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Controlling Ultra-high Speed Electron Tunneling Waves

Wednesday, November 22, 2017

Fig. 1: Scanning transmission electron microscopy (STEM) image of the thin layers within a resonant tunneling diode (RTD). An overlay of the electron waves shows the quantum states (in red) that enable resonant tunneling transport.

Resonant tunneling diodes are semiconductor devices which enable electrons to travel at ultra-high speeds. However, engineering this quantum transport regime within the III-Nitride family of semiconductors has remained challenging for almost twenty years. Now, advances in epitaxial growth and device fabrication at Cornell, have led to groundbreaking observations into the resonant tunneling physics of III-Nitride semiconductors. These results not only raise hopes for future practical applications, but also reveal fundamental properties of semiconductor quantum heterostructures.

Resonant tunneling diodes (RTDs), being the fastest active electronic devices to date, are candidates for developing compact solid-state sources which will be at the heart of future ultra-high speed communication networks. The versatility of these devices has been already demonstrated by achieving wireless data transmission rates 100 faster than current standards [1]. This milestone has been possible due to the intrinsic fast speed of these devices which are capable of generating high-frequency signals within the terahertz (THz) band. Ultra-high speed data transmission is not the only important application enabled by RTDs. THz Imaging [2] and spectroscopy [3] systems have also been envisioned for security scanners and explosives detectors. Furthermore, non-destructive quality control systems for medical and pharmaceutical industries have also become an increasingly important application. However, despite the broad range of practical applications enabled by these devices, their limited output power remains the major technical limitation, preventing field deployment.

In this scenario, the III-Nitride family of semiconductors has emerged as a potential solution to the limited output power available from current RTD-sources. This revolutionary family of semiconductors are routinely synthesized by a group of researchers led by Cornell engineering professors Debdeep Jena and Grace Xing. III-Nitride semiconductors, characterized by its efficient light emission, have already garnered the Nobel prize in physics three years ago. However, its versatility could be further expanded with the demonstration of light-emitters within the infrared and THz bands. These applications, however, rely on engineering artificial electronic levels and devising electron transport mechanisms to populate them. Nonetheless, for almost twenty years, resonant tunneling, a quantum transport process capable of solving these limitations, has remained elusive in III-Nitride semiconductors.

Now, in a recent study, the Cornell research group, has shown that III-Nitride semiconductors are capable of robust resonant tunneling transport which persists even at room temperature. These results, described by a reviewer as a “highly significant achievement”, have been recently published in Physical Review X [4].

Advances in crystal growth quality combined with the use of state-of-the-art tools, has enabled the observation of unprecedented quantum transport signatures. These unique results not only raise hopes for future practical applications, but also reveal fundamental properties of semiconductor quantum heterostructures which have remained masked over decades.

Resonant tunneling relies on quantum mechanical effects which dictate that electron waves are allowed to tunnel across a double-barrier structure such as the one shown in Fig. . This image shows the atomic layers that comprise the device under study, across which, electrons tunnel at ultra-high speeds. Quantum confinement introduced by the barriers generates artificial electronic levels in the central layer which enhance electron transport. Nonetheless, these artificial levels, depicted by the red wavefunctions, are extremely sensitive to thickness variations and structural defects. The high density of these imperfections have prevented previous research groups to engineer resonant tunneling in III-Nitrides.

Using molecular beam epitaxy (MBE), a technique which is the specialty of the Jena-Xing laboratory, precise control over the layer thickness and high quality interfaces were attained. This has led to robust quantum transport effects that allowed researchers to detect and measure several unexpected tunneling features. It turns out that these features are intimately connected with the crystal structure of these materials. By uncovering this connection, the team came up with a new metrology scheme which can be employed to probe internal electric fields with unprecedented highest precision. Such internal fields, known as polarization fields, play a crucial role in the performance of photonic and electronic devices. Thus, this novel technique, pioneered at Cornell, might enable the design of more efficient light emitting diodes (LEDs) and transistors.

Looking forward, these findings pave the way for realizing III-Nitride-based high-speed electronic oscillators and room temperature THz quantum cascade lasers which can reach wavelengths that have remained unachievable by other semiconductors. Finally, by introducing a novel metrology tool, researchers have further expanded the versatility of III-Nitride semiconductors, which could potentially span yet unexplored fields in the near future.

References

1 R. Izumi, S. Suzuki, and M. Asada, “1.98 THz resonant- tunneling-diode oscillator with reduced conduction loss by thick antenna electrode,” in 42nd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) (2017) pp. 1–2.

2 T. Miyamoto, A. Yamaguchi, and T. Mukai, “Terahertz imaging system with resonant tunneling diodes,” Jpn. J. Appl. Phys 55, 032201 (2016).

3 S. Kitagawa, M. Mizuno, S. Saito, K. Ogino, S. Suzuki, and M. Asada, “Frequency-tunable resonant-tunneling-diode terahertz oscillators applied to absorbance measurement,” Jpn. J. Appl. Phys 56, 058002 (2017).

4 J. Encomendero, F. A. Faria, S. M. Islam, V. Protasenko, S. Rou- vimov, B. Sensale-Rodriguez, P. Fay, D. Jena, and H. G. Xing, “New tunneling features in polar III-Nitride resonant tunneling diodes,” Phys. Rev. X 7, 041017 (2017).

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