# Optical Study on Temperature-Dependent Absorption Edge of \(\gamma\)-InSe-Layered Semiconductor
## 3 Results and Discussion  
The XRD pattern and Raman spectrum of the as-grown bulk InSe crystals were measured at room temperature and are depicted in Figure 1a,b, respectively. It is known that the five XRD peaks at about 10.51\({}^{\circ}\), 21.31\({}^{\circ}\), 32.20\({}^{\circ}\), 43.96\({}^{\circ}\), and 67.94\({}^{\circ}\), as labelled and shown in Figure 1a, can be assigned to define the crystal structure of the as-synthesized InSe. Referring to the reported literature [3; 4; 5; 6], [PERSON] et al. [4] [PERSON] et al. [5] have attributed these peaks to the (0, 0, 2\(n\)) planes for the hexagonal \(\beta\)- or \(\varepsilon\)-polytype, but [PERSON] et al. [3] and [PERSON] et al. [6] have attributed these peaks to the (0, 0, 3\(n\)) planes for the \(\gamma\)-polytype. The present uncertainty on the structural phase of the as-grown InSe sample can be resolved by analyzing the two additional diffraction peaks at about 29.41\({}^{\circ}\) and 47.05\({}^{\circ}\) (marked by the red arrow in Figure 1a). By comparing the JCPDS (Joint Committee on Powder Diffraction Standards) standard database, the two diffraction peaks at about 29.41\({}^{\circ}\) and 47.05\({}^{\circ}\) are unable to be defined by the standard data file of PDF 34-1431 for \(\beta\)- or \(\varepsilon\)-polytype [4; 15], but are matched to the standard data file of PDF 71-0447 for \(\gamma\)-polytype and defined as (104) and (0111) plane, respectively [6]. The calculated lattice parameters a = b = 4.32 A and c = 24.96 A are in good agreement with the JCPDS #71-0447 (a = b = 4 A and c = 24.96 A) [6]. Additionally, Raman spectroscopy can be employed to further clarify the structural phase of the as-synthesized InSe. As shown in Figure 1b, the Raman spectrum exhibits  
Figure 1: (**a**) XRD patterns (**b**) Raman spectra of the grown InSe crystals. three significant peaks at 117, 178, and 229 cm\({}^{-1}\), as well as a weak feature at 200 cm\({}^{-1}\). These observed features have been attributed to the A\({}^{1}_{\text{1g}}\), E\({}^{1}_{\text{2g}}\), A\({}^{2}_{\text{1g}}\), and A\({}^{1}_{\text{1g}}\)(LO) modes, respectively [3,4]. It should be noted that the A\({}^{1}_{\text{1g}}\)(LO) mode has been identified as a significant characteristic for the non-centrosymmetric structure of either \(\varepsilon\)-polytype or \(\gamma\)-polytype [3,4]. The observation of the A\({}^{1}_{\text{1g}}\)(LO) mode signal provides additional evidence to exclude the occurrence of \(\beta\)-polytype. Therefore, by integrating the data from both XRD and Raman analysis, it can be inferred that the structural phase of the as-prepared bulk InSe sample is \(\gamma\)-polytype [6]. Figure 2 shows the normalized PC spectra of \(\gamma\)-InSe in the energy range \(\sim\)1.0-1.4 eV at selected temperatures of 15, 100, 200, 260, and 300 K.  
It is observed that the lower-temperature PC spectra at 15, 100, and 200 K exhibit an initial decrease in PC intensity in the energy range 1.1-1.2 eV. A minimum is observed in the range 1.2-1.25 eV, which increases to a maximum in the range 1.25-1.3 eV, and then finally decreases monotonically with any further increase in photon energy within the investigated energy range. The higher-temperature PC spectra at 260 and 300 K show an almost constant below-band edge PC intensity and increase to a maximum as the photon energy sweeps across the band edge, and then decrease with any further increase in the photon energy. It is noted that the existence of impurity or defect states may play an important role in the PC spectra at low temperature, leading to a larger PC intensity in the vicinity of 1.1 eV. Thermal ionization of the defect states is evident in the observed PC spectra at higher temperature. The temperature dependence of the change in PC intensity can provide information about the fundamental absorption properties of \(\gamma\)-InSe. As stated earlier, correlation of our PC intensity measurements to the relevant absorption coefficient can be found in references [11,12] and will not be elaborated on here.