The main results achieved during the project are summarized here below.
I) Development of a modelling tool capable of predicting the evolution of the 3D morphology of microcrystals under a specified deposition rate, substrate temperature and patterned substrate geometry. This required the recasting, in the formalism of phase-field theory, of the fundamental physical mechanism taking place during growth such as the inhomogeneous and time variable incoming flux of adatoms and facet dependent incorporation. The theoretical work was supported by extensive campaigns of scanning and transmission electron microscopy measurements (SEM and TEM) performed on a set of Si and Ge microcrystals grown under different deposition conditions. These results can find applications in the modelling of fin FET structures, oxide recess filling and horizontal nanowire formation which lay the heart of several microelectronic and photonic devices.
II) Growth of SiGe microcrystals on few nanometres wide Si pillars to induce the elastic relaxation of misfit strain.
III) Growth of GaAs microcrystals on Ge‑on‑Si microcrystals to reduce the impact of anti-phase domains (APDs) formation.
IV) Fabrication of a single photon avalanche diode (SPAD) based on Si microcrystals (Si-µSPADs). These device has been designed using a TCAD tool taking as an input the crystal morphology evolution as obtained by a 2D rate-equations model. The fabrication required a combination of optical lithography (for substrate patterning), epitaxial growth (for microcrystal formation), e-beam lithography and implantation (for top contact formation). The fabricated devices do show photon counting capabilities at cryogenic temperatures (e.g. at 100 K).
V) Fabrication of a p‑i‑n photodetectors based on Ge microcrystals. The fabricated photodiodes are based on Ge microcrystal arrays connected by a suspended graphene layer. Interestingly, these devices show an enhanced absorption in the infrared region comprised between the direct and indirect gap of Ge (λ = 1550‑1800 nm) when compared with conventional Ge‑on‑Si photodetectors. This effect can be ascribed to light‑trapping phenomena associated to the faceted morphology of microcrystals and has to be expected even in forthcoming devices operating in single photon counting mode.
The enhanced infrared absorption observed in Ge microcrystal arrays can be beneficial for a wealth of application fields such as, automotive and machine vision, security, material recognition, pharmaceutics and heritage preservation.
