Material Properties and Applications of Sapphire (Al2O3)
Sapphire is a single crystal form of corundum, Al2O3. This material has unique material properties, especially for thermal stability, chemical stability, mechanical strength and optical properties.
Recently, Gallium Nitride (GaN) is widely used for fabrication of light-emitting-diodes (LEDs) to achieve high light emittance. GaN is grown on substrates such as sapphire, silicon, Silicon Carbide (SiC) by metal-organic-chemical-vapor-phase-deposition (MOCVD).
There are advantages and disadvantages on these substrates for GaN epitaxial growth.
|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|
|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|
Sapphire has 16 % of lattice mismatch with GaN. However, due to its low price, chemical stability and stable product qualities, this material has been used for the substrate of LEDs over decades.
Another application of sapphire is silicon-on-sapphire (SoS). This material has good heat resistance and insulating properties. Therefore, SoS can be used as an alternative of Gallium Arsenide (GaAs). Recently, SoS is being used as the substrate of radio-frequency (RF) devices for telecommunication.
Patterned Sapphire Substrate (PSS) Fabrication
Patterned sapphire substrates (PSS) are fabricated and used in GaN-based LEDs in order to increase the internal quantum efficiency by reducing GaN crystal defects. They also can improve external quantum efficiency by increasing the reflection area.
Regarding electroluminescence (EL) intensity, performing PSS etching improves the luminous efficiency by about 13% compared to processing without PSS. Therefore, PSS fabrication is one of the key processes for LED manufacturing. PSS shape, aspect ratio, and pattern density are important factors to improve the light extraction.
SAMCO offers PSS fabrication processes using chlorine chemistry.
The PSS cone profile is highly controllable in height and width depending on customers’ requests.
Square pyramid and dome profile are also available.
Sidewall Angle: 41°
Sidewall Angle: 55°
Sidewall Angle: 61°
The detail on this technology can be found on the paper below.
Chlorine-Based ICP Etching for Improving the Luminance Efficiency in Nitride LEDs
Anisotropic Sapphire Etching
Anisotropic sapphire etching was performed using Ni mask.
Etch Depth : 100 µm
Etch Rate : 280 nm/min
Photo courtesy of National Tsing Hua University, Taiwan