Metasurface-Enhanced Silicon-Based Germanium Photodetector

The appetite for data traffics is heightened by the rapid developments of the 5G,artificial intelligence,Internet of Things and other applications.With the rapid increase of data transmission in the communication networks,the optical interconnection gradually replaces the traditional electrical interconnection in short-distance communication,in order to meet the higher capacity and rate of signal transmission.In optical interconnection,silicon-based photonic will be one of the main solutions.The silicon-based photonic has the advantages of low cost,high-density integration,and compatibility with complementary metal-oxide semiconductor(CMOS)technology,which makes it possible to fabricate photonic circuits and microelectronic devices on the same chip.Silicon-based photodetector is the core device of photoelectric conversion in silicon-based photonic,which has important research significance.In this dissertation,a series of theoretical research and experimental exploration are carried out on the silicon-based germanium photodetector,including:growing high quality germanium films epitaxially on silicon using molecular beam epitaxy,exploiting and optimizing the fabrication technology of silicon-based germanium photodetectors,and designing resonant dielectric metasurface structures to improve the performance of the photodetectors.The details of the research are summarized as follows:(1)The growth of an ultra-thin germanium buffer layer is exploited to overcome the lattice mismatch between silicon and germanium.Using molecular beam epitaxy,the germanium buffer layer is grown at low temperature to introduce defects,which can reduce penetrating dislocations;and then the germanium buffer layer is annealed at appropriate temperature,which can improve the crystal quality.A germanium buffer layer with a thickness of only 100 nm is realized.Based on the germanium buffer layer,the high-quality germanium layer is grown at high temperature and annealed,which has a low dislocation density of 1.8×10⁵ cm^-2 and a low surface roughness of 1.9 nm.(2)The fabrication process of silicon-based germanium photodetectors is exploited.The n-type and p-type doping processes of silicon and germanium thin films are explored.The doping concentration from the highly doped layers to the intrinsic layer achieves a steep distribution of three orders of magnitude within a few nanometers.The fabrication process of opening window of the devices based on dry etching is exploited,which avoids the lateral corrision of chemical solutions,resulting in reduced the dark current density of the photodetectors by an order of magnitude.(3)A scheme for improving responsivity of normal-incident silicon-based germanium photodetectors using resonant dielectric metasurface is proposed.Resonant metasurfaces can limit locally the optical field to enhance the interaction between matter and light.Two kinds of metasurface with shallow hole and deep hole are designed for the devices.The optical field distribution of the resonant modes in the metasurface with shallow hole is analyzed.The resonant metasurface is designed to support multiple resonances in the target wavelength range.With an intrinsic absorption layer thickness of 350 nm,the device achieved a high responsivity of 0.67 A/W at 1550 nm,along with an enhancement more than 300%in the band of 1500?1560 nm.After optimizing the size of the devices,the devices exhibit a high performance.The dark current is 58 n A,the 3-d B bandwidth is up to33 GHz,and the responsivity is still up to 0.62 A/W at 1550 nm.(4)A method to manipulate the electric field and optical field distribution of a germanium avalanche photodetector using the resonant metasurface with air-hole is proposed.The metasurface structure can separate the absorption region from the multiplication region.Photogenerated carriers are generated and accelerated in the absorption region,and obtain sufficient energy before entering the multiplication region,which making more photogenerated carriers occur impact ionization.With the metasurface structure,the germanium avalanche photodetector achieves a gain-bandwidth product of137 GHz.The sensitivities of the device are-14 and-11 d Bm at 25 Gbit/s and 40 Gbit/s,respectively.