Sit Kerdsongpanya

Anomalously high thermoelectric power factor in epitaxial ScN thin films

Thermoelectric properties of ScN thin films grown by reactive magnetron sputtering on Al2O3(0001) wafers are reported. X-ray diffraction and elastic recoil detection analyses show that the composition of the films is close to stoichiometry with trace amounts (∼1 at. % in total) of C, O, and F. We found that the ScN thin-film exhibits a rather low electrical resistivity of ∼2.94 μΩm, while its Seebeck coefficient is approximately ∼−86 μV/K at 800 K, yielding a power factor of ∼2.5 × 10−3 W/mK2. This value is anomalously high for common transition-metal nitrides.

Highly oriented δ-Bi2O3 thin films stable at room temperature synthesized by reactive magnetron sputtering

We report the synthesis by reactive magnetron sputtering and structural characterization of highly (111)-oriented thin films of δ–Bi2O3. This phase is obtained at a substrate temperature of 150–200 °C in a narrow window of O2/Ar ratio in the sputtering gas (18%–20%). Transmission electron microscopy and x-ray diffraction reveal a polycrystalline columnar structure with (111) texture. The films are stable from room temperature up to 250 °C in vacuum and 350 °C in ambient air.

Transition-metal-nitride-based thin films as novel energy harvesting materials

We review experimental and theoretical research on ScN- and CrN-based transition-metal nitride materials for thermoelectrics, drawing parallels with piezoelectricity.

Phase stability of ScN-based solid solutions for thermoelectric applications from first-principles calculations

We have used first-principles calculations to investigate the trends in mixing thermodynamics of ScN-based solid solutions in the cubic B1 structure. 13 different Sc1−xMxN (M = Y, La, Ti, Zr, Hf, V, Nb, Ta, Gd, Lu, Al, Ga, In) and three different ScN1−xAx (A = P, As, Sb) solid solutions are investigated and their trends for forming disordered or ordered solid solutions or to phase separate are revealed. The results are used to discuss suitable candidate materials for different strategies to reduce the high thermal conductivity in ScN-based systems, a material having otherwise promising thermoelectric properties for medium and high temperature applications. Our results indicate that at a temperature of T = 800 °C, Sc1−xYxN; Sc1−xLaxN; Sc1−xGdxN, Sc1−xGaxN, and Sc1−xInxN; and ScN1−xPx, ScN1−xAsx, and ScN1−xSbx solid solutions have phase separation tendency, and thus, can be used for forming nano-inclusion or superlattices, as they are not intermixing at high temperature. On the other hand, Sc1−xTixN, Sc1−xZ...

Phase-stabilization and substrate effects on nucleation and growth of (Ti,V)(n+1)GeC(n) thin films

Phase-pure epitaxial thin films of (Ti,V)(2)GeC have been grown onto Al(2)O(3)(0001) substrates via magnetron sputtering. The c lattice parameter is determined to be 12.59 A, corresponding to a 50/50 Ti/V solid solution according to Vegards law, and the overall (Ti,V): Ge: C composition is 2:1:1 as determined by elastic recoil detection analysis. The minimum temperature for the growth of (Ti,V)(2)GeC is 700 degrees C, which is the same as for Ti(2)GeC but higher than that required for V(2)GeC (450 degrees C). Reduced Ge content yields films containing (Ti,V)(3)GeC(2) and (Ti,V)(4)GeC(3). These results show that the previously unknown phases V(3)GeC(2) and V(4)GeC(3) can be stabilized through alloying with Ti. For films grown on 4H-SiC(0001), (Ti,V)(3)GeC(2) was observed as the dominant phase, showing that the nucleation and growth of (Ti,V)(n+1)GeC(n) is affected by the choice of substrate; the proposed underlying physical mechanism is that differences in the local substrate temperature enhance surface diffusion and facilitate the growth of the higher-order phase (Ti,V)(3)GeC(2) compared to (Ti,V)(2)GeC.

Experimental and theoretical investigation of Cr1-xScxN solid solutions for thermoelectrics

We investigate the trends in mixing thermodynamics of Cr1-xScxN solid solutions in the cubic B1 structure and their electronic density of state by first-principle calculations, and thin-film synthe ...

