8:30 AM *Y1.1
SESSION Y1: FUNDAMENTALS OF ABLATION
Chair: Rajiv K. Singh
Monday Morning, April 13, 1998
Golden Gate B1
SURFACE MODIFICATION AND ABLATION OF INSULATORS USING A TUNABLE, PICOSECOND MID-INFRARED LASER.* R.F. Haglund, Jr
, M.R. Papantonakis, H.K. Park and O. Yavas, Vanderbilt University, Nashville, TN.
Ultrashort-pulse lasers are increasingly being used for laser-induced surface modification of insulators, including ablation. Ti:sapphire chirped-pulse amplifier systems, with fundamental wavelengths in the near infrared, couple laser light into electronic material excitations, either by multiphoton absorption processes or by one-photon transitions on laser harmonics. The relaxation of electronic excitation into vibrational modes can produce efficient ablation and other desirable surface modifications with little collateral damage because the laser energy is deposited on a time scale much shorter than thermal diffusion times. Little is known, however, about how ultrashort pulses interact with insulators at wavelengths in the vibrational infrared. This paper describes surface modifications achieved by picosecond laser irradiation in the 2-10 m range. The laser source was a tunable, free-electron laser (FEL) with 1-ps micropulses spaced 350 ps apart in a macropulse lasting up to 5 s, with an average power of up to 3 W. This unusual pulse structure makes possible novel tests of the influences vs fluence and intensity, as well as the effects of resonant vibrational excitation. As model materials systems, we studied calcium carbonate, its isoelectronic cousin sodium nitrate, and fused silica. Particularly intriguing are surface modifications achieved by tuning the laser into vibrational resonances and overtones of the target materials and tailoring the energy content of the pulse. We observe quasi-blackbody plume emission near the laser ablation threshold, reproducible micron-size surface dents or protrusions, laser-induced transitions to polymorphic phases, and even the creation of a flexible glass wool with sub-100-nm-diameter fibers, depending on irradiation parameters. Ablated ion and neutral species show substantial wavelength dependences. The mechanisms underlying these effects, and their implications for materials-modification strategies, are discussed.
9:00 AM Y1.2
THE ORIGIN OF ENERGETIC IONS AND ELECTRONS FROM LASER IRRADIATION OF WIDE BANDGAP INSULATORS. Tom Dickinson
, David Ermer, and Steve Langford, Washington State University, Department of Physics and Materials Science Program, Pullman, WA.
A critical aspect of thin film growth by pulsed laser deposition methods with nanosecond pulses is the mixture of atomic and molecular neutral species accompanied by high densities of energetic electrons and ions. These species are incident upon substrates with varied kinetic energy as well as temporal and density distributions and both the formation of metastable structures as well as the overcoming of numerous activation barriers are facilitated. The origin of energetic particles from laser irradiated dielectrics is not well understood. Here we examine plasma generation from laser interactions with surfaces under conditions where inverse bremsstrahlung (often proposed to explain plasma formation) is not possible due to insufficient photon and electron densities. We present new measurements addressing the interaction between the emitted particles in the near surface region of ionic crystals. These emissions include photoelectrons, energetic positive ions, and neutral atoms. We first establish experimentally, in vacuum, that there is overlap in space and time of portions of the distributions in the near surface region. We then present a new model for the collected motion of these particles including experimental proof of negative velocities (particles returning to the target). We show that as laser fluence is increased, sufficient densities, overlap, and kinetic energies are available to explain the onset of plume formation. A simulation of the kinetic consequences from electrostatic interactions between these particles provides a reasonable explanation of the origin of high energy ions generated by laser irradiation of wide bandgap insulators. These studies aid our understanding and possible control of energetic and reactive species desired for efficient and directed thin film growth.
9:15 AM Y1.3
CLUSTER ION EMISSION IN THE INTERACTION OF SLOW HIGHLY CHARGED IONS WITH SURFACES. Alex V. Hamza
, Thomas Schenkel, Alan V. Barnes, and Dieter Schneider, University of California, Lawrence Livermore National Laboratory, Livermore, CA.
