Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Single-wall carbon nanotubes are formed by Nd:YAG laser vaporization of a graphite/(1 at. % Ni, 1 at. % Co) target into flowing argon (500 Torr) within a quartz tube furnace (1000 °C). Here, this process is investigated for the first time with time-resolved laser-induced luminescence imaging and spectroscopy of Co atoms, C2 and C3 molecules, and clusters. These measurements under actual synthesis conditions show that the plume of vaporized material is segregated and confined within a vortex ring which maintains a ˜1 cm3 volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 μs) and cobalt (2 ms). This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs during several seconds of time from a feedstock of mixed nanoparticles in the gas-suspended plume. Using these in situ diagnostics to adjust the time spent by the plume within the high-temperature zone, single-walled nanotubes of controlled length were grown at an estimated rate of 0.2 μm/s.

The dynamics of gas phase nanoparticle formation by pulsed laser ablation into background gases are revealed by imaging photoluminescence and Rayleigh-scattered light from gas-suspended SiOx nanoparticles following ablation of c-Si targets into 1-10 Torr He and Ar. Two sets of dynamic phenomena are presented for times up to 15 s after KrF-laser ablation. Ablation of Si into heavier Ar results in a uniform, stationary plume of nanoparticles while Si ablation into lighter He results in a turbulent ring of particles which propagates forward at 10 m/s. The effects of gas flow on nanoparticle formation, photoluminescence, and collection are described. The first in situ time-resolved photoluminescence spectra from 1-10 nm diameter silicon particles were measured as the nanoparticles were formed and transported. Three spectral bands (1.8, 2.5 and 3.2 eV) similar to photoluminescence from oxidized porous silicon were measured, but with a pronounced vibronic structure. The size and composition of individual gas-condensed nanoparticles were determined by scanning transmission electron microscopy and correlated with the gas-phase photoluminescence. Weblike-aggregate nanoparticle films were collected at room temperature and 77K on c-Si substrates. After standard passivation anneals, the films exhibited strong room temperature photo-luminescence consisting of 3 spectral bands in agreement with the gas-phase measurements, however lacking the vibronic structure. These techniques demonstrate new ways to study and optimize the luminescence of novel optoelectronic nanomaterials during synthesis in the gas phase, prior to deposition.

Spectroscopic ellipsometry is used to characterize amorphous diamond, also known as tetrahedral amorphous carbon, (ta-C) films grown by pulsed laser ablation. The ellipsometry data is collected with the two-modulator generalized ellipsometer, which measures all three parameters required to characterize isotropic samples, as well as additional parameters used to characterize strain-induced birefringence of the focusing optics. Lenses are used to focus the light spot to an ellipse 0.7 × 2.0 mm2, allowing us to perform several ellipsometric measurements across the profile of ta-C films grown on 7.5 cm diameter Si wafers. The spectroscopic ellipsometry data are fit using a model of the ta-C dielectric function based on the Tauc band edge and the Lorentz expression of the dielectric function for an ensemble of atoms. These fits are used to determine the thicknesses of the rough surface layer, the ta-C film, and the interface layer, as well as the energy gap of the film. Comparisons are made with fits to an earlier formulation due to Forouhi and Bloomer [Phys. Rev. B 34, 7018 (1986).]. In addition to being Kramers-Kronig consistent, the Tauc-Lorentz formulation fits the ta-C data better.

A systematic study has been made of changes in the bonding and optical properties of hydrogen-free tetrahedral amorphous carbon (ta-C) films, as a function of the kinetic energy of the incident carbon ions measured under film-deposition conditions. Ion probe measurements of the carbon ion kinetic energies produced by ArF and KrF laser ablation of graphite are compared under identical beam-focusing conditions. Much higher C+ kinetic energies are produced by ArF-laser ablation than by KrF for any given fluence and spot size. Electron energy loss spectroscopy and scanning ellipsometry measurements of the sp3 bonding fraction, plasmon energy, and optical properties reveal a well-defined optimum kinetic energy of 90 eV to deposit ta-C films having the largest sp3 fraction and the widest optical (Tauc) energy gap (equivalent to minimum near-gap optical absorption). Tapping-mode atomic force microscope measurements show that films deposited at near-optimum kinetic energy are extremely smooth, with rms roughness of only ~ 1 Å over distances of several hundred nm, and are relatively free of particulates.

