Superlattice-like Structure Research Articles

Impact wear behavior of WC/a-C nanomultilayers

A novel design of WC/a-C multilayers with ultrathin modulation period ranging from 1.3 to 11.5 nm was fabricated by unbalanced magnetron sputtering process, and the influence of modulation period on the impact wear behavior of the films was investigated from impact dynamic response and typical wear analysis. Results show that the peak impact force, impact energy absorption rate and impact wear volume loss of the films decrease firstly and then increase with increasing the modulation period, and both reach the minimum value at Λ = 9.1 nm. High hardness, toughness and plastic deformation resistance originated from the ‘superlattice-like structure’ of the films at appropriate modulation period are proved to be responsible for the low peak impact force, low impact energy absorption rate and then the improved impact wear resistance. While, the films with low hardness and H3/E2 have severe plastic deformation and impact-induced graphitization during impact process which in turn further reduce the hardness and H3/E2 of the film itself, thus eventually resulting in severe impact wear.

Twisted graphene nanoribbons as nonlinear nanoelectronic devices

We argue that twisted graphene nanoribbons subjected to a transverse electric field can operate as a variety of nonlinear nanoelectronic devices with tunable current-voltage characteristics controlled by the transverse field. Using the density-functional tight-binding method to address the effects of mechanical strain induced by the twisting, we show that the electronic transport properties remain almost unaffected by the strain in relevant cases and propose an efficient simplified tight-binding model which gives reliable results. The transverse electric field creates a periodic electrostatic potential along the nanoribbon, resulting in a formation of a superlattice-like energy band structure and giving rise to different remarkable electronic properties. We demonstrate that if the nanoribbon geometry and operating point are selected appropriately, the system can function as a field-effect transistor or a device with nonlinear current-voltage characteristic manifesting one or several regions of negative differential resistance. The latter opens possibilities for applications such as an active element of amplifiers, generators, and new class of nanoscale devices with multiple logic states.

Optimal Bandgap in a 2D Ruddlesden–Popper Perovskite Chalcogenide for Single-Junction Solar Cells

Transition metal perovskite chalcogenides (TMPCs) are explored as stable, environmentally friendly semiconductors for solar energy conversion. They can be viewed as the inorganic alternatives to hybrid halide perovskites, and chalcogenide counterparts of perovskite oxides with desirable optoelectronic properties in the visible and infrared part of the electromagnetic spectrum. Past theoretical studies have predicted large absorption coefficient, desirable defect characteristics, and bulk photovoltaic effect in TMPCs. Despite recent progresses in polycrystalline synthesis and measurements of their optical properties, it is necessary to grow these materials in high crystalline quality to develop a fundamental understanding of their optical properties and evaluate their suitability for photovoltaic application. Here, we report the growth of single crystals of a two-dimensional (2D) perovskite chalcogenide, Ba3Zr2S7, with a natural superlattice-like structure of alternating double-layer perovskite blocks and single-layer rock salt structure. The material demonstrated a bright photoluminescence peak at 1.28 eV with a large external luminescence efficiency of up to 0.15%. We performed time-resolved photoluminescence spectroscopy on these crystals and obtained an effective recombination time of ~65 ns. These results clearly show that 2D Ruddlesden-Popper phases of perovskite chalcogenides are promising materials to achieve single-junction solar cells.

GeTe/Sb4Te films: A candidate for multilevel phase change memory

In this paper, GeTe and Sb4Te layers were alternately deposited to form superlattice-like structure (SLL) for multilevel phase change memory (PCM). It is shown that SLL GeTe/Sb4Te film can realize a multilevel storage with various appropriate thickness ratios between GeTe and Sb4Te layers. The crystallization behavior of SLL GeTe/Sb4Te film can be tuned by the thickness ratios which is associated closely with the thermal stability. GeTe(4 nm)/Sb4Te(6 nm)-based device demonstrated the minimum voltage pulse for the RESET operation of 3.2 V-5 ns, suggesting the low power consumption in comparison with that of Ge2Sb2Te5 (4.1 V). An electric pulse as short as 5 ns can trigger different operations among three stable resistance states for GeTe(4 nm)/Sb4Te(6 nm)-based device, which can be explained by the asynchronous crystallization of GeTe and Sb4Te. These results demonstrate that SLL GeTe/Sb4Te films are promising candidate for multilevel phase-change memory.

