Deposition of aluminum nitride thin film on Si(1 1 1) by KrF excimer laser and its characterizations

We present the deposition of aluminum nitride (AlN) thin film by KrF excimer laser sputtering and the study of the effects of substrate temperature and laser fluences. Deposition rate of AlN thin film at 0.3 Å/pulse has been achieved with laser fluence of 1500 mJ/cm² and at substrate temperature of 250 K, and this shows the enhancement of the deposition rate at low substrate temperature. Surface morphology of the deposited films is characterized by atomic force microscopy (AFM). In addition, the electrical performance of the MIS devices with AlN thin films prepared in this experiment has been characterized.     

Polymer self-assembled nano-structures and surface relief gratings induced with laser at 157 nm

Surface relief gratings (SRG) and self-organized nano-structures induced by laser light at 157 nm on the fluoropolymer poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA), films were obtained under well-controlled light exposure conditions. Regular and semi-regular spaced self-organized grating-like structures were created on polymeric films for ∼7.5-100 mJ/cm² laser energy fluence. For lower laser fluence, the surface morphology of light exposed/non-exposed areas exhibited irregular-like structure morphologies, while polymer surface irradiation with energy fluence higher than 150 mJ/cm² causes progressively fading out of the regular patterns. Under the specific experimental conditions, the SRG and self-organization patterning have their origin in the development of a surface thermal instability (Rayleigh's instability), which is resolved itself into regular patterns on the surface of the fluoropolymer film. The thermal instability is due to the explosive polymer surface photo-dissociation at 157 nm and the build up of longitudinal and periodic surface stress, which eventually create the SRG and the self-assembled structures on the polymer.     

Pulsed-laser crystallization and epitaxial growth of metal-organic films of Ca-doped LaMnO3 on STO and LSAT substrates

Ca-doped LaMnO₃ (LCMO) thin films have been successfully prepared on SrTiO₃ (STO) and [(LaAlO₃)_0.3-(SrAlTaO₆)_0.7] (LSAT) substrates using the excimer laser assisted metal-organic deposition (ELAMOD) process. The crystallization and the epitaxial growth of the amorphous metal-organic LCMO thin films have been achieved using a KrF excimer laser irradiation while the substrates were kept at constant temperature of 500 °C. Epitaxial films were obtained using laser fluence in the interval of 50-120 mJ/cm². The microstructure of the LCMO films was studied using cross-section transmission electron microscopy. High quality of LCMO films having smooth surfaces and sharp interfaces were obtained on both the STO and the LSAT substrates. The effect of the laser fluence on the temperature coefficient of resistance (TCR) was investigated. The largest values of TCR of the LCMO grown on the LSAT and the STO substrates of 8.3% K⁻¹ and 7.46% K⁻¹ were obtained at different laser fluence of 80 mJ/cm² and 70 mJ/cm², respectively.     

Direct observation of phase transitions by time-resolved pyro/reflectometry of KrF laser-irradiated metal oxides and metals

New experimental results are obtained by coupling both time-resolved reflectivity and rapid infrared pyrometry under a hemispherical reactor. The heating source KrF laser beam (28 ns, 248 nm) is homogenized and as for probing, a CW He-Ne laser beam (10 mW, 633 nm) is used.Using both methods infrared pyrometry with an IR detector cooled with liquid nitrogen and sensitive in the spectral range 1-12 μm, and time-resolved reflectivity with a rapid photodiode, we were able to study complex thermodynamic transitions with nanosecond time resolution. Three different materials are studied by varying the KrF fluence (energy/surface) from 100 to 2000 mJ/cm²: thin films melting (Au/Ni), the threshold of plasma formation (Ti), and complex liquid phase segregation under semi-conductor state (ZnO). The formation of a liquid Zn film induced by temperature gradient is well evidenced by our signals. Also melting of thin films irradiated by low laser fluences (less than 500 mJ/cm²) translates the typical thermodynamic behavior. Finally, wide fluence dynamic (400-2000 mJ/cm²) is analyzed in the case of Ti surface, and results show two distinguished regimes: first one bellow 1000 mJ/cm² corresponding to the early stage plasma initiation, and second one over 1000 mJ/cm² to the dynamics of plasma expansion.     

