InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0 1 0) and on semipolar (1 0 1 2), (1 1 2 2), (1 0 1 1) as well as (2 0 2 1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0 1 1) < (1 1 2 2) = (0 0 0 1) < (2 0 2 1) < (1 0 1 0) = (1 0 1 2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0 1 0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0 1 0) and the semipolar (1 0 1 2) and (2 0 2 1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1 2 2) and (1 0 1 1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

Based on the atomic arrangement of the ð1010Þ m-plane sapphire surface, we have developed a model for the initial nucleation process of gallium-nitride (GaN). This model describes why ð1122Þ and ð101 3Þ are the preferred orientations of GaN on the ð1010Þ sapphire. The experimental results from high-resolution X-ray diffraction measurements, like the crystallographic relations and the twinning of the ð1103Þ orientation are explained by the model too. Our model also predicts that ð1122Þ thin films are metal-polar and ð1103Þ thin films are nitrogen-polar. and their ternary alloys are of great interest for optoelectronic applications such as multiple-quantum-well (MQW) LEDs and laser diodes emitting in the blue and green wavelength region. Such devices are commonly grown on the polar (0001) plane. Heterostructures grown on the polar planes exhibit strong piezoelectric and spontaneous polarization fields resulting in a reduction of the radiative recombination efficiencies and a red-shift in the emission wavelength due to the quantum confined Stark effect (QCSE) [1]. The QCSE can be strongly reduced or even avoided by using non-and semi-polar crystal orientations, which exhibit only small polarization fields in the growth direction [2].First non-and semi-polar bulk GaN crystals are available. However these substrates are still very small and rather costly. Non-polar and semi-polar orientated layers can be realized on sapphire substrates, which are available in 2

( 11 2 ¯ 2 ) GaN layers were grown by metal-organic vapor phase epitaxy on (112¯2) bulk GaN substrates and (101¯0) sapphire substrates. The growth temperature was varied between 950 and 1050 °C and the total reactor pressure between 50 and 600 mbar. The growth conditions show a strong impact on the yellow band luminescence properties, while weak impact on the threading dislocation density was observed. The layer morphologies exhibit undulations with two periods along GaN [11¯00] and one period along [112¯3¯]. The different period lengths are connected to anisotropic adatom surface diffusion lengths. Arrow like features on the surface originate from the interference of the undulations along [112¯3¯] and [11¯00].

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Polarized photoluminescence of strained quantum wells grown on c-plane, semipolar (1012), (1122), (1011), (2021) planes, and nonpolar GaN substrates was studied experimentally and in theory. The observed optical polarization switching between the substrate orientations (1012) and (1122) is in accordance with our general model of polarization switching, based on a k x p model of arbitrary substrate orientation. Spectrally resolved measurements of the polarization degree stemming from (1012) samples show that the maximum of the polarization degree is red-shifted with respect to the maximum of the photoluminescence intensity. We ascribe this effect to an increased polarization of the transitions for higher indium content