Positron annihilation signals from VMCO-like samples grown by atomic layer deposition at different temperatures are utilized for the characterization of differences in open volume defects in TiN/TiO 2 /a-Si heterostructures. Doppler and coincidence Doppler mode of positron annihilation spectroscopy combined with a monoenergetic positron beam were used for this study. Differences observed in the Doppler parameters indicate differences in the positron trapping states of the TiO 2 epilayers grown at different temperatures. Furthermore, the coincidence-Doppler results show that these differences cannot be due to intermixing of the TiO 2 and a-Si layers and formation of thin SiO 2 layers at the interface during the growth process. The results indicate that the amount of open volume defects in the TiO 2 layer of the VMCO-structure seems to increase with an increase in the growth temperature. [ABSTRACT FROM AUTHOR]