III-nitride semiconductor materials, (Al, In, Ga) N, are excellent wide bandgap semiconductors increasingly evident in modern electronic and optoelectronic applications. Among all the III-nitride materials, aluminum nitride (AlN) has the largest bandgap, largest critical electric field, highest thermal conductivity, and most stable high temperature performance.
These superior material properties make AlN a compelling candidate for high performance electronic devices, especially for high power and high temperature operation.
What is Bandgap?
Bandgap refers to the energy difference (in electron volts, eV) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the minimum energy needed to create a free electron in the conduction band by exciting it out of the valence band and leaving behind a free hole in the valence band.
When an electron and hole combine, the recombination releases energy equal to the bandgap. This energy will result in a single photon if there are no defects in the vicinity. Where defects exist, the recombination (referred to as a non-radiative recombination center) results in lattice vibration which in turn becomes heat. When an electron and hole recombine to produce only a photon, the relationship between wavelength and bandgap is expressed as:
λ = 1240/E
where: λ is the wavelength in nanometers (nm) and E = bandgap energy in eV.
AlN’s wider bandgap capability means that devices grown on AlN can more effectively (technically and economically) emit at the deeper ultraviolet (UVC) wavelengths than devices grown on sapphire.
Defects and Dislocations
Any kind of defect that breaks up the crystal symmetry can act as a non-radiative recombination center. One such defect—edge dislocation—can occur during growth of thin-film materials on non-lattice matched substrates, as the growing crystal tries to accommodate the lattice mismatch.
In the case of UVC LEDs, devices grown on AlN substrates have approximately 106 fewer dislocation defects than devices grown on sapphire substrates.
The presence of these dislocations has a large impact on the performance of the material. More specifically, the Internal Quantum Efficiency (IQE) which is the percentage of electron-hole recombinations that result in a photon, is highly sensitive to crystalline defects. Devices grown on AlN have demonstrated >2x higher IQE at the UVC wavelengths than devices grown on sapphire.
All Klaran UVC LEDs are manufactured using AlN substrates, resulting in devices with higher power and longer lifetimes in the key germicidal wavelength range of 260 nm to 275 nm.
AlN deep UV-C advantages
The combined benefits of wider bandgap and lower dislocations are shown in figure 2 above. Effectively, sapphire efficiency and capability to emit ultraviolet energy peaks at 285 nm (UV-B) and quickly diminishes at UVC wavelengths, whereas devices grown on AlN substrates are capable of efficiently (technically and economically) emitting high output at deep UVC wavelengths. The significance of this ability becomes evident when aligned with microbe spectral sensitivity e.g. Methicillin Resistant Staphylococcus Aureus (MRSA) as shown in Figure 3. MRSA action spectrum peaks at a wavelength of ~265 nm and diminishes quickly at longer wavelengths. UVC LEDs grown on AlN emit higher germicidal output to coincide with the peak absorption of MRSA and hence provide stronger more effective disinfection than devices grown on sapphire.
Using AlN substrates
- Allow Klaran UVC LEDs to emit higher output, at peak germicidal wavelengths, than other UVC LED sources on the market today—providing effective and consistent disinfection performance across a range of pathogens.
- Result in lattice matched devices that offer superior thermal properties and high drive current operation—delivering high germicidal intensity and practical, low cost implementation.