Pulsed Laser Deposition System Research Articles

Epitaxial deposition of BaTiO3 on TiO2 buffered GaAs(001) substrate

Ferroelectric BaTiO3 thin films were epitaxially grown on rutile-TiO2 buffered GaAs(001) substrates by pulsed laser deposition system. The in-plane relationship of this heterostructure is 〈110〉BaTiO3//〈001〉TiO2//〈110〉GaAs and the out-plane relationship is BaTiO3(110)//TiO2(110)//GaAs(001). The surface of BaTiO3 is flat and the interfaces between each layer are clear. The oxygen vacancies were eliminated by ex-situ annealing in adequate O2. In short, a highly (110)-oriented BaTiO3 thin film was grown on TiO2 buffered GaAs (001) substrate with excellent electrical and structural properties. The BaTiO3(150nm)/TiO2(40nm)/GaAs system demonstrates hysteresis loops with a remnant polarization of 2.6μC/cm2 and a small leakage current density of 1×10−6A/cm2 both at 300kV/cm.

Novel AlN/Pt/ZnO Electrode for High Temperature SAW Sensors.

In order to develop a film electrode for the surface acoustic wave (SAW) devices working in high temperature, harsh environments, novel AlN/Pt/ZnO multilayers were prepared using pulsed laser deposition (PLD) systems on langasite (LGS) substrates. The AlN film was used as a protective layer and the ZnO buffer layer was introduced to improve the crystal quality of Pt films. The results show that the resistances of Pt and AlN/Pt film electrodes violently increase above 600 °C and 800 °C, respectively, while the resistances of AlN/Pt/ZnO electrodes have more stable electrical resistance from room temperature to 1000 °C. The AlN/Pt/ZnO electrode, where the ZnO film was deposited at 600 °C, has the best temperature stability and can steadily work for 4 h at 1000 °C. The mechanism underlying the stable resistance of the AlN/Pt/ZnO electrode at a high temperature was investigated by analyzing the microstructure of the prepared samples. The proposed AlN/Pt/ZnO film electrode has great potential for applications in high temperature SAW sensors.

Integrated Monolithic Inverter Using Gate-Recessed GaN-Based Enhancement-Mode and Depletion-Mode Metal-Oxide-Semiconductor High-Electron Mobility Transistors

The integrated monolithic inverters constructed by GaN-based gate-recessed enhancement mode and depletion mode metal-oxide-semiconductor high-electron mobility transistors (MOSHEMTs) were fabricated using the photoelectrochemically (PEC) etched-gate recessed structure and the LiNbO3 ferroelectric gate insulator deposited using the pulsed laser deposition system. Instead of the typical tuning gate width, to control the static voltage transfer characteristics but still keep the minimized matched area of the monolithic inverters, the drain-source current was adjusted by controlling the PEC-etched AlGaN thickness of the load-type depletion mode MOSHEMTs using the PEC etching method. Consequently, the drain-source current ratio β of the enhancement mode MOSHEMTs and the depletion mode MOSHEMTs in the monolithic inverters could be adjusted. When the β value was set at 25 and the monolithic inverters operated at VDD = 5 V, the output swing, the high noise margin and the low noise margin were 4.86 V, 1.88 V, and 1.62 V, respectively, for changing input voltage from 0 V to 5 V. From the experimental voltage transfer characteristics, the monolithic inverters with β = 25 could operate as high performance unskewed inverters.

Improved light emission from n-ZnO/p-Si heterojunction with HfO2 as an electron blocking layer

Light-emitting diodes (LEDs) based on ZnO were fabricated on a p-Si substrate by using a pulsed laser deposition system. Significant electroluminescence (EL) improvement was demonstrated with the insertion of an HfO2 electron blocking layer (EBL) in n-ZnO/p-Si heterojunctions. Distinct near-band-edge emission at around 392nm accompanying by a broadly strong visible emission was achieved when a proper thickness of HfO2 EBL was used. Current-voltage and capacitance-voltage measurements confirmed that a proper thickness of the HfO2 EBL can effectively balance the injection of electrons and holes, resulting in an increase of radiative recombination in the ZnO active layer and thus enhancing the EL performance of the devices. Five independent emissions corresponding to five different transition processes were proposed to clarify the EL origination of the n-ZnO/HfO2/p-Si heterojunction LEDs by Gaussian deconvolutions. It is hoped that results in this work should be helpful for the development of ZnO-based LEDs that can integrate ZnO with the Si planar technology.

