Claudia Wu

Microstructuring of silicon with femtosecond laser pulses

We report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in 500-Torr SF6 or Cl2. The spikes are up to 40-μm tall, and taper to about 1-μm diam at the tip. Irradiation of silicon surfaces in N2, Ne, or vacuum creates structured surfaces, but does not create sharp conical spikes.

Near-unity below-band-gap absorption by microstructured silicon

We increased the absorptance of light by silicon to approximately 90% from the near ultraviolet (0.25 μm) to the near infrared (2.5 μm) by surface microstructuring using laser-chemical etching. The remarkable absorptance most likely comes from a high density of impurities and structural defects in the silicon lattice, enhanced by surface texturing. Microstructured avalanche photodiodes show significant enhancement of below-band-gap photocurrent generation at 1.06 and 1.31 μm, indicating promise for use in infrared photodetectors.

Visible luminescence from silicon surfaces microstructured in air

We report visible luminescence from SiOx formed by microstructuring silicon surfaces with femtosecond laser pulses in air. Incorporation of oxygen into the silicon lattice occurs only where the laser beam strikes the surface. Laser microstructuring therefore offers the possibility of writing submicrometer luminescent features without lithographic masks. The amount of oxygen incorporated into the silicon surface depends on the laser fluence; the peak wavelength of the primary luminescence band varies between 540 and 630 nm and depends on the number of laser shots. Upon annealing, the intensity of the primary luminescence band increases significantly without any change in the luminescence peak wavelength, suggesting that the luminescence comes from defects rather than quantum confinement.

Surface femtochemistry of CO/O2/Pt(111): The importance of nonthermalized substrate electrons

We studied the surface femtochemistry of CO/O2/Pt(111) induced with 0.3 ps laser pulses over a wide range of wavelength and fluence. Below 10 μJ/mm2, the yields depend linearly on fluence. Above 10 μJ/mm2, the yields scale nonlinearly in the fluence. From the dependence of the yields on wavelength, we determine that the nonlinear surface femtochemistry is influenced by nonthermal substrate electrons.

Field emission from silicon microstructures formed by femtosecond laser assisted etching

Summary form only given. Field-emission arrays have been extensively investigated for device applications such as flat panel displays, electron multipliers and microelectronics. The use of silicon structures as emitters is especially attractive due to the low cost and availability of silicon. Previously, our group reported the formation of sharp, micron-sized conical structures in quasi-ordered arrays after irradiating a silicon surface with hundreds of femtosecond-laser pulses in an atmosphere of SF/sub 6/. These conical microstructures exhibit sharp tips with a radius of curvature /spl sim/0.25 /spl mu/m and a subtended angle of less than 20/spl deg/. Emission currents suitable for applications and comparable with current standards are achieved with no further processing beyond the laser irradiation required to form our microstructures. We report on the field emission properties of these conical microstructures through analysis of the current-voltage characteristics and the Fowler-Nordheim plots obtained for various anodecathode separations.

REACTION PATHWAYS IN SURFACE FEMTOCHEMISTRY : ROUTES TO DESORPTION AND REACTION IN CO/O2/PT(111)

Abstract We present new data that address the chemical pathway to desorption of O 2 and formation of CO 2 in CO/O 2 /Pt(111) induced by subpicosecond laser pulses. The data show conclusively that the O 2 desorbs molecularly. We also find that if the CO oxidizes by an atomic pathway then the capture of oxygen atoms by the CO is highly efficient; if the CO oxidizes by a molecular pathway then the oxygen atoms in the CO 3 ∗ transition state are inequivalent.

Novel conical microstructures created in silicon with femtosecond laser pulses

Summary form only given. Silicon wafers are mounted inside a vacuum chamber backfilled with gas and then irradiated with 800-nm pulses from a regeneratively amplified Ti:sapphire laser. Following laser treatment, the samples are removed from the chamber and analyzed with a scanning electron microscope (SEM).

Femtosecond laser activation of surface reactions

Laser induced formation of CO2 and desorption of O2 are initiated with femtosecond and picosecond laser excitation of a Pt(111) surface prepared with coadsorbed CO and O2 at 90 K. The nonlinear fluence dependent reaction yields were measured for 267, 400, and 800 nm wavelengths, and for pulse durations from 80 fs to 3.6 ps. Two-pulse correlation experiments measuring total O2 desorption yield versus time delay between 80 fs pulses show a 0.9 ps HWHM central peak and a slower 0.1 ns time-scale. At 267 nm the relative yields of O2 and CO2 are found to depend on fluence. Comparison of results at different wavelengths and pulsewidths shows that nonthermalized surface electrons play a role in the laser-induced surface chemistry.