Because of its properties, twisted bilayer graphene promises applications that include RF amplifiers, optical sensors, quantum computing, and radiation detectors. Commercialization and test challenges remain.
Early in the semiconductor industry, Gordon Moore made his famous prediction that the number of transistors on an IC would double every year through 1975. Today’s chips have over one billion transistors. Innovation with silicon, however, can no longer extend Moore’s Law. New technology needs to replace silicon to continue developing more advanced, smaller, cost-effective semiconductors. Twisted bilayer graphene could extend Moore’s Law.
A major impetus for the development of 2D materials is the replacement of silicon in integrated circuits (ICs), which is reaching the limits of shrinking gate and channel sizes. Below a few nanometers, silicon transistor performance exhibits severe deterioration due to carrier scattering.
The potential of graphene
In 2004, Drs. Novosleov and Geim at the University of Manchester proved that a single atomic layer of a substance could exist in a stable state and introduced a single atomic layer of carbon, designated as graphene. They initiated the field of 2-dimensional (2D) material research. Studies of graphene yielded significantly superior performance compared with silicon and the hope that graphene transistors could extend Moore’s Law.
Table 1 compares some of graphene’s properties (in the form of twisted bilayer graphene) with those of silicon. Graphene has substantially higher carrier mobility and thermal conductivity. In addition, it allows the creation of smaller FET transistors, and it is transparent to visible light.
Table 1. Comparison of the properties of twisted bilayer graphene with the properties of silicon. |
Twisted Bilayer Graphene
Unfortunately, graphene lacks a band gap, making it unsuitable as a digital logic transistor. Researchers discovered that by adding layers of 2D materials they can create band gaps in the multi-layer assembly. Twisted bilayer graphene (TBG) is one of those assemblies. Not only can it create a band gap, but it creates some surprising properties.
TBG is created by stacking two graphene layers on top of each other, with one layer rotated by a small angle (typically between 0.1° and 5°) relative to the other. This twisting leads to the formation of a moiré superlattice, a periodic variation in atomic alignment where, for example, atoms in one layer are above hexagonal voids in the other layer. This moiré pattern gives rise to new electronic states, including the ability to tune the material’s electronic band structure and its density of states. Figure 1 illustrates a moiré superlattice.
Figure 2 shows how an electric field applied to a TBG structure can create a bandgap.
TBG exhibits remarkable properties such as superconductivity and enhanced electron correlation effects at specific twist angles, particularly near a “magic angle” (around 1.1°). These phenomena have triggered significant interest in TBG for both fundamental physics and practical applications, particularly in electronics and optoelectronics.
RF applications
The RF and microwave frequency range (typically above 1 GHz) plays a crucial role in modern telecommunications, radar systems, and other communication technologies. Graphene, and particularly TBG, offers several advantages that make it suitable for ultra-high frequency applications. One of the key advantages of TBG in RF applications is high electron mobility. This high mobility allows for faster signal processing and high-speed operation, which is crucial for RF devices that require low loss and high efficiency. Additionally, TBG exhibits minimal resistance at the interface between the two graphene layers, enabling better performance in high-frequency electronic circuits.
Twisted bilayer graphene’s electronic properties, such as its carrier density and band structure, are tunable through either external electric fields or the twist angle between the graphene layers. This tunability makes TBG a suitable candidate for adaptive RF components, such as tunable filters, amplifiers, and oscillators. For instance, by adjusting the twist angle or applying a gate voltage, it is possible to modify the material’s electronic states to optimize its response at different frequencies, improving the performance of RF devices in a dynamic environment.
RF power amplification relies on field-effect transistors (FETs) for signal amplification. TBG-based transistors could significantly improve performance in this area due to their high mobility, efficient heat dissipation and low noise characteristics. Furthermore, the use of TBG in combination with other materials, such as two-dimensional (2D) semiconductors, could enable the development of highly efficient transistors that operate at ultra-high frequencies, paving the way for advanced communication systems.
Optical sensing applications
TBG’s unique optical properties also make it a promising candidate for optical sensor applications. Optical sensors are used in various applications, ranging from environmental monitoring to medical diagnostics and industrial sensing. The tunable electronic properties of TBG can be leveraged to create sensors with high sensitivity and selectivity across different wavelengths of light.
Figure 3 shows an implementation of a TBG photodetector whose optical sensitivity varies with the twist angle of the twisted bilayer graphene. Graphic “a” shows the structure of the device. In “b,” the plot displays the calculated low energy bands when the twist angle is 4.1°. Plot “c” indicates optical conductivity as a function of twist angle for a 1550 nm incident wavelength. Plot “d” compares light absorbance as a function of channel length for TBG and single-layer graphene. In “e,” an optical microscopy image shows the TTBG layers, and “f” shows a transmission electron microscope image of the moiré pattern with the red dashed lines.
The electronic structure of TBG, especially near the magic angle, leads to unusual optical responses, such as enhanced light absorption and photodetection. This property is crucial for developing next-generation photodetectors that operate in a broad spectral range, from the ultraviolet to the infrared. The ability of TBG to absorb light efficiently across a wide range of wavelengths makes it ideal for applications for optical sensors, including those used in spectroscopy and imaging as well as sensing.
