Showing papers on "Pulsed laser deposition published in 2007"

Leakage mechanisms in BiFeO3 thin films

Abstract: The authors report results of transport studies on high quality, fully epitaxial BiFeO3 thin films grown via pulsed laser deposition on SrRuO3∕DyScO3 (110) substrates. Ferroelectric tests were conducted using symmetric and asymmetric device structures with either SrRuO3 or Pt top electrodes and SrRuO3 bottom electrodes. Comparison between these structures demonstrates the influence of electrode selection on the dominant transport mechanism. Analysis of film electrical response suggests Poole-Frenkel emission as the limiting leakage current mechanism in the symmetric structure. Temperature dependent measurements yield trap ionization energies of ∼0.65–0.8eV. No clear dominant leakage mechanism was observed for the asymmetric structure.

Origin of Charge Density at LaAlO3 on SrTiO3 Heterointerfaces: Possibility of Intrinsic Doping

Abstract: As discovered by Ohtomo and Hwang, a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic, and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.

Blue luminescent centers and microstructural evaluation by XPS and Raman in ZnO thin films annealed in vacuum, N2 and O2

Abstract: Zinc oxide (ZnO) thin films prepared on sapphire substrates by employing a pulsed laser deposition (PLD) have been annealed in vacuum, N 2 , and O 2 ambient at annealing temperature 600 ° C. X-ray diffraction (XRD) and atomic force microscopy (AFM) observation show that the annealed thin films possess (0 0 0 2) textured feature and form better crystal grains with a large size. X-ray photoelectron spectroscopy (XPS) and Raman analysis show the defects of ZnO thin films annealed in different ambient is distinctly different. The film annealed in vacuum possesses large oxygen vacancies (V o ) and Zn interstitials (Zn i ); while large surface defects exist in film annealed in N 2 ambient. The concentrations of the intrinsic and extrinsic defects are the lowest in ZnO thin film annealed in oxygen gas. Photoluminescence (PL) spectra also reveal that the ultraviolet (UV) emission is the best for thin film annealed in O 2 . The blue emission (2.66 eV) is ascribed to the electronic transition from the donor energy level of Zn interstitials to acceptor energy level of Zn vacancies rather than the O vacancy.

Room temperature ferromagnetism in Zn1−xCuxO thin films

Abstract: Here the authors report systematic studies on the epitaxial growth and properties of Zn1−xCuxO (x=0.02–0.1) thin films deposited onto sapphire c-plane single crystals using pulsed-laser deposition. X-ray diffraction and high resolution transmission electron microscopy (HRTEM) were employed to study the epitaxial relations of Zn1−xCuxO with the substrate, and x-ray photoelectron spectroscopy was used to establish the bonding characteristics and oxidation states of copper inside the ZnO host. Room temperature ferromagnetism was observed in the Zn1−xCuxO films with magnetic moment per Cu atom decreasing with an increasing Cu content. The presence of any magnetic phase was ruled out using HRTEM. Thus, the ferromagnetism was attributed to Cu ions substituted into the ZnO lattice.

Generation of nanoparticle colloids by picosecond and femtosecond laser ablations in liquid flow

Abstract: Fabrication of silver nanoparticle colloids using ultrashort pulse laser ablation in water is studied. Ablation in liquid flow improves the reproducibility and increases the nanoparticle productivity by 380% compared to stationary liquid. Femtosecond laser ablation in water is 20% more efficient than picosecond laser ablation, but due to higher picosecond laser power (higher repetition rate), the nanoparticle productivity at the same pulse fluence is three times higher for picosecond laser ablation. With picosecond laser pulses, the maximum productivity of 8.6μg∕s is achieved at a pulse energy of 110μJ and repetition rate of 50kHz.

Properties of nanoparticles generated during femtosecond laser machining in air and water

Abstract: Femtosecond laser ablation is used to generate nanoparticle aerosols and colloids from solid targets of various materials (Ti, Ag, Au, Co, etc.) in air and water ambience. We determine the influence of different laser parameters (pulse energy, pulse overlap) and properties of media (air, airbrush, water) on the rate of production and size distribution of the laser-generated nanoparticles. It is shown that the pulse overlap and laser fluence are the parameters determining the nanoparticle size. At optimum conditions the nanoparticle productivity can be increased by 150–300%. The generation of multimaterial nanoparticle dispersions is demonstrated. Being free of toxic impurities, the laser-produced nanoparticles may be promising for biomedical applications.

