Photoluminescence above the Tauc gap in a-Si:H

We report the observation of photoluminescence (PL) significantly above the Tauc gap in a-Si:H. The dependence on temperature, incident intensity, excitation energy, deep-defect concentration, and time-resolved measurements are presented. The PL begins at the excitation energy, decreases in intensity as the photon energy decreases, reaches a minimum at 2 eV, and then increases exponentially until it approaches the PL peak at 1.4 eV. The luminescence above 2 eV is weak and temperature independent. It is attributed to the recombination of nonthermal electrons with nonthermal holes. Below 2 eV, the luminescence depends on temperature and deep-defect density. It results from the recombination of electrons and holes in the band tails. At low temperature, the slope of the luminescence indicates that the radius of the electron wave function in the band tail is 10.