Conduction mechanisms & Field effect mobility in amorphous oxide TFT

1. Conduction mechanisms & Field effect mobility in amorphous oxide TFT

2. Amorphous IGZO Localized state in amorphous semiconductors By random potential distribution, density of states have localized states (Bandtail) By qualitative modeling, bandtailhas exponential distribution of density of states

3. Amorphous IGZO Conduction in amorphous oxide Conduction by spherical s orbital overlap No angular disorder – distance distribution & compositional disorder and metal-metal distance – almost constant → much weaker MTR characteristic than a-Si:H Covalent bonding vs. ionic bonding Ionic bonding does not form non-bonding states (i.e. no dangling bonds) No Staebler-Wronski effect No auto-compensation of doping (High doping → High mobility)

4. Amorphous IGZO Temperature-dependent field-effect measurement RF-sputtered a-IGZO TFT As temperature increases, (20ºC ~ 80ºC) Field effect mobility ↑ Threshold voltage ↓ Subthreshold swing ↑ (These parameters are related to density of states) S and IOFF are insensitive to temperature, unlike Si:H TFT.

5. Amorphous IGZO Analysis of band bending and charge density Density of states N(E) is calculated from Differential value of this plot

6. Multiple trapping & release MTR conduction Electrons are trapped in localized states and, Are released and move forward by thermal excitation Percolation condiction In high temperature: Shorter but higher-barrier path (more localized state are occupied) (higher thermal energy) In low temperature: Longer but lower-barrier path

7. Solution-processed ZTO Transition point Solution-processed Zinc tin oxide TFT Combination of thermally activated & extended state transport Mobility edge is well defined Band transport dominates at high carrier concentration Data fit well with MTR transport model

8. Bilayer oxide TFT IZO/IGZO transistor High carrier concentration of IZO front channel Combining high mobility of IZO/positive vth of IGZO Both layers are amorphous IZO on SiO2 – Amorphous <20nm thickness

9. Trap limited conduction Band structure of amorphous oxide semiconductor Exponential distribution of localized tail states Conduction is controlled by MTR μFE = nfree / (nfree + ntrap) Em : Conduction band minima EM : Maximum minima level WB : Barrier width ΦB0 : Barrier potential DB : Barrier interdistance

10. Trap limited conduction 1) EF < Em : Fermi level resides in localized tail state Barrier height ΦB0 Variance Bandtail parameter Bandtail characteristic temperature 2) Em < EF< EM : Fermi level resides in extended state Percolation conduction – Bandtailterm is eliminated Gate voltage induced barrier height change

11. Trap limited conduction Field effect mobility Debye length of IGZO : ~120nm 55nm channel thickness : Fully acumulated Conduction in IGZO is large

12. Trap limited conduction General Fermi-Dirac statistics Fermi-Dirac statistics for localized state DOS of localized state ~ exp(E) Solution of Poisson’s equation Transition voltage

13. Trap limited conduction Carrier densities nfree/ntrap ratio increases as gate voltage increases Transition voltage: Vg ~ 13V At vth, nfree/ntrap ~ 0.2 → MTR limited conduction When Vg > Vp nfree/ntrap> 0.8 → Percolation dominant

14. Trap limited conduction Density of trap state (Ntc) Modeling and experiment confirms that Ntc of a-IGZO <<< Ntc of a-Si:H High nfree/ntrap ratio → High field effect mobility

15. Trap limited conduction Field effect mobility Mobility is 1) inversely proportional to temperature in general semiconductor physics 2) Proportional to temperature in hopping conduction At vth, Vg > Vp μFE is limited by localization When Vg > Vp μFE is limited by scattering

16. Compositional disorder Amorphous indium oxide TFT 2011 JACS μFE ~ 22.14 at 300°C No compositional disorder High carrier density 2011 Nature materials μFE ~ 2.3 at 300°C

17. Compositional disorder TiO2/SiO2 sol-gel reaction 1) With isopropanol 2) Without isopropanol Homocondensation Vs. Heterocondensation Heterocondensation Increases Compositional disorder

18. Fermi energy level Donor level Shallow level is preffered Oxygen vacancy level In2O3 > SnO2 > ZnO High fermi level Low Vp Easier to establish percolation path

19. Conclusion Field effect mobility of amorphous oxide is dependent to 1) Compositional disorder 2) Fermi energy level Fermi energy level near conduction band edge → Easy percolation at low gate voltage Effects on MTR conduction – localized state density & energy level

20. Future work Field effect mobility of amorphous oxide is dependent to 1) Uniform composition Precursor mixing sequences, chemical effect of precursors *(Homo-condensation & Hetero-condensation) 2) Fermi energy level Defect chemistry Studying hybrid DFT calculations