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Discovery in gallium nitride a key enabler of energy efficient electronics

Discovery in gallium nitride a key enabler of energy efficient electronics

Gallium nitride, a semiconductor that revolutionized energy-efficient LED lighting, could also transform electronics and wireless communication, thanks to a discovery made by Cornell researchers.

Their paper, “A Polarization-Induced 2D Hole Gas in Undoped Gallium Nitride Quantum Wells,” was published Sept. 26 in Science.

Silicon has long been the king of semiconductors, but it has had a little help. The pure material is often augmented, or “doped,” with impurities like phosphorus or boron to enhance current flow by providing negative charges (electrons) or positive charges (“holes,” the absence of electrons) as needed.

In recent years, a newer, sturdier family of lab-grown compound semiconductor materials has emerged: group III-nitrides. Gallium nitride (GaN) and aluminum nitride (AlN) and their alloys have a wider bandgap, allowing them to withstand greater voltages and higher frequencies for faster, more efficient energy transmission.

“Silicon is very good at switching off and on and controlling electrical energy flow, but when you take it to high voltages it doesn’t operate very well because silicon has a weak electric strength, whereas GaN can sustain much higher electric fields,” said co-senior author Debdeep Jena, the David E. Burr Professorin Electrical and Computer Engineering and in Materials Science and Engineering. “If you’re doing very large amounts of energy conversion, then wide-bandgap semiconductors such as GaN and silicon carbide are the solutions.”

Since the 1990s, researchers have doped GaN by adding magnesium impurities to create holes, but the process is highly inefficient. For every hundred magnesium atoms introduced into the crystal, only three or four holes might appear at room temperature, and even fewer at low temperatures.

Rather than using impurities, Ph.D. student Reet Chaudhuri stacked a thin GaN crystal layer – called a quantum well – atop an AlN crystal, and the difference in their crystal structures was found to generate a high density of mobile holes. Compared with magnesium-doping, the researchers discovered that the resulting 2D hole gas makes the GaN structures almost 10 times more conductive.

“In 1992, researchers discovered that when aluminum nitride is deposited on top of gallium nitride, you get free electrons at the interface. Having electrons conduct inside GaN makes what we call n-type electronic devices,” said Chaudhuri, the paper’s lead author. “The polarization theory that explains why we get mobile electrons in this structure, which in fact was conceptualized and validated by Cornell researchers in late ’90s, also predicts that we should expect holes when the structure is flipped. But to date, there had not been any report of holes in an undoped III-nitride semiconductor structure. And that’s what we have found in this work.”

Using the new material structure created by Reet, co-author and Ph.D. student Samuel James Bader recently demonstrated some of the most efficient p-type GaN transistors in a collaborative project with Intel. Now that the team has the capability to make hole-channel transistors – which are called p-type – they plan to pair them with n-type transistors to form more complex circuits, opening up new possibilities in high-power switching, 5G cellular technology and energy efficient electronics, including phone and laptop chargers.

“It’s very difficult to simultaneously achieve n-type and p-type in a wide bandgap semiconductor. Right now, silicon carbide is the only other one that has both besides GaN. But the mobile electrons in silicon carbide are more sluggish than those in GaN,” said co-senior author Huili Grace Xing, the William L. Quackenbush Professor in electrical and computer engineering and in materials science and engineering. “Using these complementary operations enabled by both n-type and p-type devices, much more energy efficient architecture can be built.”

Another advantage of the 2D hole gas is that its conductivity improves as the temperature is lowered, meaning that researchers will now be able to study fundamental GaN properties in ways that haven’t been previously possible. Equally important is its ability to retain energy that would otherwise be lost in less efficient power systems.

“Gallium nitride caused a revolution in the lighting industry,” Jena said. “It enabled the white lighting that is in our cellphones, laptops, and LED bulbs that are replacing the incandescent bulbs in our homes. With a regular 100W incandescent light bulb, which is about 4% energy efficient, you might get 4 watts of light and the rest is heat. You know this very well if you touch the bulb when it is on. LEDs on the other hand can be almost 80% efficient, and only 20% is heat. A similar change in energy-efficiency of electronics has not yet happened. And maybe this finding is a step in that direction.”

A patent application has been filed through the Center for Technology Licensing for the discovery. Other contributors included David Muller, the Samuel B. Eckert Professor in Applied and Engineering physics; and Zhen Chen, a postdoctoral researcher in Muller’s lab.

The research was supported in part by Intel, the Air Force Office of Scientific Research, the National Science Foundation and the Cornell Center for Materials Research.

