Showing papers on "Pulsed laser deposition published in 2014"

Advances in thermochromic vanadium dioxide films

Abstract: Vanadium dioxide is a thermochromic material that undergoes a semiconductor to metal transitions at a critical temperature of 68 °C. This phase change from a low temperature monoclinic structure to a higher temperature rutile structure is accompanied by a marked change in infrared reflectivity and change in resistivity. This review presents the fundamental chemical principles that describe the electronic structure and properties of solids, and the chronological developments in the theory behind the thermochromic transitions such as, the effects of electron–electron interactions and structural phase changes due to lattice distortions. An extensive discussion and observations on the current understanding of the nature of the semiconductor-to-metal transition exhibited by vanadium dioxide is detailed. The possibility of manipulating the transition temperature by introducing various dopants, additional layers or by size effects into the vanadium dioxide lattice are examined. Thermochromic vanadium dioxide materials may be exploited in areas such as microelectronics, data storage, or intelligent architectural glazing, thus are required to be synthesised as thin films for use in such applications. The numerous synthetic techniques (physical vapour deposition, sol–gel method, pulsed laser deposition, chemical vapour deposition), for making metal oxide thermochromic thin films are described in reference to the production of vanadium dioxide with a particular focus on recent results.

Pulsed laser deposition of epitaxial yttrium iron garnet films with low Gilbert damping and bulk-like magnetization

Abstract: Yttrium iron garnet (YIG, Y 3Fe5O12) films have been epitaxially grown on Gadolinium Gallium Garnet (GGG, Gd3Ga5O12) substrates with (100) orientation using pulsed laser deposition. The films were single-phase, epitaxial with the GGG substrate, and the root-mean-square surface roughness varied between 0.14 nm and 0.2 nm. Films with thicknesses ranging from 17 to 200 nm exhibited low coercivity (<2 Oe), near-bulk room temperature saturation moments (∼135 emu cm−3), in-plane easy axis, and damping parameters as low as 2.2 × 10−4. These high quality YIG thin films are useful in the investigation of the origins of novel magnetic phenomena and magnetization dynamics.

Multi-wavelength Raman scattering of nanostructured Al-doped zinc oxide

Abstract: In this work we present a detailed Raman scattering investigation of zinc oxide and aluminum-doped zinc oxide (AZO) films characterized by a variety of nanoscale structures and morphologies and synthesized by pulsed laser deposition under different oxygen pressure conditions. The comparison of Raman spectra for pure ZnO and AZO films with similar morphology at the nano/mesoscale allows to investigate the relation between Raman features (peak or band positions, width, relative intensity) and material properties such as local structural order, stoichiometry, and doping. Moreover Raman measurements with three different excitation lines (532, 457, and 325 nm) point out a strong correlation between vibrational and electronic properties. This observation confirms the relevance of a multi-wavelength Raman investigation to obtain a complete structural characterization of advanced doped oxide materials.

Wide bandgap engineering of (AlGa)2O3 films

Abstract: Bandgap tunable (AlGa)2O3 films were deposited on sapphire substrates by pulsed laser deposition (PLD). The deposited films are of high transmittance as measured by spectrophotometer. The Al content in films is almost the same as that in targets. The measurement of bandgap energies by examining the onset of inelastic energy loss in core-level atomic spectra using X-ray photoelectron spectroscopy is proved to be valid for determining the bandgap of (AlGa)2O3 films as it is in good agreement with the bandgap values from transmittance spectra. The measured bandgap of (AlGa)2O3 films increases continuously with the Al content covering the whole Al content range from about 5 to 7 eV, indicating PLD is a promising growth technology for growing bandgap tunable (AlGa)2O3 films.

Pulsed laser-deposited MoS₂ thin films on W and Si: field emission and photoresponse studies.

Abstract: We report field electron emission investigations on pulsed laser-deposited molybdenum disulfide (MoS2) thin films on W-tip and Si substrates. In both cases, under the chosen growth conditions, the dry process of pulsed laser deposition (PLD) is seen to render a dense nanostructured morphology of MoS2, which is important for local electric field enhancement in field emission application. In the case of the MoS2 film on silicon (Si), the turn-on field required to draw an emission current density of 10 μA/cm2 is found to be 2.8 V/μm. Interestingly, the MoS2 film on a tungsten (W) tip emitter delivers a large emission current density of ∼30 mA/cm2 at a relatively lower applied voltage of ∼3.8 kV. Thus, the PLD-MoS2 can be utilized for various field emission-based applications. We also report our results of photodiode-like behavior in (n- and p- type) Si/PLD-MoS2 heterostructures. Finally we show that MoS2 films deposited on flexible kapton substrate show a good photoresponse and recovery. Our investigations t...

