Photoluminescence spectroscopy has been used to study the optical properties of multiple stacked Ge/Si quantum dots (QDs) with different thickness of Si spacers inserted between the Ge dot layers. According to the emission energy of the stacked Ge/Si QDs, we found that thinner spacer will lead to significant material intermixing. Such intermixing degrades the interface sharpness and the hole confinement depth of the dots. The thermal activation energy of PL intensity quenching for different spacer thicknesses also confirms this finding. We point out that thinner spacer is in fact detrimental to the emission properties of the stacked Ge/Si QDs. To obtain better luminescence efficiency at room temperature, the influence of the material intermixing on stacked Ge/Si QDs should be minimized. (C) 2003 Elsevier B.V. All rights reserved.