Adequate precursor design for UV photoannealing -assisted low-temperature solution-process

1. Adequate precursor design for UV photoannealing-assisted low-temperature solution-process

2. Outline • Strategies for low-temperature solution process Chemical Physical Sol-gel on chip Combustion Prehydrolyzed Microwave Plasma UV Photoannealing High pressure • To achieve lower process temperature, precursor and annealing method should be considered simultaneously Metal citrato-peroxo complex + UV photoannealing • Limit of the method & new idea

3. Citrato-peroxo complex • Aqueous solution: environment-friendly & cost-effective 2-Methoxyethanol - Possible human teratogen - Contribute to photochemical smog - Substance to be avoided (listed in the U.S. Clean Air Act) • Aqueous sols are often highly acidic (pH 1~2) or basic (pH 11~12) To avoid formation and pricipitation of metal hydroxides - Hfperoxo-complex solution (pH 0.7) - Amine-hydroxo zinc complex solution (pH 13.5)

4. Citrato-peroxo complex • Citrato-peroxo complex solutions: aqueous solution with moderate pH Citric acid 235˚C – ammonium citrate decomposition 385˚C – decomposition of coordination compound 515˚C – decomposition of residual organics (Nitrogen containing organic compounds with a high thermal stability) → High processing temperature

5. UV photoannealing • UV irradiation has only been applied with external UV absorber species. ZTO = Zn acetate (precursor) + Sn chloride (precursor) + Acetylacetone (Stabilizer, UV absorber) Cl 2p Acac Electrochemical and Solid-State Letters, 15 (4) H91-H93 (2012) Titanium isopropoxide + Acetylacetone = βDIK-Ti (Chelation complex) Advanced materials, 16 (18) 1620 (2004)

6. Aqueous solution for UV photoannealing • Films derived from aqueous precursor systems are UV-active in a wide temperature window • → makes citrato-peroxo complex to be adequate precursor system for UV photoannealing • Effects of UV photoannealing • UV photons form O3 and active oxygen → Oxidation of organic compounds • Precursor decomposition → release and photolysis of H2O → reactive H+ and OH- • Amide decomposition → release and photolysis of NH3 → H, NH, and NH2 radicals

7. Aqueous solution for UV photoannealing • Combination of UV-active precursor and photoannealing : Organic decomposition a) UV-vis absorption spectra UV absorbance maintains in RT~350˚C temperature range b) FTIR spectra UV irradiation results in a decrease in the organic content of the film Effective organic decomposition was confirmed by film characterization Decreased thickness by UV-treatment 1) Less porosity 2) Lower organic contents 3) Higher crystallinity

8. Aqueous solution for UV photoannealing • Annealing temperature of 400˚C may seems to be insignificant, but it was required for crystallization (ferroelectric application) UV-treatment at low temperature (140˚C and 190˚C) At low temperature, UV active organic content of the film is still too high. Due to the abundant presence of UV absorbing species, film blistering occurs.

9. Aqueous solution for UV photoannealing • Summary • - For effective UV photoannealing, UV absorbents should be maintained at targeted annealing temperature • - Organic residues can be decomposed effectively by UV photons (O3 and N,H radicals) • - Excitation of photoactive groups formed during decomposition induces the dissociation of chemical bonds and the low-temperature formation of M-O-M bonds • Citrato-peroxo complex optimized for 400˚C process temperature • - Ferroelectric applications (PbTiO3, BiFeO3) • Have potential for ~200˚C low-temperature process, but adequate precursor design is required • idea – oxalate precursor

10. Photosensitive precursor • Precursor design • - Inorganic anions (Cl-, NO3-): cannot be decomposed by photoannealing • - Organic anions or chelates: with conjugate double bonds • Number of double bonds (C=C) ~ Absorption edge wavelength • ex) beta-carotene (orange color) with 11 C=C bonds • Conjugated systems of fewer than eight conjugated double bonds absorb only in the ultraviolet region Ideal molecule At least 1 C=C bonds Minimum carbon Citric acid Acetylacetone

11. Photosensitive precursor • Available precursors + Citrato-peroxo complex Acetate 2-ethylhexanoate Acetylacetonate • Thermal decomposition of precursors - Precursors like tin 2-ethylhexanoate or zinc acetylacetonate have low decomposition temperature - Problem is: because their limited solubility, stabilizer is required Zn acetylacetonate Sublimation

12. Photosensitive precursor • Use of 2-ethylhexanoate FTIR After 30min UV-irradiation, hydrocarbons were eliminated Ligand-to-metal charge transfer Density Additional annealing is required for high-quality dense oxide film

13. Photosensitive precursor • Use of acetylacetonate – RT deposited ZrOx XPS C 1s and N 1s were eliminated after UV-treatment Film was dense and has dielectric constant (k) of ~10 Its leakage current is high and additional organic layer is required (Phosponic acid) ※ UV lamp uncluded UVV (100–185 nm) → Acetylacetonate (2 C=C bonding) requires deep UV

14. Future works • Idea Oxalate precursor: 2 carbon and 2 C=O double bonding (minimize carbon and volume reduction) High decomposition temperature over 400 ˚C (will be overcomed by UV annealing) ! Critical limit: Oxalate is insoluble Preparation of ammonium zirconium oxalate Precursor can be prepared as an aqueous solution TGA curve for Sn precursor ~250˚C thermal decomposition → Promising!

15. Future works • Experimental procedure • Step 1. Synthesis of ammonium metal oxalate precursor • Step 2. Optical analysis (UV absorption) • Thermal analysis (decomposition T) of precursor • Step 3. UV-photoannealing • Step 4. Film characterization • Step 5. Application (active layer, gate dielectric, TCO, solar cell, etc.)

16. Experimental • Al2O3 gate dielectric • Anomalously high µFE • Possible way to overestimation of µFE • 1) Leakage current • 2) Fringe field current • 3) Underestimation of Capacitance

17. Experimental Anomalous electrical characteristics: are identical with ion gel dielectric → mobile ions can be related with anomalous phenomenon

18. Experimental Vg=0V Forward Clockwise hysteresis in output curve ID Reverse VD Mobility 0.6 cm2/Vs Calculated with ~30µF/cm2 Capacitance measurement of ion dielectric MIS structure Very low frequency

19. Experimental 20Hz High-frequency polarization SiO2 (Thermal oxidation) Al2O3 (Solution-processed) Low-frequency polarization 100Hz

20. Experimental Humidity sensor based on amorphous alumina nanotube H2O chemisorption : forms H3O+ by proton transfer → enables proton hopping (Low RH mechanism) 1) High concentration of residual OH 2) Water chemisorption Can induce ion conduction H3O+ H3O+ H3O+ OH OH O OH O OH OH O Alumina

21. Experimental

22. Experimental General trends: Channel width vs. transistor characteristics Experiment: nanoimprinted channel (1µm line pattern)

23. Experimental Nonhydrolytic sol-gel (ZTO) Alkoxide + chloride (Zinc 2-methoxyethoxide,SnCl2) Alkoxides Chlorides

24. Experimental Nonhydrolyticsol-gel (IZO) Nonhydrolytic reaction can be hindered by chelating alcohol For confirmation, Acetonitrile and Isopropanol were used as solvents As a result, Mobility of IZO film was not degraded by 2-ME 2-ME Acetonitrile Isopropanol

25. Experimental • Next week • GC-MS (Delayed: equipment contamination) • TGA-MS (for alkyl-halide ZTO) • Al2O3 – Polarization measurement or MIS capacitance • Precursor recipe reconsideration • In2O3nanomesh transistor - ~100nm patterning?