Quantitative Comparison of Mixed Conducting SOFC Cathode Materials by Means of Thin Film Model Electrodes

Abstract: Geometrically well-defined model electrodes have been employed to unambiguously elucidate the individual resistive and capacitive processes of various solid oxide fuel cell cathodes by means of impedance spectroscopy. The measurements were performed on dense, thin film-type microelectrodes of La 1-x Sr x Co 1-y Fe y O 3-δ and related perovskite-type materials prepared by pulsed laser deposition and photolithography. It was found that the substitution of the A-site cation La in La 1-x Sr x Co 1-y Fe y O 3-δ by Sm and especially by Ba leads to a strong enhancement of the surface exchange kinetics, whereas a variation of the Co/Fe ratio between 0 and 1 has only little effect on this quantity at temperatures around 750°C. Furthermore, it has been studied how the electrochemical activation effect, i.e., the strong reduction of the surface exchange resistance after application of a large dc bias, depends on composition.

Effect of thickness on structural, electrical, and optical properties of ZnO: Al films deposited by pulsed laser deposition

Abstract: To evaluate the influence of thickness on structural, electrical, and optical properties of ZnO:Al (AZO) films, a set of polycrystalline AZO samples with different thicknesses were deposited by pulsed laser deposition. X-ray diffraction measurement shows that the crystal quality of AZO films was improved with the increase of film thickness. Film surface morphology reveals that a transition of growth mode from vertical growth to lateral growth exists when the films become thicker. The resistivity decrease of AZO films with increase of film thickness owes to the change of carrier concentration for 80% regardless of film thickness in the visible region. The photoluminescence spectra of AZO films can be fitted well by seven emissions, and the emi...

Temperature-dependent leakage mechanisms of Pt/BiFeO3/SrRuO3 thin film capacitors

Abstract: Epitaxial c-axis oriented BiFeO3 (BFO) thin films were deposited on conductive SrRuO3 (SRO) on (001) SrTiO3 substrates by pulsed laser deposition. A Pt/BFO/SRO capacitor was constructed by depositing a top Pt electrode. The leakage current density versus. electric field characteristics were investigated from 80to350K. It was found that the leakage mechanisms were a strong function of temperature and voltage polarity. At temperatures between 80 and 150K, space-charge-limited current was the dominant leakage mechanism for both negative and positive biases. On the other hand, at temperatures between 200 and 350K the dominant leakage mechanisms were Poole-Frenkle emission and Fowler-Nordheim tunneling for negative and positive biases, respectively.

Fabrication of Cu2ZnSnS4 Thin-Film Solar Cell Prepared by Pulsed Laser Deposition

Abstract: A thin-film solar cell based on a Cu2ZnSnS4 (CZTS) absorber layer deposited by pulsed laser deposition has been fabricated with an Al:ZnO (n-type) window layer and a CdS buffer layer. Some peaks attributed to (112), (200), (220), and (312) planes of CZTS appeared in an X-ray diffraction pattern of a thin film. The composition of the film was Sn-rich and the band gap energy was approximately 1.5 eV. A CZTS film annealed at 500 °C in an atmosphere of N2 had optical characteristics suitable for use in an absorber layer of a thin-film solar cell and was used for a solar cell. The CZTS thin-film solar cell with an active area of 0.092 cm2 showed an open-circuit voltage of 546 mV, a short-circuit current of 6.78 mA/cm2, a fill factor of 0.48, and a conversion efficiency of 1.74%.

The role of oxygen vacancies in epitaxial-deposited ZnO thin films

Abstract: ZnO thin films were epitaxial deposited on sapphire (0001) substrates at various temperatures by using the pulsed laser deposition (PLD) technique. An x-ray diffractometer (XRD) was used to investigate the structural properties of the thin film. It was found that all of the thin films were (0002) oriented and the intensity of (0002) peak increased with the increasing growth temperature. The ϕ-scans for the thin films indicated that the thin film grown at a temperature higher than 400 °C had an epitaxial relation with the substrate. An atomic force microscope (AFM) was used to investigate the surface morphologies of the thin films. The surface roughness and grain size of the thin films increased with increasing growth temperature. A double-beam spectrophotometer was used to measure the transmittances of the thin films. The band gap energies of the thin films were calculated by linear fitting the absorption edges for high-quality thin films. A spectrometer was used to investigate the photoluminescent (PL) p...

