Substitutional sulfur and its vibrational fingerprints in Sb$_2$Se$_3$

TL;DR

This work combines infrared spectroscopy and density functional theory to identify substitutional sulfur defects in $Sb_2Se_3$ and to assign their local vibrational fingerprints. Four sulfur-related local vibrational modes appear at 249, 273, 283, and 312 cm$^{-1}$, arising from S substituting the three inequivalent Se sites; their origin is confirmed by diffusion experiments with sulfur sources and isotopic substitution using $^{34}$S. DFT calculations support the assignment by showing low formation energies for $S_{ m Se}$ under Se-poor conditions and requiring a modest uniform shift of ~+$23$ cm$^{-1}$ to match observed frequencies, consistent with site-specific modes. The results provide a robust spectroscopic fingerprint for isolated sulfur in $Sb_2Se_3$, enabling defect-aware understanding and tuning of sulfur’s role in Sb$_2$(S$_x$Se$_{1-x}$)$_3$ for photovoltaics and related applications.

Abstract

The configurational behavior of sulfur in antimony triselenide (Sb$_2$Se$_3$) is investigated by combining infrared absorption spectroscopy with density functional theory. Four sulfur-related local vibrational modes are identified at 249, 273, 283, and 312~cm$^{-1}$ in melt-grown single crystals prepared from Sb$_2$Se$_3$ granulate. Their assignment to sulfur is confirmed through controlled indiffusion experiments using Sb$_2$S$_3$ and elemental sulfur, as well as isotope-substitution studies with $^{34}$S, which produce the expected frequency shifts. Polarization-resolved measurements, together with theoretical calculations of local vibrational modes, demonstrate that the observed spectral features are fully consistent with substitutional sulfur on the three inequivalent selenium sites of Sb$_2$Se$_3$.

Figures (7)

• Figure 1: Structural model of Sb$_2$Se$_3$ viewed along the $c$ direction, indicating the three inequivalent Se sites. Red: Se, gray: Sb
• Figure 2: Polarized infrared absorption spectra in the two-phonon transition region, measured at 20 K for a nominally undoped "elemental-grown" Sb$_2$Se$_3$ single crystal. The spectra were recorded for different orientations of the electric field vector $E$ relative to the crystallographic axes. Baseline offsets were applied for clarity. The upper two spectra were obtained on a $b$-oriented sample, while the bottom spectrum was acquired on a $c$-oriented sample.
• Figure 3: Temperature-dependent infrared absorption spectra in the two-phonon transition region, measured on a nominally undoped "elemental-grown" Sb$_2$Se$_3$ single crystal. The spectra were recorded with $k \parallel b$ and $E \parallel a$ and are offset for clarity.
• Figure 4: (a) Infrared absorption spectra of an "elemental-grown" (blue) and "granulate-grown" (red) Sb$_2$Se$_3$ sample, recorded with $k \parallel b$ and $E \parallel a$. (b) Polarization-resolved differential absorption spectra of a granulate-grown sample, obtained by cross-subtracting spectra of an elemental-grown sample. Baseline offsets were applied for clarity. The upper and lower spectra were obtained on a $b$-oriented sample, while the mid spectrum was acquired on a $c$-oriented sample.
• Figure 5: (a) Polarized differential absorption spectra of three granulate-grown Sb$_2$Se$_3$ samples. A nominally undoped elemental-grown Sb$_2$Se$_3$ crystal was used as the reference for subtraction. (b) Correlation between the integrated absorption coefficient of the 249 cm$^{-1}$ mode and those at 273, 283, and 312 cm$^{-1}$, obtained across eight granulate-grown Sb$_2$Se$_3$ samples.