The startling success of GaN-based light emitting diodes despite the high density of dislocations found in typical heteroepitaxial material has been attributed to localisation of carriers at non-uniformities in the quantum wells which form the active region of such devices. Here, we review the different possible structures within the quantum wells which could act as localisation sites, at length scales ranging from the atomic to the tens of nanometre range. In some quantum wells several localisation mechanisms could be operational, but the challenge remains to optimise the quantum wells' structure to achieve improved quantum efficiencies, particularly at high excitation powers.

Gallium nitride (GaN) is a semiconductor used to make light emitting diodes, a technology that could decrease global energy demands significantly if used worldwide. Yet there are barriers to making high efficiency GaN based devices: defects, including threading dislocations (TDs), hamper the quality of the GaN crystalline film. The hypotheses proposed to explain the origin of TDs are critically reviewed. It has been suggested that TDs form upon GaN island coalescence during initial stages of crystalline film growth, yet some transmission electron microscopy and atomic force microscopy studies have shown few TDs at coalescence boundaries. Although harmful, TDs have a lesser effect on nitride based devices than on other compound semiconductors. Thus, GaN based devices are able to produce light despite high dislocation densities. This phenomenon has led to debate over the role of TDs in charge carrier recombination, which is reviewed. Some suggest that charge carriers arrive at TDs and recombine in a non-radiative manner, whereas others claim that they are repelled from the dislocations because the dislocation cores are electrically charged. The reduction of TDs in GaN films furthers the drive towards high efficiency devices. The final sections of this review address ways to effect reductions in TD density. Methods include changing growth conditions (including temperature and pressure), dosing the substrate with silane, and the exploitation of interlayers deposited during growth.

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This study addresses the ongoing debate concerning the distribution of indium in InxGa1−xN quantum wells (QWs) using a combination of atom probe tomography (APT) and transmission electron microscopy (TEM). APT analysis of InxGa1−xN QWs, which had been exposed to the electron beam in a TEM, revealed an inhomogeneous indium distribution which was not observed in a control sample which had not been exposed to the electron beam. These data validate the effectiveness of APT in detecting subtle compositional inhomogeneities in the nitrides.

Phone/Fax: þ00 441 223 331 952The structural and optical properties of trench defects, which are poorly understood yet commonly occurring defects observed on the surfaces of InGaN multiple quantum wells (MQW), are reported. These defects comprise near-circular trenches which enclose areas of MQW which give rise to a red shift in peak photoluminescence emission and a change in cathodoluminescence intensity with respect to the surrounding material. Atomic force microscopy shows that the height of trench-enclosed areas differs from that of the surrounding quantum well structure, and that trenches are unrelated to the commonly observed V-defects in InGaN films, despite being occasionally intersected by them. Cross-sectional electron microscopy analysis of trenches with raised centres suggests that the red shift in the observed cathodoluminescence peak emission may be due to the quantum wells being thicker in the trenchenclosed regions than in the surrounding quantum well area. The mechanism of trench formation and its implication for the control of the emission properties of light-emitting diodes is discussed.