In the realm of computing, bigger is better: scalability allows computers to perform increasingly demanding tasks using fewer resources. From the first vacuum-tube computers to pushing the limits of today’s silicon chips, boosts in computing power have often been thanks to breakthroughs in material synthesis.
Quantum computers, which are governed by the laws of quantum physics as opposed to classical physics, also depend on material development. Scaled-up quantum computers require an increasing number of qubits, their basic building blocks, but supporting this number of qubits is no easy feat. While qubits are highly sensitive to changes in the environment, quantum computers that are too protected from these external factors risk leaving their qubits inaccessible, which in turn renders the system impractical to perform real calculations.
In the search for a material that strikes a balance between stability and accessibility, Kuan Eng Johnson Goh, a Principal Investigator at A*STAR’s Institute of Materials Research and Engineering (IMRE), turned to make qubits out of a two-dimensional transitional dichalcogenide (TMDC) semiconductor material called tungsten disulfide (WS2).
Characterised by a crystal structure made up of two different metal atoms, WS2 exhibits unique properties that can give quantum devices optimal characteristics such as higher carrier mobility. However, conventional methods used to synthesise WS2, such as mechanical exfoliation, are inconsistent and prone to contamination.
To overcome these limitations, the research team married two different industrial processes to make WS2 semiconductor crystals. In this novel two-step process, the research team first enabled the formation of a thin layer of WS2 through chemical vapour deposition (CVD). Compared to conventional methods, CVD allows the synthesis of WS2 on a large, scalable area. The second step entailed the uniform encapsulation of the WS2 crystals with a protective dielectric layer made of hafnium oxide (HfO2).
Through this technique, the team managed to synthesise stable WS2 crystals over an area hundreds of times larger than that of mechanical exfoliation. “This provides enough real-estate to fabricate many tens of quantum devices so that we can effectively test them to further enhance the synthesis process,” said Goh.
The researchers also discovered that imperfections between the WS2 and HfO2 layers are detrimental to the working efficiency of the quantum devices. The study marks the first time this property, called “interface roughness”, has been systematically measured and quantified.
Moving forward, the research team plans to carry out further work to reduce and eliminate interface roughness that affects the performance of quantum devices. “Further optimisation of the qubit design will allow us to synthesise high-quality single- and double-qubit gates,” Goh said.
The A*STAR researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and Institute of High Performance Computing (IHPC).