Evaluation of carbon films electrodeposited on different substrates from different organic solvents

Diamond-like-carbon (DLC) films have been deposited on Si, aluminum and indium tin oxide-coated glass from several organic solvents with pulse-modulated power. The films are characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS spectra show that the main composition of the films is carbon and Raman spectra show that the films are typical DLC films and a high potential is preferable in the formation of sp ³-structure carbon. Comparing the results from different solvents and different substrates we deduce that the methyl group of the solvents has a critical function in forming the DLC films. However, the formation process and the characters of the films, such as appearance, resistivity and thickness, are mainly determined by the substrate. We may call this deposition a substrate-controlled reaction. Received: 31 May 2000 / Accepted: 9 January 2001 / Published online: 3 April 2001     

Effects of entanglement concentration on Tg and local segmental motions

The process of spin-coating to fabricate thin polymer films with high molecular weight can produce samples with entanglement concentrations that are far below the equilibrium value. It is not clear whether or not such low entanglement concentrations are responsible for the depression of the glass temperature in thin polymer films. In this work, we measure the calorimetric glass temperature and viscoelastic response of polystyrenes with molecular weights ranging from 3×10³ to 43.7×10⁶ g/mol, for both bulk material and for samples freeze-dried from dilute solution. We conclude that the reduction of the glass temperature observed in thin polymer films cannot be due to the reduced entanglement concentration in the samples. Received 15 August 2001 and Received in final form 2 March 2002     

Effect of laser fluence on the deposition and hardnessof boron carbide thin films

We deposited amorphous thin films of boron carbide by pulsed laser deposition using a B₄C target at room temperature. As the laser fluence increased from 1 to 3 J/cm², the number of 0.25–5 μm particulates embedded in the films decreased, and the B/C atomic ratio of the films increased from 1.8 to 3.2. The arrival of melt droplets, atoms, and small molecular species depending on laser fluence appeared to be involved in the film formation. In addition, with increasing fluence the nanoindentation hardness of the films increased from 14 to 32 GPa. We believe that the dominant factor in the observed increase in the films’ hardness is the arrival of highly energetic ions and atoms that results in the formation of denser films. Received: 23 March 2001 / Accepted: 1 July 2001 / Published online: 2 October 2001     

Pulsed laser deposition of conductive metallo-dielectric optical filters

We describe the fabrication by room-temperature pulsed laser deposition of a transparent conductor comprising alternating layers of silver and aluminum oxide, forming a metallo-dielectric filter. Transmittances of 0.7 over specific wavelength bands were achieved with resistivities as low as 6.0×10^-6 Ω cm, almost two orders of magnitude lower than that of the best single-substrate thin films, such as indium tin oxide. The resistivity can be predicted without adjustable parameters and designed using a simple parallel-circuit model; the optical properties are well described by standard matrix transmission calculations. This demonstrates that pulsed laser deposition may be used to fabricate prototypes of high-quality transparent conductors with predictable properties for conducting windows where low-temperature deposition is critical, as in organic light-emitting diodes and for non-linear optical films. Received: 10 June 2001 / Accepted: 9 August 2001 / Published online: 20 December 2001     

Diamond-like-carbon films produced by magnetically guided pulsed laser deposition

We have compared the quality of carbon films deposited with magnetically guided pulsed laser deposition (MGPLD) and conventional pulsed laser deposition (PLD). In MGPLD, a curved magnetic field is used to guide the plasma but not the neutral species to the substrate to deposit the films while, in conventional PLD, the film is deposited with a mixture of ions, neutral species and clusters. A KrF laser pulse (248 nm) was focused to intensities of 10 GW/cm² on a carbon source target and a magnetic field strength of 0.3 T was used to steer the plasma around a curved arc to the deposition substrate. Electron energy loss spectroscopy was used in order to measure the fraction of sp³ bonding in the films produced. It is shown that the sp³ fraction, and hence the diamond-like character of the films, increased when deposited only with the pure ion component by MGPLD compared with films produced by the conventional PLD technique. The dependence of film quality on the laser intensity is also discussed. Received: 7 December 2000 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Rapid joule heating of metal films used to crystallize silicon films

