Angled Germanium Photodiode Structure for Low-Capacitance Detection
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Solution Overview
Problem
Current opto-electric converters, such as modulators and demodulators, limit the speed of data transfer through optical fibers to approximately 25 Gbps, and there is a need to increase this speed.
Innovation Solution
A photodiode with a germanium region in a silicon layer, where the germanium region has only two sides in contact with the layer, forming an angle greater than 10° with the orthogonal direction, is used in an opto-electric converter with a waveguide and trenches of insulating material to enhance data transfer speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional photodiode structures are used, then manufacturing is simpler, but data transfer speed is limited to approximately 25 Gbps
Solution Approach 1:
The photodiode structure is segmented into distinct functional regions: a first germanium region with specific angular contact geometry, a second germanium region, and separated active areas. This segmentation allows optimization of each region for specific functions (light absorption, carrier collection) enabling higher data transfer speeds up to 50 Gbps while maintaining manufacturability through modular design
Solution Approach 2:
Different regions of the photodiode are assigned different material compositions and geometric properties. The first germanium region has a specific angle (10°-20°) with the silicon layer to minimize dislocations, while the second region provides additional light absorption. This local optimization of properties in different zones enables enhanced performance without requiring complete restructuring of the entire device
2Reliability
If germanium width is increased to improve light absorption, then sensitivity improves, but capacitance increases reducing transfer speed
Solution Approach 1:
The first germanium region is configured with a specific angular orientation (10°-20°) relative to the silicon layer plane, transitioning from a simple planar geometry to a three-dimensional angular structure. This dimensional change allows the germanium to extend further into the light path for improved absorption while maintaining a narrow effective width at the contact point, thus reducing capacitance and enabling faster transfer speeds
Solution Approach 2:
The photodiode uses a composite structure combining silicon and germanium layers with different optical and electrical properties. The silicon layer provides mechanical support and a portion of the light absorption, while the germanium regions provide enhanced absorption in the 1310nm and 1550nm wavelength ranges. This composite approach allows optimization of the germanium width for sensitivity while the silicon contributes to the overall function, managing the capacitance-sensitivity tradeoff
3Ease of manufacture
If germanium region contacts the silicon layer on multiple sides, then manufacturing is easier, but dislocations increase reducing device reliability
Solution Approach 1:
The first germanium region is designed with asymmetric contact geometry, contacting the silicon layer on only two sides with a specific angle (10°-20°) rather than symmetrically on all sides. This asymmetric configuration minimizes the formation of dislocations at the germanium-silicon interface by reducing lattice mismatch stress, thereby improving device reliability and performance while remaining compatible with standard fabrication 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 described structure allows for accurate control of the germanium width, reducing capacitance and increasing data transfer speed to potentially 50 Gbps by minimizing dislocations and optimizing the active area for improved signal transmission.
Implementation Method 1
a photodiode comprises an active area, the active area including at least one first germanium region in a first silicon layer
Data Source
AI summary
A photodiode includes an active area formed by intrinsic germanium. The active area is located within a cavity formed in a silicon layer. The cavity is defined by opposed side walls which are angled relative to a direction perpendicular to a bottom surface of the silicon layer. The angled side walls support epitaxial growth of the intrinsic germanium with minimal lattice defects.