The structure and properties of amorphous indium oxide

Abstract: A series of In2O3 thin films, ranging from X-ray diffraction amorphous to highly crystalline, were grown on amorphous silica substrates using pulsed laser deposition by varying the film growth temperature. The amorphous-to-crystalline transition and the structure of amorphous In2O3 were investigated by grazing angle X-ray diffraction (GIXRD), Hall transport measurement, high resolution transmission electron microscopy (HRTEM), electron diffraction, extended X-ray absorption fine structure (EXAFS), and ab initio molecular dynamics (MD) liquid-quench simulation. On the basis of excellent agreement between the EXAFS and MD results, a model of the amorphous oxide structure as a network of InO x polyhedra was constructed. Mechanisms for the transport properties observed in the crystalline, amorphous-to-crystalline, and amorphous deposition regions are presented, highlighting a unique structure-property relationship.

Control of the Electrical Properties in Spinel Oxides by Manipulating the Cation Disorder

Abstract: In this work, the impact of cation disorder on the electrical properties of biaxially textured Co2ZnO4 and Co2NiO4 thin films grown by pulsed laser deposition are investigated using a combination of experiment and theory. Resonant elastic X-ray diffraction along with conductivity measurements both before and after post-deposition annealing show that Co2ZnO4 and Co2NiO4 exhibit opposite changes of the conductivity with cation disorder, which can be traced back to their different ground-state atomic structures, being normal and inverse spinel, respectively. Electronic structure calculations identify a self-doping mechanism as the origin of conductivity. A novel thermodynamic model describes the non-equilibrium cation disorder in terms of an effective temperature. This work offers a way of controlling the conductivity in spinels in a quantitative manner by controlling the cation disorder and a new design principle whereby non-equilibrium growth can be used to create beneficial disorder.

Pulsed Laser Deposition of Photoresponsive Two‐Dimensional GaSe Nanosheet Networks

Abstract: Here we explore pulsed laser deposition (PLD), a well known and versatile synthesis method principally used for epitaxial oxide thin film growth, for the synthesis of functional metal chalcogenide (GaSe) nanosheet networks by stoichiometric transfer of laser vaporized material from bulk GaSe targets in Ar background gas Uniform coverage of interconnected, crystalline, few-layer, photoresponsive GaSe nanosheets in both in-plane and out-of-plane orientations were achieved under different ablation plume conditions over ~15 cm2 areas Plume propagation was characterized by in situ ICCD-imaging High (1 Torr) Ar background gas pressures were found to be crucial for the stoichiometric growth of GaSe nanosheet networks Individual 1-3 layer GaSe triangular nanosheets of ~ 200 nm domain size were formed within 30 laser pulses, coalescing to form nanosheet networks in as few as 100 laser pulses The thickness of the deposited networks increased linearly with pulse number, adding layers in a two-dimensional (2D) growth mode while maintaining a surface roughness of 2 GaSe layers for increasing overall thickness Field effect transistors using these interconnected crystalline GaSe networks showed p-type semiconducting characteristics with mobilities reaching as high as 01 cm2V-1s-1 Spectrally-resolved photoresponsivities and external quantum efficiencies ranged from 04 AW-1 and 100% at 700 nm, tomore » 14 AW-1 and 600 % at 240 nm, respectively Pulsed laser deposition under these conditions appears to provide a versatile and rapid approach to stoichiometrically transfer and deposit photoresponsive networks of 2D nanosheets with digital thickness control and substrate-scale uniformity for a variety of applications« less

Determination of the mean and the homogeneous barrier height of Cu Schottky contacts on heteroepitaxial β‐Ga2O3 thin films grown by pulsed laser deposition

Abstract: We have investigated the electrical properties of Cu Schottky contacts (SCs) on (2¯01)-oriented β-Ga2O3 thin films, which have been grown by pulsed laser deposition (PLD). The I–V characteristics of two different sample structures exhibit rectification ratios at ±2 V up to 7 orders of magnitude. The dominant current transport mechanism is thermionic emission. By fitting the I–V characteristics, we obtained the ideality factor n and the effective barrier height ΦBeff at temperatures between 50 and 320 K. Considering a Gaussian barrier height distribution, we determined a mean barrier height of 1.32 eV. The contacts are stable at high temperatures up to at least 550 K. At this temperature a homogeneous barrier height of 1.32 eV is found, consistent with the determined mean barrier height. The ideality factor for this temperature is 1.03 and barrier inhomogeneities do not influence current transport, making the contact close to ideal. Schematic band diagram of a Cu/β-Ga2O3 Schottky contact at a temperature of 550 K. The inset shows a photographic image of the sample.