Effect of Ablation Rate on the Microstructure and Electrochromic Properties of Pulsed-Laser-Deposited Molybdenum Oxide Thin Films.

Molybdenum trioxide (MoO3) is a well-known electrochromic material. In the present work, n-type α-MoO3 thin films with both direct and indirect band gaps were fabricated by varying the laser repetition (ablation) rate in a pulsed laser deposition (PLD) system at a constant reactive O2 pressure. The electrochromic properties of the films are compared and correlated to the microstructure and molecular-level coordination. Mixed amorphous and textured crystallites evolve at the microstructural level. At the molecular level, using NMR and EPR, we show that the change in the repetition rate results in a variation of the molybdenum coordination with oxygen: at low repetition rates (2 Hz), the larger the octahedral coordination, and greater the texture, whereas at 10 Hz, tetrahedral coordination is significant. The anion vacancies also introduce a large density of defect states into the band gap, as evidenced by XPS studies of the valence band and supported by DFT calculations. The electrochromic contrast improved remarkably by almost 100% at higher repetition rates whereas the switching speed decreased by almost 6-fold. Although the electrochromic contrast and coloration efficiency were better at higher repetition rates, the switching speed, reversibility, and stability were better at low repetition rates. This difference in the electrochromic properties of the two MoO3 films is attributed to the variation in the defect and molecular coordination states of the Mo cation.

Catalytically activated quantum-size Pt/Pd bimetallic core–shell nanoparticles decorated on ZnO nanorod clusters for accelerated hydrogen gas detection

This paper presents theoretical and experimental investigations of quantum-sized (3/3nm) Pt/Pd bimetallic core-shell nanoparticle (NP)-decorated ZnO nanorod (NR) clusters (Pt/Pd-ZnO NRs) on silicon (Si) substrate for accelerated hydrogen (H2) gas detection. Quantum-size core–shell NP synthesis, metal loading, and Pt/Pd bimetal composition were all carefully controlled by varying the deposition conditions used in the pulsed laser deposition (PLD) system. Theory suggests that the structural arrangement of bimetallic Pt shell around a Pd core can be advantageous in H2 sensing compared to the converse arrangement. Experimental analysis showed that the as-fabricated sensor exhibited a high response magnitude of 58% (10,000ppm), a fast response time of 5s, and a broad detection range from 0.2 to 40,000ppm at the operating temperature of 100°C. The fabricated sensor also exhibited a good selectivity and a negligible humidity effect over the entire detection range. The superior sensing features of the sensor can be attributed to the enhancement of hydrogen-induced changes in the work function of the Pt/Pd-ZnO NR network. More importantly, the atomic arrangements and chemical potentials of the core–shell interfacial region play vital role in accelerating the H2 sensing properties.

Surface Morphology of Cerium Oxide Layer and its Effect on the Performance of Superconducting Layer

So far, huge progress has been achieved in coated superconductor wires with high performance, where cerium oxides (CeO2) usually figure as superior buffer layer. However, the influence of CeO2 layer’s surface morphology on the performance of YBCO layer is still illusive. In this work, we fabricated CeO2 layers with similar texture and diverse surface characteristics by altering process parameters on IBAD-MgO substrate and then deposited YBCO layers on these CeO2 layers by a reel-to-reel pulsed laser deposition (PLD) system. The abnormal relationship between the quality of YBCO layer and the surface morphology of CeO2 layer was investigated. Instead of texture quality, the critical current density of YBCO-coated layer is highly susceptible to the surface morphology of CeO2 layer, once their surface roughness root mean square (RMS) outstrips the threshold of atomic flat. And our study indicates RMS and flat area fraction are all key issues for PLD epitaxial multilayers but play in different priority. The possible defect formation process of YBCO/CeO2 was also discussed on the basis of the SEM images.

Effect of particle clustering of silver nanoparticles on ultrathin silicon solar cell

Particle clustering is a major concern for uniform dispersal of nanoparticles in various deposition procedures. Well separated uniform distribution of metal nanoparticles is essential for effective coupling of surface plasmons. This work experimentally and theoretically, discusses the effect of nanoparticle clustering on the light trapping efficiency of silver nanoparticles. Pulsed laser deposition system has been used for deposition of silver nanoparticles, and substrate heating has been used to promote uniform distribution of nanoparticles. Pre-heated substrate depositions are compared with corresponding post-annealed samples. XRD, FESEM, Photoluminescence and UV-visible spectroscopy have been used to study the variations in their structural and optical properties. Mono-dispersal of silver nanoparticles for pre-heated substrates results in sharper surface plasmon resonance in comparison to post-annealed samples. Mie theory is used to estimate the particle size of the nanoparticles and findings are in accordance with quantitative analysis of FESEM images. Finite-difference time domain technique is used to discuss the effect of particle distribution on an ultrathin film silicon solar cell. Device degradation is observed as a result of clustering of silver nanoparticles. Hence, mono-dispersal of plasmonic nanostructures is important for required results and pre-heated deposition of metal nanoparticles by pulsed laser deposition can effectively solve the problem of particle clustering.

