基于FTO/VO_2/FTO结构的VO_2薄膜电压诱导相变光调制特性

Optical modulation characteristics of VO_2thin film due to electric field induced phase transition in the FTO/VO_2/FTO structure

采用直流磁控溅射和后退火工艺在掺氟的SnO2(FTO)导电玻璃衬底上制备VO2薄膜,研究了不同退火时间和不同比例的氮氧气氛对VO2薄膜性能的影响,对VO2薄膜的结晶取向、表面形貌、表面元素的相对含量和透过率随波长变化进行了测试分析,结果表明在最佳工艺条件下制备得到了组分相对单一的VO2薄膜.基于FTO/VO2/FTO结构在VO2薄膜两侧的透明导电膜上施加电压并达到阈值电压时,观察到了明显的电流突变.当接触面积为3 mm×3 mm时,阈值电压为1.7 V,阈值电压随接触面积的增大而增大.与不加电压的情况相比,FTO/VO2/FTO结构在电压作用下高低温的红外透过率差值可达28%,经反复施加电压,该结构仍保持性能稳定,具有较强的电致调控能力.

VO2 thin films have been studied for their semiconductor–metal reversible transition from the monoclinic to the rutile structure, where the electrical and optical properties undergo a drastic change by increasing the temperature or by applying a voltage. VO2 film is becoming a promising material for optical switch, optical storage, optical modulator, smart window, and micro-bolometer. The preparation procedures of the FTO/VO2/FTO structure in detail are as follows：First, the F-doped SnO2 conductive glass （FTO） substrates are cleaned sequentially in acetone, ethanol, and deionized water for 10 min using an ultrasonic cleaning equipment at a frequency of 20 kHz. When the FTO substrates was cleaned, they are dried with nitrogen. Second, the dried FTO substrates are placed in the chamber of a DC magnetron sputtering system equipped with a high-purity metal target of V （99.9%）. After argon （99.999%） of 80 sccm flux was discharged with the current of 2 A and the voltage of 400 V for 2 min, the vanadium films are deposited on the FTO substrates. Third, the prepared vanadium films are annealed for different annealing time in an atmosphere composed of different proportions of nitrogen-oxygen. Then another layer thickness of 350 nm of FTO conductive film is deposited on the VO2 thin film by using the plasma enhanced chemical vapor deposition method. Finally, different sizes of the FTO/VO2/FTO structure are prepared by using photolithography and chemical etching processes. The effect of different annealing time and different proportions of nitrogen-oxygen atmosphere on the VO2 thin films has been studied. X-ray diffraction （XRD）, scanning electron microscope （SEM）, atomic force microscope （AFM）, X-ray photoelectron spectroscopy （XPS） and spectrophotometer are then used to test and analyze the crystal structure, surface morphology, surface roughness, the relative content of the surface elements, and transmittance of the VO2/FTO composite films. Results show that a relatively single component VO2 thin film can be obtained under the optimum condition. The current abrupt change can be seen at the threshold voltage when the FTO/VO2/FTO structure is applied to voltage on both the transparent conductive films of the VO2 thin film. The threshold voltage is 1.7 V when the contact area is 3 mm × 3 mm, and the threshold voltage increases as the contact area increases. When the contact area is 6 mm × 6 mm, the threshold voltage of the thin film phase transition is 4.3 V; when the contact area is 8 mm × 8 mm, the threshold voltage of the thin film phase transition is 9.3 V. Compared with the no voltage situation, the infrared transmittance difference of the FTO/VO2/FTO structure under the effect of voltage is up to 28%before and after the transition. The structure remains stable with a strong electrochromic capacity when it is applied with voltage repeatedly. This brings about many new opportunities for optoelectronic devices and industrial production.