V2O5 and WO3 or Ta2O5 mixed powders were pressed as sputtering target. The doped vanadium oxide thin films were deposited on SiO2 substrates by a modified Ion Beam Enhanced Deposition (IBED) method. The XRD results showed that IBED polycrystalline doped VO2 films had a single <002> orientation, and the crystal lattice constant d was elongated about 0.34% and 0.24% for W- and Ta-doped films relative to un-doped VO2 power. Temperature of phase transition from semiconductor phase to metal phase was 32°C and 48°C, and the phase hysteresis was 2.1°C and 1.2°C; the resistivity change magnitude was 1.6 and 1.2 and the temperature coefficient of resistance (TCR) of the doped films at 300K was 10.1%/K and 3.4%/K for V0.97W0.03O2 and V0.97Ta0.03O2 polycrystalline films respectively. The reasons would be synthetically in consideration of many factors, such as the differences of atomic size, valence number and bond length between W and Ta. The effect of W doping was more evident than the effect of Ta-doing.
The new sol-gel derived multi-layer LiTaO3 thin films with Ta2O5 buffer layer were prepared on Pt/Ti/SiO2/Si substrate using lithium ethoxide and tantalum ethoxide as starting materials. The sol of Ta2O5 was firstly covered on the substrate by spin coating at 6500rpm for 50s and then a rapid annealing at 650°C for 2min to form an about 20nm thick Ta2O5 buffer layer. Multi-layer LiTaO3 thin films were made over Ta2O5 buffer by repeated spin coating at 4000rpm for 30s and then a rapid annealing process at 700°C for 3min. The spectrum of XRD show the crystalline orientation of thin film type Ta2O5 is different compared to powder type Ta2O5. The SEM micrograph of the cross section shows the prepared sample is uniform, smooth and crack-free on the surface and the thickness of LiTaO3 thin film is 0.341μm. The ferroelectric hysteresis loop and leakage current of the prepared sample have been measured using Al/LiTaO3/Ta2O5/Pt structure electrode by a ferroelectric material analyzer PLC-100. The remanent polarization and coercive field of the prepared sample polarized at 13V were 3.4μC/cm2 and 185kV/cm respectively. The leakage current of the prepared sample was 2.66x10-7A at 71.43kV/cm .Experimental results show the prepared sample of LiTaO3 thin film with Ta2O5 buffer has good ferroelectric properties. Ta2O5 buffer introduction between LiTaO3 thin film and Pt substrate can effectively decrease the leakage current and improve the properties of uncooled LiTaO3 infrared device.
In-N codoped ZnO nanocrystalline films were prepared on Si and glass substrates by Ion beam enhanced deposition method. The In-N codoped ZnO nanocrystalline films deposited on Si substrates are found to have a preferred (002) orientation, smooth surface and high density. P-type ZnO thin films were obtained. Room temperature photoluminescence measurements indicated that the ZnO nanocrystalline films had two UV emission peaks and several visible emission peaks.
V2O5 and Ta2O5 mixed powders were pressed as sputtering target. Ta-doped vanadium oxide thin films were deposited on SiO2 substrates by modified Ion Beam Enhanced Deposition (IBED) method. The VO2 film with a preferential orientation (002) was formed after post-annealing in nitrogen or argon atmosphere. The measurement results of resistance dependence on temperature indicated that the vanadium dioxide films showed a typical characteristic of phase transition. The phase transition temperature of the IBED VO2 film doped with 3 atm.% Ta decreased from 68oC to about 48oC. The reasons why the Ta-doping decrease the phase transition temperature were as followed: the atom radius of Ta larger than that of V atom, which introduce strain in the grain; the replacement of When V4+ replaced by Ta5+ the ionic bond could elongate and introduce superfluous electrons in the d valence shell, then make the gap of d energy band to decrease.
A new method of modified Ion Beam Enhanced Deposition (IBED) was designed to form VO2 polycrystalline film with good properties. While argon ion beam sputtered V2O5 powder target to deposit vanadium oxide film, a high current Ar+/H+ mixing beam with a high dose was implanted into the deposited film. The VO2 film with the Temperature Coefficient of Resistance (TCR) as high as 4%/K was obtained after subsequent appropriate annealing at the temperature above 500°C. The formation mechanism of the IBED VO2 film was discussed as following: the damage effect of the argon ion beam implantation broken some of V-O bands; The deoxidization effect of implanted hydrogen reduced V2O5 to VO2. The mixing effect could make IBED film adhere to the substrate firmly. The doping effect of the implanted argon introduced stress in the film to decrease the phase transition temperature. The bombardment effect made the film more compact, decreased oxygen vacancy density, reduced the grain boundary width, and increased the TCR of the IBED VO2 polycrystalline films.
The plasma surface treatment and ion implantation are utilized to improve the stability of charge storage of the SiO2 film electret. The effects are obviously different when the different kinds of plasma are used, and when the ions with different energy and dose implanted into the SiO2 film. To the plasma treatment, the best effect is obtained with argon plasma. After argon plasma treatment with the arcing at 700V and 15 minutes for the SiO2 film electret, its charge storage has a similar stability as one after the chemical surface modification. To the ion implantation method, the remanent surface potential is 95%of primary one after ion implantation by Ar+ with a energy of 150KeV and a dose of 2E11/cm2. The experimental results show that argon plasma treatment and Ar+ implantation change the hydrophilicity of the surface of SiO2 film and prevent the electret charge from leaking, which is owning to the increase of the surface conductance by the vapor adherence on the surface. In addition, the plasma bombardment and ion implantation induce traps into the near-surface, which make the trapped charge stored stable.
Different annealing conditions were adopted to anneal the vanadium oxide films prepared by modified Ion Beam Enhanced Deposition (IBED) method. An X-Ray Diffraction (XRD) was used to analyze the orientation of the IBED films and the resistance was tested with temperature change to measure the Temperature Coefficient of Resistance (TCR). Experiments indicated that there existed a critical temperature for crystallization of VO2, which changed with the different deposition conditions of the IBED method. It is very difficult to obtain VO2 structure if the annealing temperature was lower than the critical temperature. If the temperature is much higher than the critical temperature or annealing time is too long, the valence of vanadium in VO2 film will easily reduce from 4 to low value. The TCR of the IBED VO2 polycrystalline films annealed in appropriate condition could reach higher than 4%/K.
The phase transition characteristic of the vanadium dioxide (VO2) film prepared by ion beam enhanced deposition (IBED) method was studied. The lattice distortion hypothesis was supposed to simulate resistance change of the VO2 polycrystalline film with temperature increasing and the simulation result was explained based on Landau theory. Due to the present of argon atom in interstitial site of VO2 lattice or grain boundary, the semiconductor- to-metal phase transition began at 48°C in some grains, obviously lower than the phase transition temperature of VO2 single crystal.
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