Single crystal metal films on insulating substrates are attractive for microelectronics and other applications, but they are difficult to achieve on macroscopic length scales. The conventional approach to obtaining such films is epitaxial growth at h
igh temperature using slow deposition in ultrahigh vacuum conditions. Here we describe a different approach: sputter deposition at modest temperatures followed by annealing to induce secondary grain growth. We show that polycrystalline as-deposited Cu on alpha-Al2O3(0001) can be transformed into Cu(111) with centimeter-sized grains. Employing optical microscopy, x-ray diffraction, and electron backscatter diffraction to characterize the films before and after annealing, we find a particular as-deposited grain structure that promotes the growth of giant grains upon annealing. To demonstrate one potential application of such films, we grow graphene by chemical vapor deposition on wafers of annealed Cu and obtain epitaxial graphene grains of 0.2 mm diameter.
Films of (111)-textured Cu, Ni, and Cu$_x$Ni$_y$ were evaluated as substrates for chemical vapor deposition of graphene. A metal thickness of 400 nm to 700 nm was sputtered onto a substrate of $alpha-$Al$_2$O$_3$(0001) at temperatures of 250 C to 650
C. The films were then annealed at 1000 C in a tube furnace. X-ray and electron backscatter diffraction measurements showed all films have (111) texture but have grains with in-plane orientations differing by $60^{circ}$. The in-plane epitaxial relationship for all films was $[110]_{metal}$||$[10bar{1}0]_{{Al}_{2}{O}_{3}}$. Reactive sputtering of Al in O$_2$ before metal deposition resulted in a single in-plane orientation over 97 % of the Ni film but had no significant effect on the Cu grain structure. Transmission electron microscopy showed a clean Ni/Al$_2$O$_3$ interface, confirmed the epitaxial relationship, and showed that formation of the $60^{circ}$ twin grains was associated with features on the Al$_2$O$_3$ surface. Increasing total pressure and Cu vapor pressure during annealing decreased the roughness of Cu and and Cu$_x$Ni$_y$ films. Graphene grown on the Ni(111) films was more uniform than that grown on polycrystalline Ni/SiO$_2$ films, but still showed thickness variations on a much smaller length scale than the distance between grains.
Rotational misalignment of two stacked honeycomb lattices produces a moire pattern that is observable in scanning tunneling microscopy as a small modulation of the apparent surface height. This is known from experiments on highly-oriented pyrolytic g
raphite. Here, we observe the combined effect of three-layer moire patterns in multilayer graphene grown on SiC ($000bar{1}$). Small-angle rotations between the first and third layer are shown to produce a double-moire pattern, resulting from the interference of moire patterns from the first three layers. These patterns are strongly affected by relative lattice strain between the layers. We model the moire patterns as a beat-period of the mismatched reciprocal lattice vectors and show how these patterns can be used to determine the relative strain between lattices, in analogy to strain measurement by optical moire interferometry.
