Strain accommodation and interfacial structure of AlN interlayers in GaN

Abstract

The strain accommodation mechanisms at AlN interlayers in GaN, grown by radio‐frequency plasma assisted molecular beam epitaxy, are studied using transmission electron microscopy techniques and atomistic modelling. Interlayers of various thicknesses grown within GaN epilayers deposited on both sapphire and silicon substrates have been employed. Interlayers of thickness below 6 nm do not exhibit line defects although local roughness of the upper interlayer interface is observed as a result of the Al adatom kinetics and higher interfacial energy compared to the lower interface. Above 6 nm, introduction of a‐type misfit and threading dislocations constitutes the principal relaxation mechanism. Due to strain partitioning between AlN and GaN, threading dislocations adopt inclined zig‐zag lines thus contributing to the relief of alternating compressive‐tensile elastic strain across the AlN/GaN heterostructure. The observed dislocation configurations are consistent with a model of independent motion by climb or ancillary glide in response to their localized three‐dimensional strain environment. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Initiating polarity inversion in GaN growth using an AlN interlayer

“…A similar study was initiated on employing a HT AlN IL grown on a N‐polar MOCVD‐grown template since the addition of HT AlN interlayers is a familiar method in strain engineering 14. However, for strain engineering the AlN thickness is normally under 10 nm, and no polarity switch in the succeeding layers has been reported for such thin layers.…”

In‐situ growth condition analysis of AlN interlayers for wafer curvature control in GaN MOVPE on Si (111)

“…The regions of such coalescence have been proposed as sites of introducing misfit dislocation. It also has been postulated that deep grooves between grains can evolve into cracks with increasing AlN IL thickness [7]. After cracking, AlN ILs cannot sustain enough shear stress to strain the overlying GaN layers on them highly compressively.…”

“…On the other hand, the theoretical critical thickness for AlN on GaN is 7.56 nm [14]. Experimentally, in MBE grown samples, no line defect could be observed at AlN ILs on GaN with thickness below 6 nm [7], although the detailed growth process may differs slightly between MOCVD and MBE grown materials. Due to the lower misfit dislocation density at AlN ILs, threading dislocation propagated into following GaN may be less thus the relaxation of it may be also slower.…”

“…As the mentioned AFM results above, for LT-AlN ILs, although the 3D structure make themselves more relaxed than HT ones, which is beneficial, but the quality is poor with very high defect density. The large density of grain boundaries could also be the source of dislocations [7]. When the high density defects propagate into the overlying GaN or cause misfit dislocations at the interface, GaN would relax much quicker than that on HT-AlN ILs.…”

“…They studied the effect of growth temperature and showed that low-temperature (LT) AlN interlayer acts as a separating layer between neighbouring GaN layers and is more effective in controlling stress. Dimitrakopulos investigated the strain accommodation and relaxation mechanisms at AlN interlayers with thickness from 3 to 100 nm in GaN grown by MBE on Si using Transmission Electron Microscopy (TEM) [7]. No line defect was observed when the thickness was less than 6 nm, while above it a-type misfit and threading dislocations occur and cause relaxation principally.…”