation is controlled in the nanometer scale in order to control energy transfer between ions. One series of films was prepared in the standard on-axis configuration, i.e. a static substrate being centred with respect to the plasma expansion axis. A second series of films was prepared by rotating the substrate with respect to a shifted axis parallel to the plasma expansion one (off-axis configuration). The latter configuration leads to films with enhanced thickness and Er related photoluminescence intensity uniformity. More interestingly, the Er related photoluminescence lifetime in as-grown films increases up to 2.5 ms, which is much higher than the maximum value of 1 ms obtained for the on-axis configuration films. This enhancement is discussed in terms of a decrease of defect density when using the off-axis configuration.

Structural and pinning properties ofY2Ba4CuMOy (M = Nb,Zr)/YBa2Cu3O7−δ quasi-multilayers fabricated by off-axis pulsed laser deposition

Power applications based on YBa2Cu3O7−δ coated conductors demand an enhancement of the critical current densityJc in magnetic fields to be achieved using artificial pinning centres. A well-known approach tostudying pinning by second phase nanoparticles is based on quasi-multilayer deposition ofYBa2Cu3O7−δ (Y123) and a dopant material. In this study we use theY2Ba4CuMOy (M = Nb, Zr; M2411) phase as a dopant material, which proved to be very successful in increasingJc for bulk samples. Although stability of the M2411 phase in bulk Y123 hasbeen reported, experimental evidence of its stability in thin films is stilllacking. We found that during the quasi-multilayer deposition, yttrium dopedBa(M1−xYx)O3 forms instead of the M2411 secondary phase, and this causes an increase ofJc at high magnetic fields as well as an increase in the irreversibility field below 80 K ascompared against pure Y123 thin films.

Surface enhanced Raman scattering properties using Au-coated ZnO nanorods grown by two-step, off-axis pulsed laser deposition

ZnO nanorod arrays on Si (1 0 0) substrate were grown by the pulsed laser deposition (PLD) method, and then coated with Au. Two samples of Au-coated nanorod arrays with different average diameters of 150 and 400 nm were prepared to investigate the size dependence of the surface enhanced Raman scattering (SERS). The diameter of the nanorods was controllable by the substrate position during PLD. High SERS enhancement was observed from both Au-coated ZnO nanorod arrays. The Raman spectra of rhodamine 6G (R6G) as low as 1 nM were measured with an average diameter of 400 nm at an excitation wavelength of 532 nm. The SERS was explained by the field enhancement effect induced by surface plasmon polaritons of Au-coated nanorods (nanoshells).

Structural and magnetic properties of wurtzite CoO thin films

Hexagonal CoO thin films on (0 0 0 1) Al2O3 and ZnO are prepared by off-axis and on-axis pulsed laser deposition. The structural properties were studied by in situ reflection high-energy electron diffraction and ex situ x-ray diffraction. The magnetic properties of the films depend on the mode of growth. The films grown in a normal geometry are a mixture of Co3O4 and CoO and show ferromagnetism at room temperature, whereas the films grown off-axis are single phase and present a Curie–Weiss behaviour. Wurtzite CoO does not order magnetically because the antiferromagnetic Co2+–O2−–Co2+ superexchange is geometrically frustrated. It is proposed that defective Co3O4 in thin films is ferrimagnetic, due to the appearance of high spin Co3+ on B sites.

Catalyst-Free Growth of High-Quality ZnO Nanorods on Si(100) Substrate by Two-Step, Off-Axis Pulsed-Laser Deposition

We demonstrate the growth of ZnO nanorods on a Si(100) substrate through a two-step process, annealing and off-axis pulsed laser deposition (PLD), without a catalyst. ZnO powder was first dispersed on the Si substrate, and annealed to form seeds for ZnO nanorod growth for 2.5 h. In the second step, ZnO was deposited on the seeds by PLD for 1 h. It was experimentally confirmed that the surface morphology after annealing acts as growth nuclei of nanorods. Controlling growth parameters during deposition enables the adjustment of the sizes of nanorods, namely, diameter and length. The diameters of the grown nanorods are 50–700 nm and the lengths are 2–10 µm. Cathode luminescence (CL) spectra were used to evaluate the states of defects within the ZnO nanorods. According to the CL measurement results, the thinnest nanorod arrays were found to have fewer defects, and more defects were introduced as nanorods became thicker.