"In the post-Moore era, let Graphene go. This is what Liu Zhongfan, academician of the Chinese Academy of Sciences and president of the Beijing Graphene Research Institute, said two years ago. Graphene, a "king of new materials", a material that was once positioned as the next generation of new semiconductors at the "Global IEEE (Institute of Electrical and Electronics Engineers) International Chip Wire Technology Conference" in 2021, has set off a lot of trends.
But at that time, all kinds of concepts were raging, graphene electric heating, graphene cosmetics, and even graphene underwear, just like nano water, photocatalysis and negative oxygen ion air purifiers, once made people think that graphene products were "deceptive". It can be said that graphene has long been "played badly" by marketing.
And, more importantly, until last year, graphene had no "band gap", and a band gap of 0 means that graphene is a conductor. In other words, graphene did not even have the ability to allow semiconductors to turn on and off, let alone trigger a revolution in semiconductors and electronics, and this problem was stuck for decades.
At that time, most of the so-called "graphene materials" had no more than 60% carbon content, which meant that more than 40% of the graphene materials were not even carbon.
But recently, there is hope that this problem will finally be solved, and a functional semiconductor made of graphene has finally arrived.
Electronics open new doors
A few days ago, the team of Walter de Heer, a professor of physics at Georgia Institute of Technology, and Professor Ma Lei of Tianjin University created the world's first functional graphene semiconductor made of graphene.
The research team made a breakthrough in growing graphene on silicon carbide (SiC) wafers using a special furnace. They produced epitaxial graphene, which is a monolayer that grows on the crystal plane of silicon carbide. It has been found that when properly manufactured, epitaxial graphene chemically bonds to silicon carbide and begins to exhibit semiconductor properties. This breakthrough opens the door to the development of completely new electronic products. The study was published in the journal Nature.
According to the researchers, they want to introduce three special properties of graphene into electronics, so that they can handle very large currents and achieve higher efficiency without the temperature rising to outrageous levels.
Graphene is a single-layer graphite with carbon atoms formed by sp2 hybrid orbitals to form a hexagonal shape and arranged in a honeycomb crystal lattice. It has the characteristics of ultra-thin (1 mm thick flake graphite = 3 million pieces of graphene), ultra-light, (animal hair can support graphene aerogel the size of red dates), and super strong (perfect graphene film is made into plastic wrap and an elephant can sit on a cup to make it break).
Graphene is the "genius" of 2D materials, with electrical properties far exceeding that of any other 2D semiconductor currently being developed. It has the characteristics of high charge carrier mobility (15000 c㎡/V·S), bipolar field effect, high thermal conductivity (3000 W/m·k), and excellent electrical and mechanical properties. The excellent properties of graphene make it suitable for many fields such as nanoribbon transistors, gas sensors, supercapacitors and transparent conductive electrodes.
Solve the key problem of graphene semiconductors
A long-standing problem in graphene electronics is that graphene does not have the correct band gap, and intrinsically intact graphene has zero band gap and appears metallid. Its special corrugated valence band and conduction band are practically linked together and cannot be switched on and off in the correct proportion. All transistors and silicon electronics require a bandgap to work, and over the years, many people have tried to solve this problem in various ways.
Scientists have created band gaps by making graphene into exotic shapes, such as bands, and have also changed band gaps through quantum confinement or chemical functionalization. However, before the release of this result, no viable semiconductor graphene was successfully fabricated, either because it was too difficult to operate or too small (e.g., around 100 meV), which is still too small for electronic engineering applications.
Research on the bandgap problem of graphene semiconductors, Watchmaking|Electronic Engineering World
The result is a solution to the bandgap problem, and by annealing graphene on a specific silicon carbide crystal plane, allowing graphene to work like silicon, is a critical step in realizing graphene-based electronics, paving the way for a new era of technology that harnesses graphene's extraordinary capabilities. At the same time, they are working on the ability to further enable quantum computing.
The team said that they demonstrated that semiconductor epitaxial graphene (SEG) on a monocrystalline silicon carbide substrate has a band gap of 0.6 eV and a room-temperature electron mobility of 5500 c㎡/V·S (more than 5000 c㎡/V·S in the abstract, compared with 4000 c㎡/V·S in the first paper in February 2023), which is 3 times higher than silicon and 20 times higher than other 2D semiconductors.
