Attaining Low Lattice Thermal Conductivity in Half-Heusler Sublattice Solid Solutions: Which Substitution Site Is Most Effective?

Low thermal conductivity is an important materials property for thermoelectricity. The lattice thermal conductivity (LTC) can be reduced by introducing sublattice disorder through partial isovalent substitution. Yet, large-scale screening of materials has seldom taken this opportunity into account. The present study aims to investigate the effect of partial sublattice substitution on the LTC. The study relies on the temperature-dependent effective potential method based on forces obtained from density functional theory. Solid solutions are simulated within a virtual crystal approximation, and the effect of grain-boundary scattering is also included. This is done to systematically probe the effect of sublattice substitution on the LTC of 122 half-Heusler compounds. It is found that substitution on the three different crystallographic sites leads to a reduction of the LTC that varies significantly both between the sites and between the different compounds. Nevertheless, some common criteria are identified as most efficient for reduction of the LTC: The mass contrast should be large within the parent compound, and substitution should be performed on the heaviest atoms. It is also found that the combined effect of sublattice substitution and grain-boundary scattering can lead to a drastic reduction of the LTC. The lowest LTC of the current set of half-Heusler compounds is around 2 W/Km at 300 K for two of the parent compounds. Four additional compounds can reach similarly low LTC with the combined effect of sublattice disorder and grain boundaries. Two of these four compounds have an intrinsic LTC above ∼15 W/Km, underlining that materials with high intrinsic LTC could still be viable for thermoelectric applications.

Deposition of Low-Resistivity Aluminum Thin Films Via Application of Substrate Bias During Magnetron Sputtering

In this study, the critical role of substrate bias during the sputter deposition of Al thin films is discussed. Two sets of Al thin films having a nominal thickness of 300 nm were deposited at sputtering pressures of 4.1 and 1.5 mTorr, respectively, with an applied negative substrate bias in the range of 0-200 V. It was found that the microstructure, surface roughness, film resistivity and grain size were greatly altered by the combination of bias magnitudes and sputtering pressures. The sputtering pressure of 4.1 mTorr resulted in greater changes in the film properties with the application of substrate bias, and a lesser but still significant degree was observed for the films deposited at 1.5 mTorr. The resistivity values for the films deposited at 1.5 mTorr were found to be significantly lower, with the lowest resistivity value of 3.1 µΩcm achieved at a substrate bias of 50 V. Based on grain size measured by the line intercept method and MayadasShatzkes grain boundary scattering model, the resistivity contribution of grain boundary scattering for the lowest-resistivity film was found to be 0.37 µΩcm, which indicates that the film resistivity in the optimized condition is close to the known bulk resistivity of 2.65 µΩcm.

Enhancing the average thermoelectric figure of merit of elemental Te by suppressing grain boundary scattering

Suppressed grain boundary scattering contributes to enhanced electrical conductivity and device zT in elemental Te based thermoelectric materials.

Impacts of the thickness on the electron mobility of gallium and hydrogen co– doped zinc oxide thin films

this work, impacts of the thickness on electron mobility of Ga and H2 co-doped ZnO (HGZO) thin films were investigated. The HGZO films were prepared on glass substrate by using magnetron sputtering from ceramic Ga-doped ZnO (GZO) target in the gas mixture of argon and hydrogen. Based on the Hall measurement, the mobility enhanced fastly from 44.6 to 53.4cm2/Vs with the increasing thickness from 350 to 900 nm, then tends to be saturated at ~55cm2/Vs with further thickness. Most of the films achieve the mobility of >50cm2/Vs, which is very high value for sputtered TCOs thin films. The thicknessdependent mobility is explained in term of grain boundary scattering. The improvement of crystalline quality reduced grain boundary scattering, which lead to the fast increase in mobility of the films with 350–900nm in thickness. When the thickness increased more than 900nm, however, the appearance of many defects increased scattering centers and saturates the mobility. Furthermore, the results showed the HGZO films with optimum thickness of 800nm obtained low resistivity (5.3 10-4􀁛cm), high average transmittance (83.3%) in the wide wavelength range of 400–1100nm, and the highest figure of merit (10.3 103􀁛-1cm-1) corresponding to high mobility (51.1cm2/Vs).