Improved yield in molecular electronic devices using amino-style molecules

Molecular electronic devices were fabricated with amino-style derivatives as redox-active components. These molecules are amphiphilic to allow monolayer formation by the Langmuir–Blodgett (LB) method, and this LB monolayer is inserted between two metal electrodes. On measuring the current–voltage (I–V) characteristics, it was found that the Al/amino style LB monolayer/Al devices show remarkable hysteresis and switching behavior, so that they can be used as memory devices at ambient conditions, when an aluminum oxide layer exists on the bottom electrode. From the results of I–V measurements, we acquired values of the switching voltage and some large on/off ratios in the case of the ASBC-18 molecule. Also, we improved the yield of the molecular electronic device by reducing the area of the device and by inserting a Ti protecting layer between the top metal electrode and the amino style LB monolayer.     

The application of conventional photolithography to microscale organic resistive memory devices

We demonstrate the application of conventional photolithography to fabricate organic memory devices in an array structure with a cell area of 4 × 4 μm² without damaging the underlying organic memory layer. Applying photolithography to organic electronic devices is not trivial because the solvents used during lithography may dissolve and damage the previously coated organic layers. The application of photolithography to our organic devices was possible because of the introduction of polymethyl methacrylate (PMMA)/polyvinyl alcohol (PVA) onto the memory active layer, where PMMA functions as a buffer layer to prevent dissolution of the PVA layer during developing process, and PVA acts as a striped layer during metal lift-off process. Embedded Al bottom electrodes were particularly constructed to minimize the switching failure. The completed organic memory devices exhibited typical unipolar switching behavior and excellent memory performance in terms of their statistical memory parameters (ON and OFF currents and threshold voltages), ON/OFF ratio (>10²), endurance (>230 cycles), and retention (>10⁴ s). This convenient photolithography patterning technique is applicable for the further scaling of many types of organic devices.     

Conduction mechanism of metal-TiO2–Si structures

The conduction model has been proposed for the metal-TiO₂–Si (MIS) structures. Rutile films have been prepared on Si substrates by magnetron sputtering of TiO₂ target and annealing in the air at temperatures T?=?800 and 1050 K. The current-voltage (CVC) and capacitance-voltage characteristics of the structures have been measured over the range of T?=?283–363 K. At positive potentials on the gate, the conductivity of the MIS structures is determined by the space charge-limited current in the dielectric layer.     

The importance of the series resistance in calculating the characteristic parameters of the Schottky contacts

Cd/p-Si Schottky barrier diodes (SBDs) with and without the native oxide layer have been fabricated to determine the importance of the fact that the series resistance value is considered in calculating the interface state density distribution (ISDD) from the forward bias current-voltage (I-V) characteristics of the Cd/p-Si SBDs. The statistical analysis yielded mean values of 0.71 ± 0.02 eV and 1.24 ± 0.12 for the BH and ideality factor of the Cd/p-Si SBDs (15 dots) without the native oxide layer (MS), respectively, and mean values of 0.79 ± 0.02 eV and 1.36 ± 0.06 eV for the Cd/p-Si SBDs (28 dots) with the native oxide layer (metal-insulating layer-semiconductor (MIS)). The interface state density (Nss) distributions of the devices were calculated taking into account their series resistance values. At the same energy position near the top of the valence band, the interface state density values without taking into account the series resistance value of the devices are almost one order of magnitude larger than Nss obtained taking into account series resistance value.     

