Strand Jack, Kaviani Moloud, Afanas'ev Valeri V, Lisoni Judit G, Shluger Alexander L
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
Nanotechnology. 2018 Mar 23;29(12):125703. doi: 10.1088/1361-6528/aaa77a.
We demonstrate that electron trapping at intrinsic precursor sites is endemic in non-glass-forming amorphous oxide films. The energy distributions of trapped electron states in ultra-pure prototype amorphous (a)-HfO insulator obtained from exhaustive photo-depopulation experiments demonstrate electron states in the energy range of 2-3 eV below the oxide conduction band. These energy distributions are compared to the results of density functional calculations of a-HfO models of realistic density. The experimental results can be explained by the presence of intrinsic charge trapping sites formed by under-coordinated Hf cations and elongated Hf-O bonds in a-HfO. These charge trapping states can capture up to two electrons, forming polarons and bi-polarons. The corresponding trapping sites are different from the dangling-bond type defects responsible for trapping in glass-forming oxides, such as SiO, in that the traps are formed without bonds being broken. Furthermore, introduction of hydrogen causes formation of somewhat energetically deeper electron traps when a proton is immobilized next to the trapped electron bi-polaron. The proposed novel mechanism of intrinsic charge trapping in a-HfO represents a new paradigm for charge trapping in a broad class of non-glass-forming amorphous insulators.
我们证明，在非玻璃形成的非晶氧化物薄膜中，本征前驱体位点处的电子俘获是普遍存在的。通过详尽的光去填充实验获得的超纯原型非晶（a）-HfO绝缘体中俘获电子态的能量分布表明，在氧化物导带以下2-3 eV的能量范围内存在电子态。将这些能量分布与实际密度的a-HfO模型的密度泛函计算结果进行了比较。实验结果可以通过a-HfO中由配位不足的Hf阳离子和拉长的Hf-O键形成的本征电荷俘获位点来解释。这些电荷俘获态最多可以捕获两个电子，形成极化子和双极化子。相应的俘获位点与导致玻璃形成氧化物（如SiO）中俘获的悬空键型缺陷不同，因为这些陷阱是在键未断裂的情况下形成的。此外，当质子固定在俘获电子双极化子旁边时，氢的引入会导致形成能量上稍深的电子陷阱。所提出的a-HfO中本征电荷俘获的新机制代表了一类广泛的非玻璃形成非晶绝缘体中电荷俘获的新范式。
