Formation and Strain Analysis of Stacked Ge Quantum Dots With Strain-Compensating Si_1−xC_x Spacer

To stack Ge quantum dots (QDs) in a multilayer structure without undesirable enlargement of the QDs, the effects of both Si_1−xC_x spacer on a strain compensation of the embedded QDs and a sub-monolayer (ML) carbon (C) mediation on a formation of the Volmer-Weber (VW)-mode Ge QDs on the Si_1−xC_x spacer were investigated. In a Si_1−xC_x/Ge/Si(100) structure, lattice rexation of the embedded QDs was kept about 80% at x = 0.015. This maintaining the state of high relaxation attributed to a tensile strain from the Si_1−xC_x layer grown on a surface of a Si substrate around the QDs. In addition, by utilizing an analysis of Kelvin probe force microscopy, it was revealed that the sub-ML C mediation of 0.25 ML and over is effective to form the VW-mode Ge QDs on the Si_1−xC_x spacer. This is because the promotion of subdivision effect for the formation of the QDs via C mediation was also effective on the Si_1−xC_x surface. At C = 0.25 and 0.5 ML, diameter and density of second QDs were about 22 nm and 1.5 × 10¹¹cm⁻², respectively. These results pave the way to stack the VW-mode Ge QDs in the multilayer structure without enlargement of the QDs.