Fast-response hydrogen sensors based on discrete Pt/Pd bimetallic ultra-thin films

This study presents the fast response-recovery characteristics of an ultra-thin discrete platinum/palladium (Pt/Pd) bimetallic film-based hydrogen (H2) sensor. Pt/Pd bimetallic nanoparticles (NPs) with a size of 2–11nm were deposited on an alumina (Al2O3) substrate using a pulsed laser deposition (PLD) method. The metal loading and the composition of the Pt/Pd NPs were carefully controlled by varying the deposition conditions of the PLD system. The resultant discrete Pt/Pd bimetallic ultra-thin film was applied as a new H2 detection material, in which the fabricated Pt (3nm)/Pd (3nm) film showed noteworthy advantages, such as a large detection range of 10–40,000ppm, high response magnitude, and fast response-recovery time. For a 10,000ppm (1vol.%) H2 concentration at 150°C, a maximum sensor response of 13.56% and a response-recovery time of 4/5s were observed. The observed fast response-recovery time characteristics can be attributed to the enhanced hydrogen-induced changes in the work function of the Pt/Pd ultra-thin film. The fabricated sensor also exhibits good reproducibility and a negligible humidity effect within the entire detection range.

Superconducting properties enhancement of YBa2Cu3O7−δ thin films by BaSnO3 doping

YBa2Cu3O7−δ (YBCO) thin films, doped or undoped with BaSnO3, are deposited on single crystal SrTiO3 substrates using a pulsed laser deposition system. The optimized temperatures for BSO–YBCO and pure YBCO are different, while BSO–YBCO needs higher growth temperature of 850 °C. And the films being grown at different temperatures exhibit very different surface morphologies. The optimized BSO–YBCO film shows higher critical current density values at different measured temperatures compared to the pure YBCO film, either with or without applied magnetic field. The doped sample obtains self-field Jc of 38.5 MA/cm2 at 40 K, 3.2 MA/cm2 at 77 K, which suggests that BSO doping has effective pinning effects for YBCO superconducting properties improvement.

The influence of substrate temperature on the structural and luminescent properties of as-deposited SrGa2S4:Ce3+ thin films coated with a TaSi2 thin layer

SrGa2S4:Ce3+ thin films coated with a very thin layer of TaSi2 were deposited on Si(100) substrates utilizing the pulsed laser deposition system. During the ablation of the targets for thin films preparation, the Si(100) substrate temperature varied in the range of 400–600 °C. The as-deposited SrGa2S4:Ce3+ films showed an orthorhombic crystal structure, and the TaSi2 coating layer showed a hexagonal structure. The films maintained the crystal structures within the temperature ranges of 400–600 °C. The distribution of particles with irregular sizes contributing to a difference in surface roughness of the as-prepared thin films was observed by atomic force microscopy. The films showed a broad photoluminescence (PL) peak at 416 nm, and the maximum PL intensity, as compared to other films, was observed for a film deposited at a substrate temperature of 450 °C. Two broad cathodoluminescence peaks (440 and 490 nm) due to Ce3+ emission were observed for a film deposited at a substrate temperature of 450 °C. The presence of the TaSi2 coating layer on the SrGa2S4:Ce3+ thin films was confirmed with Auger electron spectroscopy surface elemental analysis, and Auger depth profiles showed uniform concentrations of the main elements in the deposited films.

Development of Combinatorial Pulsed Laser Deposition for Expedited Device Optimization in CdTe/CdS Thin-Film Solar Cells

A combinatorial pulsed laser deposition system was developed by integrating a computer controlled scanning sample stage in order to rapidly screen processing conditions relevant to CdTe/CdS thin-film solar cells. Using this system, the thickness of the CdTe absorber layer is varied across a single sample from 1.5 μm to 0.75 μm. The effects of thickness on CdTe grain morphology, crystal orientation, and cell efficiency were investigated with respect to different postprocessing conditions. It is shown that the thinner CdTe layer of 0.75 μm obtained the best power conversion efficiency up to 5.3%. The results of this work shows the importance that CdTe grain size/morphology relative to CdTe thickness has on device performance and quantitatively exhibits what those values should be to obtain efficient thin-film CdTe/CdS solar cells fabricated with pulsed laser deposition. Further development of this combinatorial approach could enable high-throughput exploration and optimization of CdTe/CdS solar cells.

