Early stage of CVD graphene synthesis on Ge(001) substrate

Abstract

In this work we shed light on the early stage of the chemical vapor deposition of graphene on Ge(001) surfaces. By a combined use of μ-Raman and x-ray photoelectron spectroscopies, and scanning tunneling microscopy and spectroscopy, we were able to individuate a carbon precursor phase to graphene nucleation which coexists with small graphene domains. This precursor phase is made of C aggregates with different size, shape and local ordering which are not fully sp² hybridized. In some atomic size regions these aggregates show a linear arrangement of atoms as well as the first signature of the hexagonal structure of graphene. The carbon precursor phase evolves in graphene domains through an ordering process, associated to a re-arrangement of the Ge surface morphology. This surface structuring represents the embryo stage of the hills-and-valleys faceting featured by the Ge(001) surface for longer deposition times, when the graphene domains coalesce to form a single layer graphene film.

Introduction

Catalyzed chemical vapor deposition (CVD) on metallic substrates has been largely predicted as one of the most promising techniques for the scalable synthesis of highly crystalline graphene, which is necessary for the development of graphene based electronics [1]. However, the graphene integration in standard complementary metal oxide semiconductor (CMOS) technology is hindered by metallic impurities and defects which are introduced by the growth process itself [2] or successively in the transfer process on Si wafers [3,4]. A significant improvement toward the compatibility of CVD graphene with CMOS-technology is represented by the recent achievement of metal contamination-free graphene grown directly on Ge or Ge/Si substrates [5,6], in particular on the technology relevant (001) surface orientation [[7], [8], [9], [10], [11], [12], [13]]. Despite this remarkable breakthrough, the quality of graphene deposited on Ge(001) should still be improved for “real-world” technological applications. To this end, it is necessary to investigate, understand, and acquire a full control over the adsorption and nucleation mechanisms of the carbon species on the substrate at the early stage of graphene growth, which deeply influence the quality of the graphene at all the subsequent stages of the deposition.

Theoretical studies of hydrocarbon decomposition mechanisms and nucleation on metal surfaces (such as Ir, Cu, Ni, Ru), have focused on competing roles of the C-C and carbon-metal interaction at the graphene-substrate interface on graphene nucleation [[14], [15], [16], [17], [18]]. In different substrates, this competition leads to different structures acting as stable precursors [14,16,19]. Early stage of graphene synthesis on metal substrates (e.g. Cu, Ir) was experimentally investigated in Refs. [[20], [21], [22], [23], [24], [25], [26], [27]]. A binding between the graphene precursor phase and the substrates has been evidenced by x-ray photoelectron spectroscopy (XPS) [21,27]. Gao et al. reported that precursors of the graphene growth on Ru were made of chains of C dimers [20], while in Ref. [24] was reported that graphene nuclei expand their lateral sizes by the addition of clusters comprising ∼5 C atoms. Concerning the graphene-Cu system, whose similarity with the graphene-Ge one has been recently evidenced in Ref. [10], different carbon clusters were identified by scanning tunneling microscopy (STM) as “growth intermediates” prior to the graphene formation and their evolution after the saturation of the surface in defected graphene was observed [22].

As for the CVD growth on Ge(001) substrate, the first stage of graphene nucleation has not yet been clarified yet. The quality and characteristics of the deposited graphene depend strongly on the Ge surface orientation [5,6,8,10,11], indicating that the C atom interaction with the Ge surface is of paramount importance in determining the graphene quality. Although Ge forms no stable carbide phase (GeC), theoretical calculations [28] suggest that, in graphene growth from solid C sources, the interaction between the Ge(001) surface and C atoms results in the immobilization of a C atom by substitution of a Ge atom in a surface dimer, so that either carbon dimers trapped by Ge dimer vacancies or longer C chains trapped between dimer rows act as graphene seed. As for CVD graphene, the same study [28] predicts that CH_x diffusing species can react with one another leading to the formation of polymeric carbon rings stabilized on the Ge(001) by GeC bonds.

In this work we experimentally investigate the early stage of CVD graphene synthesis on Ge(001) substrate using methane as precursor gas. To this end, we combine spectroscopic measurements (Raman spectroscopy and x-ray photoelectron spectroscopy) and an atomic scale characterization by means of scanning tunneling microscopy. Our analyses reveal that at the early stage of graphene growth a carbon precursor phase still not organized in the hexagonal atomic structure of graphene is present and covers a large part of the Ge surface. This C precursor phase evolves in graphene nuclei that in turn coalesce to form a single layer graphene (SLG).