We analyzed the rapid heating properties of 50-nm-thick silicon films via 250-nm-thick SiO₂ intermediate layers by heat diffusion from joule heating induced by electrical current flow in chromium strips. Numerical heat-flow simulation resulted in that the silicon films were heated to the melting point by a joule-heating intensity above 1 MW/cm². A marked increase in electrical conductance associated with silicon melting was experimentally detected. Taper-shaped chromium strips detected the temperature gradient in the lateral direction caused by the spatial distribution of the joule-heating intensity. Crystallization occurred according to the temperature gradient. A 2–4-μm lateral crystalline grain growth was demonstrated for the silicon films. Received: 20 November 2001 / Accepted: 22 November 2001 / Published online: 20 March 2002     

Strong ultraviolet and violet photoluminescence from Si-based anodic porous alumina films

We have investigated photoluminescence (PL) from Si-based anodic porous alumina films formed by real-time controlled anodization of electron-beam evaporated Al films. As-anodized samples show three strong PL bands at 295, 340, and 395 nm. These bands blueshift and their intensities decrease after the samples are annealed. When the annealing temperature increases to 1000 °C, the blueshift becomes specially pronounced and meanwhile the structures of the films develop toward crystalline Al₂O₃. Based on discussions on the thermal annealing behaviors of the PL and PL excitation spectra, we suggest that optical transitions in oxygen-related defects, F⁺ (oxygen vacancy with one electron) centers, are responsible for the observed ultraviolet and violet PL. Received: 24 July 2000 / Accepted: 24 February 2001 / Published online: 3 May 2001     

Deposition and characterization of ITO films produced by laser ablation at 355 nm

Indium tin oxide (ITO) films have been deposited by pulsed laser deposition (PLD) at 355 nm. Even though the absorption of laser light at the wavelength 355 nm is much smaller than that of the standard excimer lasers for PLD at 248 nm and 193 nm, high-quality films can be produced. At high fluence and at high substrate temperatures, the specific resistivity of the films, 2–3×10^-4 Ω cm, is comparable to values obtained with excimer lasers, whereas the resistivities obtained at room temperature are somewhat higher than those of films produced by excimer lasers. The transmission coefficient of visible light, about 0.9, is also comparable to values for films deposited by excimer lasers. The crystalline structure of films produced at 355 nm is similar to that of samples produced by these lasers. Received: 16 January 2001 / Accepted: 24 July 2001 / Published online: 17 October 2001     

Infrared transmission study of Pr<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>CuO<Subscript><Emphasis Type="Bold">4</Emphasis></Subscript> crystal-field excitations

We present an infrared crystal-field study of Pr₂CuO₄ single crystals and thin films. Excitations from the ground state multiplet ³H₄ to the ³H₅, ³H₆, ³F₂ and ³F₃ excited multiplets are observed in both single crystals and thin films. A precise set of crystal-field parameters, that reproduces the energy and the symmetry of the levels, is determined. Received 25 April 2001     

Transparent conducting antimony-doped tin oxide films deposited on flexible substrates by r.f. magnetron-sputtering

Transparent conducting antimony-doped tin oxide (SnO₂:Sb) films were deposited on organic substrates by r.f. magnetron-sputtering. Polycrystalline films with a resistivity of ≈ 6.5×10^-3 Ω cm, a carrier concentration of≈ 1.2×10²⁰ cm^-3 and a Hall mobility of ≈ 9.7 cm² v^-1 s^-1 were obtained. The average transmittance of these films reached 85% in the wavelength range of the visible spectrum. Received: 20 April 2001 / Accepted: 23 July 2001 / Published online: 17 October 2001     

Structural and electrical properties of polycrystalline PbZr0.5Ti0.5O3 films deposited on La0.5Sr0.5CoO3 coated silicon by sol-gel process

La_0.5Sr_0.5CoO₃ (LSCO) films have been grown on Si (100) by a metalorganic chemical liquid deposition (MOCLD) technique using lanthanum acetate, strontium acetate and cobalt acetate as the starting materials. Subsequent PbZr_0.5Ti_0.5O₃ (PZT) films were deposited onto LSCO films by a modified sol-gel method. Field-emission scanning electron microscopy and X-ray diffraction analysis show that PZT and LSCO films are polycrystalline and entirely perovskite phase. At an applied electric field of 250 kV/cm, the Pt/PZT/LSCO capacitor shows no polarization fatigue after 3×10⁹ switching cycles and an internal electric field; the remnant polarization P_r and the coercive field E_c are about 22 μC/cm² and 73 kV/cm, respectively. The dielectric constant of PZT films is 650 at a frequency of 1 kHz. Received: 20 February 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001     

