Aberration-corrected scanning transmission electron microscopy was used to investigate the core structures of threading dislocations in undoped GaN films with both high and low dislocation densities, and in a comparable high dislocation density Mg-doped GaN film. All a-type dislocations in all samples have a 5/7-atom core structure. In contrast, most (a+c)-type dislocations in undoped GaN dissociate due to local strain variations from nearby dislocations. In contrast, Mg doping prevents (a+c)-type dislocation dissociation. Our data indicate that Mg affects dislocation cores in GaN significantly.

Dislocations are one-dimensional topological defects which occur frequently in functional thin film materials and which are known to degrade the performance of In x Ga 1-x N-based optoelectronic devices. Here, we show that large local deviations in alloy composition and atomic structure re expected to occur in and around dislocation cores in In x Ga 1-x N alloy thin 2 films. We present energy-dispersive X-ray spectroscopy data supporting this result. The methods presented here are also widely applicable for predicting composition fluctuations associated with strain fields in other inorganic functional material thin films. KEYWORDSDislocations, III-nitrides, Monte Carlo, alloy segregation, atomistic modeling, STEM-EDX MAIN TEXTDislocations are ubiquitous one-dimensional topological defects that are found within thin films of nitride semiconductors, originating at the interface with the substrate, and threading up through the active region of the device before terminating at the crystal surface 1 . These dislocations can severely degrade device efficiencies 2 , and lifetimes 3 and are responsible for a broad range of undesirable behavior such as leakage currents 4 and properties such as reduced internal quantum efficiencies 5 and defect states 6,7,8,9,10 that can act as non-radiative recombination centers. In x Ga 1-x N-based alloy semiconductors are used in light-emitting diodes 11 , laser diodes 12 and solar cells 13 , which can be tuned to emit or absorb respectively over the entire visible spectrum by varying the In composition 14 . In x Ga 1-x N is subject to very high threading dislocation densities of up to 10 11 cm -2 and typically around 10 9 cm -2 when grown by metalorganic vapourphase epitaxy 15 (MOVPE), of which the majority have a-type ('edge') or (a+c)-type ('mixed') Burgers vectors with < 1% 16 being c-type ('screw'). High dislocation densities are associated with short lifetimes in InGaN-based optoelectronic devices 17 . The electronic properties of dislocations are determined by the local bonding in the region of the dislocation core 8 . It is therefore important to determine whether or not there are local differences in the alloy composition near dislocation cores in In x Ga 1-x N. Such composition fluctuations are likely to 3 affect the electronic properties of the dislocations and would therefore affect device performance.Each dislocation is associated with a strain field determined by its Burgers vector. Since the In atom is larger than the host Ga atom, it is expected that if the In atoms are sufficiently mobile during growth, then they will segregate to the tensile part of the dislocation strain field 18 .Previous theoretical work has shown that the extreme case of a pure InN c-type dislocation core in an In x Ga 1-x N alloy is more energetically favorable compared to the equivalent In x Ga 1-x N core 19 , and also that it is favorable for In atoms to bind to a c-type dislocation core in GaN 20 . Due to the sensitivity required to detect small variations in alloy concentration on sh...

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The formation of trench-defects is observed in 160 nm-thick In x Ga 1-x N epilayers with x ≤ 0.20, grown on GaN on (0001) sapphire substrates using metalorganic vapour phase epitaxy. The trench-defect density increases with increasing indium content, and high resolution transmission electron microscopy shows an identical structure to those observed previously in InGaN quantum wells, comprising meandering stacking mismatch boundaries connected to an I 1-type basal plane stacking fault. These defects do not appear to relieve in-plane compressive strain. Other horizontal sub-interface defects are also observed for these samples and are found to be pre-existing threading dislocations which form half-loops by bending into the basal-plane, and not basal-plane stacking faults, as previously reported by other groups. The origins of these defects are discussed, and are likely to originate from a combination of the small in-plane misorientation of the sapphire substrate and the thermal mismatch strain between the GaN and InGaN layers grown at different temperatures. range of In compositions absorb light across the whole solar spectrum in a multi-junction solar cell [5]. While thicker layers of around 100 nm are grown for solar cell applications [6], strained InGaN layers of between 2 and 5 nm are commonly deposited by metalorganic vapour-phase epitaxy (MOVPE) as multiple quantum wells (MQWs) in LED and LD devices. Moreover, 24 to 80 nm thick In x Ga 1-x N (0.03 < x < 0.05) underlayers deposited directly beneath multiple quantum wells in LEDs have been observed to increase LED external quantum efficiencies [6-8]. The growth of relatively thick In x Ga 1-x N epilayers on GaN on (0001) sapphire by MOVPE is challenging since the InN bond is unstable at the high temperatures of around 1000 °C necessary for high-quality GaN growth. Despite a mature growth technology for the deposition of thin In x Ga 1-x N QWs by MOVPE, the growth of thick In x Ga 1-x N epilayers on GaN remains challenging, resulting in films with high strain and a rich defect microstructure [9]. V-defects appear as V-shaped voids in cross-sectional transmission electron microscopy (TEM) images of In x Ga 1-x N films and are the most extensively studied defects in both In x Ga 1x N MQW [10-16] and epilayer [17-19] samples. They are formed when a threading dislocation, usually generated at the interface between the GaN buffer and the substrate [20] propagates along <0001> into the InGaN layer and ultimately opens up as an inverted hexagonal pyramidal pit at the film surface. The six hexagonally-oriented facets of the pit comprise inclined {10-11}-type planes. In the case of MQWs, V-defects tend to originate in the first InGaN quantum well and can permeate through the entire MQW stack depending on the (In)GaN quantum barrier growth condition [21]. Owing to their distinct crater-like shape when viewed using TEM, it was initially suggested that V-defects may be surface terminations of hollow-core screw-type dislocations, first described by Frank [22]. However, since...