Highly charged ions (charge state >30+) at low kinetic energy deposit several hundred keV of electronic energy into a nanometer-sized surface volume within femtoseconds. The deposited power (J/cm3/s), because of the small volume and short time, in a HCI interaction is comparable to the most powerful short-pulse lasers. Cluster ion emission induced by slow highly charged ion impact from a variety of surfaces is investigated by time-of-flight secondary ion mass spectrometry. The yield of cluster ions as a function of cluster size for SiO2, CaF2, UO2 and C84 surface follow a power law decline with exponent approaching the -2 limit of the equilibrium and shock wave cluster emission models. While the decline of the cluster ion emission with cluster size is an exponential decay for highly oriented pyrolytic graphite upon Th70+ impact, the decline is more gradual than for Cs1+ impact, such that at C16 the relative cluster yield is 1000 times higher. The effects of ion charge and ion energy are also investigated for UO2 cluster yields.
9:30 AM Y1.4
DOES THE SUBSURFACE SUPERHEATING EFFECT REALLY EXIST? Valentin Craciun
, Doina Craciun, Laser Dept, NILPRP, Bucharest, ROMANIA.
The subsurface superheating effect (SSSH), firstly proposed more than two decades ago, was suggested to be one of the major cause of droplet emission during pulsed laser deposition (PLD). Recently, it has been shown that for Al samples irradiated by an excimer laser, the SSSH effect is a mathematical artefact which appeared because an incorrect boundary condition was used. It has been further suggested that the SSSH effect should be replaced with the concept of phase explosion. However, this cannot explain either the huge increase of the droplet density onto the surface of PLD grown films generally observed whenever a longer wavelength laser radiation, which is usually poorly absorbed by the target, was used, or the presence of circularly shaped cavities, most likely formed by subsurface boiling, observed by scanning electron microscopy investigations of various targets after the ablation process. It is shown here that such round-shaped cavities always appeared when the laser radiation was poorly absorbed by the target, even for monocrystalline Si or Ge targets ablated by the 1064 nm radiation of a Nd:YAG laser at moderate fluences. Therefore, there is a clear connection between the high droplet density on the grown films surface, cavity formation and volume absorption of the laser radiation, which could be easily explained taking into account the subsurface boiling predicted by the SSSH effect. Numerical estimation of the temperature depth-profiles inside various target materials during laser irradiation, obtained by emplying the correct boundary conditions, which support the occurrence of a SSSH effect, though not as high as initially estimated, are also presented.
9:45 AM Y1.5
MESOSCOPIC MODEL FOR THE LASER-TARGET INTERACTION. R.Mendes Ribeiro
, Marta M.D. Ramos, Departamento FIsica, Universidade do Minho, Brago, PORTUGAL; A.M. Stoneham, Physics Department, University College London, UNITED KINGDOM.
Mesoscopic modelling is an adequate technique for the study of complex multiscale phenomena like Pulsed Laser Deposition (PLD). In fact, PLD involves both atomic processes in the absorption of the radiation, evaporation and ionization, and microscopic and macroscopic phenomena such as the desorption of large aggregates in the submicron range and the dynamic flow of a plasma in vacuum. Both atomistic and thermodynamical models have been developed for several years, but only a partial picture is possible in these frameworks. We developed a mesoscopic model which enables the understanding of several features that happen at the surface of a transparent target and which play an important role in the evolution of the evaporation process. The model predicts the generation of high intensity electric fields in places with high defect concentration, such as grain boundaries. The dependence of the density of generated electrons, the evaporated species and energy on the material properties is included.
10:30 AM Y2.1
SESSION Y2: FUNDAMENTALS OF PULSED, HYPERTHERMAL GROWTH
Chair: Douglas H. Lowndes
Monday Morning, April 13, 1998
Golden Gate B1
EVOLUTION OF SURFACE MORPHOLOGY IN EPITAXIAL Si GROWTH BY PULSED LASER DEPOSITION AND MOLECULAR BEAM EPITAXY. M.E. Taylor
and Harry A. Atwater, California Institute of Technology, Pasadena, CA.
