Schematic of neutral atom detection system with electrodes to remove emitted ions and apertures to select forward directed atoms.
Zn° intensity vs. pulse number at a laser fluence of 200 mJ/cm2, showing a decrease in intensity with exposure. Although low, the intensity levels off at a non-zero value, suggesting that the laser continues to make the defects necessary for Zn° emission.
(a)-(c) Examples of low fluence Zn° TOF curves at 150, 200, and 250 mJ/cm2. Curve fits for transforming them to energy distributions are shown. (d) The three normalized TOF curves superimposed, showing their similar shapes. (e)-(g) The corresponding energy distributions, n(E), and (h) the three normalized n(E) curves are superimposed. Again, note the near coincidence.
The relative intensities of atomic Zn ° emission at the same fluence (150 mJ/cm2) for an untreated sample (pristine) and three mechanically treated samples with a diamond scribe: single scratched, indented, and abraded (many scratches over the irradiated spot).
Zn° TOF curves acquired at a fluence of 300 mJ/cm2; shown are data due to exposure of two pristine surfaces (indistinguishable) and the TOF after 10 000 pulses. (These curves are not normalized; changes in peak height are directly proportional to intensity changes.)
Zn ° time-of-flight and energy distributions for fluences between 300 and 700 mJ/cm2. Note the shift of the leading edge of the TOF curves to shorter times and the corresponding broadening of the n(E) curves to higher energies as the laser fluence is increased.
Zn° time-of-flight and energy distributions at high fluences (0.9–1.5 J/cm2)—still below threshold for plume formation. Note the energy scale has been expanded to show the broadening of the n(E) curves to high kinetic energies at the highest laser fluence. (g) and (h) show the normalized curves superimposed for comparison.
Normalized Zn ° energy distributions at laser fluences of 0.25, 0.55, and 1.5 J/cm2. Significant growth of higher energy components with fluence is evident.
O° time-of-flight and energy distributions over a range of fluences (0.3–1.0 J/cm2)—still below threshold for plume formation. Similar to Zn°, we see an increase in kinetic energy of the O atoms with fluence.
Log-log plots of the emission intensities of neutral Zn (▲) and O (•) atoms during 193 nm irradiation of single-crystal ZnO. The relative intensities of these two data sets can be compared. The log-log plot of the renormalized crystal etch rate data is shown (◼) for comparison.
Images of irradiated ZnO crystals at 193 nm. (a) SEM image which shows an etch pit after 3000 pulses at a fluence of 650 mJ/cm2; etch rate ∼4 nm/pulse; (b) SEM image of a hole through an ∼90 μm thick sample produced by 3000 pulses at a fluence of 2 J/cm2 (etch rate ∼30 nm/pulse). (c) Optical image of a hole drilled through another sample at 1 J/cm2 showing clean etching and no melting.
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