Simulation of the effects of space charge and Schottky barriers on ferroelectric thin film capacitor using Landau Khalatnikov theory

The role of space charge induced in a ferroelectric thin film and the presence of Schottky barriers at the two electrode/film interfaces are studied by numerical simulation using Landau-Khalatnikov theory. In this work, the whole film is considered as the stacking of dipolar layers, each of which contains multilayers of perovskite cells. In the presence of a local electric field, the double-well thermodynamic potential of each layer is modified in an asymmetric manner. The local electric field distribution is determined both by the space charge and the boundary conditions imposed by the Schottky barrier heights. Asymmetric and skewed hysteresis loops are generated     

Electroluminescence of new dyed polymer films in sandwich structures

Electroluminescence (EL) from sandwich structures with emitting layers based on poly(N-epoxypropylcarbazole) (PEPC) doped with a boron difluoride organic dye complex was discovered and studied. The EL is implemented in heterostructures with Al-or In-based injecting heterojunctions possessing better technological properties as compared to those of conventional magnesium-silver electrodes. It is established that EL is controlled by the bulk recombination of charge carriers, whereby dye molecules act as the recombination centers. The EL efficiency is maximum in systems with a 50 mass % dye content in a submicron polymer film. The growth in EL quenching with an increase in the polymer film thickness is related to accumulation of the bulk space charge within a short time after the application of electric field.     

Modulating the performance of carbon nanotube field-effect transistors via Rose Bengal molecular doping

A simple, reliable, and large scale ambient environment doping method for carbon nanotubes is a highly desirable approach for modulating the performance of nanotube based electronics. One of the major challenges is doping carbon nanotubes to simultaneously offer a large shift in threshold voltage and an improved subthreshold swing. In this paper, we report on modulating the performance of carbon nanotube field-effect transistors (CNTFETs) by rationally selecting doping molecules. We demonstrated that Rose Bengal sodium salt (RB-Na) molecular doping can effectively shift the threshold voltage (ΔVth) of CNTFETs up to ～6 V, decrease the subthreshold swing down to 130 mV/decade, and increase the effective field-effect mobility to 5 cm2 V(-1) s(-1). It is also shown that CNTFETs doped with Rose Bengal lactone (RBL) show a smaller variation in ΔVth (～2 V) and subthreshold swing than those doped by RB-Na, which can be attributed to the difference in their molecular structures. The observed right-shift of the threshold voltage is attributed to the positive charge doping of the nanotube conduction channel from Rose Bengal molecules. The resultant lowering of the subthreshold swing is due to the reduced Schottky barrier at the CNT/metal/molecule interface. This room temperature chemical doping approach provides an efficient, simple, and cost-effective method to fabricate highly reliable and high-performance nanotube transistors for future nanotube based electronics.     

TiN as a high temperature diffusion barrier for arsenic and boron

The use of TiN thin films as high temperature diffusion barrier layers for arsenic and boron was investigated. The TiN films are formed by reactive evaporation at room temperature and then annealed at a higher temperature. TiN and TiN/TiSi₂ films are placed between heavily doped polycrystalline silicon films and single-crystal silicon substrates of opposite doping polarity for secondary ion mass spectrometry analysis and electrical measurements of Schottky and ohmic contacts respectively. The results indicate that TiN is a good diffusion barrier for arsenic at 900°C. The effectiveness of this property is degraded as the temperature exceeds 900°C and it becomes ineffective at 1000°C. TiN is a better diffusion barrier for boron than for arsenic. It allows limited diffusion of boron at temperatures of up to 1000°C. The TiN/TiSi₂ composite forms good ohmic contacts when the substrates are heavily doped. The ohmic contacts can survive after annealing at temperatures of up to 1000°C. It also forms good Schottky contacts when the substrates are lightly doped. The Schottky contacts can survive after annealing at temperatures of up to 950°C in one case and of up to 1000°C in another case.     

Electrical transport in ethyl cellulose-chloranil system

The charge-transport behaviour in pure and chloranil (Chl) doped ethyl cellulose (EC) system has been studied by measuring the dependence of current on field, temperature, electrode material and dopant concentration. The role of doping molecular concentration in the polymer matrix and modification in the conduction characteristics are studied. Lowering of the activation energy due to doping was observed. The current was found to increase with an increase in the chloranil concentration. An explanation for this has been attempted on the basis of formation of molecular aggregates between chloranil molecules and ethoxy groups of ethyl cellulose. It is suggested that chloranil occupies interstitial positions between the polymer chains and assists in carrier transportation by reducing the hopping barriers. The current-voltage characteristics of different samples are analyzed using space charge limited current theory and quantitative information about the transport parameters is derived. The values of effective drift mobility and trapped charge carrier concentration which result in the build up of space charge have been calculated.     

