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micro-crystals Single Photon InfraREd detectors

Periodic Reporting for period 2 - microSPIRE (micro-crystals Single Photon InfraREd detectors)

Reporting period: 2018-11-01 to 2021-10-31

µSPIRE aims at developing a key enabling technology and processing that will empower the integration on silicon of active devices, using many different semiconductor heterostructures based on materials such as Ge, GaAs and AlGaAs. In fact, many electronic and optoelectronic devices will benefit from µSPIRE fabrication technology, which provides a high degree of flexibility in the integration of dissimilar materials to the well-established silicon technology.
The work carried out over the 48 months duration of microSPIRE can be branched in two closely interlinked thematic:

I) Reaching a deeper understanding of the physical mechanism leading to the formation of semiconductor microcrystals on patterned Si substrates, and highlighting the role played by microcrystal faceting in the nucleation of crystal defects (dislocations, antiphase boundaries) during heteropitaxy.
II) Developing photodetectors based on Si and Ge microcrystals epitaxially grown on Si patterned substrates.
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.
µSPIRE ambition is to radically change how epitaxial heterostructures are integrated on Si. The novelty of µSPIRE's approach resides in the control of out-of-equilibrium epitaxial growth under geometrical constraints imposed by the substrate patterning. It is therefore a growth mode that is strongly kinetically controlled and thus allows for extreme flexibility in material properties engineering, from morphology to composition.
In µSPIRE, by exploiting VHE we aim at radically changing how photon-counting detectors are designed and fabricated. The combination of a novel fabrication platform and the innovative design of detectors made of many microcrystals will give birth to a non-incremental and revolutionary approach for designing active devices with the introduction of other (Ge, GaAs, GaAlAs, GaN, SiC) materials.
Eptaxially grown semiconductor microcrystals: modeling and experiments