Direct Observation of Bandgap Shift in the Dislocations Using High Resolution Cathodoluminescence in Transmission Electron Microscopy

InGaN/GaN multi-quantum well (MQW) is the leading structure for the next generation of lighting systems. Even though silicon and silicon carbide are used as substrates to grow GaN thin films, majority of the GaN are being grown onto sapphire substrates, whose techniques were well exercised for the past decades and had wide variation to reduce the strain induced by the lattice mismatch. Dislocations are unavoidable because of the large lattice mismatch between GaN and substrate, regardless of the substrates being used. Dislocations formed in InGaN/GaN MQW have different lattice periodicity and should deliver different emission characteristics compared to the perfect periodic lattice. Dislocations can be seen using transmission electron microscopy (TEM) and in order to link the emission characteristics with the dislocations, cathodoluminescence capability in TEM. Cathodoluminescence in transmission electron microscopy (CL-TEM) delivers superior performance to the CL-SEM because of its higher spatial resolution with short diffusion length and the capability of one-to-one mapping of the interested microstructural area with point-luminescence and the internal microstructure. In this study, luminescence characteristics were investigated using CL-TEM to identify the shift of the emission wavelength with different types of dislocations. Dislocations revealed short wavelength shift in the region with QW and threading dislocations. No emission was observed with the edge dislocations, but the screw dislocations either close to the surface or close to the substrate delivered shift of wavelength toward the higher energy side. Plane-view TEM revealed the fluctuations of the wavelengths, which gave low and high band gap area and local minima might be the contribution of the high efficiency of InGaN/GaN light emitting system.