The room-temperature current-voltage data of hole transport in ITO/PFB(400 nm)/Al organic diode (Khodabakhsh et al., 2004) where PFB is polyfluorene copolymer, and the related 5 devices of different self-assembled monolayer (SAM) with permanent molecular dipole moments chemisorbed onto the indium-tin-oxide (ITO) electrode are investigated. It is shown that the intrinsic hole mobility, mu max, within the diode is Ea independent but it is strongly dependent on the magnitude and the orientation of the SAM ' s permanent molecular dipole moments. The current-density data jointly with the reported values of 4 SAM's molecular dipole moments and their orientation with respect to the ITO normal at the contact enable to provide an estimate of the PEDOT:PSS, as well as pristine ITOP/PFB molecular dipole moments and their orientations. The dipolar electric field that is induced at the ITO/SAM/PFB interface, Eint, turns out to be a linear function of the applied field Ea of approximately SAM`s independent slope. The current-density induced dipolar electric field occurs even at the pristine ITO/PFB interface. The intrinsic hole mobility, mu max, multiplied by the given algebraic function of the Ea dependent interfacial electric field, Eint, combine into the Ea dependent hole drift mobility, mu d, Influenced jointly by the dipolar and interfacial effects, it is the parameter that is amenable in the current-density-voltage measurements. The relevance of the stated findings to the current switching mechanism in the self-assembled supramolecular ferroelectric organic semiconductor diode is also briefly discussed.

It was shown recently that the hole current-density is in organic semiconductor structures modulated by the hole drift mobility, mu(d). It explicitly depends on the externally applied electric field, E-a, on the electric field at the hole injecting electrode/organic interface, E-int, and on the E-a independent intrinsic hole drift mobility, mu max. Based on this relationship the published current-voltage measurements of two ferroelectric-like diode structures are examined. The on-state current-density of each diode is within the interval of E-a up to the value of the coercive electric field, E-C, represented by the space-charge limited current, SCLC, specified by E-a independent, values of E-int and mu max. The polarization remains constant and small. Within the stated interval each off-state curve is described in terms of the linear E-a dependent E-int, which causes the polarization change. For Ea asymptotic to EC the current density behavior depends on the location of the ferroelectric polarization within the sample. For the polarization confined within the volume of the electrode/organic interface then the off-state abruptly transits into the SCLC segment of the off-state curve that is followed by the steep unidentified current-density increase at the highest E-a to the intersection of both states. For the bulk polarized sample, the off-state close to EC deviates into the nearly linear E-a dependent current-density segment incorporating a weak E-a dependent hole density at the interface. The maximum difference between the off-, and on-states occur at E-a close to E-C which is at about the point of the off-state interface polarization maxima.

Two different sets of published temperature dependent current-voltage N-N'-Di(1-naphthyl)-NX-diphenyl-(1,1'-biphenyl)-4,4'-diamine organic semiconductor data are analyzed. The general solution of Poisson equation with the imbedded charge traps reduced to the low trap concentration limit provides the simple current density voltage relationship on which analyses are based. The relationship is expressed in terms of two parameters: the electric field at the hole injecting interface, E-int and the hole mobility, mu(max), determined by the measured current density at the maximum value of the externally applied electric field, E-a in a given experiment. It is shown that explicitly depends upon the current density, j, on the negative interfacial charge (areal) density proportional to E-int - E-a and on the concentration of the embedded holes. For cases where E-int is constant the current density within the organic diode structure is space charge limited (SCLC). Then the calculated weak E-a dependent hole drift mobility, mu(d), attains the shape of well-known exponential bias dependent mobility. From the current-voltage data not complying with SCLC characteristic the E-a dependence of E-int is calculated and its detrimental effect on the hole drift mobility is presented. An estimate of E-a dependent thickness of the interface slab is made. The effect of the deduced E-a dependent interfacial electric field, E-int on Gaussian disordered states is examined.

