Pulsed Laser Deposition System Research Articles

Effect of RF acetylene plasma on the composition and dynamics of a titanium plasma plume in a plasma enhanced pulsed laser deposition system

We report the effect of radio frequency (RF) acetylene plasma on the dynamics and composition of titanium (Ti) plasma plume in a plasma-enhanced pulsed laser deposition (PEPLD) system. The titanium target, mounted inside a capacitively coupled RF discharge, was ablated by using a nanosecond Nd:YAG pulsed laser at 1064 nm with a power density of 2.65 GW/cm2. The experiments were performed at different operating pressures of acetylene. Fast imaging and optical emission spectroscopy were employed to study the physics behind the pulsed laser deposition in both (PLD) and PEPLD systems. A nonlinear dependence of the plasma plume evolution was observed over a range of pressure. Different expansion regimes correspond to the pressure of the experiments. The plume expansion velocity ranges between 6 × 103 m/s and 30 × 103 m/s. Emission spectra reveal the presence of C II and Ti II lines depending on the experimental conditions. The presence of background RF plasma leads to substantial enhancement of the emission intensity of the C II spectral lines. In addition, with increasing RF power and background pressure, the intensities of the C II spectral lines increase; whereas the intensities of the Ti II spectral lines decrease with the increase in RF power. Plasma temperature was estimated from the Ti II lines using the Boltzmann plot method, whereas the electron density was estimated from the Stark-broadened Ti II line at 454.9 nm. The calculated densities and temperatures lie between 1017–1018 cm−3 and 0.8–2.0 eV, respectively. These results show the effects of the different backgrounds (either neutral or RF plasma) on the propagation of the laser-produced plasma (LPP), which we propose to be useful in the thin film deposition process using PLD.

Ferroelectric properties and current mechanisms of BaTiO3/Nb:SrTiO3 thin films

[Formula: see text] thin film oxides have been deposited on [Formula: see text] substrates using a pulsed laser deposition system. Different annealing methods were thereafter used to assess the ferroelectricity of the grown films. The residual polarization value, 2Pr, of 4.205 [Formula: see text]C/cm2 and saturated residual polarization value, 2Pmax, of 15.484 [Formula: see text]C/cm2 were obtained using in-situ annealing. After performing rapid thermal annealing (RTA) annealing, the residual polarization value rose to 4.676 [Formula: see text]C/cm2 and the saturated residual polarization value rose to 18.723 [Formula: see text]C/cm2. Furthermore, a comparison of results showed that the saturation test frequency of the in-situ annealing was 10 times higher than the saturation test frequency of the RTA. This could be explained by the fact that the secondary high-temperature annealing led to a decrease in the conductivity of the doped oxide and, thereby, a decrease in the effective electric field that was applied to both ends of the BTO ([Formula: see text]) film. Thus, a lower frequency was required to ensure a flip of all ferroelectric domains within the BTO film. By testing the fatigue frequency at 100 kHz, [Formula: see text] stable cycles in both annealing methods and the results confirmed the good stability of the device performance. A linear fit analysis of the [Formula: see text] – [Formula: see text] curves showed under in-situ annealing the presence of both a bulk-limited current mechanism that was dominated by the space charge limited current (SCLC) and Pool–Frenkel (PF) mechanisms, and an interface-limited current that was dominated by the Fowler–Nordheim (FN) mechanism in the structure. After RTA annealing, the conducting mechanism is ohmic contact at low electric field and the SCLC mechanism at high field strength. The results show that after RTA annealing, the quality and ferroelectric properties of the films are significantly improved compared to those under in-situ annealing. These results showed that the BTO/NSTO/STO ([Formula: see text]) structure had a standard test frequency as well as reliable stability. The BTO/NSTO/STO structure showed, therefore, promising applications in future nonvolatile information memory devices.

