Strong Electron–Phonon Coupling in β-Ga₂O₃: A Huge Broadening of Self-Trapped Exciton Emission and a Significant Red Shift of the Direct Bandgap

Abstract

The temperature dependence of self-trapped exciton (STE) emission and the optical absorption edge of monoclinic gallium oxide (β-Ga 2 O 3 ) has been carefully studied.According to this research, it is found that as temperature increases (from 10 to 300 K), the STE and the direct bandgap of β-Ga 2 O 3 exhibit a huge broadening (∼120 meV) and a significant red shift (∼250 meV), respectively. Combined with theoretical analysis, these temperature-dependent change trends are found related to the strong electron−phonon coupling (EPC) effect in crystal, which is caused by the high localization (i.e., self-trapping) of carriers in β-Ga 2 O 3 . This finding further indicates that in the transition process of carriers' absorbing and releasing photons, the influence of lattice vibration needs to be considered and described by the configuration coordinate model. The strength of EPC can be measured by the Huang−Rhys factor, which is about S ≈ 9 for β-Ga 2 O 3 with the polar longitudinal optical phonon mode of lower energy (∼31 meV) being involved.

Observation of Phonon Cascades in Cu-Doped Colloidal Quantum Wells

“…Figure c displays fitting results of E copper ( T ) based on eq with parameters of A TE = −0.017 meV/K, A EP = −54.5 meV, and E phonon = 25.9 meV, from which it can be seen that (i) the extracted E phonon is very close to the frequency of LO phonons, (ii) the energy matching between LO phonons and k B T (∼26 meV) at room temperature ensures its largest phonon occupancy and the dominant role in EPC processes, and (iii) the overwhelming EPC implies a large electron–phonon interaction strength in Cu-doped CQWs, which is further supported by analyzing the temperature-dependent line width of the broad Cu-emission band (i.e., full width at half-maximum, fwhm). Generally, the Huang–Rhys factor ( S ) is utilized to describe how strongly electrons are coupled to phonons, which can be calculated from fwhm( T ) based on the configuration coordinate model: , fwhm ( T ) = 2.36 S E normalp normalh normalo normaln normalo normaln coth ( E p h o n o n / 2 k B T ) From the satisfactory fitting shown in Figure d, we have deduced a giant Huang–Rhys factor, S ≈ 12.42.…”

Bandgap, Mobility, Dielectric Constant, and Baliga’s Figure of Merit of 4H-SiC, GaN, and β-Ga2O3 from 300 to 620 K

“…The corresponding physical mechanism will be discussed in detail as below. Generally speaking, as temperature changes, factors including the electron–phonon coupling (EPC) and the thermal expansion (TE) in the lattice will contribute to the temperature dependence of the bandgap, so the relation between bandgap and temperature can be described as follows: , where E g (0) is the energy value of bandgap when T = 0 K; (Δ E g ) EPC stands for the contribution from electron–phonon coupling interaction; (Δ E g ) TE exhibits the change of bandgap owing to thermal expansion; ( i = a , b , c implies that along various crystal axes) represents linear expansion coefficient; B is the bulk modulus; Θ E indicates the Einstein temperature; β is a fitting parameter whose value is in reference to the intensity of electron–phonon coupling in a crystal. At high temperature, the linear temperature dependence of (Δ E g ) EP ∼β T can be shown by the term (Δ E g ) EPC .…”

“…In recent years, wide-bandgap semiconductors have been universally applied in various device fields such as ultraviolet detectors, − radiation detectors, etc. On behalf of the third-generation wide-bandgap semiconductors, silicon carbide (4H-SiC, E g ∼ 3.3 eV), gallium nitride (GaN, E g ∼ 3.4 eV), and monoclinic gallium oxide (β-Ga 2 O 3 , E g ∼ 4.9 eV) have attracted wide attention − and become the most promising kind of materials applicable to high-power electronic devices due to the advantages of a wide bandgap and high electron mobility. − …”

Self-trapped excitons in soft semiconductors

“…39 From the temperature-dependent PL spectra, the thermal quenching and evolution of the STEs emission, as well as the free exciton emissions, can be well characterized. 25,59,61…”