Abrupt changes in the graphene on Ge(001) system at the onset of surface melting

Abstract

By combining scanning probe microscopy with Raman and x-ray photoelectron spectroscopies, we investigate the evolution of CVD-grown graphene/Ge(001) as a function of the deposition temperature in close proximity to the Ge melting point, highlighting an abrupt change of the graphene's quality, morphology, electronic properties and growth mode at 930 °C. We attribute this discontinuity to the incomplete surface melting of the Ge substrate and show how incomplete melting explains a variety of diverse and long-debated peculiar features of the graphene/Ge(001), including the characteristic nanostructuring of the Ge substrate induced by graphene overgrowth. We find that the quasi-liquid Ge layer formed close to 930 °C is fundamental to obtain high-quality graphene, while a temperature decrease of 10 degrees already results in a wrinkled and defective graphene film.

Driving with temperature the synthesis of graphene on Ge(110)

“…Finally, we suggest that the nucleation of embryos graphene bilayer observed at TD= 930 ° is also in line with the formation of a quasi-liquid Ge surface layer which boosts the mobility of Ge atoms and their diffusional flow up to the surface of the first graphene layer, in line with what was observed on Ge(001) [13,20].…”

“…Drawing a parallel between the data reported here for graphene/Ge(110) and the graphene synthesis Ge(001) [20], we deduce that the intimate connection of incomplete melting of Ge to the quality of graphene synthesis appears to be the general and leading driving force for quality enhancement of graphene on Ge substrates.…”

“…While growing, graphene tends to follow the potential energy minimum of the van der Waals interaction [47]: the presence of a mobile, liquid Ge layer hinders strong surface corrugations of the substrate, thus preventing the rippling of graphene. In addition, on a quasi-liquid substrate, the healing of defects is enhanced [20,48,49]. These mechanisms, acting above the onset of Ge surface melting, favor the formation of a flat, high-quality graphene film at high deposition temperature, as evidenced by Raman fingerprints (i.e.…”

Epitaxial graphene/Ge interfaces: a minireview

“…For gr/Ge(110) the single-domain graphene growth is observed at high temperatures as found by LEED and transmission electron microscopy (TEM) 55,62 , whereas for gr/Ge(111) the polycrystalline graphene layer is formed with a weak domain orientation preference. In case of the gr/Ge(001) interface, the strong faceting of the Ge surface under graphene is observed with the preferential formation of the {107} facets [57][58][59]61,63,[65][66][67] , as it is revealed in SEM and STM experiments ( Fig. 2(c,f)).…”

“…In all studied cases for the preparation of gr/Ge(110) 54,64,73 and gr/Ge(001) 54,63,66,67 , the increase of the synthesis temperature between 910 • C and 930 • C leads to the abrupt changes in the quality of graphene as indicated by the significant reduction of the intensity of the D peak as well as by the increase of the ratio of intensities of the 2D and G peaks ( Fig. 3(a)).…”