Evolution of surface morphology in epitaxial Si growth by pulsed laser deposition and molecular beam epitaxy was studied by simulation and experiment to gain insight into kinetic mechanisms that affect epitaxy. The simulation is a solid-on-solid, rate-equation, Monte Carlo code. Modeled energetic effects include transient enhanced surface diffusion and cluster breakup. Simulated film surface morphology was evaluated using surface images, surface height standard deviations, and height-height correlation functions. The results indicate that in comparison to molecular beam epitaxy, pulsed laser deposition produces smoother films at low substrate temperatures and rougher films at high substrate temperatures. The experiments will utilize Ge marker layers and cross-sectional transmission electron microscopy to correlate surface morphology with deposition method, time-averaged deposition rate, deposition coverage per pulse, and substrate temperature. Experimental results will be critically compared with simulation results.
10:45 AM Y2.2
GROWTH MODE TRANSITION IN PULSED LASER DEPOSITED COMPLEX OXIDE FILMS. B. Dam
, J. Rector, J. Huijbregtse, R. Griessen, Vrije Universiteit. Amsterdam, NETHERLANDS.
In pulsed laser deposited YBa2
(YBCO) films one finds a preference for repeated 2D-nucleation and growth instead of the spiral growth mechanism found on the same films when grown by sputtering. We relate this effect to the pulsed nature of the ablation process and the high surface diffusivity of the impinging adatoms. As the step distance in spiral growth is related to the supersaturation of adatoms, a pulsed deposition process can only result in a stable growth spiral, if the surface diffusion is slow enough to maintain a certain adatom density between two laser shots. In YBCO we can reproducibly induce the transition to spiral growth by increasing the substrate target distance or the background pressure. From the step distances as observed by Scanning Probe Microscopy, we estimate that the change in growth conditions involves a 2 orders of magnitude change in surface diffusivity. We attribute this to a difference in kinetic energy of the impinging adatoms. According to the literature, the kinetic energy of the plume changes around 0.5 eV when changing the conditions as we did.Upon impingement most of this energy will be lost to the lattice. However, the observed change in surface diffusivity requires only a change in temperature of about 0.02 eV. Thus, even if only a few percent of the kinetic energy is maintained by the adatoms upon impingement, a change in growth conditions may result in a large change in diffusivity. Thus, our surface morphological study re-establishes the old notion that growth by pulsed laser deposition benefits from the impingement energy of the adatoms.
11:00 AM Y2.3
GROWTH OF SINGLE PHASE PrBa2-x
THIN FILMS BY PULSED LASER DEPOSITION AND THE ROLE OF LATTICE STRAIN ON THE TRANSPORT PROPERTIES. Y G. Zhao
, Z.W. Dong, M. Rajeswari, R.P. Sharma, T. Venkatesan, Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, MD.
The mechanism for the absence of superconductivity in PrBa2
(PBCO) remains fully unresolved. In high Tc
devices where PBCO has been widely used as the barrier layer, a significant non-reproducibility of the device property has been encountered. In a previous paper, we have succeeded in decreasing the hybridization between the Pr 4f and O 2p states in the CuO2
plane by Sr doping at the Ba sites in PBCO bulk samples. The solid solubility limit of Sr in PBCO was found to be about 0.8, which hindered further doping. Here we report epitaxial growth of single phase PrBa2-x
(x=1.3, 1.6) thin films with the Sr dopant concentration exceeding the solid solubility limit. The localization of the carriers is greatly reduced and the resistivity (4.2 K)/(300 K) 1.5 for the PrBa0.4
, in contrast to (4.2 K)/rho
(300 K) 107
for PBCO. Moreover the resistivity of x=1.3 thin films grown on SrTiO3
is even higher than that of PBCO, while films grown on LaAlO3
show lower resistivity than that of PBCO. This dramatic difference can be explained by the effect of lattice strain on the transport properties, i.e. films grown on SrTiO3
is subject to a tensile stress in the plane of the film and a compressive stress in the c direction, perpendicular to the substrate. The reverse process occurs for films on LaAlO3
. In the former case, the localization of the carriers is enhanced due to the increase of the Pr 4f and O 2p hybridization, and the situation is the reverse in the latter case. Such strain effects may shed light on the origin of the non-reproducibility of the devices. This work shows the potential for PLD to grow thin films which cannot be obtained in single phase form by the conventional solid state reaction method.
