Conductivity Of Superlattices Research Articles

Simulation study of phonon transport at the GaN/AlN superlattice interface: Ballistic and non-equilibrium phenomena

GaN/AlN superlattice structures have potential for high electron mobility transistors (HEMTs) applications; however, nonequilibrium phonon transport mechanisms within the superlattice have not been systematically investigated. Therefore, we investigate in detail the non-Fourier thermal conductivity of superlattice structures in the perpendicular interface direction and in the parallel interface direction around the phonon BTE numerical solution in this paper, taking the kinetic parameter based on the first principle as the input parameter of the equations and based on the DOM numerical solution scheme. The study reveals that mode temperature non-equilibrium in superlattices arises from ballistic transport within the layers and interface scattering between the layers. The thermal conductivity (TC) of superlattices can be modulated by scale effects, and in superlattices with small thicknesses, phonons exhibit quasi-ballistic transport. The interface thermal resistance predominantly originates from contributions by acoustic phonon (AP) and the first optical phonon (OP1). With increasing interface density, the combined effects of interface scattering and boundary scattering lead to strong phonon scattering, resulting in a decrease in TC parallel to the interfaces. This work provides valuable insights into the thermal transport properties of GaN/AlN superlattice semiconductors and offers useful theoretical guidance for thermal management through phonon particle properties.

Influence of interposed graphene sheets on mechanical and electronic properties of Al/graphene superlattice

Graphene-reinforced aluminum matrix composites have drawn remarkable attention in several fields of high-tech industries, but the understanding of their material properties remains unclear. This work reports a first-principles study of interface binding nature, mechanical strength, and electronic properties of aluminum/graphene (Al/G) composites using superlattice models as varying graphene content. Our calculations reveal the weak binding between Al and graphene layers with no new chemical bonding at the interface and the gradual decrease in binding strength as increasing graphene content. While demonstrating the enhancement of mechanical strength by interposing graphene layers, the critical value of graphene content for keeping ductility is determined to be 14.7%. Atom-projected band structures and local density of states are analyzed to get an insight into electronic conductance of superlattices.

Two-dimensional graphene superlattice conductivity control by transverse electric fields

The possibility of controlling the conductivity of a graphene superlattice by transverse constant and alternating electric fields due to the nonadditivity of its energy spectrum is investigated. A technique is proposed for identifying the parameter areas of the applied fields in which conductivity control is most effective.

Further decrease of the thermal conductivity of superlattice through embedding nanoparticle

Utilizing different nanostructures can effectively control phonons transport of their corresponding macrostructures, and this will contribute to the development of thermoelectric energy harvesting and microelectronic thermal management. This work tends to decrease the thermal conductivity (TC) of Si/Ge superlattices (SLs) by embedding germanium nanoparticles (GNPs). The thermal transport property in such structure was investigated by non-equilibrium molecular dynamics (NEMD) method. Results showed that embedding NPs can significantly reduce the TC of SLs, but only when the particle number is less than half of the SL's periods, above which the reduction effect will be greatly decreased. In addition, we also investigated the effects of temperature, system length, particle size and number on the TC. The thermal transport properties of the system are deeply revealed by calculating and analyzing phonons parameters such as density of states, participation ratio, mean free paths (MFPs) and spectral heat flow.

Ultralow In-Plane Thermal Conductivity in 2D Magnetic Mosaic Superlattices for Enhanced Thermoelectric Performance.

Lowering thermal conductivity via introducing heterointerfaces of heterophase fillings (HPFs) is a common strategy for optimizing thermoelectric performance, but it is always accompanied by deterioration of electrical conductivity. Here we report an ordered magnetic HPF system in a CoSe2-SnSe mosaic heterostructure superlattice synthesized by van der Waals confined epitaxial growth (vdWCEG), which realizes a maximized filling amount to decrease in-plane thermal conductivity of SnSe layers and maintain the intact in-plane carrier transport path. The in-plane thermal conductivity of CoSe2-SnSe superlattice reaches the lowest range among SnSe-based materials with a value of 0.27 W m-1 K-1 at 850 K, which can be attributed to abundant interfaces between CoSe2 nanocrystals and SnSe layers. Moreover, the CoSe2 nanocrystals show superparamagnetic behavior, by which the rotation of magnetic domains provides additional phonon scattering to further decrease in-plane thermal conductivity. By combination with the preserved in-plane electrical conductivity of SnSe layers, an enhanced in-plane ZT value of 0.62 is achieved at 850 K. This vdWCEG approach can also be generally applied to fabricate various other two-dimensional (2D) mosaic heterostructures, providing an avenue for artificial 2D heterostructures with desired functionalities.

