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Low-temperature, solution-processed gate dielectric

Low-temperature, solution-processed gate dielectric. Introduction. All-solution processed metal oxide TFT Processing temperature: 350 ° C IGZO active layer AlPO gate dielectric ( ɛ r ~ 5) Polymer(Acryl) etch stopper. AlPO gate dielectric

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Low-temperature, solution-processed gate dielectric

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  1. Low-temperature, solution-processed gate dielectric

  2. Introduction All-solution processed metal oxide TFT Processing temperature: 350°C IGZO active layer AlPO gate dielectric (ɛr ~ 5) Polymer(Acryl) etch stopper AlPO gate dielectric Aqueous solution processed medium-k oxide dielectric Low temperature processibility Smooth surface (RMS ~ 0.1nm) Low leakage current µFE ~ 4.50 cm2/V·s Vth ~ 2.34 V S.S ~ 0.46 V/dec

  3. Gate dielectric Rough surface typically results in a high electric field, leading to exponentially increased Jleak Jpf : Leakage current by Poole-Frankel mechanism (Trend is same for Schottky emission) ∆ = w/h0∝ ρrms L : lateral correlation length ∝ 1/ρrms α : roughness wavelength Surface roughness can induce over x102 leakage current

  4. AlPO Solution-processed AlPO dielectric Al(OH)3 + 2HCl + xH3PO4 (x: 0.25 ~ 1) → Concept Origin of surface roughness : disruptive volume loss following from these requisite high-temperature anneals Solution stabilization by pH control : preventing formation of large sol particles limiting non-functional counterions 2) General approach – High-volume ligand (-OCH2CH3, -NO3) to low-volume product (Al2O3) AlPO – Low-volume ligand (-OH) to high-volume product (AlPO) Aluminum hydroxide (or chlorohydrate) to form polymerized hydroxo networks in aqueous solution Cl Al O H hexamer trimer monomer dimer

  5. AlPO Aluminum-phosphate interactions enhance 1) aqueous solubility, 2) morphology, 3) and curing characteristics NMR study on Aluminum-phosphate aqueous solution 27Al NMR Al complex Chemical shifts Al3+(H2O)6 0 Al3+(H2O)5(PO3-) -3.5 Al3+(H2O)4(PO3-)2 -6.6 Al3+(H2O)3(PO3-)3 -10.0 (All Al complex are hexacoordinated) (P/Al was controlled by NaPO3 addition) Gelification occurred when complexes with two or three (PO3-) ligands were more abundant than complexes with one (PO3-) ligand and free aluminum ions.

  6. AlPO NMR study on Aluminum-phosphate aqueous solution 31P NMR (P/Al = 3) Q1(chain-end groups) : -9.3 ppm Q2 (linear chains) : -21, -21.2, and -21.8 ppm Q2 (tri- and tetrametaphosphates) : -20.6 and -22.8 ppm (Al3+)-(PO3-)-(Al3+) bonds formation was confirmed by both methods (NMR, FTIR) Large amount of bonded waterand P-OH groups were detected FTIR study 1300-700 cm-1 range spectrum of metaphosphate skeletons Water and hydroxyl vibration: 3700 and 1600 cm-1 AlPO gel AlPO gel AlPO gel (Dried) NaPO4 NaPO4

  7. AlPO Thermal stability of AlPO thin film P/Al = 0.5 : optimal P/Al ratio – stable amorphous phase until 1000°C P/Al = 1 : form mixed tridymiteand cristobalite AlPO4 phases (P/Al < 0.25: limited solubility) Covalent binary compounds (AlPO) substituted into more ionic (Al2O3)systems resist film crystallization e.g., SiO2 and Al2O3 in HfO2. P2O5 (silica with glass formers) FTIR study upon annealing 3500 cm-1 : O-H stretching mode 1640 cm-1 : H-O-H bending mode H2O, -OH, and -Cl are decompose by annealing (relatively small ligands)