Deposition, evaluation and control of 4H and 6H SiC epitaxial layers for device applications

We report an experimental investigation of the deposition, optical characterization and electrical properties of 6H and 4H-SiC epitaxial layers grown by atmospheric pressure chemical vapor deposition in a home made ‘cold wall’ reactor. From a growth kinetic study performed using our deposition conditions (1 atm, 1700 K) we show that our results can be very well explained using a stagnant layer model. We also underline that the decrease of the growth efficiency for high molar fractions of silane comes from the occurrence of a gaseous phase nucleation. The electronic properties of the resulting layers have been studied by Hall effect measurements. The values of electrons mobility (900 cm² Vs⁻¹ for a low doped layer) compare well with those of other groups. Finally, Schottky diodes have been processed with good forward characteristics.     

Generalized far-infrared magneto-optic ellipsometry for semiconductor layer structures: determination of free-carrier effective-mass,mobility, and concentration parameters in n-type GaAs

We report for the first time on the application of generalized ellipsometry at far-infrared wavelengths (wave numbers from 150 cm(-1) to 600 cm(-1)) for measurement of the anisotropic dielectric response of doped polar semiconductors in layered structures within an external magnetic field. Upon determination of normalized Mueller matrix elements and subsequent derivation of the normalized complex Jones reflection matrix r of an n-type doped GaAs substrate covered by a highly resistive GaAs layer, the spectral dependence of the room-temperature magneto-optic dielectric function tensor of n-type GaAs with free-electron concentration of 1.6 x 10(18) cm(-3) at the magnetic field strength of 2.3 T is obtained on a wavelength-by-wavelength basis. These data are in excellent agreement with values predicted by the Drude model. From the magneto-optic generalized ellipsometry measurements of the layered structure, the free-carrier concentration, their optical mobility, the effective-mass parameters, and the sign of the charge carriers can be determined independently, which will be demonstrated. We propose magneto-optic generalized ellipsometry as a novel approach for exploration of free-carrier parameters in complex organic or inorganic semiconducting material heterostructures, regardless of the anisotropic properties of the individual constituents.     

InP based semiconductor structures for radiation detection

We report the preparation of semi-insulating InP single crystals of p-type conductivity and intentionally undoped p-type epitaxial layers for radiation detection. We focus on (i) the growth of InP single crystals doped with copper by the Czochralski technique and their subsequent temperature annealing to convert them to a semi-insulating (SI) state of p-type conductivity, and (ii) the growth of thick (>10 μm) p-type InP layers by liquid phase epitaxy with an admixture of Pr and Dy. Grown layers and single crystals were examined by low-temperature photoluminescence spectroscopy, capacitance-voltage and temperature dependent Hall measurements. An efficient purification due to rare earth (RE) admixture has been observed and layers grown with the addition of Pr and Dy exhibit the change of electrical conductivity from n to p at certain RE concentration in the melt. Dominant acceptors responsible for conductivity conversion have been identified. Three types of detection structures exploiting the Schottky or Schottky like contacts on pure and SI p-type InP or exploiting the p–n junction were designed.     

Scanning electron microscope EBIC and CL micrographs of dislocations in GaP

The SEM EBIC (or charge collection) method using a surface Schottky barrier was applied to GaP specimens to obtain dark spot micrographs revealing dislocations present in the specimens. We describe the experimental procedures and electron probe parameters necessary to obtain such micrographs for specimens ranging from LEC substrate material to doped and undoped VPE layers, and compare the results obtained with analogous results for the SEM CL method. A one-to-one correspondence between the dark spots in corresponding EBIC and CL micrographs was demonstrated. Factors affecting the spatial resolution of the micrographs are discussed; a best resolution of 1m was obtained.     

Investigation of the effect of rare earth doping on the electrical and photovoltaic properties of chromotrope 2R thin film devices

Praseodymium (Pr) doped and Samarium (Sm) doped chromotrope 2R (CHR) were used for the fabrication of Schottky devices, by the spin coating technique. The diode in which doped CHR behaves as an n-type organic semiconductor exhibits rectification behaviour in the dark. Doping with rare earths imparts an accelerated improvement in the n-type conductivity as well as in the rectification effect. The observed rectification effects are explained by n-type semiconductivity of the doped CHR thin films. The formation of a blocking contact (Schottky barrier) indium tin oxide with (ITO) electrode and an ohmic contact with Al or ln, also confirms its n-type behaviour. The position of the Fermi level shifts toward the conduction band edge on rare earth doping. Additionally, the concentration of free carriers and mobility of electrons also increase upon doping, with the simultaneous decrease in trap concentration. Various electrical parameters such as barrier height (b), density of ionized donor (Ns) and depletion layer width (W) were calculated from the detail capacitance–voltage analysis of (C–V) characteristics. The photo-action spectra of the devices and absorption spectra of doped CHR layer reveal the formation of a Schottky barrier at the ITO-doped CHR interface and an Ohmic contact at the Al-doped CHR interface. Photovoltaic measurements of these devices provide parameters such as short circuit photocurrent (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (). The effect of rare earth doping on the electrical and photovoltaic parameters are discussed in detail. © 1998 Chapman & Hall     

