Absorption measurement and Urbach’s rule in InSe and InSe:Ho0.0025, InSe:Ho0.025 single crystals

InSe and InSe:Ho single crystals were grown by the Bridgman–Stockberger method. Ho rare earth element are added to the InSe crystal as rate of 0.0025 and 0.025 g. Absorption measurements have been investigated as a function of temperature and Ho concentration. The high Ho concentration caused an increasing of intensity in absorption spectra and shifted towards to longer wavelengths. On the other hand low Ho concentration caused a decreasing of intensity in absorption spectra and shifted towards to longer wavelengths too. At 10 K and 320 K, the first exciton energies were calculated as 1.35 and 1.308 eV for InSe, 1.327 and 1.243 eV for InSe:Ho0.0025, 1.333 and 1.245 eV for InSe:Ho0.025, respectively. The exponentially increasing of absorption tail are explained as an Urbach–Martienssen’s (U–M’s) tail for InSe, InSe:Ho0.0025 and InSe:Ho0.025 samples in the 10–320 K temperature range. The Urbach’s energy (Eu) are increased in both InSe:Ho0.0025 and InSe:Ho0.025 samples. But increasing rate is higher in InSe:Ho0.0025 than InSe:Ho0.025 sample.