Femtosecond pulsed-laser deposition of BaTiO<Subscript>3</Subscript>

The elemental composition and the surface morphology of thin films grown by laser ablation of barium titanate with femtosecond pulses at 620 nm laser wavelength have been systematically studied according to the experimental pulsed-laser deposition parameters : laser energy density, oxygen pressure, substrate temperature, target–substrate distance and substrate position (in- and off-axis geometry). Firstly, even at high temperature (700 °C), the deposits consist of coalesced particles up to 1-μm in size, mixed in a poorly crystallised tetragonal BaTiO₃ thin film. The particles formed in femtosecond pulsed-laser deposition induce a high surface roughness, which is observed whatever the experimental growth conditions and does not correspond to the droplets often observed during laser ablation in the nanosecond regime. As shown by plasma expansion dynamics, these particles propagate toward the substrate in the plasma plume with a low velocity, and are assumed to be produced by gas-phase reactions. Moreover, the cationic concentration evaluated through the Ba/Ti ratio strongly depends on the oxygen pressure in the ablation chamber and the angular position of the substrate along the normal to the target at laser impact. Indeed, the films appear to be enriched in the heavy element (Ba) when the substrate is located at high angular deviation. This fact is correlated to an increase in the lighter species (i.e. Ti) in the central part of the plasma plume. Received: 30 April 2002 / Accepted: 26 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +33-1/4354-2878, E-mail: millon@gps.jussieu.fr RID="**" ID="**"Also at: LSMCL, Université de Metz, 57078 Metz Cedex 3, France     

Optical properties of SrTiO<Subscript>3</Subscript> thin films by pulsed laser deposition

SrTiO₃ thin films were prepared on a fused-quartz substrate by pulsed laser deposition (PLD). Dense and homogeneous films with a thickness of 260 nm were prepared. Optical constants (refractive index n and extinction coefficient k) were determined from the transmittance spectra using the envelope method. The optical band gap energy of the films was found to be 3.58 eV, higher than the 3.22 eV for bulk SrTiO₃, attributable to the film stress exerted by the substrate. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The refractive index and the packing density for the PLD-prepared SrTiO₃ thin films are higher than those for the SrTiO₃ films prepared by physical vapor deposition, sol–gel and RF sputtering. Received: 18 March 2002 / Accepted: 7 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Photoluminescence behavior of pulsed laser deposited ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript> thin-film phosphors grown on various substrates

ZnGa₂O₄ thin-film phosphors have been grown on Si(100), Al₂O₃(0001) and MgO(100) substrates using pulsed laser deposition. The structural characterization was carried out on a series of ZnGa₂O₄ films grown on various substrates under various substrate temperatures and oxygen pressures. The films grown on these substrates not only have different crystallinity and surface morphology, but also different Zn/Ga composition ratio. The crystallinity and photoluminescence (PL) of the ZnGa₂O₄ films are highly dependent on the deposition conditions, in particular the stoichiometry ratio of Zn/Ga and the kind of substrate. The variation of Zn/Ga in the films also depends on not only the oxygen pressure but also the substrate temperature during deposition. The PL properties of pulsed laser deposited ZnGa₂O₄ thin films have indicated that Al₂O₃(0001) and MgO(100) are promising substrates for the growth of high-quality ZnGa₂O₄ thin films and that the luminescence brightness depends on the substrate. The luminescence spectra show a broad band extending from 350 to 600 nm and peaking at 460 nm. Received: 11 July 2002 / Accepted: 31 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +82-51-6206356, E-mail: jhjeong@pknu.ac.kr     

Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC

We report observation of nanostructures formed on thin TiN and DLC films that were irradiated by 800- and 267-nm, femtosecond (fs) Ti:sapphire laser pulses at an energy fluence slightly above the ablation threshold. On the ablated thin-film surfaces, the linearly polarized fs pulses produce arrays of fine periodic structures that are almost oriented to the direction perpendicular to the laser polarization, while the circularly polarized light forms fine-dot structures. The size of these surface structures is 1/10–1/5 of the laser wavelength and decreases with a decrease in the laser wavelength. Received: 3 September 2002 / Accepted: 4 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-778/62-3306, E-mail: yasuma@fukui-nct.ac.jp     