Note: Epigraphene refers to graphene that forms spontaneously on silicon carbide crystals, causing a carbon-rich surface to recrystallize into graphene when silicon sublimates from the surface at high temperatures.
At the same time, the turn-on ratio of the prototype FET fabricated with SEG-on-SiC can reach 10^4, and the turn-on ratio of the optimized device is 10^6. (In comparison, the current high-performance GaN HEMT device turn-on ratio can reach 2 x 10^9~1 x 10^10)
Source: Nature
"For us, we're like the Wright brothers of the past, who built an airplane that could fly 300 feet in the air. But the skeptics asked, 'Trains and ships are already fast, why do we still want to soar into the sky?' despite this, the Wright brothers persevered, and the graphene semiconductor we worked on was exactly that, a technology that could take people through the oceans. Walter de Heer said.
Can the preparation puzzle be solved?
Although there is a solution to the problem of graphene energy gap, there is still a second problem in order to make graphene semiconductors truly applied to the industry - how to produce them on a large scale.
The key point in the study is that the SEG lattice is aligned with the SiC substrate, which is chemically, mechanically and thermally robust, and can be patterned using traditional semiconductor fabrication techniques and seamlessly attached to semi-metallic epitaxial graphene.
In human terms, the advantages of direct growth of graphene on silicon carbide substrates are:
- The graphene transfer step is omitted, and the pollution and damage caused by the transfer process to the graphene film are avoided.
- Compatible with current silicon processes, making it easy to achieve mass production;
- Both the growth temperature and the graphene formation rate are controllable.
In general, in the study, the large-scale application of graphene is more convenient, but at this stage, there is still a big gap to achieve large-scale application, far less than silicon, and there are three main problems:
- In the semiconductor field, graphene can only be prepared by CVD method, which is expensive and has a low yield, so how to achieve large-scale production of graphene is an urgent problem to be solved.
- As a 2D planar material, graphene has a serious quantum effect, and both edge and crystalline states greatly affect the electronic structure and electrical properties.
- It is necessary to study the conductivity of graphene in depth to make graphene integrated circuits have better performance.
The above problems are reflected in the industry, that is, it is expensive to prepare: the process of preparing graphene by chemical deposition method is expensive and cannot be produced on a large scale, the number of graphene layers prepared by epitaxial growth method cannot be accurately controlled, the mechanical exfoliation method is inefficient and expensive, and the structure of graphene prepared by Hummer method is damaged. The difficulty lies in the inconsistency, instability and low quality of graphene stripping, growth and large-scale preparation.
That is to say, so far we do not have the ability to prepare graphene products on a large scale, and the products need to have high quality and strong consistency and stability in order to go out of the laboratory and develop on a large scale. Only by finding a low price, and at the same time making breakthroughs in technology maturity and ease of access, can graphene truly enter the industry.
Revisit the victims of overhype
Andre Geim, a Nobel laureate in physics known as the "father of graphene", said a while ago that graphene is a victim of excessive hype.
In fact, we have come a long way to explore graphene, theoretically, graphene research has a history of more than 60 years, along the way, we have overcome many difficulties:
At first, researchers pointed out that two-dimensional crystals were thermodynamically unstable and could not exist on their own, and graphene had always been regarded as a theoretical material. Until 2004, British physicist Andrei Geim and Russian physicist Konstantin Novoselov repeatedly peeled off graphite with scotch tape in the process of studying graphene in the laboratory, until only a single layer of graphite remained, which is graphene. For this, the two were jointly awarded the 2010 Nobel Prize in Physics.
Although graphene has been "demonized" by exaggerated propaganda, in fact, the future of graphene is still broad. These include a variety of applications such as centralized circuits, field-effect transistors, high-power LED heat dissipation, wearable electronics, graphene chemical sensors, and more.
This time, one of the key problems of graphene has been solved, that is, there is real hope for graphene semiconductors. At this point, we need to re-examine graphene as a material.
Of course, it does not mean that it is necessary to make a big fuss, and once again stage a "graphene marketing fever", graphene materials should still find their own real application breakthrough, and the ultimate goal is not to completely replace silicon, but to create their own way. Just like silicon carbide and gallium nitride, there are more options for semiconductor manufacturing and downstream products.
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