Investigation of the Reverse Leakage Behavior and Substrate Defects in Vertical GaN Schottky and PIN Diodes

In this work, the effects of the substrate defect density and distribution on the reverse leakage behavior of GaN vertical Schottky diodes and p-i-n diodes are investigated. A direct connection between the reverse leakage behavior of GaN based vertical devices and the dislocation density of the underlying material was determined. The difference in the leakage current for devices on different locations of the wafer can be as high as 6 orders of magnitude (for p-i-n diodes) at -200V, for HVPE substrate with inhomogeneous but predictable defect distributions (GaN substrates with dot-core inversion domain features). For comparison, using HVPE substrates with uniform defect distribution (but with no cores), the p-i-n diodes show much more uniform leakage behavior, varying within only an order of magnitude, and that range fell within the much greater range of that for the inhomogeneous substrates. The substrates with inhomogeneous defect distribution proved to be useful to show the direct correlation. The topography measurements confirmed that the wafers with inhomogeneous defect distribution possess periodically patterned core-centers with higher defect density and larger lattice distortions surrounded by other regions, which have very low defect concentrations. Devices located away from the defective core-centers result in a reduction of the reverse bias leakage by over two orders of magnitude at -10 V for Schottky diodes. Similar trends are also observed in the p-i-n diodes; the devices close to the core centers show the highest reverse leakage (>0.01 A cm(-2) at -200V). Devices further away from the core-centers (lower dislocation density) show lower reverse leakage current. Moreover, the p-i-n diodes on regions more than 300 mu m away from the core-centers show the best leakage behavior (<10(-7) A cm(-2) at -200V) of all the devices, outperforming the devices on the substrates with uniform defect distribution (similar to 10(-6) A cm(-2) at -200V). The results from this study show that the substrate defect density and distribution play important roles in the device leakage current. X-ray topography is extremely effective for studying defect characteristics underneath individual devices. The use of the wafers with inhomogeneous, but predictable defect density clearly demonstrated the importance of low defect densities for high device performance.