Finite Refractive Spectroscopy is Useful for Determining the Graphene Bilayer Bandgap

Simultaneous control over band gap and charge carrier density in semiconductors is desired in photodetectors, highly adjustable transistors, and lasers. Bernal stacked bilayer graphene is a van-der-Waals material that allows bandgap adjustment by applying an out-of-plane electric field.

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YouStudy: Transport Spectroscopy of Ultraclean Tunable Band Gaps in the Graphene Bilayer. Image Credit: Kateryna Kon/Shutterstock.com

Apart from the invention of the adjustable band gap, the fabrication of a clean heterostructure with an electrically adjustable band gap is a recent achievement applied to finite charge carriers. An article published in Advanced Electronic Materials discusses gated bilayer graphene with adjustable bandgap, which is characterized by finite-bias transport spectroscopy and temperature-activated transport measurements.

Limited-bias transport spectroscopy helps to compare different gate materials and corresponding device technologies, influencing the interference potential in bilayer graphene. Graphite-fenced bilayer graphene exhibits low perturbation with no subgap states, leading to an adjustable band gap of up to 120 millielectronvolts.

The melodious band gap at Graphena Bilayer

Graphene is a two-dimensional (2D) crystalline form of carbon, widely used in electronics and photonics. Monolayer and bilayer graphene structures have zero band gaps and are fabricated by exfoliation from graphite or by chemical vapor deposition (CVD).

The bilayer graphene present in the AB or Bernal-stacked form has half of the associated atoms located directly above the center of the hexagon of the lower graphene sheet, and the other half of the atoms located above the other atoms where the layers are perfectly aligned.

Bilayer graphene with zero bandgap behaves like a semi-metal where bandgap recognition can be made possible through the generation of an electric transfer field between the two layers. Although Bernal stacked bilayer graphene is a 2D semi-metal, the application of an out-of-plane electric field can turn it into a 2D semiconductor, with an electronic band gap proportional to the strength of the displacement field.

Although a tunable band gap was observed using scanning tunneling spectroscopy, the subgap state in the transport measurements caused by the disturbance could not suppress electron conduction completely, making such bilayer graphene devices unsuitable for semiconductor applications.

This drawback is not solved by fabrication of a double gate structure based on suspended graphene bilayer or by encapsulation of bilayer graphene by hexagonal boron nitride (hBN). To this end, a graphite gate-based fabrication technology enables a gate-controlled band gap, leading to a true band isolation state in bilayer graphene.

Transport Spectroscopy in the Graphene Bilayer

In this study, graphite gate-based fabrication was used to introduce a tunable band gap in the observed bilayer graphene through finite refractive transport spectroscopy measurements. The obtained band gaps are in agreement with the theory and values ​​obtained from thermally activated transport.

Restricted refractive transport spectroscopy was used to compare the device technology of hBN and double-door graphene bilayer which allows to investigate jump transport due to possible interference or impurity states resulting in effective tail and subgap states. Studies show that gate-based fabrication technology impacts maximum device resistance.

In addition, this fabrication technology also affects the conductance that is suppressed by the bias voltage when measuring the transport through the graphene bilayer with the electrostatic gap and the bandgap tunability with the electric transfer field.

The results reveal that the behavior of graphite gate-based bilayer graphene devices as anticipated through theoretical predictions for ideal bilayer graphene, reveals the behavior of the device semiconductors under an applied electric displacement field.

In graphite gate-based bilayer graphene devices, a maximum resistance value of 100 gigaohms is observed in the gap regime with no subgap energy due to the trap or impurity state. However, devices with gold and silicon gates appear to be affected by interference and subgap states.

Therefore, the gold gated device exhibits a high gap induced resistance wherein the band gap is reduced in the limited refractive measurements, while no band gap is observed in the silicon gated device. The overall results confirm that bilayer graphene is crawlable in a graphite-doored BLG/hBN heterostructure, revealing its robustness in releasing tunable 2D semiconductor potentials.

Conclusion

To conclude, finite refractive transport spectroscopy was demonstrated as a versatile method for characterizing the band gap of bilayer graphene. The high sensitivity of this transport spectroscopic method allows comparative studies based on the effect of electrostatic potential for various gating technologies.

Measurements from different gate technologies show that graphite gate devices, which are part of the van der Waals heterostructure, outperform gold and silicon gate devices and behave close to the theoretical predictions obtained for ideal bilayer graphene.

Band gaps of up to 120 millielectronvolts are achieved in graphite-doored devices with resistances of up to 100 gigaohms. These results highlight the importance of graphite-based bottom gates for bilayer graphene-based van-der-Waals heterostructures. In addition, the excellent quality of the graphene/hBN/bilayer graphene device is demonstrated in this work to address the wide application of bilayer graphene.

Reference

Icking, E., Banszerus, L., Wörtche, F., Volmer, F., Schmidt, P., Steiner, C., Engels, et al. (2022) Transport Spectroscopy of the Ultraclean Tunable Band Slit in the Graphene Bilayer. Advanced Electronic Material. https://onlinelibrary.wiley.com/doi/10.1002/aelm.202200510

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