Search

The high-temperature resilience of quantum-dot (QD) laser materials is exploited to realize a broad spectral bandwidth emitter in the near infrared. For an InAs/GaAs-based QD-superluminescent light emitting diode (SLEDs), we introduced a 2000 μm long, 5 μm width ridge waveguide that is tilted by 7° and composed of eight multi-sections. With increased temperature operation over 160 °C, the spectral bandwidth is dramatically increased by thermally excited carrier transition in ES1 and ES2. Additionally, the positive net-modal gain is demonstrated at the high operating temperatures, and this is exploited in the QD-SLEDs operating at 180 °C, which exhibit a −3 dB linewidth of 270 nm and a power of 0.34 mW. The simplicity of this approach, utilizing heat alone, is contrasted with other approaches for the extremely broad spectral bandwidth emitter.

Thermoelectric devices convert thermal energy to electrical energy and are particularly well-suited for energy harvesting from waste heat. Even as the number of electronic devices used in daily life proliferates, technical advances diminish the average power such devices require to perform a given function. Localized thermal gradients that abound in our living environments, despite having modest energy densities, are therefore becoming increasingly viable and attractive to power such devices. With this motivation, we report the design, fabrication, and characterization of single-wall carbon nanotube thermoelectric devices (CNT-TDs) on flexible polyimide substrates as a basis for wearable energy converters. Our aqueous-solution-based film fabrication process could enable readily scalable, low-cost TDs; here, we demonstrate CNT-hydroxypropyl cellulose (HPC) composite thermoelectric films by aerosol jet printing. The electrical conductivity of the composite films is controlled through the number of CNT/HPC layers printed in combination with control of the annealing conditions. The HPC initially disperses the CNTs in deionized water, the greenest of solvents, and is subsequently partially eliminated from the film by annealing, with concomitant morphological changes that we characterized by TEM. HPC removal is key to obtaining good electrical conductivity (0.94 to 1.10 × 105 S/m) and Seebeck coefficients (36 to 43 μV/K). We also report a power factor of 208 μW m−1 K−2 for a CNT-TD composed of 15 layers of CNT/HPC, promising performance for CNT-based flexible TDs that are deposited from aqueous solution, stable in air, and require no additional doping or sorting processes.

When we applied colloidal quantum dots (QDs) for quantum dot light emitting diodes, it was well known that shell thickness played an important role in core protection, confinement of electrons and holes, and charge injection efficiency. However, although the shell thickness dependence of electroluminescence properties was reported, carrier injection efficiency has not been discussed in detail. In this paper, we investigated the effect of shell thickness on the carrier injection efficiency that was evaluated by photoelectrochemical measurements. By comparing the product of internal quantum yield of photoluminescence and the evaluated carrier injection efficiency with external quantum efficiency (EQE) for QDs with various shell thicknesses, we found that the optimal shell thickness for increasing EQE is determined by the balance between protection of QD's surface and carrier injection efficiency.

We report the engineering of air-voids embedded in GaAs-based photonic crystal surface emitting lasers realised by metalorganic vapour-phase epitaxy regrowth. Two distinct void geometries are obtained by modifying the photonic crystal grating profile within the reactor prior to regrowth. The mechanism of void formation is inferred from scanning transmission electron microscopy analysis, with the evolution of the growth front illustrated though the use of an AlAs/GaAs superlattice structure. Competition between rapid lateral growth of the (100) surface and slow diffusion across higher index planes is exploited in order to increase void volume, leading to an order of magnitude reduction in threshold current and an increase in output power through an increase in the associated grating coupling strength., 7 pages, 4 figures

