High-mobility two-dimensional hole gas at the SrTiO₃ interface formed by depositing an ultrathin metal film at room temperature

Despite intensive studies on the two-dimensional electron gas (2DEG) at the SrTiO₃ (STO) interface [1], forming a 2D hole gas (2DHG) at the STO interface is extremely difficult [2], although both are essential for the realization of high-speed oxide-based electronics. Here, we demonstrate a very simple method to realize a 2DHG with an ultrahigh mobility of 24,000 cm²V^–1s^–1 at an STO interface. The 2DHG is obtained by depositing a sub-nm-thick Fe layer (thickness t ≦ 0.2 nm) on STO substrates at room temperature in an ultrahigh vacuum chamber. The Fe layer is oxidized and becomes insulating amorphous FeO_x. Magnetotransport measurements reveal the existence of high-mobility carriers in the STO side, and the carrier type changes from a pure p-type (t ≦ 0.2 nm) to n-type (t > 0.3 nm) by varying the Fe thickness. In a p-type sample (t = 0.1 nm), the Shubnikov - de Haas oscillation is clearly observed in out-of-plane magnetic field but disappears in in-plane field. These results clearly demonstrate the 2D nature of the high-mobility hole carriers.
[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). [2] H. Lee et al., Nat. Mater. 17, 231 (2018).