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     

Formation of CNx thin films by reactive pulsed laser deposition assisted by electron cyclotron resonance microwave discharge

Amorphous carbon nitride thin films were synthesized by pulsed laser deposition combined with electron cyclotron resonance (ECR) microwave discharge in nitrogen gas. The ECR discharge supplies active nitrogen species in the deposition environment and to the growing film surface, enhancing the film growth in complex processes accompanied by chemical reaction. The synthesized films were characterized by Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy. The films were determined to consist purely of carbon and nitrogen with a nitrogen concentration of 42%, and have a thickness of 550 nm over which carbon and nitrogen are well distributed. Structural characterizations based on XPS, FTIR and Raman analysis showed that these films appear to contain several bonding configurations between carbon and nitrogen with a small amount of C≡N bonds compared with other bonding states. Received: 31 August 2000 / Accepted: 12 December 2000 / Published online: 23 May 2001     

Nitrogen incorporation in amorphous SiCN layers prepared from electron cyclotron resonance plasmas

Electron cyclotron resonance plasma chemical vapor deposition with nitrogen, methane, and argon-diluted silane as precursors has been used to prepare SiCN thin films. Optical emission from CN species in the plasma has been observed. Infrared measurements show that most of the nitrogen is incorporated to the thin solid films in the form of Si-N, C=N and C≡N bonds suggesting a basic structure of incomplete SiN tetrahedra with C=N and C≡N bridging bonds. The deposited films are nearly transparent in the visible range with a weak absorption threshold between 2.2 and 3.5 eV. Received: 3 April 1998 / Accepted: 5 January 1999 / Published online: 31 March 1999     

Prompt electron emission and collisional ionization of ambient gas during pulsed laser ablation of silver

17 cm^-3 and it is found that the density increases as a function of distance away from the target. Dependence of probe current on laser intensity shows plasma shielding at high laser intensities. Received: 5 January 1998/Accepted: 3 July 1998     

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     

Growth of stoichiometric and textured LiNbO3 films on Si by pulsed laser deposition

Laser ablation of single-crystal LiNbO₃ in a gas environment is used to grow films on (100) Si substrates heated to 650 °C. The film composition and crystallinity are studied as a function of the nature (reactive, O₂, or inert, Ar) and pressure of the gas environment applied during deposition and cooling-down processes, the laser energy density and the target–substrate distance. Experimental results show that a gas pressure close to 1 mbar is required to produce stoichiometric films in either O₂ or Ar. The modification of the laser energy density and the target–substrate distance allows us to improve the crystallinity of the films that become textured along the (006) direction. The influence of the experimental parameters on the film properties is discussed in the frame of the formation of a blast wave, that leads to the focusing of the expanding Li species and thus, to the increase of the Li content in the films. Received: 8 February 2001 / Accepted: 9 February 2001 / Published online: 3 May 2001     

Influence of the growth conditions on the composition of Al nitride films by laser ablation

We have studied the growth of Al nitride films by laser ablation in order to check the potential of the method. The influence of process parameters such as nature of the target, laser energy density, nitrogen partial pressure, etc. on the composition, chemical nature and structure of the films has been investigated. Rutherford backscattering spectrometry, nuclear reaction analysis, X-ray diffraction and X-ray photoelectron spectroscopy were used to characterize the films. Literature reports on AlN film growth by laser ablation but oxygen contamination is poorly discussed whereas it is the main problem encountered. The origin of this contamination and the mechanisms of incorporation were studied, and the crucial parameter was found to be the residual pressure during ablation. Due to the difference in chemical reactivity between oxygen and nitrogen atomic species, to obtain pure AlN films it is necessary to increase the concentration of atomic nitrogen. Thus, a RF discharge device was added allowing a better nitrogen molecule dissociation. Finally, despite composition deviations, the AlN phase can be formed in the laser-deposited films. Highly textured films presenting epitaxial relationships with crystalline Al₂O₃ substrates can be grown even with a 10% oxygen concentration. Received: 7 October 1999 / Accepted: 17 April 2000 / Published online: 13 September 2000     

A study of defects in ultra-thin transparent coatings on polymers

2 or more). Received: 1 September 1998/Accepted: 6 October 1998     

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     

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     

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     

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     

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     

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     

Evolution of the deposition rate during pulsed laser deposition of hydroxyapatite coatings and its relation with target morphology

The evolution of the deposition rate during pulsed laser deposition of hydroxyapatite coatings with a KrF excimer laser was studied. This evolution was related to the changes produced on the target morphology during laser irradiation. Laser fluences in the 1.0–5.2 J/cm² range, typical for the deposition of hydroxyapatite coatings, were investigated. Deposition rates were measured through a quartz-crystal microbalance, and the target surface was observed by scanning electron microscopy. At the lowest fluences, the deposition rate decreases as the number of pulses increases. At the same time, a cone structure is developed on the target surface. At the highest fluences, the deposition rate increases with the number of pulses. In this case, the target surface becomes rough but cones are not formed. In all cases, an enlargement of the effective irradiated area results. This enlargement produces the effect of fluence dilution and also an increase of the emitting area. These opposite effects could account for the different evolutions found for the deposition rate. At low fluences the effect of fluence dilution would predominate over the increase of the emitting area, and at high fluences the second effect would predominate over the first. Received: 28 September 2000 / Accepted: 4 October 2000 / Published online: 10 January 2001     

Optical studies on the deposition of carbon nitride films by laser ablation

2 radicals when the 355 and 1064 nm outputs of a Nd:YAG laser were applied. While for the 532 nm ablation, a relatively higher concentration of excited atomic carbon was obtained. Different Raman and FTIR spectral features were observed from the deposited films with different ablation wavelengths. The 532 nm laser ablation is proposed for the synthesis of high quality carbon nitride films. Received: 16 October 1996 / Accepted: 11 April 1997     

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     

Barium ferrite films with in-plane orientation grown on silicon by pulsed laser deposition

For the first time, barium ferrite films with in-plane orientation were prepared at 700°C by pulsed laser deposition technique (PLD) on Si(1 1 1) without any post-annealing. An amorphous Ba–Fe–O film is used as underlayer to facilitate the crystallization and improve the orientation of films. Sharp (1 1 0) and (2 2 0) peaks appeared in the XRD pattern. The surface morphologies observed by SEM are similar to the typical computer generated grain arrangements obtained by Suzuki et al. in their micromagnetics study. Furthermore, a particular kind of structure of film cross-section was identified in PLD for barium ferrite films. The grain size is about 3 μm, and the coercivity is around 1500 Oe.     

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.     

Excimer laser surface treatment of aluminum alloy in nitrogen

The excimer laser nitriding process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1-bar nitrogen gas. A vapor plasma expands from the surface and a shock wave dissociates and ionizes nitrogen. It is assumed that nitrogen from plasma in contact with the surface penetrates to some depth. Thus it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The nitrogen-concentration profiles are determined from Rutherford backscattering spectroscopy and scanning electron microscopy coupled to energy-dispersive X-ray analysis. Crystalline quality is evidenced by an X-ray diffraction technique. Transmission electron microscopy gives the in-depth microstructure. Fretting coefficient measurements exhibit a lowering for some experimental conditions. The polycrystalline nitride layer obtained is several micrometers thick and composed of a pure AlN (columnar microstructure) top layer (200–500 nm thick) standing on an AlN (grains) in alloy diffusion layer. From the heat conduction equation calculation it is shown that a 308-nm laser wavelength would be better to increase the nitride thickness, as it corresponds to a weaker reflectance R value for aluminum. Received: 17 October 2000 / Accepted: 19 October 2000 / Published online: 23 May 2001