New technique for transferring thin semiconductor films onto substrates

Scientists in the US have developed an approach for transferring thin semiconductors films onto arbitrary substrates, in research that could lead to innovations in flexible computing and photonic devices. The films, which are just one atom thick, were transferred without causing any wrinkles or cracks, not easy given their fragility.

The team from North Carolina State University, whose study was reported in the journal ACS Nano [Gurarslan et al. ACS Nano (2014) DOI: 10.1021/nn5057673], used a transfer process for the molybdenum sulfide (MoS2) atomic-scale thin films that is much quicker than the usual technique of chemical etching. To achieve this breakthrough, it was necessary to transfer the thin films, which are up to five centimeters in diameter, to a flexible substrate, as it is not possible to make the thin film on a flexible substrate as they cannot withstand the high temperatures involved.

MoS₂ is a comparatively cheap semiconductor material that has electronic and optical properties akin to materials already being used by the semiconductor industry. This new process is also better than chemical etching because the latter can damage or even contaminate the film. In this study, the method benefits from the physical properties of MoS₂ to move the thin film with only the aid of water at room temperature, with the entire process only taking a couple of minutes, as opposed to the hours that chemical etching entails.

While MoS₂ is hydrophobic and repels water, the sapphire substrate on which the thin film is grown is hydrophilic. The transfer method depends on a drop of water being placed on the thin film before manipulating the edge of the film with tweezers to ensure the water starts to penetrate between the MoS₂ and the sapphire substrate. When this happens, the water pushes into the gap, thereby floating the thin film to the top.

The team used tissue to absorb the water and then raise the thin film, positioning it on a flexible substrate. The water disrupts the adhesion between the substrate and the thin film, although it is imperative to remove the water before moving the film or it will wrinkle or fold on being picked up due to capillary action.

As senior author Linyou Cao pointed out, “This new transfer technique gets us one step closer to using MoS₂ to create flexible computers. We are currently in the process of developing devices that use this technology.”