Development of high quality and low defect density semipolar and non-polar GaN templates

摘要

Today commercially available wurtzite nitride based visible optoelectronic devices [1], grown along the polar [0001] c-direction, suffer from the presence of polarization-related electric fields inside multiple-quantum wells (MQWs). The discontinuities in both spontaneous and piezoelectric polarization at the hetero-interfaces result in internal electric fields in the quantum wells which causes carrier separation (quantum confined Stark effect (QCSE) and reduces the radiative recombination rate within quantum wells (QWs) [2–5]. This effect is particularly strong in high indium content devices such as green emitters, hence the very poor efficiency performance to date of direct green emitting devices. To decrease these polarization effects, devices have been grown on the non-polar and semi-polar planes. These have attracted significant attention because not only can they improve device efficiency in blue and violet devices but also, more significantly, they enable the development of high efficiency green emitters. To date, however, high quality non-polar or semi-polar GaN material has only been available through cutting expensive bulk GaN boules at the appropriate angle. Unfortunately, this method only yields small pieces, typically up to 10 mm by 15 mm, which are only suitable for fundamental research. Conventional ELOG (epitaxial lateral over growth) has also been employed to improve the crystal quality of non-polar and semi-polar GaN grown on sapphire substrates, however, the non-uniformities associated with this method along with the thick overgrowth (5 – 10 µm) needed to coalesce the layer makes this unsuitable as a route to commercialization of non-polar and semi-polar GaN. This paper presents the results of high quality and cost effective non-polar and semi-polar GaN templates on sapphire, employing unique nanotechnology developed by Seren Photonics and the University of Sheffield [6, 7].