Novel technique to shield 2D materials from vibrations

Monash University researchers have demonstrated a new, counterintuitive way to protect atomically thin electronics by adding vibrations to reduce vibrations.

By ‘squeezing’ a thin droplet of liquid gallium, graphene devices are painted with a protective coating of glass, gallium-oxide. The oxide is thin, less than 100 atoms, yet covers centimetre-wide scales, making it potentially applicable for industrial large-scale fabrication. Lead author Matthew Gebert said that mechanically transferring such large-area nanosheets is quite novel; the oxide provides a new method of device protection while improving device performance.

Co-author Semonti Bhattacharyya said the oxide enhances and protects devices when it is first transferred and also later, during subsequent processing and fabrication. Gallium-oxide’s enhancing performance is due in part to the material’s high-K dielectric properties, a key component in miniaturising devices and reducing power wastage. Protective gallium-oxide also reduces the electrical resistance in graphene that is caused by thermal vibrations due to heat in the surrounding materials.

“That’s surprising because in effect we are actually adding extra vibrations, to reduce total vibrations,” Gebert said.

The Monash researchers from the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET) used a liquid-metal printing technique to create gallium-oxide (Ga203) glass. This method was designed by FLEET collaborators at RMIT, who used the novel glass in a variety of electronics applications. The glass film that forms on the surface of droplets of liquid gallium metal is more than 5000 times thinner than a human hair, but can be ‘printed’ from the surface of the liquid metal to form uniform continuous layers over centimetre-sized areas. The liquid-metal method offers two advantages; the layer-printing method prevents growth damage, while the transferred layer is a good barrier for further processing.

Gallium-oxide encapsulation offers protection and can also enhance performance because of its High-K dielectric qualities. High-K dielectrics have not been easy to incorporate with graphene, as growth of these materials often involves the bombardment of highly energetic atoms. As gallium-oxide encapsulation is a mechanical transfer technique, it is different to alternative deposition methods (such as atomic layer deposition, evaporation, sputtering and vapour deposition), which have undesirable attributes such as high temperature requirements. Because gallium metal is liquid close to room temperature (30֯ C), this process has advantages for industrial adoption. Gallium-oxide can also be used as a buffer layer before further processing using these other methods.

The Monash researchers demonstrated that gallium-oxide protected the graphene from surface damage by testing their graphene devices with industrial growth tools. Depositing another oxide layer damaged the uncovered areas of graphene, while the areas that were covered by gallium-oxide retained their qualities. Electrically-insulating (dielectric) materials are important in the function of transistors, the ‘switches’ at the heart of electronics and computing. These dielectric materials allow a transistor to switch on or off without leaking power, which in turn allows users to use their phone/PC. To ‘switch’ a transistor, electrons accumulate across the dielectric material to create a voltage and influence the device. However, thinner dielectrics leak current — reducing the ability to switch — and wasting current as heat. High-K dielectrics are important because they increase the effectiveness of the switch, allowing a reduction in current leakage and consequently energy wastage.

However, High-K dielectrics devices are not impervious to size; as electronic materials get smaller, materials become strongly influenced by the surfaces of neighbouring materials, often resulting in a drop in performance. This explains why graphene is often damaged by High-K dielectrics. One of these degrading phenomena that occur at surfaces is material vibrations.

The vibrations of materials due to heat, which cause electrical resistance in materials, are called phonons. These vibrations (phonons) cause the atoms in a solid to oscillate, and flowing electrons bounce off these oscillations and change their direction, leading to electrical resistance. The thermal vibrations of the carbon atoms in graphene cause little resistance, which is one reason why graphene is a useful material for electronics. However, its thin nature means that thermal vibrations in surrounding (remote) materials can have a large effect on electrons in graphene, and these are the predominant cause of electrical resistance in graphene at room temperature. As temperatures increase, more photons are excited, increasing the resistance by scattering electrons.

“You can think of this scenario as a fence. The fence (the 2D graphene) is affected by the actions of neighbours on both sides (the insulating materials on either side of graphene). One neighbour might have a clean environment on their side of the fence (a good insulator, with few phonons) but the other neighbour might have an overgrown garden that damages the fence (a bad insulator with strong phonons) … So in the end, your fence (graphene) doesn’t serve the purpose it was intended to, perhaps not even forming a complete fence (electronic circuit) anymore,” Gebert said.

To investigate the protective qualities of the gallium-oxide, researchers mechanically transferred large areas onto graphene devices. Subsequent measurements confirmed that graphene’s electronic properties at various temperatures and electron populations were maintained and high electron mobility was preserved. Gebert said that adding the layer of Ga203 glass reduces the electrical resistance in graphene that is due to photon scattering. This is true in a target range of temperatures, which is slightly below room temperature. “This is counter-intuitive, because by adding this material, you are introducing additional phonons. So you might think: the more phonons, the higher we would expect resistance to be,” Gebert said.

However, these results agree with existing theories of photons in insulators. Ga203 hosts strong phonons, but this same property also allows it to adjust its own atomic configuration to ‘screen’ the electric field from phonons in the silicon-dioxide glass on the other side of graphene. The strong Ga203 phonons are modes that require high energy to populate. As a result, Ga203 phonons only become active at higher temperatures (with more thermal energy), resulting in lower overall resistance in graphene until a temperature of -53°C (220 K). Gallium-oxide only picks up the good vibrations.

This strategy, to reduce overall phonons content, could be used to identify better-performing hybrid materials at room temperature for 2D electronics. A similar dielectric material with higher-energy phonon modes than Ga203 could partner well with existing silicon technologies, which are currently being pushed to their quantum-scale limits. The liquid-metal printing technique is a versatile method for industrial partners. The process for touch-printing Ga203 scales to large wafer-scale areas is automatable and has shown good reproducibility, indicating its merit for industry adoptability. Gallium metal, which melts at about 30°C, and the transfer of equipment are also inexpensive compared to other oxide deposition methods which require large amounts of material or highly elevated temperatures.

The study was published in Nano Letters in November 2022.

Image credit: iStock.com/krystiannawrocki