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 investi gated in the excitonic resonance energy region and just below. Free exciton (FE) transitions associated with the direct gap of GaSe and GaSe:Gd have b een measured as a function of temperature in the range of 10-340 K. The par ameters describing the temperature variation of both the spectral position and the broadening function of the excitonic resonance confirm the dominati ng role of the A(1)'((1)) homopolar phonon mode at 134.6 cm(-1). The Gaussi an lineshape was used to fit the excitonic structures. The decreased absorp tion intensity and broadened lineshape of the excitonic resonances for GaSe :Gd crystals were attributed to the Gd dopant atoms. The exponentially incr easing absorption tails were explained as Urbach-Martienssen (U-M) tails fo r both GaSe and GaSe:Cd samples in the 10-340 K temperature range. The char acteristic tail width, Urbach's energy E-U, was obtained as a function of t emperature. The temperature dependence of E-U was interpreted based on the general models on this rule. The Urbach energy increased as a function of t emperature 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 asso ciated with the Gd atoms in the crystal lattice of GaSe.