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L L Bonilla - One of the best experts on this subject based on the ideXlab platform.
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parameter dependence of high frequency nonlinear oscillations and intrinsic chaos in short gaas al ga as Superlattices
Chaos, 2018Co-Authors: Jonathan Essen, L L Bonilla, Miguel Ruizgarcia, Ian Jenkins, M Carretero, Bjorn BirnirAbstract:We explore the design parameter space of short (5–25 period), n-doped, Ga/(Al,Ga)As Semiconductor Superlattices (SSLs) in the sequential resonant tunneling regime. We consider SSLs at cool (77 K) and warm (295 K) temperatures, simulating the electronic response to variations in (a) the number of SSL periods, (b) the contact conductivity, and (c) the strength of disorder (aperiodicities). Our analysis shows that the chaotic dynamical phases exist on a number of sub-manifolds of codimension zero within the design parameter space. This result provides an encouraging guide towards the experimental observation of high-frequency intrinsic dynamical chaos in shorter SSLs.We explore the design parameter space of short (5–25 period), n-doped, Ga/(Al,Ga)As Semiconductor Superlattices (SSLs) in the sequential resonant tunneling regime. We consider SSLs at cool (77 K) and warm (295 K) temperatures, simulating the electronic response to variations in (a) the number of SSL periods, (b) the contact conductivity, and (c) the strength of disorder (aperiodicities). Our analysis shows that the chaotic dynamical phases exist on a number of sub-manifolds of codimension zero within the design parameter space. This result provides an encouraging guide towards the experimental observation of high-frequency intrinsic dynamical chaos in shorter SSLs.
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parameter dependence of high frequency nonlinear oscillations and intrinsic chaos in short gaas al ga as Superlattices
arXiv: Mesoscale and Nanoscale Physics, 2017Co-Authors: Jonathan Essen, L L Bonilla, Miguel Ruizgarcia, Ian Jenkins, M Carretero, Bjorn BirnirAbstract:We explore the design parameter space of short (5--25 period), n-doped, Ga/(Al,Ga)As Semiconductor Superlattices (SSLs) in the sequential resonant tunneling regime. We consider SSLs at cool (77K) and warm (295K) temperatures, simulating the electronic response to variations in (a) the number of SSL periods, (b) the contact conductivity, and (c) the strength of disorder (aperiodicities). Our analysis shows that the chaotic dynamical phases exist on a number of sub-manifolds of codimension zero within the design parameter space. This result provides an encouraging guide towards the experimental observation of high-frequency intrinsic dynamical chaos in shorter SSLs.
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noise enhanced spontaneous chaos in Semiconductor Superlattices at room temperature
arXiv: Mesoscale and Nanoscale Physics, 2014Co-Authors: M Alvaro, Manuel Carretero, L L BonillaAbstract:Physical systems exhibiting fast spontaneous chaotic oscillations are used to generate high-quality true random sequences in random number generators. The concept of using fast practical entropy sources to produce true random sequences is crucial to make storage and transfer of data more secure at very high speeds. While the first high-speed devices were chaotic Semiconductor lasers, the discovery of spontaneous chaos in Semiconductor Superlattices at room temperature provides a valuable nanotechnology alternative. Spontaneous chaos was observed in 1996 experiments at temperatures below liquid nitrogen. Here we show spontaneous chaos at room temperature appears in idealized Superlattices for voltage ranges where sharp transitions between different oscillation modes occur. Internal and external noises broaden these voltage ranges and enhance the sensitivity to initial conditions in the superlattice snail-shaped chaotic attractor thereby rendering spontaneous chaos more robust.
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nonlinear electron and spin transport in Semiconductor Superlattices
Siam Journal on Applied Mathematics, 2008Co-Authors: L L Bonilla, Luigi Barletti, M AlvaroAbstract:Nonlinear charge transport in strongly coupled Semiconductor super lattices is described by two-miniband Wigner-Poisson kinetic equations with BGK collision terms. The hyperbolic limit, in which the collision frequencies are of the same order as the Bloch frequencies due to the electric field, is investigated by means of the Chapman-Enskog perturbation technique, leading to nonlinear drift-diffusion equations for the two miniband populations. In the case of a lateral superlattice with spin-orbit interaction, the corresponding drift-diffusion equations are used to calculate spin-polarized currents and electron spin polarization.