On the connection between photo catalytic activities and magnetic properties of TiO2−x films

Pure and Mn-doped TiO2−x films were grown on corning glass substrates by the pulsed laser deposition (PLD) system in vacuum at two different temperatures, 350 °C and 580 °C. The films grown at 350 °C were non-crystalline while those grown at 580 °C were semi-crystalline. The effect of Mn on TiO2−x films for photocatalytic activities and paramagnetic properties were investigated. The results revealed that the films grown at 580 °C photos-degrade Rhodamine-6G (Rh-6G) and Methyl Orange (MO) faster than those of 350 °C with the 10% Mn films showing an enhanced performance. On the contrary, the 350 °C grown films photo-degrades Methylene Blue (MB) much faster than the 580 °C films with the 20% Mn doped film being the most efficient. In both temperatures, the 20% Mn films possess highest MB degradation rate and spin concentration at 150 and 310 mT fields. The 10% Mn film at 580 °C possesses the highest degradation rate of both Rh-6G and MO but has the lowest spin concentration at 150 mT field. In addition, it was also found that the non-crystalline formation further enhances the degradation of MB dye.

Development of a high magnetic field assisted pulsed laser deposition system.

A high magnetic field assisted pulsed laser deposition (HMF-PLD) system has been developed to in situ grow thin films in a high magnetic field up to 10 T. In this system, a specially designed PLD cylindrical vacuum chamber is horizontally located in the bore configuration of a superconducting magnet with a bore diameter of 200 mm. To adjust the focused pulsed laser into the target in such a narrow PLD vacuum chamber, an ingeniously built-in laser leading-in chamber is employed, including a laser mirror with a reflection angle of 65° and a damage threshold up to 3.4 J/cm(2). A laser alignment system consisting of a built-in video-unit leading-in chamber and a low-energy alignment laser is applied to monitor and align the pulsed laser propagation in the PLD vacuum chamber. We have grown La0.7Sr0.3MnO3 (LSMO) thin films on (LaAlO3)0.3(Sr2AlTaO6)0.7 (001) [LSAT (001)] substrates by HMF-PLD. The results show that the nanostructures of the LSMO films can be tuned from an epitaxially continuous film structure without field to a vertically aligned nanorod structure with an applied high magnetic field above 5 T, and the dimension size of the nanorods can be tuned by the strength of the magnetic field. The associated magnetic anisotropy is found to be highly dependent on the nanorod structures. We show how the HMF-PLD provides an effective route toward tuning the nanostructures and the physical properties of functional thin films, giving it an important role in development of nanodevices and their application.

GaN-Based Enhancement-Mode Metal–Oxide–Semiconductor High-Electron Mobility Transistors Using LiNbO3Ferroelectric Insulator on Gate-Recessed Structure

To fabricate AlGaN/gallium nitride (GaN) enhancement-mode metal-oxide-semiconductor high-electron mobility transistors (E-MOSHEMTs), the gate-recessed structure and the LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ferroelectric film were utilized in this paper. The LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ferroelectric films deposited on the photoelectrochemically etched gate-recessed regions of the AlGaN/GaN E-MOSHEMTs as the gate insulator using a pulsed laser deposition system. The polarization-induced charges of the 2-D electron gas resided on the interface between the AlGaN and GaN layers could be modulated by the C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> domains of the crystalline (006) LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ferroelectric films annealed in an oxygen ambience at 600°C for 30 min. When the 15-nm-thick AlGaN was formed in the gate-recessed regions, the threshold voltage and the maximum transconductance of the resulting gate-recessed LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /AlGaN/GaN E-MOSHEMTs were +0.40 V and 56.0 mS/mm, respectively. Furthermore, the flicker noise was the dominant noise in the resulting E-MOSHEMTs. The associated normalized low-frequency noise power density of 8.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> was measured.

