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

Abstract

A measurement technique combining Kelvin-Probe Force Microscopy (KPFM) 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 high substrate bias, the measurements show a significantly lowered surface potential surrounding a small proportion of the 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 2DEG electrically "thinning" the epitaxy and therefore deforming the potential and increasing 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 substrate bias stress from the buffer to the channel with potential consequences for breakdown.With its excellent material properties, Gallium Nitride (GaN) is highly suitable for power switching and radio frequency

Communication—Electron-Beam Stimulated Release of Dislocations from Pinning Sites in GaN

“…[13][14][15][16][17][18] Currently, GaN-on-Si wafers are commercially available up to 8″ diameter and close to a cost target of 1.5 $/cm 2 for epitaxial material, where GaN will be cost-competitive with Si-based components at the device-level. 3,4 Currently, GaN-on-Si structures have epilayer thickness of 4-6 μm, producing a voltage handling capability of 650 V. In such GaN-on-Si structures, it has recently been reported that there are vertical field inhomogeneities associated with threading dislocations, which produce charge carrier trapping and increased leakage currents [19][20][21][22][23][24][25][26][27][28][29][30] and many studies have correlated dislocations with changes in local materials properties. [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] Dislocation densities for AlGaN/GaN-on Si are currently ∼10 9 cm 2 and the screw and mixed type dislocations create leakage paths in Schottky diodes under high voltage reverse biases.…”

The isolation feature geometry dependence of reverse gate-leakage current of AlGaN/GaN HFETs

“…As per [6], based on the information on the presence of inhomogeneity of the vertical electric field across the barrier of III-nitrides [6,9,22,23], the current due to the FN tunneling mechanism is position-dependent. Therefore, the FN-related current which leaks through a part of the barrier that boasts a higher electric field dominates the total leakage current through the barrier.…”