Electrical characterization of gadolinia-doped ceria films grown by pulsed laser deposition

Electrical characterization of 10 mol% gadolinia doped ceria (CGO10) films of different thicknesses prepared on MgO(100) substrates by pulsed laser deposition is presented. Dense, polycrystalline and textured films characterized by fine grains (grain sizes < 18 nm and < 64 nm for a 20-nm and a 435-nm film, respectively) are obtained in the deposition process. Grain growth is observed under thermal cycling between 300 and 800°C, as indicated by X-ray-based grain-size analysis. However, the conductivity is insensitive to this microstructural evolution but is found to be dependent on the sample thickness. The conductivity of the nanocrystalline films is lower (7.0×10^?4  S/cm for the 20-nm film and 3.6×10^?3  S/cm for the 435-nm film, both at 500°C) than that of microcrystalline, bulk samples ( $6\times 10^{-3}$  S/cm at 500°C). The activation energy for the conduction is found to be 0.83 eV for the bulk material, while values of 1.06 and 0.80 eV are obtained for the 20-nm film and the 435-nm film, respectively. The study shows that the ionic conductivity prevails in a broad range of oxygen partial pressures, for example down to about 10 ^?26  atm at 500°C.     

On the growth of gadolinia-doped ceria by pulsed laser deposition

In order to establish a new platform to manufacture micro-sized solid oxide fuel cells (SOFCs) with low operating temperatures, new design concepts, new preparation methods and new materials are being explored. Our studies in this paper are focused on the electrolyte material, and in particular gadolinia doped ceria (GDC), an electrolyte material, likely to replace the traditional yttria-stabilised zirconia (YSZ) for low temperature applications. GDC films were grown on a single crystal Si by pulsed laser deposition (PLD). The microstructure of the films as a function of growth time has been studied. We have found that the mean grain size increases with film thickness h as h^2/5, in agreement with theoretical results.     

Epitaxial growth of atomically flat gadolinia-doped ceria thin films by pulsed laser deposition

Epitaxial growth of Ce_0.8Gd_0.2O₂(CGO) films on (001) TiO₂-terminated SrTiO₃ substrates by pulsed laser deposition was investigated using in situ reflective high energy electron diffraction. The initial film growth shows a Stransky–Krastanov growth mode. However, this three-dimensional island formation is replaced by a two-dimensional island nucleation during further deposition, which results in atomically smooth CGO films. The obtained high-quality CGO films may be attractive for the electrolyte of solid-oxide fuel cells operating at low temperature.     

Nano-ionic thin films of gadolinia-doped ceria prepared by pulsed laser ablation

Microstructural properties of nano-ionic thin films of gadolinia-doped ceria (GDC) prepared by pulsed laser ablation from sintered targets of gadolinia (5–20 mol%) doped ceria are investigated. The ionic conductivity measurements of the sintered pellets showed a decrease in the activation energy from 1.1 to 0.65 eV for 5 and 30 mol% gadolinia-doped ceria, respectively. The microstructural properties of the GDC films as a function of substrate temperature, oxygen partial pressure, and laser energy show that the films are polycrystalline in the entire range of substrate temperature. The grain size is found to increase with increasing temperature up to 873 K. Further improved crystallinity is noticed for the films grown with oxygen partial pressure of 0.1–0.2 mbar. X-ray diffraction and transmission electron microscopy (TEM) reveal nanocrystalline grains with textured growth along <111> orientation in these films at low substrate temperature and at lower oxygen partial pressure. TEM study shows a uniform distribution of nanocrystal of 8–10 nm for energies ≤200 mJ/pulse, and nanocrystals embedded in a large crystalline matrix of doped ceria for energies in the range 400–600 mJ/pulse. Raman spectroscopy also confirms the defects in these films. The study also reveals that the substrate temperature and oxygen partial pressure could influence preferred orientation, while the laser energy could significantly influence defect concentration in these films. Invited paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006.     

Optical and electrical properties of aluminum-doped ZnO thin films grown by pulsed laser deposition

Transparent aluminum-doped zinc oxide (AZO) thin films were deposited on quartz glass substrates by pulsed laser deposition (PLD) from ablating Zn-Al metallic targets. The structural, electrical and optical properties of these films were characterized as a function of Al concentration (0-8 wt.%) in the target. Films were deposited at a low substrate temperature of 150 °C under 11 Pa of oxygen pressure. It was observed that 2 wt.% of Al in the target (or 1.37 wt.% of Al doped in the AZO film) is the optimum concentration to achieve the minimum film resistivity and strong ultraviolet emission. The presence of Al in the ZnO film changes the carrier concentration and the intrinsic defects.     

