Enhancing the Light Extraction of Ultraviolet Light Emitting Diodes
Cross-section of a UV LED with nanopixel contacts
Characteristics of LEDs with conventional contact (a) and nanopixel contacts of different sizes and distances: 4 µm (b), 2 µm (c) and 1 µm (d)
Light emitting diodes (LEDs) emitting in the ultra-violet (UV) spectral region are interesting for application fields such as water purification, curing of plastics, spectroscopy, and bioanalysis. For many of these applications the currently low efficiency of UV LEDs has to be significantly increased. One of the issues to be solved is that a large proportion of the generated UV light does not escape the chip but gets lost due to internal absorption. Following a novel design approach, a breakthrough has now been achieved with FBH’s semiconductor chip. Instead of a large-area electrical contact above the light-emitting area as commonly used in conventional UV LEDs, an array of nano contacts made of palladium (nanopixels) has been utilized whose spacing is filled with aluminum. Whereas the nanopixels ensure efficient current injection, the aluminum acts as efficient reflector for UV light, so that absorption losses at the chip surface are reduced.
The proof of the nanopixel concept was verified by means of LEDs emitting at a wavelength of 390 nm [see Lobo et al., Appl. Phys. Lett. 96, 081109 (2010)]. When size and distance of the nanopixels were in the 1 µm range, the efficiency of corresponding LEDs could be increased by up to 90% compared to conventional contact designs. Theoretical calculations have shown that only for those small dimensions the currents from the individual nanopixels overlap in the active region, and the reflector takes effect on the emission between the nanopixels. Currently, LEDs with nanopixels are developed for shorter wavelengths, and the concept is combined with other procedures in order to enhance light extraction and reduce thermal resistance.
FBH research: 26.08.2010