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In this work, we show that fast-scanning chip nanocalorimetry can be used to kinetically separate the heat flow contributions of crystallite growth and precursor decomposition that occur during the...

We present a unique compact model for oxide memristors based upon the concentration of oxygen vacancies as state variables. In this model, the increase (decrease) in oxygen vacancy concentration is similar in effect to the reduction (expansion) of the tunnel gap used as a state variable in existing compact models, providing a mechanism for the electronic current to increase (decrease) based upon the polarity of the applied voltage. Rate equations defining the dynamics of state variables are obtained from simplifications of a recent paper in which electronic processes (i.e., electron capture/emission) were combined with atomic processes (i.e., Frenkel-pair generation/recombination, diffusion) stemming from the thermochemical model of dielectric breakdown. Central to the proposed model is the effect of the electron occupancy of oxygen vacancy traps on resistive switching dynamics. The electronic current is calculated considering Ohmic, band-to-band, and bound-to-band contributions. The model includes uniform self-heating with Joule heating and conductive loss terms. The model is calibrated using experimental current–voltage characteristics for [Formula: see text] memristors with different electrode materials. Though a general model is presented, a delta-shaped density of states profile for oxygen vacancies is found capable of accurately representing experimental data while providing a minimal description of bound-to-band transitions. The model is implemented in Verilog-A and tested using read/write operations in a [Formula: see text] 1T1R nonvolatile memory array to evaluate its ability to perform circuit simulations of practical interest. A particular benefit is that the model does not make strong assumptions regarding filament geometry of which scant experimental-evidence exists to support.

Fully inkjet-printed, transparent conductive oxide thin-film transistors on polyimide are demonstrated for the first time integrating SnO2:F source/drain electrodes, ZrO2 gate dielectric, and ZnSnO semiconductor layers. A mobility of ∼0.04 cm2 V−1 s−1 was obtained at a maximum thermal processing temperature of 300 °C. In addition to electrical device characterization, SEM, TEM, and EDX analyses were performed to assess microstructure and chemical composition. Through our analysis, we show that future performance improvements require an increase in source/drain conductivity at lower annealing temperatures, the reduction in film cracking due to thermal stress, and the reduction in intermixing between layers occurring during annealing.

In this work, we utilize the high heating/cooling rates of chip calorimetry to perform in situ annealing of an inkjet-printed zinc oxide solgel ink consisting of zinc acetate, 2-methoxyethanol, and monoethanolamine and directly observe the thermodynamic signatures of its corresponding structural phase transformations. We find that rapid solvent removal achieved by annealing above the boiling point of the solvent, similar to an industrial drying technique known as flash drying, induces the formation of ≈ 98 % amorphous zinc oxide films having an unprecedented, largely pronounced glass transition while retaining its semiconductor properties evidenced by field-effect transistor measurements. Thin-films produced at comparable heating rates to those used for calorimetry experiments are used to corroborate these findings using independent measurements. Prepared films exhibit a clear amorphous halo centered about the three most prominent Bragg positions of the wurtzite phase of ZnO and remain thermally stable against crystallization until 250 ° C have a room temperature thermal conductivity of ≈ 1.03 – 1.4 W m − 1 K − 1 (consistent with recent ab initio estimates) with supporting evidence of a structural relaxation near T G consistently observed in both electronic and thermal conductivity.

Gravure printing is an attractive technique for patterning high-resolution features (5 μm) at high speeds (1 m/s), but its electronic applications have largely been limited to depositing nanoparticle inks and polymer solutions on plastic. Here, we extend the scope of gravure to a new class of materials and on to new substrates by developing viscous sol-gel precursors for printing fine lines and films of leading transparent conducting oxides (TCOs) on flexible glass. We explore two strategies for controlling sol-gel rheology: tuning the precursor concentration and tuning the content of viscous stabilizing agents. The sol-gel chemistries studied yield printable inks with viscosities of 20-160 cP. The morphology of printed lines of antimony-doped tin oxide (ATO) and tin-doped indium oxide (ITO) is studied as a function of ink formulation for lines as narrow as 35 μm, showing that concentrated inks form thicker lines with smoother edge morphologies. The electrical and optical properties of printed TCOs are characterized as a function of ink formulation and printed film thickness. XRD studies were also performed to understand the dependence of electrical performance on ink composition. Printed ITO lines and films achieve sheet resistance (Rs) as low as 200 and 100 Ω/□, respectively (ρ≈2×10(-3) Ω-cm) for single layers. Similarly, ATO lines and films have Rs as low as 700 and 400 Ω/□ with ρ≈7×10(-3) Ω-cm. High visible range transparency is observed for ITO (86-88%) and ATO (86-89%). Finally, the influence of moderate bending stress on ATO films is investigated, showing the potential for this work to scale to roll-to-roll (R2R) systems.

