Origin of 2DEG Modulation at LaAlO₃/SrTiO₃ Interface Revealed by In-situ TEM Biasing

Since its discovery the two-dimensional electron gas (2DEG) forming at LaAlO₃/SrTiO₃ (LAO/STO) interface has attracted a lot of research interests of oxide community. Many groups have demonstrated the successful incorporation of 2DEG into field effect transistors or diodes where the field-induced charge modulation is central to the device operation. Although the device technology has been advanced rapidly, the fundamental understanding of the field-induced charge modulation of 2DEG is not fully understood. Here we show comprehensive in-situ analysis of LAO/STO system under electrical stimulus in TEM. Our in-situ inline electron holography successfully visualized the field-induced charge density modulation of 2DEG by the electric field applied normal to the LAO/STO interface. While in-situ electron energy loss spectroscopy (EELS) confirmed no measureable oxygen vacancy migration, atom-resolved STEM-HAADF imaging revealed the polar distortion of B-site atoms in both LAO and STO, which induces the ionic polarization gradient across the interface in the range of -18.6 μC/cm² to 18.7 μC/cm²depending on applied voltages. The field-induced polar distortion adds additional charges at the interface with amount depending on the difference between LAO and STO. Our in-situ TEM study, as opposed to the previously suggested mechanism based on oxygen vacancy migration, demonstrates that the evolution of polar distortion of LAO under field electric field is key to the charge modulation at LAO/STO interface.