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.     

Morphological characteristics of amorphous Ge2Sb2Te5 films after a single femtosecond laser pulse irradiation

The morphology of materials resulting from laser irradiation of the single-layer and the multilayer amorphous Ge₂Sb₂Te₅ films using 120 fs pulses at 800 nm was observed using scanning electron microscopy and atomic force microscopy. For the single-layer film, the center of the irradiated spot is depression and the border is protrusion, however, for the multilayer film, the center morphology changes from a depression to a protrusion as the increase of the energy. The crystallization threshold fluence of the single-layer and the multilayer film is 22 and 23 mJ/cm², respectively.     

Femtosecond laser patterning of Ta0.1W0.9Ox/ITO thin film stack

Selective laser patterning of thin films in a multilayered structure is an emerging technology for process development and fabrication of optoelectronics and microelectronics devices. In this work, femtosecond laser patterning of electrochromic Ta_0.1W_0.9O_x film coated on ITO glass has been studied to understand the selective removal mechanism and to determine the optimal parameters for patterning process. A 775 nm Ti:sapphire laser with a pulse duration of 150 fs operating at 1 kHz was used to irradiate the thin film stacks with variations in process parameters such as laser fluence, feedrate and numerical aperture of objective lens. The surface morphologies of the laser irradiated regions have been examined using a scanning electron microscopy and an optical surface profiler. Morphological analysis indicates that the mechanism responsible for the removal of Ta_0.1W_0.9O_x thin films from the ITO glass is a combination of blistering and explosive fracture induced by abrupt thermal expansion. Although the pattern quality is divided into partial removal, complete removal, and ITO film damage, the ITO film surface is slightly melted even at the complete removal condition. Optimal process window, which results in complete removal of Ta_0.1W_0.9O_x thin film without ablation damage in the ITO layer, have been established. From this study, it is found that focusing lens with longer focal length is preferable for damage-free pattern generation and shorter machining time.     

F2 laser induced oxidation of inorganic material

Transparent SiO₂ thin films were selectively fabricated on Si wafer by 157 nm F₂ laser in N₂/O₂ gas atmosphere. The F₂ laser photochemically produced active O(¹D) atoms from O₂ molecules in the gas atmosphere; strong oxidation reaction could be induced to fabricate SiO₂ thin films only on the irradiated areas of Si wafer. The oxidation reaction was sensitive to the single pulse fluence of F₂ laser. The irradiated areas were swelled and the height was approximately 500-1000 nm at the 205-mJ/cm² single pulse fluence for 60 min laser irradiation. The fabricated thin films were analytically identified to be SiO₂ by the Fourier-transform IR spectroscopy. The SiO₂ thin films could be also removed by subsequent chemical etching to fabricate micro-holes 50 nm in depth on Si wafer for microfabrication.     

Multilayer Bi1.5Zn1.0Nb1.5O7/Ba0.6Sr0.4TiO3/Bi1.5Zn1.0Nb1.5O7 thin films for tunable microwave applications

Bi_1.5Zn_1.0Nb_1.5O₇/Ba_0.6Sr_0.4TiO₃/Bi_1.5Zn_1.0Nb_1.5O₇ tunable multilayer thin film has been fabricated by pulsed laser ablation and characterized. Phase composition and microstructure of multilayer films were characterized by X-ray diffraction, scanning electron microscopy (SEM) and atomic force microscopy (AFM). The film has very smooth surface with RMS roughness of 1.5-2 nm and grain size of 100-150 nm. Total film thickness has been measure to be 375 nm. The BZN thin films at 300 K, on Pt(1 1 1)/SiO₂/Si substrate showed zero-field dielectric constant of 105 and dielectric loss tangent of 0.002 at frequency of 0.1 MHz. Thin films annealed at 700 °C shows the dielectric tunability of 18% with biasing field 500 kV/cm at 0.1 MHz. The multilayer thin film shows nonferroelectric behavior at room temperature. The good physical and electrical properties of multilayer thin films make them promising candidate for tunable microwave device applications.     

