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This study presents findings related to the characterization of cubic SnS (π-SnS) thin films and p-SnS/n-Si heterojunction structures produced simultaneously using the ultrasonic spray pyrolysis technique. In this context, the impact of different spray solution flow rates on the morphological, structural, optical, and electrical characteristics of the films was examined. Morphological analyses revealed that higher flow rates resulted in films with denser and smoother surfaces, approximately 6 nm in roughness. Additionally, it was observed that both the thickness and the growth rate of the films could be adjusted through the modulation of the flow rate. Structural analyses determined that the crystallite size increased and micro-strain values decreased with increasing flow rates. Optical evaluations indicated a decline in the optical band gap of the thin films from about 1.8 eV to 1.7 eV as the flow rates increased. This trend was consistently observed in the data obtained using the Tauc method and the derivative of transmission with respect to wavelength versus photon energy graphs. Electrical analyses revealed that the resistivity values of the thin films increased from 5.24 × 105 Ωcm to 1.64 × 106 Ωcm with increasing flow rates. Furthermore, I-V analyses of the Au/p-SnS/n-Si/Ag heterojunction structures indicated significant variability in key electrical properties. The saturation currents displayed a broad range, suggesting varying efficiencies in charge carrier collection across different samples. Similarly, the change of ideality factors pointed to differences in charge transport mechanisms, while the shifts in barrier heights indicated changes in junction properties with different fabrication conditions. The results of this study offer valuable perspectives for future research. [ABSTRACT FROM AUTHOR]

Combining DFT-based calculations with Boltzman transport formalism, thermoelectric transport properties of semiconducting two-dimensional SiC-CrS2 and SiC-CrSe2 van der Waal heterostructures are investigated in unstrained and strained conditions. We computed electronic, optical and thermoelectric properties of these heterostructures. We find that these heterostructures are direct band semiconductors having type-II band alignment in unstrained conditions. The transition from the direct to the indirect band is observed in SiC-CrSe2 with compressional strain. Switching from type-II to type-I band alignment is also observed under strained conditions in SiC-CrS2. For thermoelectric properties, we have calculated the Seebeck coefficient, electrical conductivity per relaxation time and power factor at 300 K. Analysis of the power factor revealed a preference for n-type doping under zero strain conditions, whereas changes in PF values induced by strain underscored the potential for tuning the thermoelectric performance of these heterostructures. [ABSTRACT FROM AUTHOR]

The conditions whereby epitaxy is achieved are commonly believed to be mostly governed by misfit strain. We report on a systematic investigation of growth and interface structure of single crystalline tungsten thin films on two different metal oxide substrates, Al 2 O 3 ( 11 2 ¯ 0 ) and MgO (001). We demonstrate that despite a significant mismatch, enhanced crystal quality is observed for tungsten grown on the sapphire substrates. This is promoted by stronger adhesion and chemical bonding with sapphire compared to magnesium oxide, along with the restructuring of the tungsten layers close to the interface. The latter is supported by ab initio calculations using density functional theory. Finally, we demonstrate the growth of magnetic heterostructures consisting of high-quality tungsten layers in combination with ferromagnetic CoFe layers, which are relevant for spintronic applications. [ABSTRACT FROM AUTHOR]

In this work, ZnO/SnS/indium tin oxide (ITO)/glass functional heterostructures have been developed using a combined approach of electrodeposition of a SnSx layer and successive ionic layer adsorption and reaction (SILAR) of the ZnO layer. The high-quality 400 nm-thick orthorhombic SnS0.9–0.95 films were formed on the ITO substrates with a thickness of 130 nm and an electrical conductivity of less than 40 Ω/□. Chemical deposition of ZnO thin films by the SILAR method allowed to deposit hexagonal films with a thickness of about 200 nm. The morphology, elemental and phase composition of the films were characterized by Scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The band gap (1.4 eV for SnSx and 3.3 eV for ZnO), as well as the high light absorption coefficient of SnSx films (1–2) × 104 cm–1 were determined. The obtained ZnO/SnSx/ITO heterostructures formed by the electrodeposition–SILAR cycle showed a photoEMF value of 198 mV. These properties make ZnO/SnS heterostructure promising for low-cost solar cells based on affordable materials. [ABSTRACT FROM AUTHOR]

Magnesium oxide-based magnetic heterostructures with perpendicular magnetic anisotropy are receiving increasing attention for their applications in building high-density magnetic random memories. To obtain high thermal stability and flexible data writability, a large and tunable magnetic anisotropy constant (Ki) is required. In this paper, Ki is calculated for MgO/PtMnAs heterostructures with two different interfacial configurations using first-principles calculations. The MgO/AsMn_Pt heterostructure with interfacial atoms of Mn/As has a larger Ki of 4.74 mJ/m2. It is further found that a unilateral voltage-controlled magnetic anisotropy coefficient (VCMA) of 616 fJ/Vm is produced when the electric field is below − 0.2 V/nm for the MgO/AsMn_Pt heterostructure. The most significant contribution of the VCMA results from the Pt layers. The origin of these behaviors is analyzed by orbital-resolved magnetic anisotropy energy. The spin–orbit coupling of the dz2/dyz orbitals of Pt atoms is responsible for the Ki variation with voltage. This study offers a useful guide to designing magnesium oxide-based magnetic heterostructures with high and tunable magnetic anisotropy. [ABSTRACT FROM AUTHOR]

