www.sciencemag.org/cgi/content/full/science.1252268/DC1

Supplementary Materials for

Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium

Jae-Hyun Lee, Eun Kyung Lee, Won-Jae Joo, Yamujin Jang, Byung-Sung Kim, Jae Young Lim, Soon-Hyung Choi, Sung Joon Ahn, Joung Real Ahn, Min-Ho Park, Cheol-

Woong Yang, Byoung Lyong Choi,* Sung-Woo Hwang,* Dongmok Whang*

*Corresponding author. E-mail: dwhang@skku.edu (D.W.); swnano.hwang@samsung.com (S.-W.H.); choibl@samsung.com (B.L.C.)

Published 3 April 2014 on Science Express

DOI: 10.1126/science.1252268

This PDF file includes:

Materials and Methods Figs. S1 to S14 References

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Materials and Methods

Preparation of single crystal Ge film on Si wafer Hetero-epitaxial growth of Ge films on Si wafers was performed using the solid

phase epitaxy method (19). The Si substrates were chemically cleaned to remove all types of contaminants and to create an H-terminated surface. The samples were then loaded into the CVD chamber, which was evacuated. The base pressure of chamber is approximately 3 × 10-6 Torr. The flow rate of germane (GeH4) gas (10 % diluted in H2) as a precursor was 40 sccm, and total pressure is maintained at 30 Torr at 300 ˚C for 30 min. After deposition, the Ge layers were crystallized by thermal annealing at 600 oC for 30 min. Growth of single-crystal graphene on Hydrogen-terminated Ge surface

First, a single-crystal Ge wafer or Ge-deposited Si wafer was cleaned using the standard RCA method followed by oxygen plasma treatment for the removal of organic residues. Then, the Ge substrate was dipped into 10 % diluted HF to remove the native oxide. H-terminated Ge substrate was immediately loaded into a low-pressure chemical vapor deposition (LPCVD) chamber (SETS, S-R0D1), and fresh epitaxial Ge layer was deposited by flowing GeH4 gas (40 sccm, 10 – 30 Torr) at 500−900 ˚C for 30 min. To synthesize graphene, a mixture gas of CH4 and H2 (99.999 %, ultra high purity grade) was introduced in chamber at 900 – 930 oC. During the growth process, total pressure is maintained at 100 Torr at for 5 – 120 min. Finally, the as-grown substrate was rapidly cooled to room temperature under vacuum. Etch-free dry transfer of graphene

To transfer the graphene without using the wet etching process, we used an Au thin film layer as a supporting layer and electrode. The Au thin film was deposited on the graphene-grown Ge substrate using thermal evaporator. Then, the Au/graphene/Ge is attached to a thermal release tape (TRT, Haeun Chemtec, RP70N5) by applying slight pressure. The TRT/Au/graphene and Ge substrate were easily separated without using a wet etching process. The Au-deposited graphene film on the thermal release tape was placed on the desired substrate and exposed to appropriate temperature (100 oC, 2 min). To evaluate the properties of graphene, graphene covered Au thin film could be easily removed by dipping the sample into a KI/I2 solution.

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Graphene field effect transistor fabrication and measurement Graphene field effect transistors (GFET) were prepared using an efficient device

fabrication process. Figure S8 presents a schematic illustration of the device fabrication steps and Hall bar structures. Using dry transfer methods, the Au-coated graphene samples were transferred onto p-type degenerate Si(100) wafer with 300-nm-thick SiO2. Then, first patterning was performed using a photoresist (PR, AZ1512) coating, prebaking (95 oC, 1 min), UV exposure, development (MF319, 90 s), a DI rinse, and postbaking (100 oC, 1 min) processes. The Au thin film was then etched using KI/I2 solution. To define the GFET structure, the exposed graphene was removed using oxygen plasma. Second patterning was then employed to open the graphene channel. The electrical characteristics of the devices were measured using a Keithley SCS-4200 system and vacuum probe station (MSTECH, MST5000) under high vacuum (approximately 1 × 10-6 Torr).

To extract the mobility, we used the Drude model, in which the mobility depends on the carrier concentration (29-31). The carrier mobility (µ) and carrier density (ns) can be calculated as

1

s shne Rµ =

,

( )ox g Diracs

C V Vn

e−

=

where, ns is the variable carrier density, which depends on the back-gate voltage; Cox is the gate oxide capacitance (11.5 nF cm-2); and e is electric charge. The sheet resistance (Rsh) was calculated as

2 1( )X X

sh

V V wR

Id−

= ,

where, w is the channel width, d is the distance between the voltage leads X2 and X1, and I is a current flowing between leads 1 and 2. Characterizations

