Tunable optical properties in atomic layer deposition grown ZnO thin films

ZnO thin films with very low surface roughness and bulklike electron density were grown on Si and SiO2 by atomic layer deposition. The real and imaginary parts of the complex dielectric function of ZnO on Si show monotonically decreasing values with decreasing film thickness at and below a threshold of about 20 nm. On the other hand, x-ray reflectivity results show that the electron density of our ZnO films is close to that of bulk ZnO and does not vary considerably with film thickness. While the reduction of the dielectric function cannot be explained by the electron density of our ZnO films, the Tanguy-Elliott amplitude prefactor governing the strength of optical interband transitions can explain our results consistently through the lowering of the electron-hole overlap factor at the ZnO/Si interface. In the case of ZnO/Si, a staggered type-II (spatially indirect) quantum well, holes are scattered into the Si substrate, causing a lowering of the electron-hole overlap factor and thus the reduction of excitonic absorption, consequently a decrease in the real and the imaginary parts of the dielectric function. This hypothesis was confirmed with ZnO films grown on SiO2, where a thin type-I quantum well, consisting of a narrower-bandgap semiconductor grown on a wider-bandgap (insulator) substrate, in which both the electron and the hole are confined in the ZnO thin film, leads to an increase in the electron-hole overlap matrix element with decreasing film thickness due to confinement, resulting in enhancement of the excitonic absorption in thinner ZnO films on SiO2. (C) 2016 American Vacuum Society.