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     

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.     

Dependence of the sp3 bond fraction on the laser wavelength in thin carbon films prepared by pulsed laser deposition

3 bonds in the carbon films prepared by pulsed laser deposition of carbon obtained from graphite was investigated by electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS). The fraction of sp³ bonds increased with a decreasing laser wavelength. Energetic C⁺ ion species were effectively produced by using a short-wavelength laser. The sp³ bond fraction increased with an increasing amount of energetic C⁺ ion species. The fractions of sp³ bonds in the carbon film were 80%, 42%, 26% and 15% at wavelengths of 193, 248, 532 and 1064 nm, respectively. Received: 28 October 1997/Accepted:29 October 1997     

Pressure effects during pulsed-laser deposition of barium titanate thin films

2 , Ar) in a broad pressure range (10^-7–1 mbar) are correlated to the plasma expansion dynamics. It is found that the deposited films present an excess of Ba in the intermediate pressure range (10^-2<P<10^-1 mbar) and a peaked distribution of Ba to Ti atoms ratio, that is not related to either the substrate temperature or the nature of the gas environment. The results are discussed in terms of the dependence of the plume length (L_P) on the gas pressure and the existence of scattering processes for distances (d) from the target lower than L_P and the diffusion of the ejected species for L_P<d. Received: 7 November 1997/Accepted: 8 November 1997     

The orientation-selective growth of LaNiO3 films on Si(100) by pulsed laser deposition using a MgO buffer

Highly (100)-oriented, (110)-oriented and polycrystalline LaNiO₃ (LNO) films were successfully prepared on Si(100) using an oriented MgO film as a buffer. It was somewhat surprising to find that that the orientation relation between the LNO film and the corresponding MgO buffer was: LNO(100)\MgO(110), LNO(110)\MgO(111) and LNO(polycrystalline)\MgO(100). The crystalline quality of the LNO films was shown to be sensitive to the preparation conditions of the MgO buffer. The film surface was very smooth, without micrometer-sized droplets being observed. All LNO films were of metallic conductivity, with a room-temperature resistivities of approximately 250, 280 and 420 μΩ cm for the (110)-oriented, (100)-oriented and polycrystalline LNO, respectively. Received: 2 April 2001 / Accepted: 23 October 2001 / Published online: 3 June 2002     

Orientation selective growth of MgO films on Si (100) by pulsed laser deposition

Selective growth of single-oriented (110), (100) and (111) MgO films on Si (100) substrates without buffer layers was obtained via a two-step method by pulsed laser deposition. It was found that the orientation of the films was determined at the initial deposition stage by the substrate temperature only. The ambient pressure during deposition, the laser fluence and the etching of the Si substrates have no apparent effect on the orientation of the films, but affect their crystalline quality. Under the present deposition conditions, the surfaces of all three different single-oriented films were very smooth and devoid of any particulates. Received: 23 January 2001 / Accepted: 6 June 2001 / Published online: 2 October 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     

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.     

Artificially nanostructured Cu:Al2O3 films produced by pulsed laser deposition

The processes leading to the formation of Cu:Al₂O₃ composite films on Si (001) with a well defined nanostructure by alternate pulsed laser deposition (a-PLD) in vacuum are investigated. Alternately amorphous Al₂O₃ layers and Cu nanocrystals nucleated on the Al₂O₃ surface are formed, according to the PLD sequence. The Al₂O₃ deposited on the Cu nanocrystals fills in the space between them until they are completely buried, and subsequently, a continuous dense layer with a very flat surface (within 1 nm) is developed. The nucleation process of the nanocrystals and their resulting oblate ellipsoidal shape are discussed in terms of the role of the energetic species involved in the PLD process and the metal–oxide interface energy. Received: 4 July 2000 / Accepted: 5 July 2000 / Published online: 13 September 2000     

Ferroelectric SrxBa1-xNb2O6 optical waveguiding thin films on SiO2-coated Si(100) substrates

x Ba_1-xNb₂O₆ (x=0.5) films (abbreviated as SBN:0.5) on SiO₂-coated Si substrates are potential components for the application of integrated electro-optics devices. SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates with a very thin MgO diffusion buffer have been successfully prepared by pulsed laser deposition. The as-grown films have a refractive index of 2.28, which is close to that of bulk SBN. X-ray analysis showed that the as-grown films have a single-phase tetragonal tungsten bronze structure. The SBN:0.5 thin films prepared by PLD exhibit favorable ferroelectric and optical waveguiding properties. The composition and the morphology of the films were also examined by XPS and by SEM, respectively. Ferroelectric SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates are expected to be used in integrated electro-optic devices. Received: 27 February 1997/Accepted: 17 October 1997     

Anisotropic in-plane strain in W-doped (Ba,Sr)TiO<Subscript>3</Subscript> thin films deposited by pulsed-laser deposition on (001)MgO