Structural, electrical and mechanical properties of NiO thin films grown by pulsed laser deposition

Nickel oxide (NiO) thin films were prepared by reactive pulsed laser deposition on thermally oxidized Si substrates in 10 Pa oxygen pressure. The substrate temperature during deposition was varied and its influence on the structural, electrical and nanomechanical properties was studied. It was proved that the structural properties were affected by the increase of substrate temperature improving the crystalline structure. Furthermore, a higher substrate temperature resulted in a thicker NiO film, which was attributed to an increased grain size. This effect influenced the electrical properties, too. Resistivity measurements showed that it increased with the increase of substrate temperature. For the first time, the nanomechanical properties of NiO films were studied. The formation and improvement of crystalline structure affected the nanomechanical properties. Nanoindentation testing of NiO thin films revealed an increase of hardness (H) and elastic modulus (E) and a decrease of surface roughness when increasing the substrate temperature.     

Structural and electrical properties of lithium manganese oxide thin films grown by pulsed laser deposition

LiMn₂O₄ thin films were deposited by reactive pulsed laser deposition technique and studied the microstructural and electrical properties of the films. The LiMn₂O₄ thin films deposited in an oxygen partial pressure of 100 mTorr and at a substrate temperature of 573 K from a lithium rich target were found to be nearly stoichiometric. The films exhibited predominantly (111) orientation representing the cubic spinel structure with Fd3m symmetry. The intensity of (111) peak increased and a slight shift in the peak position was observed with the increase of substrate temperature. The lattice parameter increased from 8.117 to 8.2417 Å with the increase of substrate temperature from 573 to 873 K. The electrical conductivity of the films is observed to be a strong function of temperature. The evaluated activation energy for the films deposited at 873 K is 0.64 eV.     

Correlation between electrical and optical properties of Cr:ZnO thin films grown by pulsed laser deposition

Highly transparent and conducting Chromium doped ZnO (Cr:ZnO) thin films with preferential c-axis orientation were grown on (0 0 0 1) sapphire substrates using buffer assisted pulsed laser deposition. The resistivity of Cr:ZnO thin films was found to decrease to a minimum value of ∼1.13×10⁻³Ω cm with the increasing Cr concentration up to ∼1.9 at.% and then increase with further increase of Cr concentration. On the contrary, the band gap and carrier concentration of Cr:ZnO thin films increased up to ∼3.37 eV and ∼2×10²⁰ cm⁻³, respectively, with the increase of Cr concentration up to ∼1.9 at.%, then decreased with further increase of Cr concentration. The increase of carrier concentration and conductivity with Cr doping at low Cr concentrations (<1.9 at.%) could be attributed to the presence of Cr in +3 valence state in ZnO thus acting as donor while decrease of carrier concentration beyond ∼1.9 at.% of Cr concentration could be attributed to the charge compensating effect due to the presence of acceptor like point defects such as oxygen interstitials. This was experimentally confirmed using x-ray photoelectron spectroscopy. The observed variation in the band gap of Cr:ZnO thin films with increasing Cr doping was attributed to the competing effects of the high free carrier concentration induced Burstein-Moss blue shift and band gap narrowing.     

Microstructural study of thin films of 5 mol% gadolinia-doped ceria prepared by pulsed laser ablation

Microstructural characterization of thin films of 5 mol% gadolinia-doped ceria films deposited by pulsed laser ablation in the energy range 100–600 mJ/pulse has been investigated. As-deposited films were found to be nanocrystalline with preferred orientation. X-ray diffraction (XRD) analysis revealed that the size of the nanocrystals of doped ceria does not vary significantly with increasing laser energy, while transmission electron microscopy (TEM) study showed a uniform distribution of nanocrystals of 8–10 nm for energies ≤200 mJ/pulse and nanocrystals embedded in a large crystalline matrix of doped ceria for energies in the range 400–600 mJ/pulse. Though, the laser-ablated films were totally free from secondary phases, lattice imaging of the large grained doped ceria showed growth-induced defects such as dislocations and ledges. This artice was accidentally published twice. This is the second publication, please cite only the authoritative first one which is available at . An additional erratum is available at . An erratum to this article can be found at     

Thermal stability and dielectric properties of ultrathin CaZrOx films prepared by pulsed laser deposition