A theoretical insight into phonon heat transport in graphene/biphenylene superlattice nanoribbons: a molecular dynamic study

Manipulating the thermal conductivity of nanomaterials is an efficacious approach to fabricate tailor-made nanodevices for thermoelectric applications. To this end, superlattice nanostructures can be used to achieve minimal thermal conductivity for the employed nanomaterials. Two-dimensional biphenylene is a recently-synthesized sp2-hybridized allotrope of carbon atoms that can be employed in superlattice nanostructures and therefore further investigation in this context is due. In this study, we first determined the thermal conductivity of biphenylene at 142.8 W mK−1 which is significantly lower than that of graphene. As a second step, we studied the effect of the superlattice period () on thermal conductivities of the employed graphene/biphenylene superlattice nanoribbons, using molecular dynamics simulations. We calculated a minimum thermal conductivity of 105.5 W mK−1 at = 5.066 nm which indicates an achieved thermal conductivity reduction of approximately 97% and 26% when compared to pristine graphene and biphenylene, respectively. This superlattice period denotes the phonon coherent length at which the wave-like behavior of phonons starts prevailing over the particle-like behavior. Finally, the effects of temperature and temperature gradient on the thermal conductivity of superlattice were also investigated.

Lattice thermal conductivity in isotope diamond asymmetric superlattices

We study the lattice thermal conductivity of isotope diamond superlattices consisting of 12C and 13C diamond layers at various superlattice periods. It is found that the thermal conductivity of a superlattice is significantly deduced from that of pure diamond because of the reduction of the phonon group velocity near the folded Brillouin zone. The results show that asymmetric superlattices with a different number of layers of 12C and 13C diamonds exhibit higher thermal conductivity than symmetric superlattices even with the same superlattice period, and we find that this can be explained by the trade-off between the effects of phonon specific heat and phonon group velocity. Furthermore, impurities and imperfect superlattice structures are also found to significantly reduce the thermal conductivity, suggesting that these effects can be exploited to control the thermal conductivity over a wide range.

Utilizing twin interfaces to reduce lattice thermal conductivity of superlattice

Twin interfaces are easily formed in superlattices due to their lower interfacial energy. However, there are relatively few studies on their effect on the thermal conductivity of superlattices, and the conclusions are unclear. In particular, the degree of influence of the presence of twin interfaces on the thermal conductivity is inconsistent. Therefore, the thermal conductivities of silicon/germanium superlattices with twin interfaces were studied by non-equilibrium molecular dynamics simulations. It was found that the twin interface destroys coherent phonon transport, causes phonon localization, and leads a decrease in the thermal conductivity. The degree of influence of the twin interface on the thermal conductivity is strongly dependent on the period length, the system length, and temperature. Furthermore, phonon density of states, phonon participation rate, and spectral heat flow calculations were employed to deduce the phonon transport mechanisms in superlattices with twin interfaces.

Further Decrease of the Thermal Conductivity of Superlattice Through Embedding Nanoparticle

Further Decrease of the Thermal Conductivity of Superlattice Through Embedding Nanoparticle

Magnetic modulation of Fano resonances in optically thin terahertz superlattice metasurfaces

