Preventing kinetic roughening in physical vapor-phase-deposited films

The growth kinetics of the mostly used physical vapor-phase deposition techniques -molecular beam epitaxy, sputtering, flash evaporation, and pulsed laser deposition-is investigated by rate equations with the aim of testing their suitability for the preparation of ultraflat ultrathin films. The techniques are studied in regard to the roughness and morphology during early stages of growth. We demonstrate that pulsed laser deposition is the best technique for preparing the flattest films due to two key features [use of (i) a supersaturated pulsed flux of (ii) hyperthermal species] that promote a kinetically limited Ostwald ripening mechanism.     

Laser processing for deposition of high Tc superconducting films

Model considerations are applied to investigate the process of pulsed laser deposition of high T_c superconducting films. At low power densities, heat conduction and evaporation above a thermal threshold energy dominate. Increase of the laser flux results in the generation of a dense plasma with a mass ablation flow away from the target. According to different wave lengths the relation of the energy density to the pulse duration is estimated. Very short light pulses above a threshold energy density favour the ablation of a stoichiometric mass flow from a multicomponent target and suppress evaporation according to different vapor pressures.     

Production of two-dimensional periodical structures by laser interference irradiation on bi-layered metallic thin films

The production of one- and two-dimensional periodical structures produced by Laser Interference Metallurgy (LIMET) on bi-layered metallic films by using nanosecond pulsed laser is reported. The systems investigated are Fe/Al and Cu/Al and represent model systems where the melting point of the upper layer is higher tan the lower one. Aspect ratio, height and width of the structures are discussed as a function of laser fluence, partial energies and arrangement of the laser beams. Cross sectional analysis of the structured samples demonstrates the flux of molten metal parallel to the thermal gradient generated by the laser heating.     

Films of Mn12-acetate by pulsed laser evaporation

Films of the molecular nanomagnet, Mn₁₂-acetate, have been deposited using pulsed laser deposition and its novel variant, matrix assisted pulsed laser evaporation. The films have been characterized by X-ray photoelectron spectroscopy, mass spectrometry and magnetic hysteresis. The results indicate that an increase in laser energy and/or pulse frequency leads to fragmentation of Mn₁₂-acetate, whereas its chemical and magnetic integrity is preserved at low laser energy (200 mJ). This technique allows for the fabrication of patterned thin films of molecular nanomagnets for fundamental and applied experiments.     

Applications of the matrix-assisted pulsed laser evaporation method for the deposition of organic,biological and nanoparticle thin films: a review

The matrix-assisted pulsed laser evaporation (MAPLE) technique offers an efficient mechanism to transfer soft materials from the condensed to the vapor phase, preserving the versatility, ease of use and high deposition rates of the pulsed laser deposition (PLD) technique. The materials of interest (polymers, biological cells, proteins, …) are diluted in a volatile solvent. Then the solution is frozen and irradiated with a pulsed laser beam. Here, important results of MAPLE deposition of polymer, biomaterials and nanoparticle films are summarized. Finally, the MAPLE mechanism is discussed. A review of experimental and theoretical works points out that the simple model of individual molecule evaporation must be abandoned. Solute concentration, solubility, evaporation temperature of solvents, laser pulse power density and laser penetration depth emerge as important parameters to explain the morphology of the MAPLE-deposited films.     

A point source analytical model of inverse pulsed laser deposition

A simple analytical model for inverse pulsed laser deposition is proposed. In the model the motion of the evaporated material is assumed to emerge as from a point source located above the surface of evaporation at some distance. The obtained thickness profiles of inverse deposited films agree well with those calculated by the test particle Monte Carlo method. The proposed approach has been applied for analysis of experimental data on inverse pulsed laser deposition of graphite in nitrogen atmosphere with nanosecond pulses of laser fluences between 1 and 7 J/cm². The model describes well the thickness profiles and pressure dependence of film growth rate for inverse deposition.     

