Apr 9, 2025
5:00pm - 7:00pm
Summit, Level 2, Flex Hall C
Valerio Piazza1,Riccardo Brondolin1,Sena Türker1,Cyril Cayron2,Thomas Hagger1,Mitali Banerjee2,Anna Fontcuberta i Morral1
Ecole Polytechnique Federale de Lausanne1,École Polytechnique Fédérale de Lausanne2
Valerio Piazza1,Riccardo Brondolin1,Sena Türker1,Cyril Cayron2,Thomas Hagger1,Mitali Banerjee2,Anna Fontcuberta i Morral1
Ecole Polytechnique Federale de Lausanne1,École Polytechnique Fédérale de Lausanne2
Conventional epitaxial methods enable to grow high quality crystals by coordinating the arrangement of incoming atoms through the dangling bonds naturally present on the substrate surface [1]. Van-der-Waals epitaxy (VdWE) instead occurs when the epitaxial crystalline material is bound to the substrate by weak van der Waals interactions [2]. The absence of covalent bonds promises strain free (and ideally defect free) crystals. Unfortunately achieving a large-scale monocrystalline layer appears virtually impossible due to the lack of periodic surface potential. The tunable superperiodic nature of twisted bilayer graphene (tBLG) [3] can be the key to overcome this obstacle and merge specific aspects of both processes thus creating a universal epitaxial template.
In this work we explore the innovative concept of Moirè epitaxy, i.e. the use of bilayer graphene with controlled twist angle able to imprint a periodic surface potential in a 2D epitaxial substrate. We fabricate tBLG templates from mechanically exfoliated graphene with a “tear and stack” process [4]. The stack is fully encapsulated in hBN via dry transfer method. The top hBN is etched with XeF
2 dry chemistry [5]. Atomic force microscopy confirmes that the tBLG surface is exposed to the atmosphere. We investigated several cleaning recipes to minimize contaminations from the fabrication process, including O
2 plasma, annealing in vacuum and rapid thermal processes. We monitored the defectivity of the stack by micro-Raman mapping before and after the etching and cleaning steps.
We explored the effect of temperature and In/P fluxes on the nucleation and morphology of InP nano-crystals grown on 22°-twisted templates by molecular beam epitaxy.This angle corresponds to the 1
st order Moirè pattern with the closest superperiodicity to the lattice constant of InP(100). We were able to define the window of parameters enabling to obtain InP crystals with hexagonal symmetry on the graphene substrate. Electron back-scattering diffraction analysis reveals a predominant population of crystals with top (111) facets. Both evidences indicate a clear relationship with the hexagonal nature of graphene. So far, no indication of rotational ordering has been observed. Nonetheless, photoluminescence mapping revealed enhanced emission in the crystals grown on graphene, commonly interpreted as a proof of improved crystal quality. We therefore expanded this investigation further by fabricating templates with twist angles of 18° and 25°. These values closely match the lattice constants of InP(001) and Zn
3P
2(004). Overall we explored the effect of three different twist angles on two different compound semiconductors thus creating the base to develop a new solid state synthetic methodology.