Differential phase contrast 2.0--opening new "fields" for an established technique

Differential phase contrast microscopy has become known as a high resolution imaging technique for magnetic micro-structures in the past. The method senses the local induction by measuring the deflection of the probe beam after it passes through a specimen area carrying a magnetic field. Little attention has been paid, however, to the fact that this technique is also capable of measuring electric fields. An application of the technique to measure piezoelectric polarization fields inside multi-layered structures such as quantum wells is demonstrated. For this purpose, piezoelectric fields within non-centrosymmetric crystal structures, based on GaN/InGaN/GaN quantum wells, are investigated. It can be shown that the technique is sensitive to these fields and yields detailed information about the field distribution. The specific information and experimental limitations as well as artefacts of the technique will be discussed in detail and first measurements are shown. The main advantages turn out to be high sensitivity for electric fields, combined with a very high resolution, which is limited only by the STEM probe size. Another advantage is the large achievable field of view.