Excitonic absorption and Urbach-Martienssen tails in Gd-doped and undoped p-type GaSe

Optical absorption spectra of GaSe and GaSe:Gd single crystals were investigated in the excitonic resonance energy region and just below. Free exciton (FE) transitions associated with the direct gap of GaSe and GaSe:Gd have been measured as a function of temperature in the range of 10-340 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 A'(1)1 homopolar phonon mode at 134.6 cm-1. The Gaussian lineshape was used to fit the excitonic structures. The decreased absorption intensity and broadened lineshape of the excitonic resonances for GaSe:Gd crystals were attributed to the Gd dopant atoms. The exponentially increasing absorption tails were explained as Urbach-Martienssen (U-M) tails for both GaSe and GaSe:Gd samples in the 10-340 K temperature range. The characteristic tail width, Urbach's energy EU, was obtained as a function of temperature. The temperature dependence of EU was interpreted based on the general models on this rule. The Urbach energy increased as a function of temperature in the investigated temperature region for the Gd-doped sample. Such an increase of the Urbach energy can be explained as being due to the enhancement of electronic distortion caused by the structural disorder associated with the Gd atoms in the crystal lattice of GaSe.