Ge diffusion into GaAs by pulsed laser irradiation

Ge diffusion into GaAs from thin evaporated layers as sources is reported. Irradiation with aQ-switched ruby laser gives rise ton-type diffused layers of a thickness from 240 to 710 Å. A strong compensation of the diffused layers, that cannot be removed by thermal annealing, was observed. From the present experimental results it can be inferred that the diffusion coefficient increases at the melting point by 5 to 6 orders of magnitude.     

Carbon dendrite formation induced by pulsed laser irradiation

Porous carbon dendrite has been prepared by irradiating graphite targets with 532 nm, 10 ns laser pulses. The prepared samples were characterized by means of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The measurement results show that carbon dendrite structures with cluster diameter of 10 nm were obtained on the irradiated target surface in an inert gas atmosphere. The evolution of target surface morphology induced by different laser intensities was investigated. The formation mechanism of the dendrite structure has been discussed in detail. The laser intensity plays an important role in the formation of the nanostructures and there exists an optimum intensity to prepare the carbon dendrite.     

Structural ripple formation in Ge/Sb multilayers induced by laser irradiation

Thin multilayer films (Ge/Sb/Ge/Sb/Si substrate) have been irradiated with single nanosecond laser pulses (=193 nm). Real-Time Reflectivity (RTR) measurements have been used to follow the transformation in situ and cross-sectional transmission electron microscope analysis was used to study both the microstructure and the composition profile before and after irradiation. Melting and mixing are both found to nucleate at preferential sites in the upper Ge/Sb interface. During this process the film surface topography changes in a way not previously seen, and rippling of the film is observed due to lateral mass flow induced in the Sb layer underneath the surface, most probably arising from volume changes upon melting. For the highest irradiation energy densities, melting of the whole multilayer configuration takes place, the ripples are no longer observed, and following cooling and solidification, a mixed amorphous GeSb film is formed.     

Nanocomposite ZnO/Au formation by pulsed laser irradiation

The ZnO/Au nanocomposite formation involves synthesis of Au and ZnO colloidal solutions by 532 nm pulse laser ablation of metal targets in deionized water followed by laser irradiation of the mixed colloidal solution. The transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images show evolution of spherical particles into ZnO/Au nanonetworks with irradiation time. The formation mechanism of the nanonetwork can be explained on the basis of near resonance absorption of 532 nm irradiation by gold nanoparticles which can cause selective melting and fusion of gold nanoparticles to form network. The ZnO/Au nanocomposites show blue shift in the ZnO exciton absorption and red shift in the Au plasmon resonance absorption due to interfacial charge transfer.     

Theoretical and experimental study of laser induced damage on GaAs by nanosecond pulsed irradiation

The surface damage experiments of gallium arsenide (GaAs) single crystal irradiated by 1.06 and 0.53 μm nanosecond irradiations are carried out with fundamental and frequency-doubled Nd:YAG laser, respectively. The surface damage thresholds for both wavelengths are experimentally determined and the damaged morphologies and elementary component are analyzed with electron probe microanalyzer (EPM). It is found that the components of Ga and As almost keep constant in our experiments when the irradiated fluence is just around the surface damage threshold and no oxygen is found at all. The theoretical calculations on temperature rise for both wavelengths are carried out using the purely thermal model. It is shown that for irradiation with photon energy above the corresponding band gap the theoretical calculation is in good agreement with the experimental results; however, for that with photon energy just below the band gap, the experimental results cannot be effectively explained by the purely thermal heating mechanism. Combining with the experiment of multi-shot damage from references we finally conclude that the damage by laser irradiation with photon energy below the band gap should be explained by the micro-defect accumulation and consequently enhanced absorption heating mechanism.     

Structure formation on titanium during oxidation induced by cumulative pulsed Nd:YAG laser irradiation

We investigated local oxidation and surface structure development of Ti targets under multi-pulse, high-repetition-rate Nd:YAG ( =1.064 m, 300 ns, =30 kHz) laser irradiation in air at atmospheric pressure. The experiments were performed at laser intensity levels below the single-laser-pulse melting threshold of Ti. The morphology of the irradiated areas was studied by scanning electron microscopy and profilometry. The variation of the oxide compositions and the crystalline state with increasing laser pulse number was analysed by X-ray diffractometry. Besides the known phenomena related to laser irradiation and oxidation of metal surfaces (micro-crack or pore formation), we evidenced new morphological features such as droplet-like structures inside the surface micro-cracks and micro-columns, and with increasing laser pulse number the formation of a dome-shaped structure over the whole irradiated zone. The occurrence of melting under multi-pulse irradiation was associated with the rise in the surface temperature enhanced by the oxidation, and the progressive evolution of the surface structures was associated with the consecutive melting–solidification processes as well as with the different stages of oxidation. PACS 61.80.Ba; 68.37.Hk; 81.65.Mq     

