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Characterization of Columnar InAs/GaAs Quantum-Dot Structures Using Grazing Incidence X-ray Diffraction

Grazing incidence X-ray diffraction (GIXD) measurement using equipment available for laboratories was realized for the evaluation of columnar InAs/GaAs quantum dots (QDs). The QDs were grown by direct stacking of small self-assembled islands in the growth direction on (001) GaAs substrates, and we prepared some samples by changing the number of units stacked. Using an imaging plate for a detector, we successfully observed that QD-related signals appeared around a GaAs(220) peak. The QD signals extended in the direction of the 2θ axis, and the extension increased with increasing number of stacks in the columnar QD growths. The results of the measurements were evaluated by a simulation based on the finite element method. Using a simple model that divides the QDs into an InAs columnar core and an InGaAs transition domain, we found that the core was expanded according to the increase in the stacking number. We also found that the lattice constant increased almost uniformly toward the upper part inside the QD. GIXD evaluation in laboratories will enhance the speed of the development of QD devices.