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We report hybrid organic/inorganic complementary circuits using ink-jet-printed fluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) as the p-channel material and zinc oxide deposited by plasma enhanced atomic layer deposition (PEALD) as the n-channel material. Using a mixed solvent system, discrete ink-jet printed diF-TESADT OTFTs have field effect mobility as large as 0.4 cm2/V s. PEALD ZnO TFTs typically have field-effect mobility >15 cm2/V s. Using p-type diF-TESADT and n-type ZnO active layers in a simple, 4-mask, 1 ink jet printing step, low temperature ( ⩽ 200 ° C process we fabricated complimentary MOS (CMOS) inverters with maximum voltage gain of 35 and sub-pA leakage currents for both low and high input levels.

Oxide semiconductor TFTs are of interest where low-cost, high mobility, and compatibility with flexible substrates is required. We have previously demonstrated high quality thin-film transistors (TFTs) using ZnO and Al 2 O 3 deposited at low temperature (200°C) by weak oxidant, plasma-enhanced atomic layer deposition (PEALD) using a reactor utilizing a pump and purge mode of operation [1]. We also reported the use of thin, weak oxidant, PEALD Al 2 O 3 films as gate insulator for scaled III-V MOSFETs [2]. In this work, we report TFTs fabricated using ZnO and Al 2 O 3 deposited in a novel weak oxidant PEALD reactor using a showerhead to dispense the reactant gases and an integrated load-lock.

Oxide semiconductor electronics may enable new applications including large-area, flexible, integrated systems. ZnO thin film transistors have been reported with field-effect mobility > 100 cm2/V·s, on-current density > 700 mA/mm, and microwave operation (f T > 2 GHz, f max > 7 GHz) for ZnO deposited by pulsed laser deposition at 400°C.[1] Other oxide semiconductors, including amorphous and crystalline mixtures of I 2 O 3 , Ga 2 O 3 , ZnO, have also been widely studied, and high mobility (> 30 cm2/V·s) thin film transistors and circuits with propagation delays d = dI DS /dV DS ) observed in a range of oxide thin film transistors. In particular we find that self-heating is a significant limiting factor for the performance of oxide devices and circuits on low-cost, low-thermal conductivity substrates such as glass and plastic.

We report using a novel, weak oxidant, plasma-enhanced atomic layer deposition (PEALD) process to fabricate stable, high mobility ZnO thin film transistors (TFTs) and fast circuits on glass and polyimide substrates at 200 °C. Weak oxidant PEALD provides a simple, fast deposition process that results in uniform, conformal coatings and enhancement-mode MOSFETs from uncompensated films. Our PEALD ZnO TFTs have field-effect mobility of 20–30 cm2/V·s and simple gate-self-aligned saturated-load ring oscillators on glass with 2.5 µm gate length had propagation delay ≪10 ns/stage at V DD = 18 V. We also report the fastest oxide semiconductor circuits on polyimide substrates with propagation delay of ∼60 ns/stage at V DD = 18 V.

We report using a novel weak-oxidant plasma-enhanced atomic layer deposition (PEALD) process at 200 °C to deposit ZnO thin films for stable, high mobility, pyroelectric thin film transistors (TFTs). Our PEALD ZnO TFTs use Al 2 O 3 gate dielectric, also deposited by PEALD, and have field-effect mobility of 20–30 cm2/V⋅s. In these ZnO TFTs the mobility is found to be very weakly temperature activated (< 10 meV) down to 77 K, but the devices have a fast, linear, and reversible, threshold voltage shift with temperature which is believed to be related to pyroelectric charge in the ZnO layer.

We report combining plasma-enhanced atomic layer deposition (PEALD) ZnO thin film transistors (TFTs) with a self-aligned-gate process to fabricate high speed circuits. Sputter deposited ZnO films have been widely studied, but there have been few reports of dense thin films and high performance devices and circuits. Atomic layer deposition (ALD) of ZnO has been shown to be a very uniform and conformal process, but ALD ZnO films typically have high background carrier concentration and require doping compensation for enhancement-mode TFTs. PEALD provides the uniform conformal coating of ALD and enhancement-mode devices from uncompensated films. Our PEALD ZnO TFTs have linear field-effect mobility >20 cm2/V⋅s and saturation field-effect mobility >30 cm2/V⋅s and PEALD ring oscillators with beta ratio ~5, channel length ~2.8 µm, and ~1.5 µm gate-source/drain overlap operate at ~25 ns/stage. Recently, scaled indium-gallium-zinc oxide ring oscillator circuits on silicon substrates (0.5 µm channel length, and 0.5 µm overlap) were reported to operate at ~7 ns/stage with a saturated-load inverter design, and < 1 ns/stage with a novel bootstrapped inverter design [1]. The speed of our previous PEALD circuits [2] was largely limited by the parasitic capacitance between the gate and drain, and a self-aligned-gate process would provide higher speed devices and circuits. Here we have report a simple self-aligned-gate process for ZnO TFTs and high speed circuits.

Oxide semiconductor-based thin film transistors (TFTs) have potential as higher performance and improved stability replacements for amorphous silicon TFTs in displays and other large-area electronics. ZnO TFTs fabricated by pulsed laser deposition (PLD) at 400 °C on GaAs substrates have demonstrated mobility over 100 cm2/V⋅s [1], and five-stage ring oscillators based on 0.5 μm gate length indium-gallium-zinc-oxide (IGZO) TFTs fabricated on silicon substrates have demonstrated propagation delay below 1 ns/stage for bootstrapped ring oscillators [2].

— The impact of passivation processes on ZnO thin-film transistors is reported. In general, passivation processes result in back-channel doping, which corresponds to shifts in threshold voltage and changes in subthreshold slope. It was determined that ALD-based passivation results in considerably smaller undesirable shifts than those observed with plasma-based processes. Two approaches, one a bulk doping with ammonia and the other a surface treatment with hydrogen peroxide, to further mitigate the detrimental effects of the passivation process are described. After proper passivation, ZnO devices show negligible hysteresis, have excellent stability to bias stress, and maintain or improve the good transport properties of as-deposited devices. Although the existence of grain boundaries has been assumed to be a point of concern for device stability in polycrystalline metal oxides, no evidence was found to suggest that the grain boundaries present in these ZnO thin-film transistors have affected the device stability.

We report using a novel, weak oxidant, plasma-enhanced atomic layer deposition (PEALD) process at 200 °C to fabricate stable, high mobility ZnO thin film transistors (TFTs) and fast circuits on glass and polyimide substrates. Weak oxidant PEALD provides a simple, fast deposition process that results in uniform, conformai semiconductor and dielectric layers and enhancement-mode MOSFETs from uncompensated films. Highly conformai PEALD Al2O3 layers provide a high yield dielectric on rough plastic substrates for both PEALD ZnO TFTs and cross-overs. Our PEALD ZnO TFTs have field-effect mobility of >20 cm2/V·s on polyimide substrates with excellent bias stress stability. TFTs biased continuously for 40,000s showed