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.     

Atomic force microscopy investigation of surface roughness generated between SiO2 micro-pits in CHF3/Ar plasma

The present paper investigates the surface roughness generated by reactive ion etching (RIE) on the location between silicon dioxide (SiO₂) micro-pits structures. The micro-pit pattern on polymethyl methacrylate (PMMA) mask was created by an electron beam lithography tool. By using PMMA as a polymer resist mask layer for pattern transfer in RIE process, the carbon (C) content in etching process is increased, which leads to decrease of F/C ratio and causes domination of polymerization reactions. This leads to high surface roughness via self-organized nanostructure features generated on SiO₂ surface which was analyzed using atomic force microscopy (AFM) technique. The etching chemistry of CHF₃ plasma on PMMA masking layer and SiO₂ is analyzed to explain the polymerization. The surface root-mean-square (RMS) roughness below 1 nm was achieved by decreasing the RF power to 150 W and process pressure lower than 10 mTorr.     

Surface evolution and laser damage resistance of CO2 laser irradiated area of fused silica

A 10.6 μm CO₂ laser has been reported to effectively mitigate the laser damage growth of fused silica. Two zones of the laser irradiated area are defined in this work: the distorted zone and the laser affected zone. The parameters of the two zones are studied at different CO₂ laser beam sizes, irradiation times, and powers by microscopy, profilometry, and photoelastic method. The results show that the diameter of laser affected zone is almost completely determined by the laser beam size and the distorted zone is associated with the mitigation range of CO₂ laser beam. The diameter and depth of the distorted zone increase as the laser power and irradiation time increase. The depth grows exponentially depending on the irradiation time. The maximum residual stress discrepancy is located near the boundary of the laser affected zone. The laser damage resistance test results show that the distorted zone and the laser affected zone have a better damage resistance than the original substrate.     

Crystallization of 21.25Gd2O3-63.75MoO3-15B2O3 glass induced by femtosecond laser at the repetition rate of 250 kHz

We report the formation of β′-Gd₂(MoO₄)₃ (GMO) crystal on the surface of the 21.25Gd₂O₃-63.75MoO₃-15B₂O₃ glass, induced by 250 kHz, 800 nm femtosecond laser irradiation. The morphology of the modified region in the glass was clearly examined by scanning electron microscopy (SEM). By micro-Raman spectra, the laser-induced crystals were confirmed to be GMO phases and it is found that these crystals have a strong dependence on the number and power of the femtosecond laser pulses. When the irradiation laser power was 900 mW, not only the Raman peaks of GMO crystals but also some new peaks at 214 cm⁻¹, 240 cm⁻¹, 466 cm⁻¹, 664 cm⁻¹ and 994 cm⁻¹which belong to the MoO₃ crystals were observed. The possible mechanisms are proposed to explain these phenomena.     

Modulation of the photoluminescence of Si quantum dots by means of CO2 laser pre-annealing

Si quantum dots (QDs) embedded in SiO₂ can be normally prepared by thermal annealing of SiO_x (x < 2) thin film at 1100 °C in an inert gas atmosphere. In this work, the SiO_x thin film was firstly subjected to a rapid irradiation of CO₂ laser in a dot by dot scanning mode, a process termed as pre-annealing, and then thermally annealed at 1100 °C for 1 h as usual. The photoluminescence (PL) intensity of Si QD was found to be enhanced after such pre-annealing treatment. This PL enhancement is not due to the additional thermal budget offered by laser for phase separation, but attributed to the production of extra nucleation sites for Si dots within SiO_x by laser irradiation, which facilitates the formation of extra Si QDs during the subsequent thermal annealing.     

