Andre Zeumault

Gravure-Printed Sol-Gels on Flexible Glass: A Scalable Route to Additively Patterned Transparent Conductors.

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.

Improved Technique for Quantifying the Bias-Dependent Mobility of Metal-Oxide Thin-Film Transistors

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.

Engineering high-k La x Zr 1−x O y dielectrics for high-performance fully-solution-processed transparent transistors

Summary form only given. Metal oxide thin-film transistors based on high-k dielectrics (ZrOx, HfOx, Al2Ox) are a promising technology with attractive performance (μeff ~ 10 -100 cm2/Vs, On/Off > 107) and high transparency (80 - 90%). 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. 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 ZrOx, 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.

Interpretation of subthreshold swing and threshold voltage in solution-processed zinc oxide TFTs

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.

Anomalous process temperature scaling behavior of sol-gel ZrO x gate dielectrics: Mobility enhancement in ZnO TFTs

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 ZrOx 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 ZrOx 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 ZrOx was reduced.