Independent control of phases and defects in TiO₂ thin films for functional transistor channels

Abstract

TiO2 has various inherent advantages in practical devices: chemical stability, non‐toxicity, and abundance on earth, as highlighted in a range of applications of TiO2 surfaces in photocatalysis/optoelectronics. However, the application of TiO2 to three‐terminal devices has been limited; for example, TiO2‐channel transistors, which may potentially modulate the TiO2 surface properties by electrostatic back gating, has suffered from low field‐effect mobility (μFE < 1 cm2 V−1 s−1) irrespective of fabrication methods. The major challenge is to control phases (rutile/anatase) and defects (oxygen vacancy) simultaneously in TiO2 thin films. Here, we achieved μFE ∼ 10 cm2 V−1 s−1 in TiO2‐channel transistors, one order higher than before and even comparable to InGaZnO channels. The major improvement is the independent control of phases and defects in TiO2 thin films; we formed short‐range order of anatase phase in the amorphous deposition, and minimized the number of defects in the subsequent thermal treatment. Besides, we showed this independent control of phases and defects are underpinned by excellent thermal stability of TiO2/SiO2 interfaces. These results demonstrate TiO2 can be a transistor channel with a reasonable μFE and ON/OFF ratio (>6 orders), not only changing the conventional image of this material, but also promising new functionalities based on unique properties of TiO2.

Large mobility modulation in ultrathin amorphous titanium oxide transistors

“…The μ sat in the device under the discussion was~1.6 × 10 −4 cm 2 V −1 s −1 . While the acquired mobility is lower than the crystalline TiO 2 TFTs reported previously 25,27,29,32 , it is comparable to those reported from the TFTs fabricated using lower crystallinity TiO 2 10,33,37 . We further evaluated the gate dependence of field-effect mobility (μ FE ) using the general expression (Eq.…”

“…Given the~6 decades of on-off ratio observed in our ultrathin TiO x TFT, the current modulation is thus predominantly originating from the gateinduced mobility modulation while the carrier density modulation by gate providing only a minor contribution. We can safely exclude possible effects of contact Schottky barrier on the observed mobility modulation considering the choice of Al electrode that has been shown to form an ohmic contact on TiO x 26,29,33,37,40 and the negligible barrier height lowering effected by gate bias (Supplementary Note 1). The origin of gateinduced mobility modulation has been previously speculated to be related with the sub-bandgap trap states (localized tail states) 41,42 , typically present near the conduction band edge (E c ) of amorphous semiconductors with an exponential decrease in their density of states.…”

“…TiO 2 as a semiconductor is extensively used in solar cells [17][18][19][20] and photocatalysis [21][22][23][24] , but its application as a TFT channel material has not been much explored. For those reported so far, TiO 2 layers were mostly thicker than~10 nm and crystalline 9,[25][26][27][28][29][30][31][32][33][34] , with only a few reporting the mobility modulation 9,10 . A large extent of mobility modulation, as high as~9 decades, has been recently reported from mixed amorphous-crystalline TiO x TFT 35 , with its origin remaining unexplained nevertheless.…”

Photocatalytic Properties of Titanium Dioxide Thin Films Doped with Noble Metals (Ag, Au, Pd, and Pt)

“…TiO 2 in different types and forms has been extensively studied as a semiconductor for photocatalytic degradation of water organic pollutants, such as dyes, phenolic compounds, and for disinfecting wastewater . The great interest in the development of TiO 2 ‐mediated photocatalysis is due to the high chemical stability, non‐toxicity, and optical properties of this material . However, TiO 2 is only active upon UV excitation because of its wide band gap ( E g ) of 3.2 eV.…”

Molecularly-thin anatase field-effect transistors fabricated through the solid state transformation of titania nanosheets

“…If it is possible to replace UV irradiation by the electric field effect, 6 the range of applications will be expanded. Although TiOx field effect transistors (FETs) have been reported with a backgate structure in which the TiOx surface is available, [6][7][8][9][10][11] it is difficult to induce carriers on the TiOx surface, rather than at the interface between TiOx and the gate insulator, because the thickness of the TiOx layer is generally thicker than 20 nm in order to retain crystal quality.…”