Epitaxial growth and structural characterization of Pb(Fe1/2Nb1/2)O3 thin films

We have grown lead iron niobate thin films with composition Pb(Fe_1/2Nb_1/2)O₃ (PFN) on (0 0 1) SrTiO₃ substrates by pulsed laser deposition. The influence of the deposition conditions on the phase purity was studied. Due to similar thermodynamic stability spaces, a pyrochlore phase often coexists with the PFN perovskite phase. By optimizing the kinetic parameters, we succeeded in identifying a deposition window which resulted in epitaxial perovskite-phase PFN thin films with no identifiable trace of impurity phases appearing in the X-ray diffractograms. PFN films having thicknesses between 20 and 200 nm were smooth and epitaxially oriented with the substrate and as demonstrated by RHEED streaks which were aligned with the substrate axes. X-ray diffraction showed that the films were completely c-axis oriented and of excellent crystalline quality with low mosaicity (X-ray rocking curve FWHM?0.09°). The surface roughness of thin films was also investigated by atomic force microscopy. The root-mean-square roughness varies between 0.9 nm for 50-nm-thick films to 16 nm for 100-nm-thick films. We also observe a correlation between grain size, surface roughness and film thickness.     

Structural characterization of thermally evaporated Bi2Te3 thin films

Thermally evaporated Bi₂Te₃ thin films were deposited on glass substrates. X-ray diffraction study confirmed that the growned films are polycrystalline in nature having hexagonal structure. The film exhibits preferential orientation along the [0 1 5] direction for the films of all thickness together with other abundant planes [0 1 1 1] and [1 1 0]. Various structural parameters such as lattice constants, crystallite size, strain, and dislocation density have been calculated and they are found to be thickness dependent. The lattice parameters are found to be a=4.38 Å and c=30.40 Å. The grain size of the films increases with thickness as the dislocation density and the microstrain decreases with thickness. The mean bond energy and the average coordination number of Bi₂Te₃ thin film are found to be 1.72 eV and 2.4, respectively.     

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.     

Substrate effect on electronic sputtering yield in polycrystalline fluoride (LiF, CaF2 and BaF2) thin films

Influence of substrate on electronic sputtering of fluoride (LiF, CaF₂ and BaF₂) thin films, 10 and 100 nm thin, under dense electronic excitation of 120 MeV Ag²⁵⁺ ions irradiation is investigated. The sputtering yield of the films deposited on insulating (glass) and semiconducting (Si) substrates are determined by elastic recoil detection analysis technique. Results revealed that sputtering yield is higher, up to 7.4 × 10⁶ atoms/ion for LiF film on glass substrate, than that is reported for bulk materials/crystals (∼10⁴ atoms/ion), while a lower value of the yield (2.3 × 10⁶ atoms/ion) is observed for film deposited on Si substrate. The increase in the yield for thin films as compared to bulk material is a combined effect of the insulator substrate used for deposition and reduced film dimension. The results are explained in the framework of thermal spike model along with substrate and size effects in thin films. It is also observed that the material with higher band gap showed higher sputtering yield.     

The ferroelectric and ferromagnetic characterization of CoFe2O4/Pb(Mg1/3Nb2/3)O3-PbTiO3 multilayered thin films

The multiferroic (PMN-PT/CFO)_n (n = 1,2) multilayered thin films have been prepared on SiO₂/Si(1 0 0) substrate with LNO as buffer layer via a rf magnetron sputtering method. The structure and surface morphology of multilayered thin films were determined by X-ray diffraction (XRD) and atom force microscopy (AFM), respectively. The smooth, dense and crack-free surface shows the excellent crystal quality with root-mean-square (RMS) roughness only 2.9 nm, and average grain size of CFO thin films on the surface is about 44 nm. The influence of the thin films thickness size, periodicity n and crystallite orientation on their properties including ferroelectric, ferromagnetic properties in the (PMN-PT/CFO)_n multilayered thin films were investigated. For multilayered thin films with n = 1 and n = 2, the remanent polarization Pr are 17.9 μC/cm² and 9.9 μC/cm²; the coercivity H_c are 1044 Oe and 660 Oe, respectively. In addition, the relative mechanism are also discussed.     

