A novel free-standing resonant-cavity-enhanced (RCE) Ge thin-film absorber is designed with a bottom distributed Bragg reflector (DBR) for infrared photodetection at 1550 nm wavelength based on a Si substrate. The free-standing structure offers a high degree of freedom in optimizing the number of periods, the layer arrangement, and the thickness of each layer of the DBR. A SiO2/Si DBR that is compatible with Si processing technology is used on the bottom of the Ge thin film to construct the RCE structure. Based on our theoretical study, the SiO2 low-index layer must be placed next to the Ge layer to utilize the large difference in the refractive indices between SiO2 and Ge to achieve high reflectance for the DBR. In this design, the structure can reach an absorbance of 85.90% with a DBR of three periods on the bottom and an antireflective oxide layer on the top surface of the Ge layer. Without the antireflective oxide layer, the absorbance is increased to 99.17%. The design paves the way for high-responsivity Ge infrared photodetectors in Si-based photonic systems.

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