Metamorphic Al0.5Ga0.5N:Si on AlN/sapphire for the growth of UVB LEDs

In this paper we investigate the growth of metamorphic Al0.5Ga0.5N:Si on c-plane AlN/sapphire. The structural properties of the AlGaN:Si pseudo substrates and the electro-optical characteristics of subsequently grown UVB LEDs are being examined. We demonstrate, that superlattices allow the controlled strain relaxation of Al0.5Ga0.5N by rearrangement of threading dislocations, thus preventing the formation of cracks. This study investigates AlN/GaN superlattices with a nominal GaN layer thickness between 1.0 nm and 2.5 nm at a fixed AlN layer thickness of 2.5 nm. The number of superlattice-periods was also varied between 20 and 120. It was found that beyond a GaN layer thickness of 1.5 nm three-dimensional structures are formed. Additionally, these three-dimensional structures reduce the local defect density of the subsequently grown Al0.5Ga0.5N layer. Although the Al0.5Ga0.5N layer appears to be almost fully relaxed, the relaxation state of this pseudo substrate, was found to be dependent on the GaN layer thickness in the superlattice. After optimizing the superlattice structure we were able to grow crack free 4 μm thick Si-doped Al0.5Ga0.5N layers and on top UVB LEDs with a fully strained active region emitting at 310 nm with output powers of more than 18 mW at 500 mA.