Ultrafast Carrier Recombination from Quantum Pillars in Black Silicon

We report on ultrafast photoluminescence(PL) phenomena from black silicon produced by ion etching using chlorine plasma. An ultrafast blue PL component competing with non-radiative recombination at surface defects was quantified as originating from the no-phonon transition. This component involves two decay processes with a peak energy at 480 nm: a fast component of 6 ps followed by a component of 48 ps decay time constant. It exhibits also a slow component in the red spectral region with a time constant of about 2.5 ns. When it is oxidized, slow band at around 600 nm is enhanced in intensity to the detriment of blue emission band. This process results in a much slower sates assuming 3-components exponential decay. Ultrafast PL decay leads to transfer of carriers to long-lived defect states as evidenced from red emission at around 2 eV. Time-correlated single photon counting revealed a life-time of about 5 ns for these states. The results are discussed in terms of electronic band structure modification at reduced sizes and surface point defects.