Lateral heterostructures have created new architecture design of two-dimensional (2D) materials. In this work, ZrSe 2 and HfSe 2 monolayers, and their lateral heterostructure are explored using first-principles calculations based on the projector-augmented wave (PAW) method. Materials' stability is examined through phonon dispersion curves and ab initio molecular dynamic (AIMD) simulations. Density of states, electron localization function, and Bader charge analysis indicate the charge transfer from Zr and Hf atoms to Se atoms. Consequently, the chemical bonds are predominantly ionic. Electronic band structures assert their relative wide-gap semiconductor character with energy gaps between 1.17 and 1.26 eV. The studied 2D materials exhibit good light absorption from visible to ultraviolet regime with large absorption coefficient up to 19.45 (10 4 /cm). In addition, interline defects, including vacancies and antisites, are also examined. It has been found that Zr and Hf single vacancies magnetize significantly the lateral heterostructures, and weaker magnetization is also observed with a Se single vacancy at Zr side. Magnetic properties appear as a result of the considerable modification of the charge transfer in the outermost orbital (Zr-4d, Hf-5d, and Se-4p). Meanwhile, the non-magnetic semiconductor nature is preserved upon creating a Se single vacancy at Hf-size and antisite defects. Results presented herein introduce new lateral heterostructures as prospective candidates for optoelectronic applications and clarify new features induced by the interline defects, which frequently appear during the synthesis in experiments. [ABSTRACT FROM AUTHOR]

To overcome environmental problems and the energy crisis, water splitting through solar energy for the production of hydrogen fuel is a promising technique. Efficient and low-cost photocatalyst are highly desired. In the recent era, van der Waal (vdW) heterostructures have gained considerable attention due to their promising properties. Hybrid density functional theory is used to calculate the electronic and photocatalytic properties of BX-MSSe (X = P, As and M = Mo, W) vdW heterostructures. Most stable stacking is figured out among different stacking from their relaxation energy. The stability of the stable stacking is confirmed through phonon calculations. Calculated band structure shows that BAs-MoSSe and BP-WSSe have direct band gap nature while the BP-MoSSe and BAs-WSSe have indirect band gap nature. BAs-MoSSe has Type-I band alignment while BP-MoSSe, BAs-WSSe and BP-WSSe have type-II band alignment. The creation of heterostructures broadened the optical absorption from the infrared to the ultraviolet region of the solar spectrum. To tune the properties of these heterostructures compressive and tensile strain is applied. Switching in band alignment is predicted in these heterostructures with the applications of strain. Based on valence and conduction band edges potentially the photocatalytic behavior for pH = 0 is investigated, which confirms that in unstrained conditions BP-MSSe (M = Mo, W) heterostructures are efficient photocatalyst. [ABSTRACT FROM AUTHOR]

Effective modulation of electronic and optical properties for van der Waals heterostructures is beneficial to spintronics and optoelectronic devices. We use first-principle calculations to investigate the effects of intralaminar biaxial strain on the Rashba spin splitting and optical absorption of MoSSe/WSe 2 heterostructures. First, considering the different stacking manners, the most stable configuration is confirmed from six heterostructures. The results show that this is in type II energy band alignment, which effectively reduces the carrier recombination rate. Second, Rashba splitting strength can be effectively modulated by biaxial strain, and splitting occurs at the Γ point, in which the tensile and compressive strains can suppress and promote the strength, respectively. Third, we calculate the dielectric constant and optical absorption of the MoSSe/WSe 2 heterostructure and find a higher optical absorption coefficient in visible light scope than two monolayers, and the maximum value can reach 6.11 × 10 5 cm - 1 . The first absorption peak of the heterostructure increases with increasing tensile strain and the absorption edge is red shifted. On the contrary, there is an opposite effect under compressive strain. These results suggest that MoSSe/WSe 2 heterostructures have potential applications in strain-tunable spintronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Following in graphene's wake, the scotch tape method became the key enabler for the preparation of 2D materials, providing easy access to high-quality materials mainly limited by low yield. At this time, transition metal dichalcogenides (TMDC) received tremendous attention as a promising class of two-dimensional (2D) semiconductors. The motivation to reach the 2D limit of TMDCs and many other layered materials has long been set, and with the rise of gold-mediated exfoliations towards the millimeter scale, the stacking of these 2D single-layer building blocks into artificial 3D lattices is more relevant than ever. On this note, this review presents the recent developments in gold-mediated exfoliations beyond scotch tape, accompanied by a methods walkthrough for such a process. These matured gold exfoliations unlock a whole palette of 2D building blocks, ready for the assembly of macroscopic van der Waals heterostructures, or twistronics. Ultimately, mechanical exfoliation as a key enabler for high-quality single layers, evolved from scotch tape to gold, and became an even more potent tool in the process. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) layered materials and its heterostructures grab a lot of attention because of their outstanding electronic properties for wide area of applications. In this work, the structural, mechanical, electronic and optical properties of a vertically stacked MoS 2 /MoSe 2 heterostructure have been studied using first-principles-based density functional theory (DFT) calculations. The relative rotation between the monolayers leads to the formation of beautiful Moiré patterns, specifically at a twist angle of 21. 79 ∘ and 30 ∘ . The low formation energy and Young's modulus reflect the thermodynamic and mechanical stability of this heterostructure, respectively. Furthermore, there is a phase change from direct to indirect band-gap semiconductors, as well as band-gap modulation caused by interlayer coupling between the monolayers. We also observed that the optical sensitivity of the MoS 2 /MoSe 2 heterostructures is extremely enhanced at a twist angle of 60 ∘ in the visible and infrared regions as compared to its monolayers. The twist-assisted electronic and optical properties of this heterostructure open a novel route to design the 2D stacked nanostructures for next-generation nano- and opto-electronic devices. [ABSTRACT FROM AUTHOR]

