Vertical field inhomogeneity associated with threading dislocations in GaN high electron mobility transistor epitaxial stacks

A measurement technique combining Kelvin-probe force microscopy with substrate bias is developed and demonstrated on AlGaN/GaN-on-Si device structures under conditions relevant to the effect of off-state drain bias stress in transistors. For a high substrate bias, the measurements show a significantly lowered surface potential surrounding a small proportion of dislocations imaged with atomic force microscopy (AFM), laterally extending on a scale of up to a micrometer. Both the density and the size of those features increase with substrate bias; however, conductive AFM measurements under the same bias conditions showed no leakage reaching the surface associated with those features. Our model considers localized conductive paths that end a certain distance below the 2D electron gas electrically “thinning” the epitaxy and, therefore, deforming the potential and increasing the electric field under off-state stress bias. The conclusion is that the vertical electric field in the buffer is laterally highly non-uniform with an enhanced vertical field in the vicinity of those dislocations. This non-uniformity redirects the substrate bias stress from the buffer to the channel with potential consequences for breakdown.