Albert J. Paul

Composition and gas dynamics of laser ablated AlN plumes

Abstract The formation, composition and propagation of pulsed laser produced plasma plumes from an AlN target have been studied in real time by spatially and temporally resolved optical spectroscopy. The mean front velocity has been measured and appears to be slightly slower in the presence of added gas than in vacuum. In the initial stage of expansion, this velocity is almost the same for all neutral particles in the plume in the initial stage of expansion. Velocities of neutral and ionic species have been measured by their time of arrival from the target to a quadrupole mass spectrometer and their expansion beam Maxwell-Boltzmann (MB) temperature determined. The plume electronic temperature has been determined by assuming a local thermodynamic equilibrium of the emitting species.

Particulate reduction in the pulsed laser deposition of barium titanate thin films

Abstract A particulate-reduction approach has been developed, based on a pulsed supersonic gas-jet that selectively deflects particulates from the deposition stream during pulsed laser deposition of BaTiO 3 thin films. In situ imaging, using an intensified CCD camera, has shown that the desirable atomic, ionic, and molecular species move toward the substrate with velocities on the order of 10 6 cm s −1 , while the undesirable particulates move at velocities on the order of 10 4 cm s −1 and slower. This separation of velocities is sufficient that the pulsed gas-jet can be timed to impact the particulates after the vapor species reach the substrate, but while the particulates are still near the target, allowing for near-maximum deflection. Key parameters with this approach are the sharpness and the timing of the valve opening and closing. Initial results show that at least an order of magnitude reduction in the number of film particulates is achieved.

High-accuracy determination of the dependence of the photoluminescence emission energy on alloy composition in AlxGa1−xAs films

In an effort to improve the accuracy of photoluminescence (PL) measurements of the Al mole fraction (x) of AlxGa1−xAs alloys, the PL peak emission energy, EPL,peak, was measured at room temperature for molecular-beam epitaxy-grown AlxGa1−xAs films with 0⩽x<0.37, and correlated with independent measurements of x by in situ reflective high-energy electron diffraction (RHEED) and also by ex situ wavelength-dispersive x-ray spectroscopy in an electron microprobe analyzer (WDS/EMPA). The measurement uncertainty of EPL,peak was minimized through the following procedures: Accurate calibration of the photon energy (or wavelength) scale, correction of the measured spectra for the spectrometer response function, fitting the data with a well-chosen line shape function, and compensation for the effect of ambient temperature drift. With these procedures, the 2σ measurement uncertainty of EPL,peak was of the order 5×10−4 eV for most samples. From correlation of the PL and WDS/EMPA composition data, the slope ∂EPL,peak/...

Imaging and gasdynamic modeling of pulsed laser film deposition plumes

Optical multichannel emission spectroscopy and intensified charge-coupled device (ICCD) imaging have been applied to real-time, in situ gas-phase species identification during the pulsed excimer or Nd:YAG laser deposition of various ceramic thin films. A plume gasdynamic expansion model has been developed and used to predict the outer-edge plume front locations for comparison with those observed in the ICCD images. Good agreement was found between the model and ICCD images, with plume temperatures indicated by the model to be typically between 10,000 and 50,000 K. The systems studied include PbZr0.53Ti0.47O3 (PZT), BaTiO3 , AlN, and BN. When high laser fluences were used, ICCD imaging also revealed strong evidence for interactions between the laser and the near-surface plume. Plume particulates were also noted at long times following the laser pulse.

Species Temporal and Spatial Distributions in Laser Ablation Plumes

The intermediate species present in laser ablation plumes, particularly those formed during pulsed laser deposition (PLD) of thin films, have been identified for a variety of advanced materials systems. Optical multichannel analysis spectroscopy has been used to monitor the atomic and ionic species present, via their spectral emissions. Molecular beam-sampling mass spectrometry has been used to monitor the molecular species present, in addition to atoms and ions. With both monitoring approaches, temporal and spatial species distribution information has been obtained. Velocity distributions, obtained from the time-dependent mass spectral studies, show the effects of isentropic expansion to be predominant when compared with gasdynamic models of the plume evolution process. Also, the plume structure was found to be particularly sensitive to target elemental distribution. Examples of systems studied include high temperature superconductors, refractory compounds, ferroelectrics and nanostructured magnetic films.

