Techniques of Synthesizing Wafer-scale Graphene Zhaofu ZHANG

Outline  Graphene and its applications  Synthesizing graphene  Mechanical exfoliation  CVD on metal substrate  Epitaxial growth on substrate  Synthesizing wafer-scale graphene  Conclusion 2

Graphene is a single atomic layer of graphite and consists of sp 2 bonded carbon atoms arranged in two-dimensional honeycomb (hexagonal) lattice. Figure 1. Structure (a) and Raman spectrum (b) of Graphite and Graphene. Graphene and its application 3 (a)(b) [1] [1] A. C. Ferrari et al, Phys. Rev. Lett. 97, 2006,

Figure 2. Future applications of graphene. (a) novel transistors; (b) Bio-sensor; (c) Transparent conductor; (d) MEMS (b) 4 Graphene and its application (d) (a) (c)

Outline  Graphene and its applications  Synthesizing graphene  Mechanical exfoliation  CVD on metal substrate  Epitaxial growth on substrate  Synthesizing wafer-scale graphene  Conclusion 5

6 Synthesizing graphene

CON:  Difficulty in controlling layers  Not suitable for large scale synthesizing Figure 4. Characteristic of graphene with (a) AFM, (b) TEM and (c) SEM image [2] 7 Mechanical exfoliation (b) (a) (c) Figure 3. Mechanical exfoliation graphene and transition. [2] A. K. Geim et al, Nature Materials, 6, 2007, 183.

1. CVD On Copper [3]  25μm thick Cu foils  Fused silica tube, back fill with hydrogen, 1000 ℃ and 40 mTorr;  Introduce CH 4 (g), total pressure of 500 mTorr;  Cooled to room temperature Process Flow B. Transfer Process  Copper etching in an aqueous solution of iron nitrate;  Coated with PDMS or PMMA, lifted from the solution;  Transfer to other substrate such as Si or SiO 2. Figure 5. (A) SEM image of graphene on a Cu foil. (B) HRSEM image showing a Cu grain boundary and steps. (C & D) Graphene films transferred onto a SiO 2 /Si substrate and a glass plate. 8 CVD on metal substrate [3] X. Li et al, Science, 324, 5932, 2009, 1312.

9 [4] K. S. Kim et al, Nature, 457, 2009, CVD On Nickel [4]  Thin layer nickel (less than 300 nm) deposited on SiO 2 /Si substrate  Heated to 1000 ℃ (Ar atmosphere)  Flowing reaction: gas mixtures (CH 4 :H 2 :Ar = 50:65:200)  Rapidly cooled to room temperature (25 ℃ ) at the rate of 10 ℃ /s (Ar) A. Process Flow B. Transfer Process  Attaching the PDMS substrate to the graphene  Nickel layer is etched using FeCl 3 solution  Transfer to a SiO 2 substrate Figure 6. Synthesis, etching and transfer processes for the largescale and patterned graphene films. SEM image A centimetre- scale graphene film grown on a Ni/SiO 2 /Si substrate.

Figure 7. (B) AFM image of graphitized SiC; (D). STM image of one monolayer 10 Epitaxial growth on substrate 1. Thermal decomposition (or graphitization) of SiC [5]  Si terminated (0001) face of single-crystal 6H- SiC  Surface oxidized/de- oxidized, H 2 etching  Heated to temperatures ranging from 1250 ℃ to 1450 ℃ for 1–20 min  Thin graphite layers are formed determined predominantly by the temperature Process Flow [5] C. Berger et al, Science, 312, 2006, 1191.

 On-axis 6H–SiC  CMP polished  CVD chamber, baked at 600 ℃ for 30 min  Final growth point (1350–1650 ℃ ), Chamber pressure: ~500 Torr (Ar)  Propane (C 3 H 8 ) was supplied as a carbon source. 5min (C 3 H 8 and Ar)  Cooled down (150 ℃ /min) right after growth (Ar) Figure 8. Raman spectra of samples grown on C-face SiC 11 2.CVD on SiC [6] [6] J. Hwang et al, J. Crystal Growth, 312, 2010, Process Flow

Outline  Graphene and its applications  Synthesizing graphene  Mechanical exfoliation  CVD on metal substrate  Epitaxial growth on substrate  Synthesizing wafer-scale graphene  Conclusion 12

1. Polycrystalline CVD on Cu [7] Figure 9. (a) I D /I G and (b) I 2D /I G Raman mapping at the center of a 300 mm growth substrate shows the high quality of graphene with a negligible defect peak. (c) View of the 300 mm substrate used for this study. (d) I 2D /I G of Raman spot scans performed along the radial direction of the 300 mm substrate. 13 Synthesizing wafer-scale graphene [7] S. Rahimi et al, ACS Nano, 8(10), 2014,  Carried out in a CVD system at ℃  Substrates consist of ∼ nm Cu film on SiO 2 (300 nm)/Si wafer  2 min of annealing (H 2 ambient, flow rate 1000 sccm, pressure 25 mbar)  3 min of growth (CH 4 ambient, flow rate 10 sccm) with an automated wafer transfer at 600 ℃ Process Flow

