(Invited) Large Area Semipolar GaN Grown on Foreign Substrates

Green light emitting diodes based on group-III nitrides still suffer from fairly low performance as compared to shorter wavelength blue emitters. One possible reason is the lattice mismatch induced strain of the GaInN quantum wells in the active region in such devices having a comparably large In content. This causes the formation of huge piezoelectric fields within the GaInN quantum wells separating electrons and holes locally and hence reducing their recombination probability. By changing the main epitaxial growth direction from the conventional polar c-direction into less polar crystal directions, the internal fields can be strongly reduced. Therefore, currently some groups use GaN substrates cut from thick c-plane wafers grown by hydride vapour phase epitaxy (HVPE) in various non- and semipolar directions. Such substrates are, however, limited to small sizes in the range of few millimetres owing to the limited thickness of the HVPE samples. The growth on foreign wafers with adequate orientation in order to get semipolar GaN typically leads to layers with low quality, in particular with high densities of stacking faults. That is why we currently study some hetero-epitaxial approaches which can overcome this problem by still making use of c-directional growth on such foreign substrates, eventually providing nevertheless semipolar surfaces for subsequent deposition of optoelectronic device structures.

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.

Fig. 3: GaN grown by HVPE over a (20-21) MOVPE template similar as shown in Fig. 2.