In brief

Scandium-doped aluminium nitride thin films have the potential to enhance electronic devices, but oxidation of their air-exposed surfaces should be considered during device design and integration.

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Scratching the surface of electrical films

2 Aug 2023

A close look at the surface chemistry of thin films used in electronic devices reveals how air exposure affects their performance.

Leftover avocados in the fridge rapidly transition from a vibrant green into an unappetising shade of brown. This is the result of a chemical reaction called oxidation, which also frustrates materials scientists developing state-of-the-art modules for electronics.

Minghua Li and colleagues at A*STAR’s Institute of Microelectronics (IME) have been developing scandium-doped aluminium nitride (ScAlN) thin films for use as sensors in engineering applications. With their unmatched ability to convert mechanical stress into electrical signals, ScAIN films hold the potential to transform the design of advanced electrical device components and semiconductors.

However, like avocados, ScAIN films are prone to oxidation when exposed to air, which can alter their properties. “Oxidised ScAlN exhibits a lower piezoelectric response because of its partial amorphous characteristics,” said Li. “The oxide overlayer also creates a higher energy barrier which impacts ScAIN’s electrical conduction.”

To better understand how surface oxidation affects ScAIN film performance, a research team led by Li looked at the film’s microstructure to identify how its atomic arrangements and the chemical states of its core elements change over time.

They were surprised to find that while the bulk film had a crystalline microstructure, ScAIN film exposed to air developed an oxide layer with an amorphous and indistinct structure studded with small crystallites. Using X-ray photoelectron spectroscopy core-level analysis, the team identified aluminium-oxygen and scandium-oxygen bonds in the oxide layer.

Li said that these results provide an explanation for why electrical conductivity tapers off in oxidised ScAINs. “The higher the scandium content, the faster the oxidation,” Li explained. “Oxygen in air diffuses into the lattice by bonding with scandium [more easily] than [with] aluminium, creating a higher energy barrier which [changes] the electrical conduction.”

The team also found that ScAIN oxidation reaches a self-imposed limit—once the oxide layer is formed, it stabilises and stops growing. These insights fill critical gaps in our understanding of how to control and optimise the properties of ScAIN films for durable, reliable and high-performing electrical devices.

“Eliminating the surface exposure to air during device integration is a major implication of this work that can positively impact the electronic device industry,” Li concluded, adding that the team is now investigating other factors that control ScAIN reliability.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Microelectronics (IME).

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Li, M., Lin, H., Hu, K. and Zhu, Y. Oxide overlayer formation on sputtered ScAlN film exposed to air. Applied Physics Letters 121, 111602 (2022). | article

About the Researcher

Minghua Li trained as a specialist in material science at the Peking University and the Harbin Institute of Technology, China. Subsequently, she moved to Singapore and joined A*STAR Data Storage Institute in 1997 where she worked as a research scientist to study the electronic thin film deposition and microstructure characterisation. In 2018 Li joined the Institute of Microelectronics (IME) and shifted her research focus to piezoelectric film development.

This article was made for A*STAR Research by Wildtype Media Group