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

Session

PSA.3 - Semiconductors & Devices

Authors

Dr Thomas Walther (1)

Affiliations

1. University of Sheffield

Keywords

monolayers, segregation, STEM, X-ray mapping

Abstract text

We investigate AlGaAs/GaAs superlattices as well as InGaAs/GaAs quantum wells (QWs) 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 in a scanning transmission electron microscope (STEM) in a JEOL F200 with cold field-emssion gun and two sillicon drift detectors that give a combined X-ray collection angle of 1.7srad is capable to detect 0.3-0.4 fractional Al monolayers with a resolution of just under 1nm.

   

Figure 1: X-ray maps of  Al_K, Ga_K, In_L, As_K (from left to right). 

A key problem has been that compositional maps never gave zero base line values for elements not in the sample. We therefore used the minimum experimental background found for any element not in the sample (Ti K) to subtract 1.3 and 0.7 counts for the two different samples. This worked well for the In L line but less so for the Al K line, and we need to investigate further whether this problem is due to insufficient shielding to stray X-rays (a possible collimator problem), the line fitting routine used (a software problem) or simply due to the fact that all signals recorded are positive definite (i.e. non-negative) maps that cut off small negative while retaining small positive values, thereby skewing  the background by some small amount.

References

[1] T Walther, J Nutter, JP Reithmaier and E-M Pavelescu, Semicond Sci Technol (2020) under review