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nonlinear electron and spin transport in Semiconductor Superlattices
arXiv: Mesoscale and Nanoscale Physics, 2008Co-Authors: L L Bonilla, Luigi Barletti, M AlvaroAbstract:Nonlinear charge transport in strongly coupled Semiconductor Superlattices is described by Wigner-Poisson kinetic equations involving one or two minibands. Electron-electron collisions are treated within the Hartree approximation whereas other inelastic collisions are described by a modified BGK (Bhatnaghar-Gross-Krook) model. The hyperbolic limit is such that the collision frequencies are of the same order as the Bloch frequencies due to the electric field and the corresponding terms in the kinetic equation are dominant. In this limit, spatially nonlocal drift-diffusion balance equations for the miniband populations and the electric field are derived by means of the Chapman-Enskog perturbation technique. For a lateral superlattice with spin-orbit interaction, electrons with spin up or down have different energies and their corresponding drift-diffusion equations can be used to calculate spin-polarized currents and electron spin polarization. Numerical solutions show stable self-sustained oscillations of the current and the spin polarization through a voltage biased lateral superlattice thereby providing an example of superlattice spin oscillator.
H T Grahn - One of the best experts on this subject based on the ideXlab platform.
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non linear dynamics of Semiconductor Superlattices
Reports on Progress in Physics, 2005Co-Authors: L L Bonilla, H T GrahnAbstract:In the last decade, non-linear dynamical transport in Semiconductor Superlattices (SLs) has witnessed significant progress in theoretical descriptions as well as in experimentally observed non-linear phenomena. However, until now, a clear distinction between non-linear transport in strongly and weakly coupled SLs was missing, although it is necessary to provide a detailed description of the observed phenomena. In this review, strongly coupled SLs are described by spatially continuous equations and display self-sustained current oscillations due to the periodic motion of a charge dipole as in the Gunn effect for bulk Semiconductors. In contrast, weakly coupled SLs have to be described by spatially discrete equations. Therefore, weakly coupled SLs exhibit a more complex dynamical behaviour than strongly coupled ones, which includes the formation of stationary electric field domains, pinning or propagation of domain walls consisting of a charge monopole, switching between stationary domains, self-sustained current oscillations due to the recycling motion of a charge monopole and chaos. This review summarizes the existing theories and the experimentally observed non-linear phenomena for both types of Semiconductor SLs.
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explosive bifurcation to chaos in weakly coupled Semiconductor Superlattices
Physical Review Letters, 1998Co-Authors: K J Luo, K Ploog, H T Grahn, L L BonillaAbstract:The bifurcation scenario to chaos has been studied for vertical transport in an incommensurately driven superlattice system. With increasing driving amplitude, quasiperiodic, frequency-locked, and chaotic oscillations are identified by using Poincare maps, which show a variety of attractors in the chaotic regime. The dimension of the attractor is abruptly increased in the transition process, i.e., the bifurcation between frequency locking and chaos is explosive. However, the bifurcation pattern depends strongly on the applied dc bias, providing clear evidence that the system is spatially inhomogeneous in the vertical direction. [S0031-9007(98)06837-9]
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electrically tunable ghz oscillations in doped gaas alas Superlattices
Physical Review B, 1997Co-Authors: J Kastrup, L L Bonilla, Andreas Wacker, H T Grahn, K Ploog, R Hey, Manuel Kindelan, Miguel Moscoso, J GalanAbstract:Tunable oscillatory modes of electric-field domains in doped Semiconductor Superlattices are reported. The experimental investigations demonstrate the realization of tunable, GHz frequencies in GaAs-AlAs Superlattices covering the temperature region from 5 to 300 K. The orgin of the tunable oscillatory modes is determined using an analytical and a numerical modeling of the dynamics of domain formation. Three different oscillatory modes are found. Their presence depends on the actual shape of the drift velocity curve, the doping density, the boundary condition, and the length of the superlattice. For most bias regions, the self-sustained oscillations are due to the formation, motion, and recycling of the domain boundary inside the superlattice. For some biases, the strengths of the low- and high-field domain change periodically in time with the domain boundary being pinned within a few quantum wells. The dependency of the frequency on the coupling leads to the prediction of a different type of tunable GHz oscillator based on Semiconductor Superlattices.
