Bonding state of a hole in Ge/Si double quantum dots

The emission of holes from the bonding state of diatomic artificial molecules formed by vertically coupled Ge/Si(001) quantum dots is studied by the admittance spectroscopy method. It is found that, when the thickness of the Si barrier between Ge quantum dots exceeds 2.5 nm, the binding energy of a hole in an artificial molecule becomes smaller than the ionization energy of a single quantum dot. This result contradicts the predictions of the quantum-mechanical model of molecular bonds and testifies to the crucial role of mechanical stresses in the formation of the bonding orbital in a system of elastically stressed quantum dots.