In this work, aggregate epitaxial carrot distributions are observed at the crystal, wafer and dislocation defect levels, instead of individual extended carrot defect level. From combining large volumes of data, carrots are observed when both threading screw dislocations (TSD) and basal plane dislocations (BPD) densities are locally high as seen in full wafer maps. Dislocation density distributions in areas of carrot formation are shown, and suggest TSD limit the formation of carrots in regions containing BPD. These data also add support for mechanisms requiring the need for both dissociated BPD and TSD for carrot formation.

For physical studies of correlated electron systems and for realizing novel device concepts, electrostatic modulation of metal-insulator transitions (MITs) is desired. The inherently high charge densities needed to modulate MITs make this difficult to achieve. The high capacitance of ionic liquids are attractive but, voltages are needed that can be in excess of the electrochemical stability of the system. Here, we show temperature/resistivity data that suggest electrostatic modulation of the MIT temperature of NdNiO3 in a wide regime. However, additional voltammetric and x-ray photoelectron spectroscopy measurements demonstrate the electrochemical impact of the electrostatic doping approach with ionic liquids.

Electrical double layer transistors using ionic liquids as the gate and ZnO as the semiconductor exhibit stable operation in the presence of redox active additives. The characteristics of the device enable single components with the response of a Schmitt trigger.

Ionic liquids (ILs) and their gels are considered for low-voltage and flexible devices due to their ease of processing, freedom in device design, and the realization of high electrostatic fields at low bias voltages. If IL-gated devices are operated at electrochemically stable biases of their individual compounds, they are believed to be reliable. However, small instabilities at interfaces to amorphous oxide semiconductors can lead to secondary instabilities and even the decomposition of gold electrodes, in this paper shown for the case of cyano containing anions.

The electronic density of states in metal oxide semiconductors like amorphous zinc oxide (a-ZnO) and its ternary and quaternary oxide alloys with indium, gallium, tin, or aluminum are different from amorphous silicon, or disordered materials such as pentacene, or P3HT. Many ZnO based semiconductors exhibit a steep decaying density of acceptor tail states (trap DOS) and a Fermi level (EF) close to the conduction band energy (EC). Considering thin film transistor (TFT) operation in accumulation mode, the quasi Fermi level for electrons (Eq) moves even closer to EC. Classic analytic TFT simulations use the simplification EC−EF> ‘several’kT and cannot reproduce exponential tail states with a characteristic energy smaller than 1/2 kT. We demonstrate an analytic model for tail and deep acceptor states, valid for all amorphous metal oxides and include the effect of trap assisted hopping instead of simpler percolation or mobility edge models, to account for the observed field dependent mobility.

In low-temperature solution processed amorphous zinc oxide (a-ZnO) thin films, we show the thin film transistor (TFT) characteristics for the trap-filled limit (TFL), when the quasi Fermi energy exceeds the conduction band edge and all tail-states are filled. In order to apply gate fields that are high enough to reach the TFL, we use an ionic liquid tape gate. Performing capacitance voltage measurements to determine the accumulated charge during TFT operation, we find the TFL at biases higher than predicted by the electronic structure of crystalline ZnO. We conclude that the density of states in the conduction band of a-ZnO is higher than in its crystalline state. Furthermore, we find no indication of percolative transport in the conduction band but trap assisted transport in the tail-states of the band.

Using rolling as a roll-to-roll compatible compaction process for solution processable electronics, we demonstrate improved layer morphology and field effect transistor performance of nanoparticulate zinc oxide (ZnO) thin films. Semiconducting ZnO layers have been processed from a polyvinylpyrrolidone (PVP) stabilized nanoparticulate dispersion at low temperatures. Maximum saturation mobilities of 7x10^-^3cm^2/Vs, improvements in mobility of more than one order of magnitude and a reduction in threshold voltage by more than 30% are shown.

This article compares several non-vacuum-based low-temperature deposition techniques of semiconducting oxides for thin-film transistor applications. After an introduction into basic thin-film transistor theory it summarizes in short the development in the field of semiconducting oxides. Three different deposition techniques are considered in more detail: (1) a direct deposition of semiconducting oxide nanoparticles from a carrier gas stream, on the example of SnO\(_{x}\) and In\(_{2}\)O\(_{3}\), (2) a wet-deposition of nanodispersions of ZnO, and (3) a deposition of liquid precursors with subsequent transformation into the semiconducting oxide, on the example of ZnO. The advantages and disadvantages of the several methods are discussed critically also with respect to results from the literature.

Using nanoparticle dispersions for printing of semiconductors would be the easiest way to evolve from classic printing technologies towards printed electronics. However, nanoparticular thin films are unfavorable in transistor applications due to two reasons: (i) The charge transport in the thin film or at its interfaces to the gate dielectric is disturbed by the voids between the nanoparticles. (ii) These layers are highly sensitive to surface adsorbates due to their high surface to volume ratio. Atmospheric surface adsorbates, e.g. on metal oxides are known to influence the electrical properties of the thin films. In order to overcome the disadvantages of the nanoparticulate thin film, this work targets both issues with a combined approach. By choosing a qualified surface adsorbate, the perturbing surface of the nanoparticles will be passivated. By using the surface adsorbate as a linker to an electron conducting organic molecule, the n-type organic will be eligible for filling the voids between the particles. We present the synthesis of a new pyrrolidone functionalized n-type perylene diimide and its application in hetero-layer nanoparticulate zinc oxide (ZnO) field-effect transistors.