Ferroelectric and magnetic properties of Aurivillius Bim+1Ti3Fem−3O3m+3 thin films

Aurivillius Bim+1Ti3Fem−3O3m+3 (m = 4, 5, 6) thin films have been deposited by a pulsed laser deposition system. The x-ray diffraction patterns indicate the formation of orthorhombic phase. The remanent polarization (2Pr) of Bim+1Ti3Fem−3O3m+3 thin films is decreased with the m-number. Positive-up-negative-down measurements indicate the presence of ferroelectric (FE) polarization in as-obtained thin films. Piezoresponse force microscopy investigations confirm the existence of FE domains and the switchable polarization. Weak magnetic moment is detected in the Aurivillius films at room temperature. The present work suggests the possibility of Aurivillius Bim+1Ti3Fem−3O3m+3 (m = 4, 5, 6) materials as potential room-temperature multiferroics.

Effect of laser parameters on the measurement of U/Nd ratio using pulsed laser deposition followed by isotopic dilution mass spectrometry

Nd-YAG laser pulses of 1064, 532, 266nm in wavelength and of 8ns and 100ps in pulse width as well as KrF excimer laser pulses (248nm) of 30ns pulse width were employed to deposit thin films of the simulated spent nuclear fuel pellets. The deposited thin films were dissolved in nitric acid and analyzed for residual heavy element (U) to burn-up monitor (Nd) ratio using isotopic dilution mass spectrometry (employing the TIMS technique) to determine the burn-up in these simulated fuel pellets. While the use of ns-laser pulses of 1064nm wavelength resulted in high elemental fractionation with enhanced Nd concentration compared to the target composition, the 266nm ps-laser pulses produce films with least elemental fractionation in the film and hence is best suited for this analytical application. Due to the very small amount of material in the deposited film and the amenability of pulsed laser deposition system for use in radioactive shielded hot cells, this analytical technique is suitable for sampling highly radioactive materials.

Gas Sensing Properties of Epitaxial LaBaCo2O5.5+δ Thin Films.

Chemical reactivity and stability of highly epitaxial mixed-conductive LaBaCo2O5.5+δ (LBCO) thin films on (001) LaAlO3 (LAO) single-crystalline substrates, fabricated by using pulsed laser deposition system, were systematically investigated. Microstructure studies from x-ray diffraction indicate that the films are c-axis oriented with the interface relationship of [100]LBCO//[100]LAO and (001)LBCO//(001)LAO. LBCO thin films can detect the ethanol vapor concentration as low as 10ppm and the response of LBCO thin film to various ethanol vapor concentrations is very reliable and reproducible with the switch between air and ethanol vapor. Moreover, the fast response of the LBCO thin film, as the p-type gas sensor, is better than some n-type oxide semiconductor thin films and comparable with some nanorods and nanowires. These findings indicate that the LBCO thin films have great potential for the development of gas sensors in reducing/oxidizing environments.

Novel multilayer nano-composite protective coatings for metallic medical tools

Abstract Requirements for tribological protective coatings for medical tools, which would increase their wear and corrosion resistance, are very high. The presented paper deals with novel nano-composite, multilayer protective coatings for tissue interaction elaboration and their diagnosis on metallic substrates. A hybrid pulsed laser deposition system was used for coating deposition. In the presented work, nano-composite Cr/CrN + [Cr/a-C : H implanted by metallic nanocrystals] multilayer coatings were developed for surface protection. The mechanical properties of the coatings were investigated by means of micro-hardness and elasticity modulus measurements. Bio-medical tests were conducted using eukaryotic cells. Microstructural analysis by means of transmission electron microscopy indicated that chromium which was implanted into a-C : H layers reacted with carbon forming chromium carbides.

Anomalous redox properties and ultrafast chemical sensing behavior of double perovskite CaBaCo2O5+δ thin films

Perovskite oxide CaBaCo2O5+δ (CBCO) thin films with two divalent alkaline earth metal ions and the transition metal Co ion were prepared by pulsed laser deposition (PLD) system. X-ray diffraction reveals that the as-grown CBCO thin films have good crystallinity of the perovskite structure. Redox electrical properties and chemical sensing behavior on the as-grown CBCO thin films were systematically studied under various chemical environments. The electrical transport property studies indicate that the electrical resistivity increases with increase of temperature under reducing atmosphere and the temperature dependence of resistivity of the CBCO thin film in the oxidation processing above room temperature can be well fitted by small polaron hopping model. The chemical sensing property measurements reveal that the CBCO thin film is very sensitive to oxidizing/reducing environments in the temperature ranges from 620 K to 970 K. Especially, the reduced CBCO thin films with Co2+ and high oxygen vacancy concentration have a superfast oxygen sensing response from 620 to 970 K.