The method they developed could enable chipmakers to manufacture next-generation transistors from materials other than silicon. Grid of square holes on a pink chip. The chip is repeated three times. Green and white atoms are interspersed on the chip in the upper left corner. In the center, the atoms are arranged in triangular configurations inside the square holes. Close-up of precisely aligned atomic rows can be seen at right.
The number of transistors on a microchip has doubled every year since the 1960s, as predicted by Moore's Law. However, this trend is predicted to eventually plateau, as silicon loses its electrical properties below a certain device size.
Let's talk about 2D materials, which are brittle, two-dimensional layers of flawless crystals just one atom thick. 2D materials can transport electrons at the nanoscale much more effectively than silicon. Therefore, in the search for next-generation transistor materials that can replace silicon, 2D materials have received a lot of attention.
But before the electronics industry can move on to 2D materials, researchers need to figure out how to design materials on silicon chips that meet industrial standards while maintaining ideal crystal shapes. Engineers from MIT may now have a solution.
Transistors Made of 2D Materials
The team has created a technique that could allow chipmakers to create transistors made from thinner 2D materials by growing them on chips of silicon and other materials currently in use.
The innovative technique that the team first applied to create pure, defect-free 2D materials on industrial silicon wafers is a type of "non-epitaxial, single-crystal growth".
The team's technique enabled them to create a basic functional transistor from a class of 2D materials known as transition metal dichalcogenides, or TMDs, that are known to conduct electricity more effectively at the nanoscale than silicon.
According to Jeehwan Kim, an associate professor of mechanical engineering at MIT, “We think our approach could enable the development of 2D semiconductor-based, high-performance, next-generation electronic devices.” “We found a technique that would use 2D materials to capture Moore's Law.”
In a study published today in the journal Nature, Kim and colleagues describe their method.
Traditionally, researchers have used a manual procedure in which an atom-thin scale is meticulously peeled off a bulk material, similar to peeling off the layers of an onion.
However, most bulk materials are polycrystalline and consist of many crystals growing in various directions. The “grain boundary” works as an electrical barrier where the two crystals collide. Electrons passing through a crystal and encountering a crystal with a different orientation suddenly stop and reduce the conductivity of the material. Researchers have to look for “single crystal” regions even after exfoliating a 2D scale; this is a difficult, tiring and time-consuming process to implement on commercial scales.
Single-crystal 2D materials are thought to be nearly difficult to fabricate on silicon, according to Kim. “Now we can prove we can do it. And our trick is to prevent grain boundaries from forming.
It is not necessary to flake and look at the 2D material for the team's new "non-epitaxial, single-crystalline development". Instead, scientists use standard vapor deposition techniques to pump atoms onto a silicon chip. At some point, the atoms land on the chip, nucleate, and transform into two-dimensional crystal orientations. Each "core" or crystal seed will develop on the silicon chip in a random direction if left unattended. But Kim and her colleagues were able to align each crystal as it grew to produce areas of single crystals all over the chip.
To achieve this, they first applied a silicon dioxide "mask" to a silicon chip, which they structured into small pockets, each of which was intended to hold a crystal seed. Then, a gas of atoms lodged in each pocket flowed over the masking sheet to form a 2D material (a TMD in this case). The atoms were held by the pockets of the mask, which encouraged them to come together in a single, crystalline orientation on the silicon chip.
According to Kim, although there is no epitaxial relationship between the 2D material and the silicon wafer, you get single-crystal growth everywhere.
The team created a simple TMD transistor using the masking technique and showed that its electrical performance is on par with that of a pure stamp of the same material.
They also used this technique to design a multi-layer device. They first grew one type of 2D material to fill half of each square on a silicon chip covered with a patterned mask, then another type of 2D material over the first layer to fill in the remaining squares. As a result, each square contained a single-crystal, ultra-thin bilayer structure. According to Kim, a large number of 2D materials could be developed in the future and stacked in this way to create ultra-thin, flexible and multifunctional films.
Kim claims that the entire community is striving to develop next-generation processors without transferring 2D materials. “Until now, there was no technique to fabricate 2D materials in single-crystal form on silicon wafers,” he says. “Now that we have found a technique to fabricate devices smaller than a few nanometers, we have completely overcome this challenge. As a result, the Moore's Law paradigm will change.
📩 23/01/2023 18:24