For almost two decades, extensive research has been carried out on wurtzite nitride semiconductors due to their large
direct band gap which offer the possibility to produce optoelectronic devices from the infrared (InN) to the ultraviolet
range (GaN and AlN). The reduction of the defect densities and polarisation effects in nitride heterostructures are still
challenges for the production of efficient light emitting diodes and lasers past the green range. In this contribution, the
structure of the extended defects is reviewed, and their possible interaction with quantum wells and quantum dots is
discussed. The presence of threading dislocations is shown to constitute a driving force for clustering. Inside semipolar
layers, the glide planes are not available for the spreading of perfect dislocations, this results in the formation of a more
complex defect system which we have investigated in the case of (11-22) layers.
Semipolar (11-22 ) GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy
(MBE) are studied by transmission electron microscopy (TEM). The layers exhibit numerous defects with different
geometries in comparison to the growth along the c-axis, they are identified as mostly partial dislocations, basal and
prismatic stacking faults. The dislocations density is in the order of 5x109 cm-2, the corresponding Burger vectors are
b = 1/6 <20-23>, b= 1/3 <10-10> and a small fraction of perfect a type dislocations with b = 1/3 <11-20> has been
observed. The basal stacking fault density is in the order of 1x106 cm-1. In an attempt to reduce the defect density, SixNy
interlayers have been used as nanomasks for epitaxial lateral overgrowth, our analysis shows that this leads to a quite
small reduction of the defects as compared to the starting layer.
Extending the intersubband transitions in III-nitride nanostructures from near-infrared to longer wavelengths might have
significant consequences for critical applications like imaging, remote sensing and mine detection. In this work, we
analyze the potential of polar and semipolar AlGaN/GaN technologies for this relevant spectral range.
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