11:15 AM Y2.4
USE OF PULSED-LASER DEPOSITION FOR THE STUDY OF THIN-FILM SOLID-STATE REACTIONS. Matthew T. Johnson
and C. Barry Carter, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN.
Pulsed-laser deposition (PLD) has been used to grow several different oxide thin films on bulk single-crystal substrates of Al2
, MgO and other oxides in order to study fundamental processes occurring in ceramic solid-state reactions. The reaction between a thin film and a bulk substrate not only gives new insight into the early stages of such solid-state reactions but also gives new information on the kinetics of such processes. The reactants have a known controlled composition and the heterophase boundary can be forced to be planar and have a reproducible structure. Scanning and transmission electron microscopy have been used to characterize both the as-deposited thin-films and the reacted multilayers in cross-section and plan-view. This paper will illustrate the results of these studies showing the quality of films, the defect structure in the reactants, the morphology of the moving interfaces and the kinetics.
11:30 AM Y2.5
STUDIES OF IMPURITY AND SUBSTRATE DIFFUSION IN EPITAXIAL n-TYPE CdSe FILMS GROWN ON GaAs (001) BY PULSED LASER ABLATION. Jaewon Park
, Christopher M. Rouleau, and Douglas H. Lowndes, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN.
n-type CdSe films with thickness of 470 - 630 nm were grown on (001) and 2º-miscut GaAs wafers by ArF (193 nm) pulsed laser ablation of stoichiometric CdSe targets at temperatures of 250 - 425ºC in vacuum and ambient Ar gas. The substrates were attached to the heater with indium or colloidal silver paint and oriented either perpendicular or parallel to the ablation plume. Auger electron spectroscopy (AES) combined with an ion sputter gun, as well as energy dispersive x-ray fluorescent spectroscopy (EDS) were employed to investigate the CdSe film stoichiometry, In or Ag diffusion into the CdSe film, and film-substrate interdiffusion. Indium was found in films deposited at temperatures 350ºC, while little or no In was detected in films deposited at 250ºC. No silver was detected in any film. AES depth profiling showed that the In concentration was higher on the surface than inside of the CdSe films. The Cd concentration decreased with increasing the In concentration, implying that Cd substitutes for In. For films deposited at temperatures 350ºC, extensive interdiffusion with the GaAs substrate took place over distances >100 nm from the film-subsrate interface. In the absence of indium diffusion the CdSe films were nearly stoichiometric. Variation of the ambient Ar gas pressure and tilting the substrate with respect to the ablation plume were observed to slightly affect the CdSe film stoichiometry.
11:45 AM Y2.6
LASER ABLATION OF MOLYBDENUM DISULPHIDE AND THIN FILMS FORMATION. Vitali Prokopenko
, Igor Melnichenko, Gomel State University, Advanced Materials Research Laboratory, Gomel, BELARUS; Viacheslav Fominski, Moscow Engineering Physics Institute, Dept of Solid State Physics, Moscow, RUSSIA.
The influence of parameters deposition (characteristics of laser plasma, gas pressure, energy spectrum of particles etc.) on structure, composition and tribological properties of molybdenum disulphide films formed by pulsed laser deposition with various laser fluences was investigated. The films were deposited at various substrate temperature under vacuum conditions and in Ar atmosphere at a gas pressure varying from 0.5 to 5 Pa. The films thickness were of 100 to 2000 nm. The laser ablation of molybdenum disulphide target under vacuum conditions with the laser fluence at the threshold of plasma formation allowed to conserve the target stoichiometry in the films. However, according to XPS analysis data, such films contained many defects of chemical nature and exhibited unsatisfactory tribological properties. The deposition at higher laser fluences, exceeding considerably the plasma formation threshold, resulted in a substantial sulphur depletion and, under certain conditions, crystallisation of the films. These microcrystalline films exhibited a lower friction coefficient than amorphous ones. The introduction of buffer (Ar) gas during the laser ablation results to the change of the composition and microstructure of the molybdenum sulphide films. The variation of the laser fluence and the buffer gas pressure provides possibly to deposit the films with various sulphur content, including exceeding that of the target and with tribological properties superior to those of the films deposited under vacuum conditions. These films have amorphous structure and chemistry inherent to perfect molybdenum disulphide material. Process formation of the films taking into account of condensed particles energy are discussed.
