Parallel syntheses and thermoelectric properties of Ce-doped SrTiO3 thin films

Thermoelectric properties of single crystalline Ce_xSr_1−xTiO₃ films (0 ≤ x ≤ 0.5) have been studied by using combinatorial pulsed-laser deposition. Temperature gradient method was used for identifying an optimum growth temperature for SrTiO₃ homoepitaxial growth, at which both oxygen stoichiometry and persisting layer-by-layer growth mode could be accomplished. Electrical conductivity (σ) and Seebeck coefficient (S) were measured at room temperature for the composition-spread films grown at the optimized temperature and found to be considerably higher than those reported for bulk poly-crystalline compounds. Hall measurement revealed that carrier density linearly increased with increasing x, suggesting that a trivalent Ce ions substituted divalent Sr ions to supply electrons. A maximum power factor (S²σ) was obtained for the x = 0.2 film, being 7 and 14 μW/K² cm at 300 and 900 K, respectively.     

Pulsed laser deposition of (110) oriented semiconductive SrFeO3−x thin films

 The semiconductive perovskite-type oxide SrFeO_3-x (x<0.16) (SFO) thin films have been directly fabricated on (001)SrTiO₃ and (001)LaAlO₃ single crystal substrates by pulsed laser deposition(PLD) under high oxygen partial pressure of 100 Pa. The SFO thin films were (110) oriented. The x-ray photoelectron spectroscopy (XPS) analysis showed that the surface of SFO thin film has strong gas absorption capability. The resistance versus temperature has been measured in the temperature range from 77 K to 300 K. The SFO thin film showed typical semiconductive property. Dependence of resistance of SFO thin film on oxygen pressure was measured and result showed that the SFO thin film had better oxygen sensitive property. Received: 14 May 1996/Accepted: 15 August 1996     

Structural and morphological characterization of TiO2 nanostructured films grown by nanosecond pulsed laser deposition

TiO₂ has attracted a lot of attention due to its photocatalytic properties and its potential applications in environmental purification and self cleaning coatings, as well as for its high optical transmittance in the visible-IR spectral range, high chemical stability and mechanical resistance. In this paper, we report on the growth of TiO₂ nanocrystalline films on Si (1 0 0) substrates by pulsed laser deposition (PLD). Rutile sintered targets were irradiated by KrF excimer laser (λ = 248 nm, pulse duration ∼30 ns) in a controlled oxygen environment and at constant substrate temperature of 650 °C. The structural and morphological properties of the films have been studied for different deposition parameters, such as oxygen partial pressure (0.05-5 Pa) and laser fluence (2- 4 J/cm²). X-ray diffraction (XRD) shows the formation of both rutile and anatase phases; however, it is observed that the anatase phase is suppressed at the highest laser fluences. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the stoichiometry of the grown films. The surface morphology of the deposits, studied by scanning electron (SEM) and atomic force (AFM) microscopies, has revealed nanostructured films. The dimensions and density of the nanoparticles observed at the surface depend on the partial pressure of oxygen during growth. The smallest particles of about 40 nm diameter were obtained for the highest pressures of inlet gas.     

Nonlinear optical properties in SrTiO3 thin films by pulsed laser deposition

Well-crystallized 250 nm-thick SrTiO₃ thin films on fused-quartz substrate were prepared by pulsed laser deposition. The band-gap of SrTiO₃ thin film by transmittance spectra is equal to 3.50 eV, larger than 3.22 eV for the bulk crystal. The nonlinear optical properties of the films were examined with picosecond pulses at 1.064 μm excitation. A large two-photon absorption (TPA) with absorption coefficient of 87.7 cm/GW was obtained, larger than 51.7 cm/GW for BaTiO₃ thin films. The nonlinear refractive index n₂ is equal to 5.7×10⁻¹⁰ esu with a negative sign, larger than 0.267×10⁻¹¹ esu for bulk SrTiO₃. The large TPA is attributed to intermediate energy levels introduced by the grain boundaries, and the optical limiting behaviors stemming from both TPA and negative nonlinear refraction were also discussed.     