Vapor deposition of intact polyethylene glycol thin films

Thin films of polyethylene glycol (PEG) of average molecular weight, 1400 amu, were deposited by both matrix-assisted pulsed laser evaporation (MAPLE) and pulsed laser deposition (PLD). The deposition was carried out in vacuum (∼10^-6 Torr) with an ArF (λ=193 nm) laser at a fluence between 150 and 300 mJ/cm². Films were deposited on NaCl plates, Si(111) wafers, and glass slides. The physiochemical properties of the films are compared via Fourier transform infrared spectroscopy (FTIR), electrospray ionization (ESI) mass spectrometry, and matrix-assisted laser desorption and ionization (MALDI) time-of-flight mass spectrometry. The results show that the MAPLE films nearly identically resemble the starting material, whereas the PLD films do not. These results are discussed within the context of biomedical applications such as drug delivery coatings and in vivo applications where there is a need for transfer of polymeric coatings of PEG without significant chemical modification. Received: 2 March 2001 / Accepted: 5 March 2001 / Published online: 23 May 2001     

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     

Raman characteristics of carbon nitride synthesized by nitrogen-ion-beam-assisted pulsed laser deposition

Raman characteristics of carbon nitride films synthesized by nitrogen-ion-beam-assisted pulsed laser deposition were investigated. In addition to the D (disorder) band and G (graphitic) band commonly observed in carbon nitride films, two Raman bands located at 1080–1100 and 1465–1480 cm^-1 were found from our carbon nitride films. These two bands were well matched with the predicted Raman frequencies for βC₃N₄ and the observed Raman bands reported for carbon nitride films, indicating their relation to carbon-nitrogen stretching vibrations. Furthermore, the relative intensity ratio of the two Raman bands to the D and G bands increased linearly with increasing nitrogen content of the carbon nitride films. Received: 30 October 2000 / Accepted: 5 February 2001 / Published online: 2 October 2001     

Characterization of high-quality MBE-grown GaN films on intermediate-temperature buffer layers

High-quality GaN thin films are grown by rf-plasma assisted molecular beam epitaxy. The quality of the GaN epitaxial layer is significantly improved by using an intermediate-temperature GaN buffer layer (ITBL) in addition to a conventional 20-nm-thick low-temperature buffer layer. The GaN epitaxial layers demonstrate systematic improvements in the electron mobility increasing from 82 cm² V^-1 s^-1, for films grown with just the low-temperature buffer layer, to about 380 cm² V^-1 s^-1 for films grown with an ITBL of thickness 800 nm. The photoluminescence also indicates systematic improvements in the intensity and the full-width-half-maximum with the use of ITBL. Photoreflectance spectra are measured from the GaN films. Detailed analyses of the excitonic transition energy demonstrate that the residual strain relaxes rapidly with the use of ITBL, which is attributed to the observed improvements in the mobility and the PL spectra. Received: 30 November 2000 / Accepted: 4 December 2000 / Published online: 9 February 2001     

Epitaxial growth of yttria-stabilised zirconia buffer layers on X-cut LiNbO3 for superconducting electrodes

We report on the epitaxial growth of yttria-stabilised zirconia (YSZ) buffer layers on X-cut LiNbO₃ (LNO) single crystals by pulsed laser deposition. Despite the low chemical stability of the substrates at high temperature, high quality fully reproducible films were obtained over a relatively broad range of processing conditions. The films were (00h) out-of-plane single oriented and the in-plane edge of the YSZ unit cell was aligned with the polar axis of the LNO. However, the YSZ deposition also promoted the formation of the compound LiNb₃O₈. This compound is epitaxial and located at the interface. The homogeneous YSZ film presents a uniform surface, free of outgrowths and with a low roughness. These characteristics are suitable for the epitaxial growth of other oxides, as has been demonstrated with the preparation of YBa₂Cu₃O₇/CeO₂/YSZ/LNO heterostructures. The superconducting YBa₂Cu₃O₇ films were epitaxial, with the c axis perpendicular to the surface and single in-plane orientation, and presented good transport properties (critical temperatures around 86 K and critical current densities close to 10⁶ A/cm² at 77 K). Received: 5 April 2001 / Accepted: 30 July 2001 / Published online: 30 October 2001     

