An image of the aluminum oxide film showing the highly-ordered spacing of pores that gives it a more mechanically robust structure without impairing the refractive index. Image: Chih-Hao Chang.Researchers from North Carolina State University (NC State) have developed a dielectric film that has optical and electrical properties similar to air but is strong enough to be incorporated into electronic and photonic devices, making them both more efficient and more mechanically stable.
At issue is something called the refractive index, which measures how much light bends when it moves through a substance. Air, for example, has a refractive index of 1, while water has a refractive index of 1.33, which is why a straw appears to bend when put it in a glass of water.
Photonic devices require a high contrast between their component materials, with some components having a high refractive index and others having a low one. The higher the contrast between those materials, the more efficient the photonic device is, and the better it performs. Air has the lowest refractive index, but it isn't mechanically stable, while the lowest refractive index found in solid, naturally-occurring materials is 1.39.
But now researchers at NC State have developed a film made of aluminum oxide that is mechanically stiff but has a refractive index of just 1.025.
"By manipulating the structure of the aluminum oxide, which is dielectric, we've improved both its optical and mechanical properties," says Chih-Hao Chang, corresponding author of a paper on the work in Advanced Functional Materials and an assistant professor of mechanical and aerospace engineering at NC State. Dielectrics are insulator materials found in an enormous array of consumer electronics, including the capacitors that store and manage electric charge in handheld devices.
"The key to the film's performance is the highly-ordered spacing of the pores, which gives it a more mechanically robust structure without impairing the refractive index," explains Xu Zhang, lead author of the paper and a PhD student at NC State.
The researchers make the film using a nanolithography technique developed in Chang's lab for creating highly-ordered pores in a polymer substrate; they then coat this porous polymer with a thin layer of aluminum oxide using atomic layer deposition. Finally, the polymer is burned off to leave behind a three-dimensional aluminum oxide coating.
"We are able to control the thickness of the aluminum oxide, creating a coating between 2nm and 20nm thick," says Zhang. "Using zinc oxide in the same process, we can create a thicker coating. And the thickness of the coating controls and allows us to design the refractive index of the film." Regardless of the how thick the coating is, the film itself is approximately 1µm thick.
"The steps in the process are potentially scalable, and are compatible with existing chip manufacturing processes," Chang says. "Our next steps include integrating these materials into functional optical and electronic devices."
This story is adapted from material from North Carolina State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.