Structural and transport properties of cubic spinel ZnCo2O4 thin films grown by reactive magnetron sputtering

We report on the growth of cubic spinel ZnCo₂O₄ thin films by reactive magnetron sputtering and bipolarity of their conduction type by tuning of oxygen partial pressure ratio in the sputtering gas mixture. Crystal structure of zinc cobalt oxide films sputtered in an oxygen partial pressure ratio of 90% was found to change from wurtzite Zn_1−xCo_xO to spinel ZnCo₂O₄ with an increase of the sputtering power ratio between the Co and Zn metal targets, D_Co/D_Zn, from 0.1 to 2.2. For a fixed D_Co/D_Zn of 2.0 yielding single-phase spinel ZnCo₂O₄ films, the conduction type was found to be dependent on the oxygen partial pressure ratio: n-type and p-type for the oxygen partial pressure ratio below ∼70% and above ∼85%, respectively. The electron and hole concentrations for the ZnCo₂O₄ films at 300 K were as high as 1.37×10²⁰ and 2.81×10²⁰ cm⁻³, respectively, with a mobility of more than 0.2 cm²/V s and a conductivity of more than 1.8 S cm⁻¹.     

TbMnO3 epitaxial thin films by pulsed-laser deposition

Epitaxial TbMnO₃ films have been fabricated on SrTiO₃(001) and LaAlO₃(001) substrates by pulsed laser deposition (PLD), the structure and surface morphology of the films were characterized by X-ray diffraction with Cu K_α radiation and atomic force microscopy. The electrical transport and magnetic properties of the TbMnO₃ films and bulk were examined, the resistivity and the forbidden band width E_g change with epitaxial orientation, semiconductor transport properties are found in the films and bulk, the average of the E_g of the films on SrTiO₃ and on LaAlO₃ is equal to the E_g of the bulk. The two TMO films have different magnetization mode, the magnetization of the film on SrTiO₃ have an analogy to that of TbMnO₃ single crystal.     

Comparison and semiconductor properties of nitrogen doped carbon thin films grown by different techniques

Amorphous carbon nitride (a-CN_x) thin films have been synthesised by three different deposition techniques in an Ar/N₂ gas mixture and have been deposited by varying the percentage of nitrogen gas in the mixture (i.e. the N₂/Ar + N₂ ratio) from 0 to 10%. The variation of the electrical conductivity and the gap values of the deposited films versus the N₂/Ar + N₂ ratio were investigated in relation with their local microstructure. Film composition was analysed using Raman spectroscopy and optical transmission experiments. The observed variation of electrical conductivity and optical properties are attributed to the changes in the atomic bonding structures, which were induced by N incorporation, increasing both the sp² carbon content and their relative disorder. The low N content samples seem to be an interesting material to produce films with interesting properties for optoelectronic applications considering the facility to control the gas composition as a key parameter.     

Growth and structure of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 thin films prepared by pulsed laser deposition technique

Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ (BNKLT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition (PLD) technique. The films prepared were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The effects of the processing parameters, such as oxygen pressure, substrate temperature and laser power, on the crystal structure, surface morphology, roughness and deposition rates of the thin films were investigated. It was found that the substrate temperature of 600 °C and oxygen pressure of 30 Pa are the optimized technical parameters for the growth of textured film, and all the thin films prepared have granular structure, homogeneous grain size and smooth surfaces.     

Properties of conductive polymer films deposited by infrared laser ablation

Thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) were deposited by resonant infrared laser vapor deposition (RIR-LVD). The PEDOT:PSS was frozen in various matrix solutions and deposited using a tunable, mid-infrared free-electron laser (FEL). The films so produced exhibited morphologies and conductivities that were highly dependent on the solvent matrix and laser irradiation wavelength used. When deposited from a native solution (1.3% by weight in water), as in matrix-assisted pulsed laser evaporation (MAPLE), films were rough and electrically insulating. When the matrix included other organic “co-matrices” that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good electrical conductivity (0.2 S/cm), but only when irradiated at certain wavelengths. These results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process.     

Characterization of the laser ablation plasma used for the deposition of amorphous carbon

The plasma produced by laser ablation of a graphite target was studied by means of optical emission spectroscopy and a Langmuir planar probe. Laser ablation was performed using a Nd:YAG laser with emission at the fundamental line with pulse length of 28 ns. In this work, we report the behavior of the mean kinetic energy of plasma ions and the plasma density, as a function of the laser fluence (J/cm²), and the target to probe (substrate) distance. The characterized regimes were employed to deposit amorphous carbon at different values of kinetic energy of the ions and plasma density. The mean kinetic energy of the ions could be changed from 40 to 300 eV, and the plasma density could be varied from 1 × 10¹² to 7 × 10¹³ cm⁻³. The main emitting species were C⁺ (283.66, 290.6, 299.2 and 426.65 nm) and C⁺⁺ (406.89 and 418.66 nm) with the C⁺ (426.65 nm) being the most intense and that which persisted for the longest times. Different combinations of the plasma parameters yield amorphous carbon with different structures. Low levels (about 40 eV) of ion energy produce graphitic materials, while medium levels (about 200 eV) required the highest plasma densities in order to increase the CC sp³ bonding content and therefore the hardness of the films. The structure of the material was studied by means of Raman spectroscopy, and the hardness and elastic modulus by depth sensitive nanoindentation.     

