Zno Doped Film Structure And Nature Study

ZnO, a direct band gap Ⅱ―Ⅵ semiconductor, is a novel photonic material with crystalstructure similar to that of GaN. Unique properties of ZnO are a direct bandgap of3.37eV atroom temperature(RT), large exciton binding energy of60meV and a high melting temperatureof2248K. The strength of Zn-O bond is larger than that of Ga-N. These proerties make ZnO agood material for optical devices such as blue and ultra-violet optical devices, light-emittingdiodes, laser diodes, and phosphorescent display, et al. ZnO films have been studied in the pastdecade, but the quality of ZnO is not high enough for the application, so how to obtain highquality ZnO films is a hot topic. Pulse laser deposition (PLD) is an effective method to obtainhigh quality ZnO. As a good semiconductor material, its properties are influenced greatly by thedefects and the impurities. Rare earth ion Eu3+is a good red luminescence center. Becaues of theexcellent electro-optic properties, ZnO is a good candidate for host materials of rare earthelement dopants Eu3+, and Li+ions often act as the low compensation charge and luminescencesensitizer. However, to our knowledge, Eu3+and Li+co-doped ZnO has not been thoroughlyinvestigated by PLD method yet. At present, the electrodes of ZnO-based devices are usuallynoble metals such as Au, Ag and Pt. Considering of the good conductivity andanti-electromigration of Cu and its low price, Cu films are used as electrodes. Our research onZnO films, Eu3+and Li+co-doped ZnO films and ZnO/Cu films with MSM structure are asfollows:1. ZnO films are deposited on Si (111) substrates by PLD under different substratetemperature, different oxygen flux and homo-buffer with different thickness. The films werecharacterized by X-ray diffraction (XRD) and photoluminescence (PL) spectra. XRD spectrashow that all films are (002) oriented, which indicates that all films are highly c-axis oriented. Asthe substrate temperature changes from room temperature to300℃, the FWHM of ZnO (002)diffraction peaks become narrower from0.39°to0.26°,but when the substrate temperaturereaches400℃, the FWHM of ZnO (002) diffraction peak is0.33°. With the oxygen fluxincreasing from0sccm to30sccm, the the FWHM of ZnO (002) diffraction peaks becomesnarrower at first and then becomes broader; When the the oxygen flux is10sccm, the FWHM ofZnO (002) diffraction peaks is the narrowest and the crystallinity of the sample is the best. Withthe buffer thickness increasing from0pulse to1000pulse, the crystallinity of the samplesbecomes better, but the crystallinity of the sample become worse when the buffer thicknessreaches1500pulse.When irradiated under the wavelength of350nm, photoluminescence (PL)spectra of all films exhibit three emission bands, including UV band peaking,blue band,andorange-red band. With the substrate temperature increasing, the peak intensity increases at first and then decrease, and the intensity reaches highest when the substrate temperature is300℃.The rule of PL is the same for the ZnO films prepared in different oxygen flux and differentbuffer thickness. The best oxygen flux is10sccm, and the best thickness of the buffer is preparedby1000pulses. All the spectra exhibit white emission measured by CIE standard, which make itpotentional application in phosphor for white LEDs.2. Eu3+and Li+co-doped ZnO films are deposited on Si (111) substrates under differentoxygen pressure and different annealing conditions, and the films were characterized by XRDand PL spectra.(1) For the Eu3+and Li+co-doped ZnO films prepared in different oxygen pressure, theresults show that the ZnO:Eu3+,Li+films are highly c-axis, and no other crystal orientations areobserved except the ZnO crystal (002) orientation, which indicates that the doping elements ofEu3+and Li+have been incorporated into the crystal lattice of ZnO. With the oxygen pressureincreasing, the FWHM of (002) diffraction peaks become narrower from0.79°to0.34°,whichshows that the crystallinity of the samples is improved; but when the oxygen pressure reaches3.0Pa, the crystallinity of the sample becomes worse. As the oxygen pressure increases, theinterplanar space of the films becomes smaller, but is larger than that of the bulk ZnO. Whenexcited at the wavelength of325nm, the emission spectra of all samples show a UV band and agreen band; with oxygen pressure increasing, the PL intensity increases at first and thendecreases, and0.3Pa is the best oxygen pressure for the samples. When irradiated under thewavelength of395nm, obvious emission peaking at about613nm from Eu3+is observed, whichis belonging to the electric dipolar transition5D0→7F2. The intensity of613nm increases sharplywhen the oxygen pressure increases.(2) For the Eu3+and Li+co-doped ZnO films annealed at different temperature, the XRDpatterns also reveal that the films are highly c-axis oriented and Eu3+ions have been successfullyincorporated into ZnO crystals. The FWHM of (002) diffraction peak of the sample annealed inoxygen is narrower than that of the sample annealed in vacuum, which indicates that thecrystallinity of the sample in oxygen is better, and550℃is the best annealing temperature invaccum.When excited at the wavelength of395nm, the characteristic emission of613nmbelonging to Eu3+is not observed, which shows that annealing is disadvantageous for the electricdipolar transition5D0→7F2of Eu3+, and with the annealing temperature increase, the spectra issimilar. These results is a base for the Eu3+and Li+co-doped ZnO films in potential application.3. The ZnO films and the Cu films were prepared via pulsed laser deposition method on Si(111) substrates. The XRD and SEM images of ZnO/Cu films were examined and thecurrent-voltage characteristics were measured. The results exhibit that ZnO films are highly c-axis oriented, and Cu films are highly (111) oriented. SEM shows that the grains of the Cu filmare obvious and uniform after annealing, which shows that the crystallinity of the Cu film isimproved. The Ohmic contact can be obtained when the ZnO:Cu layer is involved between theZnO film and the Cu film, and the Ohmic contact properties can be improved after annealing.Study on how to improve the properties of Ohmic contact of the ZnO/Cu films is still to becontinued. The results show that Cu may be used as Ohmic contact electrodes for ZnO-baseddevices instead of Au and Pt.