Observation of interfacial strain relaxation and electron beam damage thresholds in Al0.3In0.7N/GaN heterostructures by transmission electron microscopy

The structural quality of indium-rich Al0.3In0.7N grown by metal modulated epitaxy (MME), previously demonstrating x-ray diffraction (XRD) figures of merit ∼11 times better than the previous literature is investigated to explain the origin of such a large quality improvement. Four-dimensional scanning transmission electron microscope was used to map the lattice parameter near the AlInN/GaN heterojunction and indicate a 5.4% lattice constant change, suggesting 75% relaxation within ∼2 nm from the interface. Cross-sectional TEM Moiré fringes are observed at the AlInN/GaN heterointerface, indicating that there are misfit dislocations between AlInN and GaN which, while rare, have been observed for other highly mismatched In-rich III-Nitrides. The TEM measurements show regions of contrast indicating larger scale variations in strain, but defect contrast associated with dislocations and/or intrinsic basal stacking faults was minimal, indicating a good quality AlInN film and confirming prior XRD results. Significant electron beam induced damage can occur and depended strongly on operational conditions. The damage threshold current density was estimated using time-dependent TEM to be ∼5.7 A/cm2, significantly lower than from prior studies of InGaN. Damage also strongly depends on the thickness of the TEM foil examined and occurred at thicknesses greater than found for InGaN. The present study suggests that the MME technique is an excellent candidate for growing high-quality indium-rich AlInN films as compared to the traditional molecular beam epitaxy or metal organic chemical vapor deposition techniques.