Free-exciton luminescence spectrum broadening due to excitonic complex in diamond

The excitation density evolution of a band-edge luminescence spectrum of diamond in the photon energy region 5.14 to 5.40 eV was investigated at 190, 240 and 290 K with the fifth harmonic (5.82 eV) of a pulsed YAG laser in the excitation density range 2.2 to 272 mJ/cm2. A broadening of a free-exciton luminescence (FE) spectrum with the peak energy (Epeak) of 5.28 eV due to the growth of the low-energy tail was observed with increasing excitation density. From a detailed spectrum analysis, it is found that the broadening originates in excitonic complex (EC) luminescence (Epeak = 5.26 eV) in addition to electron-hole plasma (EHP) luminescence (Epeak = 5.23 eV). The EC luminescence intensity was observed to be proportional to the 1.5 power of the FE luminescence intensity, which is the same manner as in the case of the EC luminescence in crystalline silicon. The excitation density dependence of the EHP luminescence intensity is discussed with percolation theory.