The current-voltage characteristics of resistive random-access memory cells with Ti/Pr0.7Ca0.3MnO3−δ (PCMO)/Pt stack structures were investigated. The PCMO layer on Pt had a mixed polycrystalline and amorphous structure. The cells displayed interface-type and filament-type resistive switching (RS) depending on the PCMO layer thickness. The interface-type RS was attributed to the migration of oxygen ions, which caused a redox reaction at the Ti/PCMO interface and the formation of a TiOx layer. For filament-type RS, a forming process occurred and this indicated the formation of a conductive filament in the PCMO layer. After forming, the cells showed bipolar and continuous RS similar to interface-type RS. This indicated that both the formation of a conductive filament in the PCMO layer and the redox reaction at the Ti/PCMO interface occur in the same cell. Finally, a qualitative model for the observed RS phenomenon is discussed based on conventional interface-type RS.The current-voltage characteristics of resistive random-access memory cells with Ti/Pr0.7Ca0.3MnO3−δ (PCMO)/Pt stack structures were investigated. The PCMO layer on Pt had a mixed polycrystalline and amorphous structure. The cells displayed interface-type and filament-type resistive switching (RS) depending on the PCMO layer thickness. The interface-type RS was attributed to the migration of oxygen ions, which caused a redox reaction at the Ti/PCMO interface and the formation of a TiOx layer. For filament-type RS, a forming process occurred and this indicated the formation of a conductive filament in the PCMO layer. After forming, the cells showed bipolar and continuous RS similar to interface-type RS. This indicated that both the formation of a conductive filament in the PCMO layer and the redox reaction at the Ti/PCMO interface occur in the same cell. Finally, a qualitative model for the observed RS phenomenon is discussed based on conventional interface-type RS.

Single-electron effects have been widely investigated as a typical physical phenomenon in nanoelectronics. The single-electron effect caused by trap sites has been observed in many devices. In general, traps are randomly distributed and not controllable; therefore, different current--voltage characteristics are observed through traps even in silicon transistors having the same device parameters (e.g., gate length). This allows us to use single-electron effects as fingerprints of chips. In this study, we analyze the single-electron effect of traps in conventional silicon transistors and show the possibility of their use as fingerprints of chips through image recognition algorithms. Resonant tunneling parts in the Coulomb diagram can also be used to characterize each device. These results show that single-electron effects can provide a quantum version of a physically unclonable function (quantum-PUF)., Comment: 5 pages, 8 figures

[ABSTRACT] The alignment of organic layer and metal layer energy levels within organic semiconductor devices is critical for efficient charge injection/collection at the electrodes. The electronic structure at the interface is usually examined using ultraviolet photoelectron spectroscopy (UPS) as the film thickness of the organic layer is built up incrementally (bottom-up method). As such, the morphology and crystallinity of the films that are examined using this technique may be different from the films used in actual devices because these properties are sensitive to the way in which the films are prepared. We have prepared [6]phenacene thin films with standing and lying molecular orientations on naturally oxidized silicon (SiO2) and highly oriented pyrolytic graphite (HOPG) surfaces, respectively. UPS measurements were performed on films that were incrementally decreased in thickness using gas cluster ion beam (GCIB) sputtering (top-down method). The spectral shapes and ionization energies of the films that were formed using the bottom-up and top-down methods were similar. Importantly, the characteristic features of a monolayer on the HOPG substrate were also observed when a thick film (i.e., not built up in layers using the bottom-up method) was thinned down by GCIB sputtering. We have shown that UPS combined with GCIB sputtering is a useful technique for examining the energy level alignment of interfaces within films that are fabricated using conditions similar to those used for real devices.

Threshold switches, which typically exhibit an abrupt increase in current at an onset voltage, have been used as selector devices to suppress leakage current in crosspoint arrays of two-terminal resistive switching memory devices. One of the most important metrics for selector devices is the leakage or low-voltage current, which limits the maximum achievable size of the crosspoint memory array. Here, we show that for self-heating-triggered threshold switches, there is an intrinsic lower limit to the leakage current resulting from the need to avoid an electric field-induced breakdown of the active material. We provide a quantitative theoretical estimate of this limit for NbOx threshold switches, one of the most widely studied selectors, and provide a plausible explanation for the experimentally observed leakage currents in NbOx. Our results provide some guidelines for achieving minimum leakage currents in threshold switches.