Acetone and ethanol solid-state gas sensors based on TiO2 nanoparticles thin film deposited by matrix assisted pulsed laser evaporation

Abstract: Matrix assisted pulsed laser evaporation (MAPLE) is a new promising laser-based technique thought for polymer or biomaterial thin films deposition. In this work, the MAPLE technique has been used for the deposition of titania (TiO 2 ) nanoparticle thin films to be used for gas sensor applications. For this purpose, an aqueous solution of TiO 2 nanoparticles, synthesized by a novel chemical route, was frozen at the liquid nitrogen temperature and introduced in a vacuum chamber to be irradiated with a pulsed ArF excimer laser. The volatile solvent was pumped away while the TiO 2 nanoparticles were deposited on Si and Al 2 O 3 substrates. A uniform distribution of TiO 2 nanoparticles with an average size of about 10 nm was obtained, as demonstrated by high resolution scanning electron microscopy (SEM-FEG) inspection. We realized gas-sensing devices based on resistive transduction mechanism by using the TiO 2 nanoparticles thin films deposited by the MAPLE technique onto suitable rough alumina substrates equipped with interdigitated electrical contacts (IDC) and heating elements. Electrical characterization measurements were carried out in controlled environment. The results of the gas-sensing tests towards low concentrations of ethanol and acetone vapors are reported. Typical gas sensor parameters (gas responses, response/recovery time, sensitivity, and low detection limit) towards ethanol were extracted and compared with to the corresponding parameters towards acetone.

Gallium-doped zinc oxide films as transparent electrodes for organic solar cell applications

Abstract: We report microstructural characteristics and properties of gallium-doped ZnO films deposited on glass by pulsed laser deposition. The Zn0.95Ga0.05O film deposited at 200 °C and 1×10−3 Torr showed predominant ⟨0001⟩ orientation with a metallic behavior and a resistivity of 2×10−4 Ω cm at room temperature. Low resistivity of the ZnGaO films has been explained in terms of optimal combination of carrier concentration and minimized scattering, and is correlated with the microstructure and the deposition parameters. Power conversion efficiency comparable to indium tin oxide-based devices (1.25±0.05%) is achieved on a Zn0.95Ga0.05O∕Cu-phthalocyanine∕C60 double-heterojunction solar cell.

Structure, microstructure and physical properties of ZnO based materials in various forms: bulk, thin film and nano

Abstract: ZnO is a unique material that offers about a dozen different application possibilities. In spite of the fact that the ZnO lattice is amenable to metal ion doping (3d and 4f), the physics of doping in ZnO is not completely understood. This paper presents a review of previous research works on ZnO and also highlights results of our research activities on ZnO. The review pertains to the work on Al and Mg doping for conductivity and band gap tuning in ZnO followed by a report on transition metal (TM) ion doped ZnO. This review also highlights the work on the transport and optical studies of TM ion doped ZnO, nanostructured growth (ZnO polycrystalline and thin films) by different methods and the formation of unique nano- and microstructures obtained by pulsed laser deposition and chemical methods. This is followed by results on ZnO encapsulated Fe3O4 nanoparticles that show promising trends suitable for various applications. We have also reviewed the non-linear characteristic studies of ZnO based heterostructures followed by an analysis on the work carried out on ZnO based phosphors, which include mainly the nanocrystalline ZnO encapsulated SiO2 ,an ew class of phosphor that is suitable for white light emission. (Some figures in this article are in colour only in the electronic version)

Phase Transitions and High-Voltage Electrochemical Behavior of LiCoO2 Thin Films Grown by Pulsed Laser Deposition

Abstract: The phase transitions and electrochemical behavior of LiCoO 2 thin-film cathodes prepared by pulsed laser deposition are studied for charging voltages ranging from 4.2 to 4.9 V. Two voltage plateaus at about 4.55 and 4.62 V are observed in the charge-discharge curves. Ex situ X-ray diffraction measurements confirm structural changes and a phase transition from the 03 to the H1-3 phase when LiCoO 2 is charged above 4.5 V. Thin-film LiCoO 2 electrodes show stable capacity retention up to 4.5 V but significant capacity fade above 4.5 V, which we attribute to damage from strain due to phase transitions and Li gradients. Li diffusion in the Li x CoO 2 thin film up to 4.5V is investigated by the potentiostatic intermittent titration technique. The chemical diffusion coefficient of Li reaches the maximum near x = 0.4 and starts to decrease when the Li content is further decreased.

