Supplementary Information s50

Supplementary Information Direct Growth of Graphene Film on Germanium Substrate Authors: Gang Wang1,2, Miao Zhang1, Yun Zhu1, Guqiao Ding1, Da Jiang1, Qinglei Guo1, Su

Liu2, Xiaoming Xie1, Paul K. Chu3*, Zengfeng Di1*, and Xi Wang1

Affiliations:

1 Sate Key Laboratory of Functional Materials for Informatics, Shanghai Institute of

Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning

Road, Shanghai 200050, China

2 School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China

3 Department of Physics and Materials Science, City University of Hong Kong, Tat Chee

Avenue, Kowloon, Hong Kong, China

*Correspondence to: [email protected];[email protected]

1 Experimental details

Tem:800~910℃

Heating Growth Cooling

t1 40~100min t2 Time(min)

Supplementary Figure S1. Schematic illustrating the preparation of graphene films by

APCVD.

Graphene preparation:

The Ge substrates(175 um thick, ATX) were cut into 1×1 cm2 pieces and placed at the center of the horizontal quartz tube. The synthesis was carried out in a horizontal tube furnace inside a quartz processing tube (50mm inner diameter). the quartz tube was evacuated to approximately 10-5 mbar and then filled with 200 standard cubic cm per min (sccm)

Argon(Ar, 99.9999% purity) and 50 sccm Hydrogen( H2, 99.9999% purity ). After heating to

o o the desired temperature (varied from 800 C to 910 C), CH4 (varied from 0.1 sccm to 3 sccm) was introduced to deposit the graphene film for diffferent time durations (from 40 min to 100 min). After deposition, the CH4 gas was turned off and the furnace was cooled to room temperature under flowing H2 and Ar. The process is summarized in Figure S1.

2 1.0 (a) 10 25 11 24 0.8 12 23 2 9 3 13 ) 0.6 m c

( 22 8 1 4 14

Y 0.4 7 5 15 21 6 16 0.2 20 17 19 18 0.0 0.0 0.2 0.4 0.6 0.8 1.0 X (cm)

3 1.30 I /I (c) 1.29 1.26 2D G 1.31 1.30 1.4 1.37 1.35 1.30 1.35 1.35 1.3

1.33 1.31 1.30 1.37 1.33 1.2 1.29 1.29 1.34 1.38 1.36 1.1 1.38 1.41

1.29 1.36 1.30 1.0

Supplementary Figure S2. (a) Twenty five points are selected from an area of 1×1 cm2 on the graphene grown Ge substrate for Raman measurement; (b) Raman spectra acquired from

2 2 13 of the 25 points in the 1×1 cm area; (c) I2D/IG ratio map over the area of 1×1 cm determined by the wafer viewer software.

4 T/oC

G G 3500 L 3826oC (0.1MPa) 2834oC

(C) 1500 ~938oC

500 0 10 20 30 40 50 60 70 80 90 100 Ge C Carbon(Atomic Percent)

Supplementary Figure S3. Phase diagrams of the selected binary systems: Ge-C.

o Fast cooling rate (200 C/min) o Medium cooling rate (100 C/min) o Slow cooling rate (20 C/min) 2D G ) . u . a (

t i s n e t n I

1 2 0 0 1 5 0 0 1 8 0 0 2 1 0 0 2 4 0 0 2 7 0 0 3 0 0 0 Raman Shift (cm-1)

Supplementary Figure S4. Raman spectra of graphene film grown on Ge substrate directly with different cooling rates.

5 Graphene Transfer:

Graphene films were transferred from the Ge substrate onto a highly-doped p-Si wafer with a

300 nm thermal oxide by a PMMA-assisted wet-transfer method in order to evaluate the electrical transport properties. A thin layer of polymethyl methacrylate (MicroChem 950

PMMA C, 3 % in chlorobenzene) was spin-coated onto the substrate to protect the graphene film and to act as a support which was then cured at 180 oC for 10 min. Afterwards, the Ge substrate was etched away by a mixture of HNO3:HF (1:1) allowing the PMMA/carbonic layer to float on top of the solution. After placing the layer on a filter paper, it was washed with deionized water. The PMMA/graphene layer was subsequently transferred to another substrate, followed by annealing at 180 oC for 90 min to improve adhesion. The PMMA was then dissolved gradually with acetone and deionized wafer. Finally, the graphene sample was washed with isopropanol and then annealed under flowing argon and hydrogen for 6 h at

300oC to remove the residual PMMA. Even though the transfer process was optimized, defects, wrinkles, and overlaps existed inevitably in the transferred graphene, as observed by

Raman scattering and SEM (Figures S5 (a) and (b)).

Transferred 2D (b) (a) G As grown ) . D u . a (

wrinkle overlap y

t i s n e t n I crack 1um

1 2 0 0 1 5 0 0 1 8 0 0 2 1 0 0 2 4 0 0 2 7 0 0 Raman Shift (cm-1)

Supplementary Figure S5. (a) Comparison between the Raman spectra of graphene deposited directly on Ge and that after transferring onto the SiO2/Si wafer. (b) SEM image of graphene transferred onto the SiO2/Si wafer.

6 Device fabrication and electrical transport measurement

After transferring onto a highly doped p-Si wafer covered with a 300 nm thick thermal oxide, the source and drain electrodes were defined by standard photolithography, followed by electron beam evaporation of Au/Ti (50/10 nm) and lift-off. Afterwards, another photolithographic step emplying inductively coupled plasma (ICP) was used to pattern the graphene into a field effect transistor with a channel length of 8 μm and width of 2 μm. To impove the contact of the back-gated graphene field effect transistor (GFETs) device, thermal

o annealing was performed in an H2/Ar atmosphere at 300 C for 2 h in a tube furnace. The back-gated GFETs were characterized under ambient conditions using Agilent (B1500A) semiconductor parameter analyzer. The mobility was extracted using the following equations1:

dIDS L mFET = dVG W鬃 C ox V DS

- where L and W are the channel length and width, Cox is the gate oxide capacitance (11nF cm

2 ), VDS is the source drain voltage, IDS is the source drain current, and VG is the gate voltage.

The linear regime of the transfer characteristics was used to obtain dIDS/dVG.

Supplementary References

S1.G. Liu et al. Low-frequency electronic noise in the double-gate single-layer graphene transistors. Appl. Phys. Lett. 95, 033103-1-033103-3 (2009).