Large photoconductivity and light-induced recovery of the insulator-metal transition in ultrathinLa0.7Ce0.3MnO3−δ

Abstract

Tetravalent-ion-doped lanthanum manganite films typically suffer from overoxygenation in the as-prepared state, which in turn leads to an effective hole doping instead of the nominal electron doping. This problem can be overcome by post-deposition annealing in a reducing atmosphere, which, however, suppresses the phase transition from an insulating to a metallic phase at the magnetic ordering temperature so that the films are insulating in the whole temperature range. In the present work, reduced La 0.7 Ce 0.3 MnO 3−␦ thin films were investigated with respect to their transport characteristics under photoexcitation. While the films are insulating in the dark, even the exposure to diffuse daylight recovers the insulator-metal transition ͑IMT͒. Excitation with continuous visible laser light further decreases the resistance by up to seven orders of magnitude and shifts the IMT to higher temperatures. The spectral, temporal, intensity, and temperature dependences of the photoconductivity have been investigated. The results suggest that ͑i͒ the manganite film shows a light-induced IMT and large photoconductivity, ͑ii͒ the substrate has an influence on the photoconductivity ͑through carrier injection into the film and/or substrate photoconduction͒ that grows with decreasing wavelength of the light, and ͑iii͒ an electron-doped metal state might be present under photoexcitation.

The Mn2+/Mn3+state of La0.7Ce0.3MnO3by oxygen reduction and photodoping

“…To completely clarify which of the scenarios is the dominating one, a similar investigation of LCeMO on a nonphotoconductive substrate as well as a more extended study of the spectral dependence of the photoconductivity than in ref. 22 would be illuminative.…”

“…On the other hand, as discussed in our former XPS investigation 21 , oxygen reduction by heating in an ultrahigh-vacuum environment is a suitable way to drive the Mn valence towards 2 + /3+, which is, however, connected with a decisive resistance increase and the loss of an important functional property, namely the manganitetypical metal-insulator transition (MIT) 22 . As a fur- …”

Quantifying the electrical transport characteristics of electron-doped La0.7Ce0.3MnO3 thin films through hopping energies, Mn valence, and carrier localization length

“…[8,14,17], the transport measurements of the as-prepared samples were compared to the results of a 10-nm LCeMO thin film, referred to as sample A, which was already subject of these former studies.…”

“…[14] and refs. therein), while oxygen reduction by postdeposition annealing can indeed produce Mn valences below 3+, typically followed by the loss of the manganite-typical metal-insulator transition, producing films which are insulating across the whole temperature range [8]. A recent study by Middey et al [15] demonstrated that LCeMO thin films grown layer-by-layer with pulsed laser interval deposition showed already in the as-prepared state electron doping but remained insulating in the whole temperature range, too.…”

“…In the early years, fundamental issues such as the explanation of the relation between electrical transport and magnetic order in bulk crystals were in the focus [3,4,5]. Later, especially with the decisive progress in thin film preparation [6], a number of further intriguing issues were under debate, e.g., questions related to the functionalization of heterostructures and interfaces, electronic phase separation [7], or the response to light excitation [8].…”