Band-Like Transport in High Mobility Unencapsulated Single-Layer MoS2 Transistors (1304.5567v1)

Abstract: Ultra-thin MoS2 has recently emerged as a promising two-dimensional semiconductor for electronic and optoelectronic applications. Here, we report high mobility (>60 cm2/Vs at room temperature) field-effect transistors that employ unencapsulated single-layer MoS2 on oxidized Si wafers with a low level of extrinsic contamination. While charge transport in the sub-threshold regime is consistent with a variable range hopping model, monotonically decreasing field-effect mobility with increasing temperature suggests band-like transport in the linear regime. At temperatures below 100 K, temperature-independent mobility is limited by Coulomb scattering, whereas, at temperatures above 100 K, phonon-limited mobility decreases as a power law with increasing temperature.

• The paper demonstrates band-like charge transport in single-layer MoS₂ transistors with field-effect mobility exceeding 60 cm²/Vs under high vacuum.
• The paper identifies distinct transport regimes, with variable range hopping at low carrier densities and phonon-limited mobility at higher concentrations.
• The paper highlights that environmental adsorbates significantly degrade mobility, underscoring the need for contamination control in device optimization.

Analysis of Band-Like Transport Phenomena in Single-Layer MoS₂ Transistors

The paper presents an exploration of charge transport mechanisms in high-mobility, unencapsulated single-layer molybdenum disulfide (MoS₂) transistors. Conducted by researchers from Northwestern University, the paper illuminates the intrinsic and extrinsic factors influencing electron mobility in MoS₂, aiming to enhance the performance potential of this promising two-dimensional (2D) semiconductor material in electronic and optoelectronic applications.

MoS₂, a material distinct from its 2D counterpart graphene due to its direct bandgap, has attracted interest due to its potential applications in low-power electronics and optoelectronics. The research stands out for achieving high field-effect mobility (>60 cm²/Vs at room temperature) in unencapsulated single-layer MoS₂ transistors through refined experimental conditions minimizing extrinsic contamination, notably in high vacuum. This discovery contrasts with earlier findings where unencapsulated MoS₂ exhibited significantly lower mobilities (0.2-12 cm²/Vs).

Key Findings

1. Charge Transport Mechanism: The paper identifies two dominant transport regimes dependent on temperature and carrier density:
   • Variable Range Hopping (VRH) in the sub-threshold regime, attributed to the presence of localized trap states within the bandgap, emphasizes the role disorder plays at low carrier densities.
   • Band-Like Transport in the linear regime at high carrier densities, with a field-effect mobility that increases as temperature decreases, is indicative of reduced Coulomb scattering below 100 K and phonon-limited mobility above 100 K. Above 100 K, a power-law dependence of mobility on temperature is observed, aligning with phonon scattering models exhibiting an exponent γ close to 0.62.
2. Influence of Environmental Factors: A critical observation is that atmospheric adsorbates substantially degrade mobility by inducing doping and additional trap states, which underscores the necessity of vacuum measurements. This factor highlights variability in reported data due to practical variations in environmental conditions during measurement.
3. Comparative Analysis: In juxtaposition to other studies, the findings suggest significant discrepancies owing to sample-to-sample variations and measurement conditions. Recent reports had suggested VRH-like or thermally activated behavior; however, under well-controlled conditions, this paper highlights band-like transport as a viable description in specific regimes.

Implications

This research offers deeper insights into optimizing the performance of MoS₂-based devices by focusing on controlling extrinsic contamination and choosing appropriate substrates. The observations suggest that current unencapsulated devices are still significantly below the theoretically predicted mobility of ~400 cm²/Vs, a gap attributed to the impact of extrinsic factors and crystallinity of the samples.

Future Directions

The findings pave the way for further explorations into substrate engineering and encapsulation techniques, potentially integrating technologically mature approaches like substrate-induced mobility enhancements observed in graphene. Research could extend towards investigating alternative dielectric materials and refined vacuum technology applications to suppress scattering-inducing adsorbates further, effectively pushing the performance of MoS₂ closer to its theoretical limits.

Overall, this paper underscores the critical nuances in charge transport within 2D MoS₂ and marks a step in identifying the parameters crucial for advancing the material's application in nanoelectronic devices, opening avenues for new experimental studies aimed at unlocking its full potential.