Visualization of Crystallographic Defects in InSb Micropillars by Ptychographic Topography

Investigation of the strain field and defects in crystalline materials is essential in materials characterization, fabrication and design, as they are responsible for distinct mechanical, electric and magnetic properties of a desired material. Therefore, the visualization of strain and its relation to the type and density of defects in the crystal at the nanoscale is required. A domain in which such questions are particularly relevant is the fabrication of nanodevices for microelectronics from semiconductors, such as InSb, that are used as fast transistors, detectors and sensors. Classically, transmission electron microscopy (TEM) provides imaging of the crystalline defects with atomic spatial resolution, but due to the thin sections requirement, sample preparation is invasive and can modify the strain fields to be analyzed. A conventional tool to non-invasively study strain is Laue X-ray micro-diffraction [1], which reveals the strain field in crystalline samples averaged over the direction of the beam propagation with a resolution limited by the beam size. X-ray topography (XRT) [2] has been routinely used for imaging defects based on the diffraction contrast, with the resolution being restricted by the detector pixel size. X-ray coherence methods, such as coherent diffraction imaging (CDI) and ptychography, which are based on measuring the sample's far-field diffraction patterns and using phase retrieval algorithms, permit obtaining high resolution images. If the measurements are performed close to a Bragg peak, the resulting image becomes highly sensitive to the presence of strain [3, 4]. We have recently developed ptychographic topography, in which a crystalline sample is rotated with respect to the incident beam such that a certain atomic plane is in the Bragg condition, as shown in Fig. 1a [5]. A pinhole is then placed after the sample in the forward direction and is spatially translated, providing the sufficient overlapping necessary for ptychographic reconstructions [6]. The diffraction patterns are recorded at each pinhole position with a 2D detector downstream of the pinhole and used simultaneously for the reconstruction of the wave front at the pinhole position. So far measurements were performed in forward direction due to the limited space at the beamline to fulfil the ptychographic detector sampling requirement along the Bragg-diffracted beam direction. Numerical backpropagation then enables one to obtain an image of the sample, which is sensitive to the lattice displacements caused by defects.