Optical and structural properties of Zn1-xMgxO ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn1-xMgxO composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein- Moss effect which causes a significant increase of electron concentration (2.89 x 10(18) - 5.19 x 10(20) cm(-3)). The luminescent properties of the Zn1-xMgxO pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn1-xMgxO composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities' emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X-A), neutral donor-bound excitons ((DX)-X-0) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn1-xMgxO composites is intimately connected to the film composition and surface morphology.