Hole energy spectrum in Ge/Si(001) heterostructures grown by molecular-beam epitaxy are studied using capacitance spectroscopy at a temperature range of 4.2-300 K. We find that the formation of Ge islands as the effective film thickness exceeds six monolayers lends to the appearance of the zero-dimensional hole states associated with Ge quantum dots. Analysis of the capacitance-voltage characteristics of structures containing the quantum-dot 'atoms' and the quantum-dot 'molecules' reveals the Coulomb charging effect.