Misfit dislocations in green-emitting InGaN/GaN quantum well structures
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Misfit dislocations in green-emitting InGaN/GaN quantum well structures PMFJ Costa, R Datta, MJ Kappers, ME Vickers, CJ Humphreys Department of Materials Science and Metallurgy, University of Cambridge Pembroke Street, CB2 3QZ, United Kingdom Email: [email protected]
Abstract Misfit dislocations (MDs) have been observed using transmission electron microscopy (TEM) in InGaN/GaN quantum well (MQW) structures grown under different metal-organic vapour phase epitaxy (MOVPE) regimes and with In-contents equal to or higher than 20%. These dislocations are even observed in a single quantum well 3 nm thick with an In-content of 22%. Conversely, no MDs were observed in QW structures with an In-content of 16%. The presence of MDs in the QW stack leads to strain relaxation which has been confirmed in the indium-rich structures by high resolution Xray diffraction (HRXRD).
Introduction Despite the remarkable tolerance of the optical properties of GaN-based structures to the presence of dislocations, when compared to other III-V semiconductor materials[1], structural defects remain an important subject of research for nitride-based light emitting diodes. It is known that dislocations can act as non-radiative recombination centres thus reducing the light emitting efficiency of devices[2]. Dislocations are, in fact, a strain relaxation mechanism for heterostructures that have a significant lattice mismatch between the adjacent layers. In this way, the use of relatively high In-contents (≥ 20%), central to achieving green light emission from InGaN/GaN heterostructures, will lead to considerable misfit strain and the possible formation of structural defects such as misfit dislocations. However, most theoretical models in the literature (Fischer[3], People and Bean[4], etc.) do not predict strain relaxation for 20% In structures unless thick InGaN layers (>8 nm) are considered. An exception to this is the Matthews and Blakeslee model which predicts a critical layer thickness of ~1 nm[5]. Apart from misfit dislocations, other strain relaxation mechanisms exist as, for instance, V-defects which normally have the form of an open hexagonal and inverted pyramid (frequently associated with threading dislocations). This paper reports the presence of misfit dislocations in the quantum well stack region of several high In-content (≥ 20%) InGaN/GaN structures. Besides changing the In-content or number of quantum wells of the samples analysed, three different growth conditions were also explored. Transmission electron microscopy was used to analyse the dislocations, the well/barrier interfaces and well width variations and the formation of V-defects. High resolution X-ray diffraction was employed to confirm the presence of strain relaxation in the heterostructures analysed.
Experimental A 6x2-inch Thomas Swan Close-Coupled Showerhead MOVPE reactor was used to grow the InGaN/GaN multiple quantum well samples which were deposited on GaN pseudo-substrates with a typical threading dislocation density of 4x108 cm-2 (for more details
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