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 investi gated just below and in the excitonic resonance energy region. The temperat ure dependence of the free exciton transition associated with the direct ga p 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 li neshape 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 exponent ially 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 EU, was obtained as a function of temperature. The temperature dependence of Eo 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 b e due to the reduction of the electronic distortion caused by the structura l disorders associated with the planar defects in the crystal lattice of ln Se by the Er-doping procedure.