Multilayer Device Structures Research Articles

Multilayer memristive/memcapacitive devices with engineered conduction fronts

We present a novel multilayered architecture for memristive devices which provides an alternative to conventional conductive filament switching. In conventional resistive switching, conductive filaments form and extend stochastically under applied electrical bias, with longer filaments being subjected to magnified electric fields that amplify their growth rate, producing a spatially localized and highly non-uniform conduction front of filaments. This produces devices with large variations in resistive and capacitive properties that are difficult to tune. Here, we simulate a multilayered device structure with alternating ionic mobility that predicts the development of a quasi-uniform conduction front which amplifies memcapacitive properties of the device and reduces device-to-device variability. Furthermore, this novel structure is predicted to enable fine-tuned control of switching events, an important property for analog (multibit) memory and neuromorphic computing applications.

Ternary zinc complexes as electron transport and electroluminescent materials

Ternary zinc complexes: [2-(2-hydroxyphenyl)benzoxazolato]5-chloro-8-hydroxy quinolinatozinc(II) [Zn(HPB)(Clq)] and [2-(2-hydroxyphenyl)benzoxazolato]5,7-dichloro-8-hydroxyquinolinatozinc(II) [Zn(HPB)(Cl2q)]) were synthesized and characterized as electroluminescent materials. These complexes possessed high thermal stability (>300 °C) and high glass transition temperature (>150 °C). The excited state lifetime of these complexes measured by transient photo-luminescence spectroscopy showed 4.19, 11.02 ns and 2.67, 10.7 ns for Zn(HPB)(Clq) and Zn(HPB)(Cl2q), respectively. The electroluminescence spectra showed peak emission centered at 549 and 554 nm, respectively, for these materials. Both the molecules were found to be quite efficient in a multilayered device structure ITO/α-NPD/Zinc complex/BCP/Alq3/LiF/Al with yellow emission having Internationale d'Eclairage (CIE) coordinates x = 0.37 and y = 0.57 for Zn(HPB)(Clq); and x = 0.40 and y = 0.56 for the Zn(HPB)(Cl2q) complex. Electron mobilities of these molecules were found to be 3 × 10−6–7 × 10−6 s for electric field range 1000–1200 (V/cm)1/2. These molecules showed very high efficiency as electron transport layer with device structure ITO/α-NPD/5% Ir(ppy)3 doped CBP/BCP/Zn(HPB)(Clq) or Zn(HPB)(Cl2q)/LiF/Al and therefore are better electron transporters as compared to Alq3 in a device structure of ITO/α-NPD/5% Ir(ppy)3 doped CBP/BCP/Alq3/LiF/Al.

Application of solution-processed metal oxide layers as charge transport layers for CdSe/ZnS quantum-dot LEDs

We fabricated and characterized quantum-dot light emitting devices (QLEDs) that consisted of a CdSe/ZnS quantum-dot (QD) emitting layer, a hole-transporting nickel oxide (NiO) layer and/or an electron-transporting zinc oxide (ZnO) layer. Both the p-type NiO and n-type ZnO layers were formed by using sol–gel processes. All the fabricated CdSe/ZnS QLEDs showed similar electroluminescence spectra that originated from the green CdSe/ZnS QDs. However, different combinations of hole- and electron-transporting layers resulted in efficiency variations. In addition to the control of the respective concentrations of holes and electrons within a multilayer device structure, which determines the luminance and efficiency of QLEDs, the use of metal oxide layers is advantageous for long-term stability of QLEDs because they are air stable and can block the permeation of water vapor and oxygen in ambient air to a QD emitting layer. Moreover, the wet chemistry processing for their formation makes metal oxide layers attractive for low cost and/or large area manufacture of QLEDs.