Growth and properties of epitaxial Ti1−xMgxN(001) layers

Epitaxial Ti1−xMgxN(001) layers were deposited on MgO(001) by reactive magnetron cosputtering from titanium and magnesium targets in 15 mTorr pure N2 at 600 °C. X-ray diffraction (XRD) indicates a solid solution rock-salt phase for the composition range x = 0–0.55, a lattice constant that increases monotonously from 4.251 A for TiN to 4.288 A for Ti0.45Mg0.55N, and a decreasing crystalline quality with increasing Mg content, as quantified by the XRD ω rocking curve width which increases from 0.25° to 0.80°. XRD φ-scans show that all Ti1−xMgxN layers with x ≤ 0.55 are single crystals with a cube-on-cube epitaxial relationship with the substrate: (001)TiMgN║(001)MgO and [100]TiMgN║[100]MgO. In contrast, a larger Mg concentration (x = 0.85) leads to a polycrystalline, phase-segregated, nitrogen-deficient microstructure. The room temperature electrical resistivity increases from 14 μΩ cm for x = 0 to 554 and 3197 μΩ cm for x = 0.37 and 0.49, respectively. Ti1−xMgxN layers with 0.49 ≤ x ≤ 0.55 exhibit a negative temperature coefficient of resistivity which is attributed to the decreasing electron density of states at the Fermi level and a weak carrier localization. Optical transmission and reflection measurements indicate a decreasing electron density with increasing x and absorption minima at 2.0 and 1.7 eV for Ti0.63Mg0.37N and Ti0.48Mg0.52N, respectively, suggesting an extrapolated bandgap for semiconducting Ti0.5Mg0.5N of 0.7–1.7 eV.Epitaxial Ti1−xMgxN(001) layers were deposited on MgO(001) by reactive magnetron cosputtering from titanium and magnesium targets in 15 mTorr pure N2 at 600 °C. X-ray diffraction (XRD) indicates a solid solution rock-salt phase for the composition range x = 0–0.55, a lattice constant that increases monotonously from 4.251 A for TiN to 4.288 A for Ti0.45Mg0.55N, and a decreasing crystalline quality with increasing Mg content, as quantified by the XRD ω rocking curve width which increases from 0.25° to 0.80°. XRD φ-scans show that all Ti1−xMgxN layers with x ≤ 0.55 are single crystals with a cube-on-cube epitaxial relationship with the substrate: (001)TiMgN║(001)MgO and [100]TiMgN║[100]MgO. In contrast, a larger Mg concentration (x = 0.85) leads to a polycrystalline, phase-segregated, nitrogen-deficient microstructure. The room temperature electrical resistivity increases from 14 μΩ cm for x = 0 to 554 and 3197 μΩ cm for x = 0.37 and 0.49, respectively. Ti1−xMgxN layers with 0.49 ≤ x ≤ 0.55 exhibit a negati...

Resistivity size effect in epitaxial Ru(0001) layers

Epitaxial Ru(0001) layers are sputter deposited onto Al2O3(0001) substrates and their resistivity ρ measured both in situ and ex situ as a function of thickness d = 5–80 nm in order to quantify the resistivity scaling associated with electron-surface scattering. All layers have smooth surfaces with a root-mean-square roughness <0.4 nm, exhibit an epitaxial relationship with the substrate: Ru[0001]||Al2O3[0001] and Ru [101¯0]||Al2O3 [112¯0], and show no resistance change upon air exposure, suggesting negligible resistivity contributions from geometric surface roughness and grain boundary scattering and negligible changes in the surface scattering specularity p upon oxygen exposure. The room temperature ρ vs d data are well described by the semiclassical Fuchs-Sondheimer (FS) model, indicating a bulk electron mean free path λ = 6.7 ± 0.3 nm. However, the measured ρo × λ product at 77 K is 43% lower than at 295 K, suggesting a breakdown of the FS model and/or a thickness-dependent electron-phonon coupling and/or a temperature- or environment-dependent p. Transport simulations employing the ruthenium electronic structure determined from first-principles and a constant relaxation time approximation indicate that ρ is strongly (by a factor of two) affected by both the transport direction and the terminating surfaces. This is quantified with a room temperature effective mean free path λ*, which is relatively small for transport along the hexagonal axis independent of layer orientation (λ* = 4.3 nm) and for (0001) terminating surfaces independent of transport direction (λ* = 4.5 nm), but increases, for example, to λ* = 8.8 nm for (112¯0) surfaces and transport along [11¯00]. Direct experiment-simulation comparisons show a 12% and 49% higher λ from experiment at 77 and 295 K, respectively, confirming the limitations of the semi-classical transport simulations despite correct accounting of Fermi surface and Fermi velocity anisotropies. The overall results demonstrate a low resistivity scaling for Ru, suggesting that 10 nm half-pitch Ru interconnect lines are approximately 2 times more conductive than comparable Cu lines.Epitaxial Ru(0001) layers are sputter deposited onto Al2O3(0001) substrates and their resistivity ρ measured both in situ and ex situ as a function of thickness d = 5–80 nm in order to quantify the resistivity scaling associated with electron-surface scattering. All layers have smooth surfaces with a root-mean-square roughness <0.4 nm, exhibit an epitaxial relationship with the substrate: Ru[0001]||Al2O3[0001] and Ru [101¯0]||Al2O3 [112¯0], and show no resistance change upon air exposure, suggesting negligible resistivity contributions from geometric surface roughness and grain boundary scattering and negligible changes in the surface scattering specularity p upon oxygen exposure. The room temperature ρ vs d data are well described by the semiclassical Fuchs-Sondheimer (FS) model, indicating a bulk electron mean free path λ = 6.7 ± 0.3 nm. However, the measured ρo × λ product at 77 K is 43% lower than at 295 K, suggesting a breakdown of the FS model and/or a thickness-dependent electron-phonon coupling an...