Conductivity and magnetoresistance of La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films under photoexcitation

TL;DR

This study investigates how photoexcitation affects the electrical and magnetic properties of La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films, revealing substrate contributions and effects of oxygen deficiency, with implications for interpreting previous research.

Contribution

It uncovers the significant role of SrTiO$_3$ substrate photoconductivity and electroresistance in La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films under photoexcitation, challenging prior interpretations.

Findings

Photoconductivity involves contributions from film and substrate.

Oxygen reduction decreases film resistance.

Substrate effects influence magnetoresistance measurements.

Abstract

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$_3$.