The Investigation Of Microstructure And Physical Properties Of V-doped ZnO Thin Films Prepared By RF Magnetron Sputtering

Zinc oxide is a Ⅱ-Ⅵ compound semiconductor material with large direct band gap of³.³7eV. Due to its special optical, electric and magnetic behaviors, ZnO films have wideapplication in TCO, LED, DMS, varistors and piezoceramics, etc. It is expected to be used formultiple functional electric devices by proper doping. In present work, ZnO:V thin films weredeposited on glass substrate by radio frequency magnetron sputtering technique using V₂O₅doped ZnO ceramics target with different vanadium content. The ultraviolet-visibletransmittance, band gap, microstructures, surface morphologies, room temperatureferromagnetism, V-I characteristics and dielectric properties were characterized byultraviolet-visible spectrophotometer, XRD, SEM, VSM, V-I tester and impedance microscopy.After systematic research, the conclusions were drawn as follows:（1） ZnO:V thin films were polycrystalline films with c-axis orientated wurtzite structure.The crystals were nano-sized. No second phase was detected in the films. The substratetemperature, sputtering pressure and oxygen partial pressure dramatically influenced the filmcrystallinity, orientation, crystal size, surface morphology, and film stress. SEM revealed thatwith the increase of substrate temperature, the surface morphology varied from spherical grainsat room temperature to irregular grains formed during150°C-³00°C, typical hexagonalpyramid grains at450°C, and tiny grains at600°C. The XRD results revealed that the crystalsize first decreased and then increased with the increase of substrate temperature and thesmallest value was obtained at150°C. The increase of sputtering pressure decreased the grainsize and surface roughness, while the crystal size increased, suggesting that surface grains werepolycrystalline. The increase of oxygen partial pressure reduced the deposition rate, as well asthe crystallinity. The surface grain size and crystal size reduced as the oxygen partial pressureincreased. In addition, the doping content also influenced the film microstructure andcrystallinity. The microstructures comparison of0.5mol%and1.5mol%V₂O₅doped ZnO filmssuggested that the increase of V doping deteriorated the film crystallinity and reduced thecrystal sizes.（2） The transmittance spectra in ultraviolet-visible region showed that all the ZnO:V filmsexhibit excellent transmittance above80%. The band gap of ZnO:V films derived from thetransmittance spectra was smaller than that of bulk pure ZnO.1.5mol%V₂O₅doped ZnO filmshave larger band gap than that of0.5mol%V₂O₅doping.（³） The electric behavior of ZnO:V suggested that oxygen partial pressure, substratetemperature and electrodes materials heavily influenced the V-I characteristics. The filmsdeposited in pure Ar nearly exhibited ohmic V-I behavior, while the ones deposited in themixture gas of argon and oxygen showed non-linear V-I behavior. With the increase of oxygenpartial pressure, the non-linear behavior firstly strengthened and then receded. The best non-linear behavior was obtained at the oxygen partial pressure of50%, at which the non-linearcoefficient can reach5.2and10.5, respectively for Al/ZnO:V/Al and Ti/ZnO:V/Ti multilayersamples. The switch voltage can reach2.1and2.26V for Al/ZnO:V/Al and Ti/ZnO:V/Timultilayer samples, respectively at oxygen partial pressure50%and75%. The study about theinfluence of film thickness showed that the multilayered sample showed ohmic V-I behaviorwhen the ZnO:V film was thinner than150nm, while non-linear behavior when thickness lagerthan150nm. Overall, the varistor behavior of ZnO:V films was attributed to the grain boundarybarrier formed by the coactions of V segregation and oxygen excess. The segregated V atomsbonded with the absorbed O atoms at grain boundaries and formed donor-like states. Thesestates trapped electronics from grains and formed a barrier so that the samples exhibited varistorbehaviors.（4） Pronounced ferromagnetism was observed in the ZnO:V films. The substratetemperature, sputtering pressure, oxygen partial pressure and V content heavily influenced themagnetic behavior of the ZnO:V films. For the0.5mol%V₂O₅doped ZnO films, magneticmoment were in the range27.15-40.87emu/cm³，and coercivity between126-³78Oe, as thesubstrate increased from room temperature to600°C. With the V content increased, themagnetic moment decreased due to the deterioration of films crystallinity. When deposited inpure Ar atmosphere at300°C, the variation of sputtering pressure within0.5-4.0Pa deducemagnetic moment in the interval of1.16-13.312emu/cm³. Keeping other parameters constant,with the increase of oxygen partial pressure, the film magnetic moment increased first and thendecreased at50%. The ferromagnetic behavior of ZnO:V thin films was considered to beassociated with the type and concentration of intrinsic defects in the films.