激光照射下铝靶表面汽化压力的测量

 本文分析了会聚自由振荡钕玻璃脉冲激光束辐照平面铝靶的汽化特性。在激光功率密度约10 MW/cm²下，由拍摄激光与靶相互作用产生射流发光的时间积分照片测得靶面的汽化压力约3.6 GPa。把时间积分照片与高速分幅照片比较，证明在激光与靶相互作用一段时间后确实存在着马赫盘。     

Vapor deposition of intact polyethylene glycol thin films

Thin films of polyethylene glycol (PEG) of average molecular weight, 1400 amu, were deposited by both matrix-assisted pulsed laser evaporation (MAPLE) and pulsed laser deposition (PLD). The deposition was carried out in vacuum (∼10^-6 Torr) with an ArF (λ=193 nm) laser at a fluence between 150 and 300 mJ/cm². Films were deposited on NaCl plates, Si(111) wafers, and glass slides. The physiochemical properties of the films are compared via Fourier transform infrared spectroscopy (FTIR), electrospray ionization (ESI) mass spectrometry, and matrix-assisted laser desorption and ionization (MALDI) time-of-flight mass spectrometry. The results show that the MAPLE films nearly identically resemble the starting material, whereas the PLD films do not. These results are discussed within the context of biomedical applications such as drug delivery coatings and in vivo applications where there is a need for transfer of polymeric coatings of PEG without significant chemical modification. Received: 2 March 2001 / Accepted: 5 March 2001 / Published online: 23 May 2001     

FEM simulation of the laser plasma interaction during laser nitriding of titanium

Laser nitriding of materials is based on the interaction of short pulsed laser radiation with the treated material and the hitherto formed laser plasma. The process is very promising for the fast formation of surface coatings with superior properties. Due to the short interaction times and the thin surface films an experimental observation of the underlying processes is very difficult. In order to access the basic mechanism, finite element method simulations of laser heating, evaporation, plasma formation and expansion, plasma composition and interaction with the materials surface have been performed. As a result, evaporation and expansion velocities, pressure balances and dissociation degrees have been derived. The results give a better insight into the physical processes and dependencies of the coating formation, in this case for the titanium-nitrogen system. This finally allows an optimization of the coating synthesis.     

Analysis of the laser ablation processes for thin-film silicon solar cells

A detailed analysis of the monolithical series connection of thin-film silicon modules with ZnO/Ag back contact is presented. In this study, pulsed lasers with wavelengths of 1064 nm and 532 nm were used. The influence of various laser parameters like laser power, pulse overlap, etc., on the different patterning steps is discussed. The focus of this study was on the back contact patterning process. Here (i) the flake formation process during the ablation and (ii) the influence of a NIR-laser source as an alternative approach to the green laser were investigated in detail. The latter would reduce system costs if only one NIR-laser source could be used for all patterning steps.     

Thermal model of pulsed laser ablation: back flux contribution

This paper proposes improvement of the boundary conditions for the thermal model of laser-induced solid heating and ablation. The refinement of the model takes into account the back flux from the vapor cloud to the evaporation surface. For a wide range of evaporation rates, the value of the back flux was determined by direct Monte Carlo simulation of vapor cloud expansion. The obtained back flux values are substituted to the thermal model. The improved model has been shown to be in good agreement with the experimental results on graphite as an example. PACS 52.38.Mf; 79.20.Ds; 81.15.Fg     

高功率二极管激光器相变冷却技术

根据高功率二极管激光器的散热需求,设计了一种储能式相变冷却实验系统,并开展了喷雾相变冷却器和微通道相变冷却器的设计。采用多孔微结构的换热表面,用氨做制冷剂,实现了喷雾相变冷却器表面温度37 ℃时,散热功率密度达到了511 W/cm2。采用节流汽化原理,分别设计了背冷式相变微通道冷却器和薄片型的模块式相变微通道冷却器,背冷式相变微通道冷却器采用氨做制冷剂, 散热功率密度达到了550 W/cm2,采用R124做制冷剂,散热功率密度约270 W/cm2。采用R124做制冷剂,实现了脉冲激光功率3 kW和连续激光功率100 W的相变冷却二极管激光器模块封装。     

Thermal model for nanosecond laser sputtering at high fluences

The vaporization effect and the following plasma shielding generated by high-power nanosecond pulsed laser ablation are studied in detail based on the heat flux equation. As an example of Si target, we obtain the time evolution of the calculated surface temperature, ablation rate and ablation depth by solving the heat flow equations using a finite difference method. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the variation of ablation depth per pulse with laser fluence is performed. Our numerical results are more agreed with the experiment datum than other simulated results. The result shows that the plasma shielding is very important.     

