Stimulated Recombination and the Dynamic Mott Transition in GaTe

Abstract

The dynamics of photoexcited carriers in direct gap semiconductors can now be measured directly by time-resolved spectroscopy using intense, ultra-short laser pulses. Very high carrier densities can be generated and the processes of carrier cooling and recombination can be investigated by measuring band-edge transient absorption and luminescence. At high carrier densities a Mott transition is expected to occur when the interparticle separation r_s≈2.5 a₀, where a₀ is the exciton radius, so that for lower values of r_s dramatic changes in the band-edge absorption spectrum are expected (1,2)~ In this paper we report measurements of the layered semiconductor GaTe. The chief interest in this material is, as with other III-VI semiconductors, the very strong excitonic behaviour exhibited at low temperature. At 4 K the band gap E_G = 1.796 eV (3), and in our samples the exciton absorption peak lies at E_x= 1.778 eV which gives a binding energy of 18 meV. Indeed, as is expected with such a large binding energy, relatively strong excitonic behaviour is observed at room temperature. In our experiments we observe band gap renormalization and a bleaching of the exciton absorption induced by intense picosecond illumination. The recovery of this absorption shows three distinct regions in time in which the recovery rates are substantially different. In particular, we observe the onset of a very fast process which reduces the mean recovery time to ~ 15 ps, we have identified the mechanism as being due to stimulated recombination of carriers in the electron-hole plasma.