High-detectivity and -stability multilayer-graphene/Si-quantum-dot photodetectors with TiO_x back-surface passivation layer

We first report Si quantum dots (SQDs)-embedded SiO₂ (SQDs:SiO₂) multilayers (MLs)/Si photodetectors (PDs) using multilayer graphene (MLG) transparent conductive electrode (TCE) and TiO_x back-surface passivation layer. With increasing number of graphene layer (L_n) to 5, the sheet resistance of the MLG TCE sharply decreases to ∼273 Ω/sq whilst the work function gradually increases to ∼4.71 eV, indicating p-type doping, useful for collection of photo-excited carriers at the MLG/SQDs:SiO₂ MLs interface. At L_n = 4, the PD without TiO_x exhibits best performance of 0.340 AW^-1 responsivity (R) and 3.21 × 10⁹ cm Hz^1/2 W⁻¹ specific detectivity (D*) at the peak wavelength. With TiO_x on the back side of Si wafer (L_n = 4), the R slightly increases to 0.351 AW^-1, but the D* is greatly enhanced to 9.42 × 10¹¹ cm Hz^1/2 W⁻¹, resulting from the sharp reduction of the dark current by the suppression of the carrier recombination at the Si/TiO_x interface. The PD performance is almost consistent for 1000 h in air, irrespective of the use of the TiO_x layer. These behaviors are much better than ever achieved in graphene/Si wafer PDs.