Sustainable Fabrication of Ge Freestanding Membranes and Substrate Reuse via Porosification Lift-Off
Authors: Hanus Tadeas , Ilahi Bouraoui , Provost Philippe-Olivier , Arias-Zapata Javier , Ayari Ahmed , Chapotot Alexandre , Pelletier Hubert , Cho Jinyoun , Dessein Kristof , Boucherif Abderraouf 
Published: December 2024

Recent advancements in flexible optoelectronics have spurred interest in developing nanoengineered substrates essential for the growth and fabrication of freestanding membranes (FSMs). The FSMs offer an extra degree of flexibility in applications previously unachievable through conventional techniques, such as the heterointegration of highly dissimilar materials. These membranes allow the stacking of various dissimilar materials, enabling the seamless coupling of their physical properties and integration onto a variety of substrates. Additionally, the use of FSMs instead of bulk materials significantly reduces costs and material usage during device production—a crucial advantage for non-silicon materials, which are considerably more expensive than silicon. Techniques such as substrate thinning, 2D-assisted epitaxy, and various substrate engineering approaches have demonstrated great potential for fabricating lightweight and flexible devices, underscoring the promise of FSM technology. However, the efficient production of FSMs based on germanium (Ge) and its alloys is still a challenging task.
In this work, we present the porosification lift-off process, which involves the sequential formation of porous Ge (PGe) nanostructure formation, epitaxial growth of Ge membrane, detachment, substrate reconditioning, and subsequent reuse. We demonstrate edge-to-edge formation of PGe nanostructures across 100 mm Ge wafers with excellent uniformity (Fig. 1a) using bipolar electrochemical etching[1]. Moreover, the physical properties (Thickness and porosity) of these structures can be efficiently tailored to suit specific applications by adjusting the etching parameters. These PGe structures maintain their substrate-oriented crystalline nature and exhibit smooth surface morphology, making them ideal substrates for epitaxial growth. Thin Ge FSMs are grown atop these PGe structure (Fig. 1b) at low temperature, preventing the PGe transformation into large pillars[2]. The initial growth stages on nanostructured substrate are investigated, showing the membrane formation process starting with initial 3D nucleation atop of the pore walls, followed by the coalescence of nucleation seeds into dense membrane, and concluding with the surface flattening by final 2D growth mode. This process allows for the growth of even ultrathin membranes with thickness as low as 60 nm. The high-quality monocrystalline nature of these membranes is confirmed by a thorough XRD analysis. After membrane formation, the underlying PGe structure provides a weak interface that allows easy detachment and transfer of the Ge FSM (Fig. 1c). Furthermore, the remnants of the unreconstructed PGe structure on the parent substrate can be dissolved using slow chemical etching in H2O2 solution, enabling the recovery of an epiready surface and its reporosification to produce multiple Ge FSMs. These findings provide a groundwork for engineering of Ge substrates and their application in the sustainable fabrication of high-quality Ge FSMs for lightweight and flexible optoelectronic devices.
[1] T. Hanuš et al., Adv. Mater. Interfaces (2023), 2202495 (https://doi.org/10.1002/admi.202202495)
[2] T. Hanuš et al., Mater. Today Adv. 18 (2023), 100373 (https://doi.org/10.1016/j.mtadv.2023.100373)
[3] T. Hanuš et al., Sustainability (2024), 16(4), 1444 (https://doi.org/10.3390/su16041444)

Figure 1