Oriented planar nanomaterials for high-performance TFT device

1. Oriented planar nanomaterials for high-performance TFT device

2. Introduction: ZnO thin film transistor Recent advances in electronic and display devices demand something more: High-performance ex) OLED (currently using p-si TFT) And low-temperature, in same time

3. Solution-based approaches Integration Performance Reliability Proc. Temp. Easy Moderate Good Moderate Easy Poor Moderate Low Difficult Good Poor Low Thin-film Collective nanomaterials Individual nanomaterials How can “high performance” and “low processing temperature” be accomplished? Hydrothermal method : high crystallinity from low temperature process But, it is still difficult to integrate into device

4. Nanomaterial integration Electrode separation E-beam lithography Photolithography Shadow mask ~101 nm ~1 μm ~103μm To integrate into device, nanowires have to: Ultra-long Lateral growth Wire aligning

5. Nanomaterial integration Nanoplatelet looks like to be easier to integrate than NW (without aligning) In reality, it is still unapropriate for integration Low surface energy basal plane High energy axial plane Nanowall formation is natural if considering surface energy

6. ZnO crystal morphology vs. Citarate concentration (a) When a very small quantity of citrate ions was added,the ZnO rods became shorter and fatter On average, the aspect ratio of the crystals was directly related to the citrate concentration (b) Much higher citrate concentrations can produce plate-like ZnO crystals (d,f) secondary growth in a solution without citrate ions <001> growth behaviour was restored and the 5–10-nm layered features were mostly ‘healed’

7. Citrate absorption on ZnO nanostructures: Sodium citrate 5.21Å Schematic of citrate absorbed on mineral 3.25Å 5.21Å Zn-Zn interspacing on axial plane Citrate can be absorped well

8. Aligning orientation of ZnO nanorods 3-steps for growing oriented ZnO nanorods 1. deposition of crystal seeds on the substrate surface 2. growth of randomly oriented crystals from the seeds 3. growth of aligned nanorods after extended reactions Nanorod overlap with other neighbouring crystals physically limit their misaligned growth

9. Oriented ZnO nanostructures: Secondary and tertiary growth with citrate First step: oriented growth of ZnOnanorod on seed layer (without citrate) Without seed With seed Second step: oriented growth of ZnOnanorod on seed layer (without citrate)

10. Biomimetic nanostructure ← Laterally grown ZnO nanoplatelet ← Nacre in red abalone ← Secondary growth at a much higher citrate concentration. This process produced a bilayer structure containing a first layer of ZnO nanorods and a second layer of ZnO nanoplates

11. Film growth from oriented nanocrystals Growth of ZnO on GaN-Buffered Al2O3 (0001) ZnO and GaN have the wurtzite structure and a small lattice mismatch (ca. 2%) Al2O3 have larger lattice mismatch (ca. 16.7%) ← ZnO grown at pH 7.5 at 90 °C for 24 h. discontinuous ZnO thin film ← ZnO grown at pH 10.9 at 90 °C for 24 needle shaped ZnO crystals (ca. 10 lm in length) Nothing was observed on the GaN substrate for pH values ≥ 11.0.

12. Film growth from oriented nanocrystals ← ZnO crystallites grown only in solution B on the GaN substrate ZnO hexagonal prisms (ca. 7 μm in length) ← ZnO crystallites grown in solution A and then solution B on the GaN Sharp needle shaped ZnO crystallites were converted into hexagonal prisms Without remarkable change in number and its length

13. Density of Oriented ZnO Crystallites: Nucleation at different Values of pH Significant nucleation of ZnO on the GaN surface at low pH (Fig. 1a and b) and but not at high pH → related to valence state of the soluble Zn species and the surface charge of the GaN Negatively charged Zn(OH)42– ions are generally considered as the soluble Zn species when the soluble species, Zn(OH)42– becomes supersaturated, it nucleates ZnO via the dehydration reaction Zn(OH)42– → ZnO + H2O + 2OH –

14. Density of Oriented ZnO Crystallites: Nucleation at different Values of pH The reaction of GaN with water would produce Ga–OH surface sites that would react with both H+ and OH- to produce positive (–Ga+) and negative (–GaO-) surface sites IEP of GaN and Ga2O3 < 10 At ca. pH 10.9, negative surface sites would dominate on the substrate, whereas at lower pH, positive surface sites would dominate Negatively charged soluble species, Zn(OH)42–, would be attracted by columbic forces to the surface at low pH, which results in the formation of a high density of ZnO crystallites on the substrate In addition, it appears that defects in the substrate, e.g., dislocations, might be responsible for the random nucleation of the needle-shaped, hexagonal prisms found at high pH.