In this paper, we build upon existing techniques to provide a self-consistent, physically based method particularly well suited for quantifying the mobility of solution-processed transition metal-oxide-based thin-film transistors (TFTs). The methodology is presented as a more appropriate alternative to existing square-law techniques whose assumptions are inapplicable to systems exhibiting dispersive transport. To demonstrate its utility in solution-processed electronics, this method was applied to solution-processed SnO2 and ZnO TFTs having various gate dielectrics. In addition, to account for the different operating voltages set by the differences in effective oxide thickness of the dielectric, mobility was evaluated as a function of the transverse electric field, allowing for the direct comparison of mobility independent of the choice of gate dielectric.

Solution-processed transparent conductive oxides have emerged as an attractive material system for realization of large-area and flexible electronic systems, where the scalability of solution-processing offers potential advantages [1]. Recently, the performance of solution-processed ZnO thin-film transistors (TFTs) deposited via spray pyrolysis at 400°C has become comparable to sputtering with a competitive mobility of 85 cm2/Vs on a sol-gel ZrO x gate dielectric [2]. Similar results such as these have been obtained in other TCO material systems by exploiting the commonly observed yet unexplained phenomena of mobility enhancement due to high-k dielectrics. [3] While many of these results have been obtained at process temperatures of >500°C, there is particular interest in lowering the process temperatures to expand the range of compatible substrates. We have investigated the impact of lowering the deposition temperature of sol-gel ZrO x dielectrics [4] on the electrical performance of ZnO TFTs processed at a low temperature of 250°C using a spray pyrolysis technique [2]. Surprisingly, and potentially of tremendous advantage, transistor performance improved dramatically as the processing temperature of the ZrO x was reduced.

Metal oxide thin-film transistors based on high-k dielectrics (ZrO x , HfO x , Al 2 O x ) are a promising technology with attractive performance (µ eff ∼ 10–100 cm2/Vs, On/Off > 107) and high transparency (80–90%) [1]. Solution-processed routes to oxide TFTs can potentially leverage printing technologies to enhance material utilization and throughput. However, realizing the true benefits of solution-processed oxide TFTs requires integrating dielectrics, semiconductors, and conductors that are printable and transparent. To date, there have been few reports of fully solution-processed, transparent oxide TFTs [2]. Full oxide integration is difficult because solution-processed transparent conducting oxides (TCO), such as ITO (Tin-doped indium oxide) and ATO (Antimony-doped tin oxide), reach acceptable conductivity (100–1000 S/cm) only after annealing at 400°C – 500°C, while promising high-k dielectrics, such as ZrO x , crystallize at these temperatures, resulting in high leakage and poor reliability. Here, we demonstrate that doping zirconia with lanthanum can reduce leakage and improve low-frequency dispersion, resulting in a robust dielectric for printed oxide TFTs. We apply these dielectrics in ZnO TFTs, achieving mobilities > 6 cm2/Vs and On/Off ratios > 106.