Optically thin metasurfaces operating at sub-skin depth thicknesses are intriguing because of their associated low plasmonic losses (compared to optically thick, beyond skin-depth metasurfaces). However, their applicability is restricted largely because of reduced free space coupling with incident radiations resulting in limited electromagnetic responses. To overcome such limitations, we propose enhancement of effective responses (resonances) in sub-skin depth metasurfaces through incorporation of a magneto-transport (giant magneto resistance) concept. Here, we experimentally demonstrate dynamic magnetic modulation of structurally asymmetric metasurfaces (consisting of superlattice arrangement of thin (∼10 nm each) magnetic (Ni)/nonmagnetic (Al) layers) operating in the terahertz (THz) domain. With increasing magnetic field (applied from 0 to 30 mT approximately, implies increasing superlattice conductivity), we observe stronger confinement of electromagnetic energy at the resonances (both in dipole and Fano modes). Therefore, this study introduces a unique magnetically reconfigurable ability in Fano resonant THz metamaterials, which directly improves their performances operating in the sub-skin depth regime. Our study can be explained by spin-dependent THz magneto-transport phenomena in metals and can stimulate the paradigm for on-chip spin-based photonic technology enabling dynamic magnetic control over compact, sub-wavelength, sub-skin depth metadevices.

Negligible contribution of inter-dot coherent modes to heat conduction in quantum-dot superlattice

Colloidal quantum dots (QDs) superlattice, which is made of inorganic cores and can self-assemble into various types of lattice structures, finds promising applications in optical, electrical, and optoelectronic devices. Recent inelastic neutron scattering measurement [NAT. COMMUN. 10:4236 (2019)] showed that the inter-quantum-dot vibrational frequencies can be tuned by varying the QDs shapes and ligand types, suggesting that the QDs superlattices can be a platform for phonon engineering. In this work, we quantify the impact of the second periodicity on thermal transport through full-scale molecular dynamics simulations of PbS QDs superlattice with realistic QD size and ligand morphology. The vibrational pattern analysis reveals that the vibrations can be classified into the inter-QDs coherent modes and the spatially localized modes arising from the geometry confinement. The spectral analysis indicates that spatially localized modes in the frequency range of 0.8–5 THz dominate the thermal transport and lead to an amorphous-like temperature dependence between 200 and 400 K. On the other hand, the inter-QDs coherent modes, albeit have an averaged relaxation of 10 ps, have a limited thermal conductivity value of 0.01 W/mK at room temperature due to the scarce of the vibrational states. We demonstrate that controlling the ligand morphology is more efficient than tuning the second periodicity in engineering the thermal conductivity of QDs superlattice.

Tuning Coherent-Phonon Heat Transport in LaCoO3/SrTiO3 Superlattices.

Accessing the regime of coherent phonon propagation in nanostructures opens enormous possibilities to control the thermal conductivity in energy harvesting devices, phononic circuits, etc. In this paper we show that coherent phonons contribute substantially to the thermal conductivity of LaCoO3/SrTiO3 oxide superlattices, up to room temperature. We show that their contribution can be tuned through small variations of the superlattice periodicity, without changing the total superlattice thickness. Using this strategy, we tuned the thermal conductivity by 20% at room temperature. We also discuss the role of interface mixing and epitaxial relaxation as an extrinsic, material dependent key parameter for understanding the thermal conductivity of oxide superlattices.

Slater-Pauling-like Behavior of Spin Hall Conductivity in Pt-based Superlattices

The intrinsic spin Hall effect in the bulk systems of late transition metals (Os, Ir, Pt, and Au) as well as the Pt-based superlattices were investigated by using first-principle calculations. By comparing the computed spin Hall conductivities of Pt−M superlattices (M=Os, Ir, and Au) with different compositions and those obtained from atomic bulk composition, we saw that the spin Hall conductivities (SHCs) follow the behavior described by the Slater-Pauling curve, the maximum of which is at pure Pt bulk. From the examination of the band structures of the considered systems, we found that the origin of this behavior comes from the variation of the band structures as a direct consequence of the change of the number of electrons and hybridization effects.

