Effects of compressive strain on optical properties of In _xGa _1-xN/GaN quantum wells

In0.2 Ga0.8 NGaN multiple quantum well (MQW) blue light emitting diode (LED) structure was grown on a specially designed sapphire substrate (with increasing thickness from the edge to the center within a single wafer). X-ray diffraction revealed that the GaN lattice constant c decreases continuously from the edge to the center, indicating a continuous variation in the compressive strain. The spectral peak positions of the electroluminescence (EL) spectra exhibited a blueshift when probed at the edge as compared to the center, which is a direct consequence of the continuous variation in the compressive strain across the wafer. Based on the experimental results, a ratio of elastic stiffness constants (C33 C13) for GaN was deduced to be ∼5.0±1.0, which was in agreement with the calculated value of ∼4.0. A linear relation of the EL emission peak position of LEDs with the biaxial strain was observed, and a linear coefficient of 19 meVGPa characterizing the relationship between the band gap energy and biaxial stress of In0.2 Ga0.8 NGaN MQWs was also obtained.