Pulsed laser deposition in Twente: from research tool towards industrial deposition

Abstract: After the discovery of the perovskite high Tc superconductors in 1986, a rare and almost unknown deposition technique attracted attention. Pulsed laser deposition (PLD), or laser ablation as it was called in the beginning, became popular because of the possibility to deposit complex materials, like perovskites, as thin film. By introducing in situ diagnostics and control of the laser fluence, PLD became a technique for several experimental studies of diverse complex materials. Nowadays, first steps towards industrial applications of PLD thin films on large wafers, up to 200 mm, are underway. In this paper we give a brief overview of the progress that PLD has made in our research group in Twente. Starting with control of deposition parameters, via in situ diagnostics with reflection high-energy electron diffraction and ending with the latest development in equipment for large-area deposition.

Multiferroic BaTiO 3 -BiFeO 3 composite thin films and multilayers: strain engineering and magnetoelectric coupling

Abstract: BiFeO3 and BaTiO3 were used to grow homogeneous composite thin films and multilayer heterostructures with 15 double layers by pulsed laser deposition. The perpendicular strain of the films was tuned by employing different substrate materials, i.e. SrTiO3(0 0 1), MgO(0 0 1) and MgAl2O4(0 0 1). Multiferroic properties have been measured in a temperature range from room temperature down to 2 K. The composite films show a high ferroelectric saturation polarization of more than 70 µ Cc m −2 . The multilayers show the highest magnetization of 2.3 emu cm −3 , due to interface magnetic moments and exchange coupling of the included weak ferromagnetic phases. The magnetoelectric coupling of the BaTiO3–BiFeO3 films was investigated by two methods. While the ferroelectric hysteresis loops in magnetic fields up to 8 T show only minor changes, a direct longitudinal AC method yields a magnetoelectric coefficient αME = ∂E/∂H of 20.75 V cm −1 Oe −1 with a low µ0HDC of 0.25 T for the 67% BaTiO3–33% BiFeO3 composite film at 300 K. This value is close to the highest reported in the literature.

Pulsed laser deposition of La1−xSrxMnO3: thin-film properties and spintronic applications

Abstract: Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed

Improved electrical mobility in highly epitaxial La:BaSnO3 films on SmScO3(110) substrates

Abstract: Heteroepitaxial growth of BaSnO3 and Ba1−xLaxSnO3 (x = 7%) lanthanum doped barium stannate thin films on different perovskite single crystal (SrTiO3 (001) and SmScO3 (110)) substrates has been achieved by pulsed laser deposition under optimized deposition conditions. X-ray diffraction measurements indicate that the films on either of these substrates are relaxed due to the large mismatch and present a high degree of crystallinity with narrow rocking curves and smooth surface morphology while analytical quantification by proton induced X-ray emission confirms the stoichiometric La transfer from a polyphasic target, producing films with measured La contents above the bulk solubility limit. The films show degenerate semiconducting behavior on both substrates, with the observed room temperature resistivities, Hall mobilities, and carrier concentrations of 4.4 mΩ cm, 10.11 cm2 V−1 s−1, and 1.38 × 1020 cm−3 on SmScO3 and 7.8 mΩ cm, 5.8 cm2 V−1 s−1, and 1.36 × 1020 cm−3 on SrTiO3 ruling out any extrinsic contribution from the substrate. The superior electrical properties observed on the SmScO3 substrate are attributed to reduction in dislocation density from the lower lattice mismatch.

