Methodology for the investigation of threading dislocations as a source of vertical leakage in AlGaN/GaN-HEMT heterostructures for power devices

In this work, an AlGaN/GaN-HEMT heterostructure is exemplarily studied by a strict place-to-place correlational approach in order to help clarify some open questions in the wide field of reliability topics. Especially, vertical leakage current, its relation to dislocations in general, and specific types in particular are investigated on a highly defective material. With the aid of atomic force microscopy (AFM) in tapping mode, cathodoluminescence imaging, defect selective etching, and energy dispersive X-ray, the material's defect content around the device relevant two dimensional electron gas is analyzed. The total dislocation density, as well as the density of threading screw, edge, and mixed type dislocations, is systematically investigated directly. The obtained result is statistically much more significant than is possible by conventional transmission electron microscopy studies and more precise than the results obtained by the indirect method of rocking curve analysis. The method of conductive AFM allowed mapping of variations in the vertical leakage current, which could be correlated with variations in barrier leakage or gate leakage. Spots of locally high leakage current could be observed and directly assigned to dislocations with a screw component, but with significant differences even within the same group of dislocation types. The electrical activity of dislocations is discussed in general, and a fundamental model for a potential dislocation driven vertical leakage is proposed.