Analysis of excimer laser annealing of amorphous SiGe on La2O3//Si structures

The reduction of complementary metal oxide semiconductor dimensions through transistor scaling is in part limited by the SiO₂ dielectric layer thickness. Among the materials evaluated as alternative gate dielectrics one of the leading candidate is La₂O₃ due to its high permittivity and thermodynamic stability. However, during device processing, thermal annealing can promote deleterious interactions between the silicon substrate and the high-k dielectric degrading the desired oxide insulating properties.The possibility to grow poly-SiGe on top of La₂O₃//Si by laser assisted techniques therefore seems to be very attractive. Low thermal budget techniques such as pulsed laser deposition and crystallization can be a good choice to reduce possible interface modifications due to their localized and limited thermal effect.In this work the laser annealing by ArF excimer laser irradiation of amorphous SiGe grown on La₂O₃//Si has been analysed theoretically by a numerical model based on the heat conduction differential equation with the aim to control possible modifications at the La₂O₃//Si interface. Simulations have been carried out using different laser energy densities (0.26-0.58 J/cm²), different La₂O₃ film thickness (5-20 nm) and a 50 nm, 30 nm thick amorphous SiGe layer. The temperature distributions have been studied in both the two films and substrate, the melting depth and interfaces temperature have been evaluated. The fluences ranges for which the interfaces start to melt have been calculated for the different configurations.Thermal profiles and interfaces melting point have shown to be sensitive to the thickness of the La₂O₃ film, the thicker the film the lower the temperature at Si interface.Good agreement between theoretical and preliminary experimental data has been found.According to our results the oxide degradation is not expected during the laser crystallization of amorphous Si_0.7Ge_0.3 for the examined ranges of film thickness and fluences.     

Fiber laser annealing of nanocrystalline PZT thick film prepared by aerosol deposition

Nanocrystalline PZT thick films (1 mm square and over 10 μm thick) directly deposited onto stainless-steel substrates (PZT/SUS) by aerosol deposition (AD) technique and then annealed using focused laser beam with a fiber laser to suppress thermal damage to the back sides of the PZT/SUS and substrate near the film edge and to retain the dielectric and/or ferroelectric properties of the PZT/SUS. Compared with CO₂ laser annealing, fiber laser annealing suppressed thermal damage to the substrate. Compared with PZT/SUS annealed at 600 °C using an electric furnace, PZT/SUS annealed at 600 °C using a fiber laser showed superior properties, namely, dielectric constant ? > 1200 at a frequency of 100 Hz, remanent polarization P_r > 30 μC/cm², and coercive field strength E_c < 50 kV/cm at a frequency of 10 Hz. Furthermore, the grain growth for the PZT/SUS formed by AD technique and annealed by fiber laser irradiation was occurred within the laser spot size.     

Enhanced diamond nucleation on copper substrates by graphite seeding and CO2 laser irradiation

Diamond nucleation on copper (Cu) substrates was investigated by graphite seeding and CO₂ laser irradiation at initial stages of the combustion-flame deposition. A graphite aerosol spray was used to generate a thin layer of graphite powders (less than 1 μm) on Cu substrates. The graphite-seeded Cu substrates were then heated by a continuous CO₂ laser to about 750 °C within 1 min. It was found that diamond nucleation density after this treatment was more than three times as much as that on the virgin Cu substrates. As a consequence, diamond films up to 4 μm were obtained in 5 min. The enhancement of diamond nucleation on the graphite-seeded Cu substrates was attributed to the formation of defects and edges during the etching of the seeding graphite layers by the OH radicals in the flame. The defects and edges served as nucleation sites for diamond formation. The function of the CO₂ laser was to rapidly heat the deposition areas to create a favorable temperature for diamond nucleation and growth.     

Density functional theory study on activity of α-Fe2O3 in chemical-looping combustion system

The dominant growth planes (0 0 0 1) and (1 1 0 2) have been used to investigate the activity of the natural α-Fe₂O₃ in chemical-looping combustion system based on density functional theory (DFT) calculations. In the chemical-looping combustion system, CO is selected as the probe fuel gas to detect the activities of the different surfaces. CO interacts stronger to Fe₂O₃ (1 1 0 2) than Fe₂O₃ (0 0 0 1). CO can be oxidized into CO₂ species directly on Fe₂O₃ (1 1 0 2) rather than Fe₂O₃ (0 0 0 1). The formation of CO₂ accompanying with a transformation from hematite to magnetite acted as the key step for the reduction process of hematite.     

Photocatalytic behavior of heavy La-doped TiO2 films deposited by pulsed laser deposition using non-sintered target

We have investigated the control of photocatalytic behavior under deposited conditions of non-sintered target of different molar ratios with TiO₂ and La₂O₃ from 1:0 to 1:2 for heavily La doping, and post-annealing temperature from 600 °C to 1000 °C for crystallizing by pulsed laser deposition. We have successfully crystallized heavily La-doped TiO₂ films with post-annealing temperature over 800 °C and with molar ratio of TiO₂:La₂O₃ over 1:1 on a quartz substrate. Heavily La-doped TiO₂ films are observed the decomposition of methylene blue and a water-splitting reaction in photocatalytic behavior under Xe light irradiation. When stoichiometric La-doped TiO₂ (TiO₂:La₂O₃ = 1: 1) is synthesized with heat-treatment at 900 °C, the best results are obtained under photocatalytic behavior and pure La₂Ti₂O₇ crystalline were obtained.     

