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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

Qubit Mechanical Resonator

Researchers successful in developing quantum light sources for use in optical circuits

A team of researchers led by Alexander Holleitner, Jonathan Finley, physicists at Technical University of Munich (TUM) has succeeded in placing light sources in atomically thin material layers having an accuracy of few nanometres. It allows a series of applications in quantum technology ranging from quantum sensors, transistors to encryption technology for transmission of data. The study has been published in Nature Communications

Earlier circuits on chips relied on electrons as carriers of information. However, in the coming days, photons carrying information at the speed of light will perform this task in optical circuits. The basic building blocks for such chips are quantum light sources connected with detectors and quantum fibre optic cables. 

Julian Klein, the study’s lead author said that it is a first step in making optical quantum computers. The light sources need to coupled with photon circuits for future applications to make quantum calculations based on light possible. However, the critical point is the exact controlled placement of the light sources. Quantum light sources in materials such as diamond or silicon can be created but not precisely placed in the materials. 

Physicists used a semiconductor layer, molybdenum disulfide as the initial material with a thickness of three atoms. Then they irradiated it with a beam of helium ions focused on a surface area of less than one nanometre. For generating optically active defects, molybdenum or sulfur atoms are hammered out of layer very precisely. The imperfections are traps for electron-hole pairs which emit the desired photons. The helium ion microscope at Center for Nanotechnology and Nanomaterials, Walter Schottky Institute was used for irradiating the material with accurate lateral resolution. 

Researchers from TUM, University of Bremen, Max Planck Society developed the model for describing the energy state observed at theoretical imperfections.

In future scientists want to create complex light source patterns, in two-dimensional lateral lattice structures for researching multi-exciton phenomena. This is the experimental realisation of the theory within the context of the Bose-Hubbard model, accounting for complex processes in solids. 

As the light sources have a similar underlying defect in the material they cannot be distinguished theoretically. This opens for new opportunities which are based on the quantum-mechanical principle of entanglement. Klein said that it is very much possible for the integration of quantum light sources in the photon circuits in a very elegant manner. Because of the high sensitivity, it is possible to make quantum sensors for smartphones and also make highly secure encryption technologies to transmit data. 

Journal Reference: Nature Communications

Quantum Dots with emission maxima in a 10-nm step are being produced at PlasmaChem in a kg scale

Researchers achieved near-perfect performance in low-cost semiconductors

Nowadays the whole world has become digitalized and for each and everything we have an electronic device. We have a television to entertain ourselves, an iPad to watch movies and work on the go, a mobile to receive calls when we are away from home. These electronic devices have something called as the semiconductor.

A semiconductor is a substance whose electrical conductance falls between metal and insulator. However, the conducting property can be altered by adding impurities into the crystal. Some commonly known semiconductors are silicon, germanium, and arsenide. Since it becomes very difficult to produce, semiconductor becomes very expensive.

Quantum dot is the solution and can be used in place of a semiconductor. Quantum dots are basically very small semiconductors which lie in the nanometre scale. Quantum dots change its properties even with a very small change in shape or size. The quantum dots have been used in electronic instruments like solar panels, camera sensors and medical imaging tools by researchers.

David Hanifi co-author of research on quantum dots said, “These quantum dots can be made in large number in labs in a more simple way as compared to semiconductor”.

When the research started in order to understand whether they could compete with semiconductors, the researchers focused on how efficiently the quantum dots could remit the light that they absorb, and the experiments showed that the performance of quantum dots was better as compared to a semiconductor.

This research work is the result of a collaboration between the labs of Alberto Salleo, professor of materials science and engineering at Stanford, and Paul Alivisatos, the Samsung Distinguished Professor of Nanoscience and Nanotechnology at the University of California, Berkeley, who is a pioneer in quantum dot research and senior author of the paper. However, this research is a part of the collection of projects of the Department of Energy at the Frontier Research Centre.

There are various benefits that quantum dots have. Being highly customizable, one of the biggest benefits of quantum dots is that it changes its shape due to which it can change the wavelength of light that they emit which is one of the biggest advantages in colour based applications like television.

Thus, quantum dots have hit the consumer market in the form of quantum dot TV or the QLED(where Q stands for quantum dots).

Samsung QLED TV

Samsung QLED TV 8K – 75 inches. Credit: Bretwa/ wikimedia

As we all know that everything in this universe comes with its own disadvantages, the disadvantage that the quantum dot has is that because of its smaller size – it takes many particles to come together in order to perform a particular task. In order to form so many quantum dots, the chances of something going wrong becomes highly possible, which indirectly means that the chances of some program to go wrong also becomes possible due to which there are chances of performance getting hampered.

The researchers are finding out measurement techniques in order to evaluate these particles.

The next step in the ongoing research involves even more precise measurements and if the researchers can determine that, these quantum dots could reach an efficiency of 99.9 percent or above.

With the increase in efficiency, we can have wonderful applications like:

  • New glowing dyes to enhance our ability to look at biology at the atomic scale.
  • Luminescent cooling and luminescent solar concentrators, which allow a relatively small set of solar cells to take in energy from a large area of solar radiation and many more things.

People working on these quantum dot materials have thought for more than a decade that dots could be as efficient as single crystal materials,” said Hanifi.

So, Let us hope for this research to go forward and get us many other efficient applications.

Published Researchhttp://science.sciencemag.org/content/363/6432/1199