  The nanoscale resistive switching characteristics of gallium phosphide (GaP)
thin films directly grown on Si are investigated as a function of incident
light. Firstly, as-grown GaP films show a high RON/ROFF (~10^4), shown to arise
from the formation of conductive channels along the grain boundaries. It is
proposed that point defects (most likely Ga interstitials) and structural
disorder at the grain boundaries provide the ideal environment to enable the
filamentary switching process. Next, we explored if such defects can give rise
to mid-gap states, and if so could they be activated by photonic excitation.
Both first-principles calculations as well as UV-vis and photoluminescence
spectroscopy strongly point to the possibility of mid-gap electronic states in
the polycrystalline GaP film. Photoconductive atomic force microscopy (phAFM),
a scanning probe technique, is used to image photocurrents generated as a
function of incident photon energy (ranging from sub band-gap to above
band-gap) on the GaP film surface. We observe photocurrents even for incident
photon energies lower than the band-gap, consistent with the presence of
mid-gap electronic states. Moreover the photocurrent magnitude is found to be
directly proportional to the incident photon energy with a concomitant decrease
in the filament resistance. This demonstrates GaP directly integrated on Si can
be a promising photonic resistive switching materials system.
