# Optical Study on Temperature-Dependent Absorption Edge of \(\gamma\)-InSe-Layered Semiconductor
## 1 Introduction  
In recent years, several two-dimensional layered semiconductors, including graphene, transition metal dichalcogenides (TMDCs), and III-VI semiconductors, have been investigated both experimentally and theoretically due to their unique optical, electronic and mechanical properties [1; 2; 3; 4; 5]. Among these different layered semiconductors, the indium selenide (InSe) semiconductor consisted of stacked layers of Se-In-In-Se atoms with van der Waals bonding between the quadruplet layers. There are three common polytypes described as \(\beta\)-, \(\epsilon\)- and \(\gamma\)-InSe, which differ in the symmetry and structure of the crystal lattice, and display different optical properties [1; 4]. [PERSON] et al. and [PERSON] et al. have shown that, apart from \(\varepsilon\)-InSe, which exhibited an indirect bandgap, both \(\beta\)-InSe and \(\gamma\)-InSe have a direct bandgap in the near infrared energy range [1; 2]. Accordingly, only \(\beta\) and \(\gamma\) phases of InSe can be developed as an attractive material for versatile optoelectronic devices [3; 5; 6].  
To the best of our knowledge, there are few relevant studies on the temperature dependence of the absorption edge in InSe material [7; 8]. [PERSON] et al. and [PERSON] have investigated the optical properties of InSe single crystal by absorption measurement and observed that an Urbach's tail appeared below the edge of the absorption spectrum.  
However, no information about the actual phase of their samples was provided, and hence only a qualitative comparison can be made. [PERSON]'s tail or below-band edge opticalabsorption coefficient (\(\alpha\)) can be described by an exponentially varying function with respect to photon energy [9, 10]. [PERSON] et al. indicated that the absorption tail below the absorption edge most probably originates from the temperature-independent imperfections of the material and static disorder caused by the impurities, dislocations, or stacking faults [9]. [PERSON] et al. have experimentally and theoretically explained the origin of this tail as exciton-phonon interaction in their material [10]. Therefore, the investigations on [PERSON]'s tail can provide important information on the localized states extended into the optical gap region, which are related to the structural and thermal disorder, and the electrons/excitons-phonons interactions in the layered semiconductors.  
In this work, we reported the temperature dependence of the absorption edge of \(\gamma\)-InSe by analyzing the \(\alpha\) derived from photoconductivity (PC) measurements [11, 12]. [PERSON] et al. and [PERSON] et al. have shown that properly normalized PC data can indeed give an accurate measure of \(\alpha\)[11, 12]. The energy gap estimated from the PC spectra of \(\gamma\)-InSe as a function of temperatures was analyzed by [PERSON]'s relationship [13] and the Bose-Einstein empirical equation [14]. Additionally, the effective phonon energy was further obtained based on the temperature dependence of the Urbach energy extracted from PC spectra. The detailed discussion of experimental data can provide useful information for the optoelectronic materials and solar cell devices fabricated by chalcogenides-based layered semiconductors.