Large photoconductivity of oxygen-deficient La_0.7Ca_0.3MnO₃/SrTiO₃heterostructures

Abstract

The electrical resistance of stoichiometric and oxygen-deficient epitaxial 10 nm thick La(0.7)Ca(0.3)MnO(3) thin films on SrTiO(3) under photoexcitation covering the visible to the ultraviolet range has been investigated systematically as a function of illumination intensity, wavelength and temperature. In contrast to as-prepared films, the oxygen-deficient samples exhibit large photoconductivity of several orders of magnitude at low temperatures. By our detailed comparative analysis of the electrical conductivity of the film/substrate heterostructure and the bare substrate we are able to elucidate contributions of both carrier generation in the film and carrier injection from the substrate to the observed effect.

Conductivity and magnetoresistance of La0.7Ce0.3MnO3−δthin films under photoexcitation

“…In La 0.7 Sr 0.3 MnO 3 films on SrTiO 3 , which were illuminated with a broad-band whitelight source and exhibited a negative PR, Katsu et al concluded an injection of optically generated electrons from the SrTiO 3 substrate into the hole-doped film followed by the recombination of both carrier types within the film leading to a resistance increase [19,20]. In our two previous works, where we observed positive PR in La 0.7 Ce 0.3 MnO 3−δ [7] and La 0.7 Ca 0.3 MnO 3−δ [9] films on SrTiO 3 , we gave a similar interpretation. By the comparative evaluation of the wavelength dependence of the PR and the surface photovoltage we had concluded that at least part of the photogenerated carriers must stem from interband transitions in the substrate or from carrier excitation from interface states, the latter explaining the PR below the SrTiO 3 band gap energy, and not solely from the manganite film.…”

“…[7,9,11,12], has to be questioned critically. In the future, it might be more reasonable to perform investigations of the intrinsic photoconductivity of manganites on either films grown on less photoresponsive substrates than SrTiO 3 (e.g., LaAlO 3 ) or on bulk crystals.…”

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

“…The fact that reduction and illumination can induce an electron-doped state in divalent-ion-doped manganites (whose growth is commonly easier) as well 23 questions the need of tetravalent-ion substitution in manganites.…”

Persistent photoconductivity in oxygen deficient YBa2Cu3O7−δ/La2/3Ca1/3MnO3−x superlattices grown by pulsed laser deposition

“…Here, however, for films with a thickness smaller than or comparable to the optical penetration depth (typically <100 nm for semimetallic manganites and YBa 2 Cu 3 O 7Àd (YBCO) as well), the influence due to charge injection from the substrate and/or the substrate/film interface has not been to be considered with a few exceptions. [12][13][14] Photoexcitation in underdoped cuprates, especially YBCO, shows enhancement of superconductivity as well as both transient (TPC) and persistent (PPC) photoconductivity and is most pronounced in the UV part of the spectrum. 15 The photoconductivity is intrinsic to the material and is observable in single crystals and thin films deposited onto various types of substrates (e.g., MgO, SrTiO 3 ).…”

“…17 In the case of manganites, the investigation of photoinduced phenomena is a relatively recent field and a plethora of photoinduced effects has been observed, ranging from an X-ray induced metal-insulator transition combined with a structural change in charge-ordered Pr 0.7 Ca 0.3 MnO 3 single crystals 18 to a reversible photoinduced switching between a charge ordered (CO)orbital ordered insulating (OOI) and a ferromagnetic metallic (FMM) state in Pr 0.55 (Ca 1−y Sr y ) 0.45 MnO 3 single crystals. 19 Although, no general understanding of the microscopic mechanisms of photoinduced phenomena in manganites is currently available, three main mechanisms should be distinguished as comprehensively reviewed by Beyreuther et al 13 First, in materials with a composition close to a bicritical region between the CO/OOI and the FMM phases photon induced excitations across the charge gap are the predominant mechanism for phase changes. Second, photoexcitation with photon energies above the polaron binding energy leads to a delocalization of charge carriers.…”