  Electron beam induced current (EBIC) is a powerful characterization technique
which offers the high spatial resolution needed to study polycrystalline solar
cells. Current models of EBIC assume that excitations in the $p$-$n$ junction
depletion region result in perfect charge collection efficiency. However we
find that in CdTe and Si samples prepared by focused ion beam (FIB) milling,
there is a reduced and nonuniform EBIC lineshape for excitations in the
depletion region. Motivated by this, we present a model of the EBIC response
for excitations in the depletion region which includes the effects of surface
recombination from both charge-neutral and charged surfaces. For neutral
surfaces we present a simple analytical formula which describes the numerical
data well, while the charged surface response depends qualitatively on the
location of the surface Fermi level relative to the bulk Fermi level. We find
the experimental data on FIB-prepared Si solar cells is most consistent with a
charged surface, and discuss the implications for EBIC experiments on
polycrystalline materials.
