Excitonic and electronic transitions in Me–Sb₂Se₃ structures

Abstract

The optical anisotropy of the Sb2Se3 crystals was investigated at 300 and 11 K. Excitonic features of four excitons (A, B, C, and D) were observed in the optical spectra of the Sb2Se3 single crystals and in the photoelectric spectra of the Me–Sb2Se3 structures. The exciton parameters, such as the ground (n = 1) and excited (n = 2) state positions and the binding energy (Ry), were determined. The effective mass of the electrons at the bottom of the conduction band (m c * = 0.67m 0) as well as the holes at the four top valence bands (m v1 * = 3.32m 0, m v2 * = 3.83m 0, m v3 * = 3.23m 0 and m v4 * = 3.23m 0) were calculated in the Г-point of the Brillouin zone. The magnitude of the valence band splitting V1–V2 due to the spin–orbit interaction (Δso = 35 meV) and the crystal field (Δcf = 13 meV) were estimated in the Brillouin zone center. The energy splitting between the bands V3–V4 was 191 meV. The identified features were discussed based on both the theoretically calculated energy band structure and the excitonic band symmetry in the Brillouin zone (k = 0) for crystals with an orthorhombic symmetry (Рnma). The photoelectric properties of the Me–Sb2S3 structures were investigated in the spectral range 1–1.8 eV under E||c and E⟂c polarization conditions and at different applied voltages.

Identification of Excitons and Biexcitons in Sb2Se3 under High Photoluminescence Excitation Density

“…is the reduced mass of exciton, m 0 is the electron mass, and R y is the Rydberg constant. Sb 2 Se 3 is a compound with quite high static permittivity, i.e., ε r ≥ 24 [20,25] and therefore the free exciton binding energy is expected to be very small. As already mentioned, due to the crystal field splitting of valence band we expect at least the presence of A and B excitons in this material.…”

“…The free exciton binding energy in 3D crystals is usually calculated using a simple hydrogen model [ 24 ] where is the reduced mass of exciton, m 0 is the electron mass, and R y is the Rydberg constant. Sb 2 Se 3 is a compound with quite high static permittivity, i.e., ε r ≥ 24 [ 20,25 ] and therefore the free exciton binding energy is expected to be very small. As already mentioned, due to the crystal field splitting of valence band we expect at least the presence of A and B excitons in this material.…”

“…It was additionally shown in Sb 2 Se 3 single crystals Δ cr = 13 meV and Δ so = 35 meV. [ 20 ] All these aforementioned results show that additional methods are also needed to improve the understanding of the electronic band structure of Sb 2 Se 3 crystals in more detail.…”

“…Recent optical measurements made on quasi‐1D Sb 2 Se 3 single crystals revealed that the absorption edge at low temperatures can shift about 10 meV depending on polarization conditions and even more at RT. [ 20 ] This anisotropy affects also the shape of absorption spectrum and could cause additional ambiguity to experimental results. Additionally, the valence band splitting in the center of the Brillouin zone by a crystal field and the spin–orbit interaction add extra features to the optical absorption and reflectance measurements.…”

“…Unfortunately, PR measurements were not able to detect these valence band splittings, because the broadening parameter obtained from PR spectra was too high. Very recently, the presence of four different excitonic transitions was reported by using absorption and reflectance measurements at T = 11 K and at T = 300 K. [ 20 ] These transitions are due to the crystal field (Δ cr ) and spin–orbit (Δ so ) interactions leading to valence band splitting in the center of the Brillouin zone center and are called as A, B, C, and D excitons. The low‐temperature ground state energy positions of these excitons were measured at 1.299, 1.312, 1.347, and 1.538 eV for A, B, C, and D excitons, respectively.…”