Pinhole-seeded lateral epitaxy and exfoliation of GaSb films on graphene-terminated surfaces

Abstract

Remote epitaxy is a promising approach for synthesizing exfoliatable crystalline membranes and enabling epitaxy of materials with large lattice mismatch. However, the atomic scale mechanisms for remote epitaxy remain unclear. Here we experimentally demonstrate that GaSb films grow on graphene-terminated GaSb (001) via a seeded lateral epitaxy mechanism, in which pinhole defects in the graphene serve as selective nucleation sites, followed by lateral epitaxy and coalescence into a continuous film. Remote interactions are not necessary in order to explain the growth. Importantly, the small size of the pinholes permits exfoliation of continuous, free-standing GaSb membranes. Due to the chemical similarity between GaSb and other III-V materials, we anticipate this mechanism to apply more generally to other materials. By combining molecular beam epitaxy with in-situ electron diffraction and photoemission, plus ex-situ atomic force microscopy and Raman spectroscopy, we track the graphene defect generation and GaSb growth evolution a few monolayers at a time. Our results show that the controlled introduction of nanoscale openings in graphene provides an alternative route towards tuning the growth and properties of 3D epitaxial films and membranes on 2D material masks.

Controlling the Balance between Remote, Pinhole, and van der Waals Epitaxy of Heusler Films on Graphene/Sapphire

“…This atomic step morphology is similar to bare sapphire annealed under the same conditions (Figure S3 in the Supporting Information). The 700 °C annealed graphene/sapphire also displays a much smaller concentration of pinholes than transferred graphene on III–V substrates after native oxide desorption: ∼10/μm 2 for graphene on sapphire, compared to ∼200/μm 2 for graphene on GaAs that result from amorphous oxide desorption . We attribute the reduced graphene pinhole density on sapphire to the fact that Al 2 O 3 (0001) is an air-stable crystalline surface, in contrast with III–V surfaces that are terminated with an amorphous oxide.…”

“…The 700 °C annealed graphene/sapphire also displays a much smaller concentration of pinholes than transferred graphene on III−V substrates after native oxide desorption: ∼10/μm 2 for graphene on sapphire, compared to ∼200/μm 2 for graphene on GaAs that result from amorphous oxide desorption. 8 We attribute the reduced graphene pinhole density on sapphire to the fact that Al 2 O 3 (0001) is an air-stable crystalline surface, in contrast with III−V surfaces that are terminated with an amorphous oxide. The high-temperature-annealed graphene/ sapphire interfaces provide a cleaner starting point for investigating the mechanisms for epitaxy on graphene-covered surfaces.…”

“…A pinhole mechanism occurs when pinholes or other openings in the graphene selectively nucleate the direct epitaxy of film on a substrate, followed by lateral overgrowth and coalescence . These pinholes can appear natively in the graphene or they can be created during pregrowth annealing, due to desorption of native oxides or other contaminants at the transferred graphene/substrate interface. , van der Waals epitaxy, in which a film grows with epitaxial registry to the 2D material rather than the underlying substrate, can also produce exfoliatable single crystalline films. Examples include GaN on graphene/SiC (0001) and GaN on hexagonal BN/Al 2 O 3 (0001) .…”

“…This corresponds to the expected hexagon-on-hexagon (R0) epitaxial relationship false⟨ 1 1 2̅ 0 false⟩ LaPtSb ∥ false⟨ 1 1 2̅ 0 false⟩ Al 2 normalO 3 , which is the same orientation that appears for direct epitaxy of LaPtSb on sapphire (Figure S5 in the Supporting Information). Both remote epitaxy and pinhole epitaxy provide consistent explanations for the R0 orientation, since the ability to exfoliate from graphene does not strictly exclude a pinhole growth mechanism …”

“…First, increasing the growth temperature is expected to tune the balance between remote epitaxy and pinhole-seeded epitaxy (Figure d). Pinhole epitaxy is favored at high growth temperatures, in which the surface diffusion length for Gd, Pt, and Sb adatoms is larger than the distance between graphene pinholes (λ > L ) . Here, adatoms can diffuse far enough to find the more chemically reactive pinhole sites, leading to direct nucleation of R0 domains at pinholes.…”