Control of the conductivity of Si-doped β-Ga2O3 thin films via growth temperature and pressure

Abstract: Si-doped β-Ga2O3 thin films were grown at temperatures between 400 and 650 °C and oxygen partial pressures ranging from 3 × 10−4 mbar to 2.4 × 10−2 mbar prepared by pulsed laser deposition (PLD) on c-plane sapphire substrates. For high oxygen partial pressure the samples are composed of multiple crystalline phases; for decreasing oxygen partial pressure the crystallinity improves and single phase ()-oriented thin films are obtained for an oxygen partial pressure below at a growth temperature of 650 °C. We find a correlation between surface morphology of our thin films and their conductivity; an increasing root mean square surface roughness entails increased conductivity. Further we show that the oxygen partial pressure resulting in maximal conductivity decreases with increasing growth temperature. The results provide means to realize β-Ga2O3-based devices such as rectifiers, photodetectors or thin film transistors with optimized surface roughness, structural quality, and conductivity.

Increasing the Neel temperature of magnetoelectric chromia for voltage-controlled spintronics

Abstract: Boron doped chromia (Cr2O3) thin films with substitutional doping levels between zero and 3% are grown using pulsed laser deposition in borane background gases. Magnetometry reveals a tunable increase in the Neel temperature of the (0001) textured Cr2BxO3−x thin films at a rate of about 10% with 1% oxygen site substitution preserving a net boundary magnetization. Spin resolved inverse photoemission measured after magnetoelectric annealing in subsequently reversed electric fields evidences voltage-controlled reversal of boundary magnetization and thus magnetoelectricity of Cr2BxO3−x. Conservation of magnetoelectricity far above room temperature makes ultra-low power voltage-controlled spintronic devices feasible.

Optimization of the semiconductor-metal transition in VO2 epitaxial thin films as a function of oxygen growth pressure

Abstract: High quality VO2 epitaxial thin films were deposited on sapphire single crystal substrates by pulsed laser deposition and their semiconductor-to-metal transitions (SMTs) were characterized as a function of film growth conditions. Varying the oxygen pressure during deposition affected the number of oxygen vacancies, which allowed tuning of the crystal structure and phase transition properties of the VO2 films. Films grown at optimized conditions exhibited a significant resistivity drop (>104 Ω-cm) across the SMT that is correlated with the strain due to oxygen vacancies. This resistivity drop is mainly accounted for by a large change in carrier density at the SMT.

Growth Mechanism of Pulsed Laser Fabricated Few-Layer MoS2 on Metal Substrates

Abstract: Pulsed laser deposition (PLD) on metal substrates has recently been discovered to present an alternative method for producing highly crystalline few-layer MoS2. However, not every metal behaves in the same manner during film growth, and hence, it is crucial that the ability of various metals to produce crystalline MoS2 be thoroughly investigated. In this work, MoS2 was deposited on metal substrates, Al, Ag, Ni, and Cu, using a pulsed laser. Highly crystalline few-layer MoS2 was successfully grown on Ag, but is absent in Al, Ni, and Cu under specific growth conditions. This discrepancy was attributed to either excessively strong or insufficient adlayer–substrate interactions. In the case of Al, the effects of the strong interface interactions can be offset by increasing the amount of source atoms supplied, thereby producing semicrystalline few-layer MoS2. The results show that despite PLD being a physical vapor deposition technique, both physical and chemical processes play an important role in MoS2 growth...

Compositionally Modulated Magnetic Epitaxial Spinel/Perovskite Nanocomposite Thin Films

Abstract: There is great interest in self-assembled oxide vertical nanocomposite films consisting of epitaxial spinel pillars in a single crystal perovskite matrix, due to their tunable electronic, magnetic, and multiferroic properties. Varying the composition or geometry of the pillars in the out-of-plane direction has not been previously reported but can provide new routes to tailoring their properties in three dimensions. In this work, ferrimagnetic epitaxial CoFe2O4, MgFe2O4, or NiFe2O4 spinel nanopillars with an out-of-plane modulation in their composition and shape are grown in a BiFeO3 matrix on a (001) SrTiO3 substrate using pulsed laser deposition. Changing the pillar composition during growth produces a homogeneous pillar composition due to cation interdiffusion, but this can be suppressed using a sufficiently thick blocking layer of BiFeO3 to produce bi-pillar films containing for example a layer of magnetically hard CoFe2O4 pillars and a layer of magnetically soft MgFe2O4 pillars, which form in different locations. A thinner blocking layer enables contact between the top of the CoFe2O4 and the bottom of the MgFe2O4 which leads to correlated growth of the MgFe2O4 pillars directly above the CoFe2O4 pillars and provides a path for interdiffusion. The magnetic hysteresis of the nanocomposites is related to the pillar structure.