Effect of spot size on cone formation in a XeCl laser ablation of polyethersulfone films

Radiation from the UV excimer lasers, with the fluence above the ablation threshold, can etch the polymer surfaces by photoablation. In some cases different microstructures may appear on the surface during the laser ablation. In this paper the effect of the laser spot size on the cone formation on polyethersulfone films has been investigated. The experiments have been performed with a XeCl laser at the wavelength of 308 nm and at the fluences of 70 and 100 mJ/cm² at air. For the investigation of the effect of the laser spot size on cone formation, the samples were irradiated at two different laser spot sizes of w₁ and w₂ = 0.1 w₁. The morphology of the processed surface was studied by scanning electron microscopy (SEM). It has shown that the shape, size and density of cones change with the change of the laser spot size. Also, the number of pulses and the pulse repetition rate which are needed for threshold of cone formation are affected by the laser beam spot size on the surface.     

Characteristic properties of Y2SiO5:Ce thin films grown with PLD

Three different gases (nitrogen (N₂), oxygen (O₂) and argon (Ar)) were used as background gases during the growth of pulsed laser deposition (PLD) Y₂SiO₅:Ce thin films. A Krypton fluoride laser (KrF), 248 nm was used for the PLD of the films on silicon (Si) (1 0 0) substrates. The effect of the background gases on the surface morphology, crystal growth and luminescent properties were investigated. All the experimental parameters, the gas pressure (455 mT), the substrate temperature (600 °C), the pulse frequency (8 Hz), the number of pulses (4000) and the laser fluence (1.6±0.2) J/cm² were kept constant. The only parameter that was changed during the deposition was the ambient gas species. The surface morphology and average particle sizes were monitored with scanning electron microscopy (SEM) and atomic force microscopy (AFM). X-ray diffraction (XRD) and Auger electron spectroscopy (AES) were used to determine the crystal structure and composition, respectively. Cathodo- (CL) and photoluminescence (PL) were used to measure the luminescent intensities for the different phosphor thin films. The nature of the particles, ablated on the substrate, is related to the collisions between the ejected particles and the ambient gas particles. The CL and PL intensities also depend on the particle sizes. A 144 h (coulomb dose of 1.4×10⁴ C cm⁻²) electron degradation study on the thin films ablated in the Ar gas environment resulted in a decrease in the main CL intensity peak at 440 nm and to the development of a new very broad luminescent peak spectra ranging from 400 to 850 nm due to the growth of a SiO₂ layer on the surface.     

Analysis of material modifications induced during laser damage in SiO2 thin films

The damage mechanisms in silica thin films exposed to high fluence 1064 nm nano-second laser pulses are investigated. The thin films under study are made with different techniques (evaporation and sputtering, with and without ion assistance) and the results are compared. The material morphological, optical and structural modifications are locally analyzed with optical microscopy and profilometry, photoluminescence and absorption microscopies. These observations are made for fluences near and above the laser damage threshold, and also in the case of multiple pulse irradiations. An increase in absorption in and around the damages is observed, as well as the generation of different defects that we spatially resolve with absorption and luminescence mappings.     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Stress dependence of growth mode change of epitaxial layered cobaltite γ-Na0.7CoO2

We report stress dependence of growth characteristics of epitaxial γ-Na_0.7CoO₂ films on various substrates deposited by pulsed laser deposition method. On the sapphire substrate, the γ-Na_0.7CoO₂ thin film exhibits spiral surface growth with multi-terraces and highly crystallized texture. For the γ-Na_0.7CoO₂ thin film grown on the (1 1 1) SrTiO₃ substrate, the nano-islands of ∼30 nm diameter on the hexagonal grains are observed. These islands indicate that the growth mode changes from step-flow growth mode to Stranski-Krastanow (SK) growth mode. On the (1 1 1) MgO substrate, the large grains formed by excess adatoms covering an aperture between hexagonal grains are observed. These experimental demonstrations and controllability could provide opportunities of strain effects of Na_xCoO₂, physical properties of thin films, and growth dynamics of heterogeneous epitaxial thin films.     