Fabrication of highly conductive Ti1- xNbxO2 polycrystalline films on glass substrates via crystallization of amorphous phase grown by pulsed laser deposition

Abstract: Nb-doped anatase TiO2 [Ti0.94Nb0.06O2 (TNO)] films with high electrical conductivity and transparency were fabricated on nonalkali glass using pulsed laser deposition and subsequent annealing in a H2 atmosphere. The amorphous films as deposited on unheated substrates were found to crystallize, forming polycrystalline films at around 350°C. The films annealed at 500°C showed resistivity down to 4.6×10−4Ωcm at room temperature and optical transmittance of 60%–80% in the visible region, which are comparable to those of epitaxial films. These results indicate that TNO films have the potential to be practical transparent conducting oxides that could replace indium tin oxide.

Room temperature pulsed laser deposited indium gallium zinc oxide channel based transparent thin film transistors

Abstract: Indium gallium zinc oxide deposited by pulsed laser deposition at room temperature was used as a channel layer to fabricate transparent thin film transistors with good electrical characteristics: field effect mobility of 11cm2V−1s−1 and subthreshold voltage swing of 0.20V∕decade. By varying the oxygen partial pressure during deposition the conductivity of the channel was controlled to give a low off-current of ∼10pA and a drain current on/off ratio of ∼5×107. Changing the channel layer thickness was a viable way to vary the threshold voltage. The effect of the gate dielectric on the electrical behavior was also explored.

Niobium doped TiO2: Intrinsic transparent metallic anatase versus highly resistive rutile phase

Abstract: We report on the structural, electrical, and optical properties of 5% niobium doped TiO2 thin films grown on various substrates by pulsed laser deposition. The epitaxial anatase Nb:TiO2 film on LaAlO3 is shown to be an intrinsic transparent metal and its metallic property arises from Nb substitution into Ti site as evidenced by the Rutherford backscattering channeling result. In contrast, the rutile Nb:TiO2 thin films show insulating behaviors with 2–3 orders higher room temperature electrical resistivity and ∼30 times lower mobility. A blueshift in the optical absorption edge is observed in both phases, though of differing magnitude.

ZnIr2O4, a p-type transparent oxide semiconductor in the class of spinel zinc-d6-transition metal oxide

Abstract: The authors report on the growth of spinel ZnMd62O4 M=Co, Rh, and Ir, a ρ-type wide band gap semiconductor by pulsed laser deposition. The band gap of these compounds is determined by the ligand field splitting in the subbands of the metallic d6 cation. Photoemission spectroscopy revealed that the valence band maximum is composed of occupied t2g 6 states. The observed band gap is increasing for higher quantum numbers, being as large as 3 eV for ZnIr2O4, which is expected from theoretical predictions. Grown in polycrystalline phase, films of these materials display high conductivity, well above 2 S cm−1.

CuBO2: A p-type transparent oxide

Abstract: The authors report the synthesis of CuBO2, a p-type transparent oxide belonging to Cu-delafossite family. High quality thin films of CuBO2 were deposited on c-plane sapphire substrates by pulsed laser deposition technique. Detailed structural, optical, and electrical characterizations on these films were performed. Optical transmission measurement showed the films to be highly transparent in the visible range and having indirect and direct band gaps of 2.2 and 4.5eV, respectively. Room-temperature electrical conductivity of CuBO2 films was 1.65Scm−1 and exhibited semiconductorlike temperature dependence. The Hall and Seebeck coefficients of the film were positive, indicating the p-type nature of the material.

Molecular dynamics simulation study of the ejection and transport of polymer molecules in matrix-assisted pulsed laser evaporation

Abstract: The physical mechanisms and molecular-level picture of laser-induced material ejection from frozen solutions of polymer molecules in a volatile matrix are investigated in a series of coarse-grained molecular dynamics simulations. The simulations are performed for polymer concentrations up to 6wt% and laser fluences covering the range from the regime where molecular ejection is limited to matrix evaporation from the surface up to more than twice the threshold fluence for the onset of the collective molecular ejection or ablation. The results of the simulations are related to experimental observations obtained in matrix-assisted pulsed laser evaporation (MAPLE) thin film depositions and are used to address unresolved research questions that are of direct relevance to MAPLE performance. Contrary to the original picture of the ejection and transport of individual polymer molecules in MAPLE, the simulations indicate that polymer molecules are only ejected in the ablation regime and are always incorporated into...

