  La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films of different thicknesses, degrees of
CeO$_2$-phase segregation and oxygen deficiency, grown on SrTiO$_3$ single
crystal substrates, were comparatively investigated with respect to both their
spectral and temperature-dependent photoconductivity (PC) and their
magnetoresistance (MR) behaviour under photoexcitation. While as-grown films
were insensitive to optical excitation, oxygen reduction appeared to be an
effective way to decrease the film resistance, but the film thickness was found
to play a minor role. However, from the evaluation of the spectral behaviour of
the PC and the comparison of the MR of the LCeMO/substrate-samples with a bare
substrate under illumination we find that the photoconductivity data reflects
not only contributions from (i) photogenerated charge carriers in the film and
(ii) carriers injected from the photoconductive substrate (as concluded from
earlier works), but also (iii) a decisive parallel photoconduction in the
SrTiO$_3$ substrate. Furthermore -- also by analyzing the MR characteristics --
the unexpected occurence of a strong electroresistive effect in the sample with
the highest degree of CeO$_2$ segregation and oxygen deficiency could be
attributed to the electroresistance of the SrTiO$_3$ substrate as well. The
results suggest a critical reconsideration and possibly a reinterpretation of
several previous photoconductivity and electroresistance investigations of
manganite thin films on SrTiO$_3$.
