Quantitative Atomic-Site Analysis of Functional Dopants/Point Defects in Crystalline Materials by Electron-Channeling-Enhanced Microanalysis

This novel elemental and chemical analysis scheme can be used to quantitatively derive site-dependent information for impurities or dopants within a specimen using energy-dispersive x-ray and electron energy-loss spectroscopies. The technique is simple, low cost, and quantitatively reliable compared to other analytical techniques currently available and does not require state-of-the-art equipment. This method is widely applicable to not only dopant analysis in a single crystal but also to the local structure analysis of lattice defects associated with vacancies, interstitials, and grain boundaries.

Demonstrating the procedure will be Masahiro Ohtsuka, a lecturer from my laboratory. To mount a thin film sample for transmission electron microscopy, load the sample onto a double-tilting transmission electron microscope sample holder, and insert the holder into a transmission electron microscope equipped with a scanning mode and an energy-dispersive x-ray detector. After a routine transmission electron microscope beam alignment procedure has been performed, click Attachment Scanning Image Display to navigate to STEM mode.

To perform an optical axis alignment, click the Rocking, and then click the Spot to stop beam rocking motion. When the rocking has stopped, remove the sample from the field of view, and use the magnification arrow buttons to set the beam rocking range to less than plus or minus two degrees. Turn the Brightness knob clockwise to the limit, and adjust the Object Focus Coarse knob counterclockwise to an under-focused condition.

A caustic spot will appear on the fluorescent viewing screen. Press Bright Tilt, and use the Deflector knobs to move the caustic spot to the center of the fluorescent screen. Press the Standard Focus button, and turn the Brightness knob counterclockwise until an alternative caustic spot appears on the fluorescent screen.

Press F3, and use the Deflector knobs to move the beam spot to the center of the screen. Then repeat the just demonstrated optical alignment steps until the beam position remains in the center, even if the lens condition is switched. To collimate the incident beam, first insert the third largest condenser aperture by turning the Aperture knob clockwise, and then adjust its position manually to the center of the optical axis using two attached screws.

And use the Brightness knob in conjunction with the Deflector knobs and the condenser stigmator to adjust the condenser lens stigmator until the beam shape is coaxially focused. Press High Tension Wobbler, and adjust the Brightness knob to minimize the beam size fluctuation with the change in acceleration voltage to adjust the beam convergence angle to a minimum. Press High Tension Wobbler again to stop the high tension wobbler.

To set the pivot point, activate the maintenance mode according to the manufacturer’s instructions, and select JEOLS, Scan/Focus, and Scan Control. After clicking Correction and Scan, use the Deflector and Object Focus Fine knobs to minimize the beam shift with the beam rocking. Then use the Z control keys to match the sample and pivot point height so that the sample is focused on the fluorescent screen.

To perform a final beam alignment to obtain an electron-channeling pattern for the sample, move the sample area of interest back to the center, and click Scan to start the beam rocking. Manually turn the annular dark field detector cylinder clockwise, and insert the detector. Adjust the Deflector knobs while holding the PLA key to set the detector position to the center of the beam position, and check STEI-DF.

An electron-channeling pattern will appear. Adjust the brightness and contrast to optimize the view of the pattern, slightly turning the Brightness knob to obtain the sharpest contrast as necessary. To collect the energy-dispersive x-ray spectra, in the beam rocking mode, use the spectral imaging method as a function of the beam tilting angles in the x and y directions to display the elemental intensity distribution for specified elements.

To obtain an ionizing-channeling pattern, use the line scan function to perform a 1D tilting measurement of a systematic row of reflections. Yellow arrows will appear in the electron-channeling pattern preview to specify the measuring range. Stop the measurements when sufficient data statistics are obtained.

In these representative images, experimental electron and ionization-channeling patterns for barium titanate, barium L, barium K alpha, and oxygen K alpha near the 100 and 110 zone axes, respectively, are shown. Here, the electron and ionization-channeling patterns of calcium K, tin L, O-K, europium L, and yttrium L for the europium yttrium co-doped calcium tin oxide sample near the 100 zone can be observed. The europium lanthanum ionizing-channel pattern in this analysis was closer to the calcium K pattern while the yttrium L pattern was closer to that observed for tin L.These data suggest that the europium and yttrium occupation sites could be biased, as expected.

The site occupancies of the impurities and the impurity concentrations of all of the samples are indicated in the table. As observed, for europium alone doped calcium tin oxide, europium occupied the calcium and tin sites equally, consistent with the results of the x-ray diffraction, Rietveld analysis. In contrast, europium and yttrium occupied the calcium and tin sites in the co-doped samples at ratios of approximately seven to three and four to six, respectively, significantly biased, as expected, while maintaining the charge neutrality condition within the present experimental accuracies.

It is important to carefully observe the edge of the beam and sample to determine the optimum in-focus conditions, although a final slight adjustment is possible. If you do not have a beam rocking mode in your TEM, a software plugin called QED, which runs on Gatan Microscopy Suite, can implement the same scheme.