A Systematic Study of the Temperature Dependence of the Dielectric Function of GaSe Uniaxial Crystals from 27 to 300 K

Abstract

We report the temperature dependences of the dielectric function ε = ε1 + iε2 and critical point (CP) energies of the uniaxial crystal GaSe in the spectral energy region from 0.74 to 6.42 eV and at temperatures from 27 to 300 K using spectroscopic ellipsometry. The fundamental bandgap and strong exciton effect near 2.1 eV are detected only in the c-direction, which is perpendicular to the cleavage plane of the crystal. The temperature dependences of the CP energies were determined by fitting the data to the phenomenological expression that incorporates the Bose–Einstein statistical factor and the temperature coefficient to describe the electron–phonon interaction. To determine the origin of this anisotropy, we perform first-principles calculations using the mBJ method for bandgap correction. The results clearly demonstrate that the anisotropic dielectric characteristics can be directly attributed to the inherent anisotropy of p orbitals. More specifically, this prominent excitonic feature and fundamental bandgap are derived from the band-to-band transition between s and pz orbitals at the Γ-point.

The Wannier-Mott Exciton, Bound Exciton, and Optical Phonon Replicas of Single-Crystal GaSe

“…The dominant peak can be attributed to the recombination of free excitons (FX), specifically to the n = 1 exciton state associated with the direct gap. This observation is consistent with earlier reports [21,[24][25][26]29,32,33,43,44]. At low temperatures, other peaks occur below the FX, which may be due to bound excitons (BX) or ground states of excitons with large binding energy (Frenkel excitons).…”

“…The high-energy FX is undoubtedly caused by the ground states of Wannier-Mott excitons. These Wannier-Mott excitons are formed by electrons from the bottom conduction band and holes from the top valence band at the Γpoint [32]. Figure 2a illustrates the change in absorption in a GaSe thin slab with the temperature increasing from 17 to 300 K. The absorption coefficient, α, is determined by converting the transmission spectra using the relation α = 1/dlog(I 0 /I); here d represents the sample thickness, I 0 is the transmission without the sample, and I is the transmission with the sample.…”

“…The high-energy FX is undoubtedly caused by the ground states of Wannier-Mott excitons. These Wannier-Mott excitons are formed by electrons from the bottom conduction band and holes from the top valence band at the Γ-point [32]. To further investigate the optical properties of GaSe single crystal at low temperatures, Figure 4a,b presents the PL and absorption spectra at 17 and 300 K, respectively.…”

“…GaSe single crystal was grown using the temperature-gradient method [32]. Gallium (99.99%) and selenium (99.99%) powders were weighed at a molecular ratio of 1:1 and used as precursor substances.…”

“…A comprehensive understanding of the optical properties of materials near the fundamental band edge is essential for effectively designing photonic and photovoltaic devices. Several studies have investigated the optical properties of this material using different techniques, including Raman scattering [17][18][19][20], photoluminescence (PL) [21][22][23][24][25], absorption [26][27][28], and spectroscopic ellipsometry [29][30][31][32]. In 1974, Voitchosky and Mercier [33] conducted a study on PL in various pure and doped single crystals of GaSe.…”