Flexible high-k gate dielectric for appication of 2D semiconductor nanosheets

1. Flexible high-k gate dielectric for appication of 2D semiconductor nanosheets

2. Outline : Making flexible electronic devices Nanosheets: can be answer for flexible electronics? 2D-nanosheets considered to be component for flexible electronics – Graphene,MoS2,Mica,etc… Free-standing case Nano-thin sheet: Very-low strain under significant bending High flexibility with brittle material Fixed-on-substrate case Higher strain on nanosheet with bending Very high mechanical strength is required for nanosheet → Nanosheet itself is flexible, but high mechanical strength is required for flexible electronics

3. Mechanical strength of 2D nanosheets Characterization method: Nano-indentation with AFM tip For pristine monolayer graphene: Young's modulus E = 1.0 Tpa Intrinsic strength σ= 130 GPa Breaking strain = 13% Origin of high mechanical strength GB and defect free and not from its thickness → Atomic C-C bonding should be ruptured

4. Mechanical strength of 2D nanosheets Example 1. Defective nanosheet may be weak CVD-grown BN nanosheet: 3% breaking strain (much lower than theoretical estimation ~20%) Example 2. Thicker nanowire may be strong High mechanical strength (~6% breaking strain) close to theoretical fracture strength

5. Electrical properties of MoS2nanosheet MoS2nanosheet Semiconductor nanosheet High breaking strain 6-11% for mechanically exfoliated sheet → Theoretical upper limit based on Mo-S bonding Electrical characteristics of MoS2nanosheet MoS2 transistor on SiO2 dielectric : mobility of 0.5 ~ 3 cm2/Vs (much lower than phonon-scattering limited mobility for bulk MoS2) MoS2 transistor on high-k HfO2 dielectric Significantly increased mobility of ~200 cm2/Vs Suppression of Coulomb scattering due to the high-κ dielectric environment

6. Role of high-k gate dielectric Dielectric constant and Coulomb scattering When semiconductor nanosheet contact with lower dielectric constant insulator, Coulomb scattering rate is significantly increased Dielectric constant of MoS2 ~ 7 Mobility of semiconductor nanosheet For > 1 nm thickness, the mobility can be improved by coating the film by high-k dielectrics

7. Role of high-k gate dielectric MoS2 transistor with ionic liquid gating High-capacitance ionic liquid for enhancing mobility of MoS2 ~10 nm MoS2nanoflake was used Measured at ~220 K temperature, to decrease chemical reactivity of ionic liquid MoS2 thin flake and succeeded in demonstrating ambipolaroperation in MoS2 by observing hole conductivity in a MoS2 EDLT Hall mobility Electron: 44 cm2/Vs Hole: 86 cm2/Vs

8. Role of high-k gate dielectric Flexible MoS2 transistor with ion gel dielectric Ion-gel: Solution for instability of liquid body CVD-grown trilayer MoS2 film Mobility of 12.5 cm2/Vs (higher than solid dielectric-gated CVD-grown MoS2) Bending with 0.75mm curvature (~1.67% strain) High mechanical flexibility of MoS2 and ion gel film

9. Flexible high-k gate dielectric High-k gate dielectric for flexible electronics Solid state + high-k + high mechanical strength + low temperature process Relaxor ferroelectric polymer Polymer dielectric: mechanically flexible but low dielectric constant Using ferroelectric nanodomain (PVDF) (superparaelectric) – high dielectric constant ~60 is achieved

10. Flexible high-k gate dielectric Ferroelectric nanoparticle-based hybrid nanocomposite Nanocrytalline BaTiO3 behaves like a dielectric with high dielectric constant PVA: k = 5.1 PVAIA: k = 6 (larger polar group to aid dispersion of nanoparticles) BaTiO3 (nanoparticle) PVA/BTO PVAIA/BTO k value of each nanocomposite PVA/BaTiO3 : 10.9 PVAIA/BaTio3 : 12 All gate dielectrics show leakage current density < 10-5 A/cm2 PVAIA

11. Flexible high-k gate dielectric Nanoparticle dispersion in polymer matrix For high dielectric constant, high loading of nanoparticle is prefferred Addition of surfactants could improve the dispersion of nanoparticles → But residual free surfactant may lead to high leakage current and dielectric loss Modify nanoparticles via robust chemical bonds ODPA-modified ATO nanoparticles Chemical bonding via tridentate form Stable dispersion and low residual → Smooth surface → Low leakage current High loading of nanoparticles → High dielectric constant

12. Flexible high-k gate dielectric Mechanical property With significant strain (up to 2.5%) device withstand and did not fail (correspond to ~4mm bending radii) Electrical properties under bending 1. Gate dielectric capacitance Capacitance is decreased under tensile strain (Reduce in thickness by Poisson effect) 2. Semiconductor mobility Increased mobilities of organic semiconductors under compressive strain is attributed from reduced carrier hoping distance All the changes in m and VT are reversible during the bending test - No significant damage by bending Pentacene F16CuPc

13. Conclusion 1. Mechanical property of MoS2 is preferable for flexible electronic application Not by its thickness, but its defect-free crystal structure 2. Atomically thin nanosheet requires high-k dielectric to reduce Coulomb scattering To obtain high field-effect mobility which is required for high resolution display 3. High-k dielectric should be solid-state, mechanically flexible, and low-temperature-processable Ion gel can provide high capacitance, but problematic for device integration 4. Relaxorferroelectric can provide high dielectric constant And paraelectric polymer matrix can provide mechanical flexibility Combination of gate dielectric Ferroelectric nanoparticle(BaTiO3) + Polymer matrix and nanosheet semiconductor MoS2nanosheet → Highly flexible electronic device with high carrier mobility

14. Future work / Experimental procedure Experimental procedure 1. Nanoparticle preparation BaTiO3 (purchase: from Aldrich, <100nm with cubic crystalline phase) 2. Nanoparticle modification React with PEGPA in ethanol:water = 95:5 Ultrasonication for 10 min Stirring at 80 ˚C for 1 h Washing 3. Polymer nanocomposite PVP: hydroxyl-containing polymer PMF: providing binding sites PGMEA solvent with desired NP loading

15. Future work / Experimental procedure Expected performances High NP loading up to 50% and k ~ 20 can be achieved with low leakage current < 10 nA (Porous structure for NP loading over > 50%) Effect of NP loading to mechanical flexibility – is not known yet Experimental step 1. Dielectric fabrication → 2. Optimization for mechanical flexibility → 3. MoS2 integration

16. Experimental Bandgap opening of bilayer graphene High capacitance of Al2O3 EDL Bandgap opening by high electric field Breakdown characteristics Breakdown electric field ~ 2.5 MV/cm Leakage current ~ 1 µA/cm2 Capacitance is not reciprocal to thickness in EDL dielectric: possibility for superhigh electric field 1L 2L 3L

17. Experimental Capacitance Decreased by multi-coating : increased with thickness (Origin of reduction – investigation in progress) Proton transport mechanism – may can be influenced by water absorption (moisture sensitivity) Long-term stability examination – approves practical application 3x coating 1x coating 2x coating 20Hz 100Hz 1kHz 10kHz 100kHz 1MHz 1 2 3 4 5 6 Days