Reversible multi polyelectrolyte layers on gold nanoparticles

The fabrication of semiconducting functional layers using low-temperature processes is of high interest for flexible printable electronics applications. Here, the one-step deposition of semiconducting nanoparticles from the gas phase for an active layer within a thin-film transistor is described. Layers of semiconducting nanoparticles with a particle size between 10 and 25?nm were prepared by the use of a simple aerosol deposition system, excluding potentially unwanted technological procedures like substrate heating or the use of solvents. The nanoparticles were deposited directly onto standard thin-film transistor test devices, using thermally grown silicon oxide as gate dielectric. Proof-of-principle experiments were done deploying two different wide-band gap semiconducting oxides, tin oxide, SnO _x , and indium oxide, In₂O₃. The tin oxide spots prepared from the gas phase were too conducting to be used as channel material in thin-film transistors, most probably due to a high concentration of oxygen defects. Using indium oxide nanoparticles, thin-film transistor devices with significant field effect were obtained. Even though the electron mobility of the investigated devices was only in the range of 10^?6?cm^2?V^?1?s^?1, the operability of this method for the fabrication of transistors was demonstrated. With respect to the possibilities to control the particle size and layer morphology in situ during deposition, improvements are expected.     

Studies of electrochemically grafted thin organic layers on inorganic surfaces with infrared spectroscopic ellipsometry

We present IR spectroscopic ellipsometry (IRSE) measurements of electrochemically grafted organic ultrathin films down to monolayer thickness. The formation of organic layers of 4-methoxybenzene (anisole) on TiO₂, Au, and Si(111) surfaces was confirmed from observation of the respective absorption bands in the ellipsometric spectra. We discuss the orientation of molecules in a thin film on an Au substrate and in a thick film grafted on a TiO₂ substrate. The appearance of silicon oxide related bands in the IRSE spectra shows that oxidation is a side reaction of the electrochemical grafting on Si substrates. These results demonstrate the potential of IRSE application in studies of interfacial structures and thin film engineering. PACS 68.35.Ja; 82.45.Wx; 82.45.Jn     

ToF-SIMS study of gold/phthalocyanine interface

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to characterize the interface between thin molecular film (metal-free phthalocyanine, H₂Pc) and top deposited gold layer. Such inorganic-on-organic interface is widely employed in molecular electronics and considered in numerous studies on contact phenomena in phthalocyanine-based devices. Analysis of ToF-SIMS data during the depth profiling and comparison of two different methods used for deposition of top metal layer (thermal evaporation vs. magnetron sputtering) reveal peculiar structural features of this interface. In particular, oxidation of molecules near the interface is accompanied by the mutual diffusion of components when Au layer is deposited by thermal evaporation. This diffusion leads to the saturation of metallic layer with organic species and hence to its vertical expansion. Both geometry and chemistry of Au/H₂Pc interface can be modified by varying the deposition method.     

Investigation into the cause of the exfoliation of ferroelectric PZT films from the substrate by auger electron spectroscopy

It is shown that film exfoliation in a PZT(400 nm)/Ir(50 nm)/TiO₂(10 nm)/Ti(10 nm)/SiO₂/Si system upon annealing in air at 650°C for 20 min occurs at the PZT/Ir interface along a thin intermediate oxide layer (Pb, Ir)O _x containing a significant amount of lead as compared to PZT. This layer is not continuous and exists below the PZT film at sites where the flaking does not occur. The thickness of the intermediate layer estimated by the time of ion sputtering is about 20 nm. This value and the layer composition do not change with increasing annealing time.     

The insert of zinc oxide thin film in indium tin oxide anode for organic electroluminescence devices

Zinc oxide films were prepared by rf magnetron sputtering on glass substrates with designed ZnO target using high-purity of zinc oxide (99.99%) powder. Systematic study on dependence of target-to-substrate distance (D_ts) on structural, electrical and optical properties of the as-grown ZnO films was mainly investigated in this work. XRD showed that highly preferred ZnO crystal in the [0 0 1] direction was grown in parallel to the substrate, while the D_ts did not effect to the peak position of XRD. With decreasing D_ts, the growth rate is increased while the electrical resistivity as well as crystal size in the ZnO films was decreased. The XPS data showed that the O/Zn ratio in ZnO films was increased with increasing D_ts in the films. The as-grown ZnO films have an average transmittance of above 85% at the visible region. The optical band gap of the as-grown ZnO films was changed from 3.18 to 3.36 eV with D_ts. With decreasing D_ts, the electrical resistivity was decreased, while the growth rate was increased.A bilayer is used as an anode electrode for organic electroluminescent devices. The bilayer consists of an ultrathin ZnO layer adjacent to a hole-transporting layer and an Indium tin oxide (ITO) outerlayer. We tried to bring low the barrier between the devices as deposited ZnO films on ITO substrates. We fabricated the organic EL structure consisted of Al as a cathode, Al₂O₃ as an electro transport layer, Alq₃ as a luminously layer, TPD as a hole transport layer and ZnO (1 nm)/ITO (150 nm) as an anode. The result of this experiment was not good compared with the case of using ITO, nevertheless, at this structure we obtained the lowest turn-on voltage as the value of 19 V and the good brightness (6200 cd/m²) of the emission light from the devices. Then the quantum efficiency was to be 1.0%.     

