Beyond sapphire: LED substrates from GaN to ZnO, SiC, and Si

May 14, 2012 — Light-emitting diodes (LEDs) are typically manufactured on sapphire (Al2O3) substrates, about 90% of the blue LEDs currently in production. Silicon carbide (SiC) substrates are used for virtually all the remaining 10% of blue LEDs.

Sapphire substrates will see 20-30% demand increase annually for LED fab. The supply of sapphire ingots will increase more than 20% annually, shows Displaybank. Sapphire substrates require a supply chain of sapphire ingot, or boule, growth equipment suppliers, single-crystal manufacturers, and substrate makers that cut sapphire ingots into wafers. One mega trend for the industry, in addition to capacity increases, is larger wafer diameters.

A constraint, says Displaybank, is the lumen efficiency of LEDs made on sapphire substrates. LED efficiency is defined as the multiplication of internal quantum efficiency and external light extraction efficiency. Typical LEDs glow at 60-120lm/W, which should improve to the 120-160lm/W range by 2015. Blue LEDs based on sapphire substrates are limited in efficiency, due to a lattice mismatch between the sapphire substrate and gallium nitride (GaN) LED materials — 16% or more. The smaller the lattice mismatch, the higher internal quantum efficiency can be.

Table. LED-use ingot material properties. SOURCE: LED-use Next-generation Ingot/Substrate Technology and Industry Analysis Report, Nov.2011, Displaybank.

Material Lattice mismatch (%) Growth method Strength Weakness
Sapphire 16 Cz method, Ky, Slow cooling Low price, chemical stability Large lattice mismatch
GaN 0 HVPE, Ammonothermal Homogeneous substrates Difficulty in high-quality crystal growth, high price. Currently in basic research stage.
SiC 3.5 Modified Lely Chemical properties similar to GaN Price, difficulty in large substrate growth, patent (Cree)
ZnO 2.2 Hydrothermal Small lattice mismatch, large substrate possible Long research period, expensive equipment
Si 18 Czochralski Low price, large substrate possible Difficulty in high-brightness manufacturing

Since sapphire is electrically an insulator, vertical-architecture LED chips are made by removing the sapphire from the chips once the epi process is finished. GaN and SiC substrates can be made conductive, thanks to precise injections of active ions. SiC, GaN, zinc oxide (ZnO) and silicon (Si) substrates are considered as the future LED substrates, thanks to low lattice mismatches.

Cree is currently producing epi-wafers using a SiC substrate. Gallium nitride wafers are expensive, currently used for laser diode manufacturing but not for LED manufacturing. ZnO substrates are less expensive compared to GaN ones, but suffer from volatility issues at high temperatures. LED manufacturing process temperatures must be kept low with ZnO wafers. Si substrates are relatively cheap, and benefit from the long process history of semiconductor manufacturing on Si. However, Si wafers currently offer lower brightness than sapphire-based LEDs. Several companies are developing Si technologies for LEDs.

Displaybank’s report, “Next-generation LED-use Ingot/Substrate Technology and Industry Analysis (Al2O3, GaN, SiC, ZnO)” covers growth methods for sapphire, SiC, GaN, ZnO single crystals and the substrate processing methods. It also covers commercialization efforts on new substrates, and improved sapphire-based LEDs.