Nano Today
Volume 34, October 2020, 100908
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Wafer-scale growth of single-crystal graphene on vicinal Ge(001) substrate

https://doi.org/10.1016/j.nantod.2020.100908Get rights and content

Highlights

  • Unidirectional graphene islands are achieved on the vicinal Ge(001) substrates with a miscut angle above 10°.

  • Theoretical calculations together with ingeniously designed ex situ AFM observations reveal the underlying growth mechanism of well-aligned graphene islands on the vicinal Ge(001) surface.

  • 4-inch single-crystal graphene wafer with an ultrahigh carrier mobility is obtained on the vicinal Ge(001) surface with 15° miscut angle.

  • A practical approach for commercial production of single-crystal graphene wafers.

Abstract

Wafer-scale single-crystal graphene with high carrier mobility is essential as a promising channel material for the next-generation two-dimensional nanoelectronics. However, direct synthesis of wafer-scale single-crystal graphene on complementary metal oxide semiconductor (CMOS) compatible substrates still remains a challenge. Herein, we demonstrate that single-crystal graphene film with high mobility can be synthesized on the 15° miscut Ge(001) surface by perfectly aligning all the graphene islands and this feat has never been achieved on the normal Ge(001) surface. Both experimental observations and theoretical calculations suggest unidirectional alignment of the graphene islands on the 15° miscut Ge(001) surface is caused by suppression of graphene nucleation along the miscut direction of the vicinal surface. Ex situ atomic force microscopy (AFM) verifies that no additional graphene island nucleates after the initial nucleation process and wafer-scale single-crystal graphene is formed by the seamless stitching of the preferentially oriented graphene islands. The obtained wafer-scale single-crystal graphene possesses an ultrahigh carrier mobility, opening an avenue toward scalable fabrication of two-dimensional nanoelectronic devices based on single-crystal graphene without grain boundaries.

Introduction

Graphene, a perfect two-dimensional carbon material, has attracted global interest due to its exceptional chemical stability, superior mechanical stability, and extremely high carrier mobility. In particular, wafer-scale single-crystal graphene with high mobility is crucial for massive production of graphene based nanoelectronic devices and circuits. However, grain boundaries (GBs) are usually formed inevitably in graphene during the chemical vapor deposition (CVD) process [[1], [2], [3], [4], [5], [6], [7], [8]]. Some progress has been made regarding the growth of single-crystal graphene films on metal catalysts such as Cu [[9], [10], [11], [12], [13], [14], [15], [16], [17]] and Cu-Ni alloy [[18], [19], [20], [21], [22]], but the mainstream integrated circuit (IC) technology requires metal-free single-crystal graphene on CMOS compatible substrate. In recent years, semiconducting Ge wafer has been successfully utilized as a proper substrate for the epitaxial growth of graphene due to its catalytic activity and low solubility of carbon [23]. Integration of graphene with Ge substrates by the epitaxial growth approach may address the scarcity issue of IC-compatible graphene wafers and promote the development of graphene-based nanoelectronic devices and circuits.

Wafer-scale single-crystal graphene film was firstly synthesized on 2-inch Ge substrate with the unusual (110) orientation by Lee et al. [24]. Our previous study further discovered that the formation of well-aligned graphene islands, which are seamlessly stitched to form a single-crystal graphene film, was caused by the lattice matching between the graphene islands and natural atomic steps on the Ge(110) surface [25]. However, the mainstream CMOS technology is mainly based on Si wafers with the usual (001) orientation. Considering the conventional Ge epilayer on Si substrate, the Ge(001) substrate is more compatible with the standard Si-based CMOS process compared to Ge(110). The pioneer study on graphene grown on Ge(001) found that graphene nanoribbons (GNRs) with a high aspect ratio and preferential orientation were always formed along the two perpendicular Ge<110> directions [26,27]. GNRs with an equivalent population in the orthogonal orientations lead to the formation of numerous GBs when GNRs expand to merge into a continuous wafer-scale film, which will degrade the performance of graphene-based nanoelectronic devices and circuits. Recently, much efforts, including H2/CH4 flow ratio optimization [28] and vicinal Ge(001) substrate [29,30], have been made to improve the crystal quality of graphene on Ge(001) wafers. However, the formation of GBs could not be fully suppressed by optimizing H2/CH4 flow ratio. Meanwhile, the miscut angle of vicinal Ge(001) substrate was found to affect the orientations of GNRs, however the expected well-aligned GNRs together with the consequent seamless stitching to form a single-crystal graphene wafer has not been demonstrated due to the limited variation of miscut angle of vicinal Ge(001) wafer.

