Direct and quasi-direct band gap of novel Si-Ge alloys in P-3m1 phase

Abstract

This work investigates the crystal structure, stability, mechanical properties, electronic properties, effective masses, and optical properties of Si-Ge alloys in the P-3m1 phase. The elastic constants and phonon spectra proven that the Si-Ge alloys in the P-3m1 phase have mechanical and dynamic stability. The bulk modulus, shear modulus and Young’s modulus of Si-Ge alloys in the P-3m1 phase decrease with the increase of Ge composition, and the three-dimensional diagram of Young's modulus and effective mass show that the mechanical and transport properties have anisotropy. The Si12Ge12 in the hP24 phase is a quasi-direct band gap semiconductor material with a band gap of 1.081 eV, while the Si30−x Ge x alloy (x = 6, 12, 18, 24) in the hP30 phase are all direct band gap semiconductor materials with the band gaps of 0.541 eV, 0.430 eV, 0.561 eV, and 0.387 eV, respectively. The hP30-Si6Ge24 has a very small effective electron mass. The hP24-Si12Ge12 show excellent absorptive capacity in the visible and infrared region region. Based on this work, Si-Ge alloys in the P-3m1 phase are promising materials for photovoltaic applications.

Group 14 elements in the Cmcm phase with a direct band structure for photoelectric application

“…The C ij , B, G, and E of the Cmcm space group basically decrease in the order of C 24 , Si 24 , and Ge 24 , as shown in table 2. The B and G of oC24 C 24 (B = 407 GPa, G = 458 GPa), oC24 Si 24 (B = 81 GPa, G = 52 GPa), and oC24 Ge 24 (B = 57 GPa, G = 39 GPa) are slightly less than those of diamond, diamond Si, and diamond Ge, respectively, while the B and G of oC24 Si 24 and oC24 Ge 24 are greater than those of silicon and germanium in the P4 2 /mnm [67], hP24 [40], hP12 [38], and hP30 [40] phases. It is noted that the G of Si in the oC12 phase is larger than that in the Cmcm phase, while the B of Si in the oC12 phase [24] is smaller than that in the Cmcm phase.…”

“…Saleev et al [37] proposed six novel Si and Ge allotropes, and the crystal structure, elastic property, and electronic and optical properties of Si and Ge allotropes were studied. This is attributed to the fact that semiconductor alloys could change by indirect band gap to direct band gap or quasi-direct band gap to a certain extent, for example, Si-C alloys, Si-Ge alloys and Ge-Sn alloys [38][39][40][41][42][43][44], to acquire the ideal direct band gap semiconductor material. Fan et al [38] predicted two new Ge allotropes in the hP12 and oC12 phases.…”