The thermal stability and dielectric properties of amorphous CaZrO_x film prepared by pulsed laser deposition (PLD) have been investigated. X-ray diffraction (XRD) investigation shows that CaZrO_x film still remains amorphous after rapid thermal annealing at 700 °C for 10 min. Differential thermal analysis (DTA) indicates that the crystallization temperature of CaZrO_x film is about 729.53 °C, which is significantly higher than that of amorphous ZrO₂ films prepared at the similar conditions. High-resolution transmission electron microscopy (HRTEM) and X-ray photon spectroscopy (XPS) analysis reveal there exists a Si-O transition layer between the CaZrO_x film and Si substrate. The permittivity of CaZrO_x film is about 10.5 (at 1 MHz) by measuring a Pt/CaZrO_x/Pt MIM structure. Under the optimized conditions, a small EOT=0.91 nm and a leakage current density of 125 mA/cm² at 1 V gate voltage were obtained. The enhanced thermal stability and improved electrical characteristics suggest that the amorphous CaZrO_x film may be an attractive gate dielectric alternative for next generation MOS field effect transistor applications. PACS 77.55.+f; 81.15.Fg; 73.40.Qv     

Structural and morphological properties of thin ZnO films grown by pulsed laser deposition

The pulsed laser deposition technique was used to produce zinc oxide thin films onto silicon and Corning glass substrates. Homogeneous surfaces exhibiting quite small Root Mean Square (RMS) roughness, consisting of shaped grains were obtained, their grain diameters being 40-90 nm at room temperature and at 650 °C growth respectively. Films were polycrystalline, even for growth at room temperature, with preferential crystallite orientation the (0 0 2) basal plane of wurtzite ZnO. Temperature increase caused evolution from grain to grain agglomeration structures, improving crystallinity. Compressive to tensile stresses transition with temperature was found while the lattice constant decreased.     

Morphology and photoluminescence properties of zinc oxide films grown by pulsed laser deposition

Zinc oxide films with different morphologies have been grown by pulsed laser deposition, varying substrate temperature and oxygen pressure. At low oxygen pressure and low substrate temperature continuous films with different roughness have been obtained, while at high substrate temperature a film with sparse hexagonal pyramids has been observed. Increasing the oxygen pressure the film became rougher and at 100 Pa a rod-array has been deposited. The columns of this rod-array grew along the wurtzite c-axis perpendicularly to the substrate surface as proved by X-ray diffraction measurements. Near to the sample borders the columns were slightly tilted towards the center of the sample. The possible growth mechanisms giving rise to the different morphologies have been discussed. Low-temperature photoluminescence measurements allowed to get information about the film quality, showing the variations of the excitonic peak and two defect bands (green and violet-blue) with the different deposition parameters.     

Epitaxial lithium fluoride films grown by pulsed laser deposition

Lithium fluoride (LiF) films have been prepared on LaAlO₃ (LAO), MgO, Si and TiN buffered Si substrates using a pulsed laser deposition (PLD) technique. Their surface morphology and structural qualities were studied by using scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Those films deposited on (100)MgO and (100)TiN buffered Si exhibited (200)_LiF||(200)_MgOand(200)_LiF||(200)_TiN||(200)_Si epitaxial relationships, respectively. These results suggest that LiF films can be epitaxially grown on lattice match substrates and, for the first time, on (100)Si via a buffer layer. LiF grown on bare (100)Si and (100)LAO substrates, however, yielded films of (100) preferred orientation only. Lattice mismatch and thermal effects are invoked to explain these observations. The surfaces of the LiF films are very rough and covered with large globules due to splashing of molten droplets from the target. In order to remedy such deficiencies we used a shadow mask to reduce both the size and the number of these globules while maintaining the heteroepitaxial properties. As a result optically smooth LiF films of excellent structural quality are produced. PACS 78.55.FV; 81.15.FG; 68.55.JK     

Epitaxial ZrC thin films grown by pulsed laser deposition

ZrC thin films were grown on (0 0 1)Si, (1 1 1)Si and (0 0 0 1)sapphire substrates by the pulsed laser deposition (PLD) technique. X-ray diffraction, X-ray reflectivity and Auger electron spectroscopy investigations were used to characterize the structure and composition of the deposited films. It has been found that films grown at temperatures higher than 700 °C under very low water vapor pressures were highly textured. Films deposited on (0 0 1)Si grew with the (0 0 1) axis perpendicular to the substrate, while those deposited on (1 1 1)Si and (0 0 0 1)sapphire grew with the (1 1 1) axis perpendicular to the substrate. Pole figures investigations showed that films were epitaxial, with in-plane axis aligned to those of the substrate.     

Study of the structure and electrical properties of the copper nitride thin films deposited by pulsed laser deposition

Copper nitride thin films were prepared on glass and silicon substrates by ablating a copper target at different pressure of nitrogen. The films were characterized in situ by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and ex situ by X-ray diffraction (XRD). The nitrogen content in the samples, x = [N]/[Cu], changed between 0 and 0.33 for a corresponding variation in nitrogen pressure of 9 × 10⁻² to 1.3 × 10⁻¹ Torr. Using this methodology, it is possible to achieve sub-, over- and stoichiometric films by controlling the nitrogen pressure. The XPS results show that is possible to obtain copper nitride with x = 0.33 (Cu₃N) and x = 0.25 (Cu₄N) when the nitrogen pressure is 1.3 × 10⁻¹ and 5 × 10⁻² Torr, respectively. The lattice constants obtained from XRD results for copper nitride with x = 0.25 is of 3.850 Å and with x = 0.33 have values between 3.810 and 3.830 Å. The electrical properties of the films were studied as a function of the lattice constant. These results show that the electrical resistivity increases when the lattice parameter is decreasing. The electrical resistivity of copper nitride with x = 0.25 was smaller than samples with x = 0.33.     