SESSION Y3: IN SITU DIAGNOSTICS AND NANOSCALE SYNTHESIS
Chairs: Eric Fogarassy and Jagdish Narayan
Monday Afternoon, April 13, 1998
Golden Gate B1
1:30 PM *Y3.1
NANOCLUSTERS SYNTHESIS BY CONVENTIONAL PULSED LASER ABLATION, THEORETICAL AND EXPERIMENTAL APPROACH. W. Marine, Université de la Méditerranée, GPEC, UMR CNRS, Marseill, FRANCE; B. Luk'yanchuk, General Physics Institure, Moscow, RUSSIA.
We review the basic principles of the nanoparticle condensation within the expanding vapor plume induced by UV nanosecond laser ablation of solid silicon. In the experiment, we use a conventional laser ablation technique. Ablation and deposition were performed by ArF laser ( = 193 mm, pulse duration t = 15 ns FWHM) in different background gases (He, Ar, H2 or their mixtures), at typical gas pressure 0.01-5 Torr. Optical emission, optical time of flight (TOF) spectroscopy, laser induced fluorescence and TOF mass spectrometry were used to analyse space-time resolved expansion dynamics of silicon atoms and silicon nanoclusters (Sin, where n > 20). Our theoretical treatment, based on the generalization of the Raizer's theory of condensation, is done for inhomogeneous plume where the rates of nucleation and condensation time are dependent on space coordinates. For the clusters moving together with vapor, we show three different waves propagation through the plume: 1) saturation wave, 2) supercooling wave, and 3) quenching wave. The resulting cluster size distribution depends on the inhomegeneity of the plume, where the largest clusters are formed within the center of the plume, and the smallest ones are formed near the plume edge.
2:00 PM *Y3.2
DYNAMICS OF NANOPARTICLE SYNTHESIS BY LASER ABLATION. David B. Geohegan, Alexander A. Puretzky, Gerd Duscher, and Stephen J. Pennycook, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN.
By laser-vaporizing and reassembling solids in background gases, novel photoluminescent materials can be synthesized as nanometer-sized particles where quantum size effects become important. Laser ablation is a proven technique to deposit thin films of these nanomaterials, however the plume dynamics leading to condensation in laser ablation plumes has remained a mystery. Here the dynamics of nanoparticle formation, transport, and aggregation are revealed through gated-ICCD photography of Rayleigh scattering and in situ photoluminescence from gas-suspended nanoparticles. These diagnostics were used for the first time to measure and maximize photoluminescence (PL) from nanoparticles suspended in the gas phase, prior to deposition. The dynamics of nanoparticle synthesis depend on background gas mass, pressure, and flow, and the choice of laser energy density, wavelength, and target-substrate geometry. Condensation of viable nanoparticles is governed regionally in the confined laser plasma by the temperature and the degree of saturation. PL induced by incorporation of gas-phase dopants (carried by the background gas) can be observed and optimized with these diagnostics, and compared with the efficiency of dopants incorporated into the ablation target. These effects will be demonstrated for oxidized silicon nanoparticles and doped refractory oxide materials. For silicon ablation into (1ñ10 Torr) helium and argon gas, three broad PL bands (1.8, 2.5, and 3.2 eV) with regions of pronounced structure were measured. Individual nanoparticles were swept from the gas suspension and imaged by Z-contrast TEM. The particles were spherical and 1ñ10 nm in diameter. Electron energy loss spectroscopy (EELS) of individual nanoparticles collected after bright PL in the plume indicated a silicon-rich silicon-oxide (SiO1.4) stoichiometry. EELS linescans indicate that the 5-nm particles have no observable c-Si core, but contain homogeneously-distributed regions of pure silicon and silicon oxide. As-deposited films of these particles exhibited extremely weak photoluminescence. However, following annealing and passivation, the loosely-connected, weblike films exhibited strong blue-green and violet photoluminescence bands similar to those observed in the gas phase.
