Identification and Characterization of Conductive Dislocations in p-GaN/AlGaN/GaN Heterojunctions on GaN-on-Si Substrates

Abstract

We present a study of dislocation conductivity under forward bias in p-GaN/AlGaN/GaN heterojunctions on a GaN-on-Si substrate, which are part of every p-GaN HEMT structure. Conductive atomic force microscopy (C-AFM) is combined with structural analysis by scanning transmission electron microscopy (STEM) and defect selective etching (DSE). The density of conductive TDs was found to be 5 x 106 cm-2 using semi-automatic measurements to gather larger statistics on a delayered HEMT sample. IV measurements show a shift in turn-on voltage at the leakage positions. To characterize the type of the conductive TDs, DSE with a KOH/NaOH melt was used. Three distinct etch pit sizes were observed after 5 s etch time, with large, medium and edge pits according to STEM characterization seemingly corresponding to screw, mixed and edge TDs, respectively. However, characterization by DSE etch pit size alone was found to be unreliable, as STEM TD typing of seven conductive TDs using two-beam diffraction conditions revealed mostly pure screw and mixed-type dislocations with medium-sized etch pits as origin of the observed leakage current. Our work highlights the limitations of DSE as a characterization method and recommends additional validation by STEM for each new material system, investigated layer, and etching setup. The implications of finding conductive TDs with screw-component under low forward bias conditions on device behavior and the limitations of the C-AFM method are discussed. Based on the results, it is not anticipated that the identified conductive TDs will have a substantial effect on a GaN HEMT device. Overall, this study provides important insights into the electrical properties of TDs and offers useful recommendations for future research in this area.