Two-Dimensional Phosphorus: From the Synthesis Towards the Device Integration
Creators: name: Faraone, Gabriele
Publisher: name: Zenodo
Year: 2021
Subjects: phosphorene, 2D materials, silicene

description: Phosphorus and silicon two-dimensional (2D) allotropes have been the forerunners among the post- graphene monoelemental 2D materials. The scientific and technological advantages of these materials require the development of processing methods to guarantee their effective integration in new devices for nanoelectronics. In the present thesis work, some of the unresolved bottlenecks along the device integration path of 2D elemental phosphorus allotropes have been examined considering specifically the case of the 𝛼-P (single-layer black phosphorus or phosphorene) and 𝛽-P (blue phosphorene) 2D polymorphs. The integration of the 2D 𝛼-P phase in devices has been the subject of extensive investigations and nowadays relies on an almost consolidated path that has led to applications spanning a wide range of fields. One of the few remaining obstacles on this path is the lack of a scalable method to produce 2D 𝛼-P layers on large areas and with accurate control of the thickness. In particular, such control is difficult to achieve in the exfoliation of layered black phosphorus (BP) crystals. In this respect, micro-Raman spectroscopy has been used both as a metrological tool to determine the thickness of the exfoliated flakes and as method to achieve their controllable thickness reduction employing the laser thinning technique. However, thickness determination methods based on the calibration of the intensity of the Raman bands have been poorly investigated in the case of multilayer BP flakes due to difficulties caused by optical interferences and anisotropy effects. In this thesis work, we have proposed a novel Raman spectroscopy approach that, carefully accounting for these effects, allowed the quick discrimination of the thickness of exfoliated BP flakes between 5 nm and 100 nm. Moreover, in order to achieve a better control of the laser thinning process down to the ultimate 2D limit, we have also investigated the effects of the substrate on the laser heating and ablation of multilayer BP flakes. Raman thermometry experiments and numerical calculations of the heat diffusion problem have elucidated that optical, thermal, and mechanical effects caused by the substrate may act differently on the laser heating and ablation of multilayer flakes depending on their thickness. An effective device integration route for the 2D 𝛽-P phase, instead, is still missing due to more stringent requirements in the synthesis, based on epitaxial techniques, and to the instability issue outside the UHV growth environment. These obstacles are commonly shared with other members of the family of 2D epitaxial Xenes and, in this work, have been investigated considering the case of 𝛽-P epitaxially grown on Au(111)/mica substrates. The details of its atomic structure and the chemical reactivity to ex-situ and in-situ oxygen exposure have been analyzed with the aid of Scanning Tunneling Microscopy (STM) and X-Ray Photoelectron Spectroscopy (XPS). The air-instability issues have been tackled by developing an encapsulation strategy based on the in-situ growth of an Al2O3 capping layer that, in turn, allowed the handling of epitaxial phosphorus along the preliminary steps of a device integration process. In this respect, two novel approaches for the transfer of the epitaxial membrane from the growth substrate towards target substrates have been surveyed. Both the transfer methods can be generalized to the whole class of 2D epitaxial Xenes grown on metal/mica paving the way for the establishment of methodological standards for their manipulation. In particular, the universality of such approaches has been exploited for the successful fabrication of back-gated FET and MIM devices on Al2O3/multilayer silicene/Ag(111) and Al2O3/epitaxial phosphorus/Au(111) mica-delaminated samples, respectively. The epitaxial phosphorus MIM devices may open intriguing perspectives to study of the non- volatile resistive switching in monoelemental 2D materials.