Amorphous zinc oxide thin films have been processed out of an aqueous solution applying a one step synthesis procedure. For this, zinc oxide containing crystalline water (ZnO⋅×H2O) is dissolved in aqueous ammonia (NH3), making use of the higher solubility of ZnO⋅×H2O compared with the commonly used zinc oxide. Characteristically, as-produced layers have a thickness of below 10nm. The films have been probed in standard thin film transistor devices, using silicon dioxide as dielectric layer. Keeping the maximum process temperature at 125°C, a device mobility of 0.25cm2V−1s−1 at an on/off ratio of 106 was demonstrated. At an annealing temperature of 300°C, the performance could be optimized up to a mobility of 0.8cm2V−1s−1.

We investigated the electrical effects of polyvinylpyrrolidone (PVP), used as a dispersion agent in zinc oxide (ZnO) nanodispersions. We found PVP reduces the high surface conductivity and atmospheric sensitivity. Compared with polymer free ZnO thin films, the nanoparticulate layers with PVP exhibit a smaller density of thermally active charge carriers, a reduced density of trap states, and a Fermi level shift toward the valence band, yielding improved performance, vanishing hysteresis characteristics and reduced atmospheric sensitivity in thin film transistors (TFT). In addition, we discuss the attachment of PVP to the ZnO surface.

Zinc oxide layers with a thickness of less than 10 nanometers have been synthesized from an aqueous solution for the application as active layer in thin film transistors. They have been conditioned by applying different oxidizing and reducing atmospheres during an annealing process at a temperature of 125°C. It is shown that the charge carrier mobility and threshold voltage is strongly influenced by the annealing atmosphere. Samples annealed in 10% forming gas (H 2 in N 2 - reducing atmosphere) show the highest field-effect-mobility of 0.6 cm 2 V -1 s -1 , but no saturation of the drain current, due to a high free carrier concentration. Samples treated under oxygen (strongest oxidizing atmosphere) show significantly lower mobilities. Subsequently, the samples have been exposed to synthetic air, with varying exposure times. Samples which have been annealed under hydrogen atmospheres show a pronounced decay of the drain current if exposed to synthetic air, whereas all samples conditioned under hydrogen-free atmospheres are significantly more stable under synthetic air. This enhanced sensitivity against oxygen after hydrogen treatment is attributed to residual hydrogen content in the sample that supports the formation of OH-groups which act as electron acceptors.

The electric transport properties of nanoparticulate zinc oxide (ZnO) thin films are investigated in nitrogen and ambient atmosphere with respect to the effects of polymer adsorbates, in order to study the origin of hysteresis behavior of ZnO thin film transistors. A strong dependence on the polymer adsorbate of the conductivity in nitrogen atmosphere is observed. Utilizing the space charge limited current theory, the trap depth and concentration in the films have been estimated. According to this analysis, the low conductivity of polymer free thin films in ambient atmosphere is caused by an increase in deep traps, compensating free charge carriers and not by a reduction in donorlike defect states. Furthermore, polymeric additives seem to induce similar trap states, which make the transport properties less sensitive against atmospheric influences. However, the strongly compensated semiconductor created in this way, causes a slow trap and release behavior resulting in a strong hysteresis in the transistor characteristics and long-term instabilities. It is shown, that ignoring these time-dependent characteristics, straight forward derived transistor parameters like the field effect mobility can be easily overestimated.

Solution-processed field-effect transistors (FETs) based on inorganic nano-materials are attractive system for applications in the field of printable electronics. Stabilized suspensions made of zinc oxide nano-particles were used to fabricate FETs by spin coating. The performance of such devices is strongly affected by the nature and concentration of the compounds added to stabilize the nano-dispersions. In this work we compare the field-effect mobility of zinc oxide transistors when the nano-particle dispersion is prepared using two different types of commercially available stabilizers. For the stabilizer TEGO Dispers 752 W we observed an increase in the field-effect mobility by more than one order of magnitude. In contrast, for TEGO Dispers 750 W where a decrease in the field-effect mobility of more than two orders of magnitude was observed. This stark difference is attributed to be due to the electrical conductivity of the stabilizers, the morphology and interface roughness and the stabilizer particle interaction. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In order to build printable inorganic electronic devices, semiconducting suspensions are needed, which can be processed at low temperatures using low-cost manufacturing techniques. Stabilized suspensions made of zinc oxide (ZnO) nanoparticles were used to fabricate field-effect transistors by spin coating. The performance of the devices is strongly affected by the nature and concentration of the compounds added to stabilize the nanodispersions. An increase in the field-effect mobility by more than one order of magnitude is obtained upon increasing the stabilizer concentration from 3 to 13 wt %. A further increase in the concentration above 13 wt % results in a decrease in the field-effect mobility. This behavior can be explained by changes in the morphology, the particle-particle junction, and the passivation of surface defect sites.