Genuine Unilamellar Metal Oxide Nanosheets Confined in a Superlattice-like Structure for Superior Energy Storage.

Two-dimensional (2D) metal oxide nanosheets can exhibit exceptional electrochemical performance owing to their shortened ion diffusion distances, abundant active sites, and various valence states. Especially, genuine unilamellar nanosheets with an atomic-scale thickness are expected to exhibit the ultimate energy storage capability but have not yet achieved their potential. Here, we demonstrate the utilization of genuine unilamellar MnO2 nanosheets for high-performance Li and Na storage using an alternately stacked MnO2/graphene superlattice-like structure. Different from previous reports, all unilamellar MnO2 nanosheets are separated and stabilized between the graphene monolayers, resulting in highly reversible 2D-confined conversion processes. As a consequence, large specific capacities of 1325 and 795 mA h g-1 at 0.1 A g-1, high-rate capacities of 370 and 245 mA h g-1 at 12.8 A g-1, and excellent cycling stabilities after 5000 cycles with ∼0.004% and 0.0078% capacity decay per cycle were obtained for Li and Na storage, respectively, presenting the best reported performance to date.

Superlattice-like structure and enhanced ferroelectric properties of intergrowth Aurivillius oxides.

Aurivillius oxides with an intergrowth structures have been receiving increasing interest because of their special structures and potential outstanding ferroelectric properties. In this work, Bi3LaTiNbFeO12–Bi5Ti3FeO15 and Bi3TiNbO9–Bi3LaTiNbFeO12 compounds were successfully synthetised using a simple solid-state reaction method. X-Ray diffraction patterns and scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) images confirm the 2–3 and the 3–4 intergrowth structures in Bi3TiNbO9–Bi3LaTiNbFeO12 and Bi3LaTiNbFeO12–Bi5Ti3FeO15 compounds, respectively. A superlattice-like distortion in these oxides was proposed resulting from the combination of sub-lattices with different a and b parameters, which was validated by XRD refinements and Raman spectra. Polarization-electric field tests and pulsed polarization positive-up negative-down measurements demonstrate that such superlattice-like structures can effectively enhance the intrinsic ferroelectric polarization and coercive field of these oxides, especially when compared with their component oxides Bi3TiNbO9, Bi3LaTiNbFeO12 and Bi5Ti3FeO15. Simultaneously, ferroelectric Curie temperatures of Bi3TiNbO9–Bi3LaTiNbFeO12 and Bi3LaTiNbFeO12–Bi5Ti3FeO15 oxides are lowered because of the internal stress in the superlattice-like structure. Nevertheless, the paramagnetism of the samples is hardly influenced by their structure, while mainly related to their iron content, in which iron has a similar effective magnetic moment around 3.4–3.9.

Negative Differential Resistance and Steep Switching in Chevron Graphene Nanoribbon Field-Effect Transistors

Ballistic quantum transport calculations based on the non-equilbrium Green's function formalism show that field-effect transistor devices made from chevron-type graphene nanoribbons (CGNRs) could exhibit negative differential resistance with peak-to-valley ratios in excess of 4800 at room temperature as well as steep-slope switching with 6 mV/decade subtheshold swing over five orders of magnitude and ON-currents of 88$\mu$A/$\mu$m. This is enabled by the superlattice-like structure of these ribbons that have large periodic unit cells with regions of different effective bandgap, resulting in minibands and gaps in the density of states above the conduction band edge. The CGNR ribbon used in our proposed device has been previously fabricated with bottom-up chemical synthesis techniques and could be incorporated into an experimentally-realizable structure.