SEM images were acquired with a JEOL JSM-7401F field emission scanning electron microscope. The crystallographic orientation of the Ge substrate was determined using electron backscatter diffraction (EBSD) using JSM 7000F. Bright field transmission electron microscopy (BF-TEM) images and selected-area electron diffraction (SAED) patterns were obtained using a JEOL ARM 200F at 80 kV. Raman spectroscopy was performed using two different spectrometers to investigate the properties of graphene. A Renishaw RM1000-Invia with laser excitation wavelengths of 514 nm (2.41 eV) equipped with a notch filter with 50 cm-1 cutoff frequency and a

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WITEC Raman system with excitation wavelength of 532 nm (2.33 eV) with a piezo stage were used to obtain mapping images of graphene. The laser power was lower than 2 mW to avoid laser-induced heating of graphene. X-Ray photoelectron spectroscopic (XPS) analysis was performed in an ESCA2000 spectrometer using monochromatic Al Kα radiation (1468.6 eV). The peak energies were calibrated by the C 1s peak at 284.6 eV. The thickness and surface topology of graphene were measured using an atomic force microscope (AFM, Dimension 3100, Veeco) under tapping mode at a slow scanning rate. LEED measurements of as-grown graphene on Ge surface were performed with base pressure less than 2.0 × 10-10 Torr in a UHV chamber. LEED images were measured using electron beam, with energy ranging from 60 to 120 eV. For sharp LEED patterns, all samples were annealing in the UHV chamber at 600 oC for 24 h and at 850 oC for 10 min in sequence. Supplementary References

29. Y. Zhang, Y. W. Tan, H. L. Stormer, P. Kim, Nature 438, 201-204 (2005). 30. A. Venugopal et al., J. Appl. Phys. 109, 104511 (2011). 31. C. Dimitrakopoulos et al., J. Vac. Sci. Technol. B 28, 985-992 (2010).

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Fig. S1. Simplified schemes of catalytic growth of monolayer graphene. (A) Polycrystalline graphene growth from multiple seeds with different orientations. Grain boundaries are highlighted with a green color. (B) Single crystal growth form single nucleus. (C) Single crystal growth from uni-directionally aligned multiple seeds.

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Fig. S2. Evolution of graphene islands to uniform single-crystal monolayer on Ge(110). (A-C) SEM images showing evolution of graphene islands to uniform monolayer during the CVD growth. Arrows in B point out edge fronts at which adjacent islands are merging. (D, E) TEM images of an edge front similar to the white arrows in B. (F, G, H) Lattice-resolved TEM images of the region I, II, and III in E, respectively. These images clearly show adjacent graphene islands have same orientation and merge without grain boundary defects. The merging without grain-boundary was confirmed by careful examination of four different edges.

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Fig. S3. Ge film epitaxially grown on Si(110) wafer. (A, B) Cross-sectional SEM images of the Ge film on the Si(110) wafer. An approximately 3-µm-thickness Ge film was successfully grown on Si(110) wafer without voids. (C, D) Top-view SEM image and Corresponding EBSD result of the Ge film on the Si(110) wafer. These results indicate that the entire Ge surface is flat and has (110) crystallographic orientation.

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Fig. S4. Proposed model for the catalytic graphene growth on the H-terminated Ge surface. (A) Chemisorption of carbon precursors on the H-terminated Ge surface. (B) Multiple nucleations of the graphene seeds. (C) Growth and coarsening of the graphene seeds. (D) Formation of complete monolayer graphene on the H-Ge surface.

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Fig. S5. Raman and X-ray photoelectron spectra of the graphene-grown Ge substrate. (A) Raman spectra of the Ge(110) substrate after graphene growth. (B, C) XPS spectra of C 1s and Ge 3d core peaks of the graphene-grown Ge surface. Absence of oxide-related peak for graphene/Ge substrate indicates that graphene could effectively passivate the Ge surface from oxidation.

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Fig. S6. Commensurate graphene overlayer on the H-terminated Ge(110) surface. The orange ball and green-colored grid indicate top-most Ge atom and the unit cell of the Ge(110) surface, respectively. The H atoms are omitted for clarity. Despite the large difference in their lattice constants, commensurability between graphene and the underlying crystal surface is possible by expanding the lattice to a larger superlattice. From the positions of the LEED spots of graphene and the Ge surface in Fig. 2A, the periodicity of the supercell is derived as (8 × 10) in terms of the Ge(110) unit cell. Furthermore, every 23 zigzag graphene units and perpendicular 16 armchair units almost perfectly coincide with 10 aGe along the Ge–[001] direction and 8 × √2aGe along the Ge–[1̄10] direction of the (110) facet, respectively.

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Fig. S7. Polycrystalline monolayer graphene grown on the H-terminated Ge(111) surface. (A) A SEM images of graphene seeds formed at the early stage of growth. The nucleation density was approximately 10 per µm-2. (B) A SEM image of flat monolayer graphene grown from the seeds in A. (C) HR-TEM image of polycrystalline graphene. The inset shows FFT diffraction pattern of the TEM image, indicating that the graphene obtained from the Ge(111) surface has two main orientations. (D, E) Magnified TEM images at selected regions (Region I, II and Region III, IV) in C, showing that the misorientation angle between two domains is 30 o.

