Effect of impurity compensation on optical properties of Si based on first-principles

Presently, impurity-compensated silicon (Si) has no clear potential applications due to high resistance and few carriers. Thus, it receives little attention from researchers. In our study, we find that impurity compensation makes the formation of localized state energy levels in Si bandgap, which can improve the light absorption of Si in the near infrared region. In order to fully and deeply understand the photoelectric properties of impurity-compensated Si, the localized state energy levels composed of P⁺/B^- ions are constructed in Si bandgap by the co-doping of both phosphorus (P) and boron (B) in this paper, leading to the formation of impurity-compensated Si. The first-principles based on a Density Functional Theory framework is used to study the photoelectric properties of the impurity-compensated Si (n/p-Sic) such as the density of states (DOS), dielectric function and refractive index. The DOS study reveals the following results: After the n- and p-Si with the same concentration of P and B (12.5%) are fully compensated by impurities, the Fermi energy levels of their compensated counterparts are at the valley bottom formed by the two adjacent DOS peaks, and the DOS is not zero at the valley bottom. In the study of dielectric function and refractive index, it is found that n-Sic has the largest dielectric function and refractive index in the low energy region for the doping ratio of C_B/C_P0 = 0.25. In addition, comparing intrinsic Si with its doped counterparts in the real part (Re) of their dielectric constant, the following regularity is found: In the high energy region of E ＞ 4 eV, the Re of the intrinsic Si, n/p-Si and p-Sic are negative. In the low energy region of 0.64 eV＜ E ＜ 1.50 eV, the Re of n-Sic is negative for the doping ratio of C_B/C_P0 = 0.25. The above comparison indicates that the n-Sic with C_B/C_P0 = 0.25 can achieve good metallicity in low energy region, which reveals that the electrons in valence band are easy to be excited by long-wavelength light with low energy. Theoretical studies show that the good photoelectric properties of n-Sic with C_B/C_P0 = 0.25 may be related to Si dangling bonds and localized state energy levels in Si bandgap. The Si dangling bonds are caused by the impurity compensation of B dopant for n-Si, leading to the change from part of Si-Si bonds to Si-B bonds. This study provides theoretical guidance for the application of impurity-compensated Si in the field of photodetectors such as CMOS image sensors and infrared photodetectors.