Combined effects of screening and band gap renormalization on the energy of optical transitions in ZnO and GaN

The energy positions of the optical transitions in both GaN and ZnO were in vestigated when the samples were excited simultaneously with a HeCd laser a nd an Ar+ ion laser. The increased number of free electrons excited by the Ar+ ion laser will effectively screen both the free exciton and bound excit on transitions, resulting in a blueshift. The increased number of free elec trons also produces many-body effects, which lead to a reduction of the ban d gap energy and thus a redshift. The resultant of screening and renormaliz ation results in a redshift of the optical transitions in ZnO but a nearly vanishing shift in GaN. (C) 2000 American Institute of Physics. [S0021-8979 (00)05123-9].