Fabrication And Optical Properties Of Mg_xZn_1-xO Hexagonal Nanocrystal Films

Zinc oxide is a II-VI wide band-gap (3.37 eV) compound semiconductor with wurtzite crystal structure. Due to the large exciton binding energy of 60 meV, which ensures the high efficient excitonic emission at room temperature, it is regarded as one of the most promising materials for fabricating efficient ultraviolet (UV) and blue light emitting devices. With the development of band gap engineering, people hope to find a material with similar crystal structure, crystal lattice parameters to ZnO, and even larger band gap. Alloys of this material and ZnO can form heterostructures, quantum wells and superlattice,which not only can improve the luminescent efficiency, but also can tune luminescent characteristic of ZnO. MgO whose band gap is 7.7 eV, can meet the above requirement. The band gaps of MgxZn_1-xO alloys could be tuned from 3.37 to 7.7 eV, which can be used as barrier layers in ZnO/MgxZn_1-xO quantum-well and superlattice devices.MgxZn_1-xO hexagonal nanocrystal films with wurtzite-type structure were fabricated on quartz substrates by electron beam evaporation (EBE) using Mg0.15Zn0.85O target combined with thermal annealing followed with rapidly cooling. The dependence of the microstructure and optical property of MgxZn_1-xO hexagonal nanocrystals on the annealing temperature has been investigated using X-ray diffraction (XRD), absorption, photoluminescence (PL) and x-ray photoelectron energy spectra (XPS).We also discussed the formation mechanism from scanning electron microscopy (SEM) and x-ray photoelectron energy spectra.The Mg concentration x of MgxZn_1-xO hexagonal nanocrystals was tuned in the range of 0.05-0.08 by changing annealing temperature from 300 to 700 oC. An evident blueshift and enhanced near-band-edge (NBE) ultraviolet emission were observed in photoluminescence spectra for the MgxZn_1-xO hexagonal nanocrystals. The relative maximum intensity of the green emission was observed for the MgxZn_1-xO hexagonal nanocrystals with the Mg concentration of x=0.08 and the visible emission peaks were red-shifted to lower energy side with increasing Mg concentration x.The results showed that the UV emissions mainly derived from the localized exciton recombination by fitting the photoluminescence spectra at different temperatures. The activation energy was 53 meV for the localized excitons. The interaction between excitons and LO phonons was weak, the inhomogenous broadening of PL peaks origined from the spatial fluctuation of Mg composition in the alloys and the surface defects of MgxZn_1-xO hexagonal nanocrystals.The formation of MgxZn_1-xO hexagonal nanocrystals were due to the transverse inter-diffusion along substrate surface and alloying of Zn and Mg ions in ZnO/MgO composites during the annealing process. The enhancement solubility limit of MgxZn_1-xO alloys was attributed to the non-thermal equivalent nature and the increase of the solubility limit of MgO in ZnO due to rapid cooling.MgxZn_1-xO hexagonal nanocrystal films fabricated by electron beam evaporation combined with thermal annealing followed with rapid cooling are expected to give a new method to design ZnO-based semiconductor optoelectric devices.