Deep UV laser induced fluorescence in fluoride thin films

Fluorescence experiments have been performed to study the interaction of 193-nm laser radiation with dielectric thin films of LaF₃, AlF₃, and MgF₂. Spectral- and time-resolved measurements reveal the presence of cerium in LaF₃ and the influence of hydrocarbons in MgF₂ and LaF₃. Virtually no fluorescence response is observable in the case of AlF₃. Supplementary measurements on multilayer stacks confirm the contribution of hydrocarbon and cerium emission in high-reflective UV mirrors upon ArF excimer laser irradiation. Energy density dependent measurements indicate a linear absorption process as the origin of UV laser induced fluorescence in LaF₃. Luminescence calculations are applied as a helpful tool in order to account for interference effects that are inherently to be found in the multilayer emission spectra. Received: 21 May 2002 / Accepted: 23 May 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-3641/807-601, E-mail: heber@iof.fraunhofer.de     

Pressure-controlled preparation of nanocrystalline complex oxides using pulsed-laser ablation at room temperature

Nanocrystalline thin films of complex oxides such as BaTiO₃ and LaFeO₃ were prepared by pulsed laser ablation without substrate heating. Targets under various Ar pressures were irradiated using an ArF excimer laser. The off-axis configuration of targets and substrates was used to synthesize the films. The crystallinity and chemical composition of the deposited films were strongly dependent on the processing Ar gas pressure. In case of BaTiO₃, the film deposited at 10 Pa was a single phase of BaTiO₃ with a crystallite size around 7.2 nm. With increasing Ar pressure to 200 Pa, XRD peaks of BaTiO₃ as well as BaCO₃ were observed. The by-products could be due to reaction with carbon dioxide in air after taking the sample out of the chamber. For LaFeO₃, the films deposited under 50 to 200 Pa had a single phase with a crystallite size below 10 nm. When the Ar pressure exceeded 100 Pa, the crystallite size tended to decrease for both BaTiO₃ and LaFeO₃, which could be due to formation of aggregated nanoparticles. Below 10 Pa, oxygen deficiency was observed. Over 50 Pa, the atomic concentration of all the constituent elements was almost constant, especially the [Ba]/[Ti] and [La]/[Fe] ratios, which were nearly unity. Received: 19 June 2002 / Accepted: 24 June 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +81-298/61-6355, E-mail: yoon-jw@aist.go.jp     

Interface reactions in [Fe/B]<Subscript>n</Subscript> multilayers: a way to tune from crystalline/amorphous layer sequences to homogeneous amorphous Fe<Subscript>x</Subscript>B<Subscript>100-x</Subscript> films

[Fe/B]_{n ≥2} multilayers were prepared by thermal evaporation, ion-beam sputtering and laser ablation. By applying in situ electron spectroscopies (UPS, XPS) and monitoring the electrical resistance during layer growth, evidence could be provided for the occurrence of interface reactions within the range of studied deposition temperatures (77 K ≤T ≤300 K). These reactions result in amorphous Fe_xB_100-x phases, which are spatially restricted to a width of less than 3 nm at the original interface. The amorphicity of the reacted interlayers was unequivocally proven by additional high-resolution electron microscopy (HRTEM) and their characteristically changed magnetic properties. Due to the well-defined width of the interface reaction, homogeneous amorphous Fe_xB_100-x films can be obtained by reducing the individual Fe and B layer thicknesses to below the above reaction depth, while for larger thicknesses layer sequences of the crystalline/amorphous/crystalline type will result. Received: 30 January 2002 / Accepted: 31 January 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-731/502-2963, E-mail: hans-gerd.boyen@physik.uni-ulm.de     

Pulsed-laser-deposited epitaxial aluminum nitride films on (111) Si for surface acoustic-wave applications

Epitaxial (001) aluminum nitride (AlN) thin films on (111) Si substrates are prepared using pulsed-laser deposition. The epitaxial structure of the as-prepared thin films is characterized by checking the X-ray-diffraction θ-2 θ scan and pole-figure, using scanning electron microscopy, infrared radiation (IR) spectroscopy and Raman spectroscopy. The surface acoustic-wave resonance at 345 MHz for a 1.5 μm thick AlN film on a (111) Si substrate is observed using an inter-digital electrode. Received: 18 September 2001 / Accepted: 29 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: liujm@nju.edu.cn     

High refractive index and transparent heavy metal oxide glassy thin films

Heavy metal oxide thin films of the ternary system Nb₂O₅–GeO₂–PbO have been prepared by pulsed laser deposition in an O₂ environment from either glassy or crystalline bulk samples. The range of ([Pb]+[Nb]) content in which the films are optically homogeneous and transparent is much broader (0.5–1.0) than that of the bulk samples considered in the present work (0.55–0.62). The imaginary part of the refractive index is very low in all cases (k<10^-3), whereas the real part increases linearly with the ([Pb]+[Nb]) content up to values as high as 2.35. The optical energy gap has been found to be strongly dependent on [Pb], whereas it is almost independent of [Nb]. This dependence is discussed in terms of the role of Pb and Nb as network modifiers or formers. Received: 5 August 2002 / Accepted: 8 August 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/564-5557, E-mail: j.gonzalo@io.cfmac.csic.es     

