Materials that can withstand cryogenic temperatures are a pre-requisite for practical quantum computing.

© Lau Chit Siong

Frozen: The quantum edition

14 Dec 2020

To unlock the potential of quantum computers, researchers must find semiconductor materials and metal contacts that work well at sub-zero temperatures.

Imagine a computer so blazingly fast and impenetrable that no hacker can crack it. This is not science fiction: this is a real-world application of quantum computing, an emerging field that exploits the specific energy states of semiconductor materials to create devices with incredible processing power.

Now imagine having to operate this quantum computer at temperatures as low as 4 Kelvin, or -270 degrees Celcius. These sub-zero conditions are necessary as the fragile quantum state of these devices are susceptible to a physical phenomenon—heat.

In search of novel semiconductor materials that can withstand cryogenic temperatures, a team led by Kuan Eng Johnson Goh, a Principal Investigator at A*STAR’s Institute of Materials Research and Engineering (IMRE), focused on tungsten disulfide (WS2), a two-dimensional transitional dichalcogenide (TMDC) semiconductor material.

Unlike molybdenum disulfide (MoS2), its well-studied TDMC counterpart, WS2 is expected to have higher carrier mobility due to its lower effective mass compared to MoS2. However, WS2 remains poorly understood due to the low quality of materials available and the lack of a robust contact strategy needed to probe its electronic quality. “Forming reliable contacts is the first step towards building quantum devices,” said Chit Siong Lau, the lead author of the study.

Goh and colleagues selected an indium alloy for the metal contacts in their devices, based on a technique pioneered by co-author Manish Chhowalla at the University of Cambridge. Indium alloy contacts help to improve electron transport performance in quantum devices by overcoming contact resistance.

They built two WS2 devices: single-layer and bilayer devices, and showed that both devices fared excellently down to 3 Kelvin, thanks to the high quality of the indium alloy contacts at these chilly conditions. The bilayer device, however, had one advantage: because electrons tend to travel in the top layer, the bottom layer acted as a protective layer, shielding electrons from defects in either the metal substrate or along the WS2−indium interface.

These experimental findings were supported by density-functional theory simulation studies, which provided insights into quantum transport and the properties of the WS2−indium interface.

Beyond the bilayer device, Goh’s team is now looking to design a trilayer device—a ‘sandwich’ device that will shield the WS2 layer from defects in the substrate and contaminants in the environment. “Our research unlocks the commercial potential of WS2 for diverse applications such as transistors, optoelectronics, flexible electronics, photodetectors and sensors, as well as in low-temperature quantum devices for quantum information processing,” Goh said.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).

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Lau, C.S., Chee, J.Y., Ang, Y.S., Tong, S.W., Cao, L., et al. Quantum Transport in Two-Dimensional WS2 with High-Efficiency Carrier Injection through Indium Alloy Contacts. ACS Nano 14 (10), 13700-13708 (2020) | article

About the Researcher

Kuan Eng Johnson Goh

Principal Investigator

Institute of Materials Research and Engineering
Kuan Eng Johnson Goh is Group Leader for Quantum Materials and Devices, and Principal Investigator for Valleytronics and Spin-valley Qubits at A*STAR’s Institute of Materials Research and Engineering (IMRE). His current research interests are in quantum computation, quantum transport and quantum effect devices. He obtained his PhD degree in 2007 from the Centre of Excellence for Quantum Computer Technology at the University of New South Wales, where he established an encapsulation method for phosphorus dopant-based spin qubits in silicon. He joined A*STAR in 2006 and has served as Program Manager for the ‘Ubiquituous Electronics: The Case for Semiconductor 2D Materials’ program from 2016 to 2020, and as Head of Department from 2014 to 2017.

This article was made for A*STAR Research by Wildtype Media Group