Three-dimensional imaging of threading dislocations in GaN by multimodal stimulated Raman scattering microscopy

Abstract

Emerging gallium nitride (GaN) vertical power devices require high-quality GaN crystals with reduced crystal defects, especially threading dislocations (TDs), to harness the high critical electric field and electron mobility of the material. A strong demand for characterizing TDs has driven the development of many imaging techniques, such as multiphoton excited photoluminescence (MPPL) microscopy and spontaneous Raman microscopy. However, all these techniques lack the capability of visualizing densely existing TDs in a non-destructive and three-dimensional (3D) way or distinguishing the types of TDs. Here, we propose multimodal 3D stimulated Raman scattering (SRS) and MPPL microscopy to non-destructively characterize the TDs in free-standing GaN substrates. Leveraging strong SRS signals, we achieve an imaging speed 10^2-10^3 times faster than that of spontaneous Raman microscopy. This acceleration of the imaging speed enables 3D SRS imaging of the strain fields that are induced by TDs and correlated with the edge components of the Burgers vectors. Multimodal 3D SRS and MPPL imaging reveals rich information on TD features and behavior, including propagation directions, Burgers vectors, dislocation reactions, and their relationships. Furthermore, we present a deterministic identification method for screw-type TDs, which are regarded as killer defects causing leakage current in p-n junctions, by exploiting no SRS peak shifts at screw dislocations in combination with MPPL patterns. Our multimodal imaging technique will greatly advance the understanding of dislocations toward realizing high-quality GaN crystals and high-performance GaN devices.