Study On P-I-N Ge-On-Si Waveguide Photodetector

Driven by the emerging internet technologies such as the cloud computing,the Internet of Things and the 5^th generation mobile networks,the data traffics in the network is growing exponentially,most of which will be processed in data centers.This poses a huge challenge to data centers that mainly use electrical interconnects solutions,and also brings opportunities for the developments of optical interconnects.Optical interconnects have advantages of large bandwidth,anti-electromagnetic interference,strong confidentiality,low transmission loss and low power consumption,etc.,which is the development direction of future high rate data transmission.Photodetector,which converts optical signals into electrical signals is a core component in the optical interconnects.Therefore,photodetectors with high responsibility and large bandwidth at communication wavelengths have been attracted more attentions and become a major research focus in the optical interconnect fields.Among the materials for photodetectors,germanium has received extensive attention due to its higher carrier mobility and light absorption at communication wavelengths,as well as higher thermal conductivity compared to III-V materials and compatibility with the CMOS processes.Silicon-based germanium photodetectors are currently a hot research area.In this thesis,the light absorption mechanisms in semiconductors and the principle of PIN junctions are introduced,and the main performance indicators of photodetector are given.Then,by using finite difference time domain（FDTD）method,a silicon based germanium waveguide photodetector with vertical PIN structure was designed.The structures of photodetector and it’s metal electrodes were optimized to improve the responsibility.Different from the continuous strip metal electrodes,a discrete metal electrode arranged along the length of the device was proposed,which are located in the regions with weak light intensities on the germanium surface,thereby reducing the light absorptions by the metal electrodes.The simulation results show that the light absorption can be reduced from 49%of the continuous strip metal electrode to 6.5%of the optimized discrete metal electrode.In the processes of the photodetector’s bandwidth simulations,the capacitance and resistance of the photodetector were extracted by combing the equivalent circuit and numerical fitting methods,which are 13.8f F（-1V）and 126Ω,respectively.Finally,an optimized germanium region with size of 20μm×1μm×0.4μm was obtained.When the detector is bias at-1V,30.2n A dark current,responsibility of 1.03A/W and bandwidth of 73.3GHz were achieved.Moreover,in order to improve the optical power handling capability of the photodetector,a parallel array of multiple photodetectors connected with traveling wave electrodes are considered.The electromagnetic modeling and simulation of the traveling wave electrode was carried out and the frequency responses of the photodetector itself were loaded on the coplanar waveguide traveling wave electrode as a electromagnetic model by using the equivalent circuit method.Then the characteristic impedance of the electrode was optimized by scanning the parameters.Finally,the simulation results show that for a traveling wave electrode with a 4 photodetector array,the impedance matching can be achieved and the characteristic impedance is around 50Ωin the frequency range of 0～100GHz,3d B bandwidth of 66GHz was obtained theoretically.