Effect of Several Growth Parameters on Graphene Growth on Four Types of Supported Cu Films using Cold Wall CVD and Perspective on Growth Mechanism of Graphene from Scaling Functions of Graphene Island Size Distribution

In this work, I will present results from a series of studies using a custom-automated LabVIEW controlled graphene growth method in a custom-modified multi-chamber UHV chamber transformed into a cold wall CVD system. The controlled experimental approach allowed careful study of effect of several growth parameters on graphene growth on Cu. A systematic, in depth study of graphene in industrially preferred cold wall CVD has potential to shed light on studies of other single layer 2D materials.<br/><br/>Graphene growth was explored on solid electrodeposited, recrystallized, sputter deposited and liquid Cu films supported on W or Mo refractory substrates under ambient pressure using Ar, H2 and CH4 mixtures. Among these films, electrodeposited Cu film was chosen to study the effect of total flow rate, CH4:H2 ratio and dilution of the CH4/H2 mixture by Ar at a fixed substrate temperature of 1000 °C and total pressure of 700 Torr, on the nucleation density and average size of graphene crystallites. The resulting morphological changes correspond with those that would be expected if the precursor deposition rate was varied at a fixed substrate temperature for physical deposition using thermal evaporation. The evolution of graphene crystallite boundary morphology with decreasing effective C deposition rate indicates the role of edge diffusion of C atoms along the crystallite boundaries, in addition to H2 etching on graphene crystallite shape. The results indicate that graphene grown on Cu films using cold wall CVD follows a classical two-dimensional nucleation and growth mechanism. Following nucleation at the earliest growth stages, isolated crystallites grow, impinge and coalesce to form a continuous layer. During the pre-coalescence growth regime, the size distributions of graphene crystallites exhibit scaling which is a function of island area, graphene coverage, average island area and areal density. For graphene grown on Cu surfaces that have been annealed in a reducing Ar+H2 ambient, excellent data collapse onto a monotonically decreasing universal Avrami scaling function is observed irrespective of graphene coverage, surface roughness or Cu grain size. This result is interpreted to indicate attachment limited growth and desorption of C-containing species. Graphene grown on Cu surfaces that were annealed in a nonreducing environment exhibits a qualitatively different bimodal scaling function indicating diffusion-limited growth with a lower attachment barrier combined with C detachment from the graphene edges.<br/><br/>Graphene growth on molten Cu films supported on custom-designed Mo substrates demonstrate a similar 2D nucleation and growth mechanism. The study also explores the orientation of graphene islands on molten Cu. The roles of temperature gradient, chamber pressure and rapid thermal heating in C precursor-rich environment on graphene growth morphology on thin sputtered Cu films are explained. A comparison of graphene growth on electrodeposited Cu vs recrystallized Cu is also shown.<br/><br/>The growth process and the observed results are instructive for the burgeoning graphene industry. Due to the similarities between the growth mechanism of graphene synthesis using cold wall CVD and hot wall CVD, the considerable knowledge base relevant to hot wall CVD can be exploited for graphene growth using the relatively less-explored cold wall CVD. I also anticipate that applying the study of pre-coalescence size distribution method to other 2D material systems may be useful for elucidating atomistic mechanisms of film growth that are otherwise difficult to obtain.