MBE HgCdTe for HDVIP Devices: Horizontal Integration in the US HgCdTe FPA Industry

Molecular beam epitaxy (MBE) growth of HgCdTe offers the possibility of fabricating multilayer device structures with an almost unlimited choice of infrared sensor designs for focal-plane array (FPA) fabrication. HgCdTe offers two major advantages that explain its dominance in the infrared photon detector marketplace. The thermal generation rate per unit volume of the material is lower and the quantum efficiency for photon absorption in the infrared is higher in HgCdTe than in any competing material—it yields devices with quantum efficiencies as high as 0.99. Recently, EPIR Technologies and DRS Infrared Technologies agreed to collaborate and examine: (i) the feasibility of employing MBE HgCdTe in the fabrication of high-density vertically interconnected photodiodes (HDVIPs), which are usually fabricated with liquid-phase epitaxy material, and (ii) the potential benefits of horizontal integration, with EPIR supplying the MBE materials to DRS for device and array fabrication. The team designed and developed passivation–absorber–passivation structures that are heavily used by DRS. This paper provides an overview of the characteristics of HDVIP devices and arrays fabricated from MBE HgCdTe and the anticipated advantages of horizontal integration in the industry. Material growth, device fabrication, and test results are presented.

Molecular Layer Deposition of an Organic-Based Magnetic Semiconducting Laminate

Organic-based magnets are intriguing materials with unique magnetic and electronic properties that can be tailored by chemical methodology. By using molecular layer deposition (MLD), we demonstrate the thin film fabrication of V[TCNE: tetracyanoethylene](x), of the first known room temperature organic-based magnet. The resulting films exhibit improvement in surface morphology, larger coercivity (80 Oe), and higher Curie temperature/thermal stability (up to 400 K). Recently, the MLD method has been widely studied to implement fine control of organic film growth for various applications. This work broadens its application to magnetic and charge transfer materials and opens new opportunities for metal-organic hybrid material development and their applications in various multilayer film device structures. Finally, we demonstrate the applicability of the multilayer V[TCNE](x) as a spin injector combining LSMO, an standard inorganic magnetic semiconductor, for spintronics applications.

Super-linear rectifying property of rubrene single crystal devices

We have observed a rectifying behavior in a lateral symmetrical device structure based on rubrene single crystals. Our analysis shows that the rectifying characteristics are not due to formation of Schottky junction between the electrode and the organic semiconductor, but should be attributed to a mechanism that is similar to the super-linear operation regime in organic field-effect transistors. Furthermore, we have demonstrated that this rectifying behavior can be turned on and off via modulation of the density of space charge in the organic semiconductor/substrate interface, which essentially affects the threshold voltage ( V T). Our results demonstrate a simple approach that can potentially be used to fabricate organic rectifiers without application of multi-layer device structure such as MOSFET diode.

Solar Cells Active in Complete Darkness

A graded bandgap multi-layer solar cell device structure was designed to absorb UV, visible and IR radiation, and to incorporate impact ionisation and impurity photovoltaic effects within one device. The design was experimentally tested with a well researched material system, MOVPE grown GaAs/AlGaAs. Open circuit voltages of ∼1175 mV with highest possible FF values (0.83–0.87) and Jsc∼12 mAcm−2 have been observed [1,3]. These parameters were independently verified by measuring in five different laboratories in Europe and United States including NREL. While the work is continuing to increase short circuit current density values, these devices were tested to explore the experimental evidence of impurity PV effect, as expected from this design. Responsivity measurements and PV activity in dark conditions have been carried out to investigate impurity PV effect in these devices. Responsivity measurements indicate current collection in the infra-red region confirming the contribution from IR photons. The I–V measurements in dark conditions produce open circuit voltages exceeding 750 mV confirming the contribution from surrounding heat radiation. The new features of graded bandgap devices enable impurity PV effect to dominate and create useful charge carriers, suppressing detrimental recombination process. These experimental results will be presented in this paper.

Efficiency Improvement of Solution Processed Blue Phosphorescent Organic Light-Emitting Diodes Using an Alcohol Soluble Exciton Blocking Layer

High efficiency solution processed blue phosphorescent organic light-emitting diodes (PHOLEDs) were developed using a highly polar phosphine oxide exciton blocking material spin coated from an alcohol solution. An orthogonal film could be effectively deposited due to the insolubility of the emitting layer in alcohol solvent, and a multilayer device structure could be fabricated. The quantum efficiency of the solution processed blue PHOLEDs could be improved from 0.1 to 10.5% by using the alcohol soluble exciton blocking layer. The quantum efficiency of the blue PHOLED with spin coated exciton blocking layer was comparable to that of the blue PHOLED with vacuum deposited exciton blocking layer.

