Time-resolved photoluminescence ina-Si:H: Sub-band-gap excitation

We present a comprehensive investigation of the evolution of the photoluminescence band at ∼1.3 eV in plasma-deposited $a$-Si:H as a function of excitation frequency ${\tilde{v}}_{\mathrm{ex}}$, where ${\tilde{v}}_{\mathrm{ex}}$ has been extended well into the band-tail region. When ${\tilde{v}}_{\mathrm{ex}}$ is lowered below the optical-absorption gap, the early time shift is diminished and eventually lost. No corresponding change is observed in the time decay. The most frequently cited models for the photoluminescence behavior are reexamined in the light of these new results. Models of the early time spectral shift based on a correlation between recombination energy and life-time are ruled out, and we attribute the shift to slow-electron thermalization within the conduction-band tail. The results are consistent with recombination by radiative tunneling, providing that either the electron or the hole is trapped at a charged site. A value of 0.39 eV is obtained for the sum of the Stokes shift and hole-trap binding energy.