Materials provided by Cornell University

single atomic layer graphite

Researchers discover quantum phenomenon to understand fundamental limits of graphene electronics

Scientists from the University of Manchester, Nottingham and Loughborough have discovered a quantum phenomenon for understanding the fundamental limits of graphene electronics. It describes the way electrons in one atomic-thin sheet of graphene scatter the vibrating carbon atoms that make the hexagonal crystal lattice. The study has been published in Nature Communications.

Applying magnetic field perpendicular to graphene plane, current-carrying electrons are forced to move in the “cyclotron orbits” which are closed and circular. In pure graphene, electrons escape from the orbit by bouncing off “phonon” while scattering. Phonons are particle-like bundles of momentum and energy and are “quanta” of sound waves that are associated with vibrating carbon atom. When graphene crystal is warmed from very low temperature, they are generated in large numbers.

The team passed a small electric current through graphene sheet for precisely measuring the amount of momentum and energy which is transferred between electron and phonon while scattering. It revealed that two kinds of phonons scatter electrons, transverse acoustic (TA) phonons where carbon atoms vibrate perpendicular to the direction in which phonon propagates and wave motion and longitudinal acoustic phonons (LA) where carbon atoms vibrate in the direction of phonon and wave motion. These accurately measure the speeds of two kinds of phonons which is difficult to make in a single atomic layer. It also shows that TA phonon scattering dominates LA phonon scattering.

This phenomenon is termed as magnetophonon oscillation and it was measured in many semiconductors several years before graphene was discovered. It has been known longer than the quantum Hall effect and is one of the oldest quantum transport phenomena.

Roshan Krishna Kumar and Laurence Eaves, co-authors in the work said that they were surprised to discover such magnetophonon oscillations in graphene and at the same time confused why it had not been discovered before in graphene. It had two key requirements. Scientists had to fabricate high-quality transistors of graphene having large areas at National Graphene Institute. It had not been discovered if the device dimensions were smaller than a few micrometers. 

Piranavan Kumaravadivel, University of Manchester and lead author of the paper said that macroscopic, millimeter-sized crystals were studied at the beginning of quantum transport experiments. The studied devices in most work on quantum transport on graphene are normally a few micrometers in size. Larger graphene devices are important for both applications and fundamental studies. 

The next ingredient is temperature. Graphene quantum transport experiments are carried out at ultra-cold temperatures for slowing the carbon atoms which are vibrating and “freeze-out” the phonons which break quantum coherence. So graphene is warmed so that phonons are active to cause the effect.

Mark Greenaway, Loughborough University who also worked on the quantum theory of this effect said that the result is quite exciting as it opens a new route for understanding the phonon properties in two-dimensional crystals and heterostructures. It will also help to understand electron-phonon interactions in promising materials which is vital for new devices and applications.

Research Paper: https://www.nature.com/articles/s41467-019-11379-3


Researchers a step closer in creating successor to shrinking transistors

Over the decades, computers and other electronic devices have shrunk in size and also been significantly faster. This has been possible as the makers have understood and implemented the techniques to decrease the size of individual transistors, small electrical switches which work in transmitting information.

Researchers have relentlessly worked on decreasing the size of the transistor so as to pack more in each chip. However, it seems that pursuit is almost over as scientists are rapidly approaching the minimum physical limit for the size of the transistor, with the current models measuring 10 nanometres which is equivalent to the width of 30 atoms.

Dr. Kyeongjae Cho, professor of Materials science at the University of Texas, Dallas remarked that the power of processing of electronic equipment is derived from the millions and billions of transistors which are interconnected on one chip. He also pointed out that we are very rapidly nearing the minimum scale of size.

For further making improvement on the processing speed, the industry of microelectronics is currently looking at alternative possibilities. Professor Cho’s research work has been published in the Nature Communications journal.

Normal transistors can only transmit two types of information. Being a switch, the transistor is either in the on state or off state which in binary language translates to 1 or 0.

A technique to increase the processing power without putting in additional transistors would be to ramp up the information that can be conveyed by a single transistor with the help of intermediate stages between 1 and 0. The multi-valued transistor based on this principle would make for more operations and a greater amount of information which can be processed in one device.

Cho said that the concept of multi-valued logic transistors is not very new and there have been past attempts to create similar devices.  

Cho and his research group used a unique configuration of two types of zinc oxide to make a composite layer which is incorporated with other materials inside a superlattice. They found out that the physics for multi-valued logic can be achieved by embedding crystals of zinc oxide called quantum dots in amorphous zinc oxide. The order of atoms in amorphous solid is not rigid as in crystals. Cho has applied for patenting his work as he found that it is possible to create an electronic structure for the multi-level logic structure.

The significance of this research is that it can bridge the gap between current computing and quantum computers. Cho added that quantum computing is not yet commercialized and his work is in the direction to merge the gap between binary and large degrees of freedom.