Research On High-Speed Avalanche Photodiodes For Optical Communication System

With the development of cloud computing,Internet of Things,5G broadband and other emerging network technologies,optical communication systems need to continuously improve the capacity and data processing capability.As the key device of optical receiver in optical communication system,the improvement of photodetectors is one of the important directions to enhance the rate and capacity of optical communication system.As the conventional photodetectors has no internal gain,it limits the improvement of optical receiver sensitivity,restricts the transmission distance of optical communication system,and increases the power consumption budget of optical link.Avalanche photodiodes(APDs)have internal gain due to the impact ionization,which greatly alleviates the limitation of sensitivity on transmission distance and power consumption budget.If the bandwidth of APD can be further enhanced,it is significant for building high speed and high capacity optical communication systems with long transmission distance.This dissertation focuses on photodetectors used in optical communication systems,especially high-speed avalanche photodiodes,and the main research work and results are as follows:1.A high-speed photodetector structure with a hybrid absorption layer is proposed.The introduction of the P-type gradual hybrid absorption layer into the conventional PIN photodetector shortens the drift distance of the hole,reduces the transport time of the hole,and increases the device bandwidth.With a device diameter of 12μm and a reverse bias voltage of 3 V,compared with the conventional simple PIN-PD,this design can increase the bandwidth from 24 GHz to 40 GHz and maintain the responsivity of 0.46 A/W.The doping concentration distribution of the Ptype absorption layer is optimized and three gradient absorption sub-layers is set up to accelerate the electron transport,the bandwidth has been increased to 43 GHz.The N-type field control layer and transport layer are introduced,and the field control layer are optimized to control the depletion absorption layer and the transport layer to form a step-type electric field distribution,the holes in the depletion absorption layer drift at saturation velocity and the electrons in the transport layer exhibit velocity overshoot.The obtained PIN-PD has a bandwidth of 61 GHz.2,A P-down inverted high speed APD with multi-mesa is designed.First,a double-mesa structure with P-down inverted APD is designed to limit the electric field below the top mesa and reduce the electric field at the edge of the device.Then,the P-type and N-type field control layers are optimized to improve the device performance,and the designed dual-mesa P-down inverted APD can achieve a bandwidth of 22 GHz at a gain of 9.7 and a gain bandwidth product of 214 GHz.Finally,a three-mesa P-down inverted APD is designed with a significantly reduced edge electric field of 4.6 kV/cm in the third mesa.Optimizing the mesa spacing of the third stage,the final APD achieves a bandwidth of 32 GHz at a gain of 8.9.A gain bandwidth product of 285 GHz is obtained with a dark current in the order of 0.1 nA.3.A high bandwidth APD with a p-type gradient absorption layer is proposed.The transit time of holes is shortened by using a p-type doped absorber layer.The holes in the p-doped absorber layer will enter the contact layer directly through relaxation.Simulation results show that the bandwidth of the APD is increased from 24 GHz to 29 GHz at low gain by changing the absorption layer structure.In addition,the absorption layer is design to p-type doping with almost no electric field,which avoids the effects of interband tunneling and impact ionization in the absorption layer,reduces the dark current of the APD structure to the order of picoamperes(10-12).4.A high-speed APD with p-type gradient absorption layers were prepared.A chip test and analysis system were built.High-speed APD devices were successfully fabricated using photolithography,electrode peeling,and dry etching processes.The steep and smooth side walls of the active region mesa were formed by ICP dry etching.The device with a diameter of 18 μm can reach a bandwidth of 9.2 GHz at a gain of 3,a maximum bandwidth of 13.6 GHz at a gain of 6,and a bandwidth of 11 GHz at a gain of 15.At a higher gain,the obtained gain-bandwidth product is 170 GHz,and the dark current remains at the order of 100 nA.Devices with a diameter of 24 μm can reach a bandwidth of up to 11 GHz,with a gain bandwidth product of 170 GHz.