(Invited) Large Area Semipolar GaN Grown on Foreign Substrates
In this contribution, we will concentrate on an approach, where metalorganic vapour phase epitaxial (MOVPE) growth is initiated by nucleating on inclined c-plane-like side-facets prepared by etching grooves into adequately oriented sapphire wafers (Fig. 1). After coalescence of these initially striped nitride structures, they form large area planar semipolar surfaces on which GaInN quantum wells and LED structures can be grown. By this approach, several semipolar planes including (10-11), (11-22), and (20-21) can be produced on n-plane (11-23), r-plane (10‑12), and s-plane (22-43) sapphire wafers. By carefully optimizing the growth conditions and applying various defect reduction methods already known from c-plane growth, we were able to suppress substantially the formation of the commonly observed defects like dislocations and stacking faults. Particularly for the (10‑11) and (11-22) samples, excellent structural properties have been obtained, being evident from very narrow X-ray diffraction peaks in the range of 200 – 300 arcsec and photoluminescence spectra dominated by the excitonic peaks, whereas only very weak signals of the stacking fault related signals were visible.
Due to a small relative mis-alignment of the c-planes of those sapphire wafers with respect to the GaN layers, we typically observe the development of fairly rough surfaces, particularly for the (11-22) samples. Hence, studies about growth on slightly miscut r-plane wafers are currently on the way which will be discussed in this contribution. The surface roughness of the (20-21) samples is even much worse (Fig. 2), obviously because the coalescence of such stripes is not favoured under our growth conditions.
GaInN quantum well structures grown on such semipolar (10-11) and (11-22) layers show strong luminescence at about 500 nm indicating that also on such planes large amounts of In can be incorporated. It should be noticed that even more In is needed for such semipolar quantum wells as compared to polar c-plane structures, because of the significantly reduced quantum confined Stark effect.
First doping studies indicated similar Si incorporation properties like on c-plane facets. However, the Mg incorporation was drastically reduced on (11-22) oriented GaN layers. Moreover, we observed a significantly enhanced parasitic oxygen incorporation. Hence, we could not yet obtain p-conducting GaN layers.
In order to decrease the defect density in such structures further, we have overgrown some of these GaN layers by HVPE. By applying our standard c-plane HVPE growth conditions, growth rates of more than 70µm/h could be easily realized. In particular for the extremely rough (20-21) MOVPE structures, good coalescence could be obtained with fair surface roughness (Fig. 3). Also the surface flatness of our (11‑22) layers could be drastically improved. However, the above mentioned sensitivity towards oxygen incorporation seems even stronger in HVPE.
Fig. 1: Growth of semipolar GaN out of trenches etched into the respectively oriented sapphire wafer (schematically).
Fig. 2: GaN stripes for (20-21) growth. The <20-21> direction is perpendicular to the surface, but the stripes typically form inclined (10-11) facets on top resulting in a very rough surface.