By Chen Bin, Reporter from China Science Daily
Since the two physicists from the University of Manchester in the UK first isolated graphene from graphite in 2004, this miraculous material has become a revolutionary material in many fields due to its excellent optical, electrical, and mechanical properties. Especially in the field of microelectronics, graphene is considered the key for humanity to enter the "carbon era" from the "silicon era".However, achieving this transition is not easy, as there are many key technical challenges to overcome, one of which is the "zero bandgap" characteristic of graphene.Recently, Professor Ma Lei's team from the International Research Center for Nanoparticles and Nanosystems at Tianjin University published a paper in Nature announcing the final solution to this problem. This achievement is considered the "key" to the door of graphene chip manufacturing.Functional semiconductor graphene device. Image provided by the interviewee"Zero bandgap" hinders humanity from entering the "carbon era"
As the first two-dimensional material that can stably exist at room temperature, graphene has a unique arrangement of carbon atoms and forms unique electronic characteristics.In an interview with China Science Daily, Ma Lei said that graphene has extremely high electron mobility due to its unique band structure, which means that electrons can move quickly. For example, typical suspended graphene has a mobility of up to 200,000 cm^2V^-1s^-1, while single crystal silicon has a mobility of only 1,000 cm^2V^-1s^-1. This high electron mobility means higher operational efficiency and speed.In addition to high mobility, graphene devices, as two-dimensional materials, also have the characteristics of high integration and low power consumption. These characteristics make graphene a promising core material for humanity's transition from the "silicon era" to the "carbon era".However, due to its "zero bandgap" characteristic, graphene is currently unable to be used in the manufacturing of large-scale digital circuit devices.The so-called bandgap refers to the gap between two energy bands. The presence of a bandgap is crucial for achieving a good switching ratio, which can effectively control the opening or closing of current.From this perspective, the "bandgap" can be likened to a "switch" installed on graphene. When this "switch" is present, graphene exhibits semiconductor characteristics, thereby effectively performing digital circuit functions. When this "switch" is absent, graphene is always in the "on" state, exhibiting metallic properties and cannot be used in the manufacturing of digital circuit devices.Unfortunately, although various types of graphene have been successfully prepared by humans, there is currently no graphene that has both high mobility and a bandgap. Therefore, finding graphene with this self-contained "switch" function and high mobility is the key to solving the application problems of graphene in the field of microelectronics.
The "needle" that has been found
The approach that Ma Lei's team took to solve this problem doesn't sound complicated."Simply put, we chose different silicon carbide crystal faces as substrates for graphene to 'grow' on. During this process, we precisely controlled the temperature, time, and gas flow rate for the growth of graphene, ensuring that the carbon atoms form a highly ordered structure on the silicon carbide substrate." said Ma Lei.He said that graphene is very sensitive to changes in the external environment. Different "substrates" for growth result in different properties of the produced graphene. For this reason, finding graphene that meets researchers' expected characteristics is very challenging, like finding a needle in a haystack.Fortunately, this "needle" was eventually "found" by Ma Lei's team.The semiconductor graphene developed by this team has a bandgap of about 0.6 electron volts and a room temperature mobility of up to 5,500 cm^2V^-1s^-1, which is at least one order of magnitude higher than all currently known two-dimensional semiconductors. At the same time, field-effect transistors made from this material have a switch ratio as high as 10^4.It is said that a circuit used to realizes basic logical relations is usually composed of two states, "zero" and "one". However, in order to ensure the normal transmission and manipulation of digital information, it is necessary to clearly distinguish between these two states. If the difference between "zero" and "one" is not obvious, digital information can easily be confused. Therefore, scientists use the "switch ratio" to measure the distinction between "zero" and "one". The higher the value of the switch ratio, the higher the distinction, and the less likely confusion will occur.The team members said that previously, some people had obtained graphene with a bandgap through chemical synthesis, but the mobility was significantly lower, making it unsuitable for practical applications in the field of microelectronics.