VUV F<Subscript>2</Subscript> laser ablation of sodium chloride

We report an investigation of the ablation of NaCl crystals at the 157-nm wavelength of the F₂ laser where there is very strong excitonic absorption. Probe-beam deflection and etch-rate measurements show that the interaction is characterised by a low ablation threshold (∼80 mJ cm^-2) and a capability for controllable material removal at the nanometer level. Scanning electron microscopy of the exposed surfaces show this to be microscopically smooth but with fine cracks present. It is demonstrated that micron-scale features can be formed in NaCl using 157-nm laser ablation, a result attributed to the strongly localised optical and thermal nature of the interaction. The results are discussed within the framework of a thermal vaporisation model. Received: 29 May 2002 / Accepted: 17 July 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1482/465606, E-mail: p.e.dyer@hull.ac.uk     

Production of biologically inert Teflon thin layers on the surface of allergenic metal objects by pulsed laser deposition technology

Allergic-type diseases are current nowadays, and they are frequently caused by certain metals. We demonstrated that the metal objects can be covered by Teflon protective thin layers using a pulsed laser deposition procedure. An ArF excimer laser beam was focused onto the surface of pressed PTFE powder pellets; the applied fluences were 7.5–7.7 J/cm². Teflon films were deposited on fourteen-carat gold, silver and titanium plates. The number of ablating pulses was 10000. Post-annealing of the films was carried out in atmospheric air at oven temperatures between 320 and 500 °C. The thickness of the thin layers was around 5 μm. The prepared films were granular without heat treatment or after annealing at a temperature below 340 °C. At 360 °C a crystalline, contiguous, smooth, very compact and pinhole-free thin layer was produced; a melted and re-solidified morphology was observed above 420 °C. The adhesion strength between the Teflon films and the metal substrates was determined. This could exceed 1–4 MPa depending on the treatment temperature. It was proved that the prepared Teflon layers can be suitable for prevention of contact between the human body and allergen metals and so for avoidance of metal allergy. Received: 12 June 2002 / Accepted: 13 June 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. E-mail: bhopp@physx.u-szeged.hu     

Elastic modulus-density relationship for amorphous boron suboxide thin films

Boron suboxide thin films have been deposited on Si(100) substrates by reactive RF magnetron sputtering of a sintered B target in an Ar/O₂ atmosphere. Elastic recoil detection analysis was applied to determine the film composition and density. Film structure was studied by X-ray diffraction and transmission electron microscopy. The elastic modulus, measured by nanoindentation, was found to decrease as the film density decreased. The relationship was affected by tuning the negative substrate bias potential and the substrate temperature during film growth. A decrease in film density, by a factor of 1.55, caused an elastic modulus reduction by a factor of 4.5, most likely due to formation of nano-pores containing Ar. It appears evident that the large scattering in the published data on elastic properties of films with identical chemical composition can readily be understood by density variations. These results are important for understanding the elastic properties of boron suboxide, but may also be qualitatively relevant for other B-based material systems. Received: 22 February 2002 / Accepted: 11 April 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +46-13/288-918, E-mail: denmu@ifm.liu.se     

Microstructures and dielectric properties of compositionally graded (Ba1-xSrx) TiO3 thin films prepared by pulsed laser deposition

Compositionally graded (Ba_1-xSr_x)TiO₃ (BST) (x:0.0∼0.25) thin films were grown on Pt (111)/TiO₂/SiO₂/Si (100) substrates using layer-by-layer pulsed laser deposition in the temperature range 550–650 °C. Both downgraded (Ba/Sr ratio varying from 100/0 at the bottom surface to 75/25 at the top surface) and upgraded (Ba/Sr ratio varying from 75/25 at the bottom surface to 100/0 at the top surface) BST films were prepared. Their microstructures were systematically studied by X-ray diffractometry and scanning electron microscopy. A grain morphology transition from large ‘rosettes’ (>0.30 μm) to small compact grains (70–110 nm) was observed in the downgraded BST films as the deposition temperature was increased from 550 to 650 °C. No such grain morphology transition was detected in the upgraded BST films. Dielectric measurements with metal electrodes revealed an enhanced dielectric behavior in the downgraded films. This enhancement is mainly attributed to the large compressive stress field built up near the interface between the downgraded film and substrate. Furthermore, the BaTiO₃ layer in the downgraded BST films not only serves as a bottom layer but also as an excellent seeding layer for enhancing the crystallization of the subsequent film layers in the downgraded films. Received: 10 December 2001 / Accepted: 12 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: 86-25/359-5535, E-mail: xhzhu@public1.ptt.js.cn     

