X-ray mapping in a scanning transmission electron microscope of InGaAs quantum dots with embedded fractional monolayers of aluminium

Abstract

We investigate AlGaAs/GaAs superlattices as well as InGaAs/GaAs quantum wells and epitaxial quantum dots (QDs) where during the molecular beam epitaxy of InGaAs QDs the aluminium flux cell was opened briefly to incorporate fractional monolayers of Al into the InGaAs. We show that x-ray mapping with a large collection angle is capable of detecting 0.3-0.4 fractional Al monolayers with a resolution of just under 1 nm.

Measuring Non-Destructively the Total Indium Content and Its Lateral Distribution in Very Thin Single Layers or Quantum Dots Deposited onto Gallium Arsenide Substrates Using Energy-Dispersive X-ray Spectroscopy in a Scanning Electron Microscope

“…Furthermore, the In/Ga ratio from the X-ray maps in Figure 8c is much higher than expected from the spectrum, by a factor of 3-5, which is attributed to bit-rounding errors when generating non-negative integer distribution maps: the Ga map should have had a mean of only 0.2 counts (i.e., consisting mainly of pixels with 0 and 1 counts) but actually had a mean of almost unity. This indicates that full quantification of SEM-EDXS maps with low count rates may not be possible with standard commercial software, similar to that found for STEM-EDXS [39]. Still, the observed speckle patterns in Figure 8a,b agree in length scale.…”

“…The detection limit scales with 1/ √ t, where t is the acquisition time. Error bars under ± 0.1 ML, which have been obtained for buried InGaAs layers by TEM-EDX in cross-section geometry [39,[43][44][45] and by SEM-WDX for sulphur [46] will probably not be obtainable by SEM-EDX in plan-view geometry because this would necessitate about 100 million net counts for the As L edge, which, on this tabletop SEM, would take 50-60 hrs of data acquisition and is unrealistically long for quality control. SEM instruments with field-emission sources that generate higher beam currents and larger X-ray detectors that provide higher collection angles may be able to approach this limit, which, of course, would be highly desirable for quality control purposes.…”