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.

The energy distributions of positive ions produced by exposing single-crystal MgO to pulsed 248 nm excimer laser light at fluences of 200-1200 mJ/cm2 were determined by combined quadrupole mass spectrometry and time-of-flight techniques. the dominant ionic species is Mg+, although small amounts of Mg2+, MgO+, and Mg2O+ are also observed. IN particular, the

Mg+ and Mg2+ energy distributions each show two broad peaks, with the energies of the Mg2+ peaks at significantly higher energies. Ion trajectory simulations (accounting for Coulomb forces only and assuming no surface relaxation) suggest that Mg2+ adsorbed at sites directly atop surface F-centers (oxygen vacancies with two trapped electrons) would be ejected upon photo-ionization of the F-center. the experimentally observed Mg2+ kinetic energies agree well with the energies predicted by the simulation.

The penetration of energetic pulsed ablation plumes through ambient gases is experimentally characterized to investigate a general phenomenon believed to be important to film growth by pulsed laser deposition (PLD). Under typical PLD conditions involving background gases, the ion flux in the ablation plume is observed to split into distinct fast and slow components over a limited range of distances1,2 the fast component is transmitted with near-initial velocities and high kinetic energies, potentially damaging to growing films at these distances. Formation of the second, significantly-slowed component correlates with the bright contact front3 formation observed1,4 in fast ICCD imaging studies. This general effect is explored in detail for the case of yttrium ablation into argon, a single-element target into an inert gas.5 Time-resolved optical absorption spectroscopy and optical emission spectroscopy are employed to simultaneously view the populations of both excited and ground states of Y and Y+ for comparison with quantitative intensified-CCD photography of the visible plume luminescence and ion flux measurements made with fast ion probes during this phenomenon. these measurements confirm that, in addition to the bright significantly-slowed front which has been described by shock or drag propagation models1, a fast-component of target material is transmitted to extended distances for some ambient pressures with near-initial velocities.

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.

Planar laser-induced fluorescence has been used to acquire time sequence images of ground-state, neutral Si and SiO during laser ablation of an Si target in vacuum and in the presence of a background gas at a fluence of 3-4 J/cm2. the SiO images, taken in air, strongly suggest that the observed SiO is created through reaction of silicon with oxygen at the contact front as the plume expands.

We present the results of experiments on velocity selection of fast laser ablated al, Ga, and in atoms by a novel, non-mechanical, technique. Pulses of atoms with broad velocity distributions are produced by laser ablation of a single component pure metal target in vacuum. after a delay of ~ 1 μs, there exists a strong one-to-one correlation between atomic velocity and distance traveled from the ablated surface. Thus, a second pulsed laser, delayed by ~ 1 μs and crossed at a right angle to the atomic beam, can be used to photoionize only those atoms with unwanted velocities, i.e.: atoms moving too fast or too slow to be hidden behind an opaque mask placed ~ 1 cm from the ablated surface. the photoions, and any ions surviving from the ablation event, are subsequently deflected from the beam by a static magnetic field. by a fortunate coincidence, al, Ga, and in atoms all have very large single photon photoionization cross sections at 193 nm, the output wavelength of the arF excimer laser; thus, well over 95% of the unwanted atoms can be easily photoionized and rejected. We have demonstrated velocity selected al, Ga, and in atom fluxes equivalent to Ф ~ 1011 atoms/(cm2-eV-pulse) at a working distance of 10 cm.

We have applied intensified Charge Coupled Detector (ICCD) imaging to real-time, in situ monitoring of excited vapor species emission during the pulsed excimer (248 nm wavelength) laser deposition (PLD) of BaTiO3 ferroelectric thin films. Molecular beam mass spectrometry (MS) was used for species identification and velocity distribution analysis. Gasdynamic model simulations of the plume formation and transport process are tested by comparison with the ICCD and MS results. Particular attention is given to the time scale during and immediately following the laser pulse (0 - 460 ns), and also to the effects of added 02. atomic oxygen was found to be the predominant species present at the leading edge of the vapor plume. Other, slower species found included neutral Ti, TiO, Ba, and BaO.

We present the first report of the synthesis of Cd2SnO4 films by pulsed laser deposition. Controlling the substrate temperature and the ambient atmosphere allowed for the synthesis of films ranging from amorphous to crystalline with some crystalline films exhibiting strong texture. Highly transparent films with large mobilities were obtained for both the amorphous and crystalline films. Sheet resistances of 15.5 Ω/square and mobilities of up to 44.7 cm2/V.s were observed. Typical carrier concentrations showed the crystalline films to be degenerate with carrier concentrations of 5 х 1020cm-3 while amorphous films had carrier concentrations lower by about half. Band gaps for the films ranged between 3.1-3.8 eV. these films are attractive candidates for TCO applications in thin film photovoltaic devices, flat panel displays, electrochromic windows, and as plasma filters for thermophotovoltaic devices.

