NTU team breaks semiconductor testing barrier, directly measures 2nm carrier transfer length

A research team at National Taiwan University has achieved a significant breakthrough in semiconductor testing, developing a novel technique to directly measure the carrier transfer length in 2D semiconductor transistors. This advancement addresses a critical bottleneck in the ongoing miniaturization of chips for future generations.

The new method, dubbed "in-situ cross-sectional scanning tunneling microscopy semiconductor testing technology," allows for the direct measurement of the carrier transfer length at the contact edge of 2D semiconductor transistors. The team experimentally determined this length to be approximately 2 nanometers. This direct measurement is crucial because as transistors shrink, not only the channel length but also the contact area between metal and semiconductor must be scaled down accordingly.

Traditionally, the carrier transfer length, the effective distance electrons travel to inject from metal into a semiconductor, was indirectly calculated using the Transfer Length Method (TLM). This involves fabricating a series of devices with varying contact sizes and inferring the length from electrical measurements. However, the simplified assumptions of TLM are becoming less applicable to 2D semiconductors, leading to significant discrepancies, sometimes tenfold, in results from different studies in recent years.

Led by Distinguished Professor Chiu Ya-ping of the NTU Department of Physics, the team leveraged nearly 20 years of expertise in cross-sectional scanning tunneling microscopy. They integrated semiconductor devices into an ultra-high vacuum measurement platform. In this vacuum environment, the devices were mechanically cleaved to expose a clean cross-section while remaining electrically active. Probes were then used to scan the electronic structure point-by-point at the metal-semiconductor interface. This allows for direct measurement during actual device operation, unlike the previous indirect "estimation" from external data.

In one experiment, a monolayer molybdenum disulfide (MoS₂) transistor with a semi-metal bismuth (Bi) contact metal showed a directly measured carrier transfer length of about 2 nanometers. This is substantially smaller than the 9.25 nanometers calculated using the traditional TLM method on the same device, providing empirical support for the potential of Bi/MoS₂ contact engineering in enabling next-generation technology nodes. The technique's versatility was further validated as it was successfully applied to different material components, yielding corresponding direct measurements. The research, a collaboration involving teams from NTU, National Taiwan Normal University, and the National University of Singapore, establishes a new direct testing tool for advanced semiconductor interface engineering.

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The carrier transfer length, simply put, is the effective distance required for electrons to actually complete injection when they enter a semiconductor from metal, directly affecting contact resistance and whether the component can operate normally at extremely small sizes.