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1. Temperature dependent charge transport in ferroelectrically gated graphene far from the Dirac point

   https://doi.org/10.1063/5.0096776  

   Figueroa, Kelotchi_S; Zimbovskaya, Natalya_A; Pinto, Nicholas_J; Wen, Chengyu; Johnson, A_T_Charlie (July 2022, AIP Advances)

   Charge transport in ferroelectric (FE) gated graphene far from the Dirac point (DP) was studied in the temperature range 300 K < T < 350 K. A non-monotonic/monotonic/non-monotonic behavior in the conductivity [σ(T)] was observed as one moved away from the DP. As the gate polarization increased, additional impurity charges were compensated, which reduced charge scattering. The uncompensated charges doped graphene and σ(T) switched to a monotonic increase with increasing T. However, far from the DP, the polarization reached saturation, which resulted in still lower impurity charge scattering. The carrier concentration increased, and a non-monotonic response in σ(T) reappeared, which was attributed to phonon scattering. A theoretical model is presented that combined impurity charge and phonon scattering conduction mechanisms. The top gate polarizable FE provided a novel approach to investigate charge transport in graphene via controlled compensation of impurity charges, and in the process revealed non-monotonic behavior in σ(T) not previously seen in SiO2 back gated graphene devices. 

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2. Field-effect conductivity scaling for two-dimensional materials with tunable impurity density

   https://doi.org/10.1088/2053-1583/ac72b0  

   Wang, Chulin; Peng, Lintao; Wells, Spencer A; Cain, Jeffrey D; Huang, Yi-Kai; Rhoads, Lawrence A; Dravid, Vinayak P; Hersam, Mark C; Grayson, Matthew A (June 2022, 2D Materials)

   Abstract A scaling law is demonstrated in the conductivity of gated two-dimensional (2D) materials with tunable concentrations of ionized impurity scatterers. Experimental data is shown to collapse onto a single 2D conductivity scaling (2DCS) curve when the mobility is scaled by r , the relative impurity-induced scattering, and the gate voltage is shifted by V s , a consequence of impurity-induced doping. This 2DCS analysis is demonstrated first in an encapsulated 2D black phosphorus multilayer at T = 100 K with charge trap densities programmed by a gate bias upon cooldown, and next in a Bi 2 Se 3 2D monolayer at room temperature exposed to varying concentrations of gas adsorbates. The observed scaling can be explained using a conductivity model with screened ionized impurity scatterers. The slope of the r  vs.  V s plot defines a disorder-charge specific scattering rate Γ q = d r / d V s equivalent to a scattering strength per unit impurity charge density: Γ q > 0 indicates a preponderance of positively charged impurities with Γ q < 0 for negatively charged. This 2DCS analysis is expected to be applicable in arbitrary 2D materials systems with tunable impurity density, which will advance 2D materials characterization and improve performance of 2D sensors and transistors. 

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3. Effects of temperature and oxygen partial pressure on electrical conductivity of Fe-doped β -Ga2O3 single crystals

   https://doi.org/10.1063/5.0093588  

   Ryu, Gyunghyun; Reddy, Pramod; Collazo, Ramón; Dickey, Elizabeth C. (May 2022, Applied Physics Letters)

   In this work, we measure DC and AC conductivity and Hall voltage to determine the origin of electrical insulating properties of Fe-doped β-Ga2O3 single crystals, which are measured perpendicular to the 2¯01 crystallographic plane. We find that electrical conduction is predominantly controlled by free electrons in the temperature range 230–800 °C with the mutual compensation of the impurity donor (Si) and acceptor dopant (Fe), explaining the low concentration of free electrons and Fermi level pinning over a wide range of temperatures. Furthermore, the negative temperature-dependence of the carrier mobility indicates that it is limited by optical phonon scattering. Importantly, we find electrical conductivity to be largely independent of oxygen partial pressure (pO2) from air to 10−4 atm at 600 °C, but it becomes slightly dependent on pO2 at 800 °C, as intrinsic non-stoichiometric point defects begin to influence the charge balance. 

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4. Thermal conductivity of Fe-Si alloys and thermal stratification in Earth’s core

   https://doi.org/10.1073/pnas.2119001119  

   Zhang, Youjun; Luo, Kai; Hou, Mingqiang; Driscoll, Peter; Salke, Nilesh P.; Minár, Ján; Prakapenka, Vitali B.; Greenberg, Eran; Hemley, Russell J.; Cohen, R. E.; et al (December 2021, Proceedings of the National Academy of Sciences)

   Light elements in Earth’s core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron–electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ∼100 to 110 W⋅m −1 ⋅K −1 for liquid Fe-9Si near the topmost outer core. If Earth’s core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core–mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core–mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core. 

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5. Hopping charge transport in hydrogenated amorphous silicon–germanium alloy thin films

   https://doi.org/10.1063/5.0077441  

   Stolik, L.; Eslamisaray, M. A.; Nguyen, E.; Kortshagen, U. R.; Kakalios, J. (June 2022, Journal of Applied Physics)

   Measurements of the dark conductivity and thermoelectric power in hydrogenated amorphous silicon–germanium alloys (a-Si 1- x Ge x :H) reveal that charge transport is not well described by an Arrhenius expression. For alloys with concentrations of Ge below 20%, anomalous hopping conductivity is observed with a power-law exponent of 3/4, while the temperature dependence of the conductivity of alloys with higher Ge concentrations is best fit by a combination of anomalous hopping and a power-law temperature dependence. The latter has been attributed to charge transport via multi-phonon hopping. Corresponding measurements of the Seebeck coefficient reveal that the thermopower is n-type for the purely a-Si:H and a-Ge:H samples but that it exhibits a transition from negative to positive values as a function of the Ge content and temperature. These findings are interpreted in terms of conduction via hopping through either exponential band tail states or dangling bond defects, suggesting that the concept of a mobility edge, accepted for over five decades, may not be necessary to account for charge transport in amorphous semiconductors. 

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