Nanostructure characterization of high k materials by spectroscopic ellipsometry

In this work, the optical and structural properties of high k materials such as tantalum oxide and titanium oxide were studied by spectroscopic ellipsometry, where a Tauc-Lorentz dispersion model based in one (amorphous films) or two oscillators (microcrystalline films) was used. The samples were deposited at room temperature by radio frequency magnetron sputtering and then annealed at temperatures from 100 to 500 °C. Concerning the tantalum oxide films, the increase of the annealing temperature, up to 500 °C does not change the amorphous nature of the films, increasing, however, their density. The same does not happen with the titanium oxide films that are microcrystalline, even when deposited at room temperature. Data concerning the use of a four-layer model based on one and two Tauc-Lorentz dispersions is also discussed, emphasizing its use for the detection of an amorphous incubation layer, normally present on microcrystalline films grown by sputtering.     

Color-tunable organic light-emitting devices using a thin N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine layer at heterojunction interface

Novel types of multilayer color-tunable organic light-emitting devices (OLEDs) with the structure of indium tin oxide (ITO)/N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/aluminum (III)bis(2-methyl-8-quinolinato)4-phenylphenolato (BAlq)/tris-(8-hydroxyquinolate)-aluminum (Alq₃)/Mg:Ag were fabricated. By inserting a thin layer with different thickness of a second NPB layer at the heterojunction interface of BAlq/Alq₃, the emission zone of devices shifted greatly and optoelectronic characteristics underwent large variation. Although BAlq was reported as a very good hole-blocking and blue-light-emission material, results of measurements in this paper suggested that a certain thickness of NPB layer between BAlq and Alq₃ plays an important role to modify device characteristics, which can act as recombination-controlling layer in the multilayer devices. It also provides a simple way to fabricate color-tunable OLEDs by just changing the thickness of this “recombination-controlling” layer rather than doping by co-evaporation.     

Electrochemically deposited poly(3-methylthiophene) performance in single layer photovoltaic devices

Photovoltaic devices based on electrochemically synthesized poly(3-methylthiophene) PMeT were constructed and characterized. The charge mobility for positive carriers of this polymer is quite high, 4 x 10^-4 cm²/Vs, being attractive for optoelectronic devices. In single layer photovoltaic devices with PMeT active layer electrochemically deposited onto indium-tin oxide substrate with aluminum as top electrode we have obtained short-circuit current density of 0.31 A/m², open-circuit voltage of 1 V and power conversion efficiency of 0.14% at 100 W/m² white light irradiance.Received: 17 October 2003PACS: 73.50.Pz Photoconduction and photovoltaic effects - 73.61.Ph Polymers; organic compounds - 73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths - 73.40.Sx Metal-semiconductor-metal structures     

