Variable-range hopping

About: Variable-range hopping is a research topic. Over the lifetime, 3606 publications have been published within this topic receiving 66561 citations.

Large thermoelectricity via variable range hopping in chemical vapor deposition grown single-layer MoS2.

Abstract: Ultrathin layers of semiconducting molybdenum disulfide (MoS2) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS2, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS2. Microfabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ∼30 mV/K at room temperature are observed, which are much larger than those observed in other two-dimensional crystals and bulk MoS2. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows S ∼ T(1/3), suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS2 and the high thermopower value is of interest to future thermoelectronic research and application.

Charge trapping at the MoS2-SiO2 interface and its effects on the characteristics of MoS2 metal-oxide-semiconductor field effect transistors

Abstract: The field effect transistors (FETs) based on thin layer MoS2 often have large hysteresis and unstable threshold voltage in their transfer curves, mainly due to the charge trapping at the oxide-semiconductor interface. In this paper, the charge trapping and de-trapping processes at the SiO2-MoS2 interface are studied. The trapping charge density and time constant at different temperatures are extracted. Making use of the trapped charges, the threshold voltage of the MoS2 based metal-oxide-semiconductor FETs is adjusted from 4 V to −45 V. Furthermore, the impact of the trapped charges on the carrier transport is evaluated. The trapped charges are suggested to give rise to the unscreened Coulomb scattering and/or the variable range hopping in the carrier transport of the MoS2 sheet.

Orbital magnetoconductance in the variable-range-hopping regime.

Abstract: The orbital magnetoconductance (MC) in the variable-range--hopping (VRH) regime is evaluated by use, for definiteness, of a model proposed by Nguyen, Spivak, and Shklovskii, which approximately takes into account the interference among random paths in the hopping process. Instead of logarithmic averaging which is shown to be inadequate for this case, the MC is obtained by the critical percolating resistor method of Ambegaokar, Halperin, and Langer. The small-field MC is quadratic in H; it is positive deep in the VRH regime and changes sign when the zero-field conductivity is high enough. This behavior (except for the sign change) and the relevant magnetic field scale are in agreement with recent experiments. The calculated MC is always positive for strong fields and is predicted to saturate at sufficiently large fields.

Hopping in disordered conducting polymers

Abstract: This paper addresses some important questions concerning hopping conduction in doped conducting polymers, i.e., dimensionality, homogeneity, adiabaticity, Coulomb interactions, and multiphonon character. It will be shown that electron transport in these materials is due to correlated hopping between polaronic clusters. We propose a model which describes quantitatively this process. Conductivity and magnetoresistance measurements performed on disordered polypyrroles doped with polyelectrolytes are presented in support of these ideas. This model emphasizes the role of the dopant as an attractive potential and as a tunneling bridge between neighboring chains. The dopant ions are the source of both the polaronic clusters stability and of the transverse conduction.