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Objectives
The first objective of this project is to reinforce the reliability and qualification of germanium substrates made in Europe for next‑generation space solar cell technologies. As III-V 4‑junction (4J) solar cells transition into industrial deployment, it becomes critical to ensure that substrate quality consistently meets the significantly more stringent requirements associated with metamorphic device architectures. Unlike lattice‑matched triple‑junction (3J) cells, 4J solar cells are more sensitive to substrate surface quality, meaning that features previously considered benign can adversely affect device performance. Addressing these challenges is essential to safeguard end‑of‑line solar cell quality, reduce variability across the value chain, and ensure alignment between substrate quality data generated internally and those measured by solar cell manufacturers.
The second objective of this project is to contribute to a more resilient and scalable European manufacturing base for germanium substrates by advancing process efficiency and industrial maturity. By developing leaner and more robust manufacturing flows, the project supports increased production throughput in the short term and lays the groundwork for an innovative, streamlined, and scalable process architecture suitable for future production lines. This directly underpins the long‑term sustainability, competitiveness, and strategic autonomy of the European space photovoltaic supply chain.
Benefits
Photovoltaic grade germanium ingots manufactured by Umicore using the Czochralski (Cz) method are pulled dislocation‑free, enabling the production of wafers with a highly controlled and dislocation‑free crystal structure. This intrinsic material feature is a key enabler for the growth of advanced III‑V multijunction solar cell stacks used in space applications, as it allows epitaxial layers to be deposited without the formation of threading dislocations that could degrade device performance or reliability.
For next‑generation four‑junction (4J) solar cells, which rely on metamorphic epitaxy and are therefore more sensitive to substrate characteristics, dislocation‑free germanium substrates provide a stable and predictable foundation. This contributes to improved epitaxial yield, reduced variability in device performance, and higher end‑of‑line quality consistency at solar cell manufacturers.
By combining dislocation‑free crystal growth with mature wafer manufacturing and qualification practices, the product supports the development, industrialization, and long‑term reliability of high‑efficiency space photovoltaics. As such, it strengthens the robustness, scalability, and strategic autonomy of the European space solar cell supply chain, in line with ESA and EU objectives.
Features
The product is an epi‑ready germanium wafer designed for direct use as a substrate in the manufacturing of III‑V multi‑junction (MJ) solar cells for space applications. “Epi‑ready” denotes that the wafers are fully prepared for immediate epitaxial growth of GaAs‑family III‑V compounds by metal‑organic chemical vapor deposition (MOCVD), without requiring any additional pre‑treatment prior to reactor loading.
To enable this, the wafer side to be processed must therefore be mirror-polished and clean from contamination, ensuring compatibility with high‑quality epitaxial growth. The wafers are round, with a diameter of 150 mm and a typical thickness of 225 µm, supporting established and scalable manufacturing flows within the space photovoltaic industry.
The substrates are p‑type doped, allowing the germanium wafer to serve as the bottom cell in the MJ solar cell architecture. Given the sensitivity of germanium surfaces to oxidation, the wafers are delivered in moisture‑barrier packaging to preserve surface integrity and epi‑readiness throughout handling, transport, and storage.
Challenges
The main challenge addressed by the project arises from the fact that certain substrate‑related defects only become detectable at later stages of the space solar cell manufacturing chain, once the germanium substrates have undergone epitaxial growth of the III‑V compound stack. This can make direct correlation between inspection data generated after device processing at solar cell manufacturers and the characteristics of the bare substrates assessed prior to shipping more challenging. Strengthening this correlation is essential to improve defect traceability across the value chain, reinforce confidence in early‑stage inspection methodologies, and enable earlier identification of substrates with latent risk, thereby helping to avoid solar cell rejections at later manufacturing stages.
Plan
Depending on their technology readiness level, initiatives in this project fall either under the Technology development phase or the Product development phase. Both phases will be reviewed according to a joint milestone scheme: two milestone reviews are planned during the course of the project: after 6 and 12 months. After 18 months, the final review will mark project completion.
Current Status
The project has already delivered tangible progress throughout Umicore’s manufacturing flow. In our ‘Metallurgy’ production line, we demonstrated a 20% gain in the use time of our horizontal zone refining furnaces. In our 150mm wafering flow, we achieved a 25% increase in throughput at both the polishing and the cleaning steps, which debottlenecked the end of our ‘Substrates’ production line while maintaining the quality of our substrates.
Those improvements strengthen Umicore’s ability to support the first wave of increasing demand for germanium substrates for advanced space solar cell applications. In parallel, an automated data-sharing platform has been established with a key customer, enabling direct comparison between post-epitaxy inspection results and wafer inspection data generated at the end of Umicore’s production line. This creates a stronger basis for defect traceability and continuous quality improvement across the value chain.
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