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superradiant emission from bloch oscillations in Semiconductor Superlattices
Physical Review B, 1996Co-Authors: Rainer Martini, Hartmut G Roskos, H Kurz, H T Grahn, G Klose, R HeyAbstract:The superradiant character of submillimeter-wave emission from optically excited electronic Bloch oscillations in a GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As superlattice is investigated by time-resolved terahertz emission spectroscopy. The intensity of the radiation and its decay time are studied as a function of the density of excited charge carriers. From the measured emission efficiency, the spatial amplitude of the charge oscillations is estimated. \textcopyright{} 1996 The American Physical Society.
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self oscillations of the current in doped Semiconductor Superlattices
Japanese Journal of Applied Physics, 1995Co-Authors: H T Grahn, L L Bonilla, K Ploog, J Kastrup, Manuel Kindelan, J Galan, Miguel MoscosoAbstract:Self-sustained current oscillations have been found in doped GaAs-AlAs Superlattices and are investigated experimentally and theoretically. The electric-field distribution in doped Superlattices becomes unstable if the carrier density is not sufficiently large to form stable domains. The instability results in self-oscillations of the current, which are due to an oscillatory motion of the domain boundary in the superlattice. A discrete drift model taking into account several regions of negative differential velocity and a large electron density clearly establishes the origin of these oscillations.
A G Balanov - One of the best experts on this subject based on the ideXlab platform.
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ultrafast strain induced charge transport in Semiconductor Superlattices
Physical review applied, 2020Co-Authors: M T Greenaway, T M Fromhold, Feiran Wang, Caroline Poyser, A V Akimov, R P Campion, A J Kent, A G BalanovAbstract:We investigate the effect of hypersonic ($g1$-GHz) acoustic phonon wave packets on electron transport in a Semiconductor superlattice. Our quantum-mechanical simulations demonstrate that a gigahertz train of picosecond deformation-strain pulses propagating through a superlattice can generate current oscillations the frequency of which is many times higher than that of the strain pulse train, potentially reaching the terahertz regime. The shape and polarity of the calculated current pulses agree well with experimentally measured electric signals. The calculations also explain and accurately reproduce the measured variation of the induced-current-pulse magnitude with the strain-pulse amplitude and applied bias voltage. Our results open a route to developing acoustically driven Semiconductor Superlattices as sources of millimeter and submillimeter electromagnetic waves.
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ultrafast strain induced charge transport in Semiconductor Superlattices
arXiv: Applied Physics, 2020Co-Authors: M T Greenaway, T M Fromhold, Feiran Wang, Caroline Poyser, A V Akimov, R P Campion, A J Kent, A G BalanovAbstract:We investigate the effect of hypersonic (> 1 GHz) acoustic phonon wavepackets on electron transport in a Semiconductor superlattice. Our quantum mechanical simulations demonstrate that a GHz train of picosecond deformation strain pulses propagating through a superlattice can generate current oscillations whose frequency is several times higher than that of the strain pulse train. The shape and polarity of the calculated current pulses agree well with experimentally measured electric signals. The calculations also explain and accurately reproduce the measured variation of the induced current pulse magnitude with the strain pulse amplitude and applied bias voltage. Our results open a route to developing acoustically-driven Semiconductor Superlattices as sources of millimetre and sub-millimetre electromagnetic waves.
