Ultrahigh-Purity SiC Synthesis via Sublimation
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Solution Overview
Problem
Current methods for synthesizing silicon carbide (SiC) for semiconductor applications result in materials with high nitrogen content, making them unsuitable for growing semi-insulating SiC crystals, as they contain nitrogen levels above 1×10^16 cm^-3, which is not acceptable for high-purity requirements.
Innovation Solution
A two-stage synthesis process using a graphite crucible with a reactive mixture of high-purity elemental silicon and carbon, where the first stage forms a cubic SiC charge, and the second stage converts it into high-purity hexagonal SiC through sublimation and vapor transport across a gas- and vapor-permeable carbon barrier, effectively removing impurities like nitrogen.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional SiC synthesis methods (Acheson process, CVD, direct synthesis) are used to produce SiC source material, then large-scale production is achieved, but the SiC material contains high concentrations of nitrogen and other metals (above 1×10^16 atoms/cm³), making it unsuitable for semi-insulating crystal growth
Solution Approach 1:
The synthesis process is divided into two distinct stages: first forming cubic SiC charge, then converting it to hexagonal SiC through sublimation. This segmentation allows each stage to be optimized independently, with the second stage specifically designed to achieve ultra-high purity by removing nitrogen contaminants through vapor-phase transport and condensation.
Solution Approach 2:
The invention utilizes sublimation (solid to vapor phase transition) of cubic SiC charge at high temperature, followed by condensation of SiC vapor onto hexagonal seed crystals. This phase transition process inherently purifies the material by leaving behind non-volatile nitrogen and metal impurities in the charge while transporting pure SiC vapor to form the hexagonal crystal structure.
2Manufacturing precision
If high-purity elemental silicon and carbon are used as starting materials in direct synthesis, then lower nitrogen content is achieved, but the process still produces SiC with nitrogen levels above 1×10^16 atoms/cm³, insufficient for semi-insulating applications
Solution Approach 1:
The first stage of the process preliminarily forms cubic SiC charge from high-purity elemental silicon and carbon. This preliminary action creates a suitable charge material that can then be processed in the second stage to achieve the required ultra-high purity. The preliminary formation of charge allows the subsequent sublimation-conversion process to focus specifically on purity enhancement.
Solution Approach 2:
Cubic SiC charge serves as an intermediary material between the starting elemental components and the final hexagonal SiC product. The charge is formed in the first stage, then undergoes sublimation and conversion to hexagonal structure in the second stage. This intermediary form enables the purification process to effectively remove nitrogen while maintaining the desired crystal structure.
3Reliability
If the SiC source material contains nitrogen above 1×10^16 atoms/cm³, then conventional crystal growth can proceed, but the resulting SiC single crystals cannot achieve the required semi-insulating properties (resistivity 1×10^11 to 5×10^11 Ohm-cm)
Solution Approach 1:
The sublimation-conversion process utilizes phase transitions to achieve both reliable crystal growth and precise resistivity control. By converting cubic charge to hexagonal structure through vapor-phase transport, the process ensures proper crystal structure formation while simultaneously removing nitrogen impurities to achieve the required semi-insulating properties.
Solution Approach 2:
The process changes critical parameters including temperature (heating charge to sublimation temperature), pressure (maintaining vacuum or inert atmosphere), and crystal structure (converting cubic to hexagonal). These parameter changes collectively achieve both reliable hexagonal crystal growth and the precise nitrogen purity levels needed for semi-insulating properties.
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 process produces ultra-high purity polycrystalline SiC with nitrogen levels ≤ 8×10^15 atoms/cm^-3, suitable for growing semi-insulating SiC crystals with resistivity in the range of 1×10^11 to 5×10^11 Ohm-cm, meeting the necessary purity and polytype requirements.
Implementation Method 1
heating the as-synthesized SiC charge and the carbon barrier to a third temperature sufficient to cause the as-synthesized SiC charge to sublime and produce vapors
Implementation Method 2
vapors migrate under the influence of temperature gradient into the carbon barrier
Implementation Method 3
where the vapors condense on the carbon barrier
Data Source
Figure 1
Figure 2A~2C
Figure 3
AI summary
In a method of forming polycrystalline SiC grain material, low-density, gas-permeable and vapor-permeable bulk carbon is positioned at a first location inside of a graphite crucible and a mixture of elemental silicon and elemental carbon is positioned at a second location. The mixture and the bulk carbon are heated to a first temperature below the melting point of the elemental Si to remove adsorbed gas, moisture and/or volatiles from the mixture and the bulk carbon. The mixture and bulk carbon are then heated to a second temperature that causes the elemental Si and the elemental C to react forming as-synthesized SiC inside of the crucible. The as-synthesized SiC and the bulk carbon are then heated in a way to cause the as-synthesized SiC to sublime and produce vapors that migrate into, condense on and react with the bulk carbon forming polycrystalline SiC material.