Graphene, which has been "destroyed", can really make chips this time?
Latest update time：2024-01-05
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"In the post-Moore era, let go of graphene." This is what Liu Zhongfan, an academician of the Chinese Academy of Sciences and director of the Beijing Graphene Research Institute, said two years ago. Graphene, the "King of New Materials", a material that was 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 caused quite a stir.
But at that time, various concepts were rampant, such as graphene electric heaters, graphene cosmetics, and even graphene underwear. Just like the nano water, photocatalysis and negative oxygen ion air purifiers in the past, people once thought that graphene products were They are all "liar". It can be said that graphene has long been "badly played" by marketing.
And, more importantly, until last year, graphene had no "band gap". A band gap of 0 means that graphene is a conductor. In other words, graphene did not even have the function to allow semiconductors to be turned on and off before, let alone trigger a revolution in semiconductors and electronics, and this problem has been stuck for decades.
So, what are the graphene wafers that have been "advocated" by the academic community before? At that time, most so-called "graphene materials" contained no more than 60% carbon, which meant that more than 40% of graphene materials were not even carbon.
But recently, there is finally hope that this problem can be solved, and a functional semiconductor made of graphene has finally arrived.
Fu Bin｜Author
Electronic Engineering World (ID: EEworldbbs)｜Produced by
Electronic products open new doors
Electronic products open new doors
Recently, the team of Professor Walter de Heer of Georgia Institute of Technology and Professor Marley of Tianjin University created the world’s first functional semiconductor (Functional Graphene Semiconductor) made of graphene.
The research team achieved a breakthrough when growing graphene on silicon carbide (SiC) wafers using a special furnace. They produced epitaxial graphene, a single layer grown on a silicon carbide crystal plane. The study found that when made correctly, epitaxial graphene chemically bonds to silicon carbide and begins to exhibit semiconducting properties. This breakthrough opens the door to the development of entirely new electronic products. The research was published in the journal Nature.
According to the researchers, they want to introduce three special properties of graphene into electronic products so that they can handle very large currents and obtain higher efficiency, while at the same time the temperature will not rise to an outrageous level.
Graphene is a single-layer graphite, a single-layer two-dimensional crystal in which carbon atoms are arranged in a hexagonal shape with sp2 hybrid orbitals and arranged in a honeycomb crystal lattice. It is ultra-thin (1 mm thick flake graphite = 3 million pieces of graphene), ultra-light (animal hair can support a graphene aerogel the size of a red date), and ultra-strong (a perfect graphene film is made into a plastic wrap cover It takes an elephant to sit on a cup to make it break).
Graphene is the genius of a two-dimensional material, with electrical properties that far exceed those of any other two-dimensional semiconductor currently under development. 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. Excellent properties make graphene highly applicable in many fields such as nanoribbon transistors, gas sensors, supercapacitors and transparent conductive electrodes.
Solving key issues in graphene semiconductors
Solving key issues in graphene semiconductors
A long-standing problem in graphene electronics is that graphene does not have the correct band gap. Intrinsic graphene with a complete structure has a band gap of zero and appears metallic. Its special corrugated valence and conduction bands are actually connected together and don't switch in the correct proportions. All transistors and silicon electronics require a band gap to work, and over the years many people have tried to solve this problem in various ways.
Scientists have obtained band gaps by making graphene into strange shapes, such as ribbons, and have also changed the band gaps through quantum confinement or chemical functionalization. However, before the release of this result, no viable semiconductor graphene had been successfully produced. It was either too difficult to operate or too small (for example, around 100meV), which is still too small for electronic engineering applications.
Research on the band gap problem of graphene semiconductor, tabulation | Electronic Engineering World
This achievement solves the band gap problem. By annealing graphene on a specific silicon carbide crystal surface, graphene can work like silicon. It is a key step in realizing graphene-based electronic products and provides a way to take advantage of graphene's extraordinary capabilities. A new era of technology has paved the way. At the same time, their research can further realize quantum computing.
The team stated that they demonstrated that Semiconducting Epigraphene (SEG) on a single crystal silicon carbide substrate has a band gap of 0.6 eV and a room temperature electron mobility of 5500 c㎡/V·S (marked in the abstract: More than 5000 c㎡/V·S, the first edition of the paper in February 2023 was 4000 c㎡/V·S), which is 3 times higher than silicon and 20 times higher than other two-dimensional semiconductors.
