Vanadium Dioxide Polycrystalline Films with High Temperature Coefficient of Resistance(TCR) were fabricated by
modified Ion Beam Enhanced Deposition(IBED) method. The TCR of the Un-doping VO2 was about -4%/K at room
temperature after appropriate thermal annealing. The XRD results clearly showed that IBED polycrystalline VO2 films
had a single [002] orientation of VO2(M). The TCR of 5at.%W and 7at.% Ta doped Vanadium Dioxide Polycrystalline
Films were high up to -18%/K and -12%/K at room temperature, respectively. Using 7at.% Ta and 2at.% Ti co-doping,
the TCR of the co-doped vanadium oxide film was -7%/K and without hysteresis during temperature increasing and
decresing from 0-80°C. It should indicate that the W-doped vanadium dioxide films colud be used for high sensing IR
detect and the Ta/Ti co-doped film without hysteresis is suitable for infrarid imaging application.
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.
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.
Ion beam enhanced deposition method was adopted to prepare In-N co-doped and Al-N co-doped ZnO films on Si, SiO2 and glass substrates. ZnO mixed with In2O3 or Al2O3 powder sputtering target was used and during the deposition N+/Ar+ mixed beam with an energy of 40KeV and a beam current of 2mA implanted into the deposited films. The XRD results showed that all polycrystalline In-N and Al-N co-doped ZnO films deposited on Si, SiO2 and glass substrates have a preferred (002) orientation. The as-deposited In-N co-doped ZnO film showed p-type and had a resistivity of 2.4Ωcm on SiO2 substrate. After annealed in N2, the lowest resistivity of p type In-N co-doped ZnO films was 0.8Ωcm. While Al-N co-doped ZnO film showed n type.
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.
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.
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.
Highly orientated V2O5 thin film was prepared by sol-gel method on SiO2/Si substrate. When baked at temperature higher than 400°C and pressure lower than 2Pa, the V2O5 thin film can be converted to VO2 thin film. Annealing V2O5 thin film under 1-2Pa, 480°C for 20 minutes, the VO2 polycrystalline thin film with resistance change larger than 3 orders of magnitude and 6.2°C hysteresis width was obtained. Meanwhile the transition progress from V2O5 to VO2 was discussed in detail. It was observed that the valence of V was reduced by the sequence as V2O5 to V3O7 to V4O9 to V6O13 to VO2, namely from VnO2n+1(n=2,3,4,6) to VO2. It was confirmed that the key factors to obtain VO2 thin film with good switching property were the conditions of pre-heating and vacuum baking for V2O5 thin film.
Nanosized lead titanate (PT) powder obtained by the Sol-gel method was homogeneously mixed with vinylidene fluoride-trifluoroethylene[P(VDF-TrFE)]. The nanocomposite film was used as the sensing film of PT/P(VDF-TrFE) pyroelectric sensors, whose detective merit was about 20% higher than that of sensors using pure P(VDF-TrFE). Porous silicon with porosity of 69% prepared by electrochemical method was oxidized to porous SiO2 in wet oxygen. Such porous SiO2 was chosen as substrate, which can effectively decrease the heat loss due to condition of the substrate. The voltage responsivity was increased and the thermal fluctuation noise of the pyroelectric element was decreased significantly. The experimental results indicated the specific detectivity of PT/P(VDF-TrFE) pyroelectric sensors based on porous SiO2 substrate was nearly 2 orders of magnitude higher than that of the sensors on bulk SiO2 substrate formed under the same conditions.
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