Defects in nitride semiconductors: From nanoscale imaging to macroscopic device behavior

Scanning capacitance microscopy (SCM), atomic force microscopy (AFM), and conductive AFM are used to image the spatial distribution and electronic properties of threading dislocations in AlxGa1 xN/GaN epitaxial layers grown by molecular-beam epitaxy. SCM imaging reveals that GaN growth directly on SiC substrates leads to clustering of negatively charged dislocations at nucleation island boundaries, while incorporation of an AlN buffer leads to a random spatial distribution of negatively charged dislocations. Numerical simulations demonstrate that clustered dislocations are less effective in depleting mobile carriers. AFM and conductive AFM imaging reveal the presence of highly conductive threading dislocations which lead to excessive reverse-bias leakage current flow in Schottky diodes. Temperaturedependent current–voltage spectroscopy is used to develop a model for current flow via these dislocations based on a Frenkel–Poole emission process. On the basis of this model, heterostructures are designed to suppress this emission mechanism. Conductive AFM imaging and electrical measurements then confirm the expected suppression of leakage current and specifically of the Frenkel–Poole emission process, demonstrating the validity of both the proposed mechanism of current flow and the approach for leakage current suppression. r 2006 Elsevier Ltd. All rights reserved.