The origin of the empirical linear dependence of Eint the electric field at the charge-injecting metal/poly(3-hexylthiophene), P3HT, interfaces on the externally applied bias, Ea, that governs its room temperature negative field charge drift mobility is investigated. The published electrostatic model is modified in the sense that the energetic shift φ of the Gaussian disordered hole and electron energy states is determined to be a linear function of the externally applied electric field, Ea. On this basis the distorted Gaussian shaped interfacial electric field Eint is obtained and the empirical Eint is identified as the linear interpolation function of positive slope through the inflection point of the calculated curve. The disordered energy states extend throughout the P3HT charge transport gap and the populations of interface charges follow unequal Ea dependences. The coupling between the deduced interfacial electric field Eint and the P3HT effective mobility enables the predictions to be compared to the published time-of-flight room temperature negative field drift mobility data of holes and electrons in various metal/P3HT sandwich-type organic structures.

The published (room temperature) negative hole mobility, i.e. diminishing hole mobility with an increasing electric field, Ea, of three different organic semiconductors are analyzed in terms of the processes at the hole injecting interface. The charge mobility is described by the product of the effective mobility, μeff, and the algebraic function of the argument defined by the ratio of the electric field at the charge injecting metal/organic interface, Eint, to the externally applied electric field, Ea. It is shown that the negative hole mobility is directly related to the electric field at the hole injecting metal/organic interface, Eint, determined to be a linear function of Ea while the effective mobility, μeff, remains (almost) Ea independent. In particular, for the well-defined P3HT 12 (poly(3-hexylthiophene)) sample in films of (relatively) greater crystallinity a considerably augmented but still negative hole mobility as determined by the current-voltage method has been reported recently. Based upon the above formulation of the charge mobility, it is shown that principally it is the effective mobility, μeff, which is affected by the changing degree of the sample crystallinity while its bias dependence still remains governed by the linear dependence of Eint on Ea that is a characteristic feature of the negative mobility. For the P3HT enhanced crystallinity film the range and the magnitude of the interfacial electric field are changed in a way that the ensuing hole density decrease as a function of Ea then signifies an increased transparency for hole transport within such an organic bulk. It appears that the enhanced crystallization favors conditions for augmentation of the effective hole mobility in conjunction with efficiently increased hole injection at the interface. Consequently, it is the effect occurring at the charge injecting metal/organic junction, described by the algebraic function of the argument Eint/Ea that decisively modulates the Ea dependence of the experimental negative charge mobility. Supported by the presented results this work may then provide a physically viable alternative to otherwise phenomenological descriptions of the negative mobility as described in the literature.

It is shown that the well-known empirical exponential bias-dependent mobility is an approximation function of the relevant term emerging in the Mott–Gurney space charge limited current model when the constant non-zero electric field at the hole injecting metal/organic interface Eint is taken into account. The term in question is the product of the bias-independent (but organic layer thickness-dependent) effective mobility coefficient and the algebraic function, f(λ), of the argument λ=Eint/Ea, where Ea is the externally applied electric field. On account of the non-zero interfacial field, Eint, the singularity of the spatial dependence of the hole current density, p(x), is removed. The resulting hole drift current density, j, is tested as a function of Ea against a number of published room temperature hole current j–Ea data sets, all characterized by good ohmic contact at the hole injecting interface. It is shown that the calculated current density provides a very good fit to the measurements within a high range of Ea intervals. Low values of Ea, are investigated analytically under the assumption of hole drift-diffusion. The extremely large internal electric fields at the anode/organic junction indicate drift-diffusion to be an improbable process for the structures investigated. However, a description of hole transport throughout the whole interval of experimental Ea values may be obtained at low values of Ea by an extended Mark–Helfrich drift model with traps occupying the exponentially distributed energy levels, followed by the extended Mott–Gurney model description within the remaining part of the Ea interval. In both models the same (bias-independent) effective mobility coefficient is incorporated into the calculations. The results present evidence that within the framework of the extended Mott–Gurney expression the properly derived term should replace the empirical exponential bias-dependent mobility, making it redundant in the interpretation of j–V data.

By incorporating a non-zero electric field at the charge injecting metal/organic interface, Eint, into the Mark–Helfrich model of charge traps exponentially distributed in energy, a general expression for the current density–voltage dependence of single layer metal/organic structures is presented. It is expressed in terms of the solutions of the non-linear algebraic equation relating Eint to the externally applied electric field, Ea. As a result, the finite and continuous spatial distribution of the free and trap charge density at the boundaries and within the organic layer is obtained. Arguments are presented that for an interfacial field directly proportional to the applied electric field, Eint=λ Ea, with λ bounded between 0.1≤λ≤0.99, the λ dependent current density follows an identical bias and organic thickness prediction as the original Mark–Helfrich model, but with a substantially different factor of proportionality. The resulting current density is characterized by a negligible space charge limited current (SCLC) effect. The bias independent Eint at the charge-injecting interface leads to a concave line shape (when viewed through the current axis) in the current–voltage diagram. The intense (but always) finite peak of the free charge density at this interface occurs only for Eint≪Ea, thus leading to the strong SCLC effect. These predictions are tested on some published experimental data and good agreement is observed. The combination of an interfacial electric field linearly dependent upon the applied bias up to a given value and constant thereafter results in at least two distinct line slopes in the log j−log Ea plane.

