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 In0.15Ga0.85N 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 In0.15Ga0.85N 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.
All Science Journal Classification (ASJC) codes
- Mechanics of Materials
- Mechanical Engineering