Utilizing solar energy to split water (photocatalytic water-splitting) is a good solution for global warming. However, it is difficult to find suitable materials for photocatalytic water-splitting. In this paper, through HSE06 and GW-BSE calculations, it is found that vdWHs with Se facing g-GeC (WSSe/g-GeC vdWHs) are an ideal photocatalytic material. The WSSe/g-GeC vdWHs can form a II-type band alignment, and the band gap exceeds the free energy of water splitting. The built-in electric field from Janus WSSe and vdWHs can effectively separate photo-generated electron–hole pairs and improve carrier lifetime. Compared with single-layer WSSe, WSSe/g-GeC vdWHs exhibit smaller exciton binding energy, especially for the WSSe/g-GeC vdWH I stacking (0.32 eV). Furthermore, WSSe/g-GeC vdWHs are of high absorption coefficient in the visible light regions, showing significant advantages of using solar energy. These findings imply that the WSSe/g-GeC vdWHs are promising as candidate material for photocatalytic water-splitting. [ABSTRACT FROM AUTHOR]

This work reports an extensive study of the effect of the SiC polytypes substrate on the electrical properties of heterostructures based on SiC and polyaniline (PANI) employing current–voltage (I–V), capacitance–voltage (C–V) and deep-level transient spectroscopy (DLTS) techniques. Two different SiC substrates, namely 4H-SiC and 6H-SiC, have been investigated. The I–V results revealed that the rectification ratio (IF/IR), turn-on voltage (Von) and barrier height (ϕbo) of PANI/4H-SiC hybrid device are higher than those for PANI/6H-SiC heterojunctions. The C–V measurements illustrated that the built-in potential (Vbi) of PANI deposited on 4H-SiC is greater than that of the PANI sample fabricated on 6H-SiC. Furthermore, the DLTS results showed that the energy of electrically active traps present at/close and away from the interface in devices based on the 4H-SiC substrate is lower than those of 6H-SiC substrates, corroborating with I–V results. [ABSTRACT FROM AUTHOR]

In junctionless (JL) transistors, impurity scattering limits the carrier velocity within a channel, disturbing its performance in analog/RF applications. For the first time, the Si/Si0.5Ge0.5 heterostructures and asymmetric spacers are used in this paper to improve the figures of merit of analog/RF performance of a silicon-on-insulator JL transistor (SOI-JLT). The proposed device is called hybrid heterostructure-SOI-JLT (HH-SOI-JLT), in which regions with HfO2 are referred to as the high-κ spacer, whereas regions with SiO2 are known as the low-κ spacer. According to the simulation results, Si/Si0.5Ge0.5 heterostructure in the channel of the HH-SOI-JLT device caused an offset in the conduction band energy and created quantum well in the interfacial of drain/channel and enhanced electron velocity and density of electrons significantly. Moreover, using the high-κ spacer intensified the fringing fields in the region above the threshold, and using the low-κ spacer decreased the gate-to-drain capacitance. With a 20-nm channel, maximum transconductance (Gmmax), unity gain cut-off frequency ( f T ), and intrinsic gain (AV) of the HH-SOI-JLT device are optimized by 265%, 83%, and 90%, respectively, compared to the same dimensional SOI-JLT device. In fact, the proposed HH-SOI-JLT device can be an appropriate candidate for analog/RF applications. [ABSTRACT FROM AUTHOR]

The mechanisms of tilt formation in the metal oxide multilayer heterostructures with graphoepitaxial (3DGE) growth mode were studied in the YBa2Cu3Ox–CeO2–Y:ZrO2–BaZrO3 system fabricated by pulsed laser deposition on tilted-axes NdGaO3 substrates. Every layer growing in the 3DGE mode demonstrates tilt of the small-index crystallographic plane set by surface features of the underlying layer, no matter the structure or mechanism of tilt formation in it. Introduction of a non-graphoepitaxial interface into the multilayer results in a corresponding change of tilt of the top layer. The reasons for such a change of the growth mode can be different: breach of graphoepitaxy at high tilt angles, seeding of grains of small size, or chemical interaction between neighboring layers. The epitaxial relations in the multilayer with unexpected tilt angles may be reconstructed taking into account changes of the growth step height and formation of interfaces with non-3DGE epitaxial relations. The presented results are important for analysis of the epitaxial heterostructures where 3DGE growth mode is possible, especially when trenches and slopes are patterned in some layers during fabrication of electronic devices. [ABSTRACT FROM AUTHOR]

Construction of semiconductor heterojunctions is an efficient strategy to improve photo-induced charges separation and thus enhance photocatalytic activities. Herein, g-C3N4/TiO2 heterostructures were prepared via a facile thermal procedure, with TiO2 nanorods as matrix and g-C3N4 as visible-light sensitizer. Heterojunctions formed while precursors cyanamide polymerized to g-C3N4 and protonated titanate nanotube (H-TNTs) dehydrated and shrinked to TiO2 nanorods. Notably, confined polymerization of g-C3N4 occurred at both external surface and internal space of H-TNTs with the assistant of vacuum treatment, while NH3 released from cyanamide decomposition yielded abundant oxygen vacancies (VO) in TiO2 nanorods. Compared with pristine TiO2 nanorods, the heterostructured g-C3N4/TiO2 nanorods possess 1.7 times more active in photocatalytic removal of organic dye Orange II. A mechanism was proposed for heterostructured g-C3N4/TiO2 nanorods, being attributed to synergistic increasing light harvesting by VO and charges separation by heterojunctions. [ABSTRACT FROM AUTHOR]