 Solid phase epitaxial growth of Ge films on Si wafers with H- terminated surface  Ge wafer was cleaned  Dipped Ge into 10 % diluted HF to remove the native oxide  H-terminated Ge substrate was immediately loaded into a LPCVD chamber  To synthesize graphene, a mixture gas of CH 4 and H 2 ( %) was introduced (900 – 930 ℃, 100 Torr at for 5 – 120 min)  The as-grown substrate was rapidly cooled to room temperature under vacuum. Process Flow Figure 10. Single-crystal monolayer graphene grown on a hydrogen-terminated Ge(110) surface. (B) SEM. (C) Graphene grown on a 5.08-cm Ge/Si (110) wafer. (D) HRTEM. (E) TEM. 14 Synthesizing wafer-scale graphene [8] J. H. Lee et al, Science, 334, 2014, Single-crystal CVD on H- terminated Ge [8] Figure 10. (b) Proposed model for the catalytic graphene growth on the H-terminated Ge surface.

3in.×3in. Process Flow Figure 11. (a) Schematic of catalyst engineered graphene growth process on Cu −Ni alloy (b) Cu−Ni (1200/400 nm) film before (left) and after (right) graphene growth. (c) XRD. (d) SEM (e) graphene on quartz plates (f) Time dependent graphene growth on Cu−Ni film, 15 Synthesizing wafer-scale graphene [9] W. Liu et al, Chem. Mater. 26, 2014, CVD on engineered Cu−Ni catalyst films [9] Schematic of the CVD system (with 8” heater )  Cu (thermal evaporation ) and Ni (E-beam evaporation ) were deposited on SiO 2 (300nm)/Si  Substrate was heated to 990 °C while flowing 100 sccm H 2 at 20 mbar  Annealing at 990 °C for 30 minutes  Temperature was reduced by 70 °C  Gas mixture of CH 4 and H 2 was flowed at 0.2 mbar with rates of 5 sccm and 5 sccm for 120 S  Sample was rapidly cooled 400 °C while flowing Ar under a pressure of 1 mbar.  Sample was unloaded once the temperature reached below 90 °C

.. Figure 12. Schematic of face-to-face transfer graphene mediated by capillary bridges. 16 Synthesizing wafer-scale graphene [10] L. Gao et al, Nature, 505, 2014, CVD on Cu and face-to-face transfer [10] Process Flow 8 inch or 4 inch SiO 2 /Si wafers Characterization of face-to-face transferred graphene on a SiO2/Si wafer

Outline  Graphene and its applications  Synthesizing graphene  Mechanical exfoliation  CVD on metal substrate  Epitaxial growth on substrate  Synthesizing wafer-scale graphene  Conclusion 17

18 Method Mechanical Exfoliation CVD on CuCVD on Ni Epitaxial Growth on SiC PRO High quality Highest mobility (200,000 cm 2 /V.s) Large scale synthesis Mobility of (5,000 – 10,000 cm 2 /V.s) Monolayer Large scale synthesis Mobility of (more than 3,000 cm 2 /V.s) Mobility (~1000 cm 2 /V.s at 300 K) No need for transfer CON Not for large scale synthesis Thickness and size difficult to control Has to be transferred to substrate Small grain sizes Multilayers Expensive substrate Wafer- scale NoYes No Conclusion

Thanks ! 19

CVD On Copper [8]  Thermal CVD on a Cu foil.  Methane as the precursor gas.  A quartz tube (2”) as the reaction chamber.  Cu foils were rolled up in a roll.  PMMA was spun to form PMMA/ graphene/Cu sandwich structure.  Later, Cu foil was etched away.  PMMA/graphene was transferred onto a SiO 2 (300nm)/Si wafer.  The PMMA was removed by repeatedly rinsing the film in acetone Process Flow Figure 10. Raman spectra 20 Synthesizing wafer-scale graphene [8] W. Wu et al, Sensors and Actuators B, 150, 2010, 296. A 4” × 4” graphene film was transferred onto a 6” Si wafer with a thermally grown oxide.

 Sputtering Co film on a sapphire c-plane substrate  CVD at 1000 ℃ (with CH 4 and H 2 )  Sudden cooled down after the reaction Figure 9. (d) (e) Raman mapping images (f) Corresponding Raman spectra Epitaxial growth on sapphire [7] [7] H. Ago et al, ACS Nano, 4 (12),2010, Process Flow