Light Emitting Diodes for the Ultraviolet Spectral Region
We develop light emitting diodes (LEDs) based on the semiconductor system AlN-GaN-InN with an emission in the ultraviolet (UV) spectral region, primarily in the wavelength ranges UV-B (280-315 nm) and UV-C (<280 nm). The devices offer a compact design, can be operated at low voltages, are fast switchable and do not contain toxic substances. Due to these properties, UV LEDs outperform conventional UV emitters such as gas discharge lamps in many ways – even though their efficiencies are still far below those of visible LEDs.
Our current work focuses on selected emission wavelengths:
- 300-320 nm for surface polymerization, phototherapy and plant illumination,
- 260-280 nm for water disinfection,
- <250 nm for gas sensing.
UVB LEDs at 310 nm emit through the transparent sapphire substrate, i.e. the chip back-side, with typical optical powers of 1 mW at 20 mA and maximum 13 mW at 400 mA. UVC LEDs emitting at 265 nm are still in a developmental stage; however early results promise optical output powers of approx. 1 mW at 20 mA. In cooperation with the FBH prototype engineering group, UV LEDs are used to build complete modules and test them in practical applications like water disinfection. In close collaboration with TU Berlin, FBH covers the whole process chain for fabricating UV LEDs. Several MOVPE systems are used to epitaxially grow InAlGaN heterostructures on sapphire substrates. We apply the epitaxial lateral overgrowth (ELO) technique on micro-structured AlN layers to reduce the dislocation density in the semiconductor layers. Chip technology is carried out on the process line of FBH’s cleanroom, including the fabrication of so-called nano-pixel contacts on the p-side, featuring both a low resistance and a high UV reflectivity. The LEDs are flip-chip mounted either on AlN submounts and TO headers or on ceramic packages as surface-mounted device (SMD). Optical power-current-voltage characteristics and spectra are measured on-wafer, on mounted devices and with complete UV LED modules.