In the scope of two-dimensional (2D) material study, blue phosphorus (BP) is a new graphene-like layered structure that has been successfully synthesized in the experiment after it was theoretically proved to be thermostable. These 2D structured functional materials have great potential in the next-generation nanoscale electronic devices for their unique features. Here, we composite BP and monolayer WX2 (X = S/Se/Te) based on van der Waals force (vdW) interaction to obtain well-defined type-II band alignment heterostructures. A systematic theoretic study was conducted to explore the interlayer coupling effects and the bands' re-alignment of the BP–WX2 heterostructure after the strain was applied. Nowadays, many researches have proved that 2D materials can be used to degrade pollutants or used as a potential photovoltaic cell material to obtain high performance. We here twist BP and WX2 (X = S/Se/Te) into different angles to lay a theoretical framework on the band alignment and carriers' separation. It reveals that the electronic properties of freestanding BP and WX2 can be roughly preserved in the corresponding heterostructures. Upon applying strain, band alignment exhibits significant adjustability through varying external strain. The heterostructures are type-II in a certain strain range, within which the carriers can be effectively separated spatially. These heterostructures undergo a transition from semiconductor to metal when a certain strain is imposed. This work not only provides a deep insight into the construction of heterostructures, but presents a new possibility for strain engineering that is both flexible and feasible and can be used for diverse applications. [ABSTRACT FROM AUTHOR]

The conduction electron states in step-like strained GaAs/InGaAs quantum wells are theoretically investigated under the effective mass approximation, taking into account the effects of band non-parabolicity. With such information, the intersubband three-level Raman gain is calculated looking to reveal a possible application of the studied systems as sources for THz Raman lasing. A group of high-gain intersubband transitions is identified, and the results are strongly dependent on a suitable geometric design in terms of potential well widths which can lead to values of the Raman gain between 200 and 400 cm - 1 , the latter values being close to those previously reported in GaAs-based double asymmetric quantum wells. Secondary radiation frequencies are identified within the range of few tens of THz. It is found that the influence of band non-parabolicity causes a significant reduction of the Raman gain, in comparison with the values obtained neglecting such a phenomenon. Therefore, conduction-band non-parabolicity becomes a crucial element for the accurate quantitative description of the intersubband-related optical response in low-dimensional heterostructures involving small gap materials. [ABSTRACT FROM AUTHOR]

Ultrasound provided a good dispersion of CuS on TiONTs. In order to prepare CuS/TiONTs with a good heterostructure, the CuS particles must establish contact with TiONTs uniformly. The hydrothermal process was divided into two phases, and different times of ultrasound were introduced before each stage. The effects of ultrasonic-assisted hydrothermal method on the structure and photoelectric properties of CuS/TiONTs were investigated by SEM, XRD, photocurrent, and degradation of MB. The results show that the introduction of ultrasound before the hydrothermal treatment can cause damage to the TiONTs. However, the ultrasound between the two hydrothermal stages makes CuS distributed in the TiONTs uniformly, and the amount of CuS can be increased. The photocurrent reaches 32 mA/cm at 1.5 V, and meanwhile the photocatalytic activity under visible light is also improved. [ABSTRACT FROM AUTHOR]

ZnO nanowire (NW)-CdTe quantum dot (QD) type-II heterostructures were constructed using hydrothermally grown ZnO and colloidal CdTe QDs. Photoluminescence (PL) spectroscopy was used to investigate the charge transfer and band offset between CdTe QDs and ZnO NWs. The results demonstrated that the PL shows obvious redshift and prolonged lifetime in the heterostructure, indicating that it originates from recombination between electrons localized in ZnO and holes localized in CdTe. The results reveal that the band offset and charge recombination can be tuned by the growth time or size of CdTe QDs. Our results demonstrate that PL can be a useful tool to evaluate the band alignment and charge recombination in type-II semiconductor heterostructures. [ABSTRACT FROM AUTHOR]

In the paper, to achieve total reflection in the short-wavelength near-infrared (NIR) region (780-1100 nm), a broadband NIR reflector using polymer multilayer heterostructure composed of several quarter-wave stacks was proposed. The reflector used two kinds of polymer materials: PMMA and PET with low-refractive-index contrast. Taking the solar power density as the target and the Bragg wavelength of each quarter-wave stack as the variable, a searching method based on arithmetic progression was applied to find the optimal results for broadband NIR reflection. The simulation results show that the broadband NIR reflector can obtain higher total reflectivity with the increase of the number of quarter-wave stacks, and 99.06% total reflectivity in the short-wavelength NIR region can be achieved by the polymer multilayer heterostructure with six quarter-wave stacks. The proposed polymer multilayer heterostructure can be fabricated by the micro-nanomultilayer co-extrusion technology based on torsion lamination, and could be applied in agriculture as NIR-blocking films. [ABSTRACT FROM AUTHOR]

In this work, the effect of loading Ag nanoparticles on the photocatalytic activity of SrTiO/TiO nanotube arrays (TNTAs) was investigated. TNTAs were partially transformed to SrTiO through a hydrothermal treatment, which could preserve the tubular structure of TNTAs, and then, Ag nanoparticles were well deposited on the surface of SrTiO/TNTAs heterostructure by a chemical reduction process. Compared to the TNTAs sample, the Ag-loaded SrTiO/TNTAs sample showed significantly enhanced photocatalytic activities for photodegradation of rhodamine B. The enhanced photocatalytic activity of Ag-loaded SrTiO/TNTAs could be attributed to the increased optical absorption as well as the efficient charge transfer and separation of photogenerated electron-hole pairs induced by the SrTiO/TNTAs heterojunction and the Schottky barrier between metallic Ag and SrTiO/TNTAs. On the basis of the trapping experiments, the possible photocatalytic mechanism was also discussed. [ABSTRACT FROM AUTHOR]

