TFTs with solution-processed TOS channels generally demonstrate mobility of an order of magnitude higher than a-Si, [6,7] and one common practice to achieve these high-performance TFTs is to use high-k dielectric materials as the gate insulator instead of SiO 2. [8][9][10][11] For example, Xu et al. reported nearly a 10× or 25× increase in the mobility of solution-processed In 2 O 3 or indium-zinc-oxide, respectively, by changing the gate dielectric material from SiO 2 to AlO x . [10] A good amount of similar empirical data can be found in the literature, [12][13][14][15] some examples of which are summarized in Table S1 (Supporting Information). However, despite the wide range of TOS and high-k materials being explored, there is currently no well-established explanation for the phenomenon of higher mobility in oxide TFTs that have high-k gate dielectrics. With a better understanding of the interactions of materials that enhance electron transport, the semiconductor-gate dielectric interface can be engineered to take full advantage of metal-oxide semiconductors for highperformance electronics.Various theories have been proposed to explain the boost in mobility observed in TOS/high-k devices, such as an increase in carrier concentration that fills localized states and allows bandlike transport, [8] reduced trap density at the semiconductor-dielectric interface, [16] electron donation from the gate dielectric, [17] and defect relaxation and suppression of free carriers resulting from diffusion of material from the gate dielectric to the semiconductor. [18] However, so far, only a small number of studies have been focused on explaining the fundamental mechanism behind these observations. In one such study, an analytical model was developed illustrating that field-effect mobility increases with higher gate-insulator capacitance. [19] However, the experimental verification of their model relied on devices with spin-coated HfLaO x as the gate dielectric, which may not properly account for any mobility effects seen in other more common high-k dielectrics (i.e., ZrO 2 or HfO 2 ). Furthermore, a model based on a solution-processed dielectric material is unlikely to be generalizable to explain the mobility dependence for the more commonly used vacuum-deposited dielectric materials because they tend to exhibit greater electronic disorder and nonideal insulating behavior. [20] Their incorporation into transistors has been complicated by defects such as fixed charges, electron traps, and mobile ions. [10] As shown in In metal-oxide thin-film transistors (TFTs), high-k gate dielectrics often yield a higher electron mobility than SiO 2 . However, investigations regarding the mechanism of this high-k "mobility boost" are relatively scarce. To explore this phenomenon, solution-processed In 2 O 3 TFTs are fabricated using eight different gate dielectrics (SiO 2 , Al 2 O 3 , ZrO 2 , HfO 2 , and bilayer SiO 2 /high-k structures). With these structures, the total gate capacitance can be varied independently from the semiconductor-die...
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