Advances in DC photocathode electron guns Page: 5 of 10
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TABLE 1. Summary of ion backbombardment properties
Property Result Ref
Damage produces point defects - vacancies and deep Reduced carrier concentration 
trapping levels and light absorption
More vacancies produced as E increases Higher energy does more SRIM
Damage occurs at R(E) a(E) Higher energy produces deeper 
Sputter yield is low Some NEA surface modification SRIM
Damage for E > 500 eV does not disappear with QE will not completely recover 
annealing with annealing
H2 dissociates on impact to yield 2 protons each with Damage occurs closer to the 
1/2 the energy surface
Up to 20% of the ions are backscattered out of the Reduced number of damaging SRIM
crystal for energies below 10 keV ions for low energy
50% of the ion dose occurs within 8 mm of the surface Low energy damage occurs at Calc
the laser o W
TABLE 2. Experimental observations related to ion damage
" QE degradation at the laser spot
" QE degradation along a trough towards the wafer center
* Visible wafer damage indicates a change in index of refraction
* QE away from the laser spot degrades little or not at all
* Addition of Cs improves QE over the whole wafer, but recovery is not complete
" Annealing removes most of the damage. Dimples in the QE are still visible at the previously
These observations can be used to explain the effects seen in figure 4. A cross
section through the quantum efficiency holes' in the cathode is shown in figure 4(d).
The lines plotted on the data are inverted Gaussian distributions with a width equal to
the laser spot size convoluted with the probe laser size for the QE measurements. This
demonstrates that the low energy spatial ion distribution matches the initial laser size
within a few mm of the wafer surface where half of the ions are produced.
The shape of the QE trough (see figure 4(c)) is more difficult to explain. Using
results from PARMELA and the ionization cross section, the energy distribution of the
resulting ions along the electron path can be mapped onto the surface between the
location of the laser spot and the electrostatic center of the wafer. To determine the
relative amount of damage along the wafer, several factors contributing to the quantum
yield need to be considered. The rate calculations predict that most of the ions are
produced at ]low energy, but the flat QE trough indicates that the total damage as a
function of energy is roughly constant. By folding together the properties mentioned
in Table 1 along with the fact that the absorption depth for light in GaAs is -1 m, a
qualitative fit to the QE trough can be made (solid line in figure 4(c)). Further
calculations are underway to understand the damage mechanisms in a quantitative
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Dunham, Bruce M.; Heartmann, P.; Kazimi, Reza; Liu, Hongxiu; Poelker, B. M.; Price, J. S. et al. Advances in DC photocathode electron guns, article, July 1, 1998; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc703754/m1/5/: accessed March 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.