Titanium oxide nanotubes were prepared by hydrothermal treatment of TiO powder in NaOH aqueous solution and subsequently calcined. Titanium oxide nanotubes were further decorated by TiO nanoparticles through in situ hydrolysis of titanium isopropoxide containing alcohol and ammonia in an aqueous medium to form the composite catalyst (TNP/TiONTs). The morphology and structure of TNP/TiONTs were characterized by scanning and transmission electron microscopy, X-ray diffraction, UV-Vis, and Raman spectra. The separation efficiency of photo-excited carriers was investigated by photoluminescence technique and photoelectrochemical behavior. The photocatalytic activity was evaluated by the photocatalytic degradation of methyl orange. Due to the synergy effect caused by the interaction of titanium oxide nanotubes and TiO nanoparticles, the TNP/TiONTs composite shows efficient photogenerated carriers' separation and the increased light absorption. The photocatalytic activity was enhanced. [ABSTRACT FROM AUTHOR]

Epitaxial BiEuFeO (BEFO) thin films are deposited on Nb-doped SrTiO (NSTO) substrates by pulsed laser deposition to fabricate the Pt/BEFO/NSTO (001) heterostructures. These heterostructures possess bipolar resistive switching, where the resistances versus writing voltage exhibits a distinct hysteresis loop and a memristive behavior with good retention and anti-fatigue characteristics. The local resistive switching is confirmed by the conductive atomic force microscopy (C-AFM), suggesting the possibility to scale down the memory cell size. The observed memristive behavior could be attributed to the ferroelectric polarization effect, which modulates the height of potential barrier and width of depletion region at the BEFO/NSTO interface. The continuously tunable resistive switching behavior could be useful to achieve non-volatile, high-density, multilevel random access memory with low energy consumption. [ABSTRACT FROM AUTHOR]

The observation of giant positive junction magnetoresistance (JMR) in our cobalt ferrite (CFO)/p-Si heterojunction has been reported here. The pulsed laser deposition technique has been used for fabrication of the heterojunction. The junction confirms a very good rectifying magnetic diode like behavior at low temperature, whereas at high temperatures the junction shows nonlinear I- V characteristics. The magnetic field-dependent magnetoresistance (MR) of both CFO film and across the heterojunction have been studied in detail. The CFO film shows negative MR behavior, whereas the junction shows large positive JMR behavior throughout the temperature range. The spin lifetime (142 ps) and spin diffusion length (331 nm) have been estimated of the heterostructure at 10 K. The highest JMR value (~1600 %) has been observed at 10 K, and it gradually decreases at higher temperature range. The origin of positive JMR in our heterojunction has been best explained by the standard spin injection theory. [ABSTRACT FROM AUTHOR]

The propagation of a Gaussian beam through a one-dimensional disordered media is studied. By employing the transfer matrix method, the localization length as a function of frequency is calculated for different values of transverse coordinate r. It is demonstrated that the localization length significantly depends on r in different frequency ranges. This result is in contrast to those reported for a plane wave incident on disordered structures in which the localization length is transversely constant. For some frequency regions, the peak of localization length is red-shifted and becomes smaller with increasing the transverse coordinate. At some frequencies, the system is in the localized state for particular values of r, while at other r values the system is in the extend regime at the same frequencies. It is observed that the quality of localization at each frequency depends on r. To quantify the localization behavior of the whole Gaussian beam, a modified localization length is defined in terms of the input and output powers of the Gaussian beam where the dependence of Anderson localization on the transverse coordinate is considered. It is suggested that this modified localization length is used in experiments performed for study of wave propagation in one-dimensional random media under illumination of laser beams. [ABSTRACT FROM AUTHOR]

Heterostructures which consist of ZnO and FeSO were deposited via electrochemical deposition (ECD) for application to solar cells. Galvanostatic ECD was used in FeSO deposition with a solution containing 100 mM NaSO and 30 mM FeSO. To alter the film properties, L(+)-tartaric acid (CHO) and lactic acid [CHCH(OH)COOH] were introduced as the complexing agents into the FeSO deposition solution. Larger film thickness and smaller oxygen content were obtained for the films deposited with the complexing agents. ZnO was deposited on FeSO by two-step pulse ECD from a solution containing Zn(NO). For the ZnO/FeSO heterostructures fabricated with/without complexing agents, rectifying properties were confirmed in the current density-voltage ( J- V) characteristics. However, photovoltaic properties were not improved with addition of both complexing agents. [ABSTRACT FROM AUTHOR]

In this study, we investigated the electrical properties and the strain of the AlGaN/AlN/GaN heterostructure field-effect transistors with Ni/Au gate electrodes using the measured capacitance-voltage (C-V) and current-voltage (I-V) characteristics and the polarization effect of the heterostructures. We found that the Ni/Au gate electrode showed a good thermal stability when the RTA temperature is below 400 °C; however, with further increase in the annealing temperature, the 2DEG sheet density under the Ni/Au Schottky contact started to decline dramatically, and the device started to exhibit bad pinch-off characteristics after a 700 °C RTA. We also found that the RTA process could change the strain and even damaged the crystal structure of the barrier layer under the gate electrodes. [ABSTRACT FROM AUTHOR]

Most of the potential applications of memristive devices adopt crossbar architecture for ultra-high density. One of the biggest challenges of the crossbar architecture is severe residue leakage current (sneak path) issue. A possible solution is introducing a selector device with strong nonlinear current-voltage ( I- V) characteristics in series with each memristor in crossbar arrays. Here, we demonstrate a novel selector device based on graphene-oxide heterostructures, which successfully converts a typical linear TaO memristor into a nonlinear device. The origin of the nonlinearity in the heterostructures is studied in detail, which highlights an important role of the graphene-oxide interfaces. [ABSTRACT FROM AUTHOR]

