Detection of local vibrational modes induced by intrinsic defects in undoped BaSi2 light absorber layers using Raman spectroscopy

Abstract

We fabricate BaSi 2 epitaxial films on Si(111) substrates by molecular beam epitaxy and investigate point defects inside the films using Raman spectroscopy with the help of first-principles calculation. Point defects such as Ba substituted for Si antisites, Si vacancies, and Si interstitials are considered as candidates for native point defects in BaSi 2. Vibration analysis based on firstprinciples calculation suggests that local vibrational modes caused by these point defects appear at around 430, 480, and 560 cm À1 , respectively, and are in good agreement with Raman peak positions. Comparing calculations with Raman spectra of the films formed with different Ba to Si deposition rate ratios R Ba /R Si from 1.0 to 5.1, we conclude that the density of point defects reaches a minimum at R Ba /R Si ¼ 2.2. Furthermore, the position of Raman peaks at approximately 490 cm À1 shifts to a lower wavenumber, depending on R Ba /R Si and thereby the density of point defects.

Simple way of finding Ba to Si deposition rate ratios for high photoresponsivity in BaSi2 films by Raman spectroscopy

“…The Raman peak from VSi should appear at approximately 480 cm −1 according to first-principle calculations. 31 However, this peak wavenumber of VSi is close to the most intense peak of the Ag mode in BaSi2. Therefore, we examined the FWHM value of the peak intensity of the Ag mode to confirm the presence of any contribution from VSi.…”

“…Figures 4(a) and 4(b) show the Raman spectra of the 580 and 650 °C samples, respectively, in the range between 200 and 700 cm −1 at RT. In all of the samples, we observed five distinct peaks assigned to the internal vibrations of Si tetrahedra [28][29][30][31] with Th symmetry in the lattice of BaSi2. 32 Here, focusing on the Si-rich side, the transverse optical phonon line of Si (SiTO) was observed at RBa/RSi ≤ 1.5 for the 580 °C samples, wherein the photoresponsivity reached a maximum at RBa/RSi = 2.2, i.e., slightly higher than 1.5.…”

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

“…In our previous work, defect characterizations of BaSi 2 films were conducted using deep level transient spectroscopy (DLTS), 23,24 positron annihilation spectroscopy, 25 Raman spectroscopy, 26 and electron paramagnetic resonance, 27 wherein we detected vacancy-type defects, which we regard as Si vacancies (V Si ). First-principles calculations also show that V Si are likely to occur in BaSi 2 14 and that these vacancies give rise to localized states within the bandgap.…”

Toward BaSi2/Si Heterojunction Thin‐Film Solar Cells: Insights into Heterointerface Investigation, Barium Depletion, and Silicide‐Mediated Silicon Crystallization

“…Along with the typical BaSi 2 Raman peaks, namely, F g (≈276 cm −1 ), E g (≈293 cm −1 ), E g +F g (≈355 cm −1 ), F g (≈376 cm −1 ), and A g (≈486 cm −1 ), peaks of β-FeSi 2 at 247 cm −1 and Si nanocrystals (NCs) at 519 cm −1 are observed as well. [32][33][34] In BaSi 2 /Si-0, positions (−3, 7) and (0, 0), which hold different appearances (see Figure S1a, Supporting Information), present the same Raman spectra shape, consisting of strong signals from BaSi 2 , β-FeSi 2 , and a weak peak of Si NCs, as shown in Figure 2a. By introducing Si layers, BaSi 2 /Si-10 and BaSi 2 /Si-20 present sharper color contrast in microscopic images displayed in Figure S1b,c (Supporting Information), respectively.…”

“…Along with the typical BaSi 2 Raman peaks, namely, F g (≈276 cm −1 ), E g (≈293 cm −1 ), E g +F g (≈355 cm −1 ), F g (≈376 cm −1 ), and A g (≈486 cm −1 ), peaks of β‐FeSi 2 at 247 cm −1 and Si nanocrystals (NCs) at 519 cm −1 are observed as well. [ 32–34 ]…”