Interfacial effect of Cu electrode enhanced energy density of amorphous aluminum oxide dielectric capacitor

https://doi.org/10.1016/j.jallcom.2020.157473Get rights and content

Highlights

  • Interfacial effect enhances breakdown strength and dielectric constant of capacitor.

  • Cu electrode is oxidized to Cu oxide and the structure of alumina is improved.

  • The interfacial evolution mechanism is proposed and completely understood.

  • The energy density of Cu/AmAO/Pt is enhanced from 2.9 to 6.9 J cm−3

Abstract

In this work, a novel dielectric system of Cu/amorphous aluminum oxide/Pt (Cu/AmAO/Pt) is developed for dielectric capacitor applications. The high breakdown strength (425 MV m−1), high dielectric constant (8.6) and improved leakage current density are achieved due to this effective system. Dielectric capacitors with the structure of Cu/AmAO/Pt demonstrate much higher energy density of 6.9 J cm−3 than that of Au/AmAO (2.9 J cm−3). The enhanced performance of Cu/AmAO/Pt stems intrinsically from the interfacial effect under high electric field. The interfacial effect forms copper oxides and compacts the structure of AmAO, contributing to the improvement of breakdown strength and leakage current density. Anodized copper oxide also makes contribution to the increase of dielectric constant. Moreover, the oxidation mechanism is proposed for further understanding electrochemical behaviors. Based on the extensive applications of Cu in integrated circuits, high-energy-density Cu/AmAO/Pt system promises huge potential for the integrated circuit.

Graphical abstract

CuxO is formed at the AmAO/Cu interface as the moving of oxygen ions, hydroxyl groups and copper ions under the high electric field. The interfacial effect improves the breakdown strength, dielectric constant, and energy density of dielectric capacitor. Meanwhile, the interface effect endows the system a huge potential for application of integrated circuits.

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Introduction

The integrated circuit plays an irreplaceable role in modern society. They are gradually infiltrating into all walks of our life and becoming the cornerstone of information development [1]. Dielectric capacitors, as basic energy storage components of integrated circuits [2,3], possess high pulse power density and ultrafast charging-discharging properties [[4], [5], [6]]. However, commercialized dielectric capacitors still suffer from low energy storage density of 1–2 J cm−3 compared to its counterparts, such as supercapacitor, batteries, which limits its further applications [7].

As is well-known, a dielectric capacitor consists of a top electrode, dielectrics and a bottom electrode. The energy storage density (U) of a dielectric capacitor obeys the following equation [[8], [9], [10]]:U=EdPwhere P is the electric polarization; E is the applied electric field. For linear dielectric materials, Eq. (1) can be converted as follows [[11], [12], [13]]:U=12εε0E2where ε is the dielectric constant of material; ε0 is the vacuum dielectric constant (8.85×1012Fm1). Both dielectric constant and applied electric field are key factors of dielectric capacitors for achieving high energy density [14].

Copper, featured by high malleability and superior electric/thermal conductivity, is an essential interconnect material in modern integrated circuits. Metal copper can be oxidized into copper oxides in a high humidity complex atmosphere or high electric field. Copper oxide, which has non-toxicity, presents high dielectric constant of ∼25 for polycrystalline materials, 3000 for single crystals [[15], [16], [17]]. And the improved dielectric constant of ceramics by doping CuO has been verified [18,19]. Therefore, it is feasible to improve the capacitor’s dielectric constant and further enhance the energy storage density by utilizing Cu interconnected wires in integrated circuits based on the oxidation capacity of Cu. The design idea of this paper is that the Cu electrode and dielectrics are integrated into circuit and then part of Cu wires are designed directly as the top electrode of dielectric capacitors. This design which can greatly simplify the preparation technology of capacitors as well as been useful for cost-saving.

The challenge of this work is the transformation from Cu to copper oxide. That requires special dielectrics which could oxidize Cu under the electric field. At present, dielectrics which were exploited for enhancing energy storage density are ceramics [20,21], polymers [22] and polymer-based composites [23,24]. However, no dielectrics could oxidize valve metal expect for the sol-gel-derived aluminum oxide so far [25]. Aluminum oxide is a low-cost material that has been widely used in electrical and electric energy devices owing to its outstanding thermal and chemical stability [26,27]. Meanwhile, as a dielectric material, aluminum oxide exhibits high breakdown strength (200–1000 MV m−1) and low leakage current density [28,29]. Specially, amorphous aluminum oxide shows higher breakdown strength and lower leakage current density owing to its absorbing and scattering effect of numerous defects compared with crystalline aluminum oxide [30]. Compared to polymer-based dielectrics, aluminum oxide also presents an intrinsic superiority of higher thermal endurance and thus can steady operate with the heat from the running circuits.

In this study, the amorphous aluminum oxide (AmAO) thin film was introduced as a dielectric material. Then Cu was deposited onto the AmAO film as the top electrode (Cu/AmAO/Pt). A promising energy storage density of 6.9 J cm−3 was realized, which is much higher than current applied dielectric capacitors. The excellent performance was achieved by the anodic oxide of Cu and the evolution of AmAO. The dielectric capacitor of Cu/AmAO/Pt shows tremendous application potential in integrated circuits based on low cost, simple preparation process and its high energy storage density. This work may pave the way for the practical application of Cu/AmAO/Pt in energy storage field.

Section snippets

Preparation of AmAO thin film

The amorphous aluminum oxide thin film (AmAO thin film) was fabricated by sol-gel and spin-coating technology. The process is illustrated in Fig. 1. The aluminum isopropylate (C9H21AlO3, 99%), 2-ethoxyethanol (C4H10O2, 99%), acetylacetone (C5H8O2, 99%) and acetic acid (CH3COOH, 99%) were used as raw materials. The material molar ratio is M (aluminum isopropylate): M (2-ethoxyethanol): M (acetylacetone): M (acetic acid) = 1: 25:1: 8.5. The above materials were bought from Shanghai Laboratorial

Enhanced dielectric properties

The TEM cross-sectional image of Cu/AmAO/Pt showed the AmAO film was compact without crystalline grain or cracks. And the thickness of AmAO was ∼210 nm. The Cu electrode was dense with thickness of ∼260 nm (Fig. 2a). The multi-orientations of interplanar spacings were clearly observed in Fig. 2b, suggesting Cu electrode was mainly poly-crystalline.

lnJ-E characteristics and dependable breakdown strengths of Au/AmAO/Pt and Cu/AmAO/Pt, which derived from Weibull distribution of breakdown

Conclusion

Amorphous aluminum oxide thin film (AmAO) was fabricated via sol-gel and spin-coating method. Copper top electrode was deposited onto AmAO thin film (Cu/AmAO). The properties of Cu/AmAO was improved relying on the anodic oxidation of Cu electrode and the structural evolution of AmAO thin film. Comparing to Au/AmAO, the breakdown strength increased from 293 MV m−1 to 425 MV m−1; the dielectric constant increased from 7.7 to 8.6; and the leakage current density was enormously depressed.

CRediT authorship contribution statement

Manwen Yao: Conceptualization, Methodology, Investigation, Writing - original draft, Funding acquisition. Chunyu Li: Investigation, Data curation, Software. Zhen Su: Project administration, Investigation, Writing - original draft, Funding acquisition. Zaifang Li: Writing - review & editing, Validation. Hao Wang: Visualization, Writing - review & editing. Xi Yao: Conceptualization, Writing - review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is supported by the National Natural Science Foundation of China (Youth Program, Grant No. 51902134), the National Natural Science Foundation of China (Grant No. 51872201) and the Jiaxing Public Welfare Research Program (2019).

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