Abstract
Two-dimensional electron gas or hole gas (2DEG or 2DHG) and their functionalities at artificial heterostructure interfaces have attracted extensive attention in recent years. Many theoretical calculations and recent experimental studies have shown the formation of alternating 2DEG and 2DHG at ferroelectric/insulator interfaces, such as BiFeO3/TbScO3, depending on the different polarization states. However, a direct observation based on the local charge distribution at the BiFeO3/TbScO3 interface has yet to be explored. Herein we demonstrate the direct observation of 2DHG and 2DEG at BiFeO3/TbScO3 interface using four-dimensional scanning transmission electron microscopy and Bader charge analysis. The results show that the measured charge state of each Fe/O columns at the interface undergoes a significant increase/reduction for the polarization state pointing away/toward the interface, indicating the existence of 2DHG/2DEG. This method opens up a path of directly observing charge at atomic scale and provides new insights into the design of future electronic nanodevices.
Highlights
Understanding the atomic-scale interface properties in heterostructures is critical for the engineering of oxide-based electronic devices
The combination of high-quality thin film epitaxy and atomicresolution scanning transmission electron microscopy (STEM) has led to a deep understanding of a variety of novel phenomena emergent from interfaces, such as the twodimensional electron gas (2DEG) at the LaAlO3/SrTiO3 (LAO/STO) interface.[1,2]
When the polarization points toward the interface, additional electrons will move toward the interface to screen the positive polarization bound charge, resulting in an increased charge density of electrons
Summary
Understanding the atomic-scale interface properties in heterostructures is critical for the engineering of oxide-based electronic devices. First-principles calculations have predicted this phenomenon in KNbO3/ATiO3 (A = Sr, Ba, Pb)[4] and BiFeO3/SrTiO3 (BFO/STO).[5] If all sub-layers are charge-neutral, the spontaneous polarization alone will generate a free carrier gas at the interface where free holes or electrons aggregate to screen the negative or positive polarization bound charge, forming two-dimensional hole gas (2DHG) or 2DEG, respectively. In this case, the carrier gas at the interface can be switched between 2DHG and 2DEG by switching the polarization in the ferroelectric film. Because the DFB is always pointing toward the center of the oxygen octahedra, it is opposite to the polarization vector P that is in the same plane
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