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Fig. S8. Fabricated graphene field-effect transistor. (A) Combined optical and SEM images showing a fabricated graphene field-effect transistor (GFET). Inset shows a SEM image of the graphene channel in the device. (B) A typical sheet resistance (Rsh) vs. gate voltage (Vg) curve for GFET from the single-crystal graphene grown on H-Ge(110). (C) A typical Rsh vs. Vg curve for GFET from the poly-crystalline graphene grown on H-Ge(111).

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Fig. S9. Effects of the pre-bake and the CH4/H2 ratio on graphene growth. (A) Raman spectra of graphene samples grown on Ge(110) with and without pre-bake of the CVD chamber under H2 flow. Both graphenes were grown by flowing CH4 and H2 (CH4/H2 ratio: 0.01) with total pressure of 10 Torr at 900 oC. (B, C) LEED patterns of graphene grown on H-Ge(110) with and without pre-bake of the CVD chamber, respectively. (D) Raman spectra of graphenes grown with different CH4/H2 ratio. When the C ratio was increased, I(G)/I(D) and I(G)/I(2D) were decreased. Degradation of graphene quality is correlated with crystallinity of graphene. (E, F) LEED patterns of graphenes grown on H-Ge(110) with 0.05 and 0.1 of CH4/H2 ratio, respectively. When the gas ratio is less than 0.01, uniform graphene layer was not formed. In the cases of graphene grown without pre-bake and graphene with higher C ratio, the diffraction spots at LEED patterns were azimuthally elongated and even additional set of diffraction spots was appeared. From the graphene grown without pre-bake of CVD chamber, we infer residual oxygen adsorbed on the chamber surface causes partial oxidation of Ge surface, and thus induce graphene defects during growth. Growth at high C ratio may increase graphene seeds with non-optimized binding of graphene edge on Ge surface.

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Fig. S10. Idealized atomic structures of hydrogen-terminated Ge(110) and Ge(111) surfaces. Top-view and side-view images for the H-terminated (A) Ge(110) and (B) Ge(111).

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Fig. S11. SEM, AFM, and Raman results of the graphene grown on the Ge(110) substrate. (A, C) SEM and AFM images of the as-grown graphene on Ge(110) substrate, respectively. (B, D) SEM and AFM images of graphene transferred on SiO2 substrate. These SEM and AFM images show that the graphene grown on Ge(110) is extremely flat without wrinkles. (E, F) 2D Raman mapping images of intensity ratio of I(D)/I(G) and I(2D)/I(G) acquired from graphene transferred onto SiO2/Si wafer

(wavelength of 532 nm, 50 × 50 µm2, and step size of 500 nm), Overall, the intensity ratio is uniform and the value of I(2D)/I(G) is around approximately 5, which indicates a relatively large area of graphene single layer graphene without winkles.

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Fig. S12. Surface properties of H-Ge(110) under graphene. (A) Schematic and images of dry transfer process. The Au-coated graphene film can be transferred to an arbitrary substrate without introducing structural defects. This finding indicates that the binding energy per atom between graphene and the H-Ge(110) (γGe-G) is considerably less than 60 meV, because γAu-G is known to be approximately 60 meV. (B-D) Photographs and water contact angles for (B) exposed Ge surface after graphene delamination, (C) oxidized and (D) H-terminated Ge(110) surfaces. Water droplet contact angles of H-terminated Ge surface and Ge surface exposed after graphene delamination were almost identical, demonstrating the underlying Ge surface after graphene growth is hydrogen terminated.

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Fig. S13. A Schematic illustration of the etch-free dry transfer and subsequent device fabrication. (A-C) After graphene growth, Au thin film was deposited on graphene/Ge substrate in order to protect graphene surface from organic contamination. (D) Au/graphene/Ge is attached to a thermal release tape (TRT) by applying small pressure. (E) TRT/Au/graphene and Ge substrate is delaminated from Ge substrate without wet etching. (F-G) Au deposited graphene film on the TRT is placed on a desired substrate and exposed to proper temperature to release the TRT (100 oC, 2 min). (H) Au thin film on graphene can be easily removed by dipping in KI/I2 solution. (I-K) Fabrication process of the graphene field-effect transistor (GFET). After photolithographic patterning of the device structure, exposed Au layer was etched by Au etchant (KI/I2). O2 reactive ion etching (RIE) etched unwanted graphene. Finally, the channel region of the GFET is exposed by the second pattering process.

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Fig. S14. Transmittance curve as a function of wavelength for the Ge-catalyzed single layer graphene transferred onto glass. The inset shows a photograph of graphene on the glass. Ge-catalyzed graphene exhibits a high transmittance of over 97% over the entire visible range (97.64% at 550 nm).

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