Spray-combustion synthesis: Efficient solution route to high-performance oxide transistors

Abstract

Metal-oxide (MO) semiconductors have emerged as enabling materials for next generation thin-film electronics owing to their high carrier mobilities, even in the amorphous state, large-area uniformity, low cost, and optical transparency, which are applicable to flat-panel displays, flexible circuitry, and photovoltaic cells. Impressive progress in solution-processed MO electronics has been achieved using methodologies such as sol gel, deep-UV irradiation, preformed nanostructures, and combustion synthesis. Nevertheless, because of incomplete lattice condensation and film densification, high-quality solution-processed MO films having technologically relevant thicknesses achievable in a single step have yet to be shown. Here, we report a low-temperature, thickness-controlled coating process to create high-performance, solution-processed MO electronics: spray-combustion synthesis (SCS). We also report for the first time, to our knowledge, indium-gallium-zinc-oxide (IGZO) transistors having densification, nanoporosity, electron mobility, trap densities, bias stability, and film transport approaching those of sputtered films and compatible with conventional fabrication (FAB) operations. etal-oxide (MO) semiconductors, especially in amorphous phases, represent an appealing materials family for next generation electronics owing to their high carrier mobilities, good environmental/thermal stability, mechanical flexibility, and excellent optical transparency (1-3). MO films complement organic semiconductors (4, 5), carbon/oxide nanomaterials (6), and flexible silicon (7, 8) for enabling new technologies, such as flexible displays and printed sensors. For fabricating high-performance electronics with acceptable fidelity, conventional processes require capital-intensive physical/chemical vapor deposition techniques. Capitalizing on the solubility of MO precursors in common solvents, solution methods have been used to fabricate semiconducting MO layers for thin-film transistors (TFTs). However, the fabrication process and field-effect mobilities of these TFTs are not competitive with the corresponding vapor-deposited (e.g., sputtered) devices (9), and developing routes to solution-derived MO TFTs with technologically relevant thicknesses and performance comparable to state of the art vapor-deposited devices is a critical milestone for MO electronics evolution.Sol-gel techniques are used extensively for MO film growth, including films for high-performance TFTs (10-13). However, the required sol-gel condensation, densification, and impurity removal steps typically require >400-500°C processing temperatures, which are incompatible with inexpensive glasses and typical flexible plastic substrates (14). Progress toward significantly reducing the processing temperatures of sol gel-derived MO films has afforded excellent TFT mobilities; however, achieving both reproducible high-performance and stable device operation remains an unsolved issue for Ga-containing materials (15). Sol-gel on-a-chip for indium-zinc-oxide (3) and deep-UV ...

Solution‐Processed All‐Oxide Transparent High‐Performance Transistors Fabricated by Spray‐Combustion Synthesis

“…[ 9c , 17 ] Note that the large leakage currents of the spincoated CS fi lms refl ect nanoscale pinholes/porosity reasonably caused by gas evolution during combustion. [ 7 ] Furthermore, the present SCS fi lms exhibit far higher breakdown fi elds (Figure 1 c) than spin-coated CS fi lms and spin-coated, bar-coated, or spray-pyrolysis dielectric fi lms. [ 2d , 4a , 9c , 12 ] For example, the highest breakdown fi eld for the present SCS ZrO 2 fi lms (300 °C) reaches 9.5 MV cm −1 , 3-6× greater than that of spin-coated and spray-pyrolysis ZrO 2 fi lms processed at 450 °C.…”

“…We and others have shown that well-patterned/defi ned gate/semiconductor layers yield greatly enhanced performance (turn-on characteristics, I on / I off ratios, better bias stress stability) than the crudely patterned/unpatterned TFTs based on the same materials stack. [ 7 ] Encouraged by the promising TFT performance using combined SCS-derived dielectric and semiconducting layers, conducting ZITO-coated AryLite substrates were next utilized to replace the rigid Si wafers, [ 20 ] and to realize fl exible IZO TFTs Figure S11 (Supporting Information), these devices with SCS-derived Al 2 O 3 or ZrO 2 dielectrics processed at 275 °C exhibit TFT characteristics with operating voltages below 3 V and an average mobilities of 3.9 ± 2.2 cm 2 V −1 s −1 and 6.2 ± 3.2 cm 2 V −1 s −1 for Al 2 O 3 /IZO and ZrO 2 /IZO devices, respectively. In addition, only slight decreases in mobilities for both devices were observed after bending for 10 cycles at radius of 15 mm.…”

“…However, gas evolution during the short processing times interferes with fi lm continuity and densifi cation for >5 nm fi lms, thus requiring time-consuming multi-step coating and annealing. [ 14 ] Recently, we reported a new highspeed spray-combustion synthesis (SCS) approach to MO fi lm growth, [ 7 ] producing high-density, macroscopically continuous fi lms for diverse MO semiconductors, and with carrier mobility and electrical uniformity rivaling that of magnetron-sputtered fi lms. However, low-voltage operation was only demonstrated for TFTs with a ZrO 2 dielectric, grown by sol-gel spin-coating and annealing at 500 °C.…”

“…Among these, spray pyrolysis enables the growth of dense, high-quality MO fi lms over large areas at moderate temperatures (250-400 °C) and in fabrication (FAB)-compatible processing times. [ 7 ] Thus, spray pyrolysis of In 2 O 3 …”

Patterning of Solution‐Processed, Indium‐Free Oxide TFTs by Selective Spray Pyrolysis

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by X-ray refl ectivity has shown that spray-pyrolysis fi lms of materials such as indium-gallium-zinc oxide have signifi cantly lower porosity than spin-coated fi lms of a comparable thickness, [ 12 ] ultimately leading to improved mobility and bias-stress stability.Although spray pyrolysis offers the potential to deposit highquality TCOs at a low cost over large areas, it remains a blanket deposition technique that requires more expensive patterning techniques such as photolithography and etching for fabricating multilayer patterned structures such as thin-fi lm transistors. [ 13 ] These additional patterning steps limit throughput and contribute additional process complexity and cost.

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