A review on the enhancement of figure of merit from bulk to nano-thermoelectric materials

In an ultraclean 2D electron system (2DES) subjected to crossed millimeterwave (30--150 GHz) and weak ($Bl2\text{ }\mathrm{k}\mathrm{G}$) magnetic fields, a series of apparently dissipationless states emerges as the system is detuned from cyclotron resonances. Such states are characterized by an exponentially vanishing low-temperature diagonal resistance and a classical Hall resistance. The activation energies associated with such states exceed the Landau level spacing by an order of magnitude. Our findings are likely indicative of a collective ground state previously unknown for 2DES.

Evidence for a new dissipationless effect in 2D electronic transport.

As the only non-carbon elemental layered allotrope, few-layer black phosphorus or phosphorene has emerged as a novel two-dimensional (2D) semiconductor with both high bulk mobility and a band gap. Here we report fabrication and transport measurements of phosphorene-hexagonal BN (hBN) heterostructures with one-dimensional edge contacts. These transistors are stable in ambient conditions for >300 h, and display ambipolar behavior, a gate-dependent metal?insulator transition, and mobility up to 4000 cm2 V?1 s?1. At low temperatures, we observe gate-tunable Shubnikov de Haas magneto-oscillations and Zeeman splitting in magnetic field with an estimated g-factor ?2. The cyclotron mass of few-layer phosphorene (FLP) holes is determined to increase from 0.25 to 0.31 me as the Fermi level moves towards the valence band edge. Our results underscore the potential of FLP as both a platform for novel 2D physics and an electronic material for semiconductor applications.

https://iopscience.iop.org/article/10.1088/2053-1583/2/1/011001/pdf

Gate tunable quantum oscillations in air-stable and high mobility few-layer phosphorene heterostructures

Handbook on semiconductors

The thermoelectric properties of graphene and graphene nanostructures have recently attracted significant attention from the physics and engineering communities. In fundamental physics, the analysis of Seebeck and Nernst effects is very useful in elucidating some details of the electronic band structure of graphene that cannot be probed by conductance measurements alone, due in particular to the ambipolar nature of this gapless material. For applications in thermoelectric energy conversion, graphene has two major disadvantages. It is gapless, which leads to a small Seebeck coefficient due to the opposite contributions of electrons and holes, and it is an excellent thermal conductor. The thermoelectric figure of merit ZT of a two-dimensional (2D) graphene sheet is thus very limited. However, many works have demonstrated recently that appropriate nanostructuring and bandgap engineering of graphene can concomitantly strongly reduce the lattice thermal conductance and enhance the Seebeck coefficient without dramatically degrading the electronic conductance. Hence, in various graphene nanostructures, ZT has been predicted to be high enough to make them attractive for energy conversion. In this article, we review the main results obtained experimentally and theoretically on the thermoelectric properties of graphene and its nanostructures, emphasizing the physical effects that govern these properties. Beyond pure graphene structures, we discuss also the thermoelectric properties of some hybrid graphene structures, as graphane, layered carbon allotropes such as graphynes and graphdiynes, and graphene/hexagonal boron nitride heterostructures which offer new opportunities. Finally, we briefly review the recent activities on other atomically thin 2D semiconductors with finite bandgap, i.e. dichalcogenides and phosphorene, which have attracted great attention for various kinds of applications, including thermoelectrics.

/pdf/thermoelectric-effects-in-graphene-nanostructures-1nv0pibydr.pdf

Thermoelectric effects in graphene nanostructures.

We have analyzed the current theories of the conductivity of a two-dimensional electron gas in low magnetic fields to determine the correct usage of the transport and quantum lifetimes. The analysis is shown to correctly predict the low-field amplitudes and phases of the de Haas\char21{}Shubnikov oscillation in both the longitudinal and Hall resistivities of two samples. Deviations at high fields are attributed to localization.

Low-field transport coefficients in GaAs/Ga1-xAlxAs heterostructures.

Mesure de la mobilite et des densites electroniques des sous-bandes individuelles de ces heterojonctions presentant deux occupations de sous-bandes en fonction de la densite du gaz d'electrons bidimensionnel. On fait varier la densite au moyen de la photoconductivite persistante en utilisant les rayonnements IR et rouge

Persistent photoconductivity and two-band effects in GaAs/AlxGa1-xAs heterojunctions.

We have used low-field de Haas--Shubnikov oscillations in the resistivity to determine the electron temperatures of a two-dimensional electron gas subjected to various heating currents. This was done at many values of the electron density from 5.5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ to 10\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}2}$ that were incrementally induced by the persistent-photoconductivity effect. The onset of second-subband occupation is clearly reflected in a large rise in energy-loss rates. The variations of the energy-loss rate with both temperature and electron density are well reproduced by detailed numerical calculations using values of deformation-potential and piezoelectric coupling parameters consistent with bulk measurements. The calculations also show that there is a transition from the low- to high-temperature behavior in the liquid${\mathrm{\ensuremath{-}}}^{4}$He temperature range. In this region the energy-loss rates vary roughly as the cube of the temperature, a result often observed experimentally but one which is not of fundamental significance. At the lowest temperatures the behavior should be dominated by the piezoelectric interaction which is characterized by a ${\mathit{T}}^{5}$ law. The present experimental data show the transition from ${\mathit{T}}^{3}$ to a higher-power law as the temperature is decreased, in accordance with the theoretical predictions.

Energy-loss rates of two-dimensional electrons at a GaAs/AlxGa1-xAs interface.

Electron-phonon coupling of a two-dimensional electron gas in a Si metal-oxide-semiconductor field-effect transistor in the temperature range $0.3\mathrm{K}lTl4\mathrm{K}$ has been investigated using phonon-drag thermopower ${S}^{g}$ and electron energy loss rate $F(T).$ At low temperatures (the Bloch limit) we find ${S}^{g}\ensuremath{\propto}{T}^{6},$ as expected for electron-phonon scattering mediated by a screened deformation potential, and the magnitude is in excellent agreement with a calculation using no adjustable parameters; the calculation continues to give good agreement at higher temperatures. $F(T)$ has been calculated using the same input parameters as for ${S}^{g}.$ Reasonably good agreement is found with the observed values for $Tg1.5\mathrm{K},$ but at lower temperatures the measured $F(T)$ is much larger than predicted and also exhibits a much weaker temperature dependence. Possible reasons are suggested.

Thermoelectric and hot-electron properties of a silicon inversion layer

We report the first experimental demonstration that phonon drag is present in the thermopower of a two-dimensional electron gas at a GaAs-${\mathrm{Ga}}_{0.68}$${\mathrm{Al}}_{0.32}$As heterojunction in both zero and high magnetic fields. The experiment involves polishing the rear surface of the GaAs plate on which the junction is grown in order to increase the phonon mean free path. This results in an increase of the thermopower by about a factor of 2 even though the electrical resistivities are unaffected.

R. Fletcher

Papers

Low-field transport coefficients in GaAs/Ga1-xAlxAs heterostructures.

Persistent photoconductivity and two-band effects in GaAs/AlxGa1-xAs heterojunctions.

Energy-loss rates of two-dimensional electrons at a GaAs/AlxGa1-xAs interface.

Thermoelectric and hot-electron properties of a silicon inversion layer

Evidence of phonon drag in the thermopower of a GaAs-Ga 0.68 Al 0.32 As heterojunction