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.