The tunable band gap allows TBG to function as a highly adaptable material for optical sensors, where different sensing modalities (e.g., light intensity, wavelength, polarization) can be selectively tuned by adjusting the material’s electronic structure. This adaptability can enable the development of smart optical sensors with reconfigurable properties that are capable of detecting a wide variety of optical signals.
TBG has nonlinear optical properties that could be harnessed for advanced sensor applications. Nonlinear optical effects, such as second-harmonic generation and self-focusing, are important for various applications, including high-resolution imaging and optical communication. By exploiting these nonlinear effects, TBG could be used to develop highly sensitive sensors that detect minute changes in optical signals, potentially improving the performance of optical communication systems and enabling new sensing technologies in biology and chemistry.
Quantum computing
Around the magic angle, TBG becomes a superconductor in which its electrical resistivity falls to 0 Ω at temperatures around 1 K. The energy bands near the Fermi level, the highest energy level that an electron can occupy at absolute 0 K, which lies between the valence and conduction bands in the bandgap of a semiconductor, become almost flat. The flat bands result in the electrons having a lower kinetic energy allowing enhanced effects of electron-electron interaction. As a result, TBG can be a possible quantum device, such as a quantum bit (qubit), for use in a quantum computer. Quantum computing requires materials that both exhibit superconductivity and maintain coherence over long time intervals. TBG can potentially serve as a critical material for this emerging field.
Spintronics
Spintronics, spin-based electronics, uses manipulation of electronic spin states rather than electron mobility for information storage and data processing. The spin represents the electron’s angular momentum. Spintronic devices have lower power consumption due to not having to propel electrons through material. Also, spintronic memory devices can have faster memory read and write speeds. TBG combined with other 2D materials, such as transition metal dichalcogenides (TMDs), enhances spin-orbit coupling for effective manipulation of spin states. A TBG-TMD heterostructure can create high-speed, low-power spin FETs and fast, low-power memory devices.
Radiation and particle detectors
TBG can have transformative potential for radiation and high-energy particle detectors. The flat bands created by the magic angle orientation of the two layers of graphene permit high sensitivity to small perturbations caused by radiation or particle interactions. TBG’s low electrical noise further enhances detection sensitivity. The tunable bandgap can make TBG detectors adaptable to a range of radiation energies. High carrier mobility provides fast response to incident radiation or particles. TBG-based detectors can lead to improved particle detectors, x-ray detectors, and cosmic ray detectors for applications from medicine to space.
Challenges with commercialization
Despite its potential, there are still significant challenges in realizing the full capabilities of TBG. Fabrication methods need to be refined to ensure that high-quality TBG can be reproducibly produced on a large scale. Additionally, integrating TBG with existing semiconductor technologies remains a major hurdle, as this would require compatibility with traditional device architectures and integration with other materials like dielectric layers and gate electrodes.
Test challenges
A critical element of commercializing and scaling of TBG-based specialized semiconductor devices, sensors, and integrated circuits will be for next-generation DC, RF, and optical-characterization instrumentation with signal performance suitable for these materials and devices. These nanoscale devices require testing at low voltages and currents to minimize device heating effects and alteration of device parameters.
For DC characterization of TBG devices, source instrumentation must have very low noise to avoid affecting the inherent operating characteristics of the devices. Minimizing source noise at both low (0.1 Hz to 10 Hz) and higher frequencies (10 kHz to 20 KHz and beyond) will be important as TBD-based devices are more sensitive to a wider range of signal amplitudes and frequencies. On the measurement side, fully characterizing the expanded operating ranges of TBG-based devices will require commensurate increases in DC and AC signal sensitivities. Additionally, extracting the signals from noise becomes more challenging when measuring small signals.
Instrumentation designed for today’s semiconductor testing lacks voltage sourcing sensitivity, low noise sourcing, and ability to measure small signals in the presence of noise that can have magnitudes on the order of the signal magnitudes. High-sensitivity instruments that recover small signals from noise use lock-in measurement technology and are primarily used for research applications by physicists, materials scientists, and chemists.
Lock-in technology enables extraction of a signal from a high noise level by mixing the noisy signal with a reference signal of known frequency and low-pass filtering the resultant signal. The DC component of the filtered signal is the magnitude of the signal at the frequency of the reference signal. All other frequency components and the noise in the original signal are filtered out. Lock-in technology allows measuring a signal at a frequency where the noise level is low, which maximizes measurement accuracy.
To test TBG and other 2D devices, test engineers will need to become familiar with and use instrumentation with lock-in technology that has been a staple in the researcher’s domain. Fortunately, sensitive Instrumentation that employs lock-in technology and is designed for use by test engineers is becoming available.
Concluding thoughts
In the near-term future, twisted bilayer graphene is poised to become a key material for ultra-high frequency RF applications, optical sensors, and a wide range of specialized semiconductor technologies. The tunability of TBG’s electronic and optical properties offers tremendous potential for designing adaptive devices that can operate in dynamic environments, from telecommunications to quantum computing and beyond. While challenges remain in terms of fabrication, scalability, integration with existing technologies, and testing, the ongoing research into TBG’s unique properties and the rapid advancements in fabrication techniques indicate that these hurdles are likely to be overcome in the coming years.
References
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