Enhancement of (AlGaIn)N near-UV LED efficiency using freestanding GaN substrate

The effect of freestanding GaN-substrates with low defect density (DD) on the electroluminescence (EL) characteristics of near UV-LEDs has been investigated. Three series of LED-structures with wavelengths between 377 nm and 435 nm are compared, one grown on freestanding HVPE GaN-substrates, one grown on low dislocation GaN templates on sapphire (ULD), and one grown directly on sapphire substrates with conventional low temperature nucleation. The reduction in DD results in an enhancement of the EL intensity over the whole wavelength range, which is particularly pronounced for wavelengths shorter than 390 nm. Comparing the output power of LEDs on these three different substrates special attention has to be paid to differences in light extraction efficiency. Through band-to-band absorption limits the use of freestanding GaN to wavelengths exceeding 380 nm, it offers the possibility to enhance the light extraction via simple backside surface texturing by a factor of at least 2. This way we fabricated a LED-structure on freestanding GaN with an output power measured on-wafer of 13 mW at 395 nm and 40 mA injection current, i.e. an enhancement in EL intensity of 270 % over the same LED structure grown on sapphire.