Surface characterization and luminescent properties of SrAl2O4:Eu,Dy nano thin films

SrAl₂O₄:Eu²⁺, Dy³⁺ thin films were grown on Si (1 0 0) substrates in different atmospheres using the pulsed laser deposition (PLD) technique. The effects of vacuum, oxygen (O₂) and argon (Ar) deposition atmospheres on the structural, morphological and photoluminescence (PL) properties of the films were investigated. The films were ablated using a 248 nm KrF excimer laser. Improved PL intensities were obtained from the unannealed films prepared in Ar and O₂ atmospheres compared to those prepared in vacuum. A stable green emission peak at 520 nm, attributed to 4f⁶5d¹→4f⁷ Eu²⁺ transitions was obtained. After annealing the films prepared in vacuum at 800 °C for 2 h, the intensity of the green emission (520 nm) of the thin film increased considerably. The amorphous thin film was crystalline after the annealing process. The diffusion of adventitious C into the nanostructured layers deposited in the Ar and O₂ atmospheres was most probably responsible for the quenching of the PL intensity after annealing.     

Femtosecond pulsed laser deposition of nanostructured TiO2 films

Nanostructured deposits of TiO₂ were grown on Si (1 0 0) substrates by laser ablating a TiO₂ sintered target in vacuum or in oxygen using a Ti:sapphire laser delivering 80 fs pulses. The effect of the laser irradiation wavelength on the obtained nanostructures, was investigated using 800, 400 and 266 nm at different substrate temperatures and pressures of oxygen. The composition of the deposits was characterized using X-ray photoelectron spectroscopy (XPS) and the surface morphology was studied by environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). Deposits are absent of microscopic droplets in all conditions explored. The best deposits, constituted by nanoparticles of an average diameter of 30 nm with a narrow size distribution, were obtained at the shorter laser wavelength of 266 nm under vacuum at substrate room temperature.     

Laser processing of micro-optical components in quartz

Laser induced backside wet etching combined with the diffractive gray tone phase mask has been used for the fabrication of a micro-lens array with a single lens diameter of 1 mm and a micro-prism in quartz. The micro-lens array was tested as beam homogenizer for high power XeCl excimer laser yielding a clear improvement in the quality of the laser beam.The optimum fluence range for fabrication of micro-lenses by laser induced backside wet etching using 1.4 M pyrene in THF solution and 308 nm irradiation wavelength is 1-1.6 J/cm². The etching mechanisms of LIBWE are based on a combination of pressure and temperature jumps at quartz-liquid interface.     

Mid-infrared polarization spectroscopy of C2H2: Non-intrusive spatial-resolved measurements of polyatomic hydrocarbon molecules for combustion diagnostics

Polarization spectroscopy in the mid-infrared (IRPS) has been applied to the detection of acetylene molecules making use of the asymmetric C-H stretching vibration at around 3 μm. The infrared laser pulses were produced through difference frequency generation in a LiNbO₃ crystal pumped by a Nd:YAG and dye laser system. By directly probing the ro-vibrational transitions with IRPS, sensitive detection of molecules with otherwise inaccessible electronic states was realized with high temporal and spatial resolution by using a pulsed laser and a cross-beam geometry. Detection sensitivities of 2 × 10¹³ molecules/cm³ (10 ppm in 70 mbar gas mixture) of C₂H₂ were achieved using the P(1 1) line of the (0 1 0(1 1)⁰)-(0 0 0 0⁰ 0⁰) band. The dependence of the IRPS signal on the pump laser fluence, acetylene mole fraction, and buffer gas pressure of Ar, N₂, H₂, and CO₂ has been studied experimentally. The investigation demonstrates the quantitative nature of IRPS for sensitive detection of polyatomic IR active molecules. In order to fully demonstrate the technique for combustion applications, nascent acetylene molecules were measured in a low pressure methane/oxygen flame.     