The molecular orientation and the electronic structure at molecular donor/acceptor interfaces play an important role in the performance of organic optoelectronic devices. Here, we show that graphene substrates can be used as templates for tuning the molecular orientation of pentacene (PEN), selectively driving the formation of either face-on or edge-on arrangements by controlling the temperature of the substrate during deposition. The electronic structure and morphology of the two resulting C60/PEN heterointerfaces were elucidated using ultraviolet photoelectron spectroscopy and atomic force microscopy, respectively. While the C60/PEN (edge-on) interface exhibited a vacuum level alignment, the C60/PEN (face-on) interface exhibited a vacuum level shift of 0.2 eV, which was attributed to the formation of an interface dipole that resulted from polarization at the C60/PEN boundary.The molecular orientation and the electronic structure at molecular donor/acceptor interfaces play an important role in the performance of organic optoelectronic devices. Here, we show that graphene substrates can be used as templates for tuning the molecular orientation of pentacene (PEN), selectively driving the formation of either face-on or edge-on arrangements by controlling the temperature of the substrate during deposition. The electronic structure and morphology of the two resulting C60/PEN heterointerfaces were elucidated using ultraviolet photoelectron spectroscopy and atomic force microscopy, respectively. While the C60/PEN (edge-on) interface exhibited a vacuum level alignment, the C60/PEN (face-on) interface exhibited a vacuum level shift of 0.2 eV, which was attributed to the formation of an interface dipole that resulted from polarization at the C60/PEN boundary.

The concept of the tricolor superstructure (TCS), which is a triple-layer stack structure containing two types of high dielectric constant (high-k) layers (designated HK1 and HK2) and a SiO2 layer, is proposed to control the moment and the polarity of the interface dipole layer that are induced at the high-k/SiO2 interfaces. The interface dipole layer is formed by oxygen ion migration from the layer with higher oxygen areal density (σ) to that with lower σ. When the two high-k materials are selected with the order of σHK1 > σSiO2 > σHK2 in a SiO2/HK2/HK1/SiO2 TCS, the dipole directions of the interface dipole layers at the SiO2/HK2 and the HK1/SiO2 interfaces are aligned. Additionally, in the transposed SiO2/HK1/HK2/SiO2 TCS, the total polarity is reversed. The concept is demonstrated using Al2O3 and Y2O3 layers because they offer the order of σAl2O3 > σSiO2 > σY2O3. The two stacking sequence samples composed of SiO2/Y2O3/Al2O3/SiO2 and SiO2/Al2O3/Y2O3/SiO2 that were fabricated using superlattice technique by pulsed laser deposition obviously show opposite dipole polarities. Increasing repetition of the deposited TCS unit also causes the dipole moments to increase systematically. The TCS technique enables control of the properties of the interface dipole layer at high-k/SiO2 interfaces in amorphous systems.The concept of the tricolor superstructure (TCS), which is a triple-layer stack structure containing two types of high dielectric constant (high-k) layers (designated HK1 and HK2) and a SiO2 layer, is proposed to control the moment and the polarity of the interface dipole layer that are induced at the high-k/SiO2 interfaces. The interface dipole layer is formed by oxygen ion migration from the layer with higher oxygen areal density (σ) to that with lower σ. When the two high-k materials are selected with the order of σHK1 > σSiO2 > σHK2 in a SiO2/HK2/HK1/SiO2 TCS, the dipole directions of the interface dipole layers at the SiO2/HK2 and the HK1/SiO2 interfaces are aligned. Additionally, in the transposed SiO2/HK1/HK2/SiO2 TCS, the total polarity is reversed. The concept is demonstrated using Al2O3 and Y2O3 layers because they offer the order of σAl2O3 > σSiO2 > σY2O3. The two stacking sequence samples composed of SiO2/Y2O3/Al2O3/SiO2 and SiO2/Al2O3/Y2O3/SiO2 that were fabricated using superlattice techniqu...

