Polarisation and Extraction of Light from UV-LEDs

Abstract

Ultraviolet emitters based on III-nitride of semiconductor materials have attracted significant attention for a range of applications, this is due to their compactness and more efficient use of energy. Despite the astonishing enhancement of external quantum efficiency η_EQEfor near UV light-emitting diodes (LEDs), the standard level of η_EQEfor planar UV-LEDs remains small (<10%). The principal cause of low η_EQElevels can be attributed to powerful anisotropic emission resulting from pre-eminent transverse-magnetic (TM). Three approaches that have proved themselves effective in raising levels of efficiency in extracting light from UV-LEDs are summarised thus. Nanowire structures (NW), this technique illustrates that, the effectiveness of extracting polarised light can be particularly improved in AlGaN nanowire structure by amounts in excess of 70%. The second approach is represented by patterned sapphire substrates (PSS), this shows that microdome-shaped patterning on sapphire substrate is especially helpful in improving TM-polarised output, as much as 6.1 times enhancement can be achieved for 230 nm flip-chip UV-LEDs possessing microdome-shaped patterned sapphire substrates and 2.4 times for 280 nm versions of the same. The third approach is thermal annealing, which results in concurrent growth of AlxGa1−xN layers (0≤x≤1) y on polar, semi-polar templates, in addition to non-polar AlN templates, these being grown by metal-organic vapor phase epitaxy (MOVPE) on planar sapphire substrates