Effects of gate dielectrics on channel mobility of solution-processed oxide thin-film transistor

1. Effects of gate dielectrics on channel mobility of solution-processed oxide thin-film transistor

2. Introduction Role of gate dielectric 1) Insulation 2) Capacitor Band offset as charge barrier 1 eV Gate insulator Semiconductor 1 eV C=kε0A/d High-k material can charge more electron/holes Low leakage current density Low trap density for breakdown resistance

3. Introduction High-k Gate dielectrics 1) MOSFET 2) Organic FET ex) Transfer characteristics of pentacene OFET vth tuning is rather complicated than inorganic semiconductors Threshold voltage can be reduced by high-k gate dielectric Device miniaturization → Short channel effect → Heavy doping → Larger capacitance is required

4. Gate dielectric effects on active layer mobility Key issue for gate dielectrics (with solution-processed oxide TFT) Solution processibility for all-solution-processed device Good insulation performance has to be obtained by low-temperature solution process Effects on channel field-effect- mobility Solution-processed Li-doped ZnO on various gate dielectrics (Not only channel/dielectric interface, bulk dielectric itself influences channel mobility)

5. Gate dielectric effects on active layer mobility Effects on active layer: General theory Scattering mechanisms 1) Coulomb scattering Electrons scattered by interface trap and oxide charges 2) Phonon scattering ← Electron distort around lattice. Like this electron-lattice interaction, electron can be scattered by phonon wave 3) Surface roughness scattering Interface-trap explanation: no use for material selection

6. Electromagnetic interactions “Right hand grip rule” determines direction of magnetic field Theory considering electromagnetics Magnetic field induced by channel conduction Channel as a conducting wire Strength of magnetic field H Increase with current I Decrease with distance d µ (magnetic permeability) ~ 1 in paramagnetic materials Magnetic field is applied to dielectric

7. Electromagnetic interactions External magnetic field accelerates the orbiting electrons of the atoms in the dielectric Change in external magnetic flux induce voltage, and electric field Applied force Fd (Newton’s law & Coulomb’s law)) Acceleration of orbiting electrons in the dielectric

8. Electromagnetic interactions Electrons in dielectric absorb energy from moving channel carriers Relation between ad & ae is leaded by combining equations Dielectric properties determines ad Atomic radius Rd Electron mass me Magnetic permeability µ Moving channel carriers lose energy → Carrier mobility decreases

9. BeO gate dielectrics Gate dielectric materials BeO (Beryllium oxide) Small atomic radius → Decrease in carrier mobility minimizes 2) Heavy electron mass BeO has basic properties that is required for gate dielectrics Dielectric constant of 6.9 3) High thermal conductivity → High entropy Sth Wide bandgap of 10.6 (P. G. Klemens, Int. J. Thermophys. 10, 1213) → High thermal stability

10. BeO gate dielectrics ALD-deposited BeO dielectric on InP (100) ← (Left) smooth surface of BeO (Right) Lower leakage current density and higher breakdown voltage than Al2O3 ← Mobility of InP FET was enhanced on BeO compared to Al2O3 gate dielectric

11. Conclusion • Solution-processed oxide TFTs have been developed and solution-processed gate dielectric with good insulating property is required • Channel mobility can be degraded by dielectrics, by phonon scattering and coulomb scattering at interface and dielectric layer • Add to general scattering theory, electromagnetic interaction may play a role in dielectric effect on channel field-effect-mobility • To obtain high channel mobility, gate dielectric material should have small atomic radius, high magnetic permeability, and heavy electron mass • BeO has all these properties, and also satisfy general requirement for gate dielectrics such as wide bandgap, thermal stability, and etc.

