Perovskite structured oxides have been intensively investigated over 50 years due to the discoveries of exotic physical phenomena as well as abundant functionalities for practical applications. In the past decades, the rapid advancements in heteroepitaxy growth techniques further enable controls of structure, composition, and functionality of oxide heterostructures. In this talk, I will discuss the atomic scale control of oxide thin films/surfaces and the resulting ferroelectric properties. By controlling thermodynamic variables, we were able to engineer the atomic termination sequences of ferroelectric BaTiO3 films. With the capacitor devices with symmetric and uniform interfacial termination sequences, we showed that the ferroelectric critical thickness can reach the theoretical limit value of 3.5 unit-cells [1]. With uniform TiO2 surface termination, we found an unexpected high tunneling conductance near the one-unit-cell-high terrace edges. Spatially-resolved current-voltage spectroscopies and first-principles calculations suggest that local electronic reconstruction near the terrace edge can reduce the effective tunneling barrier width and thus enhance the tunneling conductance [2]. We also fabricated high quality ferroelectric tunnel junction (FTJ) devices with a single BaTiO3 layer. We show that performance of such single layer FTJ devices can have two fundamental limits. To overcome such limits, we put extra SrTiO3 barrier and effectively modulate the barrier potential [3]. Our work demonstrates that atomic scale control of constituting layers is very important to control numerous functionalities of ferroelectric devices.
References
[1] Yeong Jae Shin, et al, Advanced Materials (10.1002/adma.201602795) (2017).
[2] Lingfei Wang, et al, submitted.
[3] Lingfei Wang, et al, Nano Letters 16, 3911 (2016).