Optical characterization of PLD grown nitrogen-doped TiO2 thin films

Nitrogen-doped TiO₂ thin films were prepared by pulsed laser deposition (PLD) by ablating metallic Ti target with pulses of 248 nm wavelength in reactive atmospheres of O₂/N₂ gas mixtures. The layers were characterized by UV-VIS spectrophotometry and variable angle spectroscopic ellipsometry with complementary profilometry for measuring the thickness of the films. Band gap and extinction coefficient values are presented for films deposited at different substrate temperatures and for varied N₂ content of the gas mixture. The shown tendencies are correlated to nitrogen incorporation into the TiO_2-xN_x layers. It is shown that layers of significantly increased visible extinction coefficient with band gap energy as low as 2.89 eV can be obtained. A method is also presented how the spectroscopic ellipsometric data should be evaluated in order to result reliable band gap values.     

Optical, structural and fluorescence properties of nanocrystalline cubic or monoclinic Eu:Lu2O3 films prepared by pulsed laser deposition

Eu³⁺-doped lutetium oxide (Eu:Lu₂O₃) nanocrystalline films were grown on fused-silica substrates by pulsed laser deposition. Depending on deposition conditions (oxygen pressure, temperature and laser energy), the structure of the films changed from amorphous to crystalline and the cubic or monoclinic phases were obtained with varying preferential orientation and crystallite size. The monoclinic phase could be prepared for the first time at temperatures as low as 240 °C and in a narrow range of parameters. Although this phase has been previously reported for powder samples, it occurs only for high pressures and high temperatures preparation conditions. The refractive indices were measured by m-lines spectroscopy for both crystalline phases and their dispersion curve fitted by the Sellmeier expression. The specific Eu³⁺ fluorescence properties of the different phases, monoclinic and cubic, were registered and show modifications due to the disorder induced by the nanometric size of the crystallites, emphasised in particular by quasi-selective excitation in the charge transfer band.     

Epitaxial growth and characterization of CaVO3 thin films

Epitaxial thin films of CaVO₃ were synthesized on SrTiO₃, LaAlO₃ and (La_0.27Sr_0.73)(Al_0.65Ta_0.35)O₃ substrates by pulsed laser deposition. All CaVO₃ films, independent of epitaxial strain, exhibit metallic and Pauli paramagnetic behavior as CaVO₃ single crystals. X-ray absorption measurements confirmed the 4+ valence state for Vanadium ions. With prolonged air exposure, an increasing amount of V³⁺ is detected and is attributed to oxygen loss in the near surface region of the films.     

An alternative procedure for the determination of the optical band gap and thickness of amorphous carbon nitride thin films

In this work, we propose an alternative procedure to obtain the optical band gap and the thickness of amorphous carbon nitride thin films that requires only the measurement of the absorbance spectrum of the samples. The method is based on an absorbance spectrum fitting (ASF) procedure using the Tauc model, which is widely applied to the study of amorphous semiconductors. With the aim of evaluating the proposed method two sets of carbon nitride samples deposited on glass substrates were analyzed; one prepared by pulsed laser ablation (PLA) and the second by magnetron sputtering. The obtained results using different conventional methods were compared with the results of the ASF method and a good agreement between the values and the tendencies with the experimental conditions used to prepare the films were observed.     

Microstructural characterization of Ti-C-N thin films prepared by reactive crossed beam pulsed laser deposition

In this work, Raman spectroscopy has been used to characterize Ti-C-N thin films in order to obtain information about the microstructure of the deposited materials, and in particular to study the effects due to the carbon incorporation into the TiN lattice. Ti-C-N thin films were prepared using a crossed plasma configuration in which the ablation of two different targets, titanium and carbon, in a reactive atmosphere was performed. With this configuration, the carbon content in the films was varied in an easy way from 5.0 at% to 40.0 at%. Thin film composition was determined from Non-Rutherford Backscattering Spectroscopy (NRBS) measurements. X-ray photoelectron spectroscopy and X-Ray diffraction measurements were also carried out in order to characterize the films in more detail, with this being used to give support to the interpretation of the Raman spectra. The Raman results revealed that at lower carbon concentrations a solid solution Ti(C, N) is formed, whilst at higher carbon concentrations a nanocomposite, consisting of nanocrystalline TiCN and TiC immersed in an amorphous carbon matrix is obtained.     