Deposition of large-area, high-quality cubic boron nitride films by ECR-enhanced microwave-plasma CVD

Large-area, 1-μm-thick cubic boron nitride (cBN) films were deposited on (001) silicon substrates by electron-cyclotron-resonance-enhanced microwave-plasma chemical vapor deposition (ECR-MP CVD) in a mixture of He-Ar-N₂-BF₃-H₂ gases. With the assistance of fluorine chemistry in the gas phase and substrate reactions, the phase purity of the sp³-configuration was improved to over 85% at a reduced substrate bias voltage of -40 V. The grown films show clear Raman transversal optical (TO) and longitudinal optical (LO) phonon vibration modes, characteristic of cBN. Such Raman spectral characteristics are the first ever observed in cBN films prepared under ECR-MP CVD conditions. Received: 3 May 2002 / Accepted: 7 May 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +852-2788/7830, E-mail: apwjzh@cityu.edu.hk     

Field emission properties of a-CN<Subscript>x</Subscript> films prepared by pulsed laser deposition

Amorphous carbon nitride (a-CN_x) films were grown on silicon substrates by ArF excimer laser ablation of a graphite target in the presence of nitrogen at various gas pressures. By working at elevated pressures (up to 100 Pa), large amounts of nitrogen can be incorporated into the films (up to 40 at. %, which leads to a strong enhancement of their field emission properties. This behaviour was demonstrated to be mainly related to changes in the surface morphology of the samples, in connection with the development of graphite-like structures. Received: 22 February 2002 / Accepted: 3 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-(0)/388-106-230, E-mail: fogarassy@phase.c-strasbourg.fr     

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     

Characteristics of SrBi2Ta2O9 ferroelectric films on GaAs with a TiO2 buffer layer

Mainly [115]-oriented SrBi₂Ta₂O₉ (SBT) films were prepared on GaAs(100) substrates with TiO₂ buffer layers. Both the SBT films and the TiO₂ buffer layers were deposited by pulsed laser deposition (PLD) using a KrF excimer laser. The depth profile of the constituent elements observed by Auger electron spectrometry (AES) shows no remarkable diffusion at both the interfaces between SBT and TiO₂ and between TiO₂ and the GaAs substrate. The electrical characteristics of the Pt/SBT/TiO₂/GaAs(100) structures show a ferroelectric hysteresis loop with a small remanent polarization (∼0.5 μC/cm²). Received: 1 March 2002 / Accepted: 3 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: xhliu81@hotmail.com RID="**" ID="**"Present address: Data Storage Institute, DSI Building, 5, Engineering Drive 1 (off Kent Ridge Crescent, NUS) 117608 Singapore     

Growth of a textured quasicrystalline phase in Ti-Ni-Zr films prepared by pulsed laser deposition

The preparation in thin film form of the known icosahedral phase in Ti-Ni-Zr bulk alloys has been investigated as a function of substrate temperature. Films were deposited by pulsed laser deposition on sapphire substrates at temperatures ranging from room temperature to 350 °C. Morphological and structural modifications have been followed by grazing-incidence and θ–2θ X-ray diffraction, transmission electron diffraction and imaging. Chemical composition has been analyzed by electron probe microanalysis. The in-depth variation of composition has been studied by secondary neutral mass spectroscopy. We show that pulsed laser deposition at 275 °C makes the formation of a 1-μm-thick film of Ti-Ni-Zr quasicrystalline textured nanocrystallites possible. Received: 7 June 2001 / Accepted: 18 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +33-3/8357-6300, E-mail: brien@mines.u-nancy.fr     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

High-power laser interactions with nanostructured materials

We use short-pulse high-power lasers to selectively modify the structure of nanolaminates and nanocrystals. It is demonstrated that femtosecond pulses can achieve excellent results for microscopic thin film removal. Laser pulses can also be used to modify the crystal structure of thin films. It is also demonstrated that coherent laser excitation promotes a selective modification of nanocrystals, resulting in changes of size, shape, and crystal structure. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-414/229-5589, E-mail: yakovlev@uwm.edu