Superlattice Effects Research Articles

Mechanical properties and epitaxial growth of TiN/AlN superlattices

TiN/AlN superlattice coatings combine the superlattice effect and the outstanding properties of AlN in its non-equilibrium face-centred cubic (NaCl-type, rs) structure. In comparison with their non-layered counterparts, the superlattice films exhibit superior and strongly bilayer-period-dependent mechanical properties. Here we analyse the structure and mechanical properties of superlattices with AlN layer thicknesses of 0.9, 1.2, and 1.5 nm and different TiN layer thicknesses. We found that the maximum AlN layer thickness, up to which AlN can be fully stabilized in its metastable rs structure, depends on the substrate material and is at least 1.5 nm for coatings on MgO (100) and 0.9 nm for the coatings grown on Si (100) or Al2O311¯02. Our data also show that with increasing thickness of the template layer, the crystallization of AlN in the stable wurtzite structure is favoured. TiN/AlN superlattice coatings on MgO (100) exhibit the highest hardness values, peaking at 37.0 ± 0.5 GPa for AlN layer thicknesses of 0.9 nm and a bilayer period of 2.5 nm. Films with ultra-small bilayer periods exhibit high hardness combined with low values of elastic modulus suggesting a high elastic-strain-to-failure for these coatings. The highest toughness was found for superlattice films grown on MgO (100), scaling inversely with the bilayer period.

Toughness enhancement in TiN/WN superlattice thin films

Transition metal nitride thin films traditionally possess a low intrinsic fracture toughness. Motivated by the recently discovered fracture toughness enhancing superlattice (SL) effect, as well as the remarkably high potential for toughness predicted by theoretical studies for TiN/WN superlattices, we synthesise a series of these materials by DC reactive magnetron sputtering. The SL coatings demonstrate a vacancy-stabilised cubic configuration throughout, as well as a marginal lattice mismatch between the TiN and WN layers. All investigated mechanical properties produced a distinct dependence on the bilayer period, featuring a hardness peak of 36.7 ± 0.2 GPa and a minimum of the indentation modulus of 387 ± 2 GPa. The toughness-related quantities of the SLs in particular show a significant enhancement compared to monolithic TiN and WN, including a tripling of the fracture energy. The fracture toughness is raised from 2.8 ± 0.1 (TiN) and 3.1 ± 0.1 (WN) to 4.6 ± 0.2 MPa√m by the SL arrangement. We relate this maximisation to the vastly disparate elastic moduli and compositional fluctuations. To complement our experimental data, we present Density Functional Theory-based models to disentangle the conspicuous trends observed for TiN/WN superlattices.

Interlayer hybridization and moiré superlattice minibands for electrons and excitons in heterobilayers of transition-metal dichalcogenides

Geometrical moir\'e patterns, generic for almost aligned bilayers of two-dimensional (2D) crystals with similar lattice structure but slightly different lattice constants, lead to zone folding and miniband formation for electronic states. Here, we show that moir\'e superlattice (mSL) effects in $\mathrm{MoSe}_2/\mathrm{WS}_2$ and $\mathrm{MoTe}_2/\mathrm{MoSe}_2$ heterobilayers that feature alignment of the band edges are enhanced by resonant interlayer hybridization, and anticipate similar features in twisted homobilayers of TMDs, including examples of narrow minibands close to the actual band edges. Such hybridization determines the optical activity of interlayer excitons in transition-metal dichalcogenide (TMD) heterostructures, as well as energy shifts in the exciton spectrum. We show that the resonantly hybridized exciton (hX) energy should display a sharp modulation as a function of the interlayer twist angle, accompanied by additional spectral features caused by umklapp electron-photon interactions with the mSL. We analyze the appearance of resonantly enhanced mSL features in absorption and emission of light by the interlayer exciton hybridization with both intralayer A and B excitons in $\mathrm{MoSe}_2/\mathrm{WS}_2$, $\mathrm{MoTe}_2/\mathrm{MoSe}_2$, $\mathrm{MoSe}_2/\mathrm{MoS}_2$, $\mathrm{WS}_2/\mathrm{MoS}_2$, and $\mathrm{WSe}_2/\mathrm{MoSe}_2$.

