Structural Modification in B-doped AlN Ferroelectric Films by STEM-DPC

Currently, a large variety of commercial ferroelectric materials have limited compatibility with semiconductor processing technology, which hinders their use in in-memory computing devices. However, the development of novel thin films with robust polarization reversal and (CMOS)-compatible processing such as HfO2 and doped wurtzite-structured materials have shown great potential as integrated ferroelectric materials. Epitaxial B-substituted AlN thin films, for instance, have been successfully grown at low temperatures (300°C) by reactive magnetron sputtering, revealing ferroelectric switching behavior with polarization values larger than 125 μC cm-2 [1]. Nonetheless, the high coercive fields currently limit their applicability. Thus, understanding the mechanisms that control and induce switchable states in such polar materials is an important goal. On this basis, we are investigating the structural modifications induced by boron-doping AlN thin films. We use scanning transmission electron microscopy (STEM) differentiated differential phase contrast (dDPC) imaging to locate projected atomic column positions to calculate structural variations and local spontaneous polarization [2] as a function of B doping in AlN thin films. STEM-DPC experimental images are acquired in a probe-corrected ThermoFisher Titan-Themis operated at 200kV, using a four-segment annular detector (DF4), a probe convergence angle of 18 mrads and a camera length that resulted in acceptance angles between 11 and 43 mrads. A set of 20 frames are collected using fast dwell time (200 ns) to avoid significant drift that can cause distortions in the images. The frames are later registered using cross-correlation and integrated to improve the signal-to-noise ratio. Final error in the cell distortion is calculated using a reference AlN lattice from a crystallographic file. Multislice image simulations are carried out using Dr. Probe V1.9 software package [3]. Simulations are carried out using the frozen-phonon configuration method at 200 keV and 1 µm of spherical aberration. Two-dimensional vector pair correlation functions (vPCFs) [4], are calculated to quantify the structure at atomic scale and the overall polarization from simulated and experimental images of AlBN films oriented along [110]. Special emphasis is given to the effect of the sample crystallographic orientation with respect to the segmented detector due to the lack of circular symmetry of the contrast transfer function [5]. The experimental data are complemented with STEM computer simulations to determine the influence of the detector orientation on the structural analysis of this wurtzite structure. Our simulations demonstrate a significant influence of the detector orientation with respect to the material structure on determining the atomic column positions, with deviations as large as 20 pm for different detector/structure orientation, as shown in Fig. 1. These differences are enhanced by the proximity between the Al-N forming dumbbell-like feature, which hinders a properly fitting the image intensities to determine the atomic column positions. Taking into account these complexities, we determine the effect of B concentration on the B-doped AlN lattice structure and the corresponding changes in spontaneous polarization and correlate it with DFT calculation [6]. (a) Al-N dumbbell angle extracted from simulated STEM – dDPC images for different 4-segment detector orientation with respect to the sample, a pixelated detector and the ground truth obtained from the model. (b-c) dDPC experimental and simulated images for Al1-xBxN, respectively.