How are LEDs made?
LEDs are typically fabricated by depositing very thin, highly crystalline layers (epi-layers) of various compositions on a crystalline wafer substrate. The wavelength emitted by an LED is defined by the material properties of the epi-layers. The deposited atoms follow the crystalline template provided by the substrate, a process called epitaxial deposition. For most material systems, there is a native substrate; for example, for diodes based on aluminum, indium, and/or gallium arsenide alloys, the native substrate is gallium arsenide (GaAs), whereas a silicon substrate is used for diodes based on silicon.
Substrates for LEDs
Aluminum nitride (AlN) and gallium nitride (GaN) alloys are typically used to fabricate ultraviolet LEDs emitting light with wavelengths in the germicidal range. Fabrication of deep ultraviolet LEDs on sapphire substrates, which are commonly used in blue and green InGaN LEDs, results in more than 108 dislocations (defects) per square centimeter and correspondingly, lower lifetime and efficiencies. An aluminum nitride (AlN) substrate is, therefore, key to reducing dislocations (defects) in deep ultraviolet LEDs.
The Crystal IS advantage
IS pioneered the development of single crystal aluminum nitride (AlN)
substrates. Next, Crystal IS scientists developed patented processes for
growing active crystal layers such as LEDs on the AlN substrates, while preserving the low defect densities of the AlN. These crystals (termed
pseudomorphic epitaxial structures) exhibit sharp interfaces, smooth surfaces
with surface roughness of less than one nanometer. These thick pseudomorphic
layers, and devices based on them, are not possible when using foreign
substrates such as sapphire.
The result has been LED devices with higher efficiencies and longer lifetimes in the 250-280 nm wavelength range than diodes fabricated from other competing technologies such as crystals grown on sapphire. In March 2013, we announced over 60 mW of light output from a single ultraviolet LED in continuous wave operation.
For our recent technical publications for the state-of-the-art in ultraviolet LEDs, please see below.
- 270 nm Pseudomorphic Ultraviolet Light-Emitting Diodes with Over 60 mW Continuous Wave Output Power, Appl. Phys. Express 6 (2013) 032101
- High Output Power from 260nm Pseudomorphic Ultraviolet Light-Emitting Diodes with Improved Thermal Performance, Appl. Phys. Express 4 (2011) 082101
- Reliability and Performance Of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk Aluminum Nitride Substrates, Phys. Status Solidi C 8, No. 5, 1528–1533 (2011)
- Properties of Mid-Ultraviolet Light Emitting Diodes Fabricated from Pseudomorphic Layers on Bulk Aluminum Nitride Substrates, Appl. Phys. Express 3 (2010) 072103
- Performance and Reliability Of Ultraviolet-C Pseudomorphic Light Emitting Diodes on Bulk AlN Substrates, Phys. Status Solidi C 7, No. 7–8, 2199–2201 (2010)
- Electron Paramagnetic Resonance of Er3+ Ions in Aluminum Nitride, Journal Of Applied Physics 105, 023714 (2009)
- The Progress of AlN Bulk Growth and Epitaxy for Electronic Applications, Phys. Status Solidi A, 1–7 (2009)
- Pseudomorphic Growth of Thick N-Type Alxga1-Xn Layers on Low-Defect-Density Bulk AlN Substrates for UV LED Applications, Journal of Crystal Growth 311 (2009) 2864–2866
Large-area AlN substrates for electronic applications: An industrial perspective, Journal of Crystal Growth 310 (2008) 4020– 4026
- Structural and Surface Characterization of Large Diameter, Crystalline AlN Substrates for Device Fabrication, Journal of Crystal Growth 310 (2008) 887–890
- Heteroepitaxy of AlGaN on Bulk AlN Substrates for Deep Ultraviolet Light Emitting Diodes, Appl. Phys. Letters 91, 051116 (2007)
- Ultraviolet Semiconductor Laser Diodes on Bulk AlN, Journal Of Applied Physics, 101, 123103 (2007)
- Fabrication and Characterization of 2-inch diameter AlN Single-Crystal Wafers cut From Bulk Crystals, Mater. Res. Soc. Symp. Proc. Vol. 955 (2007)
- Atomic Force Microscope Studies on Native AlN Substrates, Mater. Res. Soc. Symp. Proc. Vol. 892 (2006)
- Defect Content Evaluation in Single-Crystal AlN Wafers, Mater. Res. Soc. Symp. Proc. Vol. 892 (2006)
- Development of Native, Single Crystal AlN Substrates for Device Applications, Phys. Stat. sol. (a) 203, No. 7, 1667–1671 (2006)
- Electron Paramagnetic Resonance of a Donor in Aluminum Nitride Crystals, Applied Physics Letters 88, 062112 (2006)
- Surface Acoustic Wave Velocity in Single-Crystal AlN Substrates, IEEE Transactions On Ultrasonics, Ferroelectrics, And Frequency Control, vol. 53, no. 1, ( 2006)