GaN/AlN superlattice (SL) structures working as quasi-AlGaN barrier layers for GaN-based high electron mobility transistor have been grown by metal-organic chemical vapor deposition. The influences of the SL period thickness on the electrical properties of two-dimensional electron gas (2DEG) have been investigated. It is found that the sheet carrier concentration increases as the increase in period thickness at a certain equivalent Al composition, and the electron mobility is strongly dependent on the AlN thickness in a period. We consider that AlN transits from two-dimensional growth to three-dimensional (3D) growth when AlN thickness exceeds its critical thickness. The 3D growth mode results in rough interface and surface morphology, which rapidly decreases the electron mobility due to the increase in interface roughness scattering and dislocation scattering to 2DEG. [ABSTRACT FROM AUTHOR]

A heterostructure device consisting of nitrogen-doped MgZnO and gallium-doped ZnO thin films was grown on c-plane sapphire substrate using RF plasma-assisted molecular beam epitaxy. Current-voltage and photocurrent characteristics indicate the formation of a p-n junction. Random lasing behavior with lasing modes centered at 356 nm was observed. A low-threshold current of 6 mA was determined, and an output power of 34 nW was measured at an injection current of 8 mA. The film contains columnar structures with much air gaps, which assist in light scattering to achieve necessary gain for random lasing. [ABSTRACT FROM AUTHOR]

Multiwalled carbon nanotubes (MWCNTs)/ZnO heterostructures were synthesized by two different processes: (1) gas-phase transport (GPT) and nucleation of Zn powders and (2) solution-phase transport (SPT) chemical reaction of zinc nitrate solution on the MWCNTs. Transmission electron microscopy and X-ray diffraction analysis indicated that the ZnO nanostructures on the MWCNTs from the GPT and SPT processes were poly- and single-crystal hexagonal wurtzite structure, respectively. The major photoluminescence (PL) spectra of our MWCNT/ZnO hybrid, excited at 380 nm and 550 nm, were presented. The PL intensity of the MWCNT/ZnO coaxial nanostructures behaves differently depending on the ZnO synthesis methods on the MWCNTs. The MWCNT/ZnO heterostructures synthesized using the GPT process were more efficient than those synthesized by SPT process in enhancing the PL intensity around the near-band-edge emission region. However, the emission enhancement around defect region was mostly attributed to increase in the O vacancy concentration in the ZnO on the MWCNTs during the SPT process. [ABSTRACT FROM AUTHOR]

From the capacitance-voltage curves and current-voltage characteristics of the InAlN/AlN/GaN heterostructure field-effect transistors (HFETs) with side-Ohmic contacts and normal-Ohmic contacts, two-dimensional electron gas (2DEG) electron mobility was calculated. It is found that the polarization Coulomb field scattering (PCF) is closely related to the normal-Ohmic contact processing, and the PCF was weakened by side-Ohmic contact processing in InAlN/AlN/GaN HFETs, similar to that in AlGaN/AlN/GaN HFET devices. Further, due to the stronger spontaneous polarization in the thinner InAlN barrier layer, the influence of the gate bias on the PCF in InAlN/AlN/GaN HFETs is greater than that in AlGaN/AlN/GaN HFETs. As a result, the PCF in InAlN/AlN/GaN HFETs with side-Ohmic contacts is stronger than that in AlGaN/AlN/GaN HFETs with side-Ohmic contacts. Moreover, the 2DEG electron density in the InAlN/AlN/GaN HFETs with side-Ohmic contacts is increased by more than twice compared with the 2DEG electron density in the InAlN/AlN/GaN HFETs with normal-Ohmic contacts. [ABSTRACT FROM AUTHOR]

The CoFeO/Pb(ZrTi)O bilayer films were prepared by a sol-gel process, and the influence of cycling electric polarization on the multiferroic behaviors of the bilayer films was studied. The ferroelectric polarization hysteresis loops under various choices of magnetic bias were measured by an integrating current method. The results showed that after undergoing cycling electric polarization the ferroelectric polarization of the bilayer films enhanced and the suppression of ferroelectric polarization by external magnetic bias remarkably weakened. Based on the measurements of activation energy and leakage current, we confirmed that the oxygen vacancy migration in the bilayer films occurred during cycling electric polarization. Furthermore, we analyzed the mechanism of the influence of cycling electric polarization on the multiferroic behaviors of the bilayer films and attributed it to the oxygen vacancy migration, which could cause a part of ferroelectric domains to be unpinned from the oxygen vacancies and become more active under electric field and magnetic bias. [ABSTRACT FROM AUTHOR]

Strain-driven influences on the structural and optoelectronic properties of self-assembled InAs/GaAs multilayer quantum dot (MQD) heterostructures prompted our research into the growth of thermally stable MQD samples that were functional in an emission range technically favorable for communication lasers and intermediate band gap solar cells. We also studied parameter optimization by varying growth rate, capping layer thickness, seed quantum dot (QD) monolayer coverage, and post-growth annealing. A capping combination of InAlGaAs and i-GaAs was used. This combination helps in strain compensation, favors growth of multiple QD layers, functions as a strain-driven phase separation alloy, and helps increase QD stability. Photoluminescence results showed MQD sample emissions in the technologically significant range of 1.1–1.3 μm. Post-growth annealing at high temperatures led to inter-diffusion of the constituent QD materials, generation of low minimum energy states, and greater carrier involvement in intermediate band gap structures, thereby showing that annealing is a suitable method for post-growth manipulation. For one MQD sample, the annealing temperature was found to affect structural and optoelectronic properties as well as the presence of intermediate energy states. Heterostructure stability at annealing temperatures up to 750  ∘C was found for the other samples. Transmission electron microscopy and photoluminescence results supported these findings. [ABSTRACT FROM AUTHOR]