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Ultrafast strain-induced charge transport in Semiconductor Superlattices
2020Co-Authors: Feiran Wang, Caroline Poyser, Mark Greenaway, Andrey Akimov, Richard Campion, Anthony Kent, Mark Fromhold, A G BalanovAbstract:We investigate the effect of hypersonic (> 1 GHz) acoustic phonon wavepackets on electron transport in a Semiconductor superlattice. Our quantum mechanical simulations demonstrate that a GHz train of picosecond deformation strain pulses propagating through a superlattice can generate current oscillations whose frequency is many times higher than that of the strain pulse train, potentially reaching the THz regime. The shape and polarity of the calculated current pulses agree well with experimentally measured electric signals. The calculations also explain and accurately reproduce the measured variation of the induced current pulse magnitude with the strain pulse amplitude and applied bias voltage. Our results open a route to developing acoustically-driven Semiconductor Superlattices as sources of millimetre and sub-millimetre electromagnetic waves
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lyapunov stability of charge transport in miniband Semiconductor Superlattices
Physical Review B, 2013Co-Authors: Alexey A Koronovskii, A E Hramov, M T Greenaway, V A Maximenko, O I Moskalenko, Kirill N Alekseev, T M Fromhold, A G BalanovAbstract:We discuss a numerical method for the calculation of the spectrum of Lyapunov exponents for spatially extended systems described by coupled Poisson and continuity equations. This approach was applied to the model of collective charge transport in Semiconductor Superlattices operating in the miniband transport regime. The method is in very good agreement with analytical results obtained for the steady state. As an illustrative example, we consider the collective electron dynamics in the superlattice subjected to an ac voltage and a tilted magnetic field, and conclusively show that, depending on the field parameters, the dynamics can exhibit periodic, quasiperiodic, or chaotic behavior.
Bivas Saha - One of the best experts on this subject based on the ideXlab platform.
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rocksalt nitride metal Semiconductor Superlattices a new class of artificially structured materials
Applied physics reviews, 2018Co-Authors: Bivas Saha, Ali Shakouri, T SandsAbstract:Artificially structured materials in the form of superlattice heterostructures enable the search for exotic new physics and novel device functionalities, and serve as tools to push the fundamentals of scientific and engineering knowledge. Semiconductor heterostructures are the most celebrated and widely studied artificially structured materials, having led to the development of quantum well lasers, quantum cascade lasers, measurements of the fractional quantum Hall effect, and numerous other scientific concepts and practical device technologies. However, combining metals with Semiconductors at the atomic scale to develop metal/Semiconductor Superlattices and heterostructures has remained a profoundly difficult scientific and engineering challenge. Though the potential applications of metal/Semiconductor heterostructures could range from energy conversion to photonic computing to high-temperature electronics, materials challenges primarily had severely limited progress in this pursuit until very recently. In this article, we detail the progress that has taken place over the last decade to overcome the materials engineering challenges to grow high quality epitaxial, nominally single crystalline metal/Semiconductor Superlattices based on transition metal nitrides (TMN). The epitaxial rocksalt TiN/(Al,Sc)N metamaterials are the first pseudomorphic metal/Semiconductor Superlattices to the best of our knowledge, and their physical properties promise a new era in superlattice physics and device engineering.
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phonon wave effects in the thermal transport of epitaxial tin al sc n metal Semiconductor Superlattices
Journal of Applied Physics, 2017Co-Authors: Bivas Saha, Ali Shakouri, Yee Rui Koh, Sridhar Sadasivam, Timothy S Fisher, Joseph P Feser, T SandsAbstract:Epitaxial single crystalline TiN/(Al,Sc)N metal/Semiconductor superlattice metamaterials have generated significant interest in recent years for their potential applications in high temperature thermoelectric devices, optical hyperbolic metamaterials in the visible and near infrared-spectral range, and as candidates for solar-thermophotovoltaics and high temperature electronic materials. While significant progress in their structural, mechanical, and optical properties has been made, in-depth analysis and detailed understanding of their thermal transport mechanism remain to be addressed. In this article, we show that in short-period epitaxial, lattice-matched TiN/(Al,Sc)N metal/Semiconductor Superlattices, thermal transport is dominated by phonon wave effects as the wavelengths of phonons that carry significant amounts of heat become comparable to the superlattice period thickness. Due to the increasing contribution of such phonon wave-modes, the cross-plane thermal conductivity at short-periods increases...