  8. AlPO Leakage current was effectively suppressed by smooth, and dense AlPO gate dielectric (~nA/cm2) 300°C 600°C Hysteresis Clockwise: Electron traps at interface Counter-clockwise: Mobile protons in dielectric 1000°C Ref SiO2 Dehydration of AlOOH and Al2O3species requires high annealing temperature of 1200°C

  9. Al-OH • HSAB theory • acids and bases interact and the most stable interactions are hard-hard and soft-soft • hard-hard interaction (ionogenic character) • soft-soft interaction (covalent character) • Hard acid : Na+, Mg2+, Al3+ • Borderline acid : Mn2+, Zn2+ • Soft acid : Br+, Ag+, Pd2+ • Hard base : F-, OH- • Borderline base : NO3- • Soft base : SH-, CH3- I: Ionization potential A: electron affinity → expected increased softness in going down a column in the periodic table (Hardness: Ti > Zr> Hf)

  10. Aqueous HfO2 Low leakage current Jleak ~ 10nA/cm2 for 350°C annealed film TFT characteristics with sputtered IGZO µFE ~ 13.1 cm2/V·s Vth ~ 8.5 V S.S ~ 0.3 V/dec High Vth and clockwise hysteresis by plasma damage HfO2 tends to crystallize Unlike Al2O3 Roughness increases by high temperature annealing (ɛr ~ 13)

  11. Future works Summerized conclusion 1) Smooth surface, and density of thin film are important to suppress leakage current in gate dielectric 2) Aqueous solution-process is effective method for achieving those properties Microwave annealing Combination of 1) aqueous precursor solution method 2) and microwave annealing (ZnO active layer)

  12. Future works NH3 annealing NH3 annealing improves surface roughness Smooth surface can be obtained with general alcohol-based process (450°C annealing)

  13. Experiments 270°C low-temperature ZTO Increasing SnOtBu/ZnOAc ratio ZnOAc SnOtBu Substrate

  14. ZTO 0.027M SnOtBu + 0.15M ZnOAc300ºC 0.027M SnOtBu + 0.12M ZnOAc+ 0.03M SnCl2 300C 0.027M SnOtBu + 0.15M ZnOAc270ºC

  15. 270ºC ZTO 0.027M SnOtBu + 0.125M ZnOAc 0.027M SnOtBu + 0.15M ZnOAc 0.027M SnOtBu + 0.175M ZnOAc 0.027M SnOtBu + 0.2M ZnOAc

  16. 270ºC + SnCl2 30% ZTO 0.027M SnOtBu + 0.125M ZnOAc 0.027M SnOtBu + 0.15M ZnOAc 0.027M SnOtBu + 0.175M ZnOAc 0.027M SnOtBu + 0.2M ZnOAc

  17. 300ºC ZTO 0.027M SnOtBu + 0.125M ZnOAc 0.027M SnOtBu + 0.15M ZnOAc 0.027M SnOtBu + 0.175M ZnOAc 0.027M SnOtBu + 0.2M ZnOAc

  18. 300ºC + SnCl2 30% ZTO 0.027M SnOtBu + 0.125M ZnOAc 0.027M SnOtBu + 0.15M ZnOAc 0.027M SnOtBu + 0.175M ZnOAc 0.027M SnOtBu + 0.2M ZnOAc

  19. 300ºC ZTO 0.027M SnOtBu + 0.15M ZnOAc 0.027M SnOtBu + 0.175M ZnOAc 0.027M SnOtBu + 0.15M ZnOAc 0.027M SnOtBu + 0.175M ZnOAc

  20. 270ºC ZTO 0.027M SnOtBu + 0.15M ZnOAc 0.027M SnOtBu + 0.175M ZnOAc 0.027M SnOtBu + 0.15M ZnOAc 0.027M SnOtBu + 0.175M ZnOAc

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