Properties of barium hexa-ferrite thin films dependent on sputtering pressure

M-type barium ferrite thin films were deposited onto sapphire (0 0 l) substrates by radio frequency magnetron sputtering. An ultra-thin layer about 20 nm was deposited and annealed before continuous deposition of the films up to 500 nm under different sputtering pressures: 0.2, 0.5, 0.8 and 1.0 Pa, respectively. It was found that the atomic ratios of Fe to Ba increased from 9.3 to 15.0 with the increase of the pressure. The films sputtered at all pressures have c-axis normal to the film plane by a four circle X-ray diffractometer, which is an improvement of the films directly sputtered on the substrate. Needle-like grains were formed on the surface of the films under higher sputter pressure with bubble domains, which is originated from high magnetocrystalline anisotropy of the film. Magnetic hysteresis loops recorded by vibrating sample magnetometer agree with them, where in-plane and out-of-plane loops of the samples prepared under high sputtering pressures are quite different, while they are almost identical of the samples under low pressures. The influence of the sputtering pressure was understood by that with the increase of the pressure, resputtering of the films was increased. Nucleation with c-axis normal to the film plane was deteriorated. Thus samples prepared under high pressure have more needle-like crystallites which have c-axis parallel to the film plane.     

Influence of inserting AlN between AlSiON and 4H-SiC interface for the MIS structure

A control of interface between gate insulating film and semiconductor is required to achieve high-power field effect transistors (FET) using SiC. To improve the interface between the high-k layer and SiC, we propose inserting an AlN layer as an interfacial layer. The reason for selecting AlN film is that it has a wide bandgap, as well as almost the same lattice constant as that of 4H-SiC. The insertion of AlN film between 4H-SiC and the insulating film effectively reduces the interfacial roughness. The roughness of the interface between AlN and SiC can be suppressed compared with that of the thermal oxidized SiC. Moreover, the AlSiON film was deposited on the AlN layer as a high dielectric gate insulating film with low leakage current at high temperature and low space charge. The C-V characteristics of the AlSiON/AlN/SiC MIS structure with an AlN buffer layer are improved by increasing the deposition temperature of the AlN film. This demonstrates that AlSiON/AlN/SiC is one of attractive MIS structures for SiC devices.     

Electrical in‐situ characterisation of interface stabilised organic thin‐film transistors

We report on the electrical in‐situ characterisation of organic thin film transistors under high vacuum conditions. Model devices in a bottom‐gate/bottom‐contact (coplanar) configuration are electrically characterised in‐situ, monolayer by monolayer (ML), while the organic semiconductor (OSC) is evaporated by organic molecular beam epitaxy (OMBE). Thermal SiO₂ with an optional polymer interface stabilisation layer serves as the gate dielectric and pentacene is chosen as the organic semiconductor. The evolution of transistor param‐ eters is studied on a bi‐layer dielectric of a 150 nm of SiO₂ and 20 nm of poly((±)endo,exo‐bicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylic acid, diphenylester) (PNDPE) and compared to the behaviour on a pure SiO₂ dielectric. The thin layer of PNDPE, which is an intrinsically photo‐patternable organic dielectric, shows an excellent stabilisation performance, significantly reducing the calculated interface trap density at the OSC/dielectric interface up to two orders of magnitude, and thus remarkably improving the transistor performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Crystalline TiO2 grafted with poly(2-methacryloyloxyethyl phosphorylcholine) via surface-initiated atom-transfer radical polymerization

Crystalline TiO₂ films were prepared by unbalanced magnetron sputtering and the structure was confirmed by XRD. An organic layer of 11-hydroxyundecylphosphonic acid (HUPA) was prepared on the TiO₂ films by self-assembling, and the HUPA on TiO₂ films was confirmed by FTIR analysis. Simultaneously, hydroxyl groups were introduced in the phosphonic acid molecules to provide a functionality for further chemical modification. 2-Methacryloyloxyethyl phosphorylcholine (MPC), a biomimetic monomer, was chemically grafted on the HUPA surfaces at room temperature by surface-initiated atom-transfer radical polymerization. The surface characters of TiO₂ films modified by poly-MPC were confirmed by FTIR, XPS and SEM analysis. Platelet adhesion experiment revealed that poly-MPC modified surface was effective to inhibit platelet adhesion in vitro.     

