Reduced-pressure chemical vapor deposition growth of monocristalline detachable Ge-nanomembrane on mesoporous Ge substrate for sustainable space photovoltaics

Germanium (Ge) substrates are the backbone of the high-performance III-V multijunction solar cells (MJSCs). However, their high cost, representing 30-40% of MJSC production costs, restricts their use to niche markets. Only a few microns of the Ge substrate actively absorb light, while the remaining part serves as mechanical support, increasing the power-to-weight ratio critical for space applications. Ge nanomembranes (NMs) offer a promising pathway for reducing both cost and weight of MJSCs. The Porous Germanium Efficient Epitaxial Layer Release (PEELER) process has demonstrated exceptional potential for producing cost-effective Ge NMs. By creating nanostructured porous layers, this approach enables epitaxial growth and detachment of NMs, with substrates reconditioned for multiple cycles, drastically cutting costs. Transitioning from molecular beam epitaxy to chemical vapor deposition (CVD) is crucial for mass production. This study investigates Ge NM growth on mesoporous Ge substrates using reduced-pressure CVD. Process conditions, including temperature, pressure, precursor flow rates, and Ge precursor selection, were optimized to enhance nucleation and growth. Preliminary results demonstrate the production of 1 µm-thick monocrystalline Ge NMs with nanoengineered weak layers, enabling efficient detachment and substrate reuse. The CVD-based approach exhibited significant advantages, including around 4 times higher growth rate, improved scalability, and reduced operational costs, laying the groundwork for the industrial adoption of PEELER technology and the cost-effective production of high-performance photovoltaics.