SBN thin films growth by RF plasma beam assisted pulsed laser deposition

SBN thin films were grown on MgO and Silicon substrates by PLD and RF-PLD (radiofrequency assisted PLD) starting from single crystal Sr_0.6Ba_0.4Nb₂O₆ and ceramic Sr_0.5Ba_0.5Nb₂O₆ stoichiometric targets. Morphological and structural analyses were performed on the SBN layers by AFM and XRD and optical properties were measured by spectroellipsometry. The films composition was determined by Rutherford Backscattering Spectrometry. The best set of experimental conditions for obtaining crystalline, c-axis preferential texture and with dominant 31° in-plane orientation relative to the MgO (100) axis is identified.     

Surface morphology of ZnO buffer layer and its effects on the growth of GaN films on Si substrates by magnetron sputtering

ZnO thin films were first prepared on Si(111) substrates using a radio frequency magnetron sputtering system. Then the as-grown ZnO films were annealed in oxygen ambient at temperatures of 700, 800, 900, and 1000°C , respectively. The morphologies of ZnO films were studied by an atom force microscope (AFM). Subsequently, GaN epilayers about 500 nm thick were deposited on the ZnO buffer layers. The GaN/ZnO films were annealed in NH₃ ambient at 900°C. The microstructure, morphology and optical properties of GaN films were studied by x-ray diffraction (XRD), AFM, scanning electron microscopy (SEM) and photoluminescence (PL). The results are shown, their properties having been investigated particularly as a function of the ZnO layers. For better growth of the GaN films, the optimal annealing temperature of the ZnO buffer layers was 900°C.     

RF-plasma assisted pulsed laser deposition of nitrogen-doped SrTiO3 thin films

Perovskite-type nitrogen substituted SrTiO₃ thin films were deposited with a one-step process by RF-plasma assisted pulsed laser deposition from a SrTiO₃ target using a N₂ plasma, while deposition with a NH₃ plasma yields films with almost no incorporated nitrogen. The deposited films exhibit a cubic perovskite-type crystal structure and reveal oriented growth on MgO(100) substrates. The unit cell parameters of the studied N-doped SrTiO₃ films range within 3.905<a<3.918 Å, which is slightly larger than for SrTiO₃ (a=3.905 Å). The nitrogen content in the deposited films varies from 0.2 to 0.7 atom%. The amount of incorporated nitrogen in the films decreases with increasing RF-power, while the N₂ flow rate does not have any pronounced influence on the N content. Nitrogen incorporation results in an increased optical absorption at 400–600 nm, which is associated with N(2p) energy states that have a higher energy level than the valence band in strontium titanate. The optical band gap energies in the studied N-doped SrTiO₃ films are at 3.2–3.3 eV, which is very similar to that of pure strontium titanate (～3.2 eV). Films deposited with NH₃ for the RF-plasma exhibit a lower degree of crystallinity and reveal almost no nitrogen incorporation into the crystal lattice.     

Inverse pulsed laser deposition

Since the advent of pulsed laser deposition (PLD), several different target-substrate arrangements have been proposed. Besides the most common on-axis PLD, several off-axis geometries were studied, mainly to protect the substrate from the agglomerated species (clusters, droplets, particulates) of the plasma plume, which are detrimental to the homogeneity of films. Recently we introduced a novel geometry, termed inverse pulsed laser deposition (IPLD), in which the substrate is placed parallel to and slightly above the target plane. In this paper we summarize our results on this new geometry, and show how it can extend the perspectives of pulsed laser deposition, e.g., by improving the surface morphology of the films. Effects of ambient pressure are presented and exemplified on metallic and compound IPLD films, including Ti, CN _x , and Ti-oxides. AFM topographic images are used to prove that under optimized conditions IPLD is capable of growing compact and smooth films that are superior to PLD ones. A special—but easy-to-implement—IPLD arrangement is also introduced that considerably improves the homogeneity of IPLD films. In this geometry, the properties (e.g., deposition rate and roughness) of the films grown in the 1–25 Pa pressure domain are examined.     

