On the Mechanism of High Forward Voltage of InGaN Light Emitting Diodes

High forward voltage (Vf) electrical issue can be due to high parasitic series resistance within light emitting diodes (LEDs) and is challenging to isolate and identify the failing location. Such cases require careful selection of the right technique with in-depth understanding for root cause investigation to identity and resolve fabrication process anomalies. In this paper, two case studies on high Vf reject analysis in InGaN LEDs are presented. The first case study used time of flight secondary ions mass spectrometry (ToF-SIMS) and found increased hydrogen diffusion into GaN. Hydrogen is known to deactivate magnesium dopant in p-GaN resulting in increased GaN resistance. The second case study demonstrates the use of scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to reveal carbon residue at p-metal stack interface which acts as a blocking layer causing higher resistance at p-contact. Both cases demonstrate effective approaches for high V f root cause investigation and process optimizations.