Solution-Processed High Mobility Inorganic Thin-Film Transistors with a Self-Assembled Nanoscopic Gate Dielectric (27152)
The simple semiconductor and dielectric film fabrication afford TFTs with properties suitable for a variety of higher performance applications
SUMMARY: Fabrication of high performance thin-film transistors (TFTs) by solution phase processes promise commercially viable, low-cost, large-area electronics. However, efforts to develop solution-processed organic semiconducting films, for organic TFTs (OTFTs) to date afford field-effect mobilities lower than those of optimized OTFTs fabricated with vapor-deposited or single-crystal organic semiconductors by up to ~10x. This limitation precludes their use for solution-processed high speed circuits. Inorganic semiconductors can exhibit bulk field effect mobilities greater than 100 cm2 V-1s-1. However, inorganic semiconductors are typically intractable in common solvents, require soluble precursors and high annealing temperatures (>300°C) to achieve sufficient film crystallinity and charge transport properties. Furthermore, the gate insulator must be sufficiently robust to survive processing conditions and still provide low operating voltages with inorganic semiconductors. This invention provides TFT fabrication using both a solution-processed inorganic semiconductor and a solution-processed gate dielectric.
Self-assembled nanodielectrics (SANDs) are structurally well-defined hybrid multilayer thin films, compatible with both organic (solution-cast and vapor-phase) and low-temperature vapor-phase inorganic semiconductor film TFT deposition processes. SAND-based TFTs operate at extremely low voltages and exhibit greater mobilities (μFETs) than analogous TFTs fabricated with conventional SiO2 gate dielectrics. A chemical bath deposition (CBD) was employed to grow CdSe thin films from aqueous Cd2+ and sodium selenosulfate (SeSO32-) solutions on n+-Si/SAND substrates followed by 400°C annealing. The resulting hybrid TFTs proved thermally robust. Bottom-gate/top-contact CdSe-based TFTs were fabricated on commercial p+-Si/(300 nm) SiO2 (control) and n+-Si/(16.5 nm) SAND substrates following identical CBD procedures. The n+-Si/SAND/CdSe device exhibits superior properties versus the p+-Si/SiO2/CdSe control. Significantly greater mobility and reduced threshold voltage are observed in the SAND hybrid TFT (Table 1).
Metal oxides are promising candidates for solution-processed inorganic semiconductors with the advantage that sol-gel solutions of the metal oxide precursors can be spun-cast or ink-jet printed. TFTs were also prepared with films of zinc indium oxide (ZIO) spun-cast onto the SAND dielectric and SiO2 substrates. The p+-Si/SiO2/ZIO/Au device had μFET of ~0.60 cm2 V-1s-1, Ion/Ioff of 104 and a Vth equal to 30 V. In contrast the n+-Si/SAND/ZIO/Au TFT exhibited μFET of ~7.2 cm2 V-1s-1, Ion/Ioff of 104 and Vth equal to 3.3 V. Again, the improved performance of a solution-processed inorganic semiconductor is observed with the SAND gate dielectric. The demonstrated performance of the SAND dielectric solution-processed inorganic semiconductors make these devices attractive for a wide variety of applications.
Tobin Marks, Antonio Facchetti, Paul Byrne
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