Hyperdoping Silicon with Various Impurities and Doses for Enabling Infrared Absorption

Hyperdoping is used to enhance the IR absorption of silicon through two-photon absorption, benefiting high-efficiency solar cells, IR sensors, and other applications. Cz-Si and N-doped FZ-Si samples were implanted with nitrogen at 200 keV at fluences of 0.02, 0.1, 0.2, and 1 at%. Samples were characterized using 3D Confocal Raman spectroscopy at 532 nm excitation. Similarly, Se was implanted in both types of samples, and Raman spectra were collected. Non-implanted Cz-Si and FZ-Si samples were also analyzed for reference.
Raman spectra revealed structural features in N-hyperdoped silicon. Non-implanted Cz-Si and FZ-Si showed a sharp peak at 520 cm⁻¹, associated with the first-order TO mode for crystalline silicon (c-Si). In N-doped FZ-Si, this peak shifted to 514.9 cm⁻¹ at certain points, indicating residual tensile strain, likely due to nitrogen doping combined with vacancies. The slightly asymmetric shape of this peak suggested another silicon phase in FZ-Si. Swirl defects, common in FZ-Si growth, likely created inhomogeneous strain, causing the Raman shift. Disordered regions were also detected in N-doped FZ-Si as a broad band (63–207 cm⁻¹). In non-implanted Cz-Si, a slight shift to 524 cm⁻¹ indicated localized compressive strain. The c-Si peak remained symmetric in all measured locations. All as-grown samples exhibited the c-Si 2TA mode at 300 cm⁻¹ and 2TO mode at 970 cm⁻¹, shifting to 291 cm⁻¹ and 940 cm⁻¹, respectively, in some FZ-Si regions, likely due to bulk stress.
Raman spectra of N-hyperdoped NFZ-Si and NCz-Si showed both crystalline and amorphous phases (a-Si). The c-Si TO mode peak appeared around 520 cm⁻¹ in all implanted samples. In both NCz-Si and NFZ-Si, this peak had an asymmetric shape, with a shoulder band from 407 cm⁻¹ to 493 cm⁻¹ due to Si–N bond scattering, indicating amorphous silicon and SiNx nanocrystallites. This band increased with dose and was more pronounced in NFZ-Si. SiNx clusters in NFZ-Si induced strain, shifting the c-Si line to lower (509–512 cm⁻¹) and higher frequencies (521–525 cm⁻¹), indicating both tensile and compressive strain as dose increased.
NCz-Si relaxed at high nitrogen doses, returning the c-Si peak to 520 cm⁻¹, though slightly asymmetric, pointing to SiN clusters. Across all doses, the intensity of the c-Si phonon peak remained stable compared to as-grown samples.
Both NCz-Si and NFZ-Si displayed a broad peak (63–207 cm⁻¹) associated with the a-Si TA mode. This band intensified in NFZ-Si and slightly in NCz-Si with increasing doses. NFZ-Si also showed a low-intensity peak at 600–620 cm⁻¹ (a-Si 2LA mode), which only appeared in NCz-Si at high doses. Additionally, in NFZ-Si, the c-Si 970 cm⁻¹ 2TO mode shifted to 930 cm⁻¹ with higher doses, a shift not seen in NCz-Si. This is likely due to a combination of SiNx clusters and pre-existing self-interstitial aggregates in NFZ-Si, increasing stress with dose.
Overall, nitrogen-implanted Cz-Si maintained better crystalline structure compared to FZ-Si, which experienced more damage as dose increased. Ion implantation created vacancy and self-interstitial bands, trapping supersaturated nitrogen in the crystal. In previous work, we showed that oxygen and vacancies in Cz-Si interact with nitrogen to form N-V-O complexes, strengthening the material. This likely explains the relaxation observed in high-dose NCz-Si and its superior crystalline quality over NFZ-Si, which sustained more damage due to its lack of oxygen.
Raman Component Analysis of the 3D Raman mapping of Se implanted Cz Si has shown the distribution of various phases along the depth. Raman lines 132-135 cm⁻¹ are detected near the surface, in the V-rich defected zone. The c-Si line 517.4 cm⁻¹, which is red-shifted due to tensile strain, appears to be spread over much of the sample depth. Line 469 cm^-1 is suggested to be V-Se, but the tail is suggested to be due to the distribution of dislocation loops of Si_I and Se_I.