Extended defect states of Ge/Si quantum dots using optical isothermal capacitance transient spectroscopy

We investigated the hole emission processes of optically induced charges on the defect states and confined states of self-assembled Ge quantum dots (QDs) embedded in a p-i-n Si diode. Optical deep level transient spectroscopy (ODLTS) and optical isothermal capacitance transient spectroscopy (OICTS) were used to study the defect states in ten stacked Ge quantum dots. Using ODLTS and OICTS for QD-embedded samples, the peaks related to the defect states of Ge QDs could be classified distinctly; it was about 20-50 times higher in intensity than that for the bulk defect states. The charges emitted from the QD defect state were observed near 93 K, and the activation energy was calculated to be E(V)+177 meV. The defect state followed the logarithmic capture kinetics and the Arrhenius-determined apparent activation energy decreased in the band gap as the optical injection width increased. We suggest that Ge QD defect states in Si could exist as extended states.