2024-07-16 - tech

Graphene, the "ambitious" semiconductor

Not long ago, the website of the journal Nature published online a research result titled "Ultra-high Mobility Semiconductor Epitaxial Graphene Grown on Silicon Carbide."

This is a significant advancement in the field of semiconductor graphene achieved by Professor Ma Lei's team at the Tianjin International Research Center for Nanoparticles and Nanosystems at Tianjin University, who successfully prepared high-mobility semiconductor epitaxial graphene, demonstrating a performance ten times that of silicon.

This breakthrough has attracted industry attention. What is the status of graphene in the semiconductor world? Why is everyone paying attention to graphene?

01

A problem that has plagued graphene researchers for decades

Carbon is one of the earliest elements that humans have come into contact with and also one of the earliest elements utilized by humans. Graphite, as one of the allotropes of carbon, is also well-known and studied by people.

Graphene is a single layer of graphite, with its carbon atoms arranged similarly to a single atom layer of graphite, consisting of carbon atoms forming a hexagonal, honeycomb crystal lattice arrangement through sp2 hybridized orbitals in a single two-dimensional crystal layer.

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Theoretically, graphene has been studied for more than 70 years, but due to researchers such as Landau and Peierls pointing out that two-dimensional crystals are thermodynamically unstable and cannot exist alone, graphene has long been considered a theoretical material. However, in 2004, graphene was prepared at room temperature, sparking a surge of research interest in graphene.Previously, we often discussed carbon-based semiconductors, the foundation of which is graphene semiconductor materials. Graphene is an "ambitious" semiconductor; so, what advantages does graphene have over silicon-based semiconductors?

First, high mobility, which refers to the speed at which electrons move within the material. Mobility is an important indicator of semiconductor performance, determining the operating speed and power consumption of electronic devices.

Graphene's mobility is more than 10 times that of silicon, meaning that graphene semiconductors can create faster, more energy-efficient electronic devices, such as transistors, sensors, and displays. Typical suspended graphene has a mobility as high as 200,000 cm²V⁻¹s⁻¹, while the mobility of single-crystal silicon is only 1,000 cm²V⁻¹s⁻¹. This high electron mobility implies greater operational efficiency and speed.

Second, high stability, meaning the material's structure is not prone to change. Graphene is a planar structure composed of a single layer of carbon atoms, with its interatomic distances and bond angles optimized, thus it possesses strong mechanical strength and thermal stability. Graphene can maintain its performance under extreme temperatures, pressures, and electric fields without being damaged or affected by noise like silicon.

Third, high flexibility, meaning the material's shape can be altered at will. Graphene is a two-dimensional material with a thickness of only 0.34 nanometers, equivalent to 1/300 of silicon, which allows it to be easily bent, folded, stretched, and even rolled into tubes or spheres. Graphene semiconductors can adapt to various complex shapes and surfaces, offering immense potential for the manufacturing of flexible electronics, wearable devices, and biomedical applications.

These advantages make graphene a well-known candidate for the next-generation "carbon-based semiconductor."

However, there is a problem with graphene as a semiconductor—it is a zero-bandgap material. Zero bandgap means the bandgap width is zero. The bandgap is the energy difference between the lowest point of the conduction band and the highest point of the valence band. The larger the bandgap, the more difficult it is for electrons to be excited from the valence band to the conduction band, resulting in lower intrinsic carrier concentration and lower conductivity. Without a bandgap, the transistor "switch off" function, which is essential for logic circuits, cannot be fully realized.

Therefore, it is not a semiconductor but rather has metallic properties. The bandgap width of semiconductor materials is always greater than zero. In fact, graphene has a significant application prospect in the future microelectronics field, but its zero bandgap characteristic hinders its application in the semiconductor domain.

Breaking through the zero bandgap has become a decades-long challenge for graphene researchers.How to Break Through the Zero Band Gap?

 

People are trying to introduce a band gap into graphene, which will make it semiconductive, and the room temperature mobility will be an order of magnitude higher than that of silicon.

The breakthrough mentioned earlier is that Professor Ma Lei's research team, through precise control of the epitaxial growth process of graphene, has successfully introduced a band gap into graphene, creating a new type of stable semiconductor graphene. This technology ensures that carbon atoms can form a highly ordered structure on a silicon carbide substrate by strictly controlling the temperature, time, and gas flow of the growth environment. This semiconductor graphene not only has a band gap, but also has an electron mobility far exceeding silicon materials at room temperature, and possesses unique properties that silicon materials do not have.

Ma Lei said: "The semiconductor graphene has a mobility more than ten times that of silicon materials at room temperature, while having a band gap of 0.6 eV. It is a true single-crystal graphene semiconductor."

Specifically, the quasi-equilibrium annealing method is used to prepare ultra-large single-layer single-crystal domain semiconductor epitaxial graphene. At present, this method can basically meet the needs of industrial application. Compared with traditional production processes, it has a large growth area, high uniformity, simple process, low cost, and room temperature mobility that is at least an order of magnitude better than all current single-layer crystals.