Toughness Enhancement in TiN/WN Superlattice Thin Films

Transition metal nitride thin films traditionally possess a low intrinsic fracture toughness. Motivated by the recently discovered fracture toughness enhancing superlattice (SL) effect, as well as the remarkably high potential for toughness predicted by theoretical studies for TiN/WN superlattices, we synthesise a series of these materials by DC reactive magnetron sputtering. The SL coatings demonstrate a vacancy-stabilised cubic configuration throughout, as well as a marginal lattice mismatch between the TiN and WN layers. All investigated mechanical properties produced a distinct dependence on the bilayer period, featuring a hardness peak of 36.7 ± 0.2 GPa and a minimum of the indentation modulus of 387 ± 2 GPa. The toughness-related quantities of the SLs in particular show a significant enhancement compared to monolithic TiN and WN, including a tripling of the fracture energy. The fracture toughness is raised from 2.8 ± 0.1 (TiN) and 3.1 ± 0.1 (WN) to 4.6 ± 0.2 MPa√m by the SL arrangement. We relate this maximisation to the vastly disparate elastic moduli and compositional fluctuations. To complement our experimental data, we present Density Functional Theory-based models to disentangle the conspicuous trends observed for TiN/WN superlattices.

Resonantly hybridized excitons in moiré superlattices in van der Waals heterostructures.

Atomically thin layers of two-dimensional materials can be assembled in vertical stacks that are held together by relatively weak van der Waals forces, enabling coupling between monolayer crystals with incommensurate lattices and arbitrary mutual rotation1,2. Consequently, an overarching periodicity emerges in the local atomic registry of the constituent crystal structures, which is known as a moiré superlattice3. In graphene/hexagonal boron nitride structures4, the presence of a moiré superlattice can lead to the observation of electronic minibands5-7, whereas in twisted graphene bilayers its effects are enhanced by interlayer resonant conditions, resulting in a superconductor-insulator transition at magic twist angles8. Here, using semiconducting heterostructures assembled from incommensurate molybdenum diselenide (MoSe2) and tungsten disulfide (WS2) monolayers, we demonstrate that excitonic bands can hybridize, resulting in a resonant enhancement of moiré superlattice effects. MoSe2 and WS2 were chosen for the near-degeneracy of their conduction-band edges, in order to promote the hybridization of intra- and interlayer excitons. Hybridization manifests through a pronounced exciton energy shift as a periodic function of the interlayer rotation angle, which occurs as hybridized excitons are formed by holes that reside in MoSe2 binding to a twist-dependent superposition of electron states in the adjacent monolayers. For heterostructures in which the monolayer pairs are nearly aligned, resonant mixing of the electron states leads to pronounced effects of the geometrical moiré pattern of the heterostructure on the dispersion and optical spectra of the hybridized excitons. Our findings underpin strategies for band-structure engineering in semiconductor devices based on van der Waals heterostructures9.

Reconstructed Fermi surface in graphene on Ir(111) by Gd-Ir surface alloying

Graphene electronics covers a number of unique effects and the most intriguing ones are based on its interaction with other materials. Contact of graphene with the lattice-mismatched substrate clones the Dirac cone and gives rise to Hofstadter spectrum, while contact with the heavy/magnetic atoms realizes topological insulator phase. Here we study the electronic structure of graphene on Ir(111) with intercalated rare-earth Gd atoms by means of Angle-Resolved Photoemission Spectroscopy (ARPES). Gd intercalation results in the formation of the Gd-Ir surface alloy with the (2 × 2) superstructure, but the moiré superlattice of graphene persists. Strong charge transfer from Gd atoms leads to the shifting of the Dirac cone and its replicas towards the higher binding energies while closing the umklapp band gaps. The replicated Dirac cone bands cross each other near the Fermi level, that is essential for the superlattice effects application in electronics.

Exploring the DBR superlattice effect on the thermal performance of a VECSEL with the finite element method.

