Investigation of phase segregation in sol–gel derived ZnMgO thin films

Highly c-axis-oriented Zn1-xMgxO multilayered thin films have been deposited on p-type Si substrates with different concentration of Mg (x = 0.00−0.40) using a sol–gel spin-coating technique. The x-ray diffraction (XRD) shows that single-phase wurtzite thin films start showing phase segregation for a Mg content of x = 0.25 for the sol–gel-derived ZnMgO thin films. The element specific near edge x-ray fine structure (NEXAFS) collected at O K-edge also clearly evidence the phase segregation at x = 0.25. These results also show that films are deposited with wurtzite structure as dominant phase even after phase segregation. The NEXAFS spectra collected at Zn L3-edge rule out the presence of any Zn-related defect due to Mg doping. The atomic force microscopy (AFM) depicts the spherical shape of nanosized grains, and grain size varies slightly with Mg content. The single-phase ZnMgO thin films show a band gap tuning from 3.38 to 3.84 eV, which is also consistent with blue shifting of near-band edge PL emission. The electrical resistivity of thin films increases with Mg content before phase segregation. However, the optical band gap, photoluminescence and electrical resistivity show anomalous behavior at phase segregation limit which has been discussed and correlated with each other. (paper)