VERTICAL CORRELATION/ANTICORRELATION TRANSITIONCompositional Non-uniformity and Spatial Correlation in InGaAs Quantum Dots Studied by Cross-Sectional Scanning Tunneling Microscopy*
Dept of Physics, The Univ of Texas, Austin, TX
In this talk, I will describe structural studies of InGaAs quantum dots using cross-sectional scanning tunneling microscopy (X-STM). The talk will focus on two issues: (a) the compositional variations within individual quantum dots and in the wetting layers, and (b) the spatial correlation of the quantum dots (QDs) grown using migration enhanced epitaxy (MEE). In terms of the composition variations, we have found that for well-developed quantum dot, the In-concentration is very non-uniform, with the In-rich cores showing inverted triangle shape. In addition, the neighboring wetting layer also shows composition gradient: the closer to the quantum dot base, the lower the In concentration. Furthermore, this compositional gradient in the wetting layer also scale with the size of quantum dot. The implications on the growth mechanism and electronic properties will be discussed. In terms of the spatial correlation for multi-stack quantum dots, we found that depending on the growth condition, the quantum dots can be vertically correlated or anti-correlated. The controlling parameter appears to be the inter-stack spacer thickness. Comparison with the current theoretical models for anti-correlation phenomena will be discussed.
This work was supported by the National Science Foundation, DMR 0071893 and the NSF- Science In and Technology Center, CHE-8920120.
* In collaboration with X.-D. Wang, N. Liu,, S. Govindaraju, A. L. Holmes Jr. and J. Tersoff.