Herein, using vicinal Ge(001) substrate with a miscut angle of 10° or above, unidirectional alignment of graphene islands is achieved and the wafer-scale single-crystal graphene realized by the seamless stitching of well aligned graphene islands is demonstrated on the 15° miscut Ge(001) surface. Both experimental data and theoretical calculations show that the nucleation orientation selectivity of graphene islands closely correlates with the miscut angle of the vicinal Ge(001) surface. Complete suppression of GNR nucleation along the miscut direction is observed from the vicinal Ge(001) surface with a miscut angle of 15°, while aligned graphene islands perpendicular to the miscut direction are maintained. Furthermore, the designed ex situ AFM indicates no additional graphene islands nucleate after the initial nucleation process, and the wafer-scale single-crystal graphene is formed by the merging of aligned graphene islands after the island expansion. The obtained single-crystal graphene wafer possesses an ultrahigh carrier mobility comparable to that of exfoliated graphene, as evident by terahertz time-domain spectroscopy (THz-TDS), which constitutes a significant advance toward the manufacturing of graphene-based nanoelectronic devices and circuits.

Section snippets

Graphene growth

The vicinal Ge(001) substrates with various miscut angles toward [111] direction were loaded into a horizontal quartz tube. The quartz tube was evacuated to about 10−6 bar, and then refilled with argon (Ar, 99.9999 % purity) and hydrogen (H2, 99.9999 % purity) to reach atmospheric pressure. The chamber was heated to 916 °C with Ar and H2 for 1 h and then methane (CH4, 99.99 % purity) was introduced to initiate graphene growth. Afterwards, the flow of CH4 was shut off and the furnace was cooled

Results and discussion

The miscut angle α is defined as the angle between the (001) crystal plane and the vicinal surface, which is cut toward the [111] direction of (001) oriented Ge substrate, as shown in Fig. 1a. Fig. 1b–i presents the AFM friction images of the GNRs grown on the vicinal Ge(001) substrates with miscut angles varying between 0° and 15°. Consistent with the previous reports [26,27,29,30], the GNRs grown on the Ge(001) surface with 0° miscut angle (Fig. 1b) have two dominant orientations, which are

Conclusions

In conclusion, the alignment of graphene islands closely correlates to the miscut angle of the vicinal Ge(001) substrate, and wafer-scale single-crystal monolayer graphene film can be synthesized by the seamless stitching of aligned graphene islands on the vicinal Ge(001) substrate with the miscut angle of 15°. Theoretical calculations together with ex situ AFM observations reveal that the unidirectional alignment of graphene islands on the vicinal Ge(001) surface with large miscut angle is

Declaration of Competing Interest

The authors declare no competing financial interest.

Acknowledgements

The authors thank Key Research Project of Frontier Science, Chinese Academy of Sciences (QYZDB-SSW-JSC021), National Science and Technology Major Project (2016ZX02301003), National Natural Science Foundation of China (Grant Nos. 51925208, 61851401, 61774163, 61974157, and 21673075), Science and Technology Commission of Shanghai Municipality (18511110700), Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB30030000), Australian Research Council through the Discovery

Panlin Li received the B.S. degree from the Department of Material Science and Engineering, Nanchang University, Nanchang, China, in 2015. He is currently a Ph.D. candidate at Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences (CAS), Shanghai, China. His current research interests focus on the synthesis of two-dimensional materials.

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    These authors contributed equally to this work.

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