The behaviors of holes emitted from double-layer quantum dots (QDs) have been studied by deep-level transient spectroscopy (DLTS). By comparing the DLTS spectra in different samples, the decreasing coupling effect with increasing spacer layer thickness between the double layers of QDs is revealed. When the spacer layer is less than 4.5 nm, the strong coupling effect merges the two QDs layers as one. When the spacer layer is 6 nm, the coupling effect reduces greatly, but the tunneling effect makes holes transfer easily between the two layers. If the spacer is over 7.5 nm, both of the coupling and tunneling effects can be neglected. (C) 2003 Elsevier B.V. All rights reserved.