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The invention of silicon drift detectors has resulted in an unprecedented improvement in detection efficiency for energy-dispersive X-ray (EDX) spectroscopy in the scanning transmission electron microscope. The result is numerous beautiful atomic-scale maps, which provide insights into the internal structure of a variety of materials. However, the task still remains to understand exactly where the X-ray signal comes from and how accurately it can be quantified. Unfortunately, when crystals are aligned with a low-order zone axis parallel to the incident beam direction, as is necessary for atomic-resolution imaging, the electron beam channels. When the beam becomes localized in this way, the relationship between the concentration of a particular element and its spectroscopic X-ray signal is generally nonlinear. Here, we discuss the combined effect of both spatial integration and sample tilt for ameliorating the effects of channeling and improving the accuracy of EDX quantification. Both simulations and experimental results will be presented for a perovskite-based oxide interface. We examine how the scattering and spreading of the electron beam can lead to erroneous interpretation of interface compositions, and what approaches can be made to improve our understanding of the underlying atomic structure.
Achieving sub-picometer precision measurements of atomic column positions in high-resolution scanning transmission electron microscope images using nonrigid registration (NRR) and averaging of image series requires careful optimization of experimental conditions and the parameters of the registration algorithm. On experimental data from SrTiO3 , sub-pm precision requires alignment of the sample to the zone axis to within 1 mrad tilt and sample drift of less than 1 nm/min. At fixed total electron dose for the series, precision in the fast scan direction improves with shorter pixel dwell time to the limit of our microscope hardware, but the best precision along the slow scan direction occurs at 6 μs/px dwell time. Within the NRR algorithm, the “smoothness factor” that penalizes large estimated shifts is the most important parameter for sub-pm precision, but in general, the precision of NRR images is robust over a wide range of parameters.
Aberration-corrected scanning transmission electron microscopy images of the In0.15Ga0.85N active region of a blue light-emitting diode, acquired at ~0.1% of the electron dose known to cause electron beam damage, show no lateral compositional fluctuations, but do exhibit one to four atomic plane steps in the active layer’s upper boundary. The area imaged was measured to be 2.9 nm thick using position averaged convergent beam electron diffraction, ensuring the sample was thin enough to capture compositional variation if it was present. A focused ion beam prepared sample with a very large thin area provides the possibility to directly observe large fluctuations in the active layer thickness that constrict the active layer at an average lateral length scale of 430 nm.
By using aberration corrected scanning transmission electron microscopy we have found no small scale lateral In composition fluctuations exist in the In0.15Ga0.85N active region of a light emitting diode. Images were acquired at 2% of the electron dose known to create electron beam damage, so the acquired images reflect the intrinsic structure of the InGaN active region. Position averaged convergent beam electron diffraction reveals the local sample thickness where images were acquired is 4.8 nm, eliminating the possibility that the absence of composition variation was observed due to projection through a thick sample. In addition, 2-3 atomic layer steps were observed in the top surface of In0.08Ga0.92N layers and the In0.15Ga0.85N active layers, providing a possible mechanism for lateral carrier confinement.
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