Superlattice structures, consisting of SrCuO2, (Sr,Ca)CuO2, and BaCuO2 layers in the tetragonal, "infinite layer" crystal structure, have been grown by pulsed-laser deposition (PLD). Superlattice chemical modulation is observed for structures with component layers as thin as a single unit cell (~3.4 Å), indicating that unit-cell control of (Sr,Ca)CuO2 growth is possible using conventional pulsed-laser deposition over a wide oxygen pressure regime. X-ray diffraction intensity oscillations, due to the finite thickness of the film, indicate that these films are extremely flat with a thickness variation of only ~20 Å over a length scale of several thousand angstroms. Using the constraint of epitaxy to grow metastable cuprates in the infinite layer structure, novel high-temperature superconducting structural families have been formed. IN particular, epitaxially-stabilized SrCuO2/BaCuO2 superlattices, grown by sequentially depositing on lattice-matched (100) SrTiO3 from BaCuO2 and SrCuO2 ablation targets in a PLD system, show metallic conductivity and superconductivity at Tc(onset) ~70 K. these results show that pulsed-laser deposition and epitaxial stabilization have been used to effectively "engineer" artificially-layered thin-film materials.

The pulsed-laser deposition (PLD) technique utilizes one of the most energetic beams available to form thin films of the superconducting oxide YBa2Cu3O7 (YBCO). IN this study we examine the growth of YBCO at very high laser fluences (25 to 40 J/cm2); a more typical fluence for PLD would be nearer to 3 J/cm2. the use of high fluences leads to unique film microstructures which, in some cases, appear to be related to the correspondingly higher moveabilities of the adatoms. Films grown on vicinal substrates, using high laser fluences, exhibited well-defined elongated granular morphologies (with excellent transition temperature, Tc, and critical current density, Jc). Films grown on vicinal substrates using off-axis magnetron sputtering, plasma-enhanced metal organic chemical vapor deposition (PE-MOCVD), or PLD at more typical laser fluences showed some similar morphologies, but less well-defined. Under certain growth conditions, using high laser fluences with (001) oriented substrates, the YBCO films can exhibit a mixture of a- and c-axis growth where both crystallographic orientations nucleate on the substrate surface at the same time, and grow in concert. the ratio of a-axis oriented to c-axis oriented grains is strongly affected by the pulse repetition rate of the laser.

Thin films of Srl-xBaxTiO3 (SBT) with x = 0 to 0.80 have been grown in situ by pulsed laser deposition onto single crystals of (001) LaAlO3. the films were grown to thicknesses of 0.6 μ.m and found to be single phase, highly oriented, and characterized by x-ray ω scan widths of ≤ 0.5°. the temperature dependence of the dielectric constant and the relative dissipation factor were measured at 1 kHz using au interdigital capacitors deposited on top of the ferroelectric films. the capacitance measurements indicate that the temperature dependence of the dielectric constant of the film is broad and the maximum is shifted relative to the bulk material. the differences between thin film and bulk properties are attributed to strain in the film resulting from film -substrate lattice mismatch. Thick films (∼7 μm) gave direct evidence for strain through cracking and delamination. X-ray diffraction measurements have been made to determine the non-uniform strain in the thin films which was approximately 0.1%.

(Sr,Ba)Nb2O6 (SBN) is a very promising material for nonlinear optical applications because of its high electro-optic and nonlinear optical coefficients. For these applications, SBN requires ferroelectric poling along the optical axis. Conducting layers such as Pt or YBCO must therefore be deposited to provide electrodes above and below the SBN film.

We have investigated the epitaxial growth of multilayers of Sr0.61Ba0.39Nb2O6/Pt and Sr0.61Ba0.39Nb2O6/YBCO thin films on (001) MgO substrates by pulsed laser deposition. X-ray diffraction 2θ scans indicate epitaxial growth of SBN/Pt/MgO and SBN/YBCO/MgO heterostructures with their c-axes perpendicular to the substrate plane. X-ray phi scans indicate single crystal Pt and YBCO growth with one in plane orientation. atomic Force Microscopy shows surface roughness of 2.19 nm, and no evidence of particles is observed.

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.

