In this paper, we systematically investigate three different routes of synthesizing 2% Na-doped PbTe after melting the elements: (i) quenching followed by hot-pressing (QH), (ii) annealing followed by hot-pressing, and (iii) quenching and annealing followed by hot-pressing. We found that the thermoelectric figure of merit, zT, strongly depends on the synthesis condition and that its value can be enhanced to ∼2.0 at 773 K by optimizing the size distribution of the nanostructures in the material. Based on our theoretical analysis on both electron and thermal transport, this zT enhancement is attributed to the reduction of both the lattice and electronic thermal conductivities; the smallest sizes (2∼6 nm) of nanostructures in the QH sample are responsible for effectively scattering the wide range of phonon wavelengths to minimize the lattice thermal conductivity to ∼0.5 W/m K. The reduced electronic thermal conductivity associated with the suppressed electrical conductivity by nanostructures also helped reduce the total thermal conductivity. In addition to the high zT of the QH sample, the mechanical hardness is higher than the other samples by a factor of around 2 due to the smaller grain sizes. Overall, this paper suggests a guideline on how to achieve high zT and mechanical strength of a thermoelectric material by controlling nano-and microstructures of the material.waste heat recovery | energy harvesting A thermoelectric (TE) device is a solid-state device that converts heat directly into electricity and vice versa (1-5). As there are no moving parts involved and the device configuration is simple, TE devices have demonstrated long-term reliability in various space missions, usually running for tens of years without maintenance (6). However, they are not yet widely used in many other energy conversion applications on earth mainly due to their low conversion efficiencies. The conversion efficiency of a TE device largely depends on the material properties, i.e., the figure of merit (1, 3), zT = [S 2 /ρ(κ L + κ e )]T, where T is the absolute temperature, S is the Seebeck coefficient, ρ is the electrical resistivity, and κ L and κ e are, respectively, the lattice (or phonon) and electronic thermal conductivities. Increasing the zT has proven challenging because the constituent TE properties are interdependent; for example, decreasing the electrical resistivity results in decreasing the Seebeck coefficient and increasing the electronic thermal conductivity.

Direct electrostatic toner marking with poly(3,4-ethylenedioxythiophene)polystyrenesulfonate bilayer devices J. Appl. Phys. 112, 074506 (2012) Modeling the effect of top gate voltage on the threshold of a double gate organic field effect transistor J. Appl. Phys. 112, 073704 (2012) Electron transporting water-gated thin film transistors Appl. Phys. Lett. 101, 141603 (2012) Impact of gate resistance in graphene radio frequency transistors Appl. Phys. Lett. 101, 143503 (2012) Physical-gap-channel graphene field effect transistor with high on/off current ratio for digital logic applications Transistors operating well above the frequencies at which they have gain can still rectify terahertz currents and voltages, and have attracted interest as room-temperature terahertz detectors. We show that such rectifying field-effect transistors may still be treated as a lumped element device in the limit where plasma resonances of the electron gas do not occur. We derive analytic formulas for important transistor parameters, such as effective rectification length and device impedance using a transmission-line model. We draw conclusions for plasma-resonant detection where possible. We derive the THz response of a field-effect transistor with a two-dimensional electrongas channel by a Taylor expansion of the drain-source bias. We connect circuit theory to the existing theories that describe the bias in the gated region by differential equations. Parasitic effects, such as the access resistance, are included. With the approach presented in this paper, we derive the responsivity for a novel field detector that mixes a (THz) signal applied between gate and source with another signal applied between drain and source in homodyne or heterodyne operation mode. We further derive expressions for the expected noise-equivalent power (NEP) in direct detection and mixing mode, including parasitic effects, and find that sub-pW= ffiffiffiffiffiffi Hz p should be achievable for realistic device and material parameters for direct detection and less than 900 K noise temperature for mixing at 10 lW local oscillator power.

The dependence of electron mobility on growth conditions and threading dislocation density (TDD) was studied for n−-GaN layers grown by ammonia-based molecular beam epitaxy. Electron mobility was found to strongly depend on TDD, growth temperature, and Si-doping concentration. Temperature-dependent Hall data were fit to established transport and charge-balance equations. Dislocation scattering was analyzed over a wide range of TDDs (∼2 × 106 cm−2 to ∼2 × 1010 cm−2) on GaN films grown under similar conditions. A correlation between TDD and fitted acceptor states was observed, corresponding to an acceptor state for almost every c lattice translation along each threading dislocation. Optimized GaN growth on free-standing GaN templates with a low TDD (∼2 × 106 cm−2) resulted in electron mobilities of 1265 cm2/Vs at 296 K and 3327 cm2/Vs at 113 K.

AlxGa1−xN/GaN (x = 0.06, 0.12, 0.24) and AlGaN/AlN/GaN heterostructures were grown on 6 H-SiC, GaN-on-sapphire, and free-standing GaN, resulting in heterostructures with threading dislocation densities of ∼2 × 1010, ∼5 × 108, and ∼5 × 107 cm−2, respectively. All growths were performed under Ga-rich conditions by plasma-assisted molecular beam epitaxy. Dominant scattering mechanisms with variations in threading dislocation density and sheet concentration were indicated through temperature-dependent Hall measurements. The inclusion of an AlN interlayer was also considered. Dislocation scattering contributed to reduced mobility in these heterostructures, especially when sheet concentration was low or when an AlN interlayer was present.

Metal-polar In 0.17 Al 0.83 N barriers, lattice-matched to GaN, were grown under N-rich conditions by plasma-assisted molecular beam epitaxy. The compositional homogeneity of these barriers was confirmed by plan-view high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. Metal-polar In 0.17 Al 0.83 N/(GaN)/(AlN)/GaN structures were grown with a range of AlN and GaN interlayer (IL) thicknesses to determine the optimal structure for achieving a low two-dimensional electron gas (2DEG) sheet resistance. It was determined that the presence of a GaN IL was necessary to yield a 2DEG sheet density above 2 × 10 13 cm −2 . By including AlN and GaN ILs with thicknesses of 3 nm and 2 nm, respectively, a metal-polar In 0.17 Al 0.83 N/GaN/AlN/GaN structure regrown on a GaN-on-sapphire template yielded a room temperature (RT) 2DEG sheet resistance of 163 / . This structure had a threading dislocation density (TDD) of ∼5 × 10 8 cm −2 . Through regrowth on a free-standing GaN template with low TDD (∼5 × 10 7 cm −2 ), an optimized metal-polar In 0.17 Al 0.83 N/GaN/AlN/GaN structure achieved a RT 2DEG sheet resistance of 145 / and mobility of 1822 cm 2 V −1 s −1 . High-electron-mobility transistors with output current densities above 1 A mm −1 were also demonstrated on the low-TDD structure.