A BaTiO3-based flexible ferroelectric capacitor for non-volatile memories

BaTiO3 (BTO) ferroelectric films, which are renowned for their lead-free compositions, superior stability, and absence of a wake-up effect, are promising candidate materials in the field of non-volatile memories. However, the prerequisites for high-temperature conditions in the fabrication of ferroelectric thin films impose constraints on the substrate choice, which has limited the advancement in non-volatile memories based on single-crystal flexible BTO films with robust ferroelectric properties. Herein, a technique has been developed for the fabrication of flexible devices using a pulsed laser deposition system. BTO ferroelectric films have then been deposited onto a flexible mica substrate, with SrTiO3 (STO) serving as a buffer layer. The obtained flexible BTO devices exhibited excellent ferroelectricity, with a maximum polarization (2Pmax) of up to 42.58 μC/cm2 and a remnant polarization (2Pr) of up to 21.39 μC/cm2. Furthermore, even after 1000 bending cycles, the bipolar switching endurance remained high at 1012 cycles. After 104 s, the flexible BTO device still maintained excellent polarization characteristics. These results make the flexible BTO ferroelectric thin film a potential candidate for the next generation of non-volatile memories.

Microstructure and optical properties of β-Ga2O3 thin films fabricated by pulsed laser deposition

β-Ga2O3 has attracted extensive attention in the fields of optoelectronics and high-power electric devices due to its excellent electrical properties and thermal stability. Growth of epitaxial β-Ga2O3 films with good crystallinity is challenging since it is difficult to effectively control the impurities in the films. Here, β-Ga2O3 thin films were epitaxially grown on MgO (100) substrates via pulsed laser deposition system. The microstructure and optical properties of the β-Ga2O3 thin films were systematically characterized by combining high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible spectrophotometer and advanced transmission electron microscopy. Effects of the growth temperature and O2 partial pressure on the crystallinity, valance states of Ga ions and band gap of the β-Ga2O3 thin films were investigated. The light absorption wavelength of the β-Ga2O3 thin films ranges from 220 nm to 260 nm, which is close to the theoretical value. The epitaxial orientation between β-Ga2O3 film and MgO substrate is β-Ga2O3 (100) [02¯1] // MgO (100) [010]. A thin layer of γ-Ga2O3 exists in the interfacial region.

Development of a pulsed laser deposition system suitable for radioactive thin films growth

Radioactive thin films have a direct application in the development of beta-voltaic batteries. The main advantage of that kind of nuclear battery is its durability, which can range from a hundred years, depending on the half-life of the radioisotope used. In this context, Pulsed Laser Deposition (PLD) is an important tool. A relevant aspect of a system using this technique is that the main equipment is outside the chamber where the material is processed. Consequently, this feature allows the growth of radioactive thin films, as it enables the development of an arrangement where the contaminated area is controlled. In this way, the present work proposed the development of a PLD system for the growth of radioactive thin films. The PLD system was then implemented and radioactive copper targets were processed for 60 min and 120 min, resulting in radioactive thin films with an average thickness of (167.8 ± 3.7) nm and (313.5 ± 9.2) nm, respectively. Then, a study was performed about the radioactive contamination spread in the PLD system in order to prove if the filtering implemented was effective in retaining the contamination inside the vacuum chamber. Thus, it is demonstrated for the first time the feasibility of using the PLD technique in the growth of radioactive thin films, making its use possible in future studies on the development of beta-voltaic nuclear batteries.

Synthesis and Characterization of Indium-Doped SnO2-Based Impedance Spectroscopy Sensor for Real-Time Humidity Sensing Applications

Metallic transition-metal dichalcogenides are emerging as promising electrode materials for applications such as 2D electronic devices owing to their good electrical conductivity. In this study, a high-performance humidity sensor based on NbTe2 electrode material and an indium-doped SnO2 thin film sensing layer was fabricated using a pulsed laser deposition system. The morphology, structural, elemental compositions, and electrical properties of the as-deposited samples were characterized. Additionally, the humidity sensing response of the fabricated sensor with In-doped SnO2 (8:92 wt%) sensing film was evaluated in a wide range of relative humidity at room temperature. The results demonstrated that the humidity sensor based on In-doped SnO2 exhibited a high sensitivity of 103.1 Ω/%RH, fast response and recovery times, a low hysteresis value, good linearity, and repeatability. In addition, the sensor had good long-term stability, with a variation in impedance of less than 3%. The results indicated that the humidity sensor could be suitable for practical humidity sensing applications.