Phonon heat transport in two-dimensional phagraphene-graphene superlattice

In this study, we perform non-equilibrium molecular dynamics simulations to investigate phonon heat transport in a two-dimensional superlattice with equal-sized domains of graphene and phagraphene. Effects on conductivity are examined in relation to modifications of domain sizes, the length of employed nanoribbons and temperature differences between the thermal baths used with the superlattices. We have determined that effective thermal conductivity reaches a minimum value of 155W/mK for ribbons with a superlattice period of 12.85nm. This minimum thermal conductivity of graphene-phagraphene superlattices at infinite length is approximately 5%, of pure graphene thermal conductivity, and ≈50% of phagraphene thermal conductivity. Minimum thermal conductivity occurs at the transition from coherent to incoherent phonon transport, where the superlattice period is comparable to the phonon coherence length.

Thermal transport in two-dimensional C3N/C2N superlattices: A molecular dynamics approach

Nanostructured superlattices have been the focus of many researchers due to their physical and manipulatable properties. They aim to find promising materials for new electronic and thermoelectric devices. In the present study, we investigate the thermal conductivity of two-dimensional (2D) C3N/ C2N superlattices using non-equilibrium molecular dynamics. We analyze the dependence of thermal conductivity on the total length, temperature, and the temperature difference between thermal baths for the superlattices. The minimum thermal conductivity and the phonon mean free path at a superlattice period of 5.2 nm are 23.2 W/m.K and 24.7 nm, respectively. Our results show that at a specific total length, as the period increases, the number of interfaces decreases, thus the total thermal resistance decreases, and the effective thermal conductivity of the system increases. We found that at long lengths (Lx >80 nm), the high-frequency and low-wavelength phonons are scattered throughout the interfaces, while at short lengths, there is a wave interference that reduces the thermal conductivity. The combination of these two effects, i.e., the wave interference and the interface scattering, is the reason for the existence of a minimum thermal conductivity in superlattices.

Prediction of Bi2Te3-Sb2Te3 Interfacial Conductance and Superlattice Thermal Conductivity Using Molecular Dynamics Simulations.

Bismuth telluride (Bi2Te3) and its alloys with antimony telluride (Sb2Te3) have long been considered to be the best room-temperature bulk thermoelectric (TE) materials. In recent decades, proof-of-concept demonstrations on Bi2Te3-Sb2Te3 nanostructures have shown high TE performance due to reduction in lattice thermal conductivities. Particularly, ultra-low thermal conductivities have been observed in Bi2Te3-Sb2Te3 1D superlattices, leading to thermoelectric figures of merit (ZT) as high as 2.4. In contrast, very few computational studies have been performed to provide insight into the phonon transport across these nanostructures. In this work, we use non-equilibrium molecular dynamics simulations with previously developed force fields to simulate thermal transport across Bi2Te3-Sb2Te3 interfaces and superlattices. We first calculate the thermal conductance associated with a Bi2Te3-Sb2Te3 interface across a temperature range of 200-400 K. The values are also compared with thermal conductances calculated by a modified Landauer transport formalism using phonon transmission coefficients obtained from the diffuse mismatch model. Our results show that inelastic scattering processes contribute to an increase in interfacial thermal conductance at higher temperatures. Finally, we calculate the thermal conductivities of Bi2Te3-Sb2Te3 superlattices with varying period lengths from 2 to 18 nm. A minimum thermal conductivity of 0.27 W/mK is observed at a period length of 4 nm, which is attributed to the competition between incoherent and coherent phonon transport regimes. In comparison with previous experimental measurements in the literature, our results show good agreement with respect to the range of thermal conductivity values and the period length corresponding to the minimum superlattice thermal conductivity.

Вертикальная проводимость сверхрешетки в продольном сильном магнитном поле при внутризонных и межзонных переходах

The influence of interband and intraband transitions on oscillations of the vertical conductivity of superlattices in a strong magnetic field is considered. It was found that for the charged impurity scattering intraband transitions dominate in magnetic fields up to 2 T, and with a further increase in the magnetic field, interband transitions control. The conductivity oscillations are determined by a ratio of the magnetic length to the superlattice period and the effective mass anisotropy.