2:30 PM Y3.3
EMISSION SPECTRA OF PULSED-LASER-INDUCED PLASMA FROM ALUMINUM SURFACE. Y.F. Lu, Z.B. Tao, D.S.H. Chan, National Univ of Singapore, Dept of Electrical Engineering; M.H. Hong, T.S. Low, Data Storage Institute, SINGAPORE.
Emission spectra of aluminum plasma induced by pulsed Nd:YAG laser were investigated by an Optical Multichannel Analyzer (OMA). Spectroscopic study shows that aluminum plasma spectra become relatively abundant with increasing incident laser fluence. It may be due to the fact that the populations of excited aluminum atoms and ions at high energy levels increase with the laser fluence, which results in the increase of spontaneous transmission possibilities and causes the abundance of aluminum spectra. Al, Al+, Al++ spectral lines are observed successively with high laser fluence and the new spectral lines appear mainly in the short wavelength region. It is probably because one or more bound electrons are removed successively with the increasing laser fluence. Therefore, the mean charge per ion will increase. This is associated with a higher excitation energy of the atoms and ions. The spectral lines will then appear in short wavelength region. The threshold fluences for the appearance of excited Al, Al+, Al++ spectral lines as a function of energy density are measured. The spectral lines of several species are used to evaluate the plasma temperature and density for different surrounding gases and pressures. Electron temperature is measured by the intensities of the spectral lines whereas the electron density is estimated from the Stark broadening profiles of the spectral lines. The influence of gas and pressure on plasma characteristics is analyzed. Plasma temperature and density as a function of laser fluence and the time after the laser pulse are also investigated.
2:45 PM Y3.4
IN-SITU MONITORING USING TIME RESOLVED RHEED AT HIGH DEPOSITION PRESSURES. Guus J.H.M. Rijnders, Dave H.A. Blank, Gertjan Koster, and Horst Rogalla, University of Twente, Dept. of Applied Physics, Enschede, NETHERLANDS.
Time resolved in-situ RHEED diagnostic has been used to study the hetero-epitaxial growth of different materials, like REBaCuO and ACuOx, with A=Sr, Ba, or Ca. Clear oscillations of the diffracted intensity have been observed, indicating a layer by layer growth. This intensity is modulated by the pulsed nature of the deposition. Just after the laser pulse, a sharp decrease of the intensity is followed by an exponential increase. Both, amplitude and time constant of the modulation, give information about the diffusion and nucleation of the deposited material. The amplitude and relaxation time in the case of layer by layer growth (REBaCuO) are depending on the surface roughness during completion of one mono-layer. In the case of step-flow growth mode of SrCuOx, the amplitude is significantly larger and can be 30% of the actual intensity. Typical time constants are of the order of 0.5 sec. In this contribution we will discuss these time resolved RHEED experiments.
3:00 PM Y3.5
DIELECTRIC FUNCTION MODIFICATION DURING THE DEPOSITION SiOxNy BY LASER ABLATION. R. Machorro, G. Soto*, E.C. Sámano and L. Cota-Araiza, Instituto de Fisica, UNAM, Laboratorio de Ensenada, Ensenada, México; *Also at Centro de Investigación Cientifica y de Educación Superior de Ensenada, Programa de Postgrama de Posgrado en Fisica de Materiales, Ensenada, México..
Deposition of SiOxNy by laser ablation, controlling the background gas pressure, provides a powerful way to manipulate the properties of the material. A smooth and continous variation of the gas pressure might result in changes of the electrical, mechanical and optical properties of the film, which can be applied into several areas of physics and technology. For instance, inhomogeneous films are very useful in optical interference filters, providing an unlimited number of non-traditional applications. In this paper, we illustrate the effect of refractive index changes due to variations during the deposition of SiOxNy layers. A very accurate method to control the deposition is real time ellipsometry. In this work, silicon nitride films were deposited on glass and silicon substrates at room temperature by KrF (248 nm) excimer laser ablation of a Si3N4 hot pressed target in an ultra high vacuum system.