Two-dimensionally confined electrons showing unusually large thermopower (S) have attracted attention as a potential approach for developing high performance thermoelectric materials. However, enhanced S has never been observed in electric field indu
ced two-dimensional electron gas (2DEG). Here we demonstrate electric field modulation of S for a field effect transistor (FET) fabricated on a SrTiO3 crystal using a water-infiltrated nanoporous glass as the gate insulator. An electric field application confined carrier electrons up to ~2E15 /cm^2 in an extremely thin (~2 nm) 2DEG. Unusually large enhancement of |S| was observed when the sheet carrier concentration exceeded 2.5E14 /cm^2, and it modulated from ~600 (~2E15 /cm^2) to ~950 {mu}V/K (~8E14 /cm^2), which were approximately five times larger than those of the bulk, clearly demonstrating that an electric field induced 2DEG provides unusually large enhancement of |S|.
Electric field modulation analysis of thermopower (S) - carrier concentration (n) relation of a bilayer laminate structure composed of a 1.5-nm thick conducting layer, probably TinO2n-1 (n=2, 3,...) Magneli phase, and rutile TiO2 was performed. The r
esults clearly showed that both the rutile TiO2 and the thin interfacial layer contribute to carrier transport: the rutile TiO2 bulk region (mobility mu~0.03 cm2V-1s-1) and the 1.5-nm thick interfacial layer (mu~0.3 cm2V-1s-1). The effective thickness of the interfacial layer, which was obtained from the S-n relation, was below ~ 3 nm, which agrees well with that of the TEM observation (~1.5 nm), clearly showing that electric field modulation measurement of S-n relation can effectively clarify the carrier transport properties of a bilayer laminate structure.
Here we demonstrate that water-infiltrated nanoporous glass electrically switches an oxide semiconductor from an insulator to metal. We fabricated the field effect transistor structure on an oxide semiconductor, SrTiO3, using 100%-water-infiltrated n
anoporous glass - amorphous 12CaO*7Al2O3 - as the gate insulator. For positive gate voltage, electron accumulation, water electrolysis and electrochemical reduction occur successively on the SrTiO3 surface at room temperature, leading to the formation of a thin (~3 nm) metal layer with an extremely high electron concentration of 10^15-10^16 cm^-2, which exhibits exotic thermoelectric behaviour.
To clarify the electronic density of states (DOS) around the conduction band bottom for state of the art transparent amorphous oxide semiconductors (TAOSs), InGaZnO4 and In2MgO4, we fabricated TAOS-based transparent thin film transistors (TTFTs) and
measured their gate voltage dependence of thermopower (S). TAOS-based TTFTs exhibit an unusual S behavior. The |S|-value abruptly increases, but then gradually decreases as Vg increases, clearly suggesting the anti-parabolic shaped DOS is hybridized with the original parabolic shaped DOS around the conduction band bottom.
Thermopower (S) for anatase TiO2 epitaxial films (n3D: 1E17-1E21 /cm3) and the gate voltage (Vg) dependence of S for thin film transistors (TFTs) based on TiO2 films were investigated to clarify the electronic density of states (DOS) around the condu
ction band bottom. The slope of the |S|-log n3D plots was -20 {mu}V/K, which is an order magnitude smaller than that of semiconductors (-198 {mu}V/K), and the |S| values for the TFTs increased with Vg in the low Vg region, suggesting that the extra tail states are hybridized with the original conduction band bottom.
We report herein fabrication and characterization of a thin-film transistor (TFT) using single-crystalline, epitaxial SrTiO3 film, which was grown by a pulsed laser deposition technique followed by the thermal annealing treatment in an oxygen atmosph
ere. Although TFTs on the polycrystalline epitaxial SrTiO3 films (as-deposited) exhibited poor transistor characteristics, the annealed single-crystalline SrTiO3 TFT exhibits transistor characteristics comparable with those of bulk single-crystal SrTiO3 FET: an on/off current ratio >10^5, sub-threshold swing ~2.1 V/decade, and field-effect mobility ~0.8 cm^2/Vs. This demonstrates the effectiveness of the appropriate thermal annealing treatment of epitaxial SrTiO3 films.
We show herein fabrication and field-modulated thermopower for KTaO3 single-crystal based field-effect transistors (FETs). The KTaO3 FET exhibits field effect mobility of ~8 cm2/Vs, which is ~4 times larger than that of SrTiO3 FETs. The thermopower o
f the KTaO3 FET decreased from 600 to 220 microV/K by the application of gate electric field up to 1.5 MV/cm, ~400 microV/K below that of an SrTiO3 FET, clearly reflecting the smaller carrier effective mass of KTaO3.
