1. Ionic liquid gating of VO2 with a hBN interfacial barrier
Jun Takigawa
Tanaka lab
22/05/2019

2. FET = field effect transistor
Application of VO2 to a FET using an ionic liquid with BN as a gate medium
FET = field effect transistor
The devices which can control the resistance between source-drain electrodes.
TiO2
VO2
Source
Drain
Gate
BN sheet
Ionic liquid
Structure of my proposed device

3. Electrical property of VO2
Application of VO2 to a FET using an ionic liquid with BN as a gate medium
Electrical property of VO2
source-drain current ISD
gate voltage VG
objective FET
conventional FET
R-T graph of VO2 [1]
Metal-insulator transition
5×1013 ~ 1014 cm-2
injection
Band structures of VO2 [2]; insulator phase (left) , metal phase (right).
Purpose: Realize gate-controlled MIT of VO2 by ionic-liquid gating

4. Property of ionic liquids
Application of VO2 to a FET using an ionic liquid with BN as a gate medium
Property of ionic liquids
5×1013 ~ 1014 cm-2 injection
VG = 1.0 [V]
Schematic diagrams of voltage-applied ionic liquids[3]; showing hole (left) and electron (right) injection.
Relation between carrier density and capacitance
Comparison of capacitance[3]
Ionic liquid
SiO2 (300 nm)
Capacitance
~10 mF/cm2
10 nF/cm2
Potential changes in ionic liquids (a) and solid-state dielectrics (b).[3]

5. insulator phase (left) , metal phase (right).
Introduction of the paper; 『Collective bulk carrier delocalization driven by electrostatic surface charge accumulation』[2]
Purpose of this paper
Fabrication and evaluation of VO2 channel FETs by using ionic liquids.
Induce the metal-insulator transition (MIT) of VO2 by electrical fields.
Band structure of VO2 ;
insulator phase (left) , metal phase (right).
R-T graph of VO2.
Schematic device structure.

6. Introduction of the paper; 『Collective bulk carrier delocalization driven by electrostatic surface charge accumulation』[2]
Result and discussion of this paper
A hysteresis loop was observed in Rs-VG graph.
This device can be used as a nonvolatile memory.
The resistivity of metal-phase VO2 was independent of the VO2 thickness.
Rs-VG graph.
If VO2 undergoes a MIT at the surface, the MIT is induced for the entire film.
Suspicion in this research
Why does the transport curve show the hysteresis loop?
Check the next paper.
Relationship between resistivity and thickness.

7. Introduction of the paper; 『Suppression of metal-insulator transition in VO2 by electric field-induced oxygen vacancy formation』[4]
Purpose of this paper
To clarify the underlying mechanism of ionic-liquid-gated VO2 FETs.
Suspicion that the resistance change in the ionic-liquid-gated VO2 FETs was due to the chemical reaction at the interface.
Transfer curve of the ionic-liquid-gated VO2 FET
Optical image of the ionic-liquid-gated VO2 FET

8. Then, how to tackle this problem.
Introduction of the paper; 『Suppression of metal-insulator transition in VO2 by electric field-induced oxygen vacancy formation』 [4]
Result and discussion of this paper
The resistance cannot be recovered even after the ionic liquid is removed.
The behavior of the gate-dependent (Fig. 1A) and deposition O2 pressure (Fig. 1B) R-T graphs were very similar.
Fig. 1: (A) Gate-dependent and (B) O2 pressure-dependent R-T curve
Fig. 2: (A) Gated- and (B) O2 pressure-controlled-XPS spectra of VO2
The peak shifts of VO2 in the XPS spectra shows the oxygen deficiencies in gated VO2 (Fig. 2).
Hysteresis loop is due to the migration of oxygen from the VO2 film into the ionic liquid.
Then, how to tackle this problem.

9. Introduction of the paper; 『A high-mobility electronic system at an electrolyte-gated oxide surface 』[5]
Purpose of this paper
Fabrication and evaluation of ionic-liquid-gated SrTiO3 protected with boron nitride (BN)
Investigation of the impermeability of BN.
Schematic device structure without an ionic liquid.
The crystal structure of boron nitride (BN)

10. Introduction of the paper; 『A high-mobility electronic system at an electrolyte-gated oxide surface 』[5]
Result and discussion of this paper
Single-layer BN (0.3 nm) could prevent the chemical reaction at the interface.
BN could also prevent the oxygen removal from SrTiO3.
The measured carrier density reached ~9×1013 cm-2 VG = 2.7 V)
This technique can be easily applied to other systems.
Following this technique, I will fabricate a VO2 FET with an ionic liquid and BN.

11. Application of VO2 to FET using ionic liquid with BN as a gate medium
Source
Drain
Gate
BN sheet
Ionic liquid
BN can prevent chemical reaction
Structure of my proposed device

12. Future plan Get used to the device fabrication process.
Pulsed laser deposition of VO2
Photolithography
Transfer of ultrathin BN on VO2
Micro manipulation of ionic liquids
Source
Drain
VO2
VO2
TiO2
TiO2
TiO2
Ionic liquid BN BN
Source
Drain
Source
Drain
VO2
VO2
TiO2
TiO2

13. Future plan Investigate the BN property VO2 VO2 VO2 VO2
How many BN layers are needed to protect VO2 from the chemical reaction?
With thinner BN, more carriers can be introduced in VO2.
How thin BN are practically available for the device fabrication?
For thinner BN, it is more difficult to handle.
VO2
VO2
VO2
VO2
Will investigate the R-T properties of VO2 with and without the BN overlayer under high-temperature O2 atmosphere and high-vacuum annealing.

14. Future plan
Investigate the transport properties of BN-protected VO2 under ionic liquid gating
Fabrication and evaluation of the device.
VG dependence of resistance.
Carrier density
Carrier mobility
Compare the properties of ionic-liquid-gated VO2 with and without BN

15. Summary
Step 1. Will investigate the barrier property of BN on VO2 the for oxidation and reduction.
Step 2. Investigation of the barrier property of BN on VO2 under ionic-liquid-gating.
Step 3. Pursue to obtain ultrathin BN for device fabrication.
Reference
[1] 安西勇人, 大阪大学　田中研究室, 修士論文 (2019).
[2] M. Nakano et al., Nature 487, 459 (2012).
[3] T. Fujimoto et al., Phys.Chem. Chem. Phys, 15, 8983 (2013).
[4] J. Jeong et al., Science 339, 1402–1405 (2013).
[5] P. Gallagher et al., Nat Comm, 6, 6437 (2015).

16. おしまい