Competitive devices – UV-C LEDs with high injection efficiency
The output power from 265nm LEDs measured on wafer with varied thickness (left) and aluminum content of the AlxGa1-xN interlayer (right)
Output power, voltage, and spectrum of a 265nm LED mounted in flip-chip geometry on an AlN ceramic submount
UV-C radiation with a wavelength in the range of 260 – 280 nm is of great interest for many applications including water disinfection, materials processing, and medical technology. Commonly used mercury-based discharge lamps have a fixed spectrum in which this wavelength range is missing. Thus, many applications require an alternative light source. AlGaN-based UV-C LEDs are a promising candidate to fill this gap since they are compact, have a low operation voltage and, most importantly, their wavelength can be adjusted to the required application by tuning the material composition of the AlxGa1-xN semiconductor alloy. However, UV-C-LEDs have a rather low wall plug efficiency in the percentage range so far. This is due to a high density of crystal defects originating from the AlN template, a low injection efficiency, and a low light extraction efficiency.
TU Berlin and FBH have therefore started to develop UV LEDs emitting around 265 nm and achieved competitive devices by carefully optimizing the current injection efficiency. All LED structures were based on epitaxially laterally overgrown (ELO) AlN-templates on sapphire, whose threading dislocation density is reduced in comparison to conventional planar AlN templates. Thisallowed for an enhanced radiative recombination.
The injection efficiency could be significantly improved by varying the heterostructure of the electron blocking layer (EBL). An interlayer (IL) between the EBL and the last barrier of the active region was inserted and the aluminum content as well as the thickness were varied. The evaluation of the complete LED wafer was conducted by electroluminescence spectroscopy, in which indium was used for the p- and n-contacts. An LED without IL didn’t show significant luminescence from the quantum well (QW) active region, while an LED with an 1.5 nm thick AlN IL yielded approximately 1 mW output power at 60 mA injection current. A further increase of the IL thickness did not lead to higher output power. By varying the Al content of the IL between 85 and 100%, a maximum in the emission power at 92% Al content was found with 1.3 mW output power at 60 mA.
These wafers have been processed into LED chips at FBH, and single chips were diced and flip-chip mounted on ceramic AlN submounts. The LED chips show a single peak emission at 264 nm and 1.3 mW emission power at 20 mA injection current. The output power is higher than 3 mW at 50 mA, and the external quantum efficiency is about 1.4%. These numbers are comparable to those of devices available on the market.