Electronic properties of superlattices on quantum rings

We present a theoretical study of the one-electron states of a semiconductor-made quantum ring (QR) containing a series of piecewise-constant wells and barriers distributed along the ring circumference. The single quantum well and the superlattice cases are considered in detail. We also investigate how such confining potentials affect the Aharonov–Bohm like oscillations of the energy spectrum and current in the presence of a magnetic field. The model is simple enough so as to allow obtaining various analytical or quasi-analytical results. We show that the well-in-a-ring structure presents enhanced localization features, as well as specific geometrical resonances in its above-barrier spectrum. We stress that the superlattice-in-a-ring structure allows giving a physical meaning to the often used but usually artificial Born–von-Karman periodic conditions, and discuss in detail the formation of energy minibands and minigaps for the circumferential motion, as well as several properties of the superlattice eigenstates in the presence of the magnetic field. We obtain that the Aharonov–Bohm oscillations of below-barrier miniband states are reinforced, owing to the important tunnel coupling between neighbour wells of the superlattice, which permits the electron to move in the ring. Additionally, we analysis a superlattice-like structure made of a regular distribution of ionized impurities placed around the QR, a system that may implement the superlattice in a ring idea. Finally, we consider several random disorder models, in order to study roughness disorder and to tackle the robustness of some results against deviations from the ideally nanostructured ring system.

Assembled and isolated Bi5O7I nanowires with good photocatalytic activities

Assembled and isolated Bi5O7I nanowires were fast prepared through an aqueous strategy without further complex treatment. Powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and UV-vis diffuse reflectance spectroscopy (DRS) were used to characterize the obtained samples and reveal the evolution process of Bi5O7I nanomaterials. The formation of a hierarchical BiOI precursor and fast substitution of I− with OH− facilitated the evolution of assembled Bi5O7I architectures, while isolated Bi5O7I nanowires were obtained by shortening the existence time of BiOI crystals on the basis of assembled Bi5O7I nanowires. HRTEM results revealed similar crystalline orientations of nanowires along the [010] direction for the assembled and isolated Bi5O7I nanowire samples. And the nanowires in an assembled sample also exhibited a superlattice-like structure with a periodicity of 0.84 nm. On comparison to the BiOI precursor, the Bi5O7I nanowires with different assembly-styles all displayed good photocatalytic activities on the degradation of rhodamine B (RhB) dye and colorless bisphenol-A (BPA) under visible-light irradiation. The photocatalytic efficiency reached 96% in 3 h for RhB and 90% in 4 h for BPA in the degradation system at pH = 7. The assembled nanowire samples showed even better photocatalytic activities on the degradation of model organic pollutants relative to the isolated nanowire samples. Band structures and trapping experiments were studied to reveal the degradation mechanism over Bi5O7I nanowire samples. It was proposed that a valid separation of photogenerated electrons and holes and appropriate valence and conduction band positions of Bi5O7I nanowires led to good visible-light-induced catalytic properties.

Nanosecond Switching in Superlattice-Like GeTe/Sb Thin Film for High Speed and Low Power Phase Change Memory Application

Superlattice-like GeTe/Sb thin films were investigated for their potential application in phase change memory. Compared with monolayer GeTe thin film, GeTe/Sb thin film has lower crystallization temperature (208°C), crystallization activation energy (2.38 eV) and thermal conductivity (0.15 W/(m·K)). In GeTe/Sb thin film, GeTe and Sb phases exist independently. The superlattice-like structure is observed even after annealing at 250°C for 10 min. The shorter phase change time is achieved for GeTe/Sb thin film (4.3 ns). The GeTe/Sb-based phase change memory devices show an operation speed of as fast as 9 ns and a low RESET operation power of 4.09 × 10−13 J.

Superlattice-like GeTe/Sb thin film for ultra-high speed phase change memory applications

The GeTe/Sb superlattice-like thin films are systematically investigated for the potential application in phase change memory. Compared with GeTe, GeTe/Sb film has lower crystallization temperature and activation energy. Besides, the internal stress of GeTe/Sb is less than GeTe during the crystallization. The superlattice-like structure is observed by transmission electron microscopy. The polycrystalline phase is confirmed by selected area electron diffraction. The reversible resistance switching can be realized for GeTe/Sb-based phase change memory device with an electric pulse of 8ns width. In addition, GeTe/Sb shows a good endurance of 6.3×106cycles.