High fluence deposition of polyethylene glycol films at 1064 nm by matrix assisted pulsed laser evaporation (MAPLE)

Matrix assisted pulsed laser evaporation (MAPLE) has been applied for deposition of thin polyethylene glycol (PEG) films with infrared laser light at 1064 nm. We have irradiated frozen targets (of 1 wt.% PEG dissolved in water) and measured the deposition rate in situ with a quartz crystal microbalance. The laser fluence needed to produce PEG films turned out to be unexpectedly high with a threshold of 9 J/cm², and the deposition rate was much lower than that with laser light at 355 nm. Results from matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis demonstrate that the chemistry, molecular weight and polydispersity of the PEG films were identical to the starting material. Studies of the film surface with scanning electron microscopy (SEM) indicate that the Si-substrate is covered by a relatively homogenous PEG film with few bare spots.     

Indium tin oxide sol-gel precursor conversion process using the third harmonics of Nd:YAG laser

We use the third harmonics of Nd:YAG laser (λ = 355 nm) for simultaneous precursor conversion and dopant activation on sol-gel ITO thin films at a laser fluence range of 700-1000 mJ/cm². A minimum resistivity of 5.37 × 10⁻² Ω-cm with a corresponding carrier concentration of 6 × 10¹⁹ cm⁻³ is achieved at laser irradiation fluence of 900 mJ/cm². X-ray photoelectron analysis reveals that extremely high tin concentration of 19.4 at.% and above is presented in the laser-cured ITO thin films compared with 8.7 at.% in the 500 °C thermally cured counterpart. These excess tin-ions form complex defects, which contribute no free carriers but act as scattering centers, causing inferior electrical properties of the laser-cured films in comparison with the thermally cured ones.     

Magnetoresistance of magnetite thin films grown by pulsed laser deposition on GaAs(1 0 0) and Al2O3(0 0 0 1)

Magnetotransport properties of magnetite thin films deposited on gallium arsenide and sapphire substrates at growth temperatures between 473 and 673 K are presented. The films were grown by UV pulsed laser ablation in reactive atmospheres of O₂ and Ar, at working pressure of 8 × 10⁻² Pa. Film stoichiometry was determined in the range from Fe_2.95O₄ to Fe_2.97O₄. Randomly oriented polycrystalline thin films were grown on GaAs(1 0 0) while for the Al₂O₃(0 0 0 1) substrates the films developed a (1 1 1) preferred orientation. Interfacial Fe³⁺ diffusion was found for both substrates affecting the magnetic behaviour. The temperature dependence of the resistance and magnetoresistance of the films were measured for fields up to 6 T. Negative magnetoresistance values of ∼5% at room temperature and ∼10% at 90 K were obtained for the as-deposited magnetite films either on GaAs(1 0 0) or Al₂O₃(0 0 0 1).     

Femtosecond and nanosecond laser damage thresholds of doped and undoped triazenepolymer thin films

The influence of pulse duration on the laser-induced damage in undoped or infrared-absorbing-dye doped thin triazenepolymer films on glass substrates has been investigated for single, near-infrared (800 nm) Ti:sapphire laser pulses with durations ranging from 130 fs up to 540 fs and complementarily for infrared (1064 nm) Nd:YAG ns-laser single-pulse irradiation. The triazenepolymer material has been developed for high resolution ablation with irradiation at 308 nm. Post-irradiation optical microscopy observations have been used to determine quantitatively the threshold fluence for permanent laser damage. In contrast to our previous studies on a triazenepolymer with different composition [J. Bonse, S.M. Wiggins, J. Solis, T. Lippert, Appl. Surf. Sci. 247 (2005) 440], a significant dependence of the damage threshold on the pulse duration is found in the sub-picosecond regime with values ranging from ∼500 mJ/cm² (130 fs) up to ∼1500 mJ/cm² (540 fs). Other parameters such as the film thickness (50 nm and 1.1 μm samples) or the doping level show no significant influence on the material behavior upon irradiation. The results for fs- and ns-laser pulse irradiation are compared and analyzed in terms of existent ablation models.     

