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Author: Thamarasee Jeewandara, Phys.org

Researchers have used direct chemical vapor deposition (CVD) to grow wafer-level, high-quality graphene on dielectrics for a variety of applications. However, graphene synthesized in this way shows a polycrystalline film with uncontrolled defects, low carrier mobility, and high street resistance; therefore, the researchers aim to introduce new methods to develop wafer-level graphene . In a new report now published in the journal "Science Advances", Chen Zhaolong and an international research team of nanochemistry, smart materials and physics in China, the United Kingdom and Singapore described highly oriented single-layer graphene on sapphire wafer films Direct growth. They realized the growth strategy by designing electromagnetic induction CVD at high temperature. The graphene film developed in this way shows significantly improved carrier mobility and reduced sheet resistance.

The development and application of graphene in materials.

Graphene has good mechanical strength, high carrier mobility, increased optical transparency and is expected to be used in high-frequency applications and transparent conductive electrodes. The linear dispersion of graphene Dirac electrons can also be used in target devices, including photodetectors and light modulators. Most of these applications rely on the use of single-crystal, wafer-level graphene that is free of contamination or damage. Although it was easy to produce wafer-level, high-mobility graphene in the past, the uniformity of the number of layers across the wafer is still unsatisfactory. Therefore, researchers are trying to promote the direct synthesis of graphene on silicon oxide, hexagonal boron nitride (hBN) and glass by using traditional chemical vapor deposition techniques. In this work, Chen et al. Introduced the direct growth of wafer-level continuous, highly oriented single-layer graphene film on sapphire by chemical vapor deposition based on electromagnetic induction heating. This method of directly growing highly oriented graphene films on sapphire wafers paved the way for the emerging graphene electronics and photonics.

Experiment: Graphene on Sapphire

During the experiment, Chen et al. Use electromagnetic induction heating as the heat source of the chemical vapor deposition (CVD) system to expand the growth parameter space during the growth of high-quality graphene. The reactor can quickly heat up to 1400 degrees Celsius in 10 minutes. This process allows precise adjustment of the supply of activated carbon for uniform growth of single-layer graphene. In order to understand the role of sapphire in the formation of graphene, the team performed density functional theory (DFT) calculations to reveal the preferred orientation of graphene domains on sapphire. To achieve this, they simulated the adsorption of a small graphene cluster (C24H12) on an alumina plate. This model shows the possibility of growing wafer-level highly oriented graphene on sapphire after the interface coupled to guide the growth mechanism. The high temperature during the growth process promotes the full pyrolysis of methane and the effective migration of activated carbon adsorbed on the sapphire, thereby improving the growth rate and crystal quality. The continuous graphene film is covered on a 2-inch sapphire wafer with high transparency within 30 minutes.

Characterizing graphene films on sapphire wafers

Using a scanning electron microscope (SEM), Chen et al. Note the uniform contrast of the single-layer graphene when it is completely covered, without any gaps. Using the Raman spectra of graphene produced on sapphire, they determined the Raman signal representing a single layer of high-quality graphene and confirmed its uniformity on the entire wafer scale. The optical microscope results also showed uniform optical contrast without any contamination or visible secondary layer. Then, they used atomic force microscopy to determine the further characteristics of the single-layer graphene grown by the CVD (chemical vapor deposition) method. Further analysis using transmission electron microscopy (TEM) showed high uniformity without contamination. The experimental device allows single-layer graphene to grow without large carbon clusters in the gas phase, and there is a single carbon that reaches the surface of the graphene to quickly migrate to the edge of the graphene. In order to understand the lattice orientation of single-layer graphene grown on sapphire, the team performed low-energy electron diffraction characterization and revealed the highly oriented nature of wafer-sized graphene. In order to further verify the structural information of the material, they conducted selected area electron diffraction measurements and used atomic-resolved TEM images to record the honeycomb lattice structure of graphene. The experimental setup allows the nucleus to reach the most stable direction.

Chen et al. Next, a scanning tunneling microscope (STM) was performed to detect the stitched state of the graphene domains. The STM image also shows a honeycomb lattice, neatly arranged, without any defects. The atom-resolved image further highlights the existence of continuous thin films with small grain boundaries. This work also confirmed the successful climbing of the sapphire steps caused by the carbothermic reduction of the sapphire. The V-shaped density state and the characteristic Dirac cone of single-layer graphene are consistent with the honeycomb structure to re-establish the high quality and purity of the highly oriented graphene film grown from this. The scientists next performed a macro four-probe transmission measurement to evaluate the large-scale conductivity of high-quality graphene grown on a sapphire wafer. They noticed a sheet resistance graph of a 2-inch graphene/sapphire wafer with an average value as low as 587 ± 40 ohms. Compared with graphene grown directly on a glass substrate, the results are significantly superior. The team then measured the field-effect mobility of graphene on sapphire and recorded its carrier density. These values ​​are also significantly higher than those observed for graphene grown directly on dielectric substrates and metals. The results are promising in electronics and optoelectronic applications.

In this way, Zhaolong Chen and colleagues developed a method to directly grow wafer-level, continuous, highly oriented single-layer graphene films on sapphire using electromagnetic induction heating CVD routes. This synthesis method helps to quickly heat up to 1400 degrees Celsius within 10 minutes to effectively pyrolyze the carbon raw materials, thereby achieving rapid migration of active materials. This efficient and reliable high-quality single-layer graphene synthesis route on sapphire wafers is compatible with semiconductor processes, and can ultimately promote high-performance graphene electronics and industrialization. Further explore the super-large single crystal WS2 monolayer for more information: Zhaolong Chen et al., Direct growth of wafer-level highly oriented graphene on sapphire, scientific progress (2021). DOI: 10.1126/sciadv.abk0115

Yanqing Wu et al., High-frequency scaling graphene transistors on diamond-like carbon, Nature (2011). DOI: 10.1038/nature09979 Journal information: Science Advances, Nature

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