Germanium-on-Silicon Waveguide Photodetector with Rib-Shaped Mesa
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Solution Overview
Problem
Existing near-infrared photodetectors on silicon substrates face limitations in responsivity due to the small active region and high defect density of polycrystalline germanium, which restricts their performance in normal incidence detection, and waveguide configurations face challenges in maintaining efficiency while preserving speed.
Innovation Solution
A waveguide photodetector structure with a polycrystalline germanium layer deposited on a silicon substrate, featuring a rib-shaped mesa geometry that confines light both vertically and laterally, allowing for distributed absorption and efficient electron-hole pair collection, thereby enhancing responsivity and maintaining high-speed performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If normal incidence detection geometry is used with polycrystalline germanium, then fabrication compatibility with silicon is achieved, but responsivity is limited due to small active region and high defect density
Solution Approach 1:
The patent transitions from normal incidence detection (light perpendicular to surface) to waveguide geometry (light propagating laterally through the germanium layer). This dimensional change allows the light to travel a longer path through the absorptive material, increasing absorption efficiency and responsivity while maintaining fabrication compatibility with silicon substrates
2Measurement precision
If waveguide configuration is adopted to increase absorption length, then responsivity is improved, but device speed may be compromised
Solution Approach 1:
The patent creates a rib-shaped mesa structure that confines light laterally within a specific region of the waveguide. This local confinement ensures that the optical mode overlaps maximally with the germanium layer where absorption occurs, while the overall waveguide geometry maintains short carrier transit paths that preserve high device speed
3Ease of manufacture
If polycrystalline germanium is used instead of monocrystalline, then thermal budget is reduced for compatibility with CMOS, but mobility and lifetime are reduced
Solution Approach 1:
The patent uses polycrystalline germanium that replicates the absorption spectrum characteristics of monocrystalline germanium in the near-infrared region. While the crystal structure differs, the optical absorption properties are sufficiently similar to achieve high responsivity, while the polycrystalline form enables lower temperature fabrication compatible with standard CMOS processes
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The waveguide geometry increases absorption efficiency by extending the absorption length along the light propagation path, achieving higher responsivity and faster operation speeds compared to traditional normal incidence detectors, while maintaining compatibility with standard silicon CMOS technology.
Implementation Method 1
Silicon-germanium (SiGe) has been considered a promising alternative to InGaAs, due to its large absorption coefficient in the NIR
Implementation Method 2
The photocurrent generated by the selected photosensitive pixel is fed to a transimpedance amplifier
Data Source
AI summary
A photodetector structure includes a silicon-based waveguide in which optical signals to be detected travel in a given direction and are confined therein and a germanium layer disposed in contact with a portion of the silicon-based waveguide so that an evanescent tail of the propagating optical signal in the waveguide is coupled into the germanium layer. In addition, the germanium layer includes a mesa having a length along the signal propagating direction and a width in a direction substantially perpendicular to the propagating direction, in which the width of said mesa is smaller than its length. The photodetector also comprises a first and a second metal contacts, the first metallic contact being located on the germanium layer, the said second metallic contact being located on the silicon-based waveguide, the first and second contacts being used to collect electrons generated by light absorption to obtain an output electric signal.