Charge carrier traffic at self-assembled Ge quantum dots on Si

Germanium quantum dots (QDs) have been characterized by deep level transient spectroscopy (DLTS) and capacitance versus voltage (C–V) technique. Two types of dots, grown by molecular beam epitaxy (MBE) at different temperatures, were investigated and assessed with respect to morphological properties. Samples with dots grown at 350 °C, were designed as n++–p–p++ silicon junctions with the QDs positioned in the depleted p-region, while a second type of samples were Shottky diodes based on medium doped silicon with the QDs prepared at 550 °C and positioned in the Schottky depletion region. From the combined results of temperature scanned and frequency scanned DLTS, and by varying hole filling levels of the QD potentials, the energy distribution of states in the QD potentials were investigated. A wider distribution was found for the low-temperature QDs, probably related with a larger variation of size. By using a technique for separating tunneling and thermal hole emission, the average thermal activation energy for emitting holes to the valence band was found close to 0.40 eV for both types of QDs.

► Germanium quantum dots grown by MBE are characterized using two capacitance-based methods.
► The results provide information on the energy distribution of quantum dot states.
► Lower-temperature grown quantum dots had a wider distribution of energy states.
► The thermal activation energy for hole emission was found to be close to 0.40 eV.