Properties of conductive polymer films deposited by infrared laser ablation

Thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) were deposited by resonant infrared laser vapor deposition (RIR-LVD). The PEDOT:PSS was frozen in various matrix solutions and deposited using a tunable, mid-infrared free-electron laser (FEL). The films so produced exhibited morphologies and conductivities that were highly dependent on the solvent matrix and laser irradiation wavelength used. When deposited from a native solution (1.3% by weight in water), as in matrix-assisted pulsed laser evaporation (MAPLE), films were rough and electrically insulating. When the matrix included other organic “co-matrices” that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good electrical conductivity (0.2 S/cm), but only when irradiated at certain wavelengths. These results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process.     

Functional polyethylene glycol derivatives nanostructured thin films synthesized by matrix-assisted pulsed laser evaporation

We report the thin film deposition by matrix-assisted pulsed laser evaporation (MAPLE) of a polymer conjugate with an hydrophilic sequence between metronidazole molecules that was covalently attached to both oligomer ends of carboxylate poly(ethylene glycol) (PEG 1.5-metronidazole). A pulsed KrF* excimer laser was used to deposit the drug-polymer composite films. Fourier transform infrared spectroscopy was used to demonstrate that MAPLE-transferred materials exhibited chemical properties similar to the starting materials. The dependence of the surface morphology on incident laser fluence is given.     

Laser synthesis of chiral molecules in isotropic racemic media

The problem of directional laser synthesis of enantiomers in an isotropic racemic mixture of chiral molecules is analyzed taking into account rotational degrees of freedom of molecules. It is shown that the chirality of the polarization structure of a laser field is the most general necessary condition for the laser distillation of an isotropic non-racemic mixture of chiral molecules with the isotropic distribution over the Eulerian angles. In the approximation of the electric-dipole interaction, the required field configuration can be provided due to noncoplanarity of the polarization vectors of laser-pump components. A relevant experimental scheme is proposed and calculated for the transformation of an isotropic racemic mixture with the help of a three-component pulsed laser field. It is shown that the possibility of the laser control of chirality in an isotropic medium corresponds to nonzero information on coupling between the input and output in the laser field-chiral molecular state information channel.     

Explosive boiling of water induced by the pulsed HF-laser radiation

The surface evaporation and explosive boiling of water induced by the radiation of a nonchain pulsed HF laser are studied using piezoelectric acoustic pressure transducers. The evolution of pressure signals is studied and the relative contributions of thermal (photoacoustic) and evaporation mechanisms to these signals are determined for a wide range of the laser energy densities. A threshold of bulk explosive boiling with respect to laser pulse energy density (W ₀ = 0.23 J/cm²) is determined.     

Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼10⁴) at laser fluence of 1–2 J/cm² below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10⁵ Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti. Received: 12 February 2000 / Accepted: 28 March 2000 / Published online: 20 June 2001     

Optical and electrical properties of PbTe films grown by laser induced evaporation of pressed PbTe pellets

A simple and cheap method has been developed for the deposition of lead telluride thin films on glass substrates by pulsed Nd:YAG laser evaporation of lead telluride pellets made of high purity Pb and Te powders.Preliminary characterization of the crystallographic and optical properties of the films has been performed as a function of the substrate temperature.The influence of deposition conditions on the sheet resistance of these thin films has been studied. Both deposition temperature, nitrogen pressure during deposition, and addition of Ga and As impurities in the source pellets have been considered.     

Bi0.8Ba0.2FeO3/La0.7Sr0.3MnO3异质结制备及其交换偏置效应研究

采用脉冲激光沉积方法, 通过调节激光能量、激光频率、衬底温度、氧压、靶基距等工艺参数, 在(100)取向的铝酸镧单晶衬底上制备出Bi_0.8Ba_0.2FeO₃/La_0.7Sr_0.3MnO₃多铁性异质结. X射线衍射图谱表明薄膜呈钙钛矿结构, 高分辨透射电镜图谱和能量色散X射线图谱表明两相界面清晰且具有良好的匹配度, 异质结呈(00l)取向性生长. 加场冷却条件下不同温度的磁滞回线(M-H)测量结果表明样品具有明显的交换偏置效应, 交换偏置场(H_EB)随温度的线性变化可能与异质结界面处电子轨道的重构和界面处自旋、轨道自由度之间的复杂的相互作用有关.