Growth and characterization of green electroluminescent thin films

Growth and characterization of Pulsed laser deposited and sputter deposited green Zn₂GeO₄:Mn and electron beam evaporated Zn _1−xMg_xS:Mn thin films were investigated.; Experimental results show that electroluminescent brightness of Zn ₂GeO₄:Mn grown by sputter deposition was independent of working pressure and Ar/O₂ ratio. Green photo- and electroluminescence were obtained from Zn₂GeO₄:Mn films deposited on Si (for photoluminescence only), glass/ITO/ATO and Al₂O₃/Au/Pb(ZrTi)O ₃ substrates with the emission wavelength at 540 nm and CIE color coordinate at x = 0.263 and y = 0.683. The Zn₂GeO₄:Mn electroluminescent film grown on a dielectric layer of Pb(ZrTi)O₃ by pulsed laser deposition showed much higher brightness than that grown by sputter deposition (450 cd/m2 versus 120 cd/m2 at 2.5 kHz) primarily due to better film crystallinity resulting from higher substrate temperature (250°C versus R.T.) and from surface damage resulting from high energy Ar plasma. Zn₂GeO₄:Mn deposited on Al₂O ₃/Au/Pb(ZrTi)O₃ substrate showed higher EL brightness (450 cd/m2 versus 45 cd/m2 at 2.5kHz) than that deposited on glass/ITO/ATO due to better crystallinity resulting from a longer annealing time, and higher breakdown electric field for the thicker Pb(ZrTi)O₃ layer. The Zn₂GeO₄:Mn grown by pulsed laser deposition was Zn-deficient (Zn/Ge atomic ratio = 0.83 and 0.77 at a substrate temperature of 250°C and 800°C). A reaction of Zn2GeO4:Mn and Pb(ZrTi)O₃ was observed upon annealing at temperature over 800°C. Optimum annealing for Zn₂GeO₄:Mn on Pb(ZrTi)O₃ was obtained using a temperature of 700°C for 5 hours. The Zn₂GeO ₄:Mn grown on Pb(ZrTi)O₃ substrate showed extremely poor EL brightness when the film thickness was smaller than 5000Å. This was hypothesized to be due to a reduced voltage drop across the phosphor layer because Zn₂GeO₄:Mn was not continuous. In addition, a large leakage current resulted from the rough Zn₂GeO₄:Mn/Pb(ZrTi)O ₃ interface.; By partially substituting Mg for Zn, Zn_1−xMg_xS:Mn electroluminescent emission showed a green shift compared with the yellow/orange of ZnS:Mn due to crystal field reduction. Green photo- and electroluminescence were obtained from Zn₂GeO₄:Mn film deposited on glass/ITO/ATO substrate with the emission wavelength at 578 nm and CIE color coordinates at x = 0.5049, and y = 0.4900 versus ZnS:Mn emission at 592 nm, and the CIE coordinates at x = 0.5451, y = 0.4529). Film doped from a MnS source-doping exhibited EL brightness of 40 cd/m2 for 6700Å while those doped from metallic Mn source exhibit 90 cd/m2 for 13500Å both at 60Hz. Optimum Mn doping of 0.12% in the ZnMgS:Mn film was determined using SIMS. Using cross section TEM, columnar ZnMgS crystallites were observed in Zn_0.8Mg_0.2S:Mn.