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Description
Two dimensional electron gas (2DEG) refers to an electronic state in which carriers are confined at an interface or surface in two dimensions, exhibiting high mobility, strong correlation effects, and unique quantum transport properties that cannot be observed in conventional electron systems. The 2DEG associated with SrTiO3 was first reported in 2004 at the LaAlO3/SrTiO3 (LAO/STO) interface, providing a seminal example of how electrons can accumulate and become confined at an oxide interface. Subsequent studies revealed that 2DEG can also emerge at the surface of SrTiO3 (STO) itself under various conditions. STO is a prototypical perovskite oxide whose electronic properties strongly depend on surface orientation and termination. For instance, along the (001) orientation, alternating SrO and TiO2 layers yield a non-polar character, while along the (110) orientation, alternating SrTiO and O2 layers result in a polar nature. Such polar/non-polar characteristics criticlay affect charge redistribution and accumulation, thereby providing key insights into the formation of 2DEG.
In this work, we employ first-principles calculations to investigate the generation and migration of oxygen vacancies at STO surfaces. Our results demonstrate that oxygen vacancies are initially created at the surface and subsequently migrate into subsurface regions, where they stabilize the formation of 2DEG. Notably, this mechanism occurs universally, independent of surface orientation or termination, including their polar or non-polar nature. These findings elucidate the microscopic origin of 2DEG formation in SrTiO3 and provide new physical insights into electron correlation and quantum transport phenomena at oxide surfaces and interfaces.
