Bandgap Engineering of ZnO by Alloying with MgO

During last decades, the band gap modulation of ZnO films has been achieved by alloying it with much wider band gap semiconductor material such as MgO (7.8eV). However, there exists a miscibility gap in the ZnO - MgO binary system due to the structural difference and the large lattice mismatch between ZnO (hexagonal wurtzite, 3.25Å) and MgO (cubic rock salt, 4.22Å). Mg⁺² ions can replace ~ 40% of Zn⁺² ions without changing the structure of ZnO and beyond that phase segregation between hexagonal ZnO and cubic MgO has been observed in Zn_1-xMg_xO alloy. We deposited ZnMgO thin films with sintered ceramic MgZnO targets having Mg concentration up to 25% on sapphire substrates by pulsed laser deposition (PLD) technique. X-ray diffraction measurements on these films revealed formation of single phase MgZnO with high c-axis orientation for all Mg concentrations. No peaks corresponding to segregation of MgO in these films was observed. Optical transmission studies performed with UV-VIS spectrometer showed ~80% transmittance in the visible region in all films with sharp absorption edge which shifted towards blue up to 3.72 eV. Photoluminescence (PL) measurements carried out at room temperature showed distinct PL peaks in the UV and visible region for ZnMgO. The peak occurring in the UV region can be attributed to band to band emission whereas the peak at visible region could be attributed to oxygen related defects.  A blue shift in the PL peak position of the band to band emission was observed as the Mg concentration was increased. Interestingly, with increase in Mg concentration the band to band transition emission peak intensity decreased systematically and disappeared for Zn_0.75Mg_0.25O. This behavior indicates that non-radiative recombination process is dominating with increase in Mg concentration. The resonant Raman studies indicated presence of several higher order longitudinal optical phonon modes. Further studies are underway.