Tokyo Tech researchers develop new fabrication process for high-mobility AOS TFTs

Article By : Gina Roos

Researchers at Tokyo Tech solve the trade-off between mobility and stability in AOS TFTs with a new fabrication process.

Researchers at the Tokyo Institute of Technology (Tokyo Tech) have developed a novel way to fabricate thin-film transistors (TFTs) for next-generation, large flat-panel displays that overcomes the trade-off between mobility and stability in amorphous oxide semiconductor-based TFTs. This trade-off has limited the ability of these devices to replace current polycrystalline silicon technologies, said researchers.

“Amorphous oxide semiconductors (AOS) are a promising option for the next generation of display technologies due to their low costs and high electron (charge carrier) mobility. The high mobility, in particular, is essential for high-speed images. But AOSs also have a distinct drawback that is hampering their commercialization — the mobility–stability tradeoff,” said the researchers.

However, they believe by fabricating the TFTs on indium tin zinc oxide (ITZO) it will open up opportunities for new display technologies that are lower cost than current silicon-based devices, while eliminating the above-mentioned tradeoffs.

Researchers reported their findings in Nature Electronics. In the paper they describe how they fabricate the transistors with high stability and mobility.

The researchers targeted two AOS TFTs – indium gallium zinc oxide (IGZO) and ITZO. According to the researchers, the challenge was IGZO TFTs exhibit high negative-bias temperature stress (NBTS) stability, one of the core tests for stability, but poor mobility, while ITZO TFTs have the opposite characteristics.

The Tokyo Tech team focused on NBTS stability, which is explained using “charge trapping” that describes the loss of accumulated charge into the underlying substrate. They focused on “the possibility of a change in carrier density or Fermi level shift in the AOS itself,” said Junghwan Kim, assistant professor, Tokyo Tech, who headed the study, in a statement.

Kim said this was accomplished by using a “bottom-gate TFT with a bilayer active-channel structure” comprised of an NBTS-stable AOS (IGZO) layer and an NBTS-unstable AOS (ITZO) layer. The researchers characterized the TFT and compared the results with device simulations using the charge-trapping and the Fermi-level shift models. They found the experimental data agreed with the Fermi-level shift model.

Tokyo Tech's fabrication of high mobility and stability AOS TFTs

Click for a larger image. (Source: Tokyo Institute of Technology)

The next challenge was finding the biggest factor impacting the mobility in the AOSs. By understanding that the fabrication of AOS TFTs introduces impurities, including carbon monoxide (CO), into the TFT, particularly in the ITZO layer, the team found that “charge transfer was occurring between the AOSs and the unintended impurities,” causing the Fermi-level shift and NBTS instability.

“The mechanism of this CO-based electron donation is dependent on the location of the conduction band minimum, which is why you see it in high-mobility TFTs such as ITZO but not in IGZO,” said Kim.

The researchers developed a NBTS-stable ITZO TFT without CO impurities by treating the TFT at 400°C. “Super-high vision technologies need TFTs with an electron mobility above 40cm2 (Vs)-1. By eliminating the CO impurities, we were able to fabricate an ITZO TFT with a mobility as high as 70cm2 (Vs)-1,” he added.

But more work has to be done. “Any impurity that induces a charge transfer with AOSs can cause gate-bias instability. To achieve high-mobility oxide TFTs, we need contributions from the industrial side to clarify all possible origins for impurities,” Kim said.

In addition to use in display technologies, the researchers believe similar AOS TFTs could be applied to other devices such as chip input/output devices, image sensors, and power systems.

This article was originally published on Electronic Products.

Gina Roos is the editor-in-chief of Electronic Products.


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