The dynamics of nanoparticle formation by laser ablation into background gases are revealed by gated-ICCD photography of photoluminescence (PL) and Rayleigh-scattering (RS) from gas-suspended nanoparticles. These techniques, along with gated-spectroscopy of PL from isolated, gassuspended nanoparticles, permit fundamental investigations of nanomaterial growth, doping, and luminescence properties prior to deposition for thin films. Using the time-resolved diagnostics, particles unambiguously formed in the gas phase were collected on TEM grids. Silicon nanoparticles, 1-10 nm in diameter, were deposited following laser ablation into 1-10 TorrAror He. Three in situ PL bands (1.8, 2.6, 3.2 eV) similar to oxidized porous silicon were measured, but with a pronounced vibronic structure. Structureless photoluminescence bands were reproduced in the films (2.1, 2.7, 3.2 eV) only after standared annealing. The ablation of metal zinc into Ar/02 is also reported for the preparation of < 10 nm diameter hexagonal zincite nanocrystals, The particles were analyzed by bright field and Z-contrast TEM and high resolution EELS.

Sequential ion implantation followed by thermal annealing has been used to form encapsulated CdS, ZnS, and PbS nanocrystals in SiO2 and Al2O3 matrices. In SiO2, nanoparticles are nearly spherical and randomly oriented, and ZnS and PbS nanocrystals exhibit bimodal size distributions. In Al2O3, nanoparticles are facetted and oriented with respect to the matrix. Initial photoluminescence (PL) results are presented.

We have investigated the growth of GdLiF4 thin films for optical waveguide applications using pulsed-laser deposition (PLD). Epitaxial, c-axis oriented GdLiF4 films have been grown from undoped GdLiF4 targets in an on-axis PLD geometry on (100) CaF2. These films exhibit a high density of particulates on the surface which are ejected from the target in the ablation process. Growth from Nd-doped polycrystalline GdLiF4 ablation targets results in relatively smooth films with lower particulate densities, as Nd doping significantly increases the optical absorption of GdLiF4 at the ablation laser wavelength of 193 nm and permits efficient pulsed-laser deposition. Optical emission spectra of the ablation pume reveals the presence of atomic fluorine, gadolinium, and lithium, indicating the dissociation of the metal-fluorine bonds in the ablation process. In addition, we find that the residual background oxygen pressure must be sufficiently reduced to avoid the formation of Gd4O3F6 as an impurity oxy-fluoride phase in the films.

Plasma generation through vapor breakdown during ablation of a Si target by nanosecond KrF laser pulses is modeled using 0-dimensional rate equations. although there is some previous work on vapor breakdown by microsecond laser pulses, there have been no successful attempts reported on the same subject by nanosecond laser pulses. This work intends to fill the gap. a kinetic model is developed considering the following factors: (1) temperatures of both electrons and heavy-body particles (ions, neutrals, and excited states of neutrals), (2) absorption mechanisms of the laser energy such as inverse bremstrahlung (IB) processes and photoionization of excited states, (3) ionization acceleration mechanisms that include electron-impact excitation of ground state neutrals, electron-impact ionization of excited states of neutrals, photoionization of excited states of neutrals, and all necessary reverse processes. the rates of various processes considered are calculated using a second order predictor-corrector numerical scheme. the rate equations are solved for five quantities, namely, densities of electrons, neutrals, and excited states of neutrals, and the temperatures of electrons and heavy-body particles. the total breakdown times (sum of evaporation time and vapor breakdown time) at different energy fluences are then calculated. the results are compared with experimental observations of Si target ablation using a KrF laser.

Pulsed laser ablation (PLA) has several characteristics that are potentially attractive for the growth and doping of chemically complex compound semiconductors including (1) stoichiometric (congruent) transfer of composition from target to film, (2) the use of reactive gases to control film composition and/or doping via energetic-beam-induced reactions, and (3) low-temperature nonequilibrium phase formation in the laser-generated plasma “plume.” However, the electrical properties of compound semiconductors are far more sensitive to low concentrations of defects than are the oxide metals/ceramics for which PLA has been so successful. Only recently have doped epitaxial compound semiconductor films been grown by PLA. Fundamental studies are being carried out to relate film electrical and microstructural properties to the energy distribution of ablated species, to the temporal evolution of the ablation pulse in ambient gases, and to beam-assisted surface and/or gas-phase reactions. In this paper we describe results of ex situ Hall effect, high-resolution x-ray diffraction, transmission electron microscopy, and Rutherford backscattering measurements that are being used in combination with in situ RHEED and time-resolved ion probe measurements to evaluate PLA for growth of doped epitaxial compound semiconductor films and heterostructures. Examples are presented and results analyzed for doped II–VI, I–III–VI, and column-Ill nitride materials grown recently in this and other laboratories.

