Ultrafast Exciton Dynamics in Silicon Nanowires

Ultrafast exciton dynamics in one-dimensional (1D) silicon nanowires (SiNWs) have been investigated using femtosecond transient absorption techniques. A strong transient bleach feature was observed from 500 to 770 nm following excitation at 470 nm. The bleach recovery was dominated by an extremely fast feature that can be fit to a triple exponential with time constants of 0.3, 5.4, and ∼75 ps, which are independent of probe wavelength. The amplitude and lifetime of the fast component were excitation intensity-dependent, with the amplitude increasing more than linearly and the lifetime decreasing with increasing excitation intensity. The fast decay is attributed to exciton-exciton annihilation upon trap state saturation. The threshold for observing this nonlinear process is sensitive to the porosity and surface properties of the sample. To help gain insight into the relaxation pathways, a four-state kinetic model was developed to explain the main features of the experimental dynamics data. The model suggests that after initial photoexcitation, conduction band (CB) electrons become trapped in the shallow trap (ST) states within 0.5 ps and are further trapped into deep trap (DT) states within 4 ps. The DT electrons finally recombine with the hole with a time constant of ∼500 ps, confirming the photophysical processes to which we assigned the decays.