Figure 4: Sub-band gap photoresponse mechanism of Si:Au.

(a) Si:Au spectral response η as a function of photon energy measured using tunable wavelength transient photoresponse measured at room temperature for the photodiode with the highest gold dose of 10¹⁵ cm⁻². A kink in the spectral response is observed at the threshold energy E_t=0.78 eV, which corresponds to the substitutional gold donor level in silicon. For E_ph<E_t, the response is characterized by the Urbach absorption edge with a slope of E_s=42 meV. (b) Above-band gap and sub-band gap photoresponse of the Si:Au and reference silicon photodiodes for varying photon intensity, confirming that the sub-band gap response in the reference silicon is caused by two-photon absorption (TPA) in the substrate. Under standard operating conditions (~100 W cm⁻² or lower), no measurable TPA was observed in the reference silicon photodiode. (c) Possible sub-band gap photoresponse mechanism for substitutional gold in silicon through the donor level for E_ph>0.78 eV. (d) The polynomial dependence (n=0.6) of η on E_ph, for E_ph close to E_t (0.8 eV<E_ph<0.94 eV), suggesting that the defect-state wavefunction is close to that of a delta function.