In this paper the results on p-type ZnS, ZnSe, CdS and CdSe thin films grown by pulsed laser deposition will be discussed. these films were deposited on GaAs substrates. Li-doping has been shown to be effective in producing p-type II-VI thin films, while in-doping is excellent for n-type CdS and CdSe thin films. No post-annealing process was used. these preliminary results suggest a possible new approach through pulsed laser deposition to solve the doping problem of II-VI compound semiconductors.

Epitaxial and compositionally homogeneous SnxGe1-x alloy films have been grown on Si (001) by pulsed laser deposition using elemental Sn and Ge targets. these results demonstrate that pulsed laser deposition can be used to grow alloys by overcoming the strong tendency for Sn surface segregation seen in growth by other methods such as molecular beam epitaxy.

The pulsed-laser deposition technique has been used to form thin films of TiN on (100)-oriented single crystal substrates of silicon and rocksalt. Using atomic force microscopy, it was revealed that TiN films grown on silicon at substrate temperatures ranging from 50°C to 500°C were extremely smooth—the mean roughness being ~ 0.2 nm. Thin TiN films deposited on freshly cleaved NaCl were found to be epitaxial at substrate temperatures as low as 50°C. Epitaxy in this latter system is believed to be due to the structural similarity between film and substrate and the almost exact 4:3 coincident site lattice.

Control of composition and phase of a series of MoNx thin films has been accomplished by reactive pulsed laser deposition (PLD). Molybdenum foil targets were ablated in a background gas of N2/H2 (10%) at pressures ranging from 40 to 120 mTorr. the MoNx films were deposited simultaneously onto (100) MgO and fused silica substrates. the films were characterized by X-ray diffraction (XRD), temperature-dependent resistivity, and Rutherford backscattering spectroscopy (RBS). the composition, phase, and electronic transport properties were found to depend on N2/H2 pressure, substrate temperature, and substrate orientation. the highest superconducting transition temperature (Tc ) was observed in a hexagonal Mo2N film where Tc (onset) ≈ 8 K. IN general, Tc was observed to correlate most closely with the N/Mo ratio. as the ratio of N/Mo increased above optimal M02N composition, Tc decreased. Films grown on MgO generally had higher N/Mo ratios and hence lower Tc values than films deposited on silic A.

A pulsed laser deposition technique has been used to grow silicon oxynitride (SiOxNy) thin films at low deposition temperatures (25°C - 300°C). the thin films were found to be quite smooth in surface morphology, extremely inert in chemical solution and highly transparent in the optical range of 0.3 μm to 5 μm. the refractive index was tunable between 1.4 - 2.1 by the addition of oxygen in substitution of nitrogen in the film, and the dielectric constant is much larger than the similar films grown by conventional chemical vapor deposition. the high quality of the SiOxNy films deposited at such low temperatures was resulted from the large kinetic energy carried by the impinging particles created by the ablation of a high-power pulsed excimer laser. the kinetic energy of the impinged particles on the substrate provides thermal energy for surface diffusion and relaxation.

Visible light from a copper vapor laser (CVL) operating with 510 and 578 nm radiation (intensity ratio approximately 2:1), an average power of 100 W, a pulse duration of 50 ns, and a repetition frequency of 4.4 kHz has been shown to produce high quality diamond-like-carbon (DLC) films at fluences between 2xl08 and 5xl010 W/cm2. Maximum deposition rates of 2000 μm.cm2/h were obtained at 5xl08 W/cm2. DLC films with hardness values of approximately 60 GPa were characterized by a variety of techniques to confirm DLC character, hydrogen content, and surface morphology. the presence of C2 in the vapor plume was confirmed by the presence of the C2 Swan bands in emission spectra obtained during the process. Economic implications of process scale-up to industrially meaningful component sizes are presented.

Several different aspects of laser annealing and laser ablation have indicated the need for two-dimensional (2-D) modeling of heat transfer and phase-change effects. We have in mind particularly the laser-induced formation and propagation of buried liquid layers for the case of a-Si on a crystalline silicon substrate, questions related to the early stages of the laser ablation of insulators such as MgO where it is believed that the absorption of the laser radiation occurs at localized but extended regions of high concentrations of defects, and the ejection of particulate material during laser ablation. To deal with these phenomena, a 1-D computational model originally developed for laser annealing has been extended to two-dimensions. the 2-D modeling examines the heat flow and phase changes associated with localized heat sources embedded in a planar material. Concepts such as the state diagram and state array used in the 1-D work have been extended to 2-D and refined. the 2-D program has been rewritten for massively parallel machines such as the intel Paragons in ORNL's Center for Computational Sciences, thus allowing larger and more accurate calculations for complex systems to be carried out in reasonable times.