Subthreshold conduction in amorphous and nanocrystalline transparent conductive oxide (TCO) thin film transistors (TFTs) is routinely evaluated using equation frameworks based on weak inversion borrowed from those used in conventional MOSFETs [1]. Unlike traditional MOSFETs, the electrostatics of TCOs are overwhelmingly determined by the properties of localized states that predominantly consist of charged acceptors exponentially distributed in energy below the conduction band edge [2]. Furthermore, TFTs operate in accumulation, and switching occurs at the flatband voltage, where the depletion charge also vanishes. This suggests that, unlike traditional MOSFETs, no significant diffusion current is expected, since the ratio of diffusion current to the drift current is proportional to the ratio of depletion charge to the electron concentration [3]. In this work, we estimate the depletion charge and electron concentration obtained from quasi-static capacitance-voltage (QSCV) data and demonstrate that the ratio of the depletion charge to the electron concentration becomes negligibly small at and above the flatband voltage. In doing so, we provide experimental evidence demonstrating the limited contribution of diffusion current in these systems, contrary to commonly used equation frameworks, e.g. [4]. Furthermore, based on the proposed transport behavior, we propose a consistent threshold voltage definition. Recently, Qiang et al defined threshold voltage using the point of degenerate conduction, although the tail state concentrations used in their work may be substantially lower than those observed experimentally [5]. We instead offer a definition of threshold voltage that is consistent with an experimental derived density of states, namely, using the intersection between the extended and localized states, corresponding to a Mott transition (occurring at lower voltages) [6] rather than a degeneracy transition (occurring at higher voltages). This is thus more consistent with transport mechanisms in these systems in the on-state.

Low-temperature spray-deposited ZnO films are encapsulated by insulating Ga2O3 films and the resulting electronic properties of heterostructure films and transistors are analyzed and compared to un-encapsulated ZnO as a function of processing temperature. An enhancement in Hall mobility and field-effect mobility is observed similar to when high-k dielectrics are used as gate-dielectrics in transparent conductive oxide thin-film transistors except without the presence of undesirable device behavior such as gate-leakage and hysteresis. Based on a recently proposed theory consisting of electron donation from high-k dielectrics, work function measurements show that conditions for electron transfer from Ga2O3 to ZnO can be met by optimally-tuning the deposition temperature of the respective films resulting in an effective doping which is qualitatively similar to modulation doping. As a result, TFTs composed of ZnO/Ga2O3 heterostructures exhibit greatly improved switching characteristics and electron transport relative to the otherwise poor performance of low-temperature processed ZnO achieved through a simple modification to TFT device structure.

Spray-deposited nanocrystalline ZnO films are produced in order to establish empirical relationships between synthetic conditions and the density of states as a means of achieving electrostatic control. By varying the spray-pyrolysis deposition conditions, i.e., substrate temperature, precursor concentration, and flow rate, a wide range of exponentially distributed density of localized states profiles and field-effect mobility values ranging over three orders of magnitude (0.02–30 cm2 V−1 s−1) are obtained for analysis. It is found that mobility can be controlled by appropriately tuning the shape of the density of states profile, increasing the band tail slope and reducing the band edge concentration of shallow states. Most significantly, it is shown that the shape of the density of states can be modified by adjusting the spray-pyrolysis deposition conditions for electrostatic control. It is found that higher Zn precursor concentration in solution increases the slope of the band tails, leading to higher mobility. Additionally, the band edge concentration is reduced with increased substrate temperature also leading to higher mobility. These results quantify the relationship between defect electrostatics and electron transport while demonstrating electrostatic control via synthetic modification of localized states.

Additive patterning of transparent conducting metal oxides at low temperatures is a critical step in realizing low-cost transparent electronics for display technology and photovoltaics. In this work, inkjet-printed metal oxide transistors based on pure aqueous chemistries are presented. These inks readily convert to functional thin films at lower processing temperatures (T ≤ 250 °C) relative to organic solvent-based oxide inks, facilitating the fabrication of high-performance transistors with both inkjet-printed transparent electrodes of aluminum-doped cadmium oxide (ACO) and semiconductor (InOx). The intrinsic fluid properties of these water-based solutions enable the printing of fine features with coffee-ring free line profiles and smoother line edges than those formed from organic solvent-based inks. The influence of low-temperature annealing on the optical, electrical, and crystallographic properties of the ACO electrodes is investigated, as well as the role of aluminum doping in improving these properties. Finally, the all-aqueous-printed thin film transistors (TFTs) with inkjet-patterned semiconductor (InOx) and source/drain (ACO) layers are characterized, which show ideal low contact resistance (Rc < 160 Ω cm) and competitive transistor performance (µlin up to 19 cm2 V−1 s−1, Subthreshold Slope (SS) ≤150 mV dec−1) with only low-temperature processing (T ≤ 250 °C).