Raman Spectroscopy Analysis of Nitrogen Hyperdoped Cz and Fz Silicon Implanted with Nitrogen at Various Dose Levels for Infrared Absorption
Authors: Karoui Abdennaceur , Sahtout Fouzia , Wang Yongqian , Yang Shizhang 
Published: December 2024

Nitrogen hyperdoping is proposed to enable infrared absorption in silicon via two-photon absorption, which is beneficial for high-efficiency solar cells and IR sensors. Cz-Si and lightly doped FZ-Si were implanted with nitrogen at 200 keV with high fluences to achieve doping levels of 0.02, 0.1, 0.2, and 1 at% %, referred to as Dose 1, 2, 3, and 4, respectively. Confocal Raman spectroscopy with 532 nm excitation provided spectra at multiple locations and depths for hyperdoped and undoped Cz and FZ silicon.

Raman spectroscopy revealed several structural features contributing to the spectra of nitrogen hyperdoped silicon. Both non-implanted Cz-Si and FZ-Si exhibit a sharp peak at 520 cm−1, attributed to the first-order TO mode for crystalline silicon (c-Si). In certain areas of non-implanted FZ-Si samples, this peak shifts to 514.9 cm−1, indicating residual tensile strain due to nitrogen incorporation. Additionally, this peak displays a slight asymmetry, suggesting the presence of another phase of silicon in FZ-Si. The refinement process for FZ-Si introduces inhomogeneous strains and swirl defects, as revealed by etching and x-ray topography (XRT). Amorphous regions in non-implanted FZ-Si are indicated by a broad band in the Raman spectra, ranging from 63 cm−1 to 207 cm−1. Non-implanted Cz-Si samples show a small shift in the c-Si line to 524 cm−1, reflecting localized compressive strain. However, the c-Si peak remains symmetric across all measured locations. All as-grown samples display the c-Si 2TA mode at 300 cm−1 and the 2TO mode at 970 cm−1, which shift to 291 cm−1 and 940 cm−1, respectively, due to residual stress in the bulk.

Raman spectra of n hyperdoped FZ-Si (NFZ-Si) and Cz-Si (NCz-Si) samples reveal both crystalline and amorphous phases (a-Si) in the near-surface region. All spectra from implanted silicon display the intense first-order TO mode peak located around 520 cm−1, varying slightly with sample type. The broad band from 407 cm−1 to 493 cm−1 (phonon TO mode) in NCz-Si and NFZ-Si exhibits an asymmetric shoulder shape, attributed to scattering from the Si–N bond. This suggests a mixture of amorphous silicon and SiNx nanocrystallites. The band intensity increases with dose; and for a given dose, it is more prominent and wider in NFZ-Si. In this material, SiNx clusters induce strain, shifting the c-Si line to lower frequencies (509–512 cm−1) or higher frequencies (521–525 cm−1) depending on the dose, indicating the buildup of tensile or compressive strain during the implantation. As NCz-Si relaxes at high nitrogen doses, the c-Si peak shifts back to 520 cm−1, with slight asymmetry suggesting the presence of SiNx clusters.

For all implanted samples, regardless of the dose, the intensity of the c-Si phonon peak remains unchanged compared to the as-grown samples. The broad peak from 63 cm−1 to 207 cm−1 of the a-Si TA mode is also present in implanted NCz-Si and NFZ-Si. Notably, the intensity of this band increases with dose, rising significantly in NFZ-Si but only slightly in NCz-Si. NFZ-Si samples exhibit a low-intensity Raman peak at 600–620 cm−1, attributed to the a-Si 2LA mode, which appears in NCz-Si at high doses only, suggesting the involvement of oxygen interacting with nitrogen, as shown in our previous work. Additionally, we observed a shift in the c-Si 970 cm−1 2TO mode to 930 cm−1 in NFZ-Si as the dose increases, a shift not seen in NCz-Si. This may result from the combination of SiNx clusters and existing self-interstitial aggregates in NFZ-Si, which creates additional stress in the material as the implant dose rises.

Overall, N implanted Cz-Si preserves a good crystalline structure compared to FZ-Si, which undergoes more damage as the dose increases. Ion implantation generates vacancy and self-interstitial bands, leading to a supersaturated level of nitrogen trapped in the crystal. Previous work has shown that in Cz-Si, oxygen and vacancies couple with nitrogen to form N-V-O complexes, which enhance the material's strength. This could explain the relaxation observed at high doses in NCz-Si. We believe this is why N-implanted Cz-Si exhibits higher crystalline quality relative to its NFZ-Si counterpart. NFZ-Si samples sustain more damage from implantation due to their softness, suggesting that oxygen helps heal defects during implantation, up to a certain dose limit.

Figure 1