Lead-free ferroelectric BaTiO3 doped-(Na0.5Bi0.5)TiO3 thin films processed by pulsed laser deposition technique

The difficulties in synthesizing phase pure BaTiO₃ doped-(Na_0.5Bi_0.5)TiO₃ are known. In this work, we reporting the optimized pulsed laser deposition (PLD) conditions for obtaining pure phase 0.92(Na_0.5Bi_0.5)TiO₃-0.08BaTiO₃, (BNT-BT_0.08), thin films. Dielectric, ferroelectric and piezoelectric properties of BNT-BT_0.08, thin films deposited by PLD on Pt/TiO₂/SiO₂/Si substrates are investigated in this paper. Perovskite structure of BNT-BT_0.08 thin films with random orientation of nanocrystallites has been obtained by deposition at 600 °C. The relative dielectric constant and loss tangent at 100 kHz, of BNT-BT_0.08 thin film with 530 nm thickness, were 820 and 0.13, respectively. Ferroelectric hysteresis measurements indicated a remnant polarization value of 22 μC/cm² and a coercive field of 120 kV/cm. The piezoresponse force microscopy (PFM) data showed that most of the grains seem to be constituted of single ferroelectric domain. The as-deposited BNT-BT_0.08 thin film is ferroelectric at the nanoscale level and piezoelectric.     

Small size TiO2 nanoparticles prepared by laser ablation in water

Titanium dioxide nanoparticles in distilled H₂O solvent were prepared by laser ablation. The experiments were performed irradiating a Ti target with a second harmonic (532 nm) output of a Nd:YAG laser varying the operative fluence between 1 and 10 J cm⁻² and for an ablation time ranging from 10 to 30 min. Electron microscopy measurements have evidenced the predominant presence of nanoparticles with diameter smaller than 10 nm together with agglomerations of 100-200 nm whose content increases with the laser fluence. At low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while at mid and high fluences the presence of 5-7 nm nanoparticles is predominant. XPS analysis has revealed the presence of different titanium suboxide phases with the prevalence of Ti-O bonds from TiO₂ species. The optical bandgap values, determined by UV-vis absorption measurements, are compatible with the anatase phase.     

Preparation of sol-gel ZrO2-SiO2 highly reflective multilayer films and laser-induced damage threshold characteristic

Sol-gel (ZrO₂/SiO₂)¹² ZrO₂ films were prepared by spin coating method. The reflectivity spectrum of the films was measured with a Lambda 900 spectrometer. In order to investigate laser-induced damage threshold (LIDT) characteristic of highly reflective films, one-layer ZrO₂ and SiO₂ films, two-layer ZrO₂/SiO₂ and SiO₂/ZrO₂ films were also prepared by spin coating method. LIDT of each film was measured. Damage morphology after laser irradiation was characterized by optical microscopy (Nikon E600K). The experimental results showed that the reflectivity of (ZrO₂/SiO₂)¹² ZrO₂ film at 1064 nm and 355 nm wavelength is 99.7%. The LIDT results decreases as the number of layer of films increases. All the films have similar damage morphology. The experimental results are explained by the different temperature profiles of the films.     

Influence of overcoat on laser-induced damage threshold of 532 and 800 nm TiO2/SiO2 high reflectors

TiO₂/SiO₂ high reflectors with and without a SiO₂ overcoat are deposited by electron-beam evaporation. The film properties are characterized by visible spectrometry measures, structure analysis, roughness and laser-induced damage threshold (LIDT) tests, surface defects and damage morphology observation. The effects of overcoats on LIDT at 532 nm, 8 ns and 800 nm, 220 ps laser pulses are investigated. The relations between film structure, roughness, surface defects, electric field and LIDT are discussed. It is found that overcoats can increase the LIDT at these two laser wavelengths. The reduction of peak temperature, the low defects density and roughness, the low intrinsic absorption of SiO₂ and its amorphous structure are the main reasons for LIDT improvement by overcoats.     

Effect of deposition temperature on orientation and electrical properties of (K0.5Na0.5)NbO3 thin films by pulsed laser deposition

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) thin films have been prepared on Pt/TiO₂/SiO₂/Si substrates by pulsed laser deposition process. The structures, crystal orientations and electrical properties of thin films have been investigated as a function of deposition temperature from 680 °C to 760 °C. It is found that the deposition temperature plays an important role in the structures, crystal orientations and electrical properties of thin films. The crystallization of thin films improves with increasing deposition temperature. The thin film deposited at 760 °C exhibits strong (0 0 1) preferential orientation, large dielectric constant of 930 and the remnant polarization of 8.54 μC/cm².