High-accuracy determination of epitaxial AlGaAs composition with inductively coupled plasma optical emission spectroscopy

Inductively coupled plasma optical emission spectroscopy is shown to confirm a recent correlation between photoluminescence (PL) peak energy for AlGaAs epitaxial films and the Al mole fraction x of those films. These two methods also agree within their expanded uncertainties with the Al composition as determined by growth rate measurements using reflection high energy electron diffraction intensity at the time of specimen growth. No systematic variations between the three methods as a function of Al mole fraction were observed. The lowest uncertainty was found in the PL measurements, allowing certification of Al mole fraction x in standard reference materials to an expanded uncertainty of 0.003 for x<0.35. Details of the uncertainty analysis, as well as possible improvements in those uncertainties, are discussed.

Imaging and modeling of pulsed-laser thin-film-deposition plumes

Optical multichannel emission spectroscopy and intensified charge coupled device (ICCD) imaging have been applied to real-time, in situ gas phase species identification during the pulsed excimer or Nd/YAG laser deposition of selected thin films. A plume gasdynamic expansion model has been developed and used to predict the outer edge plume front locations for comparison with those observed in the ICCD images. Good agreement was found between the model and ICCD images, with plume temperatures indicated by the model to be typically between 10,000 K and 30,000 K. Molecular beam mass spectrometry has also been used for real- time, in situ species identification and velocity distribution determinations. The systems studied include PbZr0.53Ti0.47O3 (PZT), BaTiO3, AlN, BN, Al2O3, and Ag. The ICCD imaging of plumes from PZT and BN targets, in particular, revealed that particulate ejecta were present after some regions of the target surface had been modified morphologically by multiple exposures to the laser beam. These ejecta appeared long after the luminous plume had decayed. When relatively high laser fluences were used, ICCD imaging also revealed strong evidence for interactions between the laser and the near-surface plume. This interaction manifests itself as increased emission intensity in the direction of the incoming laser beam. Mass spectrometric studies showing relatively fast velocity components of both neutral and charged plume species support the imaging evidence. The laser-plume interaction results in higher kinetic energies and a much greater effective temperature for a portion of the plume species. The slower component(s) appeared to be more thermal in origin (i.e. controlled by surface vaporization), and apparently reflects the target surface temperature.

Optical Absorption, Raman, and Photoluminescence Excitation Spectroscopy of Inhomogeneous InGaN Films

InGa l-x N films with x=0.06 to x=0.49 were characterized by optical transmittance, Raman, and photoluminescence excitation spectroscopies. Previous microstructural characterizations detected phase separation only in films with x>0.2. The transmittance data suggest that compositional inhomogeneity is also present in the lower-x films (x 0.2. The composition dependence of the Raman spectra, from x=0.28 to x=0.49, is consistent with an increase in the size of the phase-separated regions with increasing x.

Monte Carlo simulations of plume evolution from laser ablation of graphite and barium titanate

Abstract Evolution of laser ablation products from graphite and barium titanate targets has been simulated using a Monte Carlo method, assuming elastic collisions. The calculated species arrival times and intensities, at a hypothetical detection plane, compare favorably with those obtained experimentally by time-resolved mass spectrometric sampling of the expanded vapor plumes. As such experiments are difficult to perform, the simulation method should prove useful for unstudied systems in defining the spatial and temporal distributions of species arriving for example, at a film deposition substrate.

Gasdynamics and Chemistry in The Pulsed Laser Deposition of Oxide Dielectric Thin Films

In the context of “chemistry and its effects on film quality,” we and a number of other research groups have developed spectroscopic and modeling approaches to better define the pulsed laser deposition process. An overview of these approaches is given here, using the results of recent work performed in our laboratory on the oxide dielectric systems of BaTiO 3 and PbZr 0.53 Ti 0.47 O 3 (PZT).