We investigate thermal transport through a quantum spin- $1\over2$ heterostructure, in which a semi-infinite chain with two-site isotropic XY interaction couples to the other semiinfinite chain with two-site isotropic XY interaction and three-site XZX+YZY interaction. By studying the dependence of the transmission ability on the model parameters, the linear and nonlinear thermal transport properties are discussed in detail. We find the rectification effects of the heat current in the case of different magnetic fields applied at the two chains; moreover, the unidirectional heat current is observed in the presence of appropriate structure parameters. With these results, we propose such a structure to be a promising candidate for heat current rectification. [ABSTRACT FROM AUTHOR]

In this work, BiO/BaTiO heterostructure were prepared through a solid milling and annealing process. It was found that Bi dissolved in the BaTiO lattice and the chemical bond was constructed between the interface of BiO and BaTiO after annealing process. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-visible absorption spectra were used to characterize the BiO/BaTiO heterostructure. Furthermore, UV-induced catalytic activities of the BiO/BaTiO heterostructure was studied by a degradation reaction of methyl orange (MO) dye. The band gap of the BiO/BaTiO heterostructure was estimated to be 3.0 eV. Compared with pure BiO powders, the BiO/BaTiO heterostructure had a much higher catalytic activity. An excellent performance of the photocatalytic property of the BiO/BaTiO heterostructure is ascribed to high mobility of species and effective separation of photogenerated carriers driven by the photoinduced potential difference generated at the BiO/BaTiO junction interface, demonstrating that the BiO/BaTiO heterostructure is a promising candidate as a photocatalyst. [ABSTRACT FROM AUTHOR]

The viability of ultrathin amorphous Ti-Al film (∼4 nm) as a diffusion barrier layer between Cu and Si for the application in Si-based ultra-large scale integration (ULSI) has been investigated. The Cu/Ti-Al/Si heterostructures are annealed in a high vacuum at various temperatures. There is no impurity peaks in the X-ray diffraction patterns for the samples up to annealing temperature of 800 C, although the island-like grains were observed on the surface of the 800 C annealed sample due to dewetting and agglomeration of the Cu film. No inter-reactions can be found from the images of transmission electron microscopy and Ti-Al is still amorphous after high-temperature annealing. These results indicate that Ti-Al film can effectively separate Cu from Si at high temperatures, and that the amorphous ultrathin Ti-Al film can be a very good barrier layer for Cu metallization. [ABSTRACT FROM AUTHOR]

We report on the growth of all-oxide SrRuO/CoFeO/LaSrMnO and LaSrMnO/CoFeO/SrRuO heterostuctures on SrTiO(001) and MgO(001) substrates by pulsed laser deposition. Structural analyses by X-ray diffraction and transmission electron microscopy clearly indicate the preservation of epitaxial relations when the LaSrMnO layer is grown first, whereas trilayers with SrRuO at the bottom are more disordered. Both the substrate material and the deposition sequence strongly influence the formation of various structural defects such as interfacial dislocations and sub-grain structures, and this is clearly reflected by a reduction of the saturation magnetization in the top electrode. When the substrate material and the deposition sequence are correctly chosen, however, the magnetic moments of the LaSrMnO and SrRuO layers reverse independently, and the LaSrMnO layer retains bulk-like magnetic properties. [ABSTRACT FROM AUTHOR]

Surface phonon and plasmon-phonon polariton characteristics of GaAs, AlGaAs/GaAs, and GaN/AlO layered structures are investigated by means of terahertz radiation reflection spectroscopy. The strong resonant absorption peaks and selective emission of the THz radiation dependent upon the lattice composition and free electron density in these layered structures are experimentally observed and analyzed. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation into the energy transport and transient wave propagation in metamaterial tunneling structures consisting of ε-negative (ENG) and μ-negative (MNG) materials. It is proved that a conjugated matched ENG/MNG bilayer and a (zero-index-material doped) photonic crystal heterostructure can work as a sub-wavelength resonator at tunneling frequency. The tunneling modes need a certain time to achieve the steady state and the charge up characteristic time increases (nearly) exponentially with the thickness of the structures. Under the steady state, the wave in the single-negative-material structures is not evanescent, but a hybrid of a traveling wave and a reactive standing wave. The phase difference between the electric field and the magnetic field varies with the position and time. The investigation of transient wave propagation in the metamaterial tunneling structures will help us to understand the interaction process between wave and metamaterial and to design special functional apparatus. [ABSTRACT FROM AUTHOR]

Tamm plasmon polaritons (TPPs) are studied theoretically and experimentally in heterostructures and sandwiched structures with the metallic film and truncated photonic crystals. Different from conventional surface plasmon polaritons, TPPs can be realized in both the transverse electric (TE) and transverse magnetic (TM) polarizations, and they can be excited from a free space. Because of nonreciprocal electromagnetic field distributions of TPPs, the structures possess strongly nonreciprocal absorption and reflection. Moreover, two tunneling peaks or a narrow transmission band can be realized in sandwiched structures for both polarizations due to the coupling effect between two TPPs. [ABSTRACT FROM AUTHOR]

We report the effect of SrTiO thickness on the capacitance-voltage ( C- V) characteristics of (La,Sr)CoO/(Pb,La)(Zr,Ti)O/SrTiO/LaVO metal-ferroelectric-insulator-semiconductor (MFIS) epitaxial heterostructures. The C- V measurement of the heterostructure exhibited the asymmetry of capacitance with respect to gate bias. Within the given thickness range (5-30 nm), the amount of capacitance reduction at positive gate bias and the rapidness of capacitance reduction decreased with increasing SrTiO thickness, which is consistent with the C- V characteristics of conventional silicon-based MFIS capacitors. These results suggest that quantitative understanding on the electrical behavior of oxide heterostructures is possible with C- V analysis, with potentially important implications on their device applications. [ABSTRACT FROM AUTHOR]

