There’s two sides to this semiconductor, and many simultaneous functions

Gallium nitride-based semiconductors have been a boon for high-frequency and power electronics. They've also revolutionized energy-efficient LED lighting. But no semiconductor wafer has been able to do both at the same time efficiently.

Now Cornell researchers, in collaboration with a team at the Polish Academy of Sciences, have developed the first dual-sided - or "dualtronic" - chip that combines its photonic and electronic functions simultaneously, an innovation that could shrink the size of functional devices, make them more energy efficient and reduce manufacturing costs.

The team's paper, "Leveraging Both Faces of Polar Semiconductor Wafers for Functional Devices," published Sept. 25 in Nature. The co-lead authors are doctoral students Len van Deurzen and Eungkyun Kim.

The project was led by Debdeep Jena, the David E. Burr Professor of Engineering in the School of Electrical and Computer Engineering and Department of Materials Science and Engineering, and Huili Grace Xing, the William L. Quackenbush Professor of Electrical and Computer Engineering and of Materials Science and Engineering, both in Cornell Engineering.

Gallium nitride (GaN) is unique among wide-bandgap semiconductors because it has a large electronic polarization along its crystal axis, which gives each of its surfaces dramatically different physical and chemical properties. The gallium, or cation, side has proved useful for photonic devices such as LEDs and lasers, while the nitrogen, or anion, side can host transistors.

The Jena-Xing Laboratoryset out to make a functional device in which a high electron mobility transistor (HEMT) on one side drives light-emitting diodes (LEDs) on the other - a feat that hasn't been achieved in any material.

"To our knowledge, nobody has made active devices on both sides, not even for silicon," van Deurzen said. "One of the reasons is that there's no additional functionality you get from using both sides of a silicon wafer because it's cubic; both sides are basically the same. But gallium nitride is a polar crystal, so one side has different physical and chemical properties than the other, which gives us extra degree in designing devices."

The project was initially conceived at Cornell by Jena and former postdoctoral researcher Henryk Turski, a co-senior author of the paper, along with Jena and Xing. Turski worked with a team at the Polish Academy of Sciences' Institute of High Pressure Physics to grow transparent GaN substrates on a single crystal wafer roughly 400 microns thick. The HEMT and LED heterostructures were then grown in Poland by molecular beam epitaxy. After the epitaxy was completed, the chip was shipped to Cornell, where Kim built and processed the HEMT on the nitrogen polar face.

"The nitrogen polar side is more chemically reactive, which means during device processing the electron channel can be damaged quite easily," Kim said. "A challenge with nitrogen polar transistor fabrication is to make sure all the plasma processes and the chemical treatment do not damage the transistors. So there was a lot of process development that had to be done for fabricating and designing that transistor."

Next, van Deurzen built the LED on the metal polar face, using a thick positive photo resist coating to protect the previously processed n-polar face. After each stage, the researchers measured their respective device characteristics and found they had not changed.

"It's actually a very feasible process," van Deurzen said. "The devices do not degrade. And this is obviously important if you want to use this as a real technology."

Since no one has made a double-sided semiconductor device before, the team had to invent a new method to test and measure it. They assembled a "crude" double-side-coated glass plate and wire-bonded one side of the wafer to it, which allowed them to probe both sides from the top. Because the GaN substrates were transparent for the entire visible range, the light was able to transmit through. The single HEMT device succeeded in driving a large LED, turning it on and off at kilohertz frequencies - plenty for a working LED display.

Currently, LED displays have a separate transistor and independent fabrication processes. An immediate application for the dualtronic chip is microLEDs: fewer components, occupying a smaller footprint and requiring less energy and materials, and manufactured quicker for lower cost.

"A good analogy is the iPhone," Jena said. "It is, of course, a phone, but it is so many other things. It's a calculator, it's a map, it lets you check the internet. So there's a bit of a convergence aspect of it. I would say our first demonstration of 'dualtronics' in this paper is convergence of maybe two or three functionalities, but really it's bigger than that. Now you may not require the different processors to perform different functions, and reduce the energy and speed lost in the interconnections between them that requires further electronics and logic. Many of those functionalities shrink into one wafer with this demonstration."

Other applications includeComplementary Metal-Oxide-Semiconductor(CMOS)devices with a polarization-induced n-channel transistor (which uses electrons) on one side and a p-channel transistor (containing holes) on the other.

In addition, because the GaN substrates have a high piezoelectric coefficient, they can be used as bulk acoustic wave resonators for filtering and amplifying radio frequency signals in 5G and 6G communications. The semiconductors could also incorporate lasers instead of LEDs for "LiFi" - i.e., light-based - transmissions.

"You could essentially extend this to enable the convergence of photonic, electronic and acoustic devices," van Deurzen said. "You're essentially limited by your imagination in terms of what you could do, and unexplored functionalities can emerge when we try them in the future."

Co-authors include doctoral students Naomi Pieczulewski; Zexuan Zhang, Ph.D. '23; David Muller, the Samuel B. Eckert Professor of Engineering in Cornell Engineering; and researchers from the Institute of High Pressure Physics, Polish Academy of Sciences, Warsaw.

The researchers made use of the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation, and the Cornell Center for Materials Researchwith support from the NSF's Materials Research Science and Engineering Centers program.

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