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microstructural evolution and thermal stability of hfn scn zrn scn and hf0 5zr0 5n scn metal Semiconductor Superlattices
Journal of Materials Science, 2016Co-Authors: Magnus Garbrecht, Bivas Saha, Jeremy L Schroeder, Jens Birch, Lars Hultman, T SandsAbstract:Nitride-based metal/Semiconductor Superlattices for possible applications as thermoelectric, plasmonic, and hard coating materials have been grown by magnetron sputtering. Since long-time thermal stability of the Superlattices is crucial for these applications, the atomic scale microstructure and its evolution under annealing to working temperatures were investigated with high-resolution transmission electron microscopy methods. We report on epitaxial growth of three cubic superlattice systems (HfN/ScN, ZrN/ScN, and Hf0.5Zr0.5N/ScN) that show long-time thermal stability (annealing up to 120 h at 950 °C) as monitored by scanning transmission electron microscopy-based energy-dispersive X-ray spectroscopy. No interdiffusion between the metal and Semiconductor layers could be observed for any of the present systems under long-time annealing, which is in contrast to earlier attempts on similar superlattice structures based on TiN as the metallic compound. Atomically resolved high-resolution transmission electron microscopy imaging revealed that even though the superlattice curves towards the substrate at regular interval column boundaries originating from threading dislocations close to the substrate interface, the cubic lattice continues coherently across the boundaries. It is found that the boundaries themselves are alloyed along the entire growth direction, while in their vicinity nanometer-size inclusions of metallic phases are observed that could be identified as the zinc blende phase of same stoichiometry as the parent rock salt transition metal nitride phase. Our results demonstrate the long-time thermal stability of metal/Semiconductor Superlattices based on Zr and Hf nitrides.
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cross plane thermal conductivity of ti w n al sc n metal Semiconductor Superlattices
Physical Review B, 2016Co-Authors: Bivas Saha, Yee Rui Koh, Jonathan Comparan, Sridhar Sadasivam, Jeremy L Schroeder, Magnus Garbrecht, Amr M S Mohammed, Jens Birch, Timothy S Fisher, Ali ShakouriAbstract:Reduction of cross-plane thermal conductivity and understanding of the mechanisms of heat transport in nanostructured metal/Semiconductor Superlattices are crucial for their potential applications ...
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thermal stability of epitaxial cubic tin al sc n metal Semiconductor Superlattices
Journal of Materials Science, 2015Co-Authors: Jeremy L Schroeder, Bivas Saha, T Sands, Magnus Garbrecht, Norbert Schell, Jens BirchAbstract:We report on the thermal stability of epitaxial cubic-TiN/(Al,Sc)N metal/Semiconductor Superlattices with the rocksalt crystal structure for potential plasmonic, thermoelectric, and hard coating applications. TiN/Al0.72Sc0.28N Superlattices were annealed at 950 and 1050 °C for 4, 24, and 120 h, and the thermal stability was characterized by high-energy synchrotron-radiation-based 2D X-ray diffraction, high-resolution (scanning) transmission electron microscopy [HR(S)/TEM], and energy dispersive X-ray spectroscopy (EDX) mapping. The TiN/Al0.72Sc0.28N Superlattices were nominally stable for up to 4 h at both 950 and 1050 °C. Further annealing treatments for 24 and 120 h at 950 °C led to severe interdiffusion between the layers and the metastable cubic-Al0.72Sc0.28N layers partially transformed into Al-deficient cubic-(Al,Sc)N and the thermodynamically stable hexagonal wurtzite phase with a nominal composition of AlN (h-AlN). The h-AlN grains displayed two epitaxial variants with respect to c-TiN and cubic-(Al,Sc)N. EDX mapping suggests that scandium has a higher tendency for diffusion in TiN/(Al,Sc)N than titanium or aluminum. Our results indicate that the kinetics of interdiffusion and the cubic-to-hexagonal phase transformation place constraints on the design and implementation of TiN/(Al,Sc)N Superlattices for high-temperature applications.
T Sands - One of the best experts on this subject based on the ideXlab platform.
