A recent study published in Advanced Energy and Sustainability Research demonstrated the effectiveness of microwave annealing in activating hydrogen within hydrogen-rich dielectrics (AlOx, SiNy, and AlOx/SiNy) deposited on polycrystalline-silicon (poly-Si) passivating contacts.
The microwave annealing method was compared to traditional hydrogenation methods, such as N2 annealing.
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Background
Hydrogenation of dangling bonds at the Si/SiO2 interface in doped poly-Si contacts has gained attention with the rise of tunneling oxide passivated contacts and poly-Si on oxide solar cells. In laboratory settings, effective hydrogenation is achieved through hydrogen-rich dielectrics or thermal processing to introduce molecular hydrogen.
In commercial silicon solar cell production, hydrogenation is often combined with the firing process for metallization. While firing occurs at high temperatures (600–700 °C), optimal hydrogenation typically requires 450–500 °C, making it challenging to balance surface passivation and metal contact formation.
Microwave annealing is a promising alternative, offering shorter processing times and cost-effective passivation.
Methods
This study used industrial n-type Czochralski-Si wafers. After saw-damage etching in an alkaline solution, the wafers were cleaned, and a 2 nm SiO2 interfacial layer was grown thermally at 600 °C in O2. An intrinsic poly-Si layer (~150 nm) was then deposited on both sides via chemical vapor deposition at 530 °C.
The wafers were diced into 40 × 40 mm samples, followed by phosphorus doping using a spin-on phosphorus glass solution. The doped samples underwent an annealing process at 950 °C for one hour to activate the dopants, ensuring proper electrical conductivity. After doping, different dielectric passivation layers were deposited, including 20 nm of AlOx, 75 nm of SiNy, or a stack of AlOx/SiNy, to study their impact on passivation.
Thermal annealing was performed under N2 at 450 °C in a quartz tube. Microwave annealing was performed at a microwave frequency of 2.45 GHz at 1000-watt power. The prepared specimens were characterized after hydrogenation and diffusion. A photoconductance lifetime tester was employed to determine their recombination parameter, effective lifetime, and implicit open-circuit voltage.
The Kane-Swanson method was used to determine the surface recombination parameter (J0) from the lifetime measurements. Photoluminescence (PL) spectra of the specimens coated with different dielectrics were recorded at identical exposure times to precisely compare passivation levels.
Finally, the micro-PL spectra of these samples were recorded using a Raman spectrometer before and after hydrogenation.
Results and Discussion
PL imaging of spin-on dopant (SOD)-poly-Si contact samples before hydrogenation revealed high non-uniformity and low PL intensity. After one-minute microwave annealing, localized improvements in PL were observed, with over 60 % of the sample area showing enhanced passivation after four minutes. Further annealing did not increase PL intensity, indicating passivation saturation.
PL intensity depended on the capping layer (AlOx, SiNy, and AlOx/SiNy) and the N2 annealing duration. AlOx samples showed little change in PL intensity over time, while SiNy and AlOx/SiNy improved significantly within the first 10 minutes, with more than half of the surface area becoming passivated. By 30 minutes, full passivation was achieved.
For N2 annealing, dielectric-coated samples showed a higher crystalline silicon (c-Si) PL peak intensity than as-deposited samples. The SiNy layer provided the best passivation for the c-Si bulk and interfaces, while AlOx was the least effective.
However, in the poly-Si signal range, the PL intensity of SiNy samples was lower than that of AlOx or AlOx/SiNy samples. Despite this, SiNy samples exhibited the highest lifetime, whereas AlOx samples had the lowest.
Microwave-annealed specimens showed c-Si PL peaks similar to those seen with N2 annealing. The poly-Si PL peaks for all three dielectrics (AlOx, SiNy, and AlOx/SiNy) had nearly identical intensity, indicating similar passivation in the poly-Si layer. Still, SiNy continued to provide superior passivation at c-Si interfaces, achieving the lowest J0 and the highest lifetime.
Compared to traditional N2 annealing, microwave annealing produced comparable and, in some cases, superior results in hydrogen activation. Notably, just one minute of microwave annealing significantly enhanced passivation in SiNy or AlOx/SiNy dielectric layers, whereas AlOx samples required around four minutes.
These findings suggest that microwave annealing is a viable alternative to conventional methods for hydrogenation in poly-Si passivating contacts.
Journal Reference
Truong, T. et al. (2025). Microwave Annealing for Fast and Effective Hydrogen Activation in Polycrystalline Silicon Passivating Contacts. Advanced Energy and Sustainability Research. DOI: 10.1002/aesr.202500004, https://onlinelibrary.wiley.com/doi/10.1002/aesr.202500004
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