First-principles study of twin grain boundaries in epitaxial BaSi2 on Si(111)

Abstract

Epitaxial films of BaSi2 on Si(111) for solar cell applications possess three epitaxial variants and exhibit a minority carrier diffusion length (ca. 9.4 μm) much larger than the domain size (ca. 0.2 μm); thus, the domain boundaries (DBs) between the variants do not act as carrier recombination centers. In this work, transmission electron microscopy (TEM) was used to observe the atomic arrangements around the DBs in BaSi2 epitaxial films on Si(111), and the most stable atomic configuration was determined by first-principles calculations based on density functional theory to provide possible interface models. Bright-field TEM along the a-axis of BaSi2 revealed that each DB was a twin boundary between two different epitaxial variants, and that Ba(II) atoms form hexagons containing central Ba(I) atoms in both the bulk and DB regions. Four possible interface models containing Ba(I)-atom interface layers were constructed, each consistent with TEM observations and distinguished by the relationship between the Si tetrahedron arrays in the two domains adjacent across the interface. This study assessed the structural relaxation of initial interface models constructed from surface slabs terminated by Ba(I) atoms or from zigzag surface slabs terminated by Si tetrahedra and Ba(II) atoms. In these models, the interactions or relative positions between Si tetrahedra appear to dominate the relaxation behavior and DB energies. One of the four interface models whose relationship between first-neighboring Si tetrahedra across the interface was the same as that in the bulk was particularly stable, with a DB energy of 95 mJ/m2. There were no significant differences in the partial densities of states and band gaps between the bulk and DB regions, and it was therefore concluded that such DBs do not affect the minority carrier properties of BaSi2.

Three-step growth of highly photoresponsive BaSi2 light absorbing layers with uniform Ba to Si atomic ratios

“…From the viewpoint of defect levels, such GBs do not work as electrically active defect sites from the potential distribution measured by Kelvin prove microscopy 42 and by first-principle calculations. 43 However, as we reported in Ref. 44 shows the highest photoresponsivity ever achieved for undoped BaSi 2 films grown by the two-step growth method.…”

Improving the photoresponse spectra of BaSi2 layers by capping with hydrogenated amorphous Si layers prepared by radio-frequency hydrogen plasma

“…Semiconducting barium disilicide (BaSi 2 ), 1,2 composed of earth-abundant elements, has all these properties such as a band gap of 1.3 eV, 3 a large α of 3 × 10 4 cm -1 at 1.5 eV, [3][4][5][6] inactive grain boundaries, 7 and a large minority-carrier lifetime (τ ∼ 10 µs). 8,9 Since BaSi 2 can be grown epitaxially on a Si substrate 10 and its band gap can be increased by adding other elements such as Sr and C, 11,12 BaSi 2 is a material of choice for targeting η > 30% in Si-based tandem structure solar cells.…”

p-BaSi2/n-Si heterojunction solar cells on Si(001) with conversion efficiency approaching 10%: comparison with Si(111)

“…28 This difference in L comes from inactive grain boundaries of BaSi2 on Si(111). 29 Although we have not evaluated L values of p-BaSi2 films by EBIC, it is reasonable to consider that they are smaller for p-BaSi2 on Si(001) than those on Si(111). Figure 1(f) shows the schematics of the band alignment of the diode simulated by automat for simulation of heterostructures (AFORS-HET).…”

“…We have used Si(111) substrates because BaSi2 epitaxial films on Si(111) have exhibited a large minority-carrier lifetime (~10 s), a large minority-carrier diffusion length (L~10 m), and inactive grain boundaries. [17][18][19][20] In addition, the lattice mismatch is as large as approximately 1% for BaSi2 on Si(111), which is much smaller than 12% for BaSi2 on Si(001). 11 However, recent achievements in BaSi2 on Si(001) such as large photoresponsivity and p-BaSi2/n-Si solar cells have renewed interest in BaSi2 on Si(001).…”