Study of hole accumulation in individual germanium quantum dots in p-type silicon by off-axis electron holography

Epitaxial germanium quantum dots (QDs) embedded in boron-doped silicon have been studied using off-axis electron holography to estimate the number of holes associated with a single QD. Holes were confined near the base of the pyramidal, 25-nm-wide Ge QDs. The resulting estimate of charge density was 0.03 holes/nm3 which corresponded to about 30 holes localized to the investigated dot. For comparison, the average number of holes confined to each Ge dot was found to be about 40, using a capacitance-voltage measurement. The difference emphasizes the value of measuring charges confined to individual nm-scale regions of a heterogeneous sample.