Note: Epigraphene refers to graphene that forms spontaneously on silicon carbide crystals when silicon sublimates from the surface at high temperatures, causing the carbon-rich surface to recrystallize into graphene.
At the same time, the on-off ratio of the prototype FET manufactured with SEG-on-SiC can reach 10^4, and the on-off ratio of the optimized device is 10^6. (For comparison, the current on-off ratio of high-performance GaN HEMT devices can reach 2 x 10^9~1 x 10^10)
Image source｜Nature
"To us, we are now like the Wright brothers in the past, who built a plane that could fly 300 feet in the air. But the skeptics ask, 'Trains and ships are already fast, why should we soar?' Sky? 'Despite this, the Wright brothers persisted, and the graphene semiconductor we studied is exactly the same. This is a technology that can take people across the ocean," said Walter de Heer.
Can this preparation problem be solved?
Although there is a solution to the graphene energy gap problem, there is still a second problem in order for graphene semiconductors to be truly applied to industry - how to produce them on a large scale.
The key point in the research is that the SEG lattice is aligned with the SiC substrate, is chemically, mechanically and thermally robust, and can be patterned using traditional semiconductor manufacturing techniques and seamlessly connected to semi-metallic epitaxial graphene. .
To explain in human terms, the advantages of growing graphene directly on silicon carbide substrates are:
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It saves the graphene transfer step and avoids contamination and damage to the graphene film caused by the transfer process;
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Compatible with current silicon processes to facilitate large-scale mass production;
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Both growth temperature and graphene formation rate are controllable.
Generally speaking, in research, large-scale application of graphene is more convenient, but at this stage, there is still a big gap in achieving large-scale application, far less than silicon. There are three main problems:
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In the semiconductor field, graphene can only be prepared by CVD method, which is expensive and has low yield. How to achieve large-scale production of graphene is an urgent problem to be solved;
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As a 2D planar material, graphene has severe quantum effects, and both edge and crystalline states greatly affect the electronic structure and electrical properties;
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In-depth research on the conductivity of graphene is needed to enable graphene integrated circuits to have better performance.
The above problems are reflected in the industry, which is that it is expensive to prepare: the process of preparing graphene using 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 peeling method is inefficient and expensive; The structure of graphene prepared by the Hummer method is damaged. The difficulty lies in the exfoliation and growth of graphene as well as the inconsistency, instability and low quality of large-scale preparation.
In other words, so far we do not have the ability to prepare graphene products on a large scale. For products to go out of the laboratory and be developed on a large scale, they need to be of high quality and extremely consistent and stable. Only by finding a product that is low-priced and has breakthroughs in terms of technological maturity and ease of acquisition can graphene truly enter the industry.
Revisiting the Victims of Overhype
Andre Geim, the Nobel Prize winner in physics who is known as the "father of graphene", once said a while ago that graphene is a victim of over-hype.
In fact, we have come a long way in exploring graphene. In theory, graphene research has a history of more than 60 years. Along the way, we have overcome many difficulties:
Initially, researchers pointed out that two-dimensional crystals are thermodynamically unstable and cannot exist alone. Graphene has always been regarded as a theoretical material. Until 2004, when British physicist Andre Geim and Russian physicist Konstantin Novoselov were studying graphene in the laboratory, they repeatedly peeled off the graphite with transparent tape until only a single layer of graphite remained. A thin layer of graphene. For this, the two jointly won the 2010 Nobel Prize in Physics.
Although graphene has been "demonized" due to exaggerated publicity, in fact, graphene's future is still broad. Including various applications such as centralized circuits, field effect transistors, high-power LED heat dissipation, wearable electronic devices, graphene chemical sensors, etc.
This time, one of the key problems of graphene has been solved, which means that graphene semiconductors are really promising. At this time, we have to re-examine the material graphene.
Of course, this does not mean that there will be a lot of hype and another "graphene marketing craze". Graphene materials should still find their own real application breakthroughs. The ultimate goal is not to completely replace silicon, but to create its own. road. Just like silicon carbide and gallium nitride, giving more choices for semiconductor manufacturing and downstream products.
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