Enhanced persistent photoconductivity in δ-doped LaAlO₃/SrTiO₃ heterostructures

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We report the effect of δ-doping at LaAlO₃/SrTiO₃ interface with LaMnO₃ monolayers on the photoconducting (PC) state. The PC is realized by exposing the samples to broad band optical radiation of a quartz lamp and 325 and 441 nm lines of a He-Cd laser. Along with the significant modification in electrical transport which drives the pure LaAlO₃/SrTiO₃ interface from metal-to-insulator with increasing LaMnO₃ sub-monolayer thickness, we also observe an enhancement in the photo-response and relaxation time constant. Possible scenario for the PC based on defect-clusters, random potential fluctuations and large lattice relaxation models have been discussed. For pure LaAlO₃/SrTiO₃, the photoconductivity appears to originate from inter-band transitions between Ti-derived 3d bands which are eg in character and O 2p - Ti t₂g hybridized bands. The band structure changes significantly when fractional layers of LaMnO₃ are introduced. Here the Mn eg bands (≈1.5 eV above the Fermi energy) within the photo-conducting gap lead to a reduction in the photo-excitation energy and a gain in overall photoconductivity.