Electronic states inGe∕Siquantum dots with type-II band alignment initiated by space-charge spectroscopy

Abstract

Space-charge spectroscopy was employed to study electronic structure of a stack of four layers of Ge quantum dots (QD's) coherently embedded in an $n$-type Si(001) matrix. Evidence for an electron confinement in Si in the vicinity of neutral Ge dots was found. From the temperature- and frequency-dependent measurements the electron binding energy was determined to be $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. Existence of localized electronic states is explained by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried Ge dots. To support experimental results we performed numerical analysis of three-dimensional strain distribution and electronic structure of the sample under investigation. The strain distribution was found in terms of atomic positions using a valence-force-field model with a Keating interatomic potential. The electronic energy levels were calculated by solving a three-dimensional effective mass Schr\"odinger equation. The carrier confinement potential in this equation is modified by the strain distribution. The calculated confined eigenenergies agree with our experimental values deduced from the admittance spectroscopy. The data obtained in this work may serve as a guideline to interpret efficient photo- and electroluminescence as well as enhanced quantum efficiency of photodetectors and solar cells with Ge QD's stacked in a multilayer structure.