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Interest in topological insulators has exploded since graphene first made its name in the scientific community. One area which has gained a lot of interest is that of Weyl semimetals (WSMs), due to their unique band and transport properties.
A team of Researchers from China have introduced defects into tungsten telluride (WTe2) samples using gallium ions to study the effects that defects have on the properties and electronic structure of the material.
Weyl semimetals were first implemented into the tantalum arsenide (TaAs) family and have since grown in popularity because of their unique band structure and transport properties.
Many theoretical forms of Weyl semimetals, such as Weyl point, Weyl cone, Fermi arc and chirality anomaly, have come to experimental fruition in recent years and the area is still expanding.
A recent discovery in the Weyl family is type II Weyl semimetals. These have been confirmed in WTe2, MoTe2 and MoxW1-xTe2 compounds and are now known to feature Weyl cones which appear at contact points between holes and electrons.
Of all the type II Weyl Semimetals, WTe2 has gathered the most attention and many of the properties in pure WTe2 have been deduced experimentally using a series of spectroscopy techniques.
One area which is lacking is how defects position themselves within and affect the band structure of WTe2, but the team of Researchers from China have now introduced gallium ions into the band structure of WTe2 and studied their relative positions and effects using a combination of theoretical and experimental approaches.
The Researchers created WTe2 flakes through mechanical exfoliation of bulk single crystals and deposited them onto a silicon wafer with a silicon dioxide layer. The Researchers then attached titanium/gold electrical contacts through conventional photolithography and electron beam evaporation methods to fabricate the device.
The Researchers introduced gallium ions into the WTe2 device using a dual beam system (FEI Helios Nanolab 600i) with Ga+ ion sources. The beam system was specifically calibrated to cover all the channel areas within the device without causing major damage to the device itself.
The process did cause some minor cascade damage resulting from ion collision, but the Researchers used thermal annealing techniques to repair the damage.
The Researchers characterized the WTe2 device using optical microscopy, atomic force microscopy (AFM), micro-Raman spectroscopy (LabRAM HR) and four-probe magnetoresistance and Hall resistivity measurements (Quantum Design PPMS-14).
The Researchers also used a combination of Vienna Ab-initio Simulation Package (VASP) code, projector-augmented-wave (PAW) and Perdew, Burke and Ernzerhof (PBE) electronic calculation methods.
Both the theoretical calculations and experimental results agreed and came to the conclusion that defect introduction into the WTe2 band structure was successful and Frenkel defects were the most dominant defect type.
Measurements and analyzes undertaken by the Researchers showed that after implementation with gallium ions, significant changes were observed in the magnetoresistance and Hall resistance. A decrease in the concentration of electron holes in the band structure was also found as a result of the presence of Frenkel defects.
Gallium ion incorporation was also found to cause changes in the Fermi surface of the WTe2 material, as well as its electronic band structure.
Using defect engineering approaches, the Researchers have demonstrated an effective way to control the electronic properties of WTe2 devices. The implementation of gallium is thought to be a useful approach to achieve electronic optimization and has the potential to extend to thin film devices of other layered transition metal dichalcogenides (TMDs).
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Source:
“Tuning the electrical transport of type II Weyl semimetal WTe2 nanodevices by Ga+ ion implantation”- Fu D., Scientific Reports, 2017, DOI:10.1038/s41598-017-12865-8
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