Correlative High-Resolution Mapping of Strain and Charge Density in a Strained Piezoelectric Multilayer

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 are known to degrade the 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 In_0.15Ga_0.85N quantum wells are compressively strained and elongated along the polar growth direction, exerting compressive stress/strain on the GaN quantum barriers. Interface sheet charges arising from a polarization gradient are obtained directly from the strain data and compared with the total charge density map, quantitatively verifying only 60% of the polarization charges are screened by electrons, leaving a substantial piezoelectric field in each In_0.15Ga_0.85N quantum well. The demonstrated capability of inline electron holography provides a technical breakthrough for future strain engineering of piezoelectric optoelectronic devices. Applying the state-of-the-art inline electron holography to a light emitting diode containing strained InGaN/GaN multiquantum wells, we show that fully quantitative maps of 2D strain and charge density can be obtained and correlated with sub-nanometer resolution. Combined analysis of the two data quantitatively evaluates the internal piezoelectric field and the electrostatic screening interaction between the polarization charges and the free charge carriers.