Transient effects in pulsed laser irradiation

Theoretical analysis of the influence of the temporal profile (rectangular, triangular, Gaussian) of the laser pulse on heating/cooling and phase transition velocities and quantity of ablated material was performed on the basis of a multifront Stephan problem. Modeling showed that material removal under stationary conditions (that correspond to long pulses) is entirely controlled by specific heat and material density, while in the case of transient regimes (short pulses) thermal conductivity and heat capacity play a predominant role. Interaction of the melting and evaporation fronts characterized by an evaporation front velocity far exceeding the melting front one is one of the examples of the transient nature of the phenomena influenced by the laser pulse parameters.     

Temperature behavior of implanted and pulsed laser irradiated GaAs

Recent studies have shown that the Low-Power Pulsed-Laser Annealing (LPPLA) of ion-implanted GaAs specimens can be realized in a power-density window in which a complete structural reordering is guaranted. As the experimentally employed conditions allow us to describe the theoretical problem in an unidimensional space domain, we describe here a method to investigate the in-depth temperature behavior during the low-power pulsed-laser irradiation of ion-implanted semiconductors. The application of this method to GaAs specimens shows that the upper limit of the energy density window is connected with the exceeding of the critical temperature T _c below which the As evaporation rate is negligible.     

Mechanism of the formation of metal nanoclusters during pulsed laser deposition

The geometrical structure of Au, Ni, Co and Cr nanoclusters self-assembled on NaCl and HOPG surfaces under pulsed laser deposition (PLD) has been experimentally investigated. The PLD technique is characterized by an extremely high instantaneous deposition rate. Unlike for the thermal evaporation (TE) process, formation of fractal nanoclusters under PLD conditions has been observed with scanning tunneling microscopy (STM). The driving mechanism for this phenomenon occurring at high deposition rate is thought to be the evolution of the initial interacting-adatom states in a system far from thermodynamic equilibrium. The obtained results can be explained by proposing a new mechanism of condensed phase formation under the conditions of strong deviation from thermodynamic equilibrium.     

Diamond-like phase formation in an amorphous carbon films prepared by periodic pulsed laser deposition and laser irradiation method

Diamond-like carbon (DLC) films were fabricated by pulsed laser ablation of a liquid target. During deposition process the growing films were exited by a laser beam irradiation. The films were deposited onto the fused silica using 248 nm KrF eximer laser at room temperature and 10⁻³ mbar pressure. Film irradiation was carried out by the same KrF laser operating periodically between the deposition and excitation regimes. Deposited DLC films were characterized by Raman scattering spectroscopy. The results obtained suggested that laser irradiation intensity has noticeable influence on the structure and hybridization of carbon atoms deposited. For materials deposited at moderate irradiation intensities a very high and sharp peak appeared at 1332 cm⁻¹, characteristic of diamond crystals. At higher irradiation intensities the graphitization of the amorphous films was observed. Thus, at optimal energy density the individual sp³-hybridized carbon phase was deposited inside the amorphous carbon structure. Surface morphology for DLC has been analyzed using atomic force microscopy (AFM) indicating that more regular diamond cluster formation at optimal additional laser illumination conditions (∼20 mJ per impulse) is possible.     

Polyethylene welding by pulsed visible laser irradiation

Laser welding of plastics is a relatively new process that induces locally a fast polymer heating. For most applications, the process involves directing a pulsed beam of visible light at the weld joint by going through one of the two parts. This is commonly referred to as “through transmission visible laser welding”. In this technique, the monochromatic visible light source uses a power ns pulsed laser in order to irradiate the joint through one part and the light is absorbed in the vicinity of the other part.In order to evaluate the mechanical resistance of the welded joint, mass quadrupole spectrometry, surface profilometry, microscopy techniques and mechanical shear tests were employed. The welding effect was investigated as a function of the laser irradiation time, nature of the polyethylene materials and temperature.     

Enhancement of pulsed laser ablation assisted with continuous wave laser irradiation

Continuous wave(CW) laser irradiation is employed to enhance the pulsed laser ablation of silicon and stainless steel(316 L)samples. Different surface temperatures generated by the CW laser irradiation are set as the initial working circumstance for the pulsed laser ablation. The diameter and depth of laser-ablated craters are measured to study threshold fluence, pulse incubation coefficient and ablation rate under different surface temperatures. Numerical simulation employing Heat Transfer in Solid and Deformed Geometry Interface modules in COMSOL is performed to estimate ablation rate theoretically based on Hertz-Knudsen equation. The realized crater-related data are analyzed to further obtain their dependences on surface temperature. The parametric and morphological studies indicate that the weakened plasma shielding effect and thermal diffusion in the ablated region induced by the CW laser irradiation lead to the enhanced pulsed laser ablation significantly.     