Charge transport mechanism and photovoltaic behaviour of undoped and I2 doped tris (1,10 phenanthroline) iron (II) complex (TPFe) thin film devices

Tris (1,10 phenanthroline) iron (II) or Fe (Phen)²⁺ ₃, a metal-to-ligand charge transfer (MLCT) type complex (TPFe), was employed in the form of thin films, for the fabrication of Schottky diodes, Al/ TPFe/ITO, where ITO is indium tin oxide. The effect of iodine doping on the electrical behaviour has been emphasized. The diodes exhibit a rectification effect which improves on iodine doping. The diodes can be classified as MIS Schottky diodes with a graded dopant profile. The current-voltage (J-V ), and capacitance-voltage (C-V ) characteristics, the photoaction spectra of the devices and the absorption spectra of the complex, reveal that both doped and undoped complexes behave as a p-type organic semiconductor which form a Schottky barrier with Al and an ohmic contact with ITO. Various electrical and photovoltaic parameters were determined from the detailed analysis of J-V and C-V characteristics and these are discussed in detail. The effect of I_{_2} doping on the rectification and photovoltaic properties is also discussed.     

Controlling Molecular Doping in Organic Semiconductors

The field of organic electronics thrives on the hope of enabling low‐cost, solution‐processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution‐processed p‐type doped polymeric semiconductors. Highlighted topics include how solution‐processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication—applications beyond those directly analogous to inorganic doping.     

Direct Imaging of Space‐Charge Accumulation and Work Function Characteristics of Functional Organic Interfaces

The tailoring of organic systems is crucial to further extend the efficiency of charge transfer mechanisms and represents a cornerstone for molecular device technologies. However, this demands control of electrical properties and understanding of the physics behind organic interfaces. Here, a quantitative spatial overview of work function characteristics for phthalocyanine architectures on Au substrates is provided via kelvin probe microscopy. While macroscopic investigations are very informative, the current approach offers a nanoscale spatial rendering of electrical characteristics which is not possible to attain via conventional techniques. Interface dipole is observed due to the formation of charge accumulation layers in thin F₁₆CuPc, F₁₆CoPc, and MnPc films, displaying work functions of 5.7, 6.1, and 5.0 eV, respectively. The imaging and quantification of interface locations with significant surface potential and work function response (<0.33 eV for material thickness <1 nm) show also a dependency on the crystalline state of the organic systems. The work function mapping suggests space‐charge carrier regions of about 4 nm at the organic interface. This reveals rich spatial electric parameters and ambipolar characteristics that may drive electrical performance at device scales, opening a realm of possibilities toward the development of functional organic architectures and its applications.     

Highly Crystalline C8‐BTBT Thin‐Film Transistors by Lateral Homo‐Epitaxial Growth on Printed Templates

Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high‐performance, low‐cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus‐guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device‐to‐device variability. Here, a double‐step method for organic semiconductor layers combining a solution‐processed templating layer and a lateral homo‐epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus‐guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high‐performance, large‐area electronics based on highly crystalline thin films of organic semiconductors.     

Change in electronic states in the accumulation layer at interfaces in a poly(3-hexylthiophene) field-effect transistor and the impact of encapsulation

The electrical properties of organic field-effect transistors (OFETs) are largely determined by the accumulation layer that extends only a few molecular layers away from the gate dielectric/organic semiconductor interface. To understand degradation processes that occur within the device structure under ambient conditions, it is thus essential to probe the interface using an architecture that minimizes the effects of bulk transport of contaminating species through upper layers of material in a thick film device. Using FETs designed with multiple voltage probes along the conducting channel and an ultrathin film of the active material, we found that the charge carrier density and the FET mobility decrease, and further, the contact and channel properties are strongly correlated. FET devices prepared with an ultrathin film of P3HT become significantly contact limited in air due to a hole diffusion barrier near the drain electrode. Encapsulation of the device with a layered organic/inorganic barrier material consisting of parylene and Al(2)O(3) appreciably retarded diffusion of molecular species from ambient air into P3HT.     