Superlattice-like Ga40Sb60/Sb films with ultra-high speed and low power for phase change memory application

Superlattice-like (SLL) Ga40Sb60/Sb (GSS) is proposed for its ultra-high speed, low power for phase change application. The excellent properties derives from the properly designed SLL structure of Sb layers with high speed and low crystallization temperature (Tc) inserting high Tc Ga40Sb60. Appropriate Tc ~ 220 °C and ultra-long data retention (151 °C for 10 years) is obtained in the SLL [Ga40Sb60(5 nm)/Sb(4 nm)]5 thin films. X-ray diffraction analysis shows that only Sb phase exists in the SLL GSS thin films and the low surface roughness is confirmed by AFM measurement. Moreover, the phase change memory devices based on Ge2Sb2Te5 (GST) and SLL [Ga40Sb60(5 nm)/Sb(4 nm)]5 thin films were fabricated and the set and reset operation was studied. The current–voltage measurement for set operations shows the threshold switching voltage (2.0 V) is smaller than the GST (4.4 V). The resistance–voltage tests indicate the reversible phase change could be realized by a pulse as short as 10 ns and the reset power of the [Ga40Sb60(5 nm)/Sb(4 nm)]5 cell calculated as 2.3 × 10−11 J is 6.5 times lower than the GST cell 1.5 × 10−10 J.

Low work function of crystalline GeTe/Sb2Te3 superlattice-like films induced by Te dangling bonds

We utilized a GeTe/Sb2Te3 superlattice-like structure (SLL) to obtain a lower crystalline work function (WF) than that for GeTe. Electrostatic force microscopy measurements demonstrated the difference in crystalline WF. Due to the lower crystalline WF, the heterojunction diodes based on the SLL obtained a better crystalline electrical performance. We preformed numerical simulation to confirm that the higher number of Te dangling bonds caused by the multilayer interface and grain boundaries in the SLL is the main reason for the decrease in WF. X-ray photoelectron spectroscopy analysis indicated that more Te–O bonds formed in SLL than GeTe after atmospheric annealing. While it is easy for the Te dangling bonds to combine dipoles, more Te dangling bonds exist in SLLs.

Multi-Level Lateral Phase Change Memory Based on N-Doped Sb70Te30 Super-Lattice like Structure

Lateral phase change memory (PCM) has attracted a lot of interest due to its simpler fabrication process and lower current. Researchers are trying to increase the storage capacity of lateral PCM. This paper proposed a novel super-lattice like (SLL) structure for lateral PCM to achieve multi-level storage. Lateral PCM with SLL structure incorporated with N-doped Sb70Te30 and ZnS-SiO2 has been fabricated and tested. Both tested static I-V curve and dynamic pulse mode R-V curve show multi states existing in the device. Electrical and thermal simulations have been done for this device. Simulation results show that different resistance level can be found in this device with different states of phase change bridges. With increasing periods of SLL structure, more states can be achieved.

Growth-Dominant Superlattice-Like Medium and Its Application in Phase Change Memory

Lateral PCM is one structure with good thermal confinement and low RESET current. Ideal growth-dominant (GD) phase change material is needed for lateral Phase change memory (PCM). We propose the concept of GD superlattice-like (SLL) phase change structure. GeTe and Sb7Te3 were selected to form the GD SLL structure. Crystallization temperature and resistivity of GeTe and Sb7Te3 were thickness dependent. The crystallization temperature of GeTe/Sb7Te3 SLL phase change structure increase with increasing thickness ratio of GeTe/Sb7Te3. GeTe/Sb7Te3 SLL structure built with a thickness ratio of 1.6:1 (GeTe to Sb7Te3) exhibited the lowest melting temperature compared to other thickness ratio. For a typical lateral PCM employing such GeTe/Sb7Te3 SLL phase change structure, stable SET/RESET operations with a low RESET current of 1.5 mA, SET and RESET speed of 30 nanoseconds, a good endurance above 106 cycles and a high RESET/SET resistance ratio window of 20 were achieved, demonstrating strong potentials in next generation non-volatile memory application.