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rocksalt nitride metal Semiconductor Superlattices a new class of artificially structured materials
Applied physics reviews, 2018Co-Authors: Bivas Saha, Ali Shakouri, T SandsAbstract:Artificially structured materials in the form of superlattice heterostructures enable the search for exotic new physics and novel device functionalities, and serve as tools to push the fundamentals of scientific and engineering knowledge. Semiconductor heterostructures are the most celebrated and widely studied artificially structured materials, having led to the development of quantum well lasers, quantum cascade lasers, measurements of the fractional quantum Hall effect, and numerous other scientific concepts and practical device technologies. However, combining metals with Semiconductors at the atomic scale to develop metal/Semiconductor Superlattices and heterostructures has remained a profoundly difficult scientific and engineering challenge. Though the potential applications of metal/Semiconductor heterostructures could range from energy conversion to photonic computing to high-temperature electronics, materials challenges primarily had severely limited progress in this pursuit until very recently. In this article, we detail the progress that has taken place over the last decade to overcome the materials engineering challenges to grow high quality epitaxial, nominally single crystalline metal/Semiconductor Superlattices based on transition metal nitrides (TMN). The epitaxial rocksalt TiN/(Al,Sc)N metamaterials are the first pseudomorphic metal/Semiconductor Superlattices to the best of our knowledge, and their physical properties promise a new era in superlattice physics and device engineering.
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phonon wave effects in the thermal transport of epitaxial tin al sc n metal Semiconductor Superlattices
Journal of Applied Physics, 2017Co-Authors: Bivas Saha, Ali Shakouri, Yee Rui Koh, Sridhar Sadasivam, Timothy S Fisher, Joseph P Feser, T SandsAbstract:Epitaxial single crystalline TiN/(Al,Sc)N metal/Semiconductor superlattice metamaterials have generated significant interest in recent years for their potential applications in high temperature thermoelectric devices, optical hyperbolic metamaterials in the visible and near infrared-spectral range, and as candidates for solar-thermophotovoltaics and high temperature electronic materials. While significant progress in their structural, mechanical, and optical properties has been made, in-depth analysis and detailed understanding of their thermal transport mechanism remain to be addressed. In this article, we show that in short-period epitaxial, lattice-matched TiN/(Al,Sc)N metal/Semiconductor Superlattices, thermal transport is dominated by phonon wave effects as the wavelengths of phonons that carry significant amounts of heat become comparable to the superlattice period thickness. Due to the increasing contribution of such phonon wave-modes, the cross-plane thermal conductivity at short-periods increases...
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microstructural evolution and thermal stability of hfn scn zrn scn and hf0 5zr0 5n scn metal Semiconductor Superlattices
Journal of Materials Science, 2016Co-Authors: Magnus Garbrecht, Bivas Saha, Jeremy L Schroeder, Jens Birch, Lars Hultman, T SandsAbstract:Nitride-based metal/Semiconductor Superlattices for possible applications as thermoelectric, plasmonic, and hard coating materials have been grown by magnetron sputtering. Since long-time thermal stability of the Superlattices is crucial for these applications, the atomic scale microstructure and its evolution under annealing to working temperatures were investigated with high-resolution transmission electron microscopy methods. We report on epitaxial growth of three cubic superlattice systems (HfN/ScN, ZrN/ScN, and Hf0.5Zr0.5N/ScN) that show long-time thermal stability (annealing up to 120 h at 950 °C) as monitored by scanning transmission electron microscopy-based energy-dispersive X-ray spectroscopy. No interdiffusion between the metal and Semiconductor layers could be observed for any of the present systems under long-time annealing, which is in contrast to earlier attempts on similar superlattice structures based on TiN as the metallic compound. Atomically resolved high-resolution transmission electron microscopy imaging revealed that even though the superlattice curves towards the substrate at regular interval column boundaries originating from threading dislocations close to the substrate interface, the cubic lattice continues coherently across the boundaries. It is found that the boundaries themselves are alloyed along the entire growth direction, while in their vicinity nanometer-size inclusions of metallic phases are observed that could be identified as the zinc blende phase of same stoichiometry as the parent rock salt transition metal nitride phase. Our results demonstrate the long-time thermal stability of metal/Semiconductor Superlattices based on Zr and Hf nitrides.
