Enhanced 400-600 nm photoresponsivity of metal-oxide-semiconductor diodes with multi-stack germanium quantum dots

Metal-oxide-semiconductor (MOS) diodes with zero-, one- or three-layer Ge quantum dots (QDs) embedded in the gate oxide are fabricated for visible to near-ultraviolet photodetection. Ge dots are formed by thermally oxidizing one or three stacks of amorphous Si (a-Si)/polycrystalline-Si_0.87Ge _0.13/a-Si multi-layers that are sandwiched by SiO₂ barriers. The current-voltage characteristics of Ge QD MOS diodes exhibit strong rectification in darkness and feature significant current enhancement in the inversion mode when illuminated. Increasing the number of Ge QD layers from zero through one to three in the gate oxide improves the responsivity from 4.64 through 482 to 812 mA W^-1 and enhances the corresponding quantum efficiency from 1.42 through 148 to 245%, respectively. The spectral response reveals a considerable blueshift in peak energies as the Ge dot size decreases from 9.1 to 5.1 nm, suggesting that the light absorption originates from the quantum confinement effect of Ge QDs. The temperature and bias dependences of the dark current indicate that the carrier transport mechanism involves percolation hopping.