Make Semiconductors Like 'Lego'... Swap Atoms to Create Insulating Layers and Electrodes All at Once [Reading Science]

IBS selectively transforms atomic layers in 0.7-nm increments, offering an alternative to complex material-stacking processes
Current-control performance nears theoretical limit... Findings published in leading journal Science

What if, instead of stacking different materials one by one to make a semiconductor, we could create the components we need simply by changing atoms? A Korean research team has succeeded in changing the properties of individual atomic layers to create a semiconductor, an insulating layer, and an electrode contact layer within a single thin film. The team has developed a next-generation transistor that reduces complex manufacturing steps while bringing current-control performance close to the theoretical limit.

The Ministry of Science and ICT and the Institute for Basic Science (IBS) announced on October 16 that a research team led by Jo Moonho, director of the IBS Center for Van der Waals Quantum Materials and a distinguished professor at POSTECH, had developed a "modular transistor fabrication technology" that selectively replaces atoms in a 2D semiconductor to create materials with different electrical properties. The findings were published that day in the international journal Science.

Schematic of modular 2D transistor fabrication using atomic-layer-selective substitution. It shows how atoms in a 2D semiconductor thin film are selectively replaced at desired layers and positions to form an insulating layer and an electrode contact layer, thereby integrating the transistor's main components into a single thin film. Provided by the research team.

As artificial intelligence (AI) becomes more widespread, technologies that increase semiconductor integration density and power efficiency are growing in importance. In particular, 2D semiconductors, which can control current even when only a few atomic layers thick, are attracting attention as next-generation materials that could overcome the limits of continued miniaturization in conventional silicon semiconductors.

The challenge lies in the manufacturing process. A transistor requires a semiconductor through which current flows, an insulating layer to control it, and electrodes through which current enters and exits. However, it is difficult to coat the surface of a 2D semiconductor uniformly with an insulating layer, and directly attaching metal electrodes can result in high contact resistance. Another approach is to transfer and stack different thin films, but the process is complex and difficult to adapt to mass production.

The research team took a different approach. Instead of bringing in and stacking other materials, they devised a way to make the required materials directly by changing atoms in the existing semiconductor.

The starting material was tungsten diselenide (WSe2), in which a layer of tungsten (W) atoms is sandwiched between layers of selenium (Se) atoms. The team left the tungsten atoms unchanged and selectively replaced the selenium atoms with oxygen (O), sulfur (S), and tellurium (Te). This produced tungsten oxide (WO?), tungsten disulfide (WS2), an n-type semiconductor, and tungsten ditelluride (WTe2), a conductive semimetal, respectively.

Changing atomic layers in 0.7-nm increments brings current-control performance close to the theoretical limit

The key is the ability to precisely control the depth and location at which atoms are replaced. The team took advantage of the fact that the tiny gaps between layers in a thin film prevent reactive atoms from moving through them. By adjusting the reaction temperature and gas pressure, they transformed the material in units of atomic layers about 0.7 nanometers (nm, one-billionth of a meter) thick. They also succeeded in changing only targeted locations using a mask for semiconductor manufacturing.

Using this technology, the team fabricated a transistor with a semiconductor, an insulating layer, and a semimetal electrode contact layer all in a single thin film. With the semimetal contact layer, the energy barrier that charge carriers must overcome when moving from a metal electrode into the semiconductor was reduced to 35.5 millielectronvolts (meV), about one-third of the level when metal electrodes were attached directly.

Current on/off performance also improved. The change in gate voltage required to vary the current by a factor of 10 was 61.3 millivolts (mV), close to the theoretical minimum of about 60 mV that conventional transistors can reach at room temperature. The ratio between the current in the on state and in the off state was also more than 10 million to one.

Key members of the research team. From left: Jo Hyunjae, student researcher at the IBS Center for Van der Waals Quantum Materials and a PhD candidate in the Department of Materials Science and Engineering at POSTECH; Center Director Jo Moonho; and postdoctoral researcher Min Seokyoung. Photo courtesy of IBS.

The equivalent oxide thickness (EOT) of the insulating layer was 0.3-0.4 nm, meeting the International Roadmap for Devices and Systems (IRDS) target of 1 nm or less. This means that a thin insulating layer can effectively control current.

The team also fabricated a complementary metal-oxide-semiconductor (CMOS) inverter, which stacks p-type and n-type transistors vertically and operates at a low voltage of 1 volt (V). An inverter is a basic component of digital circuits that reverses an input electrical signal. The result demonstrates the potential to move beyond individual devices and extend the technology to stacked integrated circuits.

However, before the technology can be used in actual semiconductor production, the uniformity and reproducibility of atomic substitution must be established in large-area processes on wafer-sized substrates. The team plans to expand the technology to a range of 2D semiconductor materials and conduct further research into its use in 3D stacked integrated circuits.

"This research is significant because it applies a fundamental materials science technique, the manipulation of materials at the atomic scale, to modern semiconductor technology, opening a path to a qualitative leap in future semiconductor technology," said Jo Moonho, director of the IBS Center for Van der Waals Quantum Materials. "The technology could be extended to ultra-low-power and ultra-miniaturized semiconductors, as well as next-generation stacked semiconductor devices."

This content was produced with the assistance of AI translation services.