If you’ve ever tried to apply a screen protector to a smartphone, you know it can be tricky to achieve a smooth, clean interface between the two layers: one without trapped bubbles or dust. Nanoelectronics engineers face a similar challenge when applying insulating films known as dielectrics onto 2D semiconductors, but an infinitely trickier one, as both layers are thinner than the eye can see.
Currently, one promising method for this task is atomic layer deposition (ALD), which can directly grow an ultrathin dielectric layer on material surfaces. However, this method is difficult to translate to 2D material surfaces and often creates tiny pinholes and bumps on the dielectric, which can disrupt the way electrons move between the two layers.
“Rough interfaces between dielectric and 2D semiconductor layers can limit a device’s overall performance,” said Aaron Lau and Johnson Goh, respectively Senior Scientist and Senior Principal Scientist at the A*STAR Quantum Innovation Centre (Q.InC).
To address the issue, Lau, Goh and colleagues at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE), including former Scientist, Dasari Venkatakrishnarao, developed a new ALD-based approach: instead of direct deposition, they first deposited the dielectric material onto a sacrificial layer of liquid metal (LM) oxide, which acted as a smooth, high-quality mould for the resulting film.
“Our previous work showed that the liquid conformal nature of LM oxides can enable atomically-smooth interfaces,” explained Lau and Goh. “We realised that LM oxides could be used as a sacrificial material to create similarly uniform interfaces between ALD-grown dielectrics and 2D semiconductors.”
Schematic of liquid metal oxide-assisted transfer of hafnium oxide (HfO2), a dielectric material. Ultrathin gallium oxide (~3 nm) is printed from liquid gallium metal onto a supporting silicon substrate, then goes through plasma treatment before atomic layer deposition (ALD) of HfO2. A polymer support layer is spincoated on before the gallium oxide is acid-etched away and washed. The HfO2/polymer stack is then transferred onto a 2D target substrate over 2D materials.
© A*STAR Research
Like a sticker’s paper backing, the LM oxide layer could be later ‘peeled’ off the dielectric film with wet etching, freeing the film for transfer elsewhere.
Using their method, the team successfully integrated hafnium oxide (HfO2), an industrial relevant dielectric material, onto a 2D layer of tungsten disulphide (WS2), using LM gallium oxide as a sacrificial layer. With support from the National University of Singapore’s Michel Bosman, the team used advanced imaging techniques to capture detailed nanoscale photos of the resulting WS2/HfO2 interfaces, which were visibly smoother than those produced by other methods.
“Our method offers several advantages. The atomically-smooth interfaces reduce flaws that can trap electrical charges, leading to better control of electric fields and less energy loss from leakage currents,” noted Lau.
The team’s method also worked well with gold electrodes embedded into the HfO2 layer, creating a simplified single-step version of a normally multi-step stack fabrication process. “By making fabrication less complex, we can reduce costs and speed up testing for new materials,” Goh added.
Next, the researchers plan to explore other dielectrics compatible with their technique and improvements in device performance. They also aim to refine the production process for large dielectric sheets, focusing on maintaining uniformity and avoiding defects like wrinkles or cracks.
“We hope our LM-assisted approach can lead to a new way of integrating dielectrics with 2D transistors, producing high-performance, energy-efficient devices,” said Lau and Goh.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Quantum Innovation Centre (Q.InC) and the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
