| Light emitting diodes (LEDs) based on the III-nitride material system (Al,In,Ga)N have been utilized in a number of commercial applications, from small Christmas tree lights to high power lightbulbs and streetlamps. Until now, all commercially available GaN-based devices have been based on the conventional c-plane (polar) orientation of the Wurtzite crystal structure, and grown heteroepitaxially on foreign substrates such as sapphire. However, the recent availability of low defect density HVPE-grown GaN substrates have opened up new possibilities to study novel crystal orientations, known as nonpolar and semipolar.;The (Al,In,Ga)N material system has bandgaps ranging from 0.7 eV in the infrared out to 6.3 eV in the deep UV---and LEDs with wavelengths from roughly 365 nm (near-UV) to 550 nm (green) are commercially available. However, although blue LEDs typically have an external quantum efficiency (EQE) > 60%, at emission wavelengths beyond 500 nm the EQE drops to ≤ 30%. Similarly, although red-emitting LEDs based on AlInGaP have high efficiency, their EQE is also reduced for yellow wavelengths. This phenomenon is known as the 'Green Gap'.;Although there are likely to be numerous factors responsible for this reduction in efficiency with emission wavelength in III-nitrides, a leading candidate is mismatch strain between the active region of the LED---consisting of thin layers of InGaN with at least 30% indium---and the GaN substrate, which have a significant lattice constant mismatch of > 3%. In order to improve the efficiency of green-emitting LEDs, strain relaxation mechanisms on semipolar orientations have been studied. By growing relaxed InGaN buffer layers, it is possible to change the lattice constant from that of the GaN substrates, reducing the mismatch strain in the active region itself.;Multiple slip systems have been observed and studied in semipolar nitrides, leading to several sets of misfit dislocations (MDs) that result in relaxation of InGaN layers. The primary mechanism is basal plane slip on inclined c-planes, which results in one-dimensional relaxation only, and does not lead to any increase in defect density. The secondary mechanism of non-basal plane slip on inclined m-planes, however, leads to two-dimensional relaxation but also to the formation of a high density of new extended defects.;Using relaxation via the formation of basal plane MDs, green LEDs have been grown on 1D-relaxed InGaN buffer layers with equivalent performance to reference samples. It was found that one-dimensionally relaxed InGaN buffer layers can enable the growth of thicker active regions, that in some cases result in improved performance. 1D relaxation also leads to changes in the valence band structure which affects the polarization properties of emitted light, and this has been studied both theoretically and experimentally.;Finally, through the use of InGaN nanostructures, the effect of 2D relaxation on light emission has been investigated, and found to have a significant link with improved indium incorporation and efficiency in green-emitting quantum wells. |