Growth and characteristics of ZnO nano-aggregates electrodeposited onto p-Si(1 1 1)

In this paper we study nanocrystalline zinc oxide thin films produced by oxidation of electrodeposited zinc nanolayers on a monocrystalline p-Si(1 1 1) substrate.The electrolyte used is ZnCl₂, an aqueous solution of 4 × 10⁻² mol/l concentration. Several deposits were made for various current densities, ranging from 13 mA/cm² to 44 mA/cm², flowing through the solution at room temperature. A parametric study enabled us to assess the effect of the current density on nucleation potential and time as well as zinc films structure. The grazing incidence X-ray diffraction (GIXD) revealed that both Zn and ZnO films are polycrystalline and nanometric. After 1-h oxidation of zinc films at 450 °C in the open air, the structural analyses showed that the obtained ZnO films remained polycrystalline with an average crystal size of about 47 nm and with (1 0 0), (0 0 2) and (1 0 1) as preferential crystallographic orientations.     

Modifications of AISI 1045 steel by picosecond Nd:YAG laser at 266 nm; comparison with 532 and 1064 nm pulses

Interaction of Nd:YAG laser, operating at 266 nm wavelength and a pulse duration of 40 ps, with AISI 1045 steel was studied. Surface damage threshold was estimated to be 0.14 J/cm². The steel surface modification was studied at the laser fluence of ∼1.0 J/cm². The energy absorbed from Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following AISI 1045 steel surface morphological changes and processes were observed: (i) intensive damage of the target in the central zone of irradiated area; (ii) appearance of periodic surface structures at nano-level, with periodicity in agreement with the used wavelength; (iii) reduction of oxygen concentration in irradiated area; and (iv) development of plasma in front of the target. Generally, interaction of laser beam with AISI 1045 steel (at 266 nm) results in a near-instantaneous creation of damage, meaning that large steel surfaces can be modified in short times.     

Formation of Al-N co-doped p-ZnO/n-Si (1 0 0) heterojunction structure by RF co-sputtering technique

Al-N co-doped ZnO (ZnO:Al-N) thin films were grown on n-Si (1 0 0) substrate by RF co-sputtering technique. As-grown ZnO:Al-N film exhibited n-type conductivity whereas on annealing in Ar ambient the conduction of ZnO:Al-N film changes to p-type, typically at 600 °C the high hole concentration of ZnO:Al-N co-doped film was found to be 2.86 × 10¹⁹ cm⁻³ and a low resistivity of 1.85 × 10⁻² Ω-cm. The current-voltage characteristics of the obtained p-ZnO:Al-N/n-Si heterojunction showed good diode like rectifying behavior. Room temperature photoluminescence spectra of annealed co-doped films revealed a dominant peak at 3.24 eV.     

Laser surface treatment of screen-printed carbon nanotube emitters for enhanced field emission

This paper investigates the surface treatment of screen-printed carbon nanotube (CNT) emitters using a 248 nm (KrF) excimer laser. The field emission characteristics of the CNT emitters are measured following irradiation using laser fluences ranging from 80 to 400 mJ/cm². The results show that the turn-on electric field, the current density, and the distribution of the emission sites are highly dependent on the value of the laser fluence and are optimized at a fluence of 150 mJ/cm². Two distinct laser fluence regimes are identified. In the low fluence regime, i.e. 80-150 mJ/cm², the surface treatment process is dominated by a photo ablation mechanism, which results in the gradual removal of the binding material from the cathode surface and leads to an improvement in the emission characteristics of the CNT cathodes with an increasing fluence. However, in the high fluence regime, i.e. 150-400 mJ/cm², the thermal ablation mechanism dominates; resulting in a removal of the CNTs from the cathode surface and a subsequent degradation in the emission characteristics.     

