Field-Effect Transistors Based on Few-Layered alpha-MoTe_2

Abstract: Here we report the properties of field-effect transistors based on few layers of chemical vapor transport grown alpha- MoTe_2 crystals mechanically exfoliated onto SiO_2. We performed field-effect and Hall mobility measurements, as well as Raman scattering and transmission electron microscopy. In contrast to both MoS_2 and MoSe_2, our MoTe_2 field-effect transistors (FETs) are observed to be hole-doped, displaying on/off ratios surpassing 106 and typical sub-threshold swings of ~ 140 mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm^2/Vs which are comparable to figures previously reported for single or bi-layered MoS_2 and/or for MoSe_2 exfoliated onto SiO_2 at room temperature and without the use of dielectric engineering. Raman scattering reveals sharp modes in agreement with previous reports, whose frequencies are found to display little or no dependence on the number of layers. Given that both MoS_2 is electron doped, the stacking of MoTe_2 onto MoS_2 could produce ambipolar field-effect transistors and a gap modulation. Although the overall electronic performance of MoTe_2 is comparable to those of MoS_2 and MoSe_2, the heavier element Te should lead to a stronger spin orbit-coupling and possibly to concomitantly longer decoherence times for exciton valley and spin indexes.

• The paper demonstrates that few-layered α-MoTe₂ FETs exhibit intrinsic hole-doping with carrier mobility ranging from 20 to 30 cm²/Vs, ideal for low-power devices.
• It employs chemical vapor transport synthesis and Raman spectroscopy to confirm high crystallinity and precise layer thickness identification.
• The research underscores the potential of strong spin-orbit coupling in α-MoTe₂ for advancing ambipolar, spintronic, and optoelectronic device applications.

Overview of Field-Effect Transistors Based on Few-Layered α-MoTe2

This paper presents a detailed investigation into the electrical and structural properties of field-effect transistors (FETs) based on few-layered molybdenum ditelluride (α-MoTe₂) synthesized via chemical vapor transport. The study focuses on α-MoTe₂ due to its unique characteristics compared to more commonly studied transition metal dichalcogenides (TMDs) like MoS₂ and MoSe₂. The research examines the potential of MoTe₂ for electronic applications, particularly in domains like spintronics, due to its hole-doping behavior and spin-orbit coupling properties.

Key Findings

The study identifies several compelling features of MoTe₂ FETs:

1. Hole-Doped Behavior: In contrast to MoS₂ and MoSe₂ which are typically electron doped, MoTe₂ displays intrinsic hole-doped characteristics. This distinct property appropriates MoTe₂ for applications in logic devices that leverage both electron and hole conduction (ambipolar characteristics).
2. Carrier Mobility: Field-effect mobility is documented ranging from 20 cm²/Vs in bilayer samples to roughly 30 cm²/Vs in seven-layer samples. These results are competitive with other TMD-based transistors, even without high-κ dielectric engineering, indicating the viability of MoTe₂ in low-power device applications.
3. Spin-Orbit Coupling: Given tellurium's large atomic mass, MoTe₂ is shown to possess strong spin-orbit coupling which facilitates longer exciton valley coherence times, bolstering its promise for spintronic and optoelectronic device innovations.
4. Structural Integrity & Raman Spectroscopy: Raman measurements showed blue and red shifts in certain modes as the number of atomic layers decreased, corroborating high crystallinity with potential use for layer thickness identification. This confirms that high-quality exfoliation can be achieved, preserving the intrinsic material properties crucial for optoelectronic applications.
5. Contrasts with Literature: The Hall mobilities in this study suggest discrepancies between previously understood mobility behavior in similar systems and point towards increasing effectiveness as the number of atomic layers increases, peaking around ten layers.

Implications and Future Directions

This research advocates for the exploration of MoTe₂ as a fundamental element in the development of novel electronic devices due to its appealing charge transport and spin properties. The study suggests that MoTe₂, especially when integrated with other TMDs, could form heterostructures that leverage TMDs' combined band-gap and spin-related properties. This could potentially enable room temperature optoelectronic and spintronic devices with enhanced efficiency or functionality not achievable with existing materials.

Future work may explore understanding the effects of stacking with other TMDs to develop ambipolar devices further. Systematic investigations into contact metal properties and their roles in determining device behavior could provide clarity on optimizing MoTe₂-based devices fully. Additionally, efforts to achieve more stoichiometrically pure samples could enhance material properties, potentially elevating the resultant FET performance metrics significantly.

Conclusion

The exploration of few-layered α-MoTe₂ FETs illustrates how advancements beyond traditional TMD counterparts like MoS₂ offer promising avenues for electronic development. With intrinsic hole-doping and advantageous spin-orbit interactions, this material can open avenues in device architectures where ambipolar transport and enhanced valley or spin coherence are critical. Such advances highlight the still burgeoning potential of TMDs for catalyzing the next generation of electronic and spintronic devices.