Modulated nanostructure characterization using aberration corrected STEM

Ray Carpenter, Ronit Sawant, Toshihiro Aoki
2020 Microscopy and Microanalysis  
Modulated nanostructures consist of short wavelength periodic composition fluctuations in metal, semiconductor, or ceramic alloys and they have strong effects on alloy properties. Their existence is often attributed to spinodal decomposition (1). The early stage nanostructure consists of periodic composition fluctuations whose amplitude and wavelength increase as the transformation progresses to equilibrium. Typical wavelength ranges 5 to 15 nm (2), but the initially small composition
more » ... must become large as the reaction progresses according to system thermodynamics. The constituent atoms are generally of different sizes so the fluctuations result in a tessellated strain field oriented in the soft directions of the host crystal lattice. Direct measurements of the composition fluctuation amplitudes and wavelengths are the best method to study the reaction kinetics. Bentley's comprehensive review (3) shows that diffraction contrast imaging/selected area diffraction produced much useful information about the periodicity and strain fields in modulated nanostructures, including defects in the fluctuation periodicity caused by more familiar lattice defects such as grain boundaries, but little direct information about the amplitude of the fluctuations, due to instrumental limitations of the time. Since then the development of field emission sources (4), aberration correctors (5), and ADF imaging for STEM microscopes should provide the necessary resolution to examine the shape of diffuse interfaces and the amplitudes of modulations directly. We used a conventional TEM/STEM (not aberration corrected) with a Schottky FEG source to examine composition modulations in Al 0.4 Ga 0.6 N layers grown by molecular beam epitaxy. The growth temperature was 750C, high enough to insure atomic mobility to form a modulated structure (6). ADF images showed that modulated structure with an average wavelength of 5.9±1.4 nm existed in the layers. We could establish the Ga and Al rich regions in the modulations using nanospectroscopy, but the spatial resolution was insufficient for quantitative analysis. We have now examined these layers in an aberration corrected D-STEM microscope. Figure 1 shows a typical modulated structure image. The modulation is one-dimensional, and the modulation wave vector is nearly perpendicular to [0001]. The layer has the wurtzite structure, and the basal plane spacing is modulated by the composition variations. The modulations are not as regular as one would expect from theory. The places where dark regions (Alrich) transform to light regions (Ga-rich) probably occur because the wavelength was growing as the layer formed during synthesis at elevated temperature. This aspect of the nanostructure is in common with those of many metal alloy modulated structures reviewed by Bentley. Whether these composition modulations are one, two, or three dimensional has been a topic of discussion. Many early investigations used satellites in x-ray diffraction patterns or images from thick specimens to conclude that they were multidimensional. We have so far only observed one dimensional composition modulations. Figure 2 shows the Al and Ga composition variations along [0001] calculated from an EELS line scan of one region of this specimen. A similar plot for N showed little variation with distance, indicating small
doi:10.1017/s1431927620016876 fatcat:ai2mdadahnhpjo3qf6rvlsfgma