In fact, before this, there were also some methods for graphene to produce a forbidden band. For example, there are direct and indirect methods of producing a forbidden band.

In terms of direct production of the forbidden band, research has shown that when the width of the constructed graphene nanoribbon is less than 10nm, the quantum effect and edge effect of nano-graphene can be used to effectively open the band gap, thereby giving it semiconductor properties. In 2008, British researchers prepared graphene quantum dot devices that were only a few nanometers wide and one atom thick. At this scale, graphene has a forbidden band width of about 0.5eV, and the device can still maintain good conductivity.

In terms of indirect production of the forbidden band, it is mainly by introducing substances with a non-zero forbidden band as a potential barrier to produce a forbidden band, constructing heterojunctions on the surface and boundaries of graphene, and forming heterojunction crystal transistors.Early Layout of Graphene

 

For new materials, failing to plan ahead means being a step behind. Therefore, the United States, the European Union, South Korea, and Japan have actually made some arrangements. Let's take a look at each of them.

The United States began exploring graphene electronic technology earlier. Starting from 2006, the National Science Foundation established numerous carbon-based electronics basic research projects, covering various fields of carbon-based electronics research and application. They have carried out multiple carbon-based electronic technology R&D projects related to graphene, carbon nanotubes, and silicon carbide, mainly covering graphene electronic devices, graphene circuits, graphene sensors, and the application of graphene in quantum switches and other quantum technologies.

In 2008, the Defense Advanced Research Projects Agency (DARPA) invested $22 million to develop carbon electronic radio frequency applications, aimed at developing new graphene transistors. In 2011, IBM fabricated a new generation of graphene transistors with an ultra-high cutoff frequency of 155GHz, featuring a 40nm gate pulse width.

Grolltex, a producer of graphene and two-dimensional materials in the United States, announced in 2019 the completion of its new capacity expansion. Its CVD single-layer graphene manufacturing plant in San Diego, California, can produce 30,000 8-inch graphene wafers (on various substrates) annually.

Looking at Europe, as early as January 2013, the European Commission planned to list the "Graphene Flagship" as one of the first "Future and Emerging Technologies Flagship Projects." It established 12 application working groups responsible for material applications, composite materials, optoelectronics, electronic devices, sensors, biomedicine, health, and environmental research directions to promote subsequent application implementation. This plan is the largest multi-party collaborative research project in European history, with a budget of 1 billion euros.

Among them, the "2D Experimental Pilot Line (2D-EPL)" project, costing 20 million euros, was launched in 2021. It aims to be the first graphene wafer factory to integrate graphene and layered materials into the semiconductor platform, guiding innovative technologies based on two-dimensional materials from the laboratory to large-scale production and commercial implementation.

Japan's research also began in the 2000s, with the Japan Society for the Promotion of Science starting to fund graphene-related materials and device technologies from 2007.

In 2019, Japan announced the precise synthesis of "graphene nanoribbons." A joint team of Japanese universities and companies developed a method to synthesize them into ribbon shapes by precisely controlling their structure and successfully produced wider "graphene nanoribbons (GNRs)."In the related news reports, it is mentioned that GNR, as a semiconductor, has exceptionally excellent electrical properties. The GNR produced this time is about 2 nanometers wide, equivalent to 17 atoms, and the "band gap" related to the easy flow of electric current is only around 0.6eV. As a semiconductor material that can be both an insulator and a conductor, it exhibits optimal characteristics.

In South Korea, recently, Samsung Electronics and LG Electronics are accelerating the development of components based on graphene, aiming to enhance the durability and energy efficiency of semiconductor and home appliance products.

It is worth noting that Samsung Electronics is one of the pioneers in the field of graphene. As early as 2014, it had successfully achieved the commercial production of graphene. Samsung Electronics has applied its own patented technology to manufacture high-performance graphene films and graphene composite materials, which are used in a wide range of applications. To date, Samsung Electronics holds more than 220 patents in the field of graphene, more than twice the number of other listed companies. These patents cover all aspects of graphene, including production, preparation, and application, giving Samsung Electronics a strong advantage in the graphene market.

Conclusion

The Moore's Law, which the traditional silicon-based integrated circuit industry relies on, is increasingly approaching the physical limit, causing concerns in academia and industry about the future development of the integrated circuit industry. In this context, graphene, with its excellent electrical properties and thermal conductivity, is regarded as one of the potential alternative materials to replace silicon-based materials.

In today's world where semiconductor sales are continuously growing, how to better reduce the investment in semiconductor chips is a problem that must be faced in the future.

In 2016, K. S. Novoselo, who won the 2010 Nobel Prize in Physics, predicted that graphene transistors might replace silicon technology after 2020. Now it seems there is still a long way to go. There are still many issues that need to be resolved for the application of graphene in logic transistors.

However, the journey is long and winding, but the destination is within reach.It seems like there is no text provided for translation. Please provide the text you would like translated into English, and I will be happy to assist you.Please provide the text you would like translated into English.