High‐Density Carrier Accumulation in ZnO Field‐Effect Transistors Gated by Electric Double Layers of Ionic Liquids

Abstract

Very recently, electric‐field‐induced superconductivity in an insulator was realized by tuning charge carrier to a high density level (1 × 1014 cm−2). To increase the maximum attainable carrier density for electrostatic tuning of electronic states in semiconductor field‐effect transistors is a hot issue but a big challenge. Here, ultrahigh density carrier accumulation is reported, in particular at low temperature, in a ZnO field‐effect transistor gated by electric double layers of ionic liquid (IL). This transistor, called an electric double layer transistor (EDLT), is found to exhibit very high transconductance and an ultrahigh carrier density in a fast, reversible, and reproducible manner. The room temperature capacitance of EDLTs is found to be as large as 34 µF cm−2, deduced from Hall‐effect measurements, and is mainly responsible for the carrier density modulation in a very wide range. Importantly, the IL dielectric, with a supercooling property, is found to have charge‐accumulation capability even at low temperatures, reaching an ultrahigh carrier density of 8×1014 cm−2 at 220 K and maintaining a density of 5.5×1014 cm−2 at 1.8 K. This high carrier density of EDLTs is of great importance not only in practical device applications but also in fundamental research; for example, in the search for novel electronic phenomena, such as superconductivity, in oxide systems.

Electric-Field-Induced Superconductivity on an Organic/Oxide Interface

“…11,12) From the capacitance and Hall measurements of ZnO and an ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium-bis(trifluoromethanesulfonyl) imide (DEME-TFSI) system, the maximum charge carrier density in a semiconductor reaches around 10 15 cm À2 . 27) Figures 3(a) and 3(b) show a schematic diagram and an image of an electric double layer transistor device fabricated on an insulating substrate. The source/drain and many voltage probes are connected to the channel area, which is in the center of the image covered by electrolyte.…”

Electric field effect on superconductivity in La2−xSrxCuO4

“…Electron accumulation using polymer electrolytes (a salt dissolved in a polymer such as polyethylene glycol) have resulted 10 in surface carrier densities as high as 10 14 cm À2 , and even higher density, 8 Â 10 14 cm À2 , has been achieved using an ionic liquid as the electrolyte. 13 This technique has recently been employed to induce superconductivity in an insulator (SrTiO 3 ) as well as a semiconductor (ZrNCl) 10,11,14 It enabled us 1 to induce a substantial modification of the carrier density of cuprates, and hence in this paper we adopt the electrolyte route to study the field effect in HTS materials.…”

Large electron concentration modulation using capacitance enhancement in SrTiO3/SmTiO3 Fin-field effect transistors

“…A major challenge in realizing all these applications is the field-effect modulation of extreme concentrations of mobile electrons, intermediate to the concentrations in semiconductors and metals. Electron concentration modulation approaching ~10 15 cm -2 has been achieved using ionic liquids in electric double layer transistors (EDLTs) [10][11][12]. Gating of phase transitions in complex oxides has also been demonstrated by this approach [13][14][15][16].…”