Zinc oxide nanostructures grown by pulsed laser deposition

We report the use of PLD to grow different ZnO nanostructures. Very different film morphologies have been observed using different laser wavelengths to ablate the target. The influence of substrate temperature and oxygen background pressure on the film morphology has been investigated too. Smooth and rough films, hexagonal pyramids and columns have been obtained by using a KrF excimer laser (248 nm) for the target ablation, while hexagonal hierarchical structures and pencils have been obtained by using ArF (193 nm). Photoluminescence and X-ray diffraction measurements revealed the good quality of the samples, in particular of those deposited using the ArF laser beam.     

Film growth mechanisms in pulsed laser deposition

This paper reviews our recent studies of the fundamentals of growth morphology evolution in Pulsed Laser Deposition in two prototypical growth modes: metal-on-insulator island growth and semiconductor homoepitaxy. By comparing morphology evolution for pulsed laser deposition and thermal deposition in the same dual-use chamber under identical thermal, background, and surface preparation conditions, and varying the kinetic energy by varying the laser fluence or using an inert background gas, we have isolated the effect of kinetic energy from that of flux pulsing in determining the differences between morphology evolution in these growth methods. In each growth mode analytical growth models and Kinetic Monte Carlo simulations for thermal deposition, modified to include kinetic energy effects, are successful at explaining much of what we observe experimentally.     

Thickness distribution of thin amorphous chalcogenide films prepared by pulsed laser deposition

Amorphous chalcogenide thin films were prepared from As₂Se₃, As₃Se₂ and InSe bulk glasses by pulsed laser deposition using a KrF excimer laser. Thickness profiles of the films were determined using variable angle spectroscopic ellipsometry. The influence of the laser beam scanning process during the deposition on the thickness distribution of the prepared thin films was evaluated and the corresponding equations suggested. The results were compared with experimental data.     

Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) is the most common solid electrolyte material used e.g. in ceramic fuel cells. Thin films of YSZ were deposited on c-cut sapphire single crystals by pulsed laser deposition using a KrF excimer laser focused on a polycrystalline 8 mol% Y₂O₃-stabilized ZrO₂ target. Depending on the substrate temperature and the oxygen background pressure during deposition, different microstructures are obtained. XRD and high-resolution SEM revealed the formation of dense amorphous films at room temperature. At 600°C preferentially (111) oriented polycrystalline films consisting of densely agglomerated nm-sized grains of the cubic phase resulted. Grain size and surface roughness could be controlled by varying the oxygen background pressure. RBS and PIXE evidenced congruent transfer only for a low number of pulses, indicating a dynamical change of the target stoichiometry during laser irradiation. The in-plane ionic conductivity of the as-deposited crystalline films was comparable to bulk YSZ. The conductivity of initially amorphous YSZ passes a maximum during the crystallization process. However, the relative changes remain small, i.e. no significant enhancement of ionic conductivity related to the formation of a nanocrystalline microstructure is found.     

Pulsed laser deposition of TiO<Subscript>2</Subscript>: diagnostic of the plume and characterization of nanostructured deposits

Promising applications of TiO₂ nanostructures include the development of optical devices, sensors, photocatalysts and self-cleaning coatings. In view of their importance, research on the synthesis of nanosized TiO₂ is a particularly active field. In this work we report on the investigation of the effect of laser irradiation wavelength (Q-switched Nd:YAG laser at 532, 355 and 266 nm), the temperature of the substrate and the atmosphere of deposition (vacuum, Ar and O₂) that are suitable for obtaining nanostructured deposits from TiO₂ sintered targets. The ablation plume emission is characterized with spectral and temporal resolution by optical emission spectroscopy (OES), while the surface morphology and chemical states of the material deposited on a Si (100) substrate are examined by environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM) and by X-ray photoelectron spectroscopy (XPS), respectively. Deposits with nanostructured morphology with grain size down to 40 nm and keeping the stoichiometry of the targets were obtained at high temperature, while the highest concentration of particulates was observed at the longest laser wavelength of 532 nm on a substrate heated up to 650°C. In situ characterization of the ablation plume, carried out by OES, indicated the presence of emissions assigned to Ti I, Ti II and O I.     