12. Gate dielectric material Atomic radius rd Be < Si < Al < Ti < Ta < Hf = Zr : Fixed value for specified metal atom

13. Gate dielectric material Electron mass md Effect on insulating property: Heavy electron mass reduce Fowler-Nordheim tunneling current Heavy fermion oxides Lowest conduction band is formed by flat f bands m* over 100 – but only at low T (Related to superconductivity) ex) Band structure of LiV2O4

14. Future works Magnetic permeability µ Non-magnetic materials have µ ~ 1 Material should be ferromagnetic to have large µ ( µ=dB/dH ) Oxide can be doped with magnetic dopants (Mn,Fe,Ni, or Co) to be ferromagnetic µ value ~5 can be obtained (right figure) ferromagnet Free space

15. Gate dielectric material ex) Fe,Co-doped ZrO2 by sol-gel method Diluted ferromagnetic oxide can be obtained by sol-gel method ZrCl2 in EG solvent – doping with 1% Fe and Co Experimental Comparison between doped and undoped gate dielectric Al Al Al Al IZO IZO (Mn,Fe, or Ni)-doped HfO2 Undoped HfO2 p++ Si p++ Si

16. Experiment HfOx aqueous solution – reference experiment 1 2 3 4 5 0.12M HfOCl2·8H2O 20mL 1M NH3 6.7mL Washing x5 10M H2O2 5mL + 2M HNO3 1.4mL Stirring 12hrs Spin coating 3000rpm 30s ~8nm per coating

17. Experiment HfOx aqueous solution 1st attempt (spin-coating x3) ~25nm k ~ 14 300°C 350°C

18. Experiment HfOx aqueous solution 1st attempt (spin-coating x3) ~25nm k ~ 14 High level of leakage currents Insufficient film thickness Poor coating quality 400°C

19. Experiment HfOx aqueous solution 2st attempt (spin-coating x10) ~82nm 350°C annealing 2nd attempt Increased film thickness (increased # of coating layer) k (1kHz) 14 Jleak (at 1MV/cm) 10 nA Breakdown 8MV/cm

20. Experiment HfOx aqueous solution Pinhole generation on film: at the stage of spin-coating Results: Low level of leakage currents are obatined only on specified dot pattern High level of leakage currents at larger electrode pattern (TFT charactersticscannot be obtained) Pinhole by dewetting Electrode dot pattern

21. Experiment HfOx aqueous solution IZO channel on HfOx gate dielectric High level of leakage current Dot pattern (d~ 250um) S/D pattern (d~ 250um)

22. Experiment HfOx aqueous solution EG-addition into HfOx solution 5%, 10% EG solution – spin-coating failed 1% EG 2% EG 5% EG 10% EG

23. Experiment HfOx aqueous solution EG-addition into HfOx solution Decreased breakdown voltage ~ 5V (k~13) Coating thicknesses were not increased 1% EG 2% EG

24. Experiment Next week Doping experiment on HfO2 Hf precursor - [Hf4(OH)12(O2)2·yH2O]m Peroxo-complex recipe is limited – for, dopant hydroxide precursor will be used Dopants: Non-magnetic – Mg, Al Magnetic – Ni, Fe, Mn, Co

25. Experiment ZTO - alkyl halide elimination At low-temperature of 270C, alkyl halide elimination is better than ester elimination Ester elimination Tin tert-butoxide + ZnOAc2 Alkyl halide elimination Tin tert-butoxide + ZnCl2

26. Experiment ZnCl2, 75mM (270°C annealing) ZnCl2, 100mM ZnCl2, 125mM ZTO - metal halide elimination 270°C annealing with 0.27MSntert-butoxide

27. Experiment ZnCl2, 75mM (270°C annealing) ZnCl2, 100mM ZnCl2, 125mM ZTO - metal halide elimination 270°C annealing with 0.23MSntert-butoxide

28. Experiment ZnCl2, 50mM (270°C annealing) ZnCl2, 75mM ZnCl2, 50mM (300°C annealing) ZnCl2, 75mM

29. Experiment ZnCl2, 50mM (330°C annealing) ZnCl2, 75mM