Excitonic absorption and Urbach-Martienssen's tails in Er-doped and undoped n-type InSe

Optical absorption spectra of InSe and InSe:Er single crystals were investigated just below and in the excitonic resonance energy region. The temperature dependence of the free exciton transition associated with the direct gap of InSe and InSe:Er were measured in the temperature range 10<T<340 K. The parameters describing the temperature variation of both the spectral position and the broadening function of the excitonic resonance confirm the dominating role of the average energy of crystal phonons. The Lorenzian lineshape was used to fit the excitonic structures. The increased absorption intensity and the narrowed lineshape of the excitonic resonances in InSe:Er crystals were attributed to the [Er] = 0.03 at% dopant atoms. The exponentially increasing absorption tail was explained as an Urbach-Martienssen's (U-M's) tail for both InSe and InSe:Er samples in the 100-340 K temperature range. The characteristic tail width, Urbach's energy E_U, was obtained as a function of temperature. The temperature dependence of E_U was interpreted based on the general models of this rule. The Urbach's energy decreased as a function of temperature in the temperature region investigated for the Er-doped sample. Such a decrease of the Urbach's energy can be explained to be due to the reduction of the electronic distortion caused by the structural disorders associated with the planar defects in the crystal lattice of InSe by the Er-doping procedure.