Subwavelength ripple formation induced by tightly focused femtosecond laser radiation

Subwavelength ripples (<λ/4) are obtained by scanning a tightly focused beam (∼1 μm) of femtosecond laser radiation (λ = 800 nm, t_p = 100 fs) over the surface of either bulk fused silica and silicon and Er:BaTiO₃. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N = 1 to 20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. Potential applications are presented.     

Tungsten ions produced by infrared pulsed laser irradiation

Energetic ions have been obtained irradiating a tungsten target with a Q-switched Nd:Yag laser, 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and power density of the order of 10¹⁰?W/cm². The laser-target interaction induces a strong metal etching with production of plasma in front of the target. The plasma contains neutrals and ions with high charge state. Time-of-flight measurements are presented for qualitative analysis of the ion production. A cylindrical electrostatic ion analyzer permits measuring of the yield of emitted ions, the charge state of detected ions and the ion energy distribution. Measurements indicate that, at a laser fluence of the order of 100?J/cm², the charge state may reach 9+ and the ion energy reaches about 5?keV. The ion energy distribution is given as a function of the charge state. Experimental results indicate that an electrical field is developed along the normal to the plane of the target surface, which accelerates the ions up to high velocity. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the author has corrected for the Coulomb interactions occurring inside the plasma.     

脉冲激光引起铜膜镜面的环形损伤波纹研究

 用波长1.06μm、半高宽10ns的脉冲Nd:YAG激光辐照铜膜镜面，在激光辐照区，用光学显微镜观察到有规律的环形波纹状损伤图案，波纹平均周期约几十μm。通过对光路系统分析，认为样品前的小孔光阑对激光产生了菲涅尔衍射，使得在样品表面光强分布变成周期性环状分布。在极短的相互作用时间内，热扩散很小，损伤图案依赖于光强分布。并依据实验参数，用柯林斯公式对样品表面的光强分布进行了计算，所得光强分布的周期与损伤波纹的周期基本一致。     

Coloration of a metal surface under pulsed laser irradiation

The regimes of irradiation using nanosecond laser pulses for creation of color images on stainless-steel and titanium surfaces upon laser engraving are studied. Parameters of radiation that correspond to the spectrum of resulting colors on the sample surface are experimentally determined. The spectral analysis of the irradiated area is performed and probe microscopy is used to study the surface relief. Complicated surface relief that results from irradiation indicates the contribution of several optical effects responsible for the surface color under laser engraving.     

脉冲激光引起铜膜镜面的环形损伤波纹研究

用波长1.06μm、半高宽10ns的脉冲Nd:YAG激光辐照铜膜镜面,在激光辐照区,用光学显微镜观察到有规律的环形波纹状损伤图案,波纹平均周期约几十μm。通过对光路系统分析,认为样品前的小孔光阑对激光产生了菲涅尔衍射,使得在样品表面光强分布变成周期性环状分布。在极短的相互作用时间内,热扩散很小,损伤图案依赖于光强分布。并依据实验参数,用柯林斯公式对样品表面的光强分布进行了计算,所得光强分布的周期与损伤波纹的周期基本一致。     

Subwavelength ripple formation on planar and nonplanar surfaces by femtosecond laser scanning

The self-formation of periodic subwavelength ripples by linear polarized femtosecond laser scanning planar and non-planar tungsten targets on the employed laser wavelength, scanning speed, and energy fluence are examined systematically. The results show that, for a certain laser wavelength, the scanning conditions have no obvious effect to the morphological features of grating structures in the threshold range of laser fluence. The spatial structured period of gratings can be self-consistently interpreted by recently presented physical model of surface two-plasmon resonance. The subwavelength structures on cylindrical surface would be a good method to realize unique surface functions on complex surface of micro-devices.     

Numerical modeling and experimental investigation of TiC formation on titanium surface pre-coated by graphite under pulsed laser irradiation

A model for carbonization of titanium surface by pulsed Nd:YAG laser was developed. The Ti substrate was covered with a relatively thick graphite layer prior to be processed under the laser beam. The experiments were performed at 15 J pulse energy with various pulse durations and overlapping factor to validate the results obtained from the numerical calculations. The model results such as temperature gradient, surface temperature, and the cooling rate were correlated with the micro-hardness of the alloyed layer. Higher pulse durations and overlapping factors which lead to the heat input increasing will result in significant rising in the micro-hardness values. The hardness values of the processed layer partially containing TiC, increased up to 10 times of the Ti substrate.