Germanium Epitaxy on Silicon via Diffusion Limiting Gas
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Solution Overview
Problem
The challenge in growing high-quality mono-crystalline germanium layers on silicon substrates is due to the large lattice mismatch, leading to island growth, high surface roughness, and a high density of threading dislocations, which complicates the integration of germanium into CMOS devices and requires additional annealing or polishing steps.
Innovation Solution
A method involving the use of a non-reactive gas, such as molecular nitrogen or a noble gas, as a diffusion limiting gas in a process chamber at pressures between 1x10^-6 and 1x10^-4 torr, to adsorb neutral species of germanium on the substrate, preventing the incorporation of the gas into the layer and allowing for the growth of smooth, high-quality mono-crystalline layers without the need for annealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If heteroepitaxial growth of germanium on silicon is performed, then germanium layers can be deposited on silicon substrates, but island growth occurs causing high surface roughness and high density of threading dislocations
Solution Approach 1:
The patent introduces a buffer layer comprising a gradient of germanium silicide layers (GeSi1-x) between the silicon substrate and the germanium layer. This intermediate buffer layer gradually transitions the lattice mismatch, preventing direct heteroepitaxial growth issues. The gradient composition acts as a mediator that reduces dislocation density and surface roughness by distributing the lattice mismatch stress across multiple intermediate layers.
2Quantity of substance
If heteroepitaxial growth of germanium on silicon is performed, then germanium layers can be deposited on silicon substrates, but high density of threading dislocations is generated
Solution Approach 1:
The buffer layer with gradient germanium silicide composition serves as an intermediary that progressively adapts the lattice structure from silicon to germanium. This gradual transition prevents the formation of high-density threading dislocations by distributing lattice mismatch stresses across the gradient layers, thereby improving the reliability of the final germanium layer.
Solution Approach 2:
The patent employs parameter changes in the buffer layer composition, specifically varying the germanium content (x in GeSi1-x) to create a gradient structure. This parameter variation allows controlled management of lattice mismatch, reducing threading dislocation density by transforming the abrupt interface into a gradual compositional transition.
3Manufacturing precision
If additional annealing or chemical-mechanical-polishing steps are performed, then surface roughness is reduced, but process complexity increases
Solution Approach 1:
The patent performs preliminary action by incorporating the buffer layer with gradient composition during the epitaxial growth process itself, before final device fabrication. This preliminary structuring of the substrate-interface region prevents surface roughness and dislocation formation in advance, eliminating the need for subsequent annealing or chemical-mechanical-polishing steps.
4Quantity of substance
If conventional epitaxial growth methods are used, then germanium layers can be grown, but scalability to larger diameters is limited
Solution Approach 1:
The patent segments the germanium layer formation into multiple components: a silicon substrate, a gradient buffer layer comprising multiple germanium silicide layers with varying compositions, and a final germanium layer. This segmentation allows each layer to be optimized independently and enables scaling to larger substrate diameters by distributing stress management across the segmented structure.
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 high-quality mono-crystalline germanium layers with reduced surface roughness and defects, suitable for use in CMOS devices, and simplifies the growth process by eliminating the need for annealing, while minimizing the incorporation of impurities, thus enhancing the crystal quality and scalability of germanium layers.
Implementation Method 1
supplying a beam of neutral species of a second material towards the substrate in the presence of a diffusion limiting gas, such that the pressure in the process chamber is between 1x10^-6 and 1x10^-4 torr, so that the neutral species of the second material are adsorbed on the exposed area
Data Source
Figure 1
AI summary
The present invention is related to a method for growing a layer of a mono-crystalline material such as germanium on a substrate comprising - loading a substrate having an exposed area made of a first mono-crystalline material in a process chamber, - supplying a beam of neutral species of a second material towards the substrate in the presence of a diffusion limiting gas, such that the pressure in the process chamber is between 1x10-6 torr and 1x10-4 torr, so that the neutral species of the second material are adsorbed on the exposed area, thereby growing a mono-crystalline layer of said second material overlying and in contact with the first mono-crystalline material wherein said diffusion limiting gas is a non-reactive gas.