Abstract
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)-HfO2 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-HfO2 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-HfO2. 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 SiO2, 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-HfO2 represents a new paradigm for charge trapping in a broad class of non-glass-forming amorphous insulators.
Highlights
We demonstrate that electron trapping at intrinsic precursor sites is endemic in non-glass-forming amorphous oxide films
We combine experimental and theoretical (Time-dependent Density Functional Theory, Timedependent Density Functional Theory (TD-DFT)) methods to demonstrate that electronic gap states responsible for a deep electron trapping in prototype a-HfO2 insulating films are intrinsic, and originate from lower coordination of ions and elongation of bonds in the amorphous phase of a-HfO2
We propose that the presence of low-coordinated ions in amorphous oxides with significant p and d character of electron states near the conduction band bottom (CBB) can lead to similar electron trapping and significantly affect characteristics of nanodevices
Summary
Thin oxide films grown on various surfaces via oxidation and deposition are ubiquitous in environment and technologies. Their structure is strongly affected by interfaces, and differs from that of bulk materials, resulting in a number of unusual electrical properties [1]. Such films can grow (poly)-crystalline or amorphous, depending on the deposition and annealing conditions. We combine experimental (exhaustive photodepopulation spectroscopy, EPDS, with improved resolution) and theoretical (Time-dependent Density Functional Theory, TD-DFT) methods to demonstrate that electronic gap states responsible for a deep electron trapping in prototype a-HfO2 insulating films are intrinsic, and originate from lower coordination of ions and elongation of bonds in the amorphous phase of a-HfO2. We propose that the presence of low-coordinated ions in amorphous oxides with significant p and d character of electron states near the conduction band bottom (CBB) can lead to similar electron trapping and significantly affect characteristics of nanodevices
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