Creation of nanostructures and porous solids by the application of etching (both chemical and laser assisted) and growth. Femtosecond and nanosecond laser-assisted etching of Si to form arrays of nano- and meso-scopic pillars.
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Studies on surface texturing by chemically enhanced laser ablation in a variety of materials, particularly silicon and
germanium are reported. The materials are exposed either to femtosecond or nanosecond laser irradiation in a variety
of vacuum or gaseous environments including air, He, sulfur hexafluoride (SF6) or hydrogen chloride (HCl). The
dynamics of pillar formation are elucidated and it is shown that the mechanisms are very different in these two pulse
length regimes. Surface texturing responds to the combined effects of laser assisted chemical etching and laser
ablation. Various processing steps either before or after laser irradiation allow us to modify the nature of the pillars
that are formed. In this way we can make ordered arrays that extend over ≥1 cm2 in just a few minutes of laser
exposure. Post-laser processing wet etching can produce Si pillars that are over 50 &mgr;m long with tips that are only 10
nm across as well as macroporous silicon with crystallographically defined pores. A process we call solidification
driven extrusion creates nanoscale spikes atop the pillars under certain circumstances - a process that is more
prevalent for Ge than Si. Pillar-covered surfaces of Si and Ge are black; that is, they exhibit very low reflectivity. For
Si this low reflectivity extends to wavelengths far below the band gap raising the possibility that we may be able to
make other transparent materials highly absorptive by laser texturing.
Sum Frequency Generation (SFG) spectra of nanocrystalline porous silicon (por-Si) exposed to different chemical treatments are studied. We report the first SFG studies of por-Si in direct contact with a liquid. SFG is excited by a regeneratively amplified Ti:sapphire system (787 nm, 120 fs, 1 kHz). The sum frequency is generated by combining this light with infrared that is generated with an optical parametric amplifier (OPA) that delivers 100-200 μJ pulses at 1370-1770 nm. Por-Si is made from a 10-20 Ω cm p-type Si(001) wafer. Comparisons are made to planar Si(001) as well as GaAs(001). First principle electronic structure theory based on density functional theory (DFT) is used to study the adsorption and optical response functions from the system of ethanol molecule adsorbed on Si(001) and Si(111) surfaces. Equilibrium atomic geometries are obtained through molecular dynamics and total energy minimization methods. Electron energy structure and optical properties are calculated using generalized gradient approximation method with ab initio pseudopotentials. Predicted differential optical absorption spectra for chemisorbed Si(001) and Si(111) surfaces are analyzed in comparison with SFG data measured on differently treated porous silicon. Substantial modifications of the surface atomic and electron energy structures of silicon surfaces due to chemisorption are shown to provide the dominant contributions to the SFG response.
The laser-induced desorption of ammonia from GaAs(100)-(4X6) and Cu(111) has been studied using (2+1)-photon resonance-enhanced multiphoton ionization as a state specific probe. A surprisingly marked isotope effect (NH3/ND3) in the photodesorption cross section of ammonia from GaAs(100) had indicated that internal degrees of freedom play a crucial role in the desorption dynamics. We find that NH3 desorbs with a mean translational energy, (Etrans), of 300 K and 600 K for GaAs(100) and Cu(111), respectively. The average extent of vibrational excitation is about 1000 K. The rotational temperatures are 520 and 170 K. These results are interpreted in terms of a direct mechanism for NH3 desorption in which a reaction path which initially follows the internal v2 coordinate and then bends towards desorption is dominating. A marked unequal population of the inversion symmetry doublet states is observed.