Enhanced persistent photoconductivity in $\delta$-doped LaAlO$_{3}$/SrTiO$_{3}$ heterostructures

We report the effect of $\delta$-doping at LaAlO$_{3}$/SrTiO$_{3}$ interface with LaMnO$_{3}$ 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 LaAlO$_{3}$/SrTiO$_{3}$ interface from metal-to-insulator with increasing LaMnO$_{3}$ 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 LaAlO$_{3}$/SrTiO$_{3}$, the photoconductivity appears to originate from inter-band transitions between Ti-derived $3d$ bands which are $e_{g}$ in character and O 2p - Ti $t_{2g}$ hybridized bands. The band structure changes significantly when fractional layers of LaMnO$_{3}$ are introduced. Here the Mn $e_{g}$ bands ($\approx1.5$ 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.