15. Film growth from oriented nanocrystals 1st growth : pH 7.5, without citrate, 90℃ for 24hrs. 2nd growth : pH 10.9, with citrate, 90℃ for 8hrs. ← Rod & film from Same nuclei ZnO film grown in 2nd solution during the second step Fully coalesced to produce a smooth surface morphology and a film approximately 2∼3 μm thick

16. Morphology of ZnO Crystallites: Growth in Different Solution ZnO thick film grown in solution B at pH 10.9 is composed of fully coalesced ZnO hexagonal prisms to produce a film with smooth surface morphology IEP of ZnO powder occurs at pH 9.4 (dominating {1120} pland surfaces) Below the IEP, the sign of the ZnO surface sites is predominately positive thus, the negative citrate ions are specifically adsorbed on all ZnO surfaces Citrate ion can have three negatively charged sites, it not only neutralizes one positive surface site, but adds two negative charges to the surface and thus shifts the apparent IEP of ZnO to lower values of pH above the iep, the {1120} planes will have many more negative surface sites relative to positive sites. Since the second stage growth occurs at pH 10.9, well above the IEP, the citrate ions that have three negative sites will not be attracted to {1120} planes, but specifically attracted to the positive (Zn terminated) basal planes of ZnO to hinder growth ¯ ¯

17. TFT application? GaN is a semiconductor material which is inhibited in ZnO TFT structure Without epitaxial layer, film can be grown (but seed layer is still required) Film is too thick for TFT application ZnO layer have to be made as a form of submicron thin-film (not micron-size thick film)

18. Future work #1 ZnO nanorods Masking 1st nanorod growth Glass substrate & ZnO seed layer Polymer mold ZnO thick film ZnO thin film 2nd growth: By molded lateral overgrowth, submicron thin film is formed

19. Lateral overgrowth ZnO grown on MgAl2O4(111) with citrate at 90℃ With masking Advantage moreover : overgrown region has less defect and better crystallinity because dislocations cannot “bend” to be incorporated carrier mobility of window region : 8.8 cm2V–1s–1 carrier mobility of wing region : 29.8 cm2V–1s–1

20. Lateral overgrowth Like making turbine blade ZnO at window LEO ZnO LEO ZnO Grown from polycrystalline ZnO, but no grain boundary for growth direction, in overgrown ZnO On more bend, perfect single crystal

21. Future work #2 Substrate #1 Nanorods Polymer Seed layer Substrate #1 Substrate #2 1. Nanorod growth on patterned seed layer 2. Polymer spin-coating on substrate #2 Imprint nanorod on polymer 3. Polymer etching reveals top part of nanorods 4. 2nd growth of ZnO Laterally overgrown ZnO thin film

22. Device integration Gate Source Drain If this overgrown length is not sufficient, then Gate Source Drain Electrodes can be prepared under seed layer (layers patterned at same time)

23. Channel diffusion problem? CdS nanowires deposited into the AAO template by CBD Diameters of pores (channels) of AAO lie in several tens of nanometers range

24. Experiment Step #1 Autoclave 24ml Solution containing Zinc nitrate & HMTA : pH~7 80 ℃ 18 hrs. Silicon substrate w. ZnO seed Holder Step #2 Same setup, with Zinc nitrate & Na citrate solution 80 ℃, 8 hrs.

25. Next week ① Many large particles Suspicion 1. Homogeneously grown particles → In reference, substarate is facing down 2. Impurities on substrate → 2nd cleaning procedure after ZnO layer deposition ② SEM work to confirm nanorod and film morphology ③ Continue to NOA filling and plasma etching experiment

26. Previous week Poor result: No JSC & power output WO3/Ag/WO3 P3HT/PCBM Al/LiF Glass Bottom electrode (Al/LiF) was most suspicious This week Bottom electrode is replaced with Al/ZnO and ITO/ZnO WO3/Ag/WO3 WO3/Ag/WO3 P3HT/PCBM P3HT/PCBM ITO/ZnO Al/ZnO Glass Glass

27. Result ITO/ZnO works properly WO3/Ag/WO3 P3HT/PCBM ITO/ZnO Glass VOC=0.58V ISC=1.15mA/cm2 FF=0.32 η=0.21% Low current seems to be due to high resistance of ZnO layer ZnO layer thickeness have to be reduced

28. Result WAW has good transmittance Significant loss of current by light absorption is not observed Light absorption from WAW side ~ 2/3 of absorption from ITO side ITO side: η=0.14% WO3/Ag/WO3 WO3/Ag/WO3 P3HT/PCBM P3HT/PCBM ITO/ZnO ITO/ZnO Glass Glass WAW side: η=0.12%

29. Result Al/ZnO did not works properly WO3/Ag/WO3 P3HT/PCBM Al/ZnO Glass Very low fill factor significantly degrades conversion efficiency Long time measurement recovered little amount of JSC

30. Next week ① Main suspicion : Al electrode 1. Al @ E-beam evaporator → Al @ Thermal evaporator (Chamber contamination or something?) 2. Zn/ZnO electrode (Interface defects and trap sites) WO3/Ag/WO3 P3HT/PCBM Al/ZnO Glass ② Device optimization (lower ZnOthickness) ③ Continue to disposable substrate preparation and device fabrication