The influence of growth temperature on structural and optical properties of sputtered ZnO QDs embedded in SiO2 matrix

Abstract

We report the influence of growth temperature on surface morphology, structural and optical properties of ZnO QDs embedded with SiO2/Si matrix fabricated by radio frequency (RF) magnetron sputtering method. The fragmentation due to elastic strain relaxation compensate adatom diffusion length at higher temperature causes the decrement of the ZnO QDs sizes from ~41 nm to ~12 nm and number density enhancement from ~0.2 to ~15.4 × 1010 cm-2 by increasing the growth temperature. ZnO QDs shows a well-defined hexagonal close packed wurtzite structure with lattice parameters close to those of bulk ZnO, confirming their high crystalline quality. Increasing growth parameters causes to decrease the lattice parameters due to change in interatomic distances explained by elimination of defects and structural relaxation. The room temperature photoluminescence (PL) spectra shows strong UV accompanied by weaker green peak originated from recombination of free excitons and dominative deep-level emissions respectively. As the growth temperature increased to 500°C, an emission intensity in the ultraviolet and green region enhanced which is due to increment in the numbers of photo-carriers by increasing the number density of QDs. Calculated band gap using an optical transmittance measurement indicates that the bandgap shift to higher energies is not taken placed because of large size of dots. The Urbach energy increases considerably in samples with increasing growth temperature which is attributed to higher degree of surface disorder in smaller QDs. The excellent features of the results suggest that our systematic analysis method may constitute a basis for the tunable growth of ZnO QDs suitable in nanophotonics.