Superlattice-like Ge8Sb92/Ge thin films for high speed and low power consumption phase change memory application

The amorphous-to-crystalline transitions of superlattice-like Ge8Sb92/Ge thin films were investigated through in situ film resistance measurement. X-ray reflectivity was used to measure the density change before and after phase change. The superlattice-like structure of the thin films was confirmed by using transmission electron microscopy. A picosecond laser pump–probe system was used to study the phase change speed. Phase change memory cells based on the SLL [Ge8Sb92(4 nm)/Ge(3 nm)]7 thin films were fabricated to test and verify the switching speed and operation consumption.

Enhanced dielectric and ferroelectric properties in the artificial polymer multilayers

Multilayers consisting of alternating ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer and relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) (P(VDF-TrFE-CFE)) terpolymer with different periodicities in thickness were prepared. A superlattice-like structure is shown in the polymer multilayer as the periodic thickness is lower than a critical value. The dielectric constant of the multilayer with a small periodic thickness is two times higher than that of the P(VDF-TrFE) copolymer over a temperature range between 300 K and 350 K. The multilayer also shows a good ferroelectricity in the same temperature range. The enhanced electrical properties of the multilayers are due to the long-range ferroelectric coupling.

Superlattice-like Sb50Se50/Ga30Sb70 thin films for high-speed and high density phase change memory application

Multi-level phase change character of superlattice-like (SLL) Sb50Se50/Ga30Sb70 thin films was investigated through in-situ film resistance measurement. SLL structure of the thin films was confirmed by using transmission electron microscopy. Three resistance states were observed during heating process, and their thermal stability was also examined. A picosecond laser pump-probe system was used to measure phase-change time of the SLL Sb50Se50/Ga30Sb70 thin films. Phase change memory cells based on the SLL [SS(5 nm)/GS(10 nm)]3 thin films were fabricated to test and verify multi-level switch between set and reset states.

Compositionally matched nitrogen-doped Ge2Sb2Te5/Ge2Sb2Te5 superlattice-like structures for phase change random access memory

A compositionally matched superlattice-like (SLL) structure comprised of Ge2Sb2Te5 (GST) and nitrogen-doped GST (N-GST) was developed to achieve both low current and high endurance Phase Change Random Access Memory (PCRAM). N-GST/GST SLL PCRAM devices demonstrated ∼37% current reduction compared to single layered GST PCRAM and significantly higher write/erase endurances of ∼108 compared to ∼106 for GeTe/Sb2Te3 SLL devices. The improvements in endurance are attributed to the compositionally matched N-GST/GST material combination that lowers the diffusion gradient between the layers and the lower crystallization-induced stress in the SLL as revealed by micro-cantilever stress measurements.

Electronic Structure and Spontaneous Polarization in ScxAlyGa1-x-yN Alloys Lattice-Matched to GaN: A First-Principles Study

We performed first-principles calculations of the spontaneous polarization and electronic band structures in ScxAlyGa1-x-yN alloys assuming their growth on freestanding GaN. We found an apparent deviation from the Vegard's law of the lattice constants of the ScxAlyGa1-x-yN alloys lattice-matched to GaN. It was supposed that this deviation comes from the different bonding properties of IIIB and IIIA nitrides, resulting in different crystal structures, such as hexagonal and wurtzite structures. As was reported in our previous report on YxAlyGa1-x-yN [K. Shimada et al.: J. Appl. Phys. 110 (2011) 074114], we also found that in ScxAlyGa1-x-yN alloys, the superlattice-like structure of Sc atoms reduced the magnitude of spontaneous polarization [K. Shimada et al.: Semicond. Sci. Technol. 27 (2012) 105014]. The magnitude of the spontaneous polarization of ScxAlyGa1-x-yN is larger than that of YxAlyGa1-x-yN in a wide mole fraction range of Ga. We found the nonlinearity and dependence of the atomic arrangement of Sc in the alloys. The band-gap energies at Γ have the same characteristics as the spontaneous polarization. The band-gap energies of ScxAlyGa1-x-yN are also larger than those of YxAlyGa1-x-yN in the wide mole fraction range of Ga. The band structures of ScxAlyGa1-x-yN have a direct gap at Γ and form a flat band around the valence band top originating from the hybridization of the Sc 3d and N 2p electrons.