The arguments are presented that the assumption of a zero electric field at the charge carrier injection electrode/organic interface, as assumed by the current-voltage model of shallow traps occupying a single energy level within the organic semiconductor charge transport band as well as by the model of traps exponentially distributed in energy, is invalid in principle. In consequence of this fact, it is shown that the value of material parameters is strongly related to the bias dependent non-zero electric field at the charge-injecting interface. A set of published room temperature current-voltage data of a single layer, electron-only, metal/organic semiconductor structure is used in order to show explicitly that the singularity of free (or total) electron charge carriers at the injecting interface and the associated space charge limited current characteristics reflects an unrealistic assumption of the above two models. For the model of traps exponentially distributed in energy, the charge traps density and the associated non-zero interfacial electric field are uniquely determined under a subsidiary condition of minimal energy of charges in an external electric field. The resulting (bias dependent) spatial distribution of the internal electric field and the spatial dependence of the electron density turns out to be practically uniform throughout the organic sample in agreement with the predictions of the model of shallow traps occupying a single energy level. The internal electric field and the charge density within the narrow midway region of the organic are, at each bias, invariant to the variation of the trap density and the associated interfacial electric field. It is also shown that the drift-diffusion electron transport interpreted in terms of the model of shallow traps occupying a single energy level is incompatible with the published measurements.

The closed solution for the internal electric field and the total charge density derived in the drift-diffusion approximation for the model of a single layer organic semiconductor structure characterized by the bulk shallow single trap-charge energy level is presented. The solutions for two examples of electric field boundary conditions are tested on room temperature current density-voltage data of the electron conducting aluminum/tris(8-hydroxyquinoline aluminum/calcium structure [W. Brütting et al., Synth. Met. 122, 99 (2001)] for which jexp∝Va3.4, within the interval of bias 0.4 V≤Va≤7. In each case investigated the apparent electron mobility determined at given bias is distributed within a given, finite interval of values. The bias dependence of the logarithm of their lower limit, i.e., their minimum values, is found to be in each case, to a good approximation, proportional to the square root of the applied electric field. On account of the bias dependence as incorporated in the minimum value of the apparent electron mobility the spatial distribution of the organic bulk electric field as well as the total charge density turn out to be bias independent. The first case investigated is based on the boundary condition of zero electric field at the electron injection interface. It is shown that for minimum valued apparent mobilities, the strong but finite accumulation of electrons close to the anode is obtained, which characterize the inverted space charge limited current (SCLC) effect. The second example refers to the internal electric field allowing for self-adjustment of its boundary values. The total electron charge density is than found typically to be of U shape, which may, depending on the parameters, peak at both or at either Alq3 boundary. It is this example in which the proper SCLC effect is consequently predicted. In each of the above two cases, the calculations predict the minimum values of the electron apparent mobility, which substantially exceed the corresponding published measurements. For this reason the effect of the drift term alone is additionally investigated. On the basis of the published empirical electron mobilities and the diffusion term revoked, it is shown that the steady state electron current density within the Al/Alq3 (97 nm)/Ca single layer organic structure may well be pictured within the drift-only interpretation of the charge carriers within the Alq3 organic characterized by the single (shallow) trap energy level. In order to arrive at this result, it is necessary that the nonzero electric field, calculated to exist at the electron injecting Alq3/Ca boundary, is to be appropriately accounted for in the computation.

