Self-consistent method for quantifying indium content from X-ray spectra of thick compound semiconductor specimens in a transmission electron microscope

Composition Analysis by STEM-EDX of Ternary Semiconductors by Internal References

A practical method to determine the composition within ternary heterostructured semiconductor compounds using energy-dispersive X-ray spectroscopy in scanning transmission electron microscopy is presented. The method requires minimal external input factors such as user-determined or calculated sensitivity factors by incorporating a known compositional relationship, here a fixed stoichiometric ratio in III–V compound semiconductors. The method is demonstrated for three different systems; AlGaAs/GaAs, GaAsSb/GaAs, and InGaN/GaN with three different specimen geometries and compared to conventional quantification approaches. The method incorporates absorption effects influencing the composition analysis without the need to know the thickness of the specimen. Large variations in absorption conditions and assumptions regarding the reference area limit the accuracy of the developed method.

A bi-layer buffer system AlN/Al1−xInxN to enable the growth of high crystal quality Al0.36In0.64N thin films on Si (111)

We report a unique AlN/AlInN bi-layer buffer design to enable the growth of textured c-axis wurtzite Al_0.36In_0.64N epilayer on a Si (111) substrate, which creates the possibility to grow high crystal quality Al_0.36In_0.64N.

Self-consistent absorption correction for quantifying very noisy X-ray maps: group III nitride nanowires as an example

Energy-dispersive X-ray mapping in a scanning transmission electron microscope is a method to visualize the spatial distribution of chemical elements in a sample. Quantification of the signal intensities depends on proper background elimination and correction of the self-absorption and fluorescence of X-ray lines in the sample. The latter become particularly relevant for slightly thicker and rough samples, small take-off angles and low-energetic X-ray lines, for which we have recently introduced a self-consistent absorption correction based on effective k*-factors selected automatically from curves simulated as function of a K/L line intensity ratio of one of the heavier elements in the sample for ranges of different compositions. This has been shown to work well for thick and for rough samples. Correcting the background intensity to sub-pixel accuracy is necessary for reliable quantification of very noisy maps. In this study, we show how this self-consistent absorption correction method can be applied to InGaN and AlGaN layers in GaN nanowires, the net maps of which can be so noisy the layers can hardly be seen by the eye (a few counts per pixel) and the background is below a single count in each spectrum channel. The result indicates that background estimation for the Ga L-line intensity using fractional counts from an interpolation of maps from neighbouring X-ray lines of elements that are not actually present in the specimen is critical for quantification. The nanowires studied were between 66 and 375 nm thick.

LAY DESCRIPTION

Energy-dispersive X-ray mapping in a scanning transmission electron microscope is a method to visualize the spatial distribution of chemical elements in a sample. Quantification of the signal intensities depends on proper background elimination and correction of the self-absorption of X-ray lines in the sample. Here we show that our previously developed method of self-consistent effective absorption factors works well even with extremely noise elemental maps of a few net counts only where the human eye can hardly discern any pattern and the background signal is typically less than a single count in each spectrum channel. Correcting the background intensity to sub-pixel accuracy is then necessary for reliable quantification.

X-ray absorption in pillar shaped transmission electron microscopy specimens

The dependence of the X-ray absorption on the position in a pillar shaped transmission electron microscopy specimen is modeled for X-ray analysis with single and multiple detector configurations and for different pillar orientations relative to the detectors. Universal curves, applicable to any pillar diameter, are derived for the relative intensities between weak and medium or strongly absorbed X-ray emission. For the configuration as used in 360° X-ray tomography, the absorption correction for weak and medium absorbed X-rays is shown to be nearly constant along the pillar diameter. Absorption effects in pillars are about a factor 3 less important than in planar specimens with thickness equal to the pillar diameter. A practical approach for the absorption correction in pillar shaped samples is proposed and its limitations discussed. The modeled absorption dependences are verified experimentally for pillars with HfO₂ and SiGe stacks.

New pathways for improved quantification of energy-dispersive X-ray spectra of semiconductors with multiple X-ray lines from thin foils investigated in transmission electron microscopy

Theoretical approaches to quantify the chemical composition of bulk and thin-layer specimens using energy-dispersive X-ray spectroscopy in a transmission electron microscope are compared to experiments investigating (In)GaAs and Si(Ge) semiconductors. Absorption correctors can be improved by varying the take-off angle to determine the depth of features within the foil or the samples thickness, or by definition of effective k-factors that can be obtained from plots of k-factors versus foil thickness or, preferably, versus the K/L intensity ratio for a suitable element. The latter procedure yields plots of self-consistent absorption corrections that can be used to determine the chemical composition, iteratively for SiGe using a set of calibration curves or directly from a single calibration curve for InGaAs, for single X-ray spectra without knowledge of sample thickness, density or mass absorption coefficients.