Giant nonlinear optical properties of bismuth thin films grown by pulsed laser deposition

Thin films of Bi were grown by pulsed laser deposition on glass substrates at room temperature. The thickness and roughness of the films were characterized by grazing-incidence x-ray reflectivity, and the complex refractive indices were measured in the range from 1.5 to 4 eV by spectroscopic ellipsometry. We performed Z-scan measurements to study the third-order optical nonlinearity of the films. It was found that the Bi films exhibited an unusually large nonlinear refractive coefficient, n(I)~1.24x10(-1) cm(2)/kW and nonlinear absorption coefficient, alpha(I)~-3.97 cm/W , at low laser intensity, ~60 kW/cm(2) . This anomaly is believed to have an origin related to melting of the Bi films at the focus spot by the laser beam.     

Study of structural and optical properties of ZnO films grown by pulsed laser deposition

Wurtzite zinc oxides films (ZnO) were deposited on silicon (0 0 1) and corning glass substrates using the pulsed laser deposition technique. The laser fluence, target-substrate distance, substrate temperature of 300 °C were fixed while varying oxygen pressures from 2 to 500 Pa were used. It is observed that the structural properties of ZnO films depend strongly on the oxygen pressure and the substrate nature. The film crystallinity improves with decreasing oxygen pressure. At high oxygen pressure, the films are randomly oriented, whereas, at low oxygen pressures they are well oriented along [0 0 1] axis for Si substrates and along [1 0 3] axis for glass substrates. A honeycomb structure is obtained at low oxygen pressures, whereas microcrystalline structures were obtained at high oxygen pressures. The effect of oxygen pressure on film transparency, band gap E_g and Urbach energies was investigated.     

Optical and waveguiding properties of Nd:KGW films grown by pulsed laser deposition

 Nd: KGd(WO₄)₂ thin films were deposited by KrF laser ablation on MgO, YAP, YAG and Si substrates at temperatures up to 800 °C. Film crystallinity, morphology, stoichiometry (WDX, RBS and PIXE), excitation spectra, fluorescence, refractive index and waveguiding properties were studied in connection with deposition conditions. The best films were crystalline and exhibited losses of approximately 5 dB cm^-1 at a wavelength of 633 nm. Received: 8 January 2001 / Accepted: 7 November 2001 / Published online: 11 February 2002     

Evolution of Ag nanocrystal films grown by pulsed laser deposition

We have conducted the first experiments under identical thermal, background, and surface preparation conditions to compare metal-on-insulator growth morphology in Pulsed Laser Deposition (PLD) and Physical Vapor Deposition (PVD). Such films deposited from a thermal vapor are known to exhibit a characteristic morphological progression beginning with isolated three-dimensional islands and ending with a percolating, continuous film that conducts electrically. We have studied this progression for Pulsed Laser Deposition, a technique that differs from PVD in that depositing species arrive in short bursts (<10 microseconds) with kinetic energy typically of 10–100 eV. Kinetic Monte Carlo (KMC) simulations that take into account only the pulsed nature of the flux predict that PLD films should advance to percolation with relatively less deposition compared with PVD under otherwise identical conditions. Our experiments, with PLD and PVD performed in the same chamber, reveal that PLD films actually require more deposition to reach percolation. We conclude that energetic effects are important in determining morphology evolution. PACS 81.15.Fg; 68.55.-a; 81.07.Bc; 82.20.Wt     

Oxidation and thermal annealing effects on native and ion-irradiated PbTe films grown by molecular beam deposition

Investigations are reported into the effect of low-pressure oxygen exposure and thermal annealing on the carrier transport properties of native and 350 eV Ar⁺ bombarded PbTe films. The electrical measurements were madein situ on MBD-grown PbTe films without breaking vacuum. On native surfaces, oxidation was initially sustained by diffusion of a donor species from the film bulk to the surface, where reaction with oxygen occured. This diffusion process was apparently inhibited on ion irradiated films and direct doping of the film surface effected a gradual reduction in the ion-induced electron accumulation resident at the film surface. The native properties and behavioural characteristics of the films could be recovered by thermal annealing of the ion-irradiated and/or oxidized films at 300–350 °C.