Electrical properties of boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser ablation

We report on electrical measurements and structural characterization performed on boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser deposition. The resistance has been measured between 77 and 300 K using four probe technique on platinum contacts for different boron doping. Different behaviours of the resistance versus temperature have been evidenced between pure DLC and boron-doped DLC. The a-C:B thin film resistances exhibit Mott variable range hopping signature with temperature. Potential applications of DLC thin films to highly sensitive resistive thermometry is going to be discussed.     

Anatase TiO2 films fabricated by pulsed laser deposition using Ti target

TiO₂ films were prepared by pulsed laser deposition using a metallic Ti target in an O₂ gas ambient. The microstructure along with optical and photocatalytic properties of the deposited films were systematically studied by changing the deposition parameters and substrates. It was found that TiO₂ films having nearly pure anatase phase grew effectively in O₂ atmosphere. When the films were fabricated at a substrate temperature of 400°C, their phase structures were greatly affected by the O₂ gas pressure, and nearly pure anatase phase with typical (101) and (004) peaks can be obtained under an O₂ pressure of 15 Pa. For the deposition at 700°C, the crystal structure of the TiO₂ films exhibited a strong anatase (004) peak and was inert to the oxygen pressures. Two modes, namely a substrate-temperature-controlled mode and an oxygen-pressure-controlled mode, were considered for the growth of the anatase TiO₂ films under different substrate temperatures. In addition, the optical and photocatalytic properties were found to be sensitive to both the microstructure and grain size of the TiO₂ films.     

A point source analytical model of inverse pulsed laser deposition

A simple analytical model for inverse pulsed laser deposition is proposed. In the model the motion of the evaporated material is assumed to emerge as from a point source located above the surface of evaporation at some distance. The obtained thickness profiles of inverse deposited films agree well with those calculated by the test particle Monte Carlo method. The proposed approach has been applied for analysis of experimental data on inverse pulsed laser deposition of graphite in nitrogen atmosphere with nanosecond pulses of laser fluences between 1 and 7 J/cm². The model describes well the thickness profiles and pressure dependence of film growth rate for inverse deposition.     

Growth regimes in pulsed laser deposition of aluminum oxide films

Alumina is technologically exploited in several forms, ranging from compact hard films as protective coatings to open microstructures of high specific area as supports for catalysts. Currently, various production processes are used to deposit the different forms. PLD has the potential of obtaining not only the different forms, but also a continuous modulation of properties, by tuning of the process parameters. This work investigates the relationship between the process parameters and the resulting film morphology, structure and properties for PLD performed with an alumina target in a background oxygen atmosphere. Three distinct growth regimes are found, leading, respectively, to compact homogeneous films, columnar structures and open microstructures. These structures are quantitatively characterized, and the ranges of the process parameters corresponding to the three regimes are identified. An empirical scaling law is proposed, which can be exploited as a guide for the design of growth processes aimed at obtaining specific film properties.     

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     

Influence of the plasma parameters on the properties of aluminum oxide thin films deposited by laser ablation

The plasma produced by the ablation of a high purity Al₂O₃ target, using the fundamental line (1064 nm) of a Nd:YAG laser, was characterized. The laser fluence was varied in order to study its effect on the characteristics of the produced plasma as well as on the properties of the material deposited. Optical emission spectroscopy (OES) was used to determine the type of excited species present in the plasma. The mean kinetic energy of the ions and the maximum plasma density were determined from the time of flight (TOF) curves, obtained with a planar Langmuir probe. The obtained results reveal that the fast peak in the probe curve could be attributed to Al III, while the slow peak corresponds to the Al II. Aluminum oxide thin films were then deposited under the same conditions of the diagnosed plasma, in an attempt to correlate the plasma parameters with the properties of the deposited material. It was found that when Al II ion energies are lower than 461.0 eV the films deposited have structural characteristics similar to that of α-Al₂O₃, whereas at ion energies greater than 461.0 eV amorphous material was obtained.     

