Three-dimensional graphene (3D-Gr) with excellent light absorption properties has received enormous interest, but in conventional processes to prepare 3D-Gr, amorphous carbon layers are inevitably introduced as buffer layers that may degrade the performance of graphene-based devices. Herein, 3D-Gr is prepared on germanium (Ge) using two-dimensional graphene (2D-Gr) as the buffer layer. 2D-Gr as the buffer layer facilitates the in situ synthesis of 3D-Gr on Ge by plasma-enhanced chemical vapor deposition (PECVD) by promoting 2D-Gr nucleation and reducing the barrier height. The growth mechanism is investigated and described. The enhanced light absorption as confirmed by theoretical calculation and 3D-Gr/2D-Gr/Ge with a Schottky junction improves the performance of optoelectronic devices without requiring pre-and post-transfer processes. The photodetector constructed with 3D-Gr/2D-Gr/Ge shows an excellent responsivity of 1.7 A W −1 and detectivity 3.42 × 10 14 cm Hz 1/2 W −1 at a wavelength of 1550 nm. This novel hybrid structure that incorporates 3D-and 2D-Gr into Ge-based integrated circuits and photodetectors delivers excellent performance and has large commercial potential.