Advances in DC photocathode electron guns Page: 3 of 10
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are accelerated straight back (due to their much larger mass) towards the wafer where
they cause crystal damage. The plot in the right side of figure 1 shows the QE profile
over part of a wafer (from this lab) from which over 20 C was extracted. The laser
beam was located at x=0 mm and y=2.3 mm and the QE trough from there to the
center of the wafer illustrates the effect of the crystal damage.
SIMPLE MODEL FOR ION DAMAGE
The ionization rate as a function of kinetic energy, R(E), can be calculated
assuming that the residual gas in the chamber is molecular hydrogen at some pressure
R(E) = Ia-(E)pAz = Iop-AE
p(m3) = 3.54 x 1022P(Torr)
where a is the ionization cross section for & on H2 (see figure 2), I is the beam current,
p is the H2 gas density, and AU/Az is the average accelerating gradient in the anode-
cathode region. From this one can calculate the total integrated ionization rate as a
function of pressure or the total rate as a function of E for a given pressure (see figure
For the present geometry, the anode and cathode are separated by about 60 mm
with an accelerating gradient of 1 kV/mm at the wafer. From the plot of integrated
rate versus energy, 50% of the ions are generated between 0 and 8 kV, or within about
8 mm of the wafer surface. These ions will be accelerated straight back and implanted
into the GaAs while the higher energy ions will be distributed between the location of
E 0.2 -
0 So. 1000 1s20 2000 2500
ElwOn Enmrgy (*V}
0 20 40 60 80 100
Electron Energy (keV)
Figure 2. Cross-section for H2 ionization by electrons .
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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/m1/3/: accessed March 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.