Germanium Photodetector Planar Surface via Selective Epitaxial Growth
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Solution Overview
Problem
The lattice mismatch between germanium and silicon substrates leads to high dislocation densities at the Ge/Si interface, affecting the electrical performance of germanium devices, and existing epitaxial lateral overgrowth techniques still generate twin planes that behave like dislocations, limiting the performance of germanium photodetectors.
Innovation Solution
A method involving selective epitaxial growth of germanium using a silicon oxide template, followed by chemical mechanical polishing to create a planar surface, and specific doping techniques to keep dislocations away from the active layer, while preventing coalescence between adjacent growth fronts to minimize interface leakage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If germanium is grown directly on silicon substrate, then the fabrication process is simple, but high dislocation density occurs at the Ge/Si interface due to lattice mismatch
Solution Approach 1:
A silicon oxide template layer is introduced as an intermediary between the silicon substrate and germanium growth regions. This template enables selective epitaxial growth through contact holes, allowing germanium to grow only in specific areas while the oxide sidewalls act as physical barriers that block dislocation propagation from the Ge/Si interface.
Solution Approach 2:
The germanium growth is segmented into isolated regions defined by contact holes in the silicon oxide template. This segmentation prevents the formation of continuous dislocation networks that would occur in conventional planar growth, as each germanium region is independently nucleated and bounded by oxide sidewalls.
2Manufacturing precision
If epitaxial lateral overgrowth technique is used to reduce dislocations, then threading dislocations are blocked, but twin planes are generated at the merging fronts
Solution Approach 1:
The silicon oxide template sidewalls, which initially seem to constrain growth, are converted into beneficial dislocation-blocking barriers. The oxide interfaces force the germanium to grow laterally along the sidewalls, and when these lateral fronts meet, the dislocations are already contained against the oxide, preventing their propagation into the bulk germanium layer.
3Manufacturing precision
If germanium ELO layer is grown on silicon substrate, then dislocation density is reduced, but interface leakage current increases due to abrupt Ge/oxide interface
Solution Approach 1:
The physical and chemical parameters at the Ge/oxide interface are modified by introducing a graded buffer layer or adjusting the oxide thickness and composition. This gradual transition in material properties reduces the abruptness of the interface, thereby minimizing band discontinuities and reducing leakage current while maintaining the dislocation-blocking function.
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
This approach results in a defect-free germanium photodetector with reduced interface leakage, enhancing the electrical performance by eliminating threading dislocations and twin planes, thereby improving the sensitivity and reliability of the device.
Implementation Method 1
selective epitaxial growth of germanium using a silicon oxide template
Implementation Method 2
chemical mechanical polishing to create a planar surface
Data Source
AI summary
A method of fabricating a germanium photo detector includes preparing a silicon substrate wafer and depositing and planarizing a silicon oxide layer on the silicon substrate. Contact holes are formed in the silicon oxide layer. An N+ epitaxial germanium layer is grown on the silicon oxide layer and in the contact holes. An N+ germanium layer is formed by ELO. The structure is smoothed and thinned. An intrinsic germanium layer is grown on the N+ epitaxial germanium layer. A P+ germanium layer is formed on the intrinsic germanium layer and a silicon oxide overcoat is deposited. A window is opened through the silicon oxide overcoat to the P+ germanium layer. A layer of conductive material is deposited on the silicon oxide overcoat and in the windows therein. The conductive material is etched to form individual sensing elements.