A comparative study of arF- and KrF-laser deposition of amorphous diamond-like carbon (DLC) films and relevant carbon plasmas has been performed. Spectroscopic ellipsometry and EELS analysis of the DLC films deposited on Si <100> and NaCl substrates were utilized to characterize the high quality arF- and KrF-laser deposited films (up to 84% of sp3 bonded carbon in 7 J/cm2 -ArF-laser DLC film). Gated ICCD imaging, luminescence and ion current probe diagnostics of the carbon plume have revealed quite different properties of carbon plasmas generated by arF- and KrF- lasers. KrF-laser (6.7 J/cm2) irradiation produces a less energetic carbon plasma containing larger amounts of C2 and probably larger clusters compared with arF-laser irradiation at the same energy fluence. We conclude that the more energetic and highly-atomized arF-laser carbon plasma results in the better diamond-like properties.

Highly p-doped ZnTe films have been grown on semi-insulating GaAs (001) substrates by pulsed-laser ablation (PLA) of a stoichiometric ZnTe target in a high-purity N2 ambient without the use of any assisting (DC or aC) plasma source. Free hole concentrations in the mid-1019 cm-3 to 1020 cm-3 range were obtained for a range of nitrogen pressures the maximum hole concentration equals the highest hole doping reported to date for any wide band gap II-VI compound. the highest hole mobilities were attained for nitrogen pressures of 50–100 mTorr (~6.5–13 Pa). Unlike recent experiments in which atomic nitrogen beams, extracted from RF and DC plasma sources, were used to produce p-type doping during molecular beam epitaxy deposition, spectroscopic measurements carried out during PLA of ZnTe in N2 do not reveal the presence of atomic nitrogen. This suggests that the high hole concentrations in laser ablated ZnTe are produced by a new and different mechanism, possibly energetic beam-induced reactions with excited molecular nitrogen adsorbed on the growing film surface, or transient formation of Zn-N complexes in the energetic ablation plume. This appears to be the first time that any wide band gap (Eg 2 eV) II-VI compound (or other) semiconductor has been impurity-doped from the gas phase by laser ablation. In combination with the recent discovery that epitaxial ZnSe1-xSx films and heterostructures with continuously variable composition can be grown by ablation from a single target of fixed composition, these results appear to open the way to explore PLA growth and doping of compound semiconductors as a possible alternative to molecular beam epitaxy.

Several physics and computational approaches have been developed to globally characterize phenomena important for film growth by pulsed laser deposition of materials. these include thermal models of laser-solid target interactions that initiate the vapor plume; plume ionization and heating through laser absorption beyond local thermodynamic equilibrium mechanisms; gas dynamic, hydrodynamic, and collisional descriptions of plume transport; and molecular dynamics models of the interaction of plume particles with the deposition substrate. the complexity of the phenomena involved in the laser ablation process is matched by the diversity of the modeling task, which combines materials science, atomic physics, and plasma physics.

Rapid transformations through the liquid and vapor phases induced by laser-solid interactions are described by our thermal model with the Clausius-Clapeyron equation to determine the vaporization temperature under different surface pressure condition. Hydrodynamic behavior of the vapor during and after ablation is described by gas dynamic equations. these two models are coupled. Modeling results show that lower background pressure results lower laser energy density threshold for vaporization. the ablation rate and the amount of materials removed are proportional to the laser energy density above its threshold. We also demonstrate a dynamic source effect that accelerates the unsteady expansion of laser-ablated material in the direction perpendicular to the solid. a dynamic partial ionization effect is studied as well. a self-similar theory shows that the maximum expansion velocity is proportional to cs/α, where 1 – α is the slope of the velocity profile. Numerical hydrodynamic modeling is in good agreement with the theory. With these effects, α. is reduced. therefore, the expansion front velocity is significantly higher than that from conventional models. the results are consistent with experiments. We further study the plume propagates in high background gas condition. Under appropriate conditions, the plume is slowed down, separates with the background, is backward moving, and hits the solid surface. then, it splits to be two parts when it rebounds from the surface. the results from the modeling will be compared with experimental observations where possible.

Various physical processes during laser ablation of solids for pulsed-laser deposition (PLD) are studied using a variety of computational techniques. In the course of our combined theoretical and experimental effort, we have been trying to work on as many aspects of PLD processes as possible, but with special focus on the following areas: (a) the effects of collisional interactions between the particles in the plume and in the background on the evolving flow field and on thin film growth, (b) interactions between the energetic particles and the growing thin films and their effects on film quality, (c) rapid phase transformations through the liquid and vapor phases under possibly nonequilibrium thermodynamic conditions induced by laser-solid interactions, (d) breakdown of the vapor into a plasma in the early stages of ablation through both electronic and photoionization processes, (c) hydrodynamic behavior of the vapor/plasma during and after ablation. The computational techniques used include finite difference (FD) methods, particle-in-cell model, and atomistic simulations using molecular dynamics (MD) techniques.

