Ameer Shaan, Jindal Kajal, Tomar Monika, Jha Pradip K, Gupta Vinay
Department of Physics and Astrophysics, University of Delhi, New Delhi-110007, India.
Phys Chem Chem Phys. 2020 May 14;22(18):10010-10026. doi: 10.1039/c9cp06614d. Epub 2020 Apr 27.
Light elements like carbon may enter unintentionally into a material during material processing owing to their ubiquitous nature, and may significantly influence its observed electronic and magnetic properties. In the present work, the energetics and kinetics of carbon impurity related defects in BiFeO (BFO) are studied using first principles calculations in order to gain insight into the ongoing controversial aspects of conductivity of BFO. The results suggest that oxygen deficient conditions provide a favorable chemical environment for incorporation of carbon in BFO. Calculations based on the formation energy predict that carbon can spontaneously occupy interstitials, O, and Fe sites in BFO (where it is found to introduce impurity induced shallow acceptor type states at an energy of 0.05 eV above the valence band maximum). Carbon occupying cationic sites (C and C) tends to ionize their vacancies (V and V), resulting in the formation of a CO cluster, whereas it induces localized electron traps with energy levels composed of impurity states near the center of the band gap (0.9 eV above the valence band maximum) when occupying interstitial sites in BFO. An understanding of the migration of C impurity in BFO is developed, which suggests the favorable incorporation of carbon impurity via a vacancy mechanism. In order to confirm the theoretical results, experimental studies are carried out where BFO and carbon doped BFO (BCFO) thin films are grown by the pulsed laser deposition technique. Polycrystalline pure phase (R3c) thin films of BFO and BCFO are obtained. The presence of defect states in the deposited thin films is optically analyzed by the photoluminescence (PL) technique. In order to highlight the critical role of carbon in modifying the electrical conductivity of BFO, a BCFO/BFO/ITO based p-i-n heterojunction is prepared. The electrical characteristics depict remarkable rectifying characteristics, thus suggesting the p-type nature of carbon dopant in otherwise intrinsic BFO.
像碳这样的轻元素由于其普遍存在的性质，在材料加工过程中可能会无意地进入材料中，并可能显著影响其观测到的电学和磁学性质。在本工作中，利用第一性原理计算研究了BiFeO（BFO）中与碳杂质相关的缺陷的能量学和动力学，以便深入了解BFO电导率目前存在争议的方面。结果表明，缺氧条件为碳在BFO中的掺入提供了有利的化学环境。基于形成能的计算预测，碳可以自发占据BFO中的间隙位置、O位和Fe位（发现它在价带最大值上方0.05 eV的能量处引入杂质诱导的浅受主型态）。占据阳离子位置（C和C）的碳倾向于使其空位（V和V）电离，从而形成CO团簇，而当它占据BFO中的间隙位置时，会诱导出具有位于带隙中心附近（价带最大值上方0.9 eV）的杂质态组成的能级的局域电子陷阱。对BFO中C杂质的迁移有了一定的认识，这表明碳杂质通过空位机制有利地掺入。为了证实理论结果，进行了实验研究，通过脉冲激光沉积技术生长了BFO和碳掺杂BFO（BCFO）薄膜。获得了BFO和BCFO的多晶纯相（R3c）薄膜。通过光致发光（PL）技术对沉积薄膜中缺陷态的存在进行了光学分析。为了突出碳在改变BFO电导率方面的关键作用，制备了基于BCFO/BFO/ITO的p-i-n异质结。电学特性显示出显著的整流特性，从而表明在原本本征的BFO中碳掺杂剂的p型性质。
