Abstract
We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states stemming from the disorder of amorphous metal oxide networks. We start from presenting the results for amorphous (a) HfO2, chosen due to the availability of highest purity amorphous films, which is vital for studying their intrinsic electronic properties. Exhaustive photo-depopulation spectroscopy measurements and theoretical calculations using density functional theory shed light on the atomic nature of electronic gap states responsible for deep electron trapping observed in a-HfO2. We review theoretical methods used for creating models of amorphous structures and electronic structure calculations of amorphous oxides and outline some of the challenges in modeling defects in amorphous materials. We then discuss theoretical models of electron polarons and bi-polarons in a-HfO2 and demonstrate that these intrinsic states originate from low-coordinated ions and elongated metal-oxygen bonds in the amorphous oxide network. Similarly, holes can be captured at under-coordinated O sites. We then discuss electron and hole trapping in other amorphous oxides, such as a-SiO2, a-Al2O3, a-TiO2. We propose that the presence of low-coordinated ions in amorphous oxides with electron states of significant p and d character near the conduction band minimum can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides. Finally, we demonstrate that bi-electron trapping in a-HfO2 and a-SiO2 weakens Hf(Si)–O bonds and significantly reduces barriers for forming Frenkel defects, neutral O vacancies and O2− ions in these materials. These results should be useful for better understanding of electronic properties and structural evolution of thin amorphous films under carrier injection conditions.
Highlights
Thin metal oxide films grown on different substrates via oxidation and deposition are ubiquitous in the environment and across a wide range of technologies
We focus on discussing exhaustive photo-depopulation spectroscopy (EPDS) measurements to provide insight into the methods used to study electron trapping in amorphous films. Combined with this we provide discussion of density functional theory (DFT) based theoretical calculations to study the atomic nature of electronic gap states responsible for charge trapping
We discuss how electron and hole trapping can be relevant to other amorphous oxides, such as a-SiO2, a-Al2O3, a-TiO2 as well as indium gallium zinc oxide and propose that the presence of lowcoordinated ions in other amorphous oxides with significant p and d character of electron states near the conduction band minimum (CBM) can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides
Summary
Thin metal oxide films grown on different substrates via oxidation and deposition are ubiquitous in the environment and across a wide range of technologies Their atomic network structures are strongly affected by interfaces and may differ significantly from those of bulk materials, resulting in a number of unusual electrical, physical and chemical characteristics recognized in earlier studies [1, 2]. It benefits from and touches upon many common issues pertaining to the structure and properties of thin amorphous oxide films studied for other purposes Native defects, such as oxygen vacancies, and impurities, e.g. hydrogen and metal ions, are known to serve as electron and hole trapping sites in crystalline oxide films. The approach demonstrated in our manuscript will advance the state-of-the-art in this field
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