Asymmetric anthracene-based blue host materials: synthesis and electroluminescence properties of 9-(2-naphthyl)-10-arylanthracenes

A series of bulky aryl-substituted asymmetric anthracene blue host materials, 9-(2-naphthyl)-10-(3-(1-naphthyl)phenyl)anthracene, where phenyl was varied from H (5a), Me (5b), Ph (5e), and 1-Naph (5f) at the 6-position and Me (5c) at the 2-position, was synthesized by Suzuki coupling reaction between 10-(2-naphthyl)anthracene-9-boronic acid and 1-(3-iodophenyl)naphthalene derivatives. A less bulky aryl-substituted anthracene, 9-(2-naphthyl)-10-(2,5-diphenyl)phenyl)anthracene (5d), was also synthesized for comparison. All asymmetric anthracenes showed high glass transition temperatures in the range of 84–153 °C. The photophysical and electrochemical properties in solution showed that the substituent at the 10-positions of the anthracene unit did not influence the blue emission of 420 nm and HOMO–LUMO energy level (5.5–2.5 eV). However, a gradual decrease of bathochromic shift in solid state PL was observed from the increaseg of the substituent bulkiness, exhibiting 5e (18 nm) ≤ 5f (19 nm) < 5c (25 nm) ≤ 5b (26 nm) < 5a (30 nm) < 5d (41 nm), respectably. When 5a was used as a blue host material in the multilayered device structure of ITO/DNTPD/NPD/host:dopant (9%)/PyPySPyPy/LiF/Al, enhanced OLED device performance was observed, showing a luminous current efficiency of 9.9 cd A−1, power efficiency of 6.3 lm W−1 at 20 mA cm−2, deep blue color coordinates of (0.14, 0.18), and a 932 h device lifetime at L0 = 3000 cd m−2.

Fabrication and Efficiency Improvement of Soluble Blue Phosphorescent Organic Light‐Emitting Diodes Using a Multilayer Structure Based on an Alcohol‐Soluble Blue Phosphorescent Emitting Layer

Highly efficient soluble blue phosphorescent organic light emitting diodes have been developed using a multilayer device structure based on alcohol soluble blue phosphorescent emitting layer. A device structure with double emitting layer and an exciton blocking layer could be fabricated due to orthogonal film formation and a high quantum efficiency of 14.1% and a current efficiency of 30.4 cd/A were achieved in the soluble blue phosphorescent organic light emitting diodes.

Core-Shell Structured Nanowire Spin Valves

Nanowire based magnetic spin valves utilizing a core-shell device architecture about free-standing Ni nanowires have been fabricated and characterized. Devices containing sequential shell layers of CoO(10 nm)-Co(5 nm)-Cu(5 nm)-Co(5 nm) deposited through sputter deposition around the chemical-vapor-deposited Ni core nanowires exhibit a giant magnetoresistance effect of ~9%, which matches that of the corresponding planar thin film multilayer. The Ni nanowires which serve as the device platforms are obtained in diameters ranging from 100 nm through 300 nm and typical heights of 20 micrometers or greater. Since the nanowires are oriented vertically upon Si/SiO2 growth substrates they allow the creation of core-shell spin valve devices with an orientation, aspect ratio, and distribution difficult to achieve with conventional thin film methodologies. The demonstrated, fully-functional, nanowire-based spin valve establishes the viability of magnetic multi-layer device structures in the core-shell implementation. Devices of this nature have potential in various applications including high-acuity magnetic field sensing.

