Silicon Carbide ProductionMagel Su
Silicon Carbide (SiC)
▪ Ceramic crystal with approximately 250 polymorphs
▪ 2 Major Polymorphs▪ α-SiC (Hexagonal Crystal Structure)
▪ Forms at >1700°C
▪ β-SiC (Cubic Crystal Structure)▪ Forms at <1700°C
α-SiC (6H)
β-SiC (3C)
Properties (6H-SiC)
▪ Colorless▪ Brown/Black caused by iron impurities
▪ Colorful sheen caused by SiO2
▪ Chemically Inert
▪ High Thermal Conductivity - 4.9 W/(cm*K) at 300K
▪ Low Coefficient of Thermal Expansion (4.3 x 10-6 °C-1)
▪ High Maximum Current Density, High Electric Field Breakdown Strength
▪ High Melting Temperature (3103 ± 40 K)
Phase Diagram
http://www.ioffe.ru/SVA/NSM/Semicond/SiC/thermal.html
Uses
▪ Semiconductor▪ n-type with Nitrogen, Phosphorus
▪ p-type with beryllium, boron, aluminum, gallium
▪ Superconductivity at 1.5 K (3C-SiC:Al, 3C-SiC:B, 6H-SiC:B)
▪ Lightning Arresters
▪ Diodes, transistors, LEDs (Replaced by GaN)▪ Yellow LED (3C-SiC). Blue LED (6H-SiC)
Uses
▪ Abrasive sandpaper, Cutting disks
▪ Harder than corundum (AI2O3), Softer than diamond
▪ Ceramic brake discs
▪ Catalyst support
▪ Steel and Graphene Production
▪ Jewelry (Diamond substitute)
Moissanite Engagement RingCarbon-Ceramic Discs
Abrasive Sandpaper
Natural Sources of SiC
▪ Moissanite-naturally occurring SiC
▪ Found in meteorites and as inclusions in diamonds, kimberlite, lamproite
▪ Extremely rare, only synthetic SiC is used
LamproiteKimberlite
Acheson Process (1896)
▪ 50% Silica (quartz sand), 40% carbon (petroleum coke), 7% sawdust, 3% NaCl heated in Acheson furnace to 2700°C, then cooled gradually
▪ Acheson furnace▪ Heated by resistivity-current passes through a graphite core
surrounded by reactants
▪ SiC layer forms around graphite core
Cross Section of Acheson Furnace
C + SiO2 → SiO + COSiO2 + CO → SiO + CO2
C + CO2 → 2CO2C + SiO → SiC + CO
Acheson Process (1896)
▪ No control over purity of crystals
▪ No polymorph/polytype control
▪ Sawdust and salt used to increase purity▪ Sawdust increases mixture porosity
▪ NaCl reacts with volatile metal impurities
Lely Process (1955)
▪ SiC lumps packed between two concentric graphite tubes
▪ Inner tube removed, leaving porous SiC layer contained within larger graphite tube (Crucible)
▪ Crucible placed in furnace with SiC , heated to 2500°C under Argon at 1 Atm
▪ SiC powder near crucible wall sublimes and decomposes due to higher temperature
▪ SiC crystals nucleate on inner SiC surface due to lower temperatures
Lely Process (1955)
Lely Process (1955)
▪ Forms high quality crystals compared to Acheson Process
▪ Low product yield
▪ Irregular crystal sizes
▪ No polytype control (Usually forms hexagonal crystals)
Seeded Sublimation Method (1978)
▪ Modified Lely Process
▪ Uses same crucible setup from Lely Process, except with inner tube left in▪ Inner graphite tube is thin and porous
▪ Crucible heated to 2200°C in Argon with <1 Atm
▪ Temperature gradient (20-40°C/cm) applied over length of crucible
▪ SiC powder temperature at bottom of crucible is greater than seed temperature
Seeded Sublimation Method (1978)
Seeded Sublimation Method (1978)
▪ Growth rate controlled by:▪ Temperature (1800-2600°C)
▪ Temperature Gradient
▪ Pressure (10-4 – 760 mmHg)
▪ Seed Crystal Quality
▪ High yield, definite size
Seeded Sublimation Method (Modern)
▪ Source placed at bottom, seed at top
▪ Used to produce bulk SiC
▪ 90% yield
Liquid Phase Epitaxy (LPE)
▪ Used to produce thin films of SiC
▪ SiC substrate attached to graphite holder dipped into liquid Si with dissolved C
▪ Holder rotated continuously to promote radial growth
▪ Slow cooling is driving force for crystal formation
▪ Performed at 1650 – 1800°C
▪ Growth rate: 2 – 7 µm/hour
Liquid Phase Epitaxy (LPE)
▪ Low solubility of C in Si (15% at 2800°C)▪ Scandium, Terbium, Praseodymium added to increase solubility to 50%
▪ Argon increases atmospheric pressure, reducing Si vapor pressure effect
Chemical Vapor Deposition (CVD)
▪ Seed crystal placed in vertical reactor made of graphite
▪ Silane (SiH4) and propane (C3H8) diluted in helium (carrier gas) enter reactor through cracking zone
▪ Cracking zone-heated walls which produce reactive radicals from gas
▪ Reactor walls covered with SiC to prevent graphite evaporation
▪ Growth rate: 0.5 – 0.8 mm/hr at 200-800 mbar, 2000 - 2300°C
Summary
Seeded Sublimation
LPE CVD
Growth Rate High Low Low
Crystal Quality Medium Medium High
Defect Quality Medium High Medium
Cost Low Medium High
References
▪ Byrappa, K., and T. Ohachi. Crystal Growth Technology. Norwich, NY: William Andrew Pub., 2003. Print.
▪ Majumdar, Arka. "BULK GROWTH OF SILICON-CARBIDE CRYSTALS." (n.d.): n. pag. 7 Mar. 2006. Web. 22 Jan. 2016.
▪ "NSM Archive - Silicon Carbide (SiC) - Thermal Properties." NSM Archive - Silicon Carbide (SiC) - Thermal Properties. N.p., n.d. Web. 20 Jan. 2016.
▪ Saddow, Stephen E., and Anant Agarwal. Advances in Silicon Carbide Processing and Applications. Boston: Artech House, 2004. Print.