Role of crystal orientation on the magnetic properties of CoFe2O4 thin films grown on Si (1 0 0) and Al2O3 (0 0 0 1) substrates using pulsed laser deposition

We report the influence of crystal orientation on the magnetic properties of CoFe₂O₄ (CFO) thin films grown on single crystal Si (1 0 0) and c-cut sapphire (Al₂O₃) (0 0 0 1) substrates using pulsed laser deposition technique. The thickness was varied from 200 to 50 nm for CFO films grown on Si substrates, while it was fixed at 200 nm for CFO films grown on Al₂O₃ substrates. We observed that the 200 and 100 nm thick CFO-Si films grew in both (1 1 1) and (3 1 1) directions and displayed out-of-plane anisotropy, whereas the 50 nm thick CFO-Si film showed only an (1 1 1) orientation and an in-plane anisotropy. The 200 nm thick CFO film grown on an Al₂O₃ substrate was also found to show a complete (1 1 1) orientation and a strong in-plane anisotropy. These observations pointed to a definite relation between the crystalline orientation and the observed magnetic anisotropy in the CFO thin films.     

Fixation of bioactive calcium alkali phosphate on Ti6Al4V implant material with femtosecond laser pulses

Bone implants made of metal, often titanium or the titanium alloy Ti6Al4V, need to be surface treated to become bioactive. This enables the formation of a firm and durable connection of the prosthesis with the living bone. We present a new method to uniformly cover Ti6Al4V with a thin layer of ceramics that imitates bone material. These calcium alkali phosphates, called GB14 and Ca10, are applied to the metal by dip coating of metal plates into an aqueous slurry containing the fine ceramic powder. The dried samples are illuminated with the 790 nm radiation of a pulsed femtosecond laser. If the laser fluence is set to a value just below the ablation threshold of the ceramic (ca. 0.4 J/cm²) the 30 fs laser pulses penetrate the partly transparent ceramic layer of 20-40 μm thickness. The remaining laser fluence at the ceramic-metal interface is still high enough to generate a thin metal melt layer leading to the ceramic fixation on the metal. The laser processing step is only possible because Ti6Al4V has a lower ablation threshold (between 0.1 and 0.15 J/cm²) than the ceramic material. After laser treatment in a fluence range between 0.1 and 0.4 J/cm², only the particles in contact with the metal withstand a post-laser treatment (ultrasonic cleaning). The non-irradiated rest of the layer is washed off. In this work, we present results of a successful ceramic fixation extending over larger areas. This is fundamental for future applications of arbitrarily shaped implants.     

Investigation on upconversion photoluminescence of Bi3TiNbO9:Er:Yb thin films

Bi₃TiNbO₉:Er³⁺:Yb³⁺ (BTNEY) thin films were fabricated on fused silica by pulsed laser deposition. It was demonstrated that different laser fluence and substrate temperature during growth of BTNEY upconversion photoluminescence (UC-PL) samples control the film’s grain size and hence influences the UC-PL properties. The average grain size of BTNEY thin films deposited on fused silica substrates with laser fluence 4, 5, 6, and 7 J/cm² are 30.8, 35.9, 40.6, and 43.4 nm, respectively. The 525 nm emission intensities increase with the deposition laser fluence and the emission intensities of BTNEY thin film deposited under 700 and 600 °C are almost 24 and 4 times, respectively, as strong as those of samples under 500 °C. The grain size of BTNEY thin film increases with the increasing temperature. UC-PL of BTNEY films is enhanced by increasing grain size of the films.     