The design criterion of thermal conductivity for the GaAs/AlAs distributed Bragg reflector (DBR) superlattice structure was thoroughly investigated to precisely analyze the thermal behaviors of the optically pumped vertically external cavity surface-emitting laser (VECSEL). A finite element model with detailed configuration of a VECSEL gain chip was constructed to fulfill the analysis. A 1060nm VECSEL with different pump conditions was further demonstrated to verify the finite element analysis. At the VECSEL thermal rollover point, the analysis results show that the model with the superlattice property predicts more precise temperature values than that using a bulk composite property. It reveals that the accurate determination of the thermal conductivity of the DBR superlattice is significantly important for the VECSEL thermal analysis.

Strain-induced broadening temperature range of electrocaloric effects in ferroelectric superlattices

Motivated by a broad temperature span, we performed a strain-induced broadening temperature range of electrocaloric effects in PbZr0.6Ti0.4O3/PbTiO3 ferroelectric superlattices based on phenomenological Landau-Devonshire thermodynamic theory. The results indicated that interlayer misfit strain and substrate misfit strain in superlattices modulate the Curie temperature. Considering two types of strains coexist in ferroelectric superlattices, there exist a balance between interlayer misfit strain and substrate misfit strain to broaden temperature range (281 K) of electrocaloric effects. The present study therefore contributes to the understanding of strain effects of electrocaloric cooling and provides guidance towards experimental design of high efficiency cooling devices.

Performance improvement of light-emitting diodes with double superlattices confinement layer * * Project supported by the National Key Research and Development Program of China (No. 2016YFB0400102), the Beijing Municipal Science and Technology Project (Nos. Z161100002116032, D12110300140000), the National Basic Research Program of China (No. 2011CB301902), the Guangzhou Science & Technology Planning Project of Guangdong Province, China (Nos. 201604016095, 201604030035), the

In this study, the effect of double superlattices on GaN-based blue light-emitting diodes (LEDs) is analyzed numerically. One of the superlattices is composed of InGaN/GaN, which is designed before the multiple quantum wells (MQWs). The other one is AlInGaN/AlGaN, which is inserted between the last QB (quantum barriers) and p-GaN. The crucial characteristics of double superlattices LEDs structure, including the energy band diagrams, carrier concentrations in the active region, light output power, internal quantum efficiency, respectively, were analyzed in detail. The simulation results suggest that compared with the conventional AlGaN electron-blocking layer (EBL) LED, the LED with double superlattices has better performance due to the enhancement of electron confinement and the increase of hole injection. The double superlattices can make it easier for the carriers tunneling to the MQWs, especially for the holes. Furthermore, the LED with the double superlattices can effectively suppress the electron overflow out of multiple quantum wells simultaneously. From the result, we argue that output power is enhanced dramatically, and the efficiency droop is substantially mitigated when the double superlattices are used.

Thermoelectric effect in superlattices; applicability of coherent and incoherent transport models

Calculations of thermoelectric transport coefficients including quantum effects are performed on superlattices using the Buttiker approximation. The results are compared to the Boltzmann transport equation with minibands present, and to an incoherent transport model. Comparisons are performed in the linear regime for the electrical conductivity, Seebeck and Lorenz coefficients. We show that at superlattice periods smaller than the typical electron mean free path, the former model and the calculations including quantum effects are in agreement. However, for longer superlattices the incoherent model is shown to be more correct.

Anisotropy of Nernst–Ettingshausen Effect in Superlattices During Scattering on Phonons

The paper focuses on anisotropy of the transverse Nernst–Ettingshausen effect in superlattices depending on the degree of mini-band filling and magnetic field direction during scattering on acoustic and polar optical phonons. The authors demonstrate that in the case of scattering on polar optical phonons one observes considerable anisotropy and, depending on the magnetic field direction, the Nernst–Ettingshausen coefficient may change its sign.

Energy minibands degeneration induced by magnetic field effects in graphene superlattices

Energy minibands are a basic feature of practically any superlattice. In this regard graphene superlattices are not the exception and recently miniband transport has been reported through magneto-transport measurements. In this work, we compute the energy miniband and transport characteristics for graphene superlattices in which the energy barriers are generated by magnetic and electric fields. The transfer matrix approach and the Landauer-Büttiker formalism have been implemented to calculate the energy minibands and the linear-regime conductance. We find that energy minibands are very sensitive to the magnetic field and become degenerate by rising it. We were also able to correlate the evolution of the energy minibands as a function of the magnetic field with the transport characteristics, finding that miniband transport can be destroyed by magnetic field effects. Here, it is important to remark that although magnetic field effects have been a key element to unveil miniband transport, they can also destroy it.