Enhanced charge trapping characteristics through composite high-k material phase separation

Charge trapping memory with the P-Si/Al2O3/LaTiO/Al2O3/Pt structure was fabricated by a pulsed laser deposition system. An innovative high-k nanocrystal-amorphous phase structure could be stably formed in the charge trapping layer. The La2O3 nanocrystals are embedded in amorphous TiO2. Numerous charge traps are generated at the phase interface, which could significantly increase the charge trapping capability. A larger memory window of 16.56 V at ±12 V sweep voltage is observed, comparing with a lower value of 5.52 V for the simple amorphous structure. The device also demonstrated excellent stability, with only a 13% charge loss rate after 10 years and an unchanged memory window after 105 program/erase cycles. It is attributed to the structure that the amorphous phase isolates the trapped electrons around the nanocrystal and, thus, is resistant to loss. This work could provide an approach to generating charge traps by phase separation of high-k materials for future nonvolatile memory applications.

High performance UV photodetectors based on W doped δ -Ta2O5 single crystalline films

Amorphous or polycrystalline tantalum pentoxide (Ta2O5) films with high resistance have been widely used in semiconductor devices as insulating dielectric layers. In this work, tungsten (W) doped δ-Ta2O5 (0001) monocrystal films were deposited on Y-stabilized ZrO2 (111) substrates using a pulsed laser deposition system. The lattice structure, heteroepitaxial relationship, and electrical properties of the films were analyzed in detail. The carrier concentration, Hall mobility, and resistivity of the 2% W doped δ-Ta2O5 film are 6.61 × 1015 cm−3, 65.2 cm2/V s and 14.5 Ω cm, respectively. High performance metal–semiconductors–metal ultraviolet (UV) detectors based on the W doped δ-Ta2O5 films were fabricated. The UV detector based on a 2% W doped δ-Ta2O5 film exhibits a high photo responsivity of 10.32 A/W and a photocurrent-to-darkcurrent ratio of 1.3 × 104. The performances of the UV detectors in this work are so high, which indicates that the W doped δ-Ta2O5 films can be applied in UV detectors as an active layer.

Structural and magnetic anisotropy in YBa$_2$Cu$_3$O$_7$/La$_{0.67}$Sr$_{0.33}$MnO$_3$ bilayer on SrTiO$_3$ substrate

We study the magnetic properties and emergence of superconductivity in YBa_22Cu_33O_77 (YBCO)/ La_{0.67}0.67Sr_{0.33}0.33MnO_33 (LSMO) heterostructures. Bilayer films of superconducting layer, YBCO, and ferromagnetic layer, LSMO were grown on SrTiO_{3}3 (STO) (001) substrate using ultrahigh vacuum Pulsed Laser Deposition system. Magnetization data at 100 K as a function of applied magnetic field shows ferromagnetic behaviour due to the LSMO layer. Cooling below 100 K leads to superconductivity in this material; the onset of superconductivity occurs at a temperature, T_{c,SC},Tc,SC, of 86 K for H_{ext}\perp cHext⊥c (in-plane) & 84 K for H_{ext}||cHext||c (out-of-plane) under 100 Oe applied field. In-plane magnetic measurements show significant suppression of diamagnetic behaviour as compared to the out-of-plane measurements. The susceptibility signals are higher for out-of-plane direction. Such strong anisotropy in magnetism and the transition temperatures reveal complex interplay of magnetism and superconductivity in this system and calls for further study in this direction.

Effect of Oxygen Plasma on β-Ga2O3 Deep Ultraviolet Photodetectors Fabricated by Plasma-Assisted Pulsed Laser Deposition

We fabricated metal–semiconductor–metal-structured β-Ga2O3 photodetectors using a plasma-assisted pulsed laser deposition system with various oxygen plasma radio frequency (RF) powers ranging from 0 to 100 W. All optoelectronic properties of the material were enhanced as the RF power increased. β-Ga2O3 photodetector with RF power of 100 W showed the best optoelectronic characteristics, such as photoresponsivity of 0.39 A/W, external quantum efficiency of 192.61%, and detectivity of 9.09 × 1013 cm Hz1/2/W. In addition, photo-switching analysis revealed the fastest photoresponse speeds (1.46 and 0.21 s) for on/off switching. These results originate from the decrease in the oxygen vacancy defect concentration in the β-Ga2O3 films by the oxygen RF power. Our results suggest that β-Ga2O3 photodetectors fabricated with oxygen plasma can optimize and improve the photodetection performance and can be applied for future deep ultraviolet detectors.