Nanoparticle Formation by Femtosecond Laser Ablation

Abstract: Ultra short femtosecond (fs) pulses for the laser ablation of materials lead to deposited films which are very different from those obtained by the well-known classical nanosecond (ns) pulsed laser deposition (PLD). In very specific cases, epitaxial thin films can be obtained, whereas in the majority of materials, the films formed by fs PLD are constituted by the random stacking of nanoparticles (Nps) in the 10–100 nm size range. As a result, fs PLD has been rapidly considered as a viable and efficient method for the synthesis of Nps of a wide range of materials presenting interesting physical properties and potential applications. The Np synthesis by fs laser ablation has been studied, and theoretical investigations have been reported to establish their formation mechanisms. Two possibilities can be assumed to explain the Np synthesis: direct cluster ejection from the target or collisional sticking and aggregation in the ablated plume flow.

Thin films of Co–B prepared by pulsed laser deposition as efficient catalysts in hydrogen producing reactions

Abstract: Hydrogen generation by catalytic hydrolysis of sodium borohydride (NaBH 4 ) is studied by using Co–B-based thin film catalyst synthesized by pulsed laser deposition (PLD) technique. Co–B nanoparticles, produced in the catalyst film after the laser ablation process, act as active centers producing significantly higher H 2 generation rate than Co–B bulk powder. Surface morphology was studied by using scanning electron microscopy and compositional analysis was established by using X-photoelectron and infrared spectroscopies. Films were deposited at different PLD set-up parameters in order to understand the possible role of size and density of the nanoparticles in the catalytic process. Cobalt has been found here to act as an efficient catalyst only when alloyed with boron which partially prevents cobalt oxidation. In particular we report that when used as suitable thin films, Co–B produces H 2 with a maximum generation rate of about 3300 ml/min per gram of catalyst.

Pulsed laser deposition of quaternary Cu2ZnSnSe4 thin films

Abstract: We report the successful growth of quaternary Cu2ZnSnSe4 (CZTSe) thin films by pulsed laser deposition (PLD) using Nd:YAG laser. It was found that CZTSe films atomic ratios were close to target atomic ratios with a slight metal excess and selenium deficiency. Quaternary CZTSe films grew and crystallized as a stannite-type structure even at room temperature. All CZTSe films showed a p-type electrical conductivity with a high absorption coefficient of 104–105 cm–1, a bandgap of 1.5 eV and a carrier concentration of the order of 1017–1018 cm–3. These results show that pulsed laser deposition could be employed as a particularly effective deposition method for the preparation of quaternary compounds thin films. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of epitaxial strain on ferroelectric polarization in multiferroic BiFeO3 films

Abstract: Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960 nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric polarization along the body diagonal of the distorted pseudo-cubic unit cells, as calculated from measurements along the normal direction, barely changes.

Reduced ferroelectric coercivity in multiferroic Bi0.825Nd0.175FeO3 thin film

Abstract: Bi0.825Nd0.175FeO3 (BNFO) thin film is grown on Pt∕TiO2∕SiO2∕Si substrate by pulsed laser deposition, and its multiferroic properties are compared with those of BiFeO3 (BFO) thin film. With limited Fe2+ ions and its twinborn oxygen vacancies, both samples show low dielectric losses of <0.026 at 100Hz and high maximum ferroelectric polarizations of ∼34μC∕cm2. The ferroelectric coercive field of BNFO is reduced by ∼40%, reaching a low value of 235kV∕cm, compared to that of BFO due to the increased ratio of 180° and curved ferroelectric domains to total ferroelectric domains and better nucleation of the ferroelectric domains at the BNFO∕Pt interface. The Raman scattering spectra confirm that the ferroelectric polarizations of both samples originate in the stereochemical activity of the Bi lone electron pair. Weak ferromagnetism is observed in both samples as a result of the limited amount of Fe2+ ions and γ‐Fe2O3 impurity.