Substrate engineering and advanced epitaxial growth for the production of group IV semiconductor freestanding membranes

Abstract : Semiconductor-based freestanding membranes (FSMs) have recently become central to the rapidly expanding frontiers of nanoscience and technology, and a highly promising area of advanced materials research. FSMs offer an extra degree of freedom for implementations that cannot be obtained by conventional methods such as heteroepitaxy, which often involves significant lattice mismatch in crystalline structures. Fabrication of FSMs from various materials allows for layer-by-layer stacking, enabling an easy coupling of the physical properties of dissimilar materials. Additionally, FSM structures offer unprecedented lightweight, and flexibility compared to conventional substrates. This demonstrates their high potential for the fabrication of novel applications, such as stretchable on-skin electronics or vertically stacked devices, flexible optoelectronics, etc., as well as a straightforward path for heterointegration. Furthermore, the use of FSMs provides significant cost savings in device production, especially for materials with orders of magnitude higher prices than that of silicon, as only a fraction of the material is being used when compared to conventional wafers. In this context, group IV materials FSM attract a lot of attention for their applications in high-performance optoelectronics and high-speed telecommunication, as well as for their biocompatibility and nontoxicity compared to III-V counterparts. However, the fabrication of high-quality group IV FSMs is still a challenging task. In this thesis, we demonstrate two promising paths for production of group IV FSMs using substrate engineering and advanced epitaxial growth. The first part of this work focuses on 2D-assisted epitaxy. We introduce the Anchor Point Nucleation approach enabling the growth of high-quality FSMs over a graphene-covered surface. Through plasma treatment defects, such as dangling bonds and nanoholes, are introduced in the graphene layer, acting as preferential nucleation sites. The experimental data unravel the nature of these defects, their role in nucleation, and the mechanisms governing this technique. Additionally, high-resolution transmission electron microscopy combined with geometrical phase analysis established that the as-grown layers are perfectly single crystalline, stress-free, and oriented by the substrate underneath the engineered graphene layer. These findings provide new insights into graphene engineering by plasma and open a universal pathway for the heterointegration of high-quality 3D semiconductors on graphene, and fabrication of FSMs. The second part of this work focuses on an alternative approach to produce group IV FSMs using nanostructured substrates. First, we demonstrate the formation of homogenous porous germanium (PGe) layers across the entire 100 mm wafer using bipolar electrochemical etching, with the possibility to tune the physical properties of the PGe structure by variation of etching parameters. The PGe nanostructure maintains the crystalline nature and orientation of the Ge substrate, and presents low surface roughness, making it an ideal substrate for the epitaxy. The low-temperature growth allows maintaining the PGe’s integrity during the formation of high-quality FSM on top. The membrane can then be easily detached through the porous interface and the substrate can be cleaned for reuse and fabrication of multiple FSMs generations. These findings provide new opportunities to produce lightweight and flexible, high-performance optoelectronics based on Ge FSMs, while also ensuring reduction of both costs and critical material consumption.

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