# The Frequency Response Characteristics of Ge-on-Si Photodetectors Under High Incident Power
## 2 Structure and Process  
Figure 1 presents a cross-sectional diagram of a vertical PIN germanium photodetector. This device is fabricated on a silicon on insulator (SOI) substrate, where the silicon layer functions as both the slab and the waveguide regions. The silicon slab consists of both lightly and heavily doped areas. The lightly doped region, spanning from the rib waveguide to the electrode, is 1.5 \(\upmu\)m wide and has a doping concentration of \(1\times 10^{-17}\) cm\({}^{-3}\). To ensure optimal ohmic contact between the slab and the metal electrode, a heavily doped contact region with a doping concentration of \(1\times 10^{-20}\) cm\({}^{-3}\) is incorporated. The thickness of the silicon slab is 60 nm. The width of the 220 nm thick rib waveguide is 1.5 \(\upmu\)m and the length is 10 \(\upmu\)m. The doping concentration for the silicon rib is \(1\times 10^{-15}\) cm\({}^{-3}\); this concentration ensures the transport of high-concentration photogenerated carriers without significant light absorption. The germanium absorption layer is epitaxially grown on the silicon rib waveguide region with a thickness of 600 nm, and its width and length match those of the silicon rib waveguide region. For ohmic contact with the electrode, the top of the germanium absorption layer is heavily doped with a concentration of \(3.8\times 10^{-18}\) cm\({}^{-3}\).  
The fabrication process begins with an SOI wafer, which then has its top layer of silicon etched, on which the non etched area is the silicon waveguide and the shallow etched area is the slab region. Next, the wafer undergoes a two-step boron implantation process that involves a light implantation followed by a heavy implantation to form p-type ohmic contacts. The dopants within the silicon are then activated using a rapid thermal annealing (RTA) process at 1030 \({}^{\circ}\)C for 5 s, which precedes the germanium epitaxy step. Subsequently, phosphorus ion implantation is carried out on the germanium layer to create the n-type ohmic contact region. Following the deposition of the cladding oxide, the fabrication process concludes with the formation of contact vias and the patterning of aluminum interconnects. The defect density of the top silicon layer in the SOI is \(3\times 10^{4}\) cm\({}^{-2}\) and the defect density of the epitaxial germanium layer is \(7\times 10^{4}\) cm\({}^{-2}\).  
An increase in the size of the germanium absorption region improves the level of high incident light power processing of the photodetector. However, it increases the device's capacitance, which reduces the bandwidth of the device.  
The 3 dB bandwidth of the photodetector was measured using a setup comprising a vector network analyzer (VNA, KEYSIGHT PNA-X Network Analyzer N5247A, Keysight, Santa Rosa, CA, USA) and a high-performance LiNbO\({}_{3}\) modulator with a 40 GHz bandwidth. A precision DC power supply (KEITHLEY 2611A SYSTEM Source Meter, Keithley,  
Figure 1: Cross-sectional diagram of the germanium PIN photodetector.  
Santa Rosa, CA, USA) was used to apply the DC bias voltage to the photodetector through a bias tee.