Research On Grating-coupled Ge-based Near-infrared Waveguide Detector

Recently,fiber-optic communications,bio-imaging,night vision,atmospheric remote sensing and other technical fields are developing rapidly,therefore,there are more higher requirements for the corresponding near-infrared photoelectric detection technology and device performance,such as light response,bandwidth,size and integration.Group IV Gebased materials have been considered as one of the candidate materials for breakthroughs in silicon-based photoelectric detection integration technology due to their large absorption coefficient in the near-infrared band,adjustable bandgap,high carrier mobility and its compatibility with silicon technology.There are broad application prospects in silicon-based infrared photodetection.Meanwhile,in many types of detectors,waveguide photodetector can solve the problem of trade-off between bandwidth and responsivity for free space normal-incidence vertical photodetectors,and it has characteristics of high speed,high response and easy integration with silicon substrate.Therefore,Ge-based waveguide photodetector has attracted increasing research attention.Based on the advantages of Ge based materials and waveguide detector structure,this thesis focuses on grating coupler and waveguide detector,and studies the grating-coupled Ge-based near-infrared waveguide detector.The research results in this thesis can be summarized as follows.1.High coupling efficiency grating coupler based on silicon on insulator(SOI)is proposed.Aiming at the low coupling efficiency of uniform grating under 1550 nm light,the grating coupler is optimized by adding amorphous-Silicon layer,bottom mirror and non-uniform grating.Under the optimal parameters,the maximum coupling efficiency of the three structures is 71.6%,63.1% and 77.4%,respectively.Furthermore,based on the SOI substrate with top Si thickness of 340 nm,a grating coupling structure with peak coupling efficiency of 85% is obtained by combining the non-uniform grating and bottom mirror,coupling efficiency increased by 31.3%.In addition,the grating structure is optimized for the wavelength of 2000 nm,and the maximum coupling efficiency is 75.5% in the non-uniform grating with 340 nm top Si thickness.Finally,considering the feasibility of the threedimensional composite structure of grating and optical waveguide,this thesis analyzes the structure of the linear taper grating,and establishes a small-scale structure suitable for focusing grating in the waveguide detector.2.Based on the wide spectral absorption of Ge Sn and the light confinement of SOI structure,a high speed and high response Ge based near infrared waveguide detector with evanescent coupling is designed.FDTD and ATLAS TCAD software were used to simulate and optimize the optical absorption and electrical performance of the device,respectively.The simulation results show that the total absorption of Ge Sn waveguide structure with 3?9% Sn is more than 98% and the optical responsivity is 1.13 A/W under 1550 nm illumination when the waveguide length is 10 ?m;When the Sn content is 9% and the waveguide length is 20 ?m,the absorptivity of 2000 nm incident light can reach more than 95% and the optical responsivity can reach 1.45 A/W.By analyzing the correlation between dark current and doping concentration,Sn composition and device size,it is found that high doping,low Sn composition and small size contribute to the suppression of dark current.Then,the intrinsic layer thickness is optimized.When the intrinsic layer thickness is 100 nm,the bandwidth at1550 nm can reach 90 GHz at-5 V bias;In order to ensure the optical absorption at the wavelength of 2000 nm,the bandwidth is 68 GHz at-5 V bias when the intrinsic layer thickness is 200 nm.3.Based on the optimized structure of grating coupler and waveguide detector,a grating coupled waveguide detector based on SOI and GOI is designed.Through the analysis of the field distribution and optical absorption,it is found that the grating structure can realize the transformation from the vertical light incident direction to the propagation direction of the optical waveguide,and the optical absorption of the detector is increased by more than 7times and 30 times respectively compared with the detector without grating structure.In conclusion,the device structures designed in this thesis can not only solve the problems of low coupling efficiency and low responsivity of the detector,but also meet the requirements of near-infrared wide spectrum detection,which has a certain guiding significance for the realization of high-performance detector and silicon-based optoelectronic integrated application.