Periodic surface structures on crystalline silicon created by 532 nm picosecond Nd:YAG laser pulses

Creation of laser-induced morphology features, particularly laser-induced periodic surface structures (LIPSS), by a 532 nm picosecond Nd:YAG laser on crystalline silicon is reported. The LIPSS, often termed ripples, were produced at average laser irradiation fluences of 0.7, 1.6, and 7.9 J cm⁻². Two types of ripples were registered: micro-ripples (at micrometer scale) in the form of straight parallel lines extending over the entire irradiated spot, and nano-ripples (at nanometer scale), apparently concentric, registered only at the rim of the spot, with the periodicity dependent on laser fluence. There are indications that the parallel ripples are a consequence of the partial periodicity contained in the diffraction modulated laser beam, and the nano-ripples are very likely frozen capillary waves. The damage threshold fluence was estimated at 0.6 J cm⁻².     

Crystallisation of TiO2 thin films induced by excimer laser irradiation

Titanium dioxide thin films have been deposited by reactive magnetron sputtering on glass substrate and subsequently irradiated by UV radiation using a KrF excimer laser. In this work, we have study the influence of the laser fluence (F) ranging between 0.05 and 0.40 mJ/cm² on the constitution and microstructure of the deposited films. Irradiated thin films are characterized by profilometry, scanning electron microscopy and X-ray diffraction. As deposited films are amorphous, while irradiated films present an anatase structure. The crystallinity of the films strongly varies as a function of F with maximum for F = 0.125 J/cm². In addition to the modification of their constitution, the irradiated areas present a strongly modified microstructure with appearance of nanoscale features. The physico-chemical mechanisms of these structural modifications are discussed based on the theory of nucleation.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

Effects of UV photon irradiation on SiOx (0 < x < 2) structural properties

Thin films of a-SiO_x (0 < x < 2) were prepared by reactive r.f. magnetron sputtering from a polycrystalline-silicon target in an Ar/O₂ gas mixture. The oxygen partial pressure in the deposition chamber was varied so as to obtain films with different values of x. The plasma was monitored, during depositions, by optical emission spectroscopy (OES) system. Energy dispersive X-ray (EDX) measurements and infra-red (IR) spectroscopy were used to study the compositional and structural properties of the deposited layers.Structural modifications of SiO_x thin films have been induced by UV photons’ bombardment (wavelength of 248 nm) using a pulsed laser. IR spectroscopy and X-ray photoemission spectroscopy (XPS) were used to investigate the structural changes as a function of x value and incident energy. SiO_x phase separation by spinodal decomposition was revealed. The IR peak position shifted towards high wavenumber values when the laser energy is increased. Values corresponding to the SiO₂ material (only Si⁴⁺) have been found for laser irradiated samples, independently on the original x value. The phase separation process has a threshold energy that is in agreement with theoretical values calculated for the dissociation energy of the investigated material.For high values of the laser energy, crystalline silicon embedded in oxygen-rich silicon oxide was revealed by Raman spectroscopy.     

Impact of 100 MeV Au on the surface of relaxed Si0.5Ge0.5 alloy films studied by atomic force microscopy

We demonstrate a gradual surface modification process of relaxed Si_0.5Ge_0.5 alloy films by 100 MeV Au beam with fluence varying between 5 × 10¹⁰ and 1 × 10¹² ions/cm² at 80 K by means of atomic force microscopy (AFM). Presence of Ge quantum dots (QDs) was found in the virgin sample. The disappearance of the QDs were noticed when the samples were irradiated with a fluence of 5 × 10¹⁰ ions/cm². Craters were found developing at a fluence of 1 × 10¹¹ ions/cm². Apart from the evolution of the craters, blisters were also detected at a fluence of 1 × 10¹² ions/cm². Variation of the average root mean square value of the surface roughness as a function of fluence was examined.