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Designing surface sol-gel process for successful deposition

Designing surface sol-gel process for successful deposition. Introduction. Sol-gel oxide for electronics. Semiconductors. Dielectrics. Surface sol-gel oxide Dielectric materials – well studied (relatively) Semiconductors – only few examples. Precursor. Valency of alkoxide.

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Designing surface sol-gel process for successful deposition

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  1. Designing surface sol-gel process for successful deposition

  2. Introduction Sol-gel oxide for electronics Semiconductors Dielectrics Surface sol-gel oxide Dielectric materials – well studied (relatively) Semiconductors – only few examples

  3. Precursor Valency of alkoxide Low valency of metal atom (<4) → steric effect is less significant → easy to form oligomer → particle-like multilayer deposition ex) tetramer of titanium ethoxide

  4. Solvent Solvent compatibility M M Alkoxide precursor Reaction product: alcohol Alcohol is matched with alkoxide in many cases ex) isopropanol for Ti isopropoxide Acetate precursor Reaction product: acetic acid

  5. Solvent Solvent effect on alkoxide Alkoxideoligomerization ex) Ti(OPri)4 in 2-propanol → Tetrahedral Ti(OEt)4in ethanol → Pentahedral Solvent effect ex) Ti(OPri)4 in 2-propanol → Monomeric Ti(OEt)4in ethanol → Dimeric complex Ti2(OC2H5)8 2C2H5OH Transesterification

  6. SnO2 coating Coating procedure Experimental details 0.1M tin isopropoxide in anhydrous isopropanol (5min) Isopropanol washing 1M NH4OH hydrolysis – 1min 15 deposition cycles ~ 50nm thickness 700ºC annealing for 2 hrs

  7. SnO2 coating SnO2 properties SnO2 deposition on biosilica (Aulacoseira diatom) Dense, conformal thin film on natural nanostructure ~5nm diameter nanocrystal-embedded film Application: NO gas sensor

  8. SnO2 coating Tin alkoxide Deposition rate was high (3nm/cycle) → due to dimeric nature of tin isopropoxide Tin tert-butoxide (monomer) Tin isopropoxide (dimer)

  9. SnO2 coating Oligomerization vs. temperature Tin isopropoxide complex: isopropanol desorption by increased temperature → decreased steric hinderence → increase reactivity -60ºC -40ºC 25ºC

  10. SnO2 coating SnO2 properties XRD measurement Cassiterite structure (Rutile) SAED pattern Polycrystalline

  11. TFT device application SnO2 TFT Ta-doped SnO2 nanowire TFT Single-crystal rutile Ta-doping Increase carrier concentration Decrease environmental sensitivity (Undoped SnO2 is more like sensor) Surface sol-gel Ta2O5 coating is available (ref: J. of Nanoparticle Res. 1, 43)

  12. ITO coating Coating procedure Precursors Indium methoxyethoxide Tin isopropoxide Solvent Methoxyethanol Isopropanol-methanol Reaction at 50ºC, 3min Washing Hydrolysis 3min Thermal annealing at 450ºC

  13. ITO coating ITO properties SEM & TEM Particle deposition Porous coating SAED pattern Rhombohedral structure (Cubic is stable for bulk ITO) Conductivity 16 S/cm (1/100 of commercial ITO)

  14. ITO coating Indium alkoxide Number of cycles and deposition rate was not clearly expressed at paper High molecular complexity of indium alkoxide → particle aggregation High temperature adsorption → related to low reactivity of polymer Indium tert-butoxide (dimer) Tin isopropoxide (Polymer)

  15. Precursor adsorption Alkoxide adsorption Proper adsorption Maintain reactivity after washing OH OH OH OH OH OH OH OH OH OH Ethanol washing without alkoxide adsorption Hydrogen bonding: hydroxyl-ethanol Lose hydrophilicity OH OH OH OH OH OH OH OH

  16. Precursor adsorption Hydrolysis rate Ethanol washing without alkoxide adsorption Metal alkoxides are usually reactive to water Water addition Immediate Precipitation Adsorption with hydrolyzed precursor Deposition rate: ~40A/cycle (~10 times higher than normal SSG)

  17. ZnO coating Deposition characteristics ZnO surface sol-gel with hydrolyzed precursor Island growth Imperfect coverage (~80%) Adsorption from colloids: particle aggregation

  18. Adsorption Reaction related to adsorption Oxolation Alcoxolation In water-inhibited surface sol-gel process, Alcoxolation determines adsorption rate

  19. Adsorption Adsorption rate In successful case, adsorption rate is very fast ex) Adsorption of Ti(Onbu)4 (saturate at 1min.) Temperature is only 16ºC Deposition rate 50Hz frequency shift indicates 4.8 atoms/nm2 8Å thickness

  20. Adsorption Adsorption rate Adsorption rate can be increased by simply elevated temperature Addition of acetic acid, monomerize hafnium n-butoxide and increase reactivity II+ 6 Hacac → 3VI+ 6nBuOH Cyclic trimer (in toluene) Monomer Addition of isopropanol or THF couldn’t monomerize hafnium n-butoxide

  21. Conclusion • There is a relation between: Molecular complexity Reactivity/ Deposition characteristic Metal valency • For successful deposition: • Monomer precursor or minimized molecular complexity • Sufficient reactivity

  22. Future work • HfO2 dielectric coating • Coating with elevated temperature (~30ºC) • SnO2 semiconductor coating • SnO2 coating (tin isopropoxideprecursor) • Ta2O5 coating • Ta-doped SnO2 coating (sequencial, mixed) • TFT fabrication

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