Growth and doping of semipolar GaN grown on patterned sapphire substrates

abstract In order to achieve large area semipolar GaN layers with high crystal quality, we have etched trenchesinto n-plane and r-plane sapphire wafers exposing c-plane-like side-walls, from which GaN stripes canbe grown by metalorganic vapor phase epitaxy mainly in c-direction, forming semipolar f1011g orf1122g surfaces after coalescence. Here, we describe how to improve such layers by optimizing the side-facet orientation and by including a SiN nanomask interlayer in situ into the growth process, eventuallyresulting in a basal plane stacking fault density below 5 10 3 cm 1 . Moreover, doping experimentshave revealed a substantially lower Mg incorporation efficiency on the f1122g surface as compared tothe c-plane, whereas Si does not show such differences.& 2014 Elsevier B.V. All rights reserved. 1. IntroductionGreen light emitting diodes based on group-III nitrides stillsuffer from fairly low performance as compared to shorterwavelength blue emitters. One possible reason is the latticemismatch induced strain of the GaInN quantum wells in the activeregion in such devices having a comparably large In content. Thiscauses the formation of huge piezoelectric fields within the GaInNquantum wells separating electrons and holes spatially and hencereducing their recombination probability. By changing the mainepitaxial growth direction from the conventional polar c-directioninto less polar crystal directions, the internal fields can be stronglyreduced. This approach is currently mainly investigated by grow-ing on semipolar GaN wafers cut from thick c-plane materialgrown by other methods like hydride vapor phase epitaxy [1] orammonothermal crystal growth [2,3]. However, owing to thelimited thickness of those c-plane wafers, semipolar bulk sub-strates cut from them are limited in size to a few squaremillimeters. On the other hand, approaches to grow semipolarGaN on flat foreign substrates of accurate orientation other thanc-plane typically result in highly defective layers (see [4] andreferences therein). Obviously, growth in the polar c-directionleads to lowest defect densities. Therefore, we currently study aheteroepitaxial approach where the epitaxial process starts fromc-plane-like sidewalls of trenches etched into sapphire wafers. Thestripes nucleating on these side facets later coalesce to a closedsurface with semipolar orientation. This technique was firstproposed by Honda et al. to grow semipolar GaN planes on Si [5]and later applied to various directions of GaN by Okada et al. (see[6–8] and references therein). This procedure can be easily appliedto large size sapphire wafers. Even first light emitting devicestructures have been successfully grown on such layers [9].In this contribution, we describe how to optimize such largearea semipolar GaN layers grown by metalorganic vapor phaseepitaxy by applying similar strategies as have been successfullydeveloped for planar c-plane layers. In particular, we have studiedthe applicability of a defect-blocking SiN nanomask layer [10–12]to our approach. Moreover, we have investigated the Si and Mgincorporation for achieving n- and p-type doping on thesesurfaces.2. ExperimentalAll samples investigated in these studies have been grown bymetalorganic vapor phase epitaxy (MOVPE) carried out in aContents lists available at ScienceDirectjournal homepage:www.elsevier.com/locate/jcrysgro