Evolution of InAs quantum dot shape on GaAsB

InAs quantum dots (QDs) were grown by molecular beam epitaxy on GaAs ( 1 ¯ 1 ¯ 4 ¯ ) B surfaces and studied by in situ scanning tunneling microscopy (STM). Atomically resolved STM images, obtained for the QDs of different size, reveal that the QDs exhibit different shapes at different stages of evolution. First, small islands develop composed mainly of flat { 1 ¯ 3 ¯ 5 ¯ } B and ( 1 ¯ 1 ¯ 2 ¯ ) B facets. Next, steeper { 2 5 ¯ 11 ¯ } A and { 2 ¯ 5 11 ¯ } A facets form at the { 1 ¯ 3 ¯ 5 ¯ } B and ( 1 ¯ 1 ¯ 2 ¯ ) B facets. Then a remarkable shape transition occurs with the aspect ratio increasing from 0.10 to 0.20. The QDs grow mainly in height now by stacking { 2 5 ¯ 11 ¯ } A layers so that new, steep { 0 1 ¯ 1 ¯ } and ( 1 ¯ 1 ¯ 1 ¯ ) B facets appear at the side and the top { 2 5 ¯ 11 ¯ } A facets shrink and finally vanish. The mature QD is terminated by five steep facets besides the flat base. Further growth results in an elongation of the mature QD along the symmetry direction.