Big-data-accelerated aperiodic Si/Ge superlattice prediction for quenching thermal conduction via pattern analysis

Thermal conductivity of material is one of the basic physical properties and plays an important role in manipulating thermal energy. In order to accelerate the prediction of material structure with desired thermal property, machine learning algorithm has been widely adopted. However, in the optimization of multivariable material structure such as different lengths or proportions, the machine learning algorithm may be required to be reconducted again and again for each variable, which will consume a lot of computing resources. Recently, it has been found that the thermal conductivity of aperiodic superlattices is closely related to the degree of the structural randomness, which can also be reflected in their local pattern structures. Inspired by these, we propose a new pattern analysis method, in which machine learning only needs to be carried out for one time, and through which the optimal structure of different variables with low thermal conductivity can be obtained. To verify the method, we compare the thermal conductivities of the structure obtained by pattern analysis, conventional machine learning, and previous literature, respectively. The pattern analysis method is validated to greatly reduce the prediction time of multivariable structure with high enough accuracy and may promote further development of material informatics.

Vertical conductivity and Poole–Frenkel-ionization of Mg acceptors in AlGaN short-period superlattices with high Al mole fraction

Mg-doped AlGaN short-period superlattices with a high aluminum mole fraction are promising to fabricate highly efficient deep UV light emitting diodes. We present a robust and easy-to-implement experimental method for quantification of the vertical component of the anisotropic short-period superlattice conductivity based on current–voltage characteristics of devices with varying short-period superlattice thicknesses. In particular, the vertical conductivity of Al0.71Ga0.29N/Al0.65Ga0.35N:Mg short-period superlattices is investigated and found to be strongly affected by the temperature and by the applied electric field. At room temperature, the vertical conductivity varies between 5.5 × 10−7 Ω −1 cm−1 at 0.05 MV cm−1 and 6.7 × 10−5 Ω−1 cm−1 at 0.98 MV cm−1 and increases by almost two orders of magnitude when the temperature increases up to 100 °C. This behavior is in very good agreement with simulations based on a 3D-Poole–Frenkel model. In addition, the zero-field ionization energy and the inter-trap distance of the Mg acceptors in the AlGaN short-period superlattices were determined to be 510 ± 20 meV and 5.1 ± 0.3 nm, respectively.

Thermal conductivities of different period Si/Ge superlattices

Thermoelectric materials, which can convert wasted heat into electricity, have attracted considerable attention because they provide a solution to energy problems. The Si/Ge superlattices have shown tremendous promise as effective thermoelectric materials. The period lengths of the Si/Ge superlattices can effectively tailor the phonon's transport behaviors and control their thermal conductivities. In this paper, three kinds of Si/Ge superlattices with different period length distributions (uniform, gradient, random) are constructed. The non-equilibrium molecular dynamics (NEMD) method is used to calculate the thermal conductivities of Si/Ge superlattices under the different period length distributions. The effect of the sample’s total length and temperature on the superlattice's thermal conductivity are studied. The simulation result shows that the thermal conductivity of gradient and random periodical Si/Ge superlattices are significantly reduced at room temperature compared with that of the uniform period Si/Ge superlattices. Phonons are transported by wave or particle properties in the different periodical superlattices. The thermal conductivity of uniform period superlattices has an obvious size effect with the increasing of the sample total length. In contrast, the thermal conductivity of gradient, random periodical Si/Ge superlattices are weakly dependent on the sample’s total length. At the same time, temperature is an important factor affecting the heat transport properties. We find that the temperature affects the thermal conductivities of the three kinds of superlattices in different ways. With the increase of the temperature, (i) the thermal conductivity of uniform periodical superlattices shows an obvious temperature effect; (ii) the thermal conductivity of the gradient and random periodical Si/Ge superlattices are nearly unchanged due to the competition between phonon localization weakness and phonon-phonon scattering enhancement. In addition, the phonon densities of states of superlattices with three different periodical length distributions are calculated. We find that in the picture of uniform periodical Si/Ge superlattices, the number of pronounced peaks quickly decreases as the period length increases, particularly at higher frequencies. This indicates that as the period length increases, fewer coherent phonons will be formed over the superlattices. Moreover, the scattering mechanisms of phonons for gradient and random periodical Si/Ge superlattices are basically the same at 100 K and 500 K. These findings provide a developmental way to further reduce the thermal conductivity of superlattices.