Dual pulsed laser deposition system for the growth of complex materials and heterostructures.

Here, we present an integrated ultra-high-vacuum (UHV) apparatus for the growth of complex materials and heterostructures. The specific growth technique is the Pulsed Laser Deposition (PLD) by means of a dual-laser source based on an excimer KrF ultraviolet and solid-state Nd:YAG infra-red lasers. By taking advantage of the two laser sources-both lasers can be independently used within the deposition chambers-a large number of different materials-ranging from oxides to metals, to selenides, and others-can be successfully grown in the form of thin films and heterostructures. All of the samples can be in situ transferred between the deposition chambers and the analysis chambers by using vessels and holders' manipulators. The apparatus also offers the possibility to transfer samples to remote instrumentation under UHV conditions by means of commercially available UHV-suitcases. The dual-PLD operates for in-house research as well as user facility in combination with the Advanced Photo-electric Effect beamline at the Elettra synchrotron radiation facility in Trieste and allows synchrotron-based photo-emission as well as x-ray absorption experiments on pristine films and heterostructures.

A novel approach to pulsed laser deposition of platinum catalyst on carbon particles for use in polymer electrolyte membrane fuel cells.

The research undertaken aimed to develop an efficient Pt-based catalyst for polymer electrolyte membrane fuel cells (PEMFCs) by using a cost-effective and efficient physical method to deposit platinum nanoparticles (PtNPs) on carbon supports directly from the platinum target. The method developed avoids the chemical functionalization of the carbon substrate and the chemical synthesis of PtNPs during catalyst fabrication. Platinum was deposited on carbon particles at room temperature using a pulsed laser deposition (PLD) system equipped with an ArF excimer laser (λ = 193 nm). The uniform deposition of PtNPs on carbon supports was achieved thanks to a specially designed electromechanical system that mixed the carbon support particles during platinum deposition. In the studies, Vulcan XC-72R carbon black powder, a popular material used as support in the anodes and cathodes of PEMFCs, and a porous carbon material with a high degree of graphitization were used as carbon supports. The best electrochemical measurement results were obtained for Pt deposited on Vulcan XC-72R. The peak power density measured for this material in a membrane electrode assembly (MEA) of a PEMFC (fed with H2/Air) was 0.41 W/cm2, which is a good result compared to 0.57 W/cm2 obtained for commercial 20% Pt Vulcan XC-72R. This result was achieved with three times less Pt catalyst on the carbon support compared to the commercial catalyst, which means that a higher catalyst utilization factor was achieved.

The role of saturable absorbers thickness in the Q-switching of the erbium-doped fiber laser

A passively Q-switched Erbium (Er3+) doped fiber laser (EDFL) based on a ZnO saturable absorber (SA) prepared using a pulsed laser deposition (PLD) technique is demonstrated. The in-situ monitoring of the thickness in the PLD system enabled the control of the SA’s thickness during the growth. The thickness of the SA was varied and the output characteristics of the fiber laser with all thicknesses are compared. This study reveals that the performance and efficiency of an EDFL including pulse repetition rates, pulse duration, pulse energy, peak power, stability, and signal-to-noise ratio can be improved by varying the thickness of the SA. Based on the thickness of the SA, the optimized results are also presented. These findings suggest that the thickness control of the SA film grown directly on the fiber ferrule facilitates much-improved EDFL that has potential applications in pulsed laser sources.