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     

Colloidal dipolar interactions in 2D smectic-C films

We use a two-dimensional (2D) elastic free energy to calculate the effective interaction between two circular disks immersed in smectic-C films. For strong homeotropic anchoring, the distortion of the director field caused by the disks generates topological defects that induce an effective interaction between the disks. We use finite elements, with adaptive meshing, to minimize the 2D elastic free energy. The method is shown to be accurate and efficient for inhomogeneities on the length scales set by the disks and the defects, that differ by up to 3 orders of magnitude. We compute the effective interaction between two disk-defect pairs in a simple (linear) configuration. For large disk separations, D, the elastic free energy scales as ∼D ^-2, confirming the dipolar character of the long-range effective interaction. For small D the energy exhibits a pronounced minimum. The lowest energy corresponds to a symmetrical configuration of the disk-defect pairs, with the inner defect at the mid-point between the disks. The disks are separated by a distance that is twice the distance of the outer defect from the nearest disk. The latter is identical to the equilibrium distance of a defect nucleated by an isolated disk. Received 26 October 2001 and Received in final form 14 December 2001     

Texture control and its impact on the properties of SrBi2Ta2O9 thin films prepared by pulsed laser deposition

SrBi₂Ta₂O₉ (SBT) ferroelectric thin films with different preferred orientations were deposited by pulsed laser deposition (PLD). Several methods have been developed to control the preferred orientation of SBT thin films. For SBT films deposited directly on Pt/TiO₂/SiO₂/Si substrates and in situ crystallized at the deposition temperature, the substrate temperature has a significant impact on the orientation and the remnant polarization (Pr) of the films; a higher substrate temperature benefits the formation of (115) texture and larger grain size. The films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C are (115)-oriented and exhibit 2Pr of 6 μC/cm². (115)- and (200)-predominant films can be formed by using a La_0.85Sr_0.15CoO₃ (LSCO) buffer layer or by annealing amorphous SBT films deposited on Pt/TiO₂/SiO₂/Si substrates at 450 °C using rapid thermal annealing (RTA). These films exhibit good electric properties; 2Pr of the films are up to 12 μC/cm² and 17 μC/cm², respectively. The much larger 2Pr of the films deposited on the LSCO buffer layer and of the films obtained by RTA than 2Pr of the films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C is attributed to a stronger (200) texture. Received: 30 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Preparation and assessment of piezo- and pyroelectric poly (vinylidene fluoride-hexafluoropropylene) copolymer films

Films of poly(vinylidene fluoride-hexafluoropropylene) copolymer [P(VDF-HFP)] were cast from a dimethylsulfoxide (DMSO)/acetone solution of Solef?85-15 P(VDF-HFP) copolymer powder grade 21508. Undrawn and uniaxially drawn cast copolymer films were investigated with respect to their piezo- and pyroelectric properties. Quasistatic charge integration was employed for the determination of the poling-field dependence of the piezoelectric d₃₁ and pyroelectric p₃ coefficients. In addition, the thermal stability of the pyroelectric effect was studied with a combination of thermally stimulated discharge (TSD) and temperature-modulation techniques. Cast copolymer films could withstand electrical poling fields of up to 400 MV/m. The maximum values of d₃₁=30 pC/N and p₃=49 μC/(m²K) for uniaxially drawn samples are similar to those found on commercial PVDF films and much higher than those on pure PVDF films cast from solution. Samples kept for 5 min at 150 °C still exhibit 30–40% of the initial pyroelectric effect [up to around 20 μC/(m²K)]. After this annealing step, no further decay of the pyroelectric coefficient could be observed during storage at 120 °C for several hours. Received: 5 July 2001 / Accepted: 27 July 2001 / Published online: 2 October 2001