Giant photovoltaic effects driven by residual polar field within unit-cell-scale LaAlO(3) films on SrTiO(3)

For polar/nonpolar heterostructures, Maxwell's theory dictates that the electric potential in the polar components will increase divergently with the film thickness. For LaAlO(3)/SrTiO(3), a conceptually intriguing route, termed charge reconstruction, has been proposed to avert such “polar catastrophe”. The existence of a polar potential in LaAlO(3) is a prerequisite for the validity of the charge reconstruction picture, yet to date, its direct measurement remains a major challenge. Here we establish unambiguously the existence of the residual polar potential in ultrathin LaAlO(3) films on SrTiO(3), using a novel photovoltaic device design as an effective probe. The measured lower bound of the residual polar potential is 1.0 V. Such a direct observation of the giant residual polar potential within the unit-cell-scale LaAlO(3) films amounts to a definitive experimental evidence for the charge reconstruction picture, and also points to new technological significance of oxide heterostructures in photovoltaic and sensing devices with atomic-scale control.