Growth of Semiconducting Graphene Nanoribbons and Single-Crystal Graphene on Germanium

The chemical vapor deposition of graphene on germanium (Ge) and germanium-on-silicon (Ge-on-Si) substrates provides a scalable, industry-compatible route for integrating graphene-based devices on traditional semiconductor platforms. Here, we will discuss our work on the wafer-scale synthesis of aligned semiconducting graphene nanoribbon arrays and single-crystal semi-metallic graphene films on Ge wafers.<br/><br/>First, we have discovered how to drive graphene crystal growth kinetics into a highly anisotropic regime, enabling the synthesis of graphene nanoribbons with tunable sub-5-nm widths, nearly atomically defined armchair edges, atomic thickness, and unidirectional alignment on Ge(001) and vicinal Ge(001).^1,2,3,4 This precise control over the nanoribbon width and edges transforms graphene into a semiconductor with technologically useful bandgap &gt; 0.5 eV. The nanoribbons also exhibit high performance in field-effect transistors in terms of their on-state conductance (~10³ μS μm^-1) and on/off current ratio (~10⁴). Initiating the nanoribbon growth from nanoscale seeds significantly reduces ribbon-to-ribbon width polydispersity and provides control over the nanoribbon placement. Such semiconducting nanoribbon arrays with narrow widths and well-defined edges promise to meet the demands of speed, energy efficiency, density, and functionality required for next-generation semiconductor electronics and promise to meet the performance metrics set by the International Roadmap for Devices and Systems for the foreseeable future.<br/><br/>Second, we have elucidated the factors controlling the crystallinity of graphene grown on Ge(110) using advanced dark-field low-energy electron microscopy (DF-LEEM) and micro-low-energy electron diffraction (μ-LEED). This insight is utilized to produce large-area graphene films with minimal rotational misalignment—a major step toward realizing state-of-the-art technologies harnessing the exceptional properties of graphene lacking defective grain boundaries. We have discovered a new phenomenon in which misoriented graphene domains nucleate from the edges of unidirectionally aligned graphene islands when the islands grow over Ge surface steps, increasing polycrystallinity throughout growth. Extensive synthesis studies provide strategies for suppressing this secondary nucleation of misoriented domains to achieve graphene films in which the predominant epitaxial orientation has high coverage &gt; 99% and low rotational spread &lt; 0.6°. These results clarify the varying crystallinity and irreproducibility of graphene grown on Ge(110) reported in the literature, demonstrate the importance of using techniques with high spatial and angular resolution when determining if 2D materials are single crystals, and provide a route towards the large-area synthesis of single-crystal graphene on commercially available and technologically useful substrates.<br/><br/>[1] R. M. Jacobberger, et al. <i>Nat. Commun.</i> <b>6</b>, 8006 (2015).<br/>[2] R. M. Jacobberger, et al. <i>ACS Nano</i> <b>11</b>, 8924-8929 (2017).<br/>[3] R. M. Jacobberger, et al. <i>Nanoscale</i> <b>11</b>, 4864-4875 (2019).<br/>[4] A. J. Way, et al. <i>Nat. Commun.</i> <b>13</b>, 2992 (2022).