Preparations And Optical Characteristic Of Cr-doping ZnO Thin Films

Wurtzite-structured ZnO of wide-band-gap semiconductor and a large excitation binding energy at room temperature is a perfect performance semiconductor material for optoelectronic devices. In the past decades, ZnO has aroused a lot of research attentions in many domains, such as physics, chemistry, material and electronic. Recently using the controlling technique, 3d transition-metal ions have been doped in ZnO film to make the diluted magnetic semiconductors (DMS). Presently more and more people have been interesting in DMS. However, the study in this field is just on the initial stage in China. We discuss that the structure, morphology and photoluminescence of ZnO:Cr films. For the development and application of the ZnO (DMS), these studies are promising.Magnetron sputtering has been the most popular technique for the growth of many materials, especially for ZnO films. In this work, pure and doped ZnO films with 2%, 3%, 5%, 6% and 7% chromium with a preferential c-axis orientation were prepared on glass substrates by magnetron sputtering. The effect of the parameters of doping concentration, substrate temperature, annealing temperature and annealing atmosphere on the surface morphologies, microstructure and optical characteristics were studied respectively.With the increase of chromium doping concentration, the peak position of the ZnO(002) plane is shifted to the high 2θvalue, interplanar spacing and lattice constant c decreased. It is found that ultraviolet emission peak and green emission band located at 375 nm and 520 nm by photoluminescence (PL) measurement at room temperature. The former PL originates from the free exciton radiative recombination and the later is generated from defect levels of oxygen vacancies and zinc vacancies. With the increasing of chromium doping, the intensity of the ultraviolet emission and green emission decreases, while the band-gap of the ultraviolet emission shifts to lower wavelength. It indicates that Zn0.98Cr0.02O thin film has better crystallization quality when the substrate temperature is 400℃. And the absorption edges have nothing with the substrate temperature. With the increasing of annealing temperature, the intensity of the ultraviolet emission increases, but the green emission increases firstly, then decreases when Zn0.98Cr0.02O thin films anneal at in oxygen. Such changes are usually attributed to better crystalline quality effected by annealing. There will be a lot of oxygen defect when ZnO thin films anneal in air. So the green emission caused by oxygen defect will be decrease even vanish when the ZnO:Cr samples anneal at 600℃in oxygen. And we can obtain superior characteristics of luminescence.