We demonstrate the synthesis and investigate the electrical and optical characteristics of 'nanocorals' (NCs) composed of CuO/ZnO grown at low temperature through the hydrothermal approach. High-density CuO nanostructures (NSs) were selectively grown on ZnO nanorods (NRs). The synthesized NCs were used to fabricate p-n heterojunctions that were investigated by the current density-voltage ( J- V) and the capacitance-voltage ( C- V) techniques. It was found that the NC heterojunctions exhibit a well-defined diode behavior with a threshold voltage of about 1.52 V and relatively high rectification factor of ∼760. The detailed forward J- V characteristics revealed that the current transport is controlled by an ohmic behavior for V≤0.15 V, whereas at moderate voltages 1.46≤ V<1.5 the current follows a J α exp( βV) relationship. At higher voltages (≥1.5 V) the current follows the relation J α V, indicating that the space-charge-limited current mechanism is the dominant current transport. The C- V measurement indicated that the NC diode has an abrupt junction. The grown CuO/ZnO NCs exhibited a broad light absorption range that is covering the UV and the entire visible parts of the spectrum. [ABSTRACT FROM AUTHOR]

We report the results of studies of rectifying behavior, positive magneoresistance (MR), the charge-transport mechanism and the effect of an electric field on a ZnO (n)/LaPrSrMnO (LPSMO) (p)/SrNbTiO (SNTO) (n) heterostructure comprising two p-n junctions fabricated using the pulsed laser deposition technique. The heterostructure exhibits rectifying behavior over a wide temperature and field range having hysteresis in I- V behavior (forward bias) due to the tunneling of charge carriers. It is also observed that, depending on the nature of the electric field bias to n-type ZnO and SNTO, the junction resistance becomes modified, which has been explained on the basis of spin injection in the heterostructure. The observation of unconventional positive MR at room temperature has been justified on the basis of interface effects and the reduction in barrier height obtained using fitting of the I- V data by a thermionic emission model. [ABSTRACT FROM AUTHOR]

An electrically pumped random laser diode was fabricated with a MgZnO/ZnO/MgZnO double heterostructure embedded in a ZnO pn junction. Gain can be achieved at very low-threshold current owing to exciton processes. Light closed loops are formed by random multiple scattering on vertical column boundaries in the thin film. The tilted and rough mesa edge planes serve as refraction mirrors, giving rise to surface emission. [ABSTRACT FROM AUTHOR]

We have investigated the effect of post growth rapid thermal annealing on self-assembled InAs/GaAs multilayer QDs (MQD) overgrown with a combination barrier of InAlGaAs and GaAs for their possible use in photovoltaic device application. The samples were characterized by transmission electron microscopy and photoluminescence measurements. We noticed a thermally induced material interdiffusion between the QDs and the wetting layer in the MQD sample up to a certain annealing temperature. The QD heterostructure exhibited a thermal stability in the emission peak wavelength on annealing up to 700 C temperature. A phenomenological model has been proposed for this stability of the emission peak. The model considers the effect of the strain field, propagating from the underlying QD layer to the upper layers of the multilayer QD and the effect of indium atom gradient in the combination barrier layer due to the presence of a quaternary InAlGaAs layer. [ABSTRACT FROM AUTHOR]

Silica nanowires, grown via the active oxidation of a silicon substrate, have been coated with TiO using two coating methods: solution-based deposition of Ti-alkoxides and atomic layer deposition. Analysis of as-deposited and annealed films shows that it is possible to produce stable conformal coatings of either the anatase or rutile phases of TiO on nanowires with diameters greater than 100 nm when annealed between 500-600°C and 800-900°C, respectively, with annealing at higher temperatures (1050°C) producing coatings with a highly facetted rutile morphology. The efficacy of the process is shown to depend on nanowire diameter, with nanowires having diameters less than about 100 nm fusing together during solution-based coating and decomposing during TiO atomic layer deposition. The use of a suitable buffer layer is shown to be an effective means of minimizing nanowire decomposition. Finally, annealing coated nanowires under active oxidation conditions (1100°C) is shown to be an effective technique for depositing additional conformal SiO coatings, thereby providing a means of fabricating multi-layered coaxial nanostructures. [ABSTRACT FROM AUTHOR]

We present subwavelength periodic gratings achieved by employing a metallic grating waveguide heterostructure (MGWHS). The mask can be designed to make one of its diffraction order (± mth) waves resonate with the surface plasmon wave supported by the MGWHS. With a finite-difference time-domain method, we numerically demonstrate that one-dimensional periodic structure of about 60 nm feature, which is far beyond the diffraction limit, can be patterned with the interference of the 3rd diffraction order waves of the mask at a wavelength of 546 nm. The technique can also be extended to two-dimensional patterns using circularly polarized incidence and for the incidence with an angle θ. [ABSTRACT FROM AUTHOR]

Rectifying and photovoltaic properties of a LaSrCoO/Si heterostructure have been experimentally studied. The heterostructure exhibits a rectifying behavior, similar to that of the traditional diode. A photovoltaic effect is observed when the heterostructure is exposed to the laser with a wavelength of 532 nm and a power of 6 mW/mm. It is interesting to note that the peak photovoltages initially increase with decreasing temperature, followed by a decrease at T<220 K. This reveals that the photovoltaic effect can be modulated by the intrinsic properties of the LaSrCoO layer. [ABSTRACT FROM AUTHOR]