The Exchange Bias of LaMnO3/LaNiO3 Superlattices Grown along Different Orientations

With the goal of observing and explaining the unexpected exchange bias effect in paramagnetic LaNiO3-based superlattices, a wide range of theoretical and experimental research has been published. Within the scope of this work, we have grown high-quality epitaxial LaMnO3(n)-LaNiO3(n) (LMO/LNO) superlattices (SLs) along (001)-, (110)-, and (111)-oriented SrTiO3 substrates. The exchange bias effect is observed in all cases, regardless of growth orientation of the LMO/LNO SLs. As a result of a combination of a number of synchrotron based x-ray spectroscopy measurements, this effect is attributed to the interfacial charge transfer from Mn to Ni ions that induces localized magnetic moments to pin the ferromagnetic LMO layer. The interaction per area between interfacial Mn and Ni ions is nearly consistent and has no effect on charge transfer for different orientations. The discrepant charge transfer and orbital occupancy can be responsible for the different magnetic properties in LMO/LNO superlattices. Our experimental results present a promising advancement in understanding the origin of magnetic properties along different directions in these materials.

Investigation of Ti0.54Al0.46/Ti0.54Al0.46N multilayer films deposited by reactive gas pulsing process by nano-indentation and electron energy-loss spectroscopy

Physical vapour deposition technology is well suited to the deposition of advanced TiAlN-based coatings. Among these thin films, multilayer systems consisting of stacked layers of metallic Ti1−xAlx and nitride Ti1−xAlxN with x around 0.5 are expected to have improved mechanical properties with respect to single nitride layers of the same composition. A set of Ti0.54Al0.46/Ti0.54Al0.46N multilayer films with five different periods Λ (from 4 to 50nm) were deposited using the reactive gas pulsing process (RGPP). This RGPP approach allows the deposition of TiAl-based alloy/nitride multilayer films by radio frequency reactive magnetron sputtering with a controlled pulsing flow rate of the nitrogen reactive gas. The coherent growth of the multilayer coatings, depending on the period, is checked by X-ray diffraction and the mechanical properties are determined by Berkovich nano-indentation and friction experiments. A model to describe the dependence of the indentation modulus M and the hardness HB on the penetration depth h, the period Λ, and the film thickness ef is proposed. The indentation modulus of the multilayer films (M at h=0 and for ef~1900nm) is found to be in the range of 340GPa<M<525GPa≈M(Ti0.54Al0.46N). For a fixed penetration depth, M follows a Hall and Petch evolution as a function of the period (4≤Λ≤50nm). The Berkovich hardness, 25GPa<HB<50GPa, also presents the same kind of evolution, and for Λ<16nm (at h=0), HB>HB (Ti0.54Al0.46N)=33GPa. Hence, a superlattice effect is clearly evidenced. Moreover, for the larger periods, the wear behaviour of these multilayered coatings seems to be dominated by the plastic deformation of the metallic layer. The multilayer coating of period Λ=10nm, which exhibits a diffraction pattern typical of superlattices and favourable mechanical properties, is more precisely investigated. Transmission electron microscopy confirms the main growth of the film along the [111] direction, and the evolution of the bonding of nitrogen in the direction normal to the rough interfaces between Ti0.54Al0.46 and Ti0.54Al0.46N layers is specified by electron energy-loss near-edge spectroscopy. Nitride nano-grains are included in the metallic layer, which attests to the mixing of nitrogen into the layers. The structure of these nano-grains presents a progressive evolution into the layer and gradually acquires a TiN-like structure near the interface. For this Λ=10nm period, the indentation modulus and hardness for different penetration depths are weakly sensitive to the multilayer film thickness.

Small-angle diffraction by heterogeneous composite nanostructures based on (Co45Fe45Zr10)35(Al2O3)65

Superlattice effects in such multilayer heterogeneous metal-containing composite nanostructures as [(Co45Fe45Zr10)35(Al2O3)65/a-Si] m and [(Co45Fe45Zr10)35(Al2O3)65/a-Si:H] m with interlayers of amorphous silicon (a-Si) and hydrogenated amorphous silicon a-Si:H are studied by means of small-angle X-ray diffraction.