Formation of surface stabilized Si nanocrystal by pulsed laser ablation in hydrogen gas

We prepared silicon nanocrystallites by pulsed laser ablation (PLA) of a Si target in hydrogen background gas. A mixture of hydrogen and helium was used as a background gas and the hydrogen partial pressure was varied. The deposited nanocrystal-film system shows a hierarchical structure composed of surface hydrogenated silicon nanocrystallites as the primary structure and aggregates of the nanocrystallites as the secondary structure. The size of the primary particles was not sensitive to the hydrogen partial pressure, while the porosity of the secondary structure constituted by the aggregation of the primary particles increased with increasing hydrogen partial pressure. This indicates that the surface is stabilized and that aggregation of the primary structure is depressed by surface hydrogenation. The optical gap energy of the deposits shifted to higher energy with increasing hydrogen partial pressure due to the formation of well-isolated nanocrystallites by surface stabilization. These results indicate that PLA in hydrogen gas is a promising technique to prepare surface stabilized and controlled silicon nanocrystallites.     

Pulsed laser deposition of BaGa<Subscript>2</Subscript>O<Subscript>4</Subscript>

This paper reports the first results obtained on monobarium gallate thin films grown on silicon and platinum coated substrates by pulsed laser deposition. The influence of oxygen background pressure and substrate (or post-annealing) temperature on the film properties was studied. The films were characterized by XRD, RHEED, AFM, photoelectron and electrical impedance spectroscopy. The structure analysis showed that the films crystallized into a hexagonal phase, most probably into (metastable) α-BaGa₂O₄. Depending on deposition conditions, films with different (from nearly epitaxial to polycrystalline) textures were obtained.     

Growth of ZnO nanoparticles and nanorods with ultrafast pulsed laser deposition

In this work, we study the application of ultrafast pulsed laser deposition (PLD) in ZnO nanomaterial synthesis, including nanoparticles and nanorods. PLD using long pulse (nanosecond) lasers has been widely used as a method for growing prototype materials. The recently-emerged ultrafast PLD is expected to be able to overcome the problem of large liquid droplet formation. Using near infrared and femtosecond laser pulses in ablation, we first characterize the ablation plume using a Langmuir probe and plasma optical emission spectroscopy. We then examine the structural properties of the nanoparticles generated during low-fluence ablation. Finally, we demonstrate that using nanoparticle aggregates as templates, assisted by plume-excited nitrogen radicals at a high fluence, high quality ZnO nanorods can be grown free of metal catalysts.     

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     

Polycrystalline silicon films obtained by hot-wire chemical vapour deposition

Silicon films were deposited at moderate substrate temperatures (280–500° C) from pure silane and a silane-hydrogen mixture (10% SiH₄, 90% H₂) in a hotwire CVD reactor. The morphology, structure and composition of the samples were studied with scanning electron microscopy, transmission electron microscopy, transmission electron diffraction, X-ray diffraction, Raman spectroscopy and secondary ion mass spectrometry. The sample deposited at 500° C with pure silane has an amorphous structure, whereas the samples obtained from silane diluted in hydrogen have a polycrystalline structure, even that grown at the lowest temperature (280° C). Polycrystalline samples have a columnar structure with 0.3–1 m crystallite sizes with preferential orientation in [220] direction. Deposition rates depend on the filament-substrate distance and range from 9.5 to 37 Å/s for the polycrystalline samples. The high quality of the polycrystalline samples obtained makes the hot-wire technique very promising. Moreover, it is expected to be easily scaled up for applications to large-area optoelectronic devices and to photovoltaic solar cells.