超薄VO2外延薄膜的制备及其金属-绝缘体相变的原位研究

摘要

Ultra-thin VO2 films were deposited on crystalline TiO2 (110) substrate by electron beam evaporation.A high-purity V bar was used as evaporation source and high-purity O2 was the reactive gas.The substrate temperature and O2 partial pressure were fixed respectively at 330 ℃ and 2 × 10-6 mbar during deposition.VO2 films with various characteristics were prepared by tuning the V evaporation flux and deposition time.The optimal growth conditions were determined by analyzing the surface morphology and crystal structure of films prepared under different conditions.The change of VO2 electronic band structure across metal-insulator transition(MIT) was also characterized.When the evaporation flux was 20 nA,the film obtained almost atomically smooth surface,giving rise to relatively bright low energy electron diffraction (LEED) patterns.With extension of deposition time,sample surface became rough,leading to hazy LEED patterns.Meanwhile,the valence state of V ions in the film changed from + 5 to + 3.MIT was observed in the sample with 10 atomic-layer thickness.%采用电子束蒸发法(EBE)在单晶TiO2(110)衬底上沉积VO2超薄薄膜.实验以高纯金属V棒为蒸发源,高纯氧气作为反应气体,固定生长温度330℃,氧压2×10-6 mbar,然后通过改变不同的V蒸发速率和生长时间制备出不同特性的VO2薄膜.利用原位的扫描隧道显微镜、低能电子衍射(LEED)和X线光电子能谱系统地分析所得样品的表面形貌、结构特征以及相转变过程中的能带结构变化,并对比找出EBE制备法的最佳生长条件.结果表明,当蒸发束流固定在20nA时,LEED点阵较亮,薄膜显示出接近于原子级平滑的表面;随着生长时间的增加,表面变粗糙,点阵变暗,V的价态逐渐降低,从+5价过渡到+3价;在薄膜厚度接近10个原子层时,薄膜存在金属-绝缘体相变行为.