Influencing factors on the size uniformity of self-assembled SiGe quantum rings grown bymolecular beam epitaxy

Abstract

The size uniformity of self-assembled SiGe quantum rings, which are formed by cappingSiGe quantum dots with a thin Si layer, is found to be greatly influenced by the growthtemperature and the areal density of SiGe quantum dots. Higher growth temperaturebenefits the size uniformity of quantum dots, but results in low Ge concentration as well asasymmetric Ge distribution in the dots, which induces the subsequently formedquantum rings to be asymmetric in shape or even broken somewhere in the ridge ofrings. Low growth temperature degrades the size uniformity of quantum dots, andthus that of quantum rings. A high areal density results in the expansion andcoalescence of neighboring quantum dots to form a chain, rather than quantum rings.Uniform quantum rings with a size dispersion of 4.6% and an areal density of7.8×108 cm − 2 are obtained at the optimized growth temperature of640°C.