Investigation on transport properties of GdMnO3 based bilayer thin films

Two heterostructures (Al-ZnO/GdMnO3/SrTiO3 and ZnO/GdMnO3/SrTiO3) were fabricated using Pulsed Laser Deposition System. The lattice parameters of the hexagonal (Al-ZnO and ZnO) and orthorhombic (GdMnO3) films are determined from the analysis of the x- ray diffraction patters. To study the surface morphology of prepared heterostructures Atomic Force Microscopy (AFM) were performed which revels that both heterostructures have homogenous island type of growth. To understand the transport behaviour of semiconductor-ferromagnetic (S-F) bilayers/films the resistivity measurement and field dependent dielectric measurements show and one Variation/Comparison in resistivity and dielectric behaviour of both heterostructures discussed based on Variable Rang Hopping (VRH) model and Universal dielectric response (UDR) Model.

Efficient Charge Transfers in Highly Conductive Copper Selenide Quantum Dot-Confined Catalysts for Robust Oxygen Evolution Reaction.

Defective quantum dots (QDs) are the emerging materials for catalysis by virtue of their atomic-scale size, high monodispersity, and ultra-high specific surface area. However, the dispersed nature of QDs fundamentally prohibits the efficient charge transfer in various catalytic processes. Here, we report efficient and robust electrocatalytic oxygen evolution based on defective and highly conductive copper selenide (CuSe) QDs confined in an amorphous carbon matrix with good uniformity (average diameter 4.25 nm) and a high areal density (1.8 × 1012 cm-2). The CuSe QD-confined catalysts with abundant selenide vacancies were prepared by using a pulsed laser deposition system benefitted by high substrate temperature and ultrahigh vacuum growth conditions, as evidenced by electron paramagnetic resonance characterizations. An ultra-low charge transfer resistance (about 7 Ω) determined by electrochemical impedance spectroscopy measurement indicates the efficient charge transfer of CuSe quantum-confined catalysts, which is guaranteed by its high conductivity (with a low resistivity of 2.33 μΩ·m), as revealed by electrical transport measurements. Our work provides a universal design scheme of the dispersed QD-based composite catalysts and demonstrates the CuSe QD-confined catalysts as an efficient and robust electrocatalyst for oxygen evolution reaction.

Characteristics of Thin High Entropy Alloy Films Grown by Pulsed Laser Deposition

Starting from solid-solutions (SS) of AlCoCrFeNix high-entropy alloys (HEAs) that have been produced with high purity constituent elements by vacuum arc remelting (VAR) method varying the nickel molar ratio x from 0.2 to 2.0, we investigated the synthesis of protective thin films of HEAs and high-entropy nitrides (HENs) with the aid of the pulsed laser deposition (PLD) system. The structure of all ten available bulk targets have been examined by means of X-Ray Diffraction (XRD), as well as their elemental composition by means of energy dispersion X-ray spectroscopy (EDS). Three targets with nickel molar composition x = 0.4, 1.2 and 2.0 corresponding to BCC, mixed BCC and FCC, and finally FCC structures were used for thin film depositions using a KrF excimer laser. The depositions were performed in residual low vacuum (10−7 mbar) and under N2 (10−4 mbar) at room temperature (RT~25 °C) on Si and glass substrates. The deposited films’ structure was investigated using grazing incidence XRD, their surface morphology, thickness and elemental composition by scanning electron microscopy (SEM), EDS and X-ray photoelectron spectroscopy (XPS), respectively. A homemade four-point probe (4PP) set-up was applied to determine layers electrical resistance. Besides, a Nanoindentation (NI) was employed to test films’ mechanical properties. XRD results showed that all deposited films, regardless of the initial structure of targets, were a mixture of FCC and BCC structures. Additionally, the quantitative and qualitative EDS and XPS results showed that the elemental composition of films was rather close to that of the targets. The depositions under an N2 atmosphere resulted in the inclusion of several percentage nitrogen atoms in a metallic nitride type compound into films, which may explain their higher electrical resistivity. The Young’s modulus, nanohardness and friction coefficient values showed that the deposited films present good mechanical properties and could be used as protective coatings to prevent damage in harsh environments.

