Yusuf Emirov

Ultrafast lasers and solids in highly excited states: results of hydrodynamics and molecular dynamics simulations

Action of ultrafast optical and X-ray lasers on metals is considered. It is known that under certain conditions surface structures appear as result of irradiation. Generation of nano-structures is usually associated with excitation of surface plasmons. But often structures do not have forms of ripples, and their spacial scales are order of magnitude less than optical wavelength. In the paper full description of surface nano-structures is given for the case of single shot laser action onto well polished boundaries. Plasmon effects are insignificant for this case and also for X-ray pulses. It is shown that structures are formed after laser illumination in a process of mechanical spallation of ultrathin surface layer of molten metal. Spallation is accompanied by a strong foaming of melt, breaking of foam, and freezing of foam remnants. Those remnants form chaotic nano-structures observed in experiments.

Surface nano-structuring produced by spallation of metal irradiated by an ultrashort laser pulse

It is known that under certain conditions the complex surface nano-structures are formed after irradiation on metals by ultrashort optical and X-ray laser pulses. In the paper the mechanism of formation and final geometry of such surface structures are discussed for the case of single pulse acting on a well-polished metal surface. The typical surface structures observed in our experiments and simulations are different from well-known ripples composing a regular pattern generated by excitation of surface plasmons. By contrast with the plasmon mechanism, the observed structures have spacial scales which are order of magnitude less than the used optical laser wavelengths. We demonstrate that such structures are formed after laser irradiation due to the thermomechanical spallation of ultrathin surface layer of melt, rather than the plasmon effects, which are found to be insignificant in given conditions of a single shot and initially smooth surface. Spallation is accompanied by a strong foaming of melt followed by breaking of the foam. After several nanoseconds the foam remnants freeze up with formation of complex nano-structures on a target surface.

A Robust Process for Ion Implant Annealing of SiC in a Low-Pressure Silane Ambient

High-dose Al implants in n-type epitaxial layers have been successfully annealed at 1600°C without any evidence of step bunching. Anneals were conducted in a silane ambient and at a process pressure of 150 Torr. Silane, 3% premixed in 97% UHP Ar, was further diluted in a 6 slm Ar carrier gas and introduced into a CVD reactor where the sample was heated via RF induction. A 30 minute anneal was performed followed by a purge in Ar at which time the RF power was switched off. The samples were then studied via plan-view secondary electron microscopy (SEM) and atomic force microscopy (AFM). The resulting surface morphology was step- free and flat.

Making carbon nanotube probes for high aspect ratio scanning probe metrology

Carbon nanotubes (CNT) have exceptional mechanical strength at small diameters needed for measuring high aspect ratio features. Manually attached carbon nanotube atomic force microscopy probes have demonstrated exceptional longevity. Unfortunately, due to the manual attachment process, and the usually arbitrary diameter and length of the used CNT, such probes are not suitable for high aspect ratio critical dimension metrology (CDM). For reproducible and accurate CDM measurements precisely defined CNT probes are necessary. We are reporting about the progress made growing carbon nanotubes (CNT) directly on top of standard Si probes. The goal is to produce well-defined long lasting probes for CDM measurements in the <100 nm pitch range. Our efforts currently focus on manufacturing precisely aligned CNT having defined locations, diameters and lengths. This is accomplished by using plasma assisted chemical vapor deposition in combination with focused ion beam (FIB) patterned catalyst films. Our results demonstrate that it is possible to manufacture 1:10 aspect ratio CNT probes at <100 nm diameters.

Using carbon nanotube cantilevers in scanning probe metrology

Carbon nanotubes (CNT) are among the candidates for atomic force microscopy probes for use in high aspect ratio critical dimension metrology (CDM). Their mechanical strength at small diameters makes them ideal probes for narrow and deep features. The synthesis of CNT has been making great progress in recent years. The use of CNT in scanning probe microscopy, however, has been limited due to a number of problems. While the CNT probes generally appear to be long lasting, the manufacture of precisely aligned CNT of defined length, diameter and number of walls poses a number of challenges. Yet, such precisely defined CNT probes seem to be required if the cantilevers are to be used for CDM. Our result demonstrate, for example, that the attachment angle of CNT with respect to the cantilever beam is crucial for their application in CDM. We report about our efforts to overcome these problems by growing well-defined CNT on standard Si cantilevers using chemical vapor deposition in combination with focused ion-beam machining techniques.

Ultrashort laser-matter interaction at moderate intensities: two-temperature relaxation, foaming of stretched melt,and freezing of evolving nanostructures

Interaction of ultrashort laser pulse with metals is considered. Ultrafast heating in our range of absorbed fluences Fabs > 10 mJjcm2 transfers matter into two-temperature (2T) state and induces expressed thermomechani cal response. To analyze our case, where 2T, thermomechanical, and multidimensional (formation of surface structures) effects are significant, we use density functional theory (DFT), solutions of kinetic equations in τ- approximation, 2T-hydrodynamics, and molecular dynamics simulations. We have studied transition from light absorption in a skin layer to 2T state, and from 2T stage to hydrodynamical motions. We describe (i) formation of very peculiar (superelasticity) acoustic wave irradiated from the laser heated surface layer and (ii) rich com plex of surface phenomena including fast melting, nucleation of seed bubbles in hydrodynamically stretched fluid, evolution of vapor-liquid mixture into very spatially extended foam, mechanical breaking of liquid membranes in foam (foam disintegration), strong surface tension oscillations driven by breaking of membranes, non-equilibrium freezing of overcooled molten metals, transition to nano-domain solid, and formation of surface nanostructures.

Biologically Engineered Quantum Dots for Biomedical Applications

We report on a short-wavelength, “blue” spectral shift of the photoluminescence (PL) spectrum in CdSeTe/ZnS core/shell quantum dots (QDs) caused by bioconjugation with several monoclonal cancer related antibodies (ABs). Scanning PL spectroscopy was performed on samples dried on solid substrates at various temperatures. The influence of the AB chemical origin on the PL spectral shift was observed. The conjugation QD-AB reaction was confirmed using the agarose gel electrophoresis technique. The spectral shift is strongly increased and the process facilitated when the samples are dried above room temperature. The PL spectroscopic mapping revealed a profile of the PL spectral shift across the dried QD-AB spot. Transmission Electron Microscopy analyses of the samples were performed to reveal the shape and size of individual QDs. A mechanism of the “blue” shift is attributed to changes in the QD electronic energy levels caused by local stress field applied to the bio-conjugated QD.

Carbon nanotube atomic force microscopy cantilevers

We report about our progress in developing a process for the manufacture of carbon nanotube (CNT) atomic force microscopy (AFM) cantilevers. Due to their exceptional mechanical properties, CNT are among the most promising materials for high aspect ratio critical dimension metrology (CDM) AFM probes. Our goal is to produce well-defined long lasting CNT probes for CDM measurements in the <100 nm pitch range. Our efforts currently focus on manufacturing precisely aligned CNT having defined locations, diameters and lengths. The CNT are grown using plasma enhanced chemical vapor deposition (PECVD). The CNT growth process is enabled by the presence of a catalyst, which allows precise definition of the growth location. Experimental data from CNT grown on Si AFM probes and catalyst patterns prepared by focused ion beam (FIB) and electron beam lithography (EBL) is being shown. Furthermore, first results from scanning experiments with CNT-AFM cantilevers are shown.