The structural and the microwave dielectric properties of Ba_xSr_(1-x)TiO₃ films (BST) with x=0.5, 0.6 and 0.7, containing 1 mol % W have been investigated. The films were grown by pulsed-laser deposition on MgO (001) substrates at a temperature of 720 °C in an oxygen pressure from 3 to 500 mTorr. The film structures were determined by X-ray diffraction. The lattice parameters were fitted to a tetragonal distortion of a cubic lattice. The out-of-plane lattice parameter (c) was calculated from the position of the (004) reflection. Using c, the in-plane lattice parameter, a, was calculated from the position of the (024) and (224) reflections. A deviation in the calculated values for a, beyond the systematic error, was found in the in-plane lattice parameter, suggesting an in-plane orthorhombic distortion (a, a’). Films with x=0.7 showed a minimum in-plane distortion due to a better lattice match with the substrate. The ratio of the in-plane and out-of-plane lattice parameters was calculated as a measure of the lattice distortion (a/c and a’/c). The dielectric properties of the films deposited were measured at room temperature at 2 GHz using gap capacitors fabricated on top of the dielectric film. For all Ba/Sr ratios investigated in the W-doped material, the dielectric Q (1/cosδ) was observed to be insensitive to the oxygen deposition pressure. A peak in the change in the dielectric constant, as a function of an applied electric field (0–80 kV/cm), was observed for films deposited in 50 mTorr of oxygen. The largest K-factor, K=(ε(0)-ε(V )/ε(0)×Q(0)), for films deposited from a BST x=0.6 (1 mol % W-doped) target was observed in the film that had a minimum in-plane strain, where a∼a’ and c was greater than a and a’. Received: 4 July 2002 / Accepted: 5 July 2002 / Published online: 17 December 2002 RID="*" ID="*"Permanent address: Nuclear Research Center–Negev, P.O. Box 9001 Beer-Sheva, Israel RID="**" ID="**"Corresponding author. Fax: +1-202/767-5301, E-mail: horwitz@ccsalpha3.nrl.navy.mil     

Influence of inert gas pressure on deposition rate during pulsed laser deposition

The deposition rates of permalloy and Ag are monitored during pulsed laser deposition in different inert gas atmospheres. Under ultrahigh vacuum conditions, resputtering from the film surface occurs due to the presence of energetic particles in the plasma plume. With increasing gas pressure, a reduction of the particle energy is accompanied with a decrease of resputtering and a rise in the deposition rate for materials with high sputtering yield. In contrast, at higher gas pressures, scattering of ablated material out of the deposition path between target and substrate is observed, leading to a decrease in the deposition rate. While in the case of Xe and Ar these processes strongly overlap, they are best separated in He. A He pressure of about 0.4 mbar should be used to reduce the kinetic energy of the deposited particles, to reach the maximum deposition rate and to avoid implantation of the particles. This is helpful for the preparation of stoichiometric metallic alloy films and multilayers with sharp interfaces. Received: 27 March 2002 / Accepted: 3 April 2002 / Published online: 5 July 2002     

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     

Growth of completely (110)-oriented Pt film on Si (100) by using MgO as a buffer by pulsed laser deposition

(110)-textured MgO films were grown on Si (100) with etching and without etching by pulsed laser deposition. The deposited MgO films were shown to be droplets-free. The MgO film was used as a buffer layer to further grow Pt film on Si (100). A completely (110)-oriented Pt film was obtained on such a buffer layer and its surface is very smooth with a roughness of about 7.5 nm over 5×5 μm. This can be used as a new oriented Pt electrode on silicon for devices. Received: 23 January 2001 / Accepted: 27 April 2001 / Published online: 27 June 2001     

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     

Amorphous hydrogenated carbon synthesized by pulsed laser deposition from cyclohexane

It is demonstrated that a liquid hydrocarbon precursor, cyclohexane, is appropriate for laser-induced carbon deposition. Amorphous hydrogenated carbon films (a-C:H) were deposited by KrF excimer laser irradiation of single-crystal silicon surface immersed under cyclohexane. The technique is simple and easy to operate. IR absorption spectra of the deposited films confirmed the presence of carbon in the diamond phase. Raman and XPS studies showed diamond-like character of the deposited films. Moreover, these two studies provided strong evidence that laser fluence played an important role in the formation of DLC bondings and the quality of the deposited films. Received: 15 September 1998 / Accepted: 5 January 1999 / Published online: 5 May 1999     

Characterization of ablation plumes and carbon nitride films produced by reactive pulsed laser deposition in the presence of a magnetic field

x ) films in a nitrogen atmosphere within the range 5×10^-4–4×10^-1 Torr. In the presence of a magnetic field, the emission intensities of N₂ (second positive system) and CN species in the graphite ablation plumes were altered significantly, depending on the pressure of the N₂ environment. Corresponding to an intense CN emission, a magnetic field-induced enhancement of N incorporation – for example, up to 37% at an N₂ pressure of 300 mTorr – and the formation of sp³ tetrahedral CN bonding were both observed in the films. This suggests that the arrival of CN species at the substrate surface with kinetic energies is important for film deposition. Received: 27 August 1997/Accepted: 8 September 1997     

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     

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     

Oxygen deficiency dependence of magnetic and magnetoresistive properties for La0.5Sr0.5CoO3-x thin films prepared by pulsed laser deposition

The complete (001)-oriented thin films of La_0.5Sr_0.5CoO_3-x (LSCO) are deposited on (001) SrTiO₃ substrates by pulsed laser deposition under reduced oxygen pressure. It is revealed that the c axis of the film stretches with depleting oxygen. The magnetic, electrical, and magnetoresistive properties of the films are characterized by means of various techniques. Significant dependence of these properties on oxygen deficiency in the films is demonstrated, with enhanced magnetoresistance recorded for the samples deposited over a wide range of reduced oxygen pressure. Received: 9 July 1998 / Accepted: 15 January 1999 / Published online: 31 March 1999