Structural and electrical transport properties of Nb-doped TiO2 films deposited on LaAlO3 by rf sputtering

We have investigated the structural and electrical transport properties of Nb-doped TiO₂ films deposited on (1 0 0) LaAlO₃ substrates by rf magnetron sputtering at temperatures ranging from 873 K to 1073 K. Films deposited below 998 K are anatase, and mixed phases between anatase and rutile exist in the film grown at higher temperatures. We find that films deposited at low temperatures exhibit semiconductor behavior, while metallic conductivity is observed in the most conducting film deposited at 998 K. For this sample, compared to electron-phonon scattering mechanism, electron-phonon-impurity interference effect plays an important role in its electron transport process. Moreover, the temperature coefficient of the resistivity for the film deposited at 1073 K is negative from 2 K to 300 K. The temperature dependence of resistivity for the film is described by ∼exp(b/T)^1/2 at temperatures from 80 K down to 30 K, and by the fluctuation induced tunneling model from 80 K to 300 K.     

Optical band-edge absorption of oxide compound SnO2

Tin oxide (SnO₂) is an important oxide for efficient dielectrics, catalysis, sensor devices, electrodes and transparent conducting coating oxide technologies. SnO₂ thin film is widely used in glass applications due to its low infra-red heat emissivity. In this work, the SnO₂ electronic band-edge structure and optical properties are studied employing a first-principle and fully relativistic full-potential linearized augmented plane wave (FPLAPW) method within the local density approximation (LDA). The optical band-edge absorption α(ω) of intrinsic SnO₂ is investigated experimentally by transmission spectroscopy measurements and their roughness in the light of the atomic force microscopy (AFM) measurements. The sample films were prepared by spray pyrolysis deposition method onto glass substrate considering different thickness layers. We found for SnO₂ qualitatively good agreement of the calculated optical band-gap energy as well as the optical absorption with the experimental results.     

Effects of growth temperature on Li-N dual-doped p-type ZnO thin films prepared by pulsed laser deposition

Li-N dual-doped p-type ZnO (ZnO:(Li,N)) thin films have been prepared by pulsed laser deposition. The introduction of Li and N was confirmed by secondary ion mass spectrometry measurements. The structural, electrical, and optical properties as a function of growth temperature were investigated in detail. The lowest room-temperature resistivity of 3.99 Ω cm was achieved at the optimal temperature of 450 °C, with a Hall mobility of 0.17 cm²/V s and hole concentration of 9.12 × 10¹⁸ cm⁻³. The ZnO:(Li,N) films exhibited good crystal quality with a complete c-axis orientation, a high transmittance (about 90%) in the visible region, and a predominant UV emission at room temperature. The two-layer-structure p-ZnO:(Li,N)/n-ZnO homojunctions were fabricated on a sapphire substrate. The current-voltage characteristics exhibited the rectifying behavior of a typical p-n junction.     

SrTiO3 homoepitaxy by the pulsed laser deposition method: island, layer-by-layer, and step-flow growth

SrTiO₃ homoepitaxy was investigated under various conditions using the pulsed laser deposition method. The growth mode was determined by in-situ reflection high-energy electron diffraction, and the surface of the films was characterized by ex-situ atomic force microscopy. At the laser fluence of 0.68 J/cm², island growth was observed below 500 °C substrate temperature, while the growth mode turned into layer-by-layer growth above 500 °C. On further raising the substrate temperature, the step-flow growth mode prevailed above 800 °C. We thus demonstrated that step-flow growth in SrTiO₃ homoepitaxy is possible at a temperature as low as 800 °C.     

Resonant infrared pulsed laser deposition of thin biodegradable polymer films

Thin films of the biodegradable polymer poly(DL-lactide-co-glycolide) (PLGA) were deposited using resonant infrared pulsed laser deposition (RIR-PLD). The output of a free-electron laser was focused onto a solid target of the polymer, and the films were deposited using 2.90 (resonant with O-H stretch) and 3.40 (C-H) μm light at macropulse fluences of 7.8 and 6.7 J/cm², respectively. Under these conditions, a 0.5-μm thick film can be grown in less than 5 min. Film structure was determined from infrared absorbance measurements and gel permeation chromatography (GPC). While the infrared absorbance spectrum of the films is nearly identical with that of the native polymer, the average molecular weight of the films is a little less than half that of the starting material. Potential strategies for defeating this mass change are discussed. Received: 22 August 2001 / Accepted: 23 August 2001 / Published online: 17 October 2001