Advances in DC photocathode electron guns Page: 4 of 10
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the laser spot and the center of the wafer. Thus the only realistic way to reduce the
number of ions (and the damage) is to reduce the pressure, since introducing any sort
of fields so close to the cathode to deflect the ions away from the surface would be
impractical and undesirable.
The next question is what is the damage mechanism for reducing the quantum
efficiency? One possibility is that the cesium and fluorine atoms deposited on the
surface to produce the NEA conditions are sputtered away by the ions. In the limited
operation history of the polarized gun, adding more cesium improves the QE in the
damaged region and the wafer as a whole, but it never fully recovers. This implies
that a substantial portion of the damage occurs below the surface. Calculation of
sputter yields for protons on GaAs using the program SRIM  indicate at most 1 or 2
sputtered atoms per 100 incident protons are produced, which may account for some
of the photoresponse reduction.
A more likely candidate is physical damage to the bulk crystal from the implanted
ions. Fortunately, since implantation of H and H2t in GaAs is a commonly used
technique to induce damage to alter its electrical and optical properties, a rich literature
exists. One important parameter is the range of the ion in the material: the range is the
average distance the ion will travel before it stops. Atomic displacements near the end
of the ion range give rise to point defects that absorb light and trap carriers . For
protons implanted in GaAs, the range varies roughly linearly from 0 (at 0 eV) to 0.8
gim (at 100 keV) with a spread of 10 to 20% in the average range (calculated using
SRIM). These numbers have been experimentally verified  and show that most of
the damage occurs when the ion stops, so a 100 keV proton will do most of its damage
0.8 0.1 tm deep in the crystal. Using the SRIM program, other quantities of interest
can be calculated and are summarized in table 1 along with other known information.
V0 41 -
E P(H2) = 10 (k g rrr ( r
0 20 40 6o so too -15 -14 i13 -12 -11 *10 -9
Ehcetron Energy (We) lop Pressure (Torr)
Figure 3. The integrated ion production rate per Coulomb of incident electrons as a function of energy
for a given pressure and total rate for all energies versus pressure.
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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. (digital.library.unt.edu/ark:/67531/metadc703754/m1/4/: accessed September 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.