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Techniques of Synthesizing Wafer-scale Graphene

Zhaofu ZHANG

2022 2056

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Outline

Graphene and its applications Synthesizing graphene

Mechanical exfoliation CVD on metal substrate Epitaxial growth on substrate

Synthesizing wafer-scale graphene Conclusion

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Graphene is a single atomic layer of graphite and consists of sp2 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

(a) (b)[1]

[1] A. C. Ferrari et al, Phys. Rev. Lett. 97, 2006, 187401.

4Figure 2. Future applications of graphene. (a) novel transistors;

(b) Bio-sensor; (c) Transparent conductor; (d) MEMS

(b)

Graphene and its application

(d)

(a)

(c)

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Outline

Graphene and its applications Synthesizing graphene

Mechanical exfoliation CVD on metal substrate Epitaxial growth on substrate

Synthesizing wafer-scale graphene Conclusion

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Synthesizing graphene

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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]

Mechanical exfoliation

(b)(a)

(c)

Figure 3. Mechanical exfoliation graphene and transition.

PRO: Great quality, large grains High mobility: up to 200,000 cm2/Vs

[2] A. K. Geim et al, Nature Materials, 6, 2007, 183.

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1. CVD On Copper[3]

25μm thick Cu foils Fused silica tube, back fill with

hydrogen, 1000 and 40 mTorr;℃ Introduce CH4 (g), total pressure of

500 mTorr; Cooled to room temperature

Process Flow

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 SiO2/Si substrate and a glass plate.

CVD on metal substrate

[3] X. Li et al, Science, 324, 5932, 2009, 1312.

9[4] K. S. Kim et al, Nature, 457, 2009, 706.

2. CVD On Nickel [4]

Thin layer nickel (less than 300 nm) deposited on SiO2/Si substrate

Heated to 1000 (Ar ℃atmosphere)

Flowing reaction: gas mixtures (CH4:H2: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 FeCl3 solution

Transfer to a SiO2 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/SiO2/Si substrate.

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Figure 7. (B) AFM image of graphitized SiC; (D). STM image of one monolayer

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, H2 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.

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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 (C3H8) was supplied as a carbon source. 5min (C3H8 and Ar)

Cooled down (150 /min) right ℃after growth (Ar)

Figure 8. Raman spectra of samples grown on C-face SiC

2.CVD on SiC[6]

[6] J. Hwang et al, J. Crystal Growth, 312, 2010, 3219.

Process Flow

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Outline

Graphene and its applications Synthesizing graphene

Mechanical exfoliation CVD on metal substrate Epitaxial growth on substrate

Synthesizing wafer-scale graphene Conclusion

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1. Polycrystalline CVD on Cu[7]

Figure 9. (a) ID/IG and (b) I2D/IG 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) I2D/IG of Raman spot scans performed along the radial direction of the 300 mm substrate.

Synthesizing wafer-scale graphene

[7] S. Rahimi et al, ACS Nano, 8(10), 2014, 10471.

Carried out in a CVD system at 750-800 ℃ Substrates consist of 500-900 nm Cu film on ∼

SiO2(300 nm)/Si wafer 2 min of annealing (H2 ambient, flow rate 1000 sccm,

pressure 25 mbar) 3 min of growth (CH4 ambient, flow rate 10 sccm) with

an automated wafer transfer at 600 ℃

Process Flow

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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 CH4 and H2 (99.999 %)

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.

Synthesizing wafer-scale graphene

[8] J. H. Lee et al, Science, 334, 2014, 286.

2. Single-crystal CVD on H-terminated Ge[8] Figure 10. (b) Proposed

model for the catalytic graphene growth on the H-terminated Ge surface.

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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,

Synthesizing wafer-scale graphene

[9] W. Liu et al, Chem. Mater. 26, 2014, 907.

3. 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 SiO2(300nm)/Si

Substrate was heated to 990 °C while flowing 100 sccm H2 at 20 mbar

Annealing at 990 °C for 30 minutes Temperature was reduced by 70 °C Gas mixture of CH4 and H2 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

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.

Figure 12. Schematic of face-to-face transfer graphene mediated by capillary bridges.

Synthesizing wafer-scale graphene

[10] L. Gao et al, Nature, 505, 2014, 190.

4. CVD on Cu and face-to-face transfer [10]

Process Flow

8 inch or 4 inch SiO2/Si wafers

Characterization of face-to-face transferred graphene on a SiO2/Si wafer

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Outline

Graphene and its applications Synthesizing graphene

Mechanical exfoliation CVD on metal substrate Epitaxial growth on substrate

Synthesizing wafer-scale graphene Conclusion

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Method Mechanical Exfoliation CVD on Cu CVD on Ni Epitaxial Growth

on SiC

PROHigh quality

Highest mobility (200,000 cm2/V.s)

Large scale synthesis

Mobility of (5,000 – 10,000 cm2/V.s)

Monolayer

Large scale synthesis

Mobility of (more than 3,000 cm2/V.s)

Mobility (~1000 cm2/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

Has to be transferred to

substrate

Small grain sizes

Multilayers

Expensive substrate

Wafer-scale No Yes Yes No

Conclusion

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Thanks !

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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

SiO2(300nm)/Si wafer. The PMMA was removed by repeatedly

rinsing the film in acetone

Process Flow

Figure 10. Raman spectra

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.

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Sputtering Co film on a sapphire c-plane substrate

CVD at 1000 (with CH℃ 4 and H2)

Sudden cooled down after the reaction

Figure 9. (d) (e) Raman mapping images (f) Corresponding Raman spectra

3. Epitaxial growth on sapphire[7]

[7] H. Ago et al, ACS Nano, 4 (12),2010, 7407.

Process Flow