Nano Today
Wafer-scale growth of single-crystal graphene on vicinal Ge(001) substrate
Graphical abstract
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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Qinghong Yuan received the Ph.D. degree from The Chinese University of Hong Kong, Hong Kong, in 2010. In 2012 she joined the Department of Physics at East China Normal University and was promoted to a Full Professor in 2015. She is currently working in the University of Queensland as a DECRA research fellow. Her current research interests focus on theoretical study of the chemical vapor deposition growth of graphene and carbon nanotubes, structure of property investigation of new nanomaterials, surface catalysis, etc.
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These authors contributed equally to this work.