The effects of pulsed XeCl (308 nm) laser radiation on polycrystalline Al2O3 (alumina, 99.6% pure) and single-crystal Al2O3 (sapphire) are studied as a function of laser fluence. No laser etching ofeither material is detected below a threshold fluence value, which is much lower for alumina than for sapphire. Above this threshold, laser etching of both materials is observed following a number of incubation (induction) pulses. This number is much larger for sapphire than for alumina but decreases with increasing fluence for both materials. Laser etching rates for the two materials are similar at high fluences and after the incubation period. Scanning electron microscope images show that alumina melts and flows under repeated irradiation at fluences ≥0.7 J/cm2. Atomic force microscopy and surface profilometry reveal significant smoothing of the as-received polycrystalline alumina surface after repeated irradiations at moderate fluences (∼1−3 J/cm2). Ion probe measurements for alumina in vacuum confirm the incubation behavior, and reveal that at fixed fluence the (positive) charge collected per pulse saturates after a sufficient number of pulses, as does the etch-plume velocity. The results are interpreted in terms of laser-generation of a sufficient concentration of absorption centers before efficient ablation/etching of these wide bandgap materials can occur.

The propagation of the laser-induced plasma formed by KrF irradiation of Y1Ba2Cu3O7 has been characterized in background pressures of oxygen and argon typically used for thin film growth. The ion current transmitted through the background gases was recorded along the normal to the irradiated pellet as a function of distance in order to measure the decreasing velocity and magnitude of the expanding plasma current due to collisional slowing and attenuation of the laser plume. The integrated ion charge delivered to a substrate at low pressures can be described by elastic scattering giving a general integral cross sections of σc[O2] = 3.2 × 10−16 cm2 and σc[Ar] = 2.7 × 10−16 cm2. At higher pressures, inelastic-scattering leads to increased recombination and reactive conversion of ions indicated by increased fluorescence of all the species, which becomes dominated by fluorescence of YO and BaO. Spatially resolved fluorescence measurements indicate that the luminous boundary to the plasma follows a weak shock front which coincides with the ion flux propagation. The ion transmission is found to drop exponentially with distance and background pressure, in agreement with a simple scattering model which yields general scattering cross sections for ion-argon σi-Ar = 2.1 × 10−16 cm2 and ion-oxygen σi-O2 = 2.3 × 10−16 cm2 interactions in background pressures up to 300 mTorr. The general features of the plume deceleration are described in terms of a drag force model.

Thin films of Ho1Ba2Cu3O7 − x and Y1Ba2Cu3O7 − x were deposited on SrTiO3 and Al2O3, substrates by pulsed laser deposition of high-Tc bulk superconductor pellets in vacuum. Following annealing in O2 at 800–900 °C the films were superconducting with typical Tc (50%) = 89 K and transition widths of 10 K. Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) were utilized to study the stoichiometry of the as-deposited films for laser energy, densities between 0.11 and 4.5 J cm−2. The films were deficient in holmium and yttrium for energy densities below 0.6 and 0.4 J cm −2, respectively. The films were stoichiometric for fluences above 0.6 J cm−2. In addition, preliminary time dependence and spectroscopic observations of the laser-produced plasma are presented. The results indicate an ablation mechanism that at high energy densities preserves stoichiometry. TEM and x-ray characterization of annealed, superconducting Ho1Ba2Cu3O7 − x films on (100) SrTiO3 showed mixed regions of epitaxially oriented 1:2:3 material with either the c axis or a axis oriented along the surface normal. The a-axis-oriented material grew preferentially in the films with b, c, twinning.

High-purity digermane (Ge2H6, 5% in He) has been used to grow epitaxially oriented crystalline Ge films by pyrolysis. Amorphous Ge:H films also have been deposited by pyrolysis and ArF (193 nm) laser-induced photolysis. The amorphous-to-crystalline transition and the film's morphology was studied as a function of deposition conditions. The film's microstructure, strain and epitaxial quality were assessed using x-ray diffraction curves and scanning and transmission electron microscopy. It was found that commensurate, coherently strained epitaxial Ge films could be grown pyrolytically on (100) GaAs at low (0.05–40 m Torr) Ge2H6 partial pressures, for substrate temperatures above 380°C.

Thin superconducting films of YBa2Cu3O7 and HoBa2Cu3O7 have been obtained by ablating material from high-Tc bulk superconductor ceramics onto a nearby substrate using focused excimer laser radiation. Subsequent annealing in oxygen at elevated temperatures was necessary to achieve the superconducting phase. A range of deposition and annealing conditions was investigated to yield films of the desired stoi chiometry and phase.