Impact of pits formed in the AlN nucleation layer on buffer leakage in GaN/AlGaN high electron mobility transistor structures on Si (111)

Limiting buffer layer current leakage is essential for the realization of high breakdown fields in GaN-on-Si high electron mobility transistors (HEMTs). In this report, we demonstrate the importance of controlling the surface morphology of the AlN nucleation layer (NL) in limiting this leakage. Testing on a self-consistent series of samples grown under two different AlN NL conditions revealed the presence of leakage paths within the epilayers grown using a single temperature AlN NL owing to the presence of surface pits. The introduction of a higher temperature AlN in the NL drastically reduced the pit density and led to a large reduction (>10(3)) in the lateral and vertical buffer leakage in HEMT structures. Using conductive atomic force microscopy, secondary ion mass spectroscopy, and temperature-dependent carrier transport measurements, we confirm that these pits-which originate in the AlN NL, thread vertically, and propagate into the device structures-are associated with leakage paths, thus reducing the field that can be dropped across the epilayers. This is explained by invoking preferential oxygen segregation at their side-facets. It is shown that when a pit-free surface is maintained, a vertical field of 1.6MV/cm can be achieved for HEMTs. This study is expected to benefit the development of high-performance GaN HEMTs in moving toward the theoretical breakdown field of III-nitrides.