We describe a high angle annular dark field scanning transmission electron microscopy study of a self-assembled InAs-GaAs quantum dot (QD) laser sample providing insight into the micro-structure of the QD ensemble. A size distribution anisotropy of the QDs is observed in the two orthogonal (110) planes, and this structural information is used to develop a density of states model for the QD ensemble which is shown to be in strong agreement with a range of optical spectroscopic measurements. This link between the micro-structure and optical properties allows routes to QD device simulation. We go on to discuss how changes to the micro-structure would affect the density of states and hence laser performance.We describe a high angle annular dark field scanning transmission electron microscopy study of a self-assembled InAs-GaAs quantum dot (QD) laser sample providing insight into the micro-structure of the QD ensemble. A size distribution anisotropy of the QDs is observed in the two orthogonal (110) planes, and this structural information is used to develop a density of states model for the QD ensemble which is shown to be in strong agreement with a range of optical spectroscopic measurements. This link between the micro-structure and optical properties allows routes to QD device simulation. We go on to discuss how changes to the micro-structure would affect the density of states and hence laser performance.

We investigated the influence of heat treatment on the crystallographic structure and ferroelectric phase transition behavior of barium titanate (BaTiO3, BT) nanocubes assembled into highly ordered monolayers, using tip-enhanced Raman spectroscopy (TERS), temperature-dependent micro-Raman spectroscopy, and scanning transmission electron microscopy (STEM). TER spectra from individual BT nanocubes with the size of about 20 nm were obtained with a side-illumination optical setup, and revealed that heat treatment enhances their tetragonality. The result of temperature-dependent micro-Raman spectroscopy showed that the ferroelectric phase transition behavior of the monolayers becomes similar to that of bulk BT through heat treatment in spite of their thickness. STEM observation for the cross-section of the heated BT nanocube monolayer showed that amorphous layers exist at the interface between BT nanocubes in face-to-face contact. These results indicate that the tetragonal crystal structure of BT nanocubes is stabilized by heat treatment and the formation of the interfacial amorphous layer during heat treatment may be a key to this phenomenon.

Niobium oxide (NbOx) two-terminal threshold switches are potential candidates as selector devices in crossbar memory arrays and as building blocks for neuromorphic systems. However, the physical mechanism of NbOx threshold switches is still under debate. In this paper, we show that a thermal feedback mechanism based on Poole-Frenkel conduction can explain both the quasi-static and the transient electrical characteristics that are experimentally observed for NbOx threshold switches, providing strong support for the validity of this mechanism. Furthermore, a clear picture of the transient dynamics during the thermal-feedback-induced threshold switching is presented, providing useful insights required to model nonlinear devices where thermal feedback is important.

During plasma processing, low-k dielectrics are exposed to high levels of vacuum ultraviolet (VUV) radiation that can cause severe damage to dielectric materials. The degree and nature of VUV-induced damage depend on the VUV photon energies and fluence. In this work, we examine the VUV-absorption spectrum of low-k organosilicate glass using specular X-ray reflectivity (XRR). Low-k SiCOH films were exposed to synchrotron VUV radiation with energies ranging from 7 to 21 eV, and the density vs. depth profile of the VUV-irradiated films was extracted from fitting the XRR experimental data. The results show that the depth of the VUV-induced damage layer is a function of the photon energy. Between 7 and 11 eV, the depth of the damaged layer decreases sharply from 110 nm to 60 nm and then gradually increases to 85 nm at 21 eV. The maximum VUV absorption in low-k films occurs between 11 and 15 eV. The depth of the damaged layer was found to increase with film porosity.

Threshold switches based on niobium oxide (NbOx) are promising candidates as bidirectional selector devices in crossbar memory arrays and building blocks for neuromorphic computing. Here, it is experimentally demonstrated that the electrical characteristics of NbOx threshold switches can be tuned by engineering the thermal insulation. Increasing the thermal insulation by ∼10× is shown to produce ∼7× reduction in threshold current and ∼45% reduction in threshold voltage. The reduced threshold voltage leads to ∼5× reduction in half-selection leakage, which highlights the effectiveness of reducing half-selection leakage of NbOx selectors by engineering the thermal insulation. A thermal feedback model based on Poole-Frenkel conduction in NbOx can explain the experimental results very well, which also serves as a piece of strong evidence supporting the validity of the Poole-Frenkel based mechanism in NbOx threshold switches.

