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Gold nanoparticles (AuNPs) have been deposited on n-type Ge photodetectors to improve the responsivity. Two different coverage ratios, including 10.5 and 30.3% of AuNPs have been prepared, and the fabricated photodetectors are compared with the control sample. The 1,310-nm responsivities at -2 V of the control, 10.5% AuNPs, and 30.3% AuNPs samples are 465, 556, and 623 mA/W, respectively. The AuNPs could increase the responsivities due to the plasmon resonance. The reflectance spectra of these samples have been measured to verify that plasmon resonance contributes to the forward scattering of incident light. The reflectance decreases with AuNP deposition, and a denser coverage results in a smaller reflectance. The smaller reflectance indicates more light could penetrate into the Ge active layer, and it results in a larger responsivity.

We have tried to improve the responsivity of germanium-based thin-film photodetectors. It has been shown that applying a mechanical strain to the detector led to a 46.6% enhancement on the 1550 nm detection. This improvement is better than the 1310 nm case, because the bandgap shrinkage is more beneficial to the small-energy photon detection. The AuNP coating is even more attractive for responsivity enhancement of thin-film germanium (Ge) detectors. The responsivity enhancement due to the AuNP deposition is as high as 89% and 47%, for the 1310 nm and 1550 nm detections, respectively. To the best of our knowledge, this is the best responsivity enhancement for the thin-film Ge detectors reported to date.

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