Features of the Experimental Results When Measuring the Charge Center Density Distribution in the Active Region of a <Emphasis Type="Italic">p-n</Emphasis>-Junction with a Latent Inverse Layer by the Dynamic Capacitance Method

A dynamic barrier capacitance method is described for measuring the distribution density of fixed electrically active centers in the relatively lightly doped region of barrier structures such as p-n-junctions, Schottky barriers and MOD-structures with averaging of the density over the depth of the profile up to 1 nm. It is shown that the presence of latent inverse layers in the active region of the barrier structures leads to an inadequate experimental result of the charge center distribution with respect to the true distribution. The proposed method enables the latent inverse layer in the active region of the barrier structures to be revealed. __________ Translated from Izmeritel'naya Tekhnika, No. 10, pp. 63–67, October, 2005.     

Conductivity of amorphous hydrogenated silicon in the planar and sandwich configurations

The temperature dependence of the conductivity of amorphous hydrogenated silicon in planar and sandwich configurations prepared under identical conditions is measured. The planar conductivity is measured on heat-dried samples; the sandwich conductivity is obtained from (i) the ohmic series resistance of forward-biased Schottky diodes and (ii) the ohmic conductivity of n⁺/n/n⁺ structures. We find that the conductivities for the two configurations compare favourably, thus ruling out any appreciable effect of space charge layers on the conductivity of the planar samples.     

High rectification ratios of Fe–porphyrin molecules on Au facets

We report room temperature measurements of current vs. voltage (I–V) from self-assembled Fe porphyrin [Fe(III) 5,15-di[4-(s-acetylthio)phenyl]-10,20-diphenyl porphine] molecular layers formed on annealed gold crystal facets on glass substrates. I–V curves were measured using an atomic force microscope with a conductive platinum tip. We observed a rectifier effect that shows asymmetric I–V curves from a monolayer of molecules. The majority rectification ratios at ±1 V obtained from hundreds of I–V lie in between 20 and 200, with the highest up to 9000. This is in contrast to the symmetric I–V curves measured from a few nm thick multilayer molecular islands. We contribute the observed rectification in ultrathin FeP molecular layers from asymmetric Schottky barriers that result from molecules in different bonding strengths to electrodes of gold and platinum.     

Nanopatched Graphene with Molecular Self‐Assembly Toward Graphene–Organic Hybrid Soft Electronics

Increasing the mechanical durability of large‐area polycrystalline single‐atom‐thick materials is a necessary step toward the development of practical and reliable soft electronics based on these materials. Here, it is shown that the surface assembly of organosilane by weak epitaxy forms nanometer‐thick organic patches on a monolayer graphene surface and dramatically increases the material's resistance to harsh postprocessing environments, thereby increasing the number of ways in which graphene can be processed. The nanopatched graphene with the improved mechanical durability enables stable operation when used as transparent electrodes of wearable strain sensors. Also, the nanopatched graphene applied as an electrode modulates the molecular orientation of deposited organic semiconductor layers, and yields favorable nominal charge injection for organic transistors. These results demonstrate the potential for use of self‐assembled organic nanopatches in graphene‐based soft electronics.     

Investigation of exciton photodissociation, charge transport and photovoltaic response of poly(N-vinyl carbazole):TiO(2) nanocomposites for solar cell applications

The photogeneration of charge carriers in spin-coated thin films of nanocrystalline (nc-)TiO(2) particles dispersed in a semiconducting polymer, poly(N-vinylcarbazole) (PVK), has been studied by photoluminescence and charge transport measurements. The solvent and the TiO(2) particle concentration have been selected to optimize the composite morphology. A large number of small domains leading to a large interface and an improved exciton dissociation could be obtained with tetrahydrofuran (THF). The charge transport mechanism and trap distribution at low and high voltage in ITO/nc-TiO(2):PVK/Al diodes in the dark could be identified by current-voltage measurements and impedance spectroscopy. The transport mechanism is space charge limited with an exponential trap distribution in the high voltage regime (1-4?V), whereas a Schottky process with a barrier height of about 0.9?eV is observed at low bias voltages (<1?V). The current-voltage characteristics under white illumination have shown a dramatic increase of the short circuit current density J(sc) and open circuit voltage V(oc) for a 30% TiO(2) volume content corresponding to the morphology exhibiting the best dispersion of TiO(2) particles. A degradation of the photovoltaic properties is induced at higher compositions by the formation of larger TiO(2) aggregates. A procedure has been developed to extract the physical parameters from the J-V characteristics in the dark and under illumination on the basis of an equivalent circuit. The variation of the solar cell parameters with the TiO(2) composition confirms that the photovoltaic response is optimum for 30% TiO(2) volume content. It is concluded that the photovoltaic properties of nc-TiO(2):PVK nanocomposites are controlled by the interfacial area between the donor and the acceptor material and are limited by the dispersion of the TiO(2) nanoparticles in the polymer.