We demonstrated transparent and flexible heaters based on single-walled carbon nanotubes (SWNTs) using solution and room temperature processing. The direct film fabrication and curing onto a plastic substrate requiring no transfer processing enable a low-cost potential and large-scale fabrication of devices while restraining defects and damage to the transparent and flexible SWNT heaters. The developed ultra-thin heaters show high saturation temperature and extremely high response speed, as indicated by reaching over 100 °C within 1 s at an input voltage of 7 V. The temperature-power performance achieved 187 °C/W·cm2. The transparent and flexible heater with low driving voltage and high response speed is expected to have innovative thermal applications such as portable medical sensors and devices.

Electrical transport in anodized single titania nanotube (TNT) free from any structural effects of titania nanotube array (TNA) was investigated. An anodized TNA was disassembled into single TNTs with two-step anodization technique. Then, single TNT bridges between gold electrodes with a gap of 500 nm were prepared by dielectrophoretic alignment. Quantitative assessment of electron mobility inside single anatase and rutile TNT was carried out by 2-probe current-voltage measurement and analysis based on a metal-semiconductor-metal circuit model with Schottky barriers. Our approach to intrinsic electrical transport of single nanotube is quite effective for understanding the electronic and optical properties of TNA.

We present simultaneous measurement of femtosecond time-resolved nonlinear phase and amplitude changes around the excitonic resonance of the AlGaAs∕GaAs quantum well using polarization-division Sagnac interferometer. The nonlinear complex refractive index of the semiconductor material for all-optical devices is successfully determined from directly measured nonlinear phase and amplitude changes.

The time-resolved photoluminescence of bound excitons in bulky single crystalline CuInS2 grown by the traveling heater method is investigated. It is found that radiative decay of the bound exciton at 1.530 eV is exponential with two characteristic decay times while that of the bound excitons at 1.525 and 1.520 eV is well-represented by a single exponent at low temperatures. The radiative lifetimes of the bound excitons at 1.530, 1.525, and 1.520 eV are obtained to be 500 ps, 2.1 ns, and 3.5 ns, respectively. According to our estimates, a neutral charge is to be assigned to the defect centers associated with the observed bound excitons.

Charge neutralization reaction in ionic salt of Fe2+C22− is found to produce carbon-skinned Fe nanocrystals. FeC2 is formed as an intermediate product in the reaction of FeCl2 solved in acetonitrile with CaC2 fine powder and also able to be isolated as black nanocrystals. Heating of FeC2 at temperature higher than 250 °C induces segregation of metallic iron. The segregated carbons grow as graphitic sheets parallel to the growing Fe lattice plane. This direct bonding is due to an accidental matching of the Fe–Fe distance (2.866 A) with that of the C1–C4 distance (2.842 A) of the hexagonal rings in graphite. The x-ray diffraction pattern indicates that the particles are composed of α-Fe and graphitic carbon. The thickness of the skin is almost constant as thick as 3.5 nm independent of the body size. The particles with an average size of 30 nm exhibit temperature dependence of the magnetic cohesive force as function of T−0.275.

During plasma processing, low-k dielectrics are exposed to high levels of vacuum ultraviolet (VUV) radiation emitted from the plasma. The porous structure of these materials makes them more sensitive to modification because of their low density and consequently deep penetration of active species into the film. Here, we investigate the changes to electrical properties of porous low-k dielectrics as a function of porosity after VUV irradiation. Organosilicate low-k films of porosities between 30% and 50% were exposed to synchrotron VUV radiation at 8 eV with a fluence of approximately 5 × 1014 photons/cm2. Capacitance-voltage measurements showed an increase in the dielectric constant along with a flat-band voltage shift. FTIR results show methyl depletion as well as water uptake after VUV treatment. These show that deterioration of the electrical properties after VUV exposure and the degree of damage are found to be higher for the more porous films.