Electrostatic control of insulating SrTiO3 surface: insulator-metal transition, negative capacitance, multi-band conduction, Kondo effect, and more.

Most of the people in the world have not given it a thought that our fully electronics-depended society is now really confronted a fatal problem called “miniaturisation limit” of transistors. By the time of Tokyo Olympic in 2020, it is anticipated a typical dimension of the transistor would reach 10 nm. Then, the channel contains only about 10 chemical dopants (origin of carriers), and the switching energy becomes smaller than the thermal noise limit (∼100 kBT ); “on/off” states are no more distinguished. The saviour from such a looming crisis is nothing but to use a chemical-doping-free FET, and to develop such electronics beyond the present Si ones is indeed an urgent challenge for condensed matter physicists.

In this talk, we demonstrate an ex- ample:   by  applying  an  electric  field, a quite insulating ‘non-doped’ SrTiO3 shows a two dimensional (2D) insulator- metal transition at the surface, and it actu- ally works as an excellent field effect tran- sistor (FET) with sufficiently good sub-threshold swing (∼ 170 mV/decade) and very large carrier mobility (∼ 10 cm2/Vs). 

SrTiO3 is well-known for its defect-prone surface, but by inserting a thin (6 nm) organic insulator Parylene-C between thesurface and the high-k gate insulator HfO2, we can avoid any damages on the surface during the device fabrication as well as during the application of the gate voltage. This clean interface between Parylene-C and SrTiO3 is not only promising for the electronic application of the SrTiO3-FET but also unexpected intriguing electronic properties of the SrTiO3 surface are getting revealed one after another.  We can accumulate 2D carriers more than 1014 cm2, which is surprisingly beyond the value expected from the capacitance of the gate insulator (the phenomenon is called “negative capacitance”[1]. By increasing the gate voltage VG, the channel resistance decreases, and the tem- perature dependence exhibits a clear insulator to metal transition with the boundary at the quantum resistance. Metallic channel shows the Kondo effect at low temperatures, as well as an anomaly of the Hall effect without hysteresis is accompanied. All these results suggest consistently the lifting of the three-fold degenerate t2g band, which is most probably caused by a strong Rashba effect at the surface. We will explain these phenomena with the experimental data, and discuss on the rich physics behind them.

[1] N. Kumar, A. Kitoh and I. H. Inoue, Scientific Reports 6, 25789 (2016).

Isao H. Inoue received BSc, MSc, and DSc degrees in Physics from the University of Tokyo in 1990, 1992 and 1998, respectively. He became a tenure researcher of the Electrotechnical Laboratory (ETL) in 1992 and a senior researcher in 1999. From 1999 to 2001, he was a visiting scholar at Cavendish Laboratory, University of Cambridge. In 2001, ETL was re-organized to the National Institute of Advanced In- dustrial Science and Technology (AIST); since then, he has been a senior researcher of AIST. He studied in a wide range of research field: from the high-energy spec- troscopies and fermiology of strongly correlated materials to the development of the Mott transistor, ReRAM, and other electronic devices, which utilise functional oxides, where electron correlations play a crucial role.