Graphene’s quantum tunnel advances the era of terahertz wireless communication

Scientists from MIPT, Moscow State Pedagogical University and University of Manchester have created a highly sensitive terahertz detector based on the effect of quantum mechanics tunneling on graphene. The sensitivity of the device is already superior to commercially available analogs based on semiconductors and superconductors, opening up prospects for graphene detector applications in wireless communication, security systems, radio astronomy and medical diagnostics. The research results are published in a prestigious journal Nature Communications.

Quantum tunnel. Image credit: Daria Sokol, MIPT Press Office

Information transmission in wireless networks is based on converting a continuous high-frequency electromagnetic wave into a discrete series of bits. This technique is known as signal modulation. To transfer bits faster, one has to increase the modulation frequency. However, this requires a simultaneous increase in the carrier frequency. A common FM radio sends at frequencies of 100 megahertz, the Wi-Fi receiver uses approximately five gigahertz signals, while 5G mobile phone networks can transmit up to 20 gigahertz signals. This is far from the limit, and a further increase in the carrier frequency allows for a proportional increase in the data transfer rates. Unfortunately, capturing signals at frequencies of one hundred GHz and higher is an increasingly difficult problem.

A typical receiver used in radio communications consists of a transistor-based amplifier for weak signals and a demodulator that corrects the bit sequence of the modulated signal. This scheme originated in the age of radio and television, and became ineffective at frequencies of hundreds of gigahertz that were desirable for mobile systems. The truth is that most of the transistors out there are not fast enough to recharge at such a high frequency.

An evolutionary way to solve this problem is only to increase the maximum operating frequency of the transistor. Most nanoelectronics professionals are working hard in this direction. A revolutionary method for solving the problem was proposed in theory at the beginning of the 1990s by physicists Michael Diakonoff And the Michael ShoreI realized, among other things, By a group of authors in 2018. It means abandoning active transistor amplification, and abandoning a discrete demodulator. What remains in the circuit is one transistor, but its role is different now. It converts the modified signal into a bit sequence or an audio signal by itself, due to the non-linear relationship between the current and voltage drop.

In the present work, the authors have demonstrated that terahertz signal detection is highly effective in so-called tunneling field-effect transistors. To understand its operation, one can only recall the principle of electromechanical relay, in which the passage of current through the control contacts leads to a mechanical connection between two conductors and, accordingly, to the appearance of current. In tunneling transistor, applying voltage to the control contact (called the “ gate ”) causes the power levels of the source and channel to align. This also causes current to flow. The defining feature of a tunneled transistor is its extremely strong voltage control sensitivity. Even a simple “deconstruction” of energy levels is sufficient to interrupt the delicate process of quantum mechanical tunneling. Likewise, a small voltage at the control gate is able to “connect” the levels and initiate the tunnel current.

“The idea of ​​a strong reaction of a tunneled transistor to lower voltages has been known for about fifteen years,” Dr. Dmitry Sventsov, One of the study authors, is head of the 2D Materials Optoelectronics Lab at the MIPT Center for Photonics and 2D Materials. “But it is only known in the low-power electronics community. Nobody before us realized that the same property of tunneling transistors could be applied in terahertz detector technology. Georgy Alimov (Study co-author) I was fortunate to work in both areas. We then realized: if the transistor is opened and closed at low power from the control signal, it should also be good at picking up weak signals from the surrounding environment. “

The device created is based on two-layer graphene, a unique material with which the position of the energy levels (more strictly, the band structure) can be controlled using an electric voltage. This allowed the authors to switch between classical and quantum tunneling within a single device, with just a change in the polarity of the voltage at the control contacts. This possibility is of prime importance for an accurate comparison of the detection capacity of the classical and quantitative tunneling transistor.

Experience has shown that the sensitivity of the device in the tunneling mode is slightly higher than in the classic transmission mode. The minimum signal that the detector distinguishes against a noisy background actually competes with that of commercially available semiconductor and superconducting pressure meters. However, this is not the limit – the sensitivity of the detector can be increased in “cleaner” devices with a low concentration of residual impurities. The developed detection theory, tested by the experiment, shows that the sensitivity of an “optimum” detector can be 100 times higher.

“The current characteristics raise great hopes for the creation of fast and sensitive detectors for wireless communication,” says the author of the work. Dr.. Denise Bandurin. This region is not limited to graphene nor is it limited to tunnel transistors. We expect that, with the same success, a wonderful detector could be constructed, for example, based on an electrically controlled phase transition. It turns out that graphene is just a good starting point here, just a door, behind a whole world of exciting new research. “

The results presented in this paper are an example of successful collaboration between several research groups. The authors note that it is this form of work that allows them to obtain universal scientific results. For example, earlier, the same team of scientists explained how waves in an electron sea from graphene could contribute to the development of terahertz technology. “In the era of rapidly developing technology, it is becoming increasingly difficult to achieve competitive results.” Comments Dr. Georgy Fedorov, Vice President of Carbon Nanomaterials Laboratory, MIPT, – “Only by combining the efforts and expertise of several groups can we successfully achieve the most difficult tasks and achieve the most ambitious goals, which we will continue to do.”

The work was supported by the Russian Science Foundation (Grant No. 16-19-10557) and the Russian Foundation for Basic Research (Grant No. 18-29-20116 M.K.).

Source: MIPT

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