The room-temperature current-voltage data of the hole conducting single layer poly-p-phenylene-vinylene indium tin oxide/poly-p-phenylene-vinylene (200 nm)/Al organic structure of Karg et al. [J. Appl. Phys. 82, 1951 (1997)], representing an archetype for (shallow) trap-controlled space charge limited conduction characterized by the current density proportional to the second power of the applied bias, j∝Va2, are used to calculate the spatial dependence of the internal electric field, the internal electric potential, and the total (free and trap) hole charge density within the organic layer. In contrast to the usual space charge limited current results, it is determined that these quantities are linearly distributed throughout the above structure. The breakdown of the Mott–Gurney approach is assigned to the usual, but in this case inappropriate, assumption of the boundary condition at the hole injecting metal/organic junction. It is shown that for the above stated structure the electric field at the anode/organic boundary is nonzero and linearly increases with the forward bias. As a consequence of this fact the product of the hole mobility and the trapping parameter turns out to be considerably larger than the value deduced on the basis of the Mott–Gurney law. The organic bulk differential capacitance is small in magnitude and exhibits a nonlinear decrease with the increasing value of the applied voltage Va.

The room-temperature differential capacitance of monolayer metal/organic-semiconductor/metal structures was derived. The derivation was based on two basic assumptions: (a) the rectifying metal/organic-semiconductor junction is characterized by the bias-dependent net excess charge density, induced at the interface, and (b) the charge flow within the organic layer is represented by the space-charge-limited current. The predictions of the derivations were compared to C-U data on an ionized cluster beam Ag-deposited 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on indium-tin-oxide (ITO), Ag∕PTCDA∕ITO, sample obtained at 1 kHz and at room temperature. In addition, thorough analyses of published, room-temperature capacitance-voltage data for Al∕pentacene(60nm)∕ITO, poly(phenylene vinylene) Al∕PPV(200nm)∕ITO, poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene], Ca∕MEH-PPV(40nm)∕Au, tris-(8-hydroxyquinoline) aluminum, Al∕Alq3(60nm)∕ITO, Ca∕Alq3(60nm)∕ITQ, and Ca∕Alq3(120nm)∕ITO organic-semiconductor structures were made, and, in all cases, a good agreement with the predictions was obtained. The quantitative and qualitative values of the interfacial charge density, the internal electric field originating from populated traps, and the nature of the current in these monolayer structures were identified. The parameters deduced from fitting to the Al∕PPV(200nm)∕ITO capacitance-voltage measurements of Meier et al. [J. Appl. Phys. 82, 1961 (1997)] were used to show, assuming Fermi-level alignment, that the energy bands of this structure, as well as all other samples investigated in this work, are well described in terms of a quadratic function of the coordinate, as measured from the cathode/organic junction.

The bias dependent interface charge is considered as the origin of the observed non-ideality in current–voltage and capacitance–voltage characteristics. Using the simplified model for the interface electronic structure based on defects interacting with the continuum of interface states, the microscopic origin of empirical parameters describing the bias dependent interface charge function is investigated. The results show that in non-ideal metal–semiconductor contacts the interface charge function depends on the interface disorder parameter, density of defects, barrier pinning parameter and the effective gap center. The theoretical predictions are tested against several sets of published experimental data on bias dependent ideality factor and excess capacitance in various metal–semicoductor systems.

Appropriate transfer of the ultrasound pulse is essential for the ultrasound based materials characterization. In the appropriate set-up, the ultrasonic pulse transfer is realized as the pulse's propagation from the transducer to the buried object to be characterized, and back to transducer again. The signal received is a complex combination of characteristics of both the transfer entity and the characterized object according to Stepanic et al. (2003 and 2004), and Krstelj and Stepanic Jr (2000). Therefore, the transfer entity influence of the ultrasonic pulse should be known in order to enable one to use the ultrasound for buried object materials characterization. That influence could be modeled and the object containing transfer medium appropriately designed. The recently proposed sensing configuration in which piezoelectric specimens of nonconventional shape are integrated in the hand-probe is presented. We show the manufacturing of the first generation sensors, and characterization of the corresponding ultrasonic fields in space-time-intensity coordinates. Specifically, the details of the ionized cluster beam deposition technique as stated by Takagi (1986) that has been employed in order to achieve the desired novel geometrical shapes of the piezoelectrical sensors is presented and discussed (6 References).

The role of the tunneling mechanisms in metal-disordered layer-semiconductor structure under spin injection at the interface is investigated. The non-ideal metal-semiconductor structure as prepared by ionized cluster beam deposition is considered, and it is shown that the depletion region of the semiconductor can be tailored to include a suitably heavily doped region near the interface. The tunneling is described within a simplified model in which the expression for the interface resistance of the metal-disordered layer-semiconductor structure is obtained. It is argued that in the case of ionized cluster beam deposited non-ideal Schottky structure a significant spin injection is achieved.

