This dissertation documents the development and demonstration of a new laser crystallization process called spot-beam annealing (SBA). The SBA method is a partial-melting-based laser-annealing method, which converts as-deposited amorphous Si films into high-mobility TFT-enabling polycrystalline films. SBA builds on the thermally additive utilization of multiple short-lived low-energy ultra-high-frequency pulses, achieved via substantially overlapped scanning of a small spot beam to incrementally and gradually heat and partially melt the beam-irradiated region. After a brief review of other laser crystallization technologies, the conceptual framework for the SBA process is introduced, and various possible implementation schemes and development paths are discussed. In the present work, the SBA method is implemented using a new class of ultra-high-frequency (>100 MHz), low-pulse energy (<1 𝜇J), short-pulse-duration (<1 ns) UV fiber lasers. The first half of the thesis (chapters 4 and 5) presents, the simulation- and calculation-based studies of the SBA process. A simple but relevant one-dimensional thermal analysis identifies the "dwell time" (associated with the overall intensity temporal profile defined by the collection of those pulses that irradiate a point in the film) as a key SBA parameter. Provided that a sufficient number of multiple shots are involved in irradiating the point in the film, this parameter dictates the overall thermal and transformation cycle of heating, primary melting, and solidification that enables the ultra-short-pulse-based SBA method to mimic the physical conditions encountered previously only using pulsed lasers with pulse duration in the range of tens to hundreds of nanoseconds; the precise range needed for optimally generating laser-annealed polycrystalline materials on glass and plastic substrates. Additionally, we also identify and examine an important differentiating feature of the SBA method, namely the highly transient temperature spikes that arise from the individual pulses incident onto a point on the film during overlapped scanning. By simultaneously considering the preliminary experimental results that are presented in this thesis (chapters 6 and 7), we suggest that these periodic temperature spikes, the specific degree of which depends on the temporal profile and energy density of individual pulses, can potentially play a key role in dictating certain important details of melting and solidification transitions encountered in SBA. In particular, we identify and elaborate on how the temperature fluctuations can affect how explosive crystallization of a-Si films is manifested in a different manner than has previously been observed. In addition, we point out how the fluctuations can control the degree to which the melting scenarios in SBA can deviate from the grain-boundary-melting-dominated 2-D transition scenario (as for instance encountered in pulsed-laser irradiation of columnar-grained polycrystalline films), where lateral melting is exclusively initiated at grain boundaries and propagates predominantly laterally into the superheated and defect-free interior of the grains. In the second half of the thesis, the experimental results that are obtained from a recently constructed research SBA system are presented, characterized, and evaluated. Specifically, the examination of single-scan and multiple-scan exposed Si films conducted using OM, AFM, and TEM material characterization techniques reveals that the method is capable of not only generating uniform polycrystalline Si films consisting of ordered grains with tight grain-size distribution around the beam wavelength, but it can furthermore be configured to produce polycrystalline films with an enhanced level of ordering as manifested in the films with a highly parallel ridge (HPR) pattern.

Call number: LD2668 .T4 1986 Y36 / Master of Science / Physics

Thesis (M.S.)--University of Alabama at Birmingham, 2008. / Description based on contents viewed May 30, 2008; title from title screen. Includes bibliographical references (p. 50-51).

Supersaturated substitutional alloys formed by ion implantation and rapid liquid-phase epitaxial regrowth induced by pulsed-laser annealing have been studied using Rutherford-backscattering and ion-channeling analysis. A series of impurities (As, Sb, Bi, Ga, In, Fe, Sn, Cu) have been implanted into single-crystal (001) orientation silicon at doses ranging from 1 x 10^15/cm2 to 1 x 10^17/cm2. The samples were subsequently annealed with a Ω-switched ruby laser (energy density ~1.5 J/cm2, pulse duration 15 x 10-9 sec). Ion-channeling analysis shows that laser annealing incorporates the Group III (Ga, In) and Group V (As, Sb, Bi) impurities into substitutional lattice sites at concentrations far in excess of the equilibrium solid solubility. Channeling measurements indicate the silicon crystal is essentially defect free after laser annealing. The maximum Group III and Group V dopant concentrations that can be incorporated into substitutional lattice sites are determined for the present laser-annealing conditions. Dopant profiles have been measured before and after annealing using Rutherford backscattering. These experimental profiles are compared to theoretical model calculations which incorporate both dopant diffusion in liquid silicon and a distribution coefficient (k') from the liquid. It is seen that a distribution coefficient (k') far greater than the equilibrium value (k0) is required for the calculation to fit the experimental data. In the cases of Fe, Zn, and Cu, laser annealing causes the impurities to segregate toward the surface. After annealing, none of these impurities are observed to be substitutional in detectable concentrations. The systematics of these alloys systems are discussed.