Lee Seung Ran, Baasandorj Lkhagvasuren, Chang Jung Won, Hwang In Woong, Kim Jeong Rae, Kim Jeong-Gyu, Ko Kyung-Tae, Shim Seung Bo, Choi Min Woo, You Mujin, Yang Chan-Ho, Kim Jinhee, Song Jonghyun
Korea Research Institute of Standards and Science , Daejeon 34113 , Republic of Korea.
Center for Correlated Electron Systems, Institute for Basic Science (IBS) & Department of Physics and Astronomy , Seoul National University , Seoul 08826 , Republic of Korea.
Nano Lett. 2019 Apr 10;19(4):2243-2250. doi: 10.1021/acs.nanolett.8b04326. Epub 2019 Mar 14.
The requirements of multifunctionality in thin-film systems have led to the discovery of unique physical properties and degrees of freedom, which exist only in film forms. With progress in growth techniques, one can decrease the film thickness to the scale of a few nanometers (∼nm), where its unique physical properties are still pronounced. Among advanced ultrathin film systems, ferroelectrics have generated tremendous interest. As a prototype ferroelectric, the electrical properties of BaTiO (BTO) films have been extensively studied, and it has been theoretically predicted that ferroelectricity sustains down to ∼nm thick films. However, efforts toward determining the minimum thickness for ferroelectric films have been hindered by practical issues surrounding large leakage currents. In this study, we used ∼nm thick BTO films, exhibiting semiconducting characteristics, grown on a LaAlO/SrTiO (LAO/STO) heterostructure. In particular, we utilized two-dimensional electron gas at the LAO/STO heterointerface as the bottom electrode in these capacitor junctions. We demonstrate that the BTO film exhibits ferroelectricity at room temperature, even when it is only ∼2 unit-cells thick, and the total thickness of the capacitor junction can be reduced to less than ∼4 nm. Observation of ferroelectricity in ultrathin semiconducting films and the resulting shrunken capacitor thickness will expand the applicability of ferroelectrics in the next generation of functional devices.
薄膜系统对多功能性的要求促使人们发现了仅以薄膜形式存在的独特物理性质和自由度。随着生长技术的进步，可以将薄膜厚度减小到几纳米（nm）的尺度，此时其独特的物理性质仍然很显著。在先进的超薄膜系统中，铁电体引起了极大的关注。作为典型的铁电体，BaTiO（BTO）薄膜的电学性质已被广泛研究，并且从理论上预测铁电性在厚度低至nm的薄膜中依然存在。然而，围绕大漏电流的实际问题阻碍了确定铁电薄膜最小厚度的研究工作。在本研究中，我们使用了生长在LaAlO/SrTiO（LAO/STO）异质结构上、表现出半导体特性的nm厚BTO薄膜。特别是，我们利用LAO/STO异质界面处的二维电子气作为这些电容器结的底部电极。我们证明，即使BTO薄膜仅有约2个晶胞厚，它在室温下仍表现出铁电性，并且电容器结的总厚度可以减小到小于4nm。在超薄半导体薄膜中观察到铁电性以及由此导致的电容器厚度缩小，将扩大铁电体在下一代功能器件中的适用性。