Ultrasonic sensor improves confirmation capability of the buried object detection using hand probe. Different parts of the sensor, e.g., matching layers, piezoelectric specimen, electrodes etc. should all be considered explicitly to realise the optimal confirmation capability. In this paper we consider the contribution of the configuration of electrodes to the spatial distribution of the pressure field of ultrasonic sensor located within the hand-probe tip. The fixing of the parameters of the geometry of the electrodes is performed following the purpose of the system. Appropriate transfer of the ultrasound pulse is essential for the ultrasound based materials characterization. In the appropriate set-up, the ultrasonic pulse transfer is realized as the pulse’ s propagation from the transducer to the buried object to be characterized, and back to transducer again. The signal received is a complex combination of characteristics of both the transfer entity and the characterized object [1 – 3]. Therefore, the transfer entity influence of the ultrasonic pulse should be known in order to enable one to use the ultrasound for buried object materials characterization. That influence could be modelled and the object containing transfer medium appropriately designed. The recently proposed sensing configuration in which piezoelectric specimens of non-conventional shape are integrated in the hand-probe is presented. We show the manufacturing of the first generation sensors, and characterisation of the corresponding ultrasonic fields in space-time-intensity co-ordinates. Specifically, the details of the Ionised Cluster Beam deposition technique [4] that has been employed in order to achieve the desired novel geometrical shapes of the piezoelectrical sensors is presented and discussed.

Analysis of the Ag/p-Si(100) metal–semiconductor interface prepared by the ionized cluster beam (ICB) technique using admittance spectroscopy is given. The observed peak in the imaginary part of the complex dielectric function spectra indicates the presence of the interface states at the metal–disordered layer and disordered layer–semiconductor interfaces. The frequency dependence of the zero-bias measured conductance and capacitance of the ICB m/s interface is well described assuming the distribution of electronic states in the semiconductor bandgap, and the frequency response of the conductance is related to the hopping conductivity and the tunnelling current to the interface states.

The origin of the excess capacitance of nonideal Schottky junctions on Si substrates is attributed to the external bias voltage dependent net charge density induced at an appropriate interface for which a functional dependence, in terms of empirical parameters, is explicitly suggested. This conjecture, resulting in the introduction of an additional term in the depletion layer differential capacitance, is verified on frequency dependent, room temperature, C–U data of our ionized cluster beam (ICB) deposited Ag/Si and Pb/Si Schottky junctions as well as on various other relevant published measurements. It is found, that the envelope of the calculated effective density of gap states of our ICB samples exhibits dense almost standing wave-like oscillations across the energy gap at the interface for which the envelope amplitude and number of nodes are sensitively dependent on the external bias voltage.

The observed variation of the Schottky barrier height in ionized-cluster-beam (ICB)-deposited Ag/Si Schottky structures is described within the framework of the disorder-induced gap stated theory (DIGS). The physical mechanisms determining the position of the Fermi level in ICB Schottky structures under different regimes of acceleration voltage are suggested, emphasizing the role of the local electronic structure.

The series of Ag/n-Si(1 1 1) Schottky junctions, prepared by the ionized cluster beam deposition method for an Ag ion acceleration voltage of Ua=0 V, 300 V, 600 V and 1 kV, exhibits strong excess capacitances, the peaks of which gradually shift from reverse to forward bias voltages. These C–U measurements, the first such reported, are interpreted on the basis of the existence of external bias-dependent induced interfacial net charge density. It has been found that either reverse or forward excess capacitance of pertinent samples of the series is directly related to the induced interfacial charges above. From the above data an effective density of interface states is extracted which is characterized by the bias-dependent sharp spikes distributed along the semiconductor energy gap.

At metal–semiconductor interfaces, the Fermi level is usually found “pinned” in the bandgap. Schottky barriers of Ag/n-Si structures deposited by the ionized cluster beam (ICB) technique show a similar Fermi level pinning behaviour with respect to the metal atom acceleration voltage. We propose a simple model for the density of interface states at the disordered interface in an ICB Schottky structure. The Fermi level position inside the semiconductor bandgap is calculated as a function of increasing disorder.