SiGe Quantum Wells with Kane-like Bands for Optical Modulation
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Solution Overview
Problem
Conventional silicon technology is unsuitable for optoelectronic devices, and integrating silicon electronics with compound semiconductor optoelectronic devices is challenging due to lattice mismatch and thermal expansion issues, leading to poor electron confinement and mechanical fragility in Si-Ge quantum wells.
Innovation Solution
The development of Si-Ge quantum wells with a conduction band energy local minimum at k=0, utilizing Kane-like bands, grown on a silicon substrate without a graded buffer layer, and annealed Ge-rich Si-Ge buffer layers to accommodate lattice mismatch and improve optical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick graded buffer layer is used to accommodate lattice mismatch, then the quantum well structure can be grown on silicon substrate, but the device chips become mechanically fragile and the fabrication cost increases
Solution Approach 1:
The patent changes the composition parameter of the buffer layer by using a SiGe alloy with intermediate Ge content (e.g., Si0.8Ge0.2) rather than a graded composition from Si to Ge. This parameter change allows the buffer layer to accommodate lattice mismatch through elastic strain while maintaining mechanical strength and reducing fabrication complexity
Solution Approach 2:
The patent extracts the graded composition requirement entirely from the buffer layer structure. By using a uniform SiGe composition instead of a graded structure, the patent eliminates the need for complex graded buffering while still achieving lattice mismatch accommodation through the intermediate Ge content
2Reliability
If a thick graded buffer layer is used to manage lattice mismatch, then epitaxial growth can proceed, but the wafer becomes susceptible to breaking or cracking during temperature variation
Solution Approach 1:
The patent changes the buffer layer composition to a uniform SiGe alloy with intermediate Ge content, which has a thermal expansion coefficient intermediate between Si and Ge. This parameter change reduces the thermal expansion mismatch stress during temperature variations, preventing wafer breaking or cracking while maintaining lattice mismatch management
3Reliability
If Si-Ge quantum wells are fabricated with conventional approaches, then the structure can be grown on silicon, but electron confinement is poor due to bandgap discontinuity being mostly in the valence band
Solution Approach 1:
The patent changes the Ge content parameter in the SiGe material system to achieve a composition range (e.g., x > 0.7 in Si1-xGex) where the conduction band edge forms a local minimum at k=0. This parameter change creates Kane-like bands that provide both electron and hole confinement, solving the electron confinement problem while maintaining quantum well structure formation
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 provides improved optical properties and mechanical robustness for Si-Ge quantum wells, enabling effective electron confinement and exploitation of physical effects like the Quantum Confined Stark Effect for optoelectronic devices, particularly optical modulators, while reducing the impact of thermal expansion mismatch.
Implementation Method 1
the lattice mismatch of about 4% between Si and Ge is a significant complication for epitaxial growth of Ge (or Ge-rich Si—Ge) on silicon. A conventional approach for managing the lattice mismatch is to grow a buffer layer having a graded composition on a Si substrate
Implementation Method 2
The well layer thickness is typically less than about 10 nm, and the energy bandgap of the well layer is less than the energy bandgap of the barrier layers. Si—Ge quantum wells tend to have poor electron confinement, since most of the quantum well bandgap discontinuity is in the valence band
Implementation Method 3
Quantum well structures that satisfy this condition have 'Kane-like' bands at and near k=0 which can provide physical effects useful for various device applications, especially optical modulators
Implementation Method 4
In many cases it is further preferred to grow a second Ge-rich Si—Ge buffer layer on top of the first buffer layer and anneal the resulting layered structure
Data Source
AI summary
Si—Ge quantum wells where the well material has a lowest conduction band energy minimum at k=0 (the Γ point of the first Brillouin zone) are provided. Quantum well structures that satisfy this condition have “Kane-like” bands at and near k=0 which can provide physical effects useful for various device applications, especially optical modulators. In the Si1−xGex material system, this condition on the band structure is satisfied for x greater than about 0.7. The quantum well barrier composition may or may not have Kane-like bands. A preferred method of providing such quantum well structures on a substrate (e.g., a silicon substrate) is to grow a first Ge-rich Si—Ge buffer layer on the substrate, and then anneal the resulting layered structure. In many cases it is further preferred to grow a second Ge-rich Si—Ge buffer layer on top of the first buffer layer and anneal the resulting layered structure.