Growth of crystalline cobalt ferrite thin films at lower temperatures using pulsed-laser deposition technique

Cobalt ferrite thin films were grown on SiO2/Si(100) substrates using pulsed-laser deposition technique at substrate temperatures ranging from 250 to 600 °C. Thermal expansion mismatch between the film and substrate appears to have a substantial effect on the magnetic properties of the cobalt ferrite films, due to the large magnetoelastic coupling of cobalt ferrite. It was shown in this study, that polycrystalline films with (111)-preferred orientation could be prepared at substrate temperatures as low as 250 °C. The growth of crystalline cobalt ferrite films at such low temperatures indicates the potential to use cobalt ferrite for microelectromechanical systems devices and sensor applications including integration with a wider range of multilayer device structures.

Efficient multilayer red fluorescent polymer light-emitting diodes by host and guest blend system

Efficient red light polymer light-emitting diodes are obtained by energy transfer from host and guest blend system. Two kinds of blend systems, SPB02:(10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1] benzopyrano [6,7,8-ij]quinolizin-11-one)(C545T):4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and SuperYellow (SY):poly(3,3 ′′′-didodecylquaterthiophene) (PQT-12), are used as the active layers. C545T is a green emitter, acting as an assistant dopant to improve the energy transfer from blue SPB02 host to red DCJTB. The efficiency can achieve to 3.3 cd/A for DCJTB and 3.5 cd/A for PQT-12 system by multilayer device structure with 2,2,2 ′′-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) (TPBi) as electron transport layer. The SuperYellow:PQT-12 device shows great luminance of 16,930 cd/m 2.

Modeling Disorder in the Crystal Structure of the α Polymorph of Alq3

Alq3, tris(quinolin-8-olato)aluminum(III) is used in the electron transport and/or electron-injecting layer in multilayer organic light-emitting diode device structures. In recent years, five crystalline phases (α, β, γ, δ, and e) of Alq3 have been identified. A structure of the α form, containing the meridional isomer, has been reported in literature based on powder XRD data. Single crystals of α Alq3 have been found to have essentially the same unit cell parameters [triclinic, space group $$P \bar {1}$$ , a = 6.2455(10) A, b = 12.8710(18) A, c = 14.739(3) A, α = 69.890(6)°, β = 89.464(5)°, and γ = 82.520(13)°] as reported previously from powder XRD: triclinic, space group $$P \bar {1}$$ , a = 6.2586(8) A, b = 12.914(2) A, c = 14.743(2) A, α = 109.66(1)°, β = 89.66(1)°, and γ = 97.68(1)°. The single-crystal XRD structure for α Alq3, however, appeared to be present mostly as the facial isomer, with a high degree of disorder. Although the structure obtained by single-crystal XRD appears to be predominantly or entirely facial, various spectroscopy results (particularly NMR) are more consistent with a meridional composition. To resolve the α Alq3 isomerism contradiction, we have reconsidered the structural conclusions that were drawn from the single crystal XRD. Based on modeling results, the hypothesis is that α Alq3 could be a disordered analog of e Alq3, with meridional conformation. Disorder in the crystal structure of the α polymorph of Alq3 was modeled with help from various spectroscopic techniques.

Amorphous selenium and its alloys from early xeroradiography to high resolution X‐ray image detectors and ultrasensitive imaging tubes

AbstractWe describe the progress in the science and technology of stabilized a‐Se from its early use in xerography and xeroradiography to its present use in commercial modern flat panel X‐ray imagers and ultrasensitive video tubes which utilize impact ionization of drifting holes. Both electrons and holes can drift in stabilized a‐Se, which is a distinct advantage since X‐ray photogeneration of charge carriers occurs throughout the bulk of the photoconductive layer. An a‐Se photoconductor has to be operated at high fields to ensure that the photogeneration efficiency is sufficiently large to provide reasonable X‐ray sensitivity. However, at high fields, the dark current is unacceptably large in simple metal/a‐Se/metal devices, and special multilayer device structures need to be designed. The dark current decays with time and increases with the nominal applied field. The reduction of the dark current to a tolerable level was one of the key factors that lead to the commercialization of a‐Se X‐ray detectors. We discuss the origin of the dark current, and highlight some of the current challenges in the design of next generation detectors. We also discuss the origin of impact ionization in a‐Se, and its fruitful utilization in ultrasensitive imaging devices, including the Harpicon, which are likely to lead to new high detective quantum efficiency detectors.

