Zhang Zhiqiang, Janotti Anderson
Department of Physics and Astronomy, University of Delaware, Newark, Deleware 19716, USA.
Department of Materials Science and Engineering, University of Delaware, Newark, Deleware 19716, USA.
Phys Rev Lett. 2020 Sep 18;125(12):126404. doi: 10.1103/PhysRevLett.125.126404.
It has been recently revealed that strontium titanate (SrTiO_{3}) displays persistent photoconductivity with unique characteristics: it occurs at room temperature and lasts over a very long period of time. Illumination of SrTiO_{3} crystals at room temperature with sub-band-gap light reduces the electrical resistance by three orders of magnitude and persists for weeks or longer [Tarun et al., Phys. Rev. Lett. 111, 187403 (2013)PRLTAO0031-900710.1103/PhysRevLett.111.187403]. Experiments indicate that oxygen vacancy and hydrogen play important roles, yet the microscopic mechanism responsible for this remarkable effect has remained unidentified. Using hybrid density functional theory calculations we show that an instability associated with substitutional hydrogen H_{O}^{+} under illumination, which becomes doubly ionized and leaves the oxygen site, can explain the experimental observations. H_{O} then turns into an interstitial hydrogen and an oxygen vacancy, leading to excess carriers in the conduction band. This phenomenon is not exclusive to SrTiO_{3}, but it is also predicted to occur in other oxides. Interestingly, this phenomenon represents an elegant way of proving the existence of hydrogen substituting on an oxygen site (H_{O}), forming an interesting, and rarely observed, type of three-center, two-electron bond.
最近有研究表明，钛酸锶（SrTiO₃）具有独特的持久光电导特性：它在室温下即可发生，且能持续很长时间。在室温下用亚带隙光照射SrTiO₃晶体，其电阻会降低三个数量级，并能持续数周甚至更长时间[Tarun等人，《物理评论快报》111, 187403 (2013)PRLTAO0031 - 900710.1103/PhysRevLett.111.187403]。实验表明，氧空位和氢起着重要作用，然而导致这种显著效应的微观机制仍未明确。通过杂化密度泛函理论计算，我们发现光照下与替代氢Hₒ⁺相关的不稳定性，它会发生双电离并离开氧位点，这可以解释实验现象。然后Hₒ转变为间隙氢和氧空位，导致导带中出现过量载流子。这种现象并非SrTiO₃所独有，预计在其他氧化物中也会发生。有趣的是，这种现象代表了一种巧妙的方式来证明氢在氧位点上替代（Hₒ）的存在，形成了一种有趣且罕见的三中心两电子键类型。