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thermal stability of epitaxial cubic tin al sc n metal Semiconductor Superlattices
Journal of Materials Science, 2015Co-Authors: Jeremy L Schroeder, Bivas Saha, T Sands, Magnus Garbrecht, Norbert Schell, Jens BirchAbstract:We report on the thermal stability of epitaxial cubic-TiN/(Al,Sc)N metal/Semiconductor Superlattices with the rocksalt crystal structure for potential plasmonic, thermoelectric, and hard coating applications. TiN/Al0.72Sc0.28N Superlattices were annealed at 950 and 1050 °C for 4, 24, and 120 h, and the thermal stability was characterized by high-energy synchrotron-radiation-based 2D X-ray diffraction, high-resolution (scanning) transmission electron microscopy [HR(S)/TEM], and energy dispersive X-ray spectroscopy (EDX) mapping. The TiN/Al0.72Sc0.28N Superlattices were nominally stable for up to 4 h at both 950 and 1050 °C. Further annealing treatments for 24 and 120 h at 950 °C led to severe interdiffusion between the layers and the metastable cubic-Al0.72Sc0.28N layers partially transformed into Al-deficient cubic-(Al,Sc)N and the thermodynamically stable hexagonal wurtzite phase with a nominal composition of AlN (h-AlN). The h-AlN grains displayed two epitaxial variants with respect to c-TiN and cubic-(Al,Sc)N. EDX mapping suggests that scandium has a higher tendency for diffusion in TiN/(Al,Sc)N than titanium or aluminum. Our results indicate that the kinetics of interdiffusion and the cubic-to-hexagonal phase transformation place constraints on the design and implementation of TiN/(Al,Sc)N Superlattices for high-temperature applications.
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bulk like laminated nitride metal Semiconductor Superlattices for thermoelectric devices
IEEE\ ASME Journal of Microelectromechanical Systems, 2014Co-Authors: Jeremy L Schroeder, Ali Shakouri, David A Ewoldt, Reja Amatya, Rajeev J Ram, T SandsAbstract:Bulk-like thermionic energy conversion devices have been fabricated from nanostructured nitride metal/Semiconductor Superlattices using a novel lamination process. 5- $\mu{\rm m}$ thick $({\rm Hf}_{0.5}{\rm Zr}_{0.5}){\rm N}$ (6-nm)/ScN (6-nm) metal/Semiconductor Superlattices with a 12 nm period were deposited on 100-silicon substrates by reactive magnetron sputtering followed by a selective tetra methyl ammonium hydroxide substrate etching and a gold-gold lamination process to yield 300 $\mu{\rm m}\times\,$ 300 $\mu{\rm m}\times\,$ 290 $\mu{\rm m}$ microscale thermionic energy conversion elements with 16,640 superlattice periods. The thermionic element had a Seebeck coefficient of ${-}{\rm 120}~\mu{\rm V}/{\rm K}$ at 800 K, an electrical conductivity of ${\sim}{2500}~\Omega^{-1}{\rm m}^{-1}$ at 800 K, and a thermal conductivity of 2.9 and 4.3 W/m-K at 300 and 625 K, respectively. The temperature dependence of the Seebeck coefficient from 300 to 800 K suggests a parallel parasitic conduction path that is dominant at low temperature, and the temperature independent electrical conductivity indicates that the $({\rm Hf}_{0.5}{\rm Zr}_{0.5}){\rm N}/{\rm gold}$ interface contact resistivity currently dominates the device. The thermal conductivity of the laminate was significantly lower than the thermal conductivity of the individual metal or Semiconductor layers, indicating the beneficial effect of the metal/Semiconductor interfaces toward lowering the thermal conductivity. The described lamination process effectively bridges the gap between the nanoscale requirements needed to enhance the thermoelectric figure of merit ZT and the microscale requirements of real-world devices. $\hfill[2013\hbox{--}0158]$