All-solution-processed blue small molecular organic light-emitting diodes with multilayer device structure

All-solution-processed multilayer blue small molecular organic light-emitting diodes are fabricated by blade coating method. Fluorescent blue host,1-(7-(9,9′-bianthracen-10-yl)- 9,9-dioctyl-9H-fluoren-2-yl)pyrene, and blue dopant, 4,4′-(1E,1′E)-2,2′-(naphthalene-2,6-diyl)bis(ethene-2,1-diyl)bis(N,N-bis(4-hexylphenyl)aniline), are used to achieve good solubility and pinhole-free thin film by solution process. The multilayer device structure with hole/electron transport layer is achieved by blade coating method without the dissolution problem between layers. The efficiency of the all-solution-processed device is 4.8 cd/A at 1200 cd/m 2, close to that by thermal deposition in high vacuum chamber. The device performance is optimized with the annealing temperature of TPBi layer at 50 °C.

P‐166: All‐Solution‐Processed Multilayered Small‐Molecule OLEDs with High Device Efficiency

AbstractAll‐solution‐processed multilayer blue small molecular organic light‐emitting diodes are fabri‐cated by blade coating method. Fluorescent blue host,1‐(7‐(9,9′‐bianthracen‐10‐yl)‐ 9,9‐dioctyl‐9H‐fluoren‐2‐yl)pyrene, and blue dopant, 4,4′‐(1E,1′E)‐2,2′‐(naphthalene‐2,6‐diyl)bis(ethene‐2,1‐diyl)bis(N,N‐bis(4‐hexylphenyl) aniline), are synthesized to achieve good solubility and pinhole‐free thin film by solution process. The multilayer device structure with hole/electron transport layer is achieved by blade coating method without the dissolution problem between layers. The efficiency of the all‐solution‐processed device is 4.8 cd/A at 1,200 cd/m2, close to that by thermal deposition in high vacuum chamber.

Research on Effective Moat Configuration for Nb Multi-Layer Device Structure

One of the most important obstacles to realize high-end single-flux quantum (SFQ) digital circuits is how to eliminate the effects of trapped magnetic flux. In this study, we investigated various moat structures for multi-layer devices with multiple ground planes to determine which moat configuration was most suitable for a cell library. We designed various test circuits with different moat structures and evaluated their effectiveness by measuring the I-V characteristics of superconducting quantum interface device (SQUID) test circuits. Tests were carried out at several milli-gauss to evaluate the moats. We concluded that the most suitable moat configuration for a multi-layer device structure was obtained by surrounding the SFQ circuits with basic via moats in all ground layers of bias ports with ground contacts and by using narrow rectangular moats in only the main ground plane.

High efficient organic light emitting diodes using new 9,10-diphenylanthracene derivatives containing bulky substituents on 2,6-positon

Novel blue emitters with two bulky substituents on the 2,6-position of 9,10-diphenylanthracene (DPAN), which can provide effective hindrance to the intermolecular packing, were synthesized, characterized, and used in the fabrication of organic light emitting diodes (OLEDs). DSC thermograms and theoretical calculations of 3-dimensional structures of Ph-DPAN, Flu-DPAN and TPA-DPAN, support that they have an amorphous and non-coplanar structure. With application of these newly non-doped, blue emitting materials in a multilayer device structure, it is possible to achieve a current efficiency of 4.6–5.8 cd/A for DPAN derivatives.

Effect of metal cathode reflectance on the exciton-dissociation efficiency in heterojunction organic solar cells

Effects of different cathode metals, such as aluminum, calcium, and silver, and difference in their reflectivity on the photocurrent generation in pentacene-C60 heterojunction solar cell are presented. Using optical transfer matrix calculations, we find that metal reflectivity has a profound impact on the electrical field confinement within the multilayer device structures. Silver as cathode offers better optical-field confinement close to the pentacene-C60 interface over generally preferred aluminum cathode. External quantum efficiency measurements confirm higher exciton dissociation efficiency and high photocurrent generation ability of silver over aluminum cathode making the choice of cathode metal an important parameter in device optimization.