The improvement design of two-colour pyrometry for pulsed laser deposition system

In this work, we develop a digital pyrometry with an automatic control system. A two-colour pyrometer is applied to measure the substrate surface temperature in the pulsed laser dep-osition (PLD) system. A digital computer and two-colour photodetector are coupled to obtain the substrate surface temperature and the output signal from the respective detectors with high accuracy. These sets enable us to determine the substrate temperature during the process of deposition remotely and potentially allow us to develop control routines for maintaining a constant temperature throughout the process. The designed system has a broad spectral response range from 0.32 to 1.7μm and achieves a deficient full-scale temperature error of <1.8°C. Factors affecting the temperature measurements are studied to identify the sensor limitations, such as possible damage on the two-colour photodetectors, spectral losses, responsivity, and the distance between the photodetector end to the substrate. Finally, this pyrometric system is applied in the PLD process to maintain a stable substrate surface temperature.

Magnetoelectric properties of three-layered composite thin film fabricated by pulsed laser deposition

Magnetostriction-piezoelectric composite films present promising application prospects due to the presence of the magnetoelectric effect. In this paper, a sandwich structure Bi4Ti3O12/CoFe2O4/Bi4Ti3O12 composite film was prepared in a pulsed laser deposition system. The morphology, EDX, XPS, ferroelectric, piezoelectric, ferromagnetic and magnetoelectric properties were investigated in detail. In the sample, the atomic percentages of Bi, Ti and O elements measured by EDX are 35.373%, 12.824% and 46.578%, respectively. The out-of-plane electric domain structure and the in-plane magnetic domain structure show that BTO and CFO phases have excellent ferroelectricity (Pr = 1.57 μC/cm−2) and ferromagnetism (Mr = 104.83 emu/cc), respectively. The piezoelectric properties are proportional to the ferroelectric properties, so the composite thin films also have good piezoelectric properties (d33max = 192 p.m./v). Moreover, a great magnetoelectric coupling coefficient (∼11.95 mV/cm·Oe) was obtained by mechanical strain transfer and interface coupling. By adding a ferroelectric layer to the composite thin film, the clamping effect can be reduced and the coupling between the ferroelectric and the ferromagnetic layers can be increased, providing the experimental basis for the research of magnetoelectric coupling effect.

Structural and optical properties of single crystal Zn2TiO4 films prepared on MgO (110) substrates

Zinc orthotitanate (Zn2TiO4) thin films were prepared on MgO (110) substrates by a pulsed laser deposition system. The structural, surface morphological and optical properties of the obtained samples were investigated. Monocrystalline Zn2TiO4 films with cubic structure were obtained through post annealing process and the film annealed at 1000 °C exhibited the optimal crystalline quality. The XRD Φ scan measurements and high resolution transmission electron microscopy results showed that the epitaxial relationship of Zn2TiO4 film on MgO substrate is Zn2TiO4 (220)∥MgO (220) and Zn2TiO4 [001]∥MgO [001]. The 1000°C-annealed Zn2TiO4 film has excellent transparency in the visible light range and a wide band gap of 4.41 eV.

Pulsed laser deposition of large-sized superlattice films with high uniformity

Oxide superlattices often exhibit emergent physical properties that are desirable for future information device applications. The most common growth technique for fabrication of oxide superlattices is pulsed laser deposition (PLD), which is convenient yet powerful for the growth of various oxide superlattices. However, the sample size prepared by PLD is rather small confined by the plasmon plume, which greatly limits its potential for device applications. Here, we design a PLD system that is capable of fabricating large-sized oxide superlattices with high uniformity. Specifically, during growth, the laser beam scans the target surface by combining the pitch and yaw angle rotation of the high reflective mirror and the linear motion of the focus lens. A SiC susceptor is placed in between the sample holder and the substrate to improve the large area infrared heating efficiency. Using such a system, droplet-free 10 × 10 mm2 [(LSMO)12/(PCMO)6]7 superlattices are epitaxially grown with the same period of superlattices across the whole sample areas. The high uniformity of the superlattices is further illustrated by near identical physical properties of all regions of the superlattice films. The present PLD system can be used to grow various kinds of oxide superlattices with the area size as large as 2 in., which is highly useful for device applications of oxides.