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Strain mapping using inline electron holography


A key to strain engineering of piezoelectric semiconductor devices is the quantitative assessment of the strain-charge relationship. This is particularly demanding in current InGaN/GaN-based light-emitting diode (LED) designs as piezoelectric effects reduce device performance. Using the state-of-the-art inline electron holography, we have obtained fully quantitative maps of the two-dimensional strain tensor and total charge density in conventional blue LEDs and correlated these with sub-nanometer spatial resolution. We show that the In0.15Ga0.85N wells are compressively strained and elongated along the polar growth direction, exerting compressive stress/strain on the GaN barriers. Interface sheet charges arising from polarization gradients is obtained directly from the stain data and compared with the total charge density map, verifying only 60% of the polarization charges are screened by electrons, leaving a substantial piezoelectric field in each In0.15Ga0.85N well. The demonstrated capability of inline electron holography provides a technical breakthrough for future strain engineering of piezoelectric semiconductor devices.