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

Abstract

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.

Probing the Internal Atomic Charge Density Distributions in Real Space

“…Figure a shows the simulated intensity distribution at the diffraction plane, and thus at the segmented area detector, when a 300 kV electron probe passes 0.2 Å to the left of a gallium atomic column in a 1.6 nm thick GaN crystal oriented along the [112̅ 0] direction. GaN is a common material in a wide variety of optoelectronic devices, such as blue light-emitting diodes, and has been extensively studied using DPC-STEM at nanometer resolutions, exploring the possibility of detecting the piezoelectric fields in optoelectronic devices. − The probe intensity at the diffraction plane is shifted toward the Ga column due to the interaction between the incident electrons and the atomic electric field: the direction of deflection shows that the attractive force exerted on the incident electrons by the nucleus is dominant, although it is moderated by the screening electrons. The simulation exposes how the traditional picture of a rigid deflection of the probe due to the atomic electric field is only a simplification of the real effects, as described in recent experimental and theoretical , reports.…”

Direct Measurement of Polarization-Induced Fields in GaN/AlN by Nano-Beam Electron Diffraction

“…Digital-DPC was performed by applying a ‘virtual’ DPC detector to the diffraction patterns, recorded with a CCD camera, in order to compare the two techniques, and it was found that the intensity variations within the transmitted beam strongly contribute to the signal, thus rendering the method ineffective for direct field measurements. Recently in-line holography36 was used to measure the piezoelectric charge density at the quantum well interfaces, where most of the limitations imposed by specimen preparation conditions for off-axis holography are not a hindrance. However, as stated earlier, it cannot be considered a direct technique and as in-line holography needs long exposure times (~10 s/image), this can create a problem while measuring the charge density or potential in a specimen due to radiation induced charge migration and a potential build up in the illuminated area of the sample37.…”

Two-dimensional misorientation mapping by rocking dark-field transmission electron microscopy

“…For reflections far from the central beam, the rocking curve maxima can more easily be identified, which is an essential feature for mapping the orientation by the proposed method. To exclude the intensities of beams other than the desired beam a small objective aperture diameter 10 m ( = μ ) was inserted, as is also used for mapping strain by dark-field inline electron holography [17,20,21]. The range of scattering angles passing through this objective aperture corresponds nearly to the size of the first Brillouin zone.…”