Simultaneous three-photon absorption induced ultraviolet upconversion in Pr:Y2SiO5 crystal by femtosecond laser irradiation

Near-infrared to ultraviolet upconversion luminescence was observed in the Pr³⁺:Y₂SiO₅ crystal with 120 fs, 800 nm infrared laser irradiation. The observed emissions at around 270 nm and 305 nm could be assigned to 5d → 4f transitions of Pr³⁺ ions. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to simultaneous three-photon absorption induced upconversion luminescence.     

Reactivity screening of silica

As a screening of the chemical activity of silica [SiO₂/Si(1 0 0)], which is one of the most often used supports for nanostructures, thermal desorption spectroscopy data have been gathered for a variety of gases such as n-nonane, n-hexane, n-butane, iso-butane, ethane, CO₂, CO, O₂, and H/H₂. Whereas, the alkanes with chain lengths larger than three adsorb with large binding energies (E_d = 50-70 kJ/mol), the activity towards the other probe molecules is negligible (<24 kJ/mol) down to adsorption temperatures of 95 K. The adsorption of n- and iso-butane has additionally been studied by molecular beam scattering and follows standard precursor mediated adsorption dynamics.     

Experimental research on axisymmetric folded-combined cavity CO2 laser

Rationality and feasibility of axisymmetric folded-combined cavity CO₂ laser which is reported in this paper have been proved by principle experiments. The laser lays a foundation to manufacture higher power CO₂ lasers whose output power can easily reach 1 kW. Faculas at different positions in free space and facula of nine beams which pass through the beam focusing optics have been obtained. Analyses of the experimental results are made. These analyses lay a foundation for beam transformation, transmission and shaping.     

The effect of P2O5 on the ultra broadband near-infrared luminescence from bismuth-doped SiO2-Al2O3-CaO glass

The effect of P₂O₅ on infrared luminescence properties of bismuth-doped SiO₂-Al₂O₃-CaO (SAC) glass was investigated. Under excitation of 690 and 808 nm LD, two infrared emissions from bismuth ions central at 1100 and 1300 nm were observed, respectively. The addition of P₂O₅ was not only found to lead to the increase of full width at half maximum (FWHM) of two infrared emissions, but also result in intensity variety of the infrared emissions. The intensity of the infrared emission located at 1300 nm is reduced by a factor of 2, while the luminescence at 1110 nm is increased by a factor of 5. We propose that the infrared emissions at 1100 and 1300 nm may originate from different valence Bi ion in glasses. Compared with emission at 1300 nm, the infrared emissions at 1100 nm is more possibly from the transition of lower valent Bi ion.     

Role of Ti in the luminescence process of Al2O3:Si,Ti

Al₂O₃:Si,Ti, prepared under oxidizing condition at high temperature, gives PL emission around 430 nm when excited with 240 nm. The Al₂O₃:C, TL/OSL phosphor, also shows emission around 430 nm, which corresponds to characteristic emission of F-center. Thus, to identify the exact nature of luminescent center in Al₂O₃:Si,Ti, fluorescence lifetime measurement studies were carried out along with the PL,TL and OSL studies. The PL and TL in Al₂O₃:Si,Ti show emission around 430 nm and the time-resolved fluorescence studies show lifetime of about 43 μs for the 430 nm emission, which is much smaller than the reported lifetime of ∼35 ms for the 430 nm emission (F-center emission) in Al₂O₃:C phosphor. Therefore, the emission observed in Al₂O₃:Si,Ti phosphor was assigned to Ti⁴⁺ charge transfer transition. Fluorescence studies of Al₂O₃:Si,Ti do not show any traces of F and F⁺ centers. Also, Ti⁴⁺ does not show any change in the charge state after gamma-irradiation. On the basis of the above studies, a mechanism for TSL/OSL process in Al₂O₃:Si,Ti is proposed.     

Surface modification of silicate glasses by nanoimprint using nanostriped NiO thin film molds

Nanoscale surface modification of silicate glasses was examined by applying nanoimprint technique using a nanostriped NiO thin film mold. The mold had the pattern composed of regularly arranged straight nanogrooves, which was formed by high-temperature annealing of the Li-doped NiO epitaxial thin film deposited on the atomically stepped sapphire (α-Al₂O₃ single crystal) substrate. The glass imprint was proceeded through the simple steps of heating (∼600 °C), pressing (∼1 kPa) and then cooling in air. The nanoimprinted glass surface transferred reversely from the mold exhibited the multi nanowire array having an interval of ∼80 nm, wire width of ∼70 nm, and wire height of ∼20 nm.