Depth dependent strain analysis in GaN-based light emitting diodes using surface-plasmon enhanced Raman spectroscopy

GaN-based LEDs often develop unintended strain, due to the lattice and thermal expansion mismatch between the sapphire substrate and GaN layers. This residual compressive strain in GaN overlayers can cause a number of critical effects, such as wafer bowing, threading dislocation generation that impact the overall properties of the epilayers, and the resulting device performance. Raman spectroscopy is known to be a useful technique for measuring strain analysis by monitoring the energy position of the E2 (high) mode. However, the local strain of a specific area cannot be obtained by conventional Raman spectroscopy since the excitation laser in the usual case is transparent and scattered throughout the whole LED structure. As a result, only the average strain of the entire structure is generally measured. We successfully analyzed the depth dependent strain distribution by applying the SERS technique and careful sample manupulation, to produce a truncated structure. Our results showed that the strain in the undoped GaN buffer layers and the n-GaN layer gradually decreased up to the quantum well region, as the thickness increased, while the strain saturated in the active and p-GaN layers. The depth depndent strain was also quantified using our analysis.