Low Voltage Si/Ge APD with Re-entrant Mirror for PICs
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Solution Overview
Problem
High operating biases and high packaging costs hinder the widespread adoption of silicon/germanium (Si/Ge) avalanche photodiodes (APDs) in photonic integrated circuits (PICs), particularly for applications requiring low voltage operation and efficient optical coupling, as they often require more than 12V and have inefficient edge coupling methods that are not suitable for high volume manufacturing.
Innovation Solution
A low voltage Si/Ge APD is integrated with a re-entrant minor (REM) waveguide, where the APD is disposed over an inverted re-entrant mirror facet at the end of the waveguide, allowing for internal reflection and efficient light coupling, and a top contact architecture with an overlying optically reflective metal film for secondary absorption, reducing operational voltage to around 8.5V or less and enhancing detector responsivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Si/Ge APD is used for high sensitivity detection, then detection sensitivity is improved, but operating voltage increases beyond 12V
Solution Approach 1:
The patent changes the structural parameters of the APD by introducing a re-entrant mirror configuration with specific geometric angles (45-55 degrees) and a multi-layer semiconductor structure with optimized thicknesses and doping concentrations. These parameter changes enable the device to achieve high sensitivity while reducing the breakdown voltage to below 12V, resolving the contradiction between detection sensitivity and operating voltage.
2Reliability
If edge coupling method is used for optical fiber to waveguide connection, then optical coupling is achieved, but manufacturing efficiency decreases due to active alignment requirements
Solution Approach 1:
The re-entrant mirror structure serves multiple functions simultaneously: it provides the optical coupling interface between waveguide and APD, performs the coupling function, and eliminates the need for separate alignment mechanisms. This self-service approach enables passive alignment during manufacturing, significantly improving productivity while maintaining reliable optical coupling through the precisely engineered mirror geometry.
3Ease of manufacture
If packaging is optimized for APD, then packaging cost decreases, but optical coupling efficiency may be compromised
Solution Approach 1:
The re-entrant mirror structure performs multiple functions within a single component: it acts as the optical coupling interface, provides structural support for the APD, and enables both edge-coupled and vertically-coupled fiber configurations. This multi-functionality allows for simplified, cost-effective packaging solutions that maintain high optical coupling efficiency without requiring complex alignment mechanisms or specialized packaging procedures.
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 solution achieves a high bandwidth-responsivity product with reduced operational voltage and improved manufacturing efficiency, enabling the use of Si/Ge APDs in a wider range of applications, including mobile computing and data server machines, with lower packaging costs and more efficient optical coupling.
Implementation Method 1
an inverted re-entrant mirror facet at an end of the waveguide to couple light by internal reflection from the waveguide to an underside of the APD
Implementation Method 2
an overlying optically reflective metal film interconnects the plurality of ohmic contacts and returns light transmitted around the ohmic contacts back to the absorption layer for secondary absorption
Implementation Method 3
Avalanche Photodiodes (APD) are useful in applications where high sensitivity is desired
Data Source
AI summary
A low voltage APD is disposed at an end of a waveguide extending laterally within a silicon device layer of a PIC chip. The APD is disposed over an inverted re-entrant mirror co-located at the end of the waveguide to couple light by internal reflection from the waveguide to an under side of the APD. In exemplary embodiments, a 45°-55° facet is formed in the silicon device layer by crystallographic etch. In embodiments, the APD includes a silicon multiplication layer, a germanium absorption layer over the multiplication layer, and a plurality of ohmic contacts disposed over the absorption layer. An overlying optically reflective metal film interconnects the plurality of ohmic contacts and returns light transmitted around the ohmic contacts to the absorption layer for greater detector responsivity.