The Er3+ photoluminescent properties of Er-doped Si/SiO2 superlattices in which the location of Er atoms and layer thickness were controlled with sub-nm precision are investigated. The superlattices were deposited either by ultra-high vaccum(UHV) ion beam sputter deposition or electron-cyclotron resonance plasma-enhanced chemical vapor deposition(ECR-PECVI) method with a subsequent anneal at 950 degreesC. Er was doped only into the SiO2 layers whose thickness was fixed. The Er-carrier interaction was controlled by either depositing nm-thin buffer layers of pure SiO2 or by varying the thickness of the Si layers from 0.6 to 3.6 nm. The structure and the composition of the films were confirmed using transmission electron microscopy(TEM) and medium energy ion scattering spectroscopy(MEIS). We find that the Er3+ luminescence increases very strongly as the buffer layer thickness is increased, even though the excitation of Er3+ ions occurs through carriers generated in Si layers. Furthermore, Er3+ luminescence can be increased even further by decreasing the Si layer thickness down to the limit of one monolayer of Si. This demonstrates that with sub-nm scale dimensional control on the environment of Er atoms, we can achieve substantial enhancement in Er3+ luminescence.