23 research outputs found

    Role of a native oxide on femtosecond laser interaction with silicon (100) near the damage threshold

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    Si (100) with and without a 14–25 Å14–25Å thick native oxide was laser machined at grazing incidence using a Ti:sapphire femtosecond pulsed laser under ultrahigh vacuum conditions. The resulting damage feature size and morphology indicate that the presence or absence of the native oxide significantly affects the mechanism for femtosecond laser-induced damage. We propose that a fluence-dependent modification of the oxide by the incident laser pulse must be considered when studying femtosecond laser damage of Si (100) with a native oxide. Data are also presented that are consistent with a dose-dependent phase transformation in the amorphous oxide. The implications of the native oxide, including relative damage thresholds of the underlying Si (100) and the role of the oxide in damage morphology are addressed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87851/2/264103_1.pd

    Pulsed laser ignition of reactive multilayer films

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    Nanostructured Al/PtAl∕Pt multilayer films were ignited by single pulse irradiation from a Ti:sapphire femtosecond laser system. Critical ignition fluences (0.9–22 J/cm2)(0.9–22J∕cm2) required to initiate a self-propagating reaction were quantified for different multilayer designs. Multilayers with smaller bilayer thickness required relatively lower fluence for ignition. Ignition threshold fluence was also found to be 1.4–3.6 times higher for Al-capped multilayers than for Pt-capped multilayers. Ablation threshold fluences were measured for Al (860±70 mJ/cm2)(860±70mJ∕cm2) and Pt (540±50 mJ/cm2)(540±50mJ∕cm2) and related to the observed difference in ignition fluences for Al- and Pt-capped multilayers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87770/2/144102_1.pd

    Observations of Screw Dislocation Driven Growth and Faceting During CVD Homoepitaxy on 4H-SiC On-Axis Mesa Arrays

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    Previous studies of (0001) homoepitaxial growth carried out on arrays of small-area mesas etched into on-axis silicon-face 4H-SiC wafers have demonstrated that spiral growth emanating from at least one screw dislocation threading the mesa is necessary in order for a mesa to grow taller in the (c-axis vertical) direction while maintaining 4H stacking sequence [1]. However, even amongst mesas containing the screw dislocation step source necessary for vertical c-axis growth, we have observed striking differences in the height and faceting that evolve during prolonged homoepitaxial growths. This paper summarizes Atomic Force Microscopy (AFM), Electron Channeling Contrast Imaging (ECCI), Scanning Electron Microscopy (SEM), and optical microscopy observations of this phenomenon. These observations support our initially proposed model [2] that the observed large variation (for mesas where 3C-SiC nucleation has not occurred) is related to the lateral positioning of a screw dislocation step source within each etched mesa. When the screw dislocation step source is located close enough to the developing edge/sidewall facet of a mesa, the c-axis growth rate and facet angle are affected by the resulting interaction. In particular, the intersection (or near intersection) of the inward-sloping mesa sidewall facet with the screw dislocation appears to impede the rate at which the spiral provides new steps required for c-axis growth. Also, the inward slope of the sidewall facet during growth (relative to other sidewalls of the same mesa not near the screw dislocation) seems to be impeded by the screw dislocation. In contrast, mesas whose screw dislocations are centrally located grow vertically, but inward sloping sidewall facets shrink the area of the top (0001) growth surface almost to the point of vanishing

    Femtosecond laser induced thermal damage in thin films: The role of heat dissipation.

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    This thesis reports investigations of the thermal modifications induced by ultra-short pulsed laser irradiation in thin films at fluences near and above ablation threshold. Specific work was aimed at understanding the role of heat dissipation in determining the nature and spatial extent of thermal processes induced by femtosecond laser irradiation. Thermal damage in the form of microstructural modifications, phase transitions, and/or chemical reactions was analyzed within, outside, and below areas of femtosecond laser irradiated surfaces. The thermal damage induced by femtosecond laser ablation of silicon thin foils was investigated by high resolution transmission electron microscopy. Extensive single-crystalline Si was observed at the ablated hole edges for irradiation near the ablation threshold. Higher fluence ablation produced hole edges exhibiting a strong amorphous content. The rate of lateral cooling following femtosecond laser irradiation is proposed to account for the resulting microstructure near femtosecond laser ablation. A nanoscale technique was developed for direct measurement of heat affected zones resulting from femtosecond laser ablation. The lateral extent of cobalt silicidation was measured by selected area diffraction and morphological investigations using transmission electron microscopy. Silicidation extending 3 mum from the edge of an ablated hole provided evidence of a zone where the local temperature reached ∼500°C. Evidence of Si recrystallization was observed within 1 mum of the ablated hole edge and validated by thermal modeling. Reactive multilayer films composed of Co/Al, Al/Pt, and Ti/Ni were irradiated by femtosecond and nanosecond pulsed lasers. Fluence thresholds for igniting self-propagating reactions were measured as a function of bilayer thickness. The resulting ignition thresholds versus bilayer thickness trends were nearly reciprocal that of reaction propagation speed versus bilayer thickness. The trend is indicative of a relationship between the ignition thresholds and the thermal damage necessary to achieve a critical volume of layer intermixing. Femtosecond laser ablation at lower fluences near ablation threshold showed evidence of single and multiple layer removal. Samples irradiated by nanosecond lasers of comparable fluence showed evidence of extensive intermixing with negligible material removal. Heat dissipation rates were observed to be an important factor for pulsed laser ignition of reactive multilayers based on numerical analysis and comparative ignition studies of three bimetallic systems.Ph.D.Applied SciencesMaterials scienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126538/2/3253383.pd

    Investigating Defect Contrast in Ge X

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    Defect Analysis in La 0.7

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