Superlattice effect for enhanced fracture toughness of hard coatings

Coherently grown nanolayered TiN/CrN thin films exhibit a superlattice effect in fracture toughness, similar to the reported effect in indentation hardness. We found –by employing in-situ micromechanical cantilever bending tests on free-standing TiN/CrN superlattice films– that the fracture toughness increases with decreasing bilayer period (Λ), reaching a maximum at Λ~6nm. For ultrathin layers (Λ~2nm), the fracture toughness drops to the lowest value due to intermixing and loss of superlattice structure. Both, fracture toughness and hardness peak for similar bilayer periods of TiN/CrN superlattices.

Multiple omnidirectional defect modes and nonlinear magnetic-field effects in metamaterial photonic superlattices with a polaritonic defect

We report the existence of multiple omnidirectional defect modes in the zero-n¯ gap of photonic stacks, made of alternate layers of conventional dielectric and double-negative metamaterial, with a polaritonic defect layer. In the case of nonlinear magnetic metamaterials, the optical bistability phenomenon leads to switching from negligible to perfect transmission around these defect modes. We hope these findings have potential applications in the design and development of multichannel optical filters, power limiters, optical-diodes and optical-transistors.

Impact of confined LO-phonons on the Hall effect in doped semiconductor superlattices

Abstract Based on the quantum kinetic equation method, the Hall effect in doped semiconductor superlattices (DSSL) has been theoretically studied under the influence of confined LO-phonons and the laser radiation. The analytical expression of the Hall conductivity tensor, the magnetoresistance and the Hall coefficient of a GaAs:Si/GaAs:Be DSSL is obtained in terms of the external fields, lattice period and doping concentration. The quantum numbers N, n, m were varied in order to characterize the effect of electron and LO-phonon confinement. Numerical evaluations showed that LO-phonon confinement enhanced the probability of electron scattering, thus increasing the number of resonance peaks in the Hall conductivity tensor and decreasing the magnitude of the magnetoresistance as well as the Hall coefficient when compared to the case of bulk phonons. The nearly linear increase of the magnetoresistance with temperature was found to be in good agreement with experiment.

The Radioelectric effect in doped superlattices under the influence of confined phonon

The Radioelectric effect in doped superlattices under the influence of confined phonon has been theoretically studied. The analytical expression for the Radioelectric field is obtained by quantum kinetic equation method. The theoretical expression shows that the Radioelectric field in doped superlattices depends on the frequencies and amplitudes of the laser and the linearly polarized electromagnetic wave, the period of the superlattices and especially the quantum number m characterizing the phonon confinement. Numerical calculation is also applied for GaAs:Si/GaAs:Be doped superlattices. It is found that the Radioelectric field is different from that in the normal bulk semiconductor as well as two-dimensional systems in case of unconfined phonon and in case of confined phonon when the contribution of confined potential of doped superlattices and confined phonon is remarkable. The Radioelectric field has multiple resonance peaks and increases as the increasing of quantum number m.

Transforming Common III–V and II–VI Semiconductor Compounds into Topological Heterostructures: The Case of CdTe/InSb Superlattices

Currently, known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. It is shown via first‐principle calculations that a heterovalent superlattice made of common semiconductor building blocks can transform its non‐TI components into a topological nanostructure, illustrated by III–V/II–VI superlattice InSb/CdTe. The heterovalent nature of such interfaces sets up, in the absence of interfacial atomic exchange, a natural internal electric field that along with the quantum confinement leads to band inversion, transforming these semiconductors into a topological phase while also forming a giant Rashba spin splitting. The relationship between the interfacial stability and the topological transition is revealed, finding a “window of opportunity” where both conditions can be optimized. Once a critical InSb layer thickness above ≈1.5 nm is reached, both [111] and [100] superlattices have a relative energy of 1.7–9.5 meV Å–2, higher than that of the atomically exchanged interface and an excitation gap up to ≈150 meV, affording room‐temperature quantum spin Hall effect in semiconductor superlattices. The understanding gained from this study could broaden the current, rather restricted repertoire of functionalities available from individual compounds by creating next‐generation superstructured functional materials.