Lead titanate ferroelectric films on single-crystal silicon

This paper reports on the results of investigations into the phase transformations observed in Pb/Ti/Si and Ti/Pb/Si thin-film heterostructures upon layer-by-layer magnetron sputtering of lead and titanium onto a single-crystal silicon substrate and subsequent annealing in an oxygen atmosphere. It is shown that the dielectric properties of lead titanate films depend on the order of sputtering of lead and titanium metal layers onto the surface of single-crystal silicon. The ferroelectric properties are revealed in 3000-nm-thick lead titanate films prepared by two-stage annealing of the Pb/Ti/Si thin-film heterostructure (with the upper lead layer) at T ₁=473 K and T ₂=973 K for 10 min. These films are characterized by the coercive field E _c=4.8 kV/cm and the spontaneous polarization P _s=16.8 μC/cm². The lead titanate films produced by annealing of the Ti/Pb/Si thin-film heterostructure (with the upper titanium layer) do not possess ferroelectric properties but exhibit properties of a conventional dielectric. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 4, 2002, pp. 745–749. Original Russian Text Copyright ? 2002 by Sidorkin, Sigov, Khoviv, Yatsenko, Logacheva.     

Formation of W/HfO2/Si gate structures using in situ magnetron sputtering and rapid thermal annealing

The W(150 nm)/HfO₂(5 nm)/Si(100) structures prepared in a single vacuum cycle by rf magnetron sputtering were subjected to rapid thermal annealing in argon. It is found that at an annealing temperature of 950°C, the tungsten oxide WO _x phase and the hafnium silicate HfSi _x O _y phase grow at the W/HfO₂ and HfO₂/Si(100) interfaces, respectively. Herewith, the total thickness of the oxide layeris 30% larger than that of the initial HfO₂ film. In addition, a decrease in the specific capacitance in accumulation C _max and in the dielectric constant k (from 27 to 23) is observed. At an annealing temperature of 980°C, intensive interaction between tungsten and HfO₂ takes place, causing the formation of a compositionally inhomogeneous Hf _x Si _y W _z O oxide layer and further decrease in C _max. It is shown that a considerable reduction in the leakage currents occurs in the W/HfO₂/X/Si(100) structures, where X is a nitride barrier layer.     

Die Analyse monomolekularer FestkörperoberflÄchenschichten mit Hilfe der SekundÄrionenemission

The use of extremely low primary ion current densities (10^?9 A cm^?2) increases the sputtering time of the original first monolayer of a solid to such a degree (>10⁴ s), that the analysis of this layer by secondary ion spectroscopy can be realized. Changes in its composition (caused by surface reactions, particle bombardment etc.) can be observed for a long period before a considerable fraction of the surface has interacted with the weak ion beam. The capacity of the method is demonstrated by analysis results of Ag and Mo surfaces. Many kinds of ions follow exactly the time dependenceN(t)=N(0)e ^?t/T, characteristic exclusively for ions originating from components present only in the first monolayer. The detection limit for several complex anions is below 1 ppm of a monolayer (<10^?15 g). This method of monolayer analysis shows some advantages compared with electron probe x-ray analysis or Auger electron spectroscopy: Isotope separation, detection of changes of concentration within the first few monolayers, and especially the detection of chemical compounds.     

Ellipsometric measurements on vapor deposited organic films

It has been demonstrated that ellipsometry can be used to study the kinetic properties of organic materials deposited, under vacuum, as thin films on optical surfaces. The organic films were deposited in a controlled manner using a vapor effusion source. Evaporation rate measurements on a DC-704 silicone oil film over a thickness range of 140 to 10 Å have shown that the film exists as two distinct layers with a transition thickness of ~18 Å. The second layer (d > 18 Å) exhibits an linear evaporation rate corresponding to that of the bulk fluid at 300 K while the first layer has a value two orders of magnitude smaller. The first layer was interpreted as a single monolayer having a thickness equal to the helix diameter of the silicone molecule. Additional measured kinetic properties were the sticking coefficients and vapor pressure tor various substrate temperatures and the molecular binding energy. The measured values of Δ and ψ were used to determine the optical constants of both DC-704 and DC-705 silicone oil films as N = 1.5 l — i0.00 and 1.48 — i0.00, respectively, where N = n — ik.