Low-Cost Synthetic Diamond Fabrication Could Step Up Quantum Tech

Using a thin tungsten layer as a restraining, conductive, removable hard mask, researchers at the University of Technology Sydney (UTS) developed a method for the fabrication of synthetic diamond. The developed material can be used for the photonic circuitry used in quantum technologies. The masking method is safe and inexpensive, and it uses the tungsten layer to pattern the diamond nanostructure that enables the creation of 1D photonic crystal cavities.

Although the tungsten layer is only 30 nm wide, it can enable the creation of a diamond etch of over 300 nm.

“For diamond to be used in quantum applications, we need to precisely engineer optical defects in the diamond devices — cavities and waveguides — to control, manipulate, and read out information in the form of qubits,” professor Igor Aharonovich said. “It is akin to cutting holes or carving gullies in a super-thin sheet of diamond, to ensure light travels and bounces in the desired direction.”

With a tungsten layer (as opposed to a more conventional silicon oxide layer), the researchers achieved high levels of repeatability and reliability in their fabrication procedures, resulting in diamond cavities with quality factors approaching 1 × 104. The researchers further showed that the cavities could be transferred onto a trenched substrate to allow for fully suspended diamond cavities.

“The use of tungsten as a hard mask addresses several drawbacks of diamond fabrication,” researcher Blake Regan said. “It acts as a uniform restraining conductive layer to improve the viability of electron beam lithography at nanoscale resolution. It also allows the post-fabrication transfer of diamond devices onto the substrate of choice under ambient conditions.”

The tungsten mask improves the safety and accessibility of the diamond nanofabrication process — the mask can be removed without having to use hazardous materials, such as hydrofluoric acid. The masking process can be further automated to create modular components for diamond-based quantum photonic circuitry, the researchers said.

To lower fabrication costs and improve scalability, the researchers also developed a way to grow single-crystal diamond photonic structures with embedded quantum defects from a polycrystalline substrate. In experiments, they used a pattern growth method to demonstrate how polycrystalline diamond could be used to grow single-crystal diamond structures with predefined shapes and sizes.

As outlined in a separate paper, the researchers developed a method to control the conditions for growth and then introduced germanium impurities during the growth. They were able to show localized, enhanced emission from fabricated, pyramid-shaped, single-crystal diamonds containing germanium vacancy (GeV) color centers. They measured a linewidth of about 500 MHz at 4 K from a single GeV center in the pyramid-shaped diamonds.

Artist's impression of a diamond building block in a future photonic circuit. The red color emphasizes the germanium vacancy centers emitting at the red spectral range and the ring illustrates the cavity. Designed by Trong Toan Tran. Courtesy of University of Technology Sydney.

“Our process relies on lower-cost large polycrystalline diamond, which is available as large wafers, unlike the traditionally used high-quality single-crystal diamond, which is limited to a few square millimeters,” researcher Milad Nonahal said. “To the best of our knowledge, we offer the first evidence of the growth of a single-crystal diamond structure from a polycrystalline material using a bottom-up approach.”

Bottom-up approaches for the development of single-crystal diamond structures with purposely introduced color centers could be valuable for quantum technologies such as quantum supercomputers, secure communications, and sensors. The method developed by the UTS team could speed the development of diamond-based quantum technology.

“Our method eliminates the need for expensive diamond materials and the use of ion implantation, which is key to accelerating the commercialization of diamond quantum hardware,” researcher Mehran Kianinia said. “This also significantly improves the safety and accessibility of the diamond nanofabrication process.”

The research was published in Nanoscale (www.doi.org/10.1039/D1NR00749A) and in Advanced Quantum Technologies (www.doi.org/10.1002/qute.202100037).