Asal Kiazadeh

Towards environmental friendly solution-based ZTO/AlOx TFTs

Solution based deposition has been recently considered as a viable option for low-cost flexible electronics. In this context research efforts have been increasingly centred on the development of suitable solution-processed materials for oxide based transistors. Nevertheless, the majority of synthetic routes reported require the use of toxic organic solvents. In this work we report on a new environmental friendly solution combustion synthesis route, using ethanol as solvent, for the preparation of indium/gallium free amorphous zinc-tin oxide (ZTO) thin film transistors (TFTs) including AlOx gate dielectric. The decomposition of ZTO and AlOx precursor solutions, electrical characterization and stability of solution processed ZTO/AlOx TFTs under gate-bias stress, in both air and vacuum atmosphere, were investigated. The devices demonstrated low hysteresis (ΔV = 0.23 V), close to zero turn on voltage, low threshold voltage (VT = 0.36 V) and a saturation mobility of 0.8 cm2 V−1 s−1 at low operation voltages. Ethanol based ZTO/AlOx TFTs are a promising alternative for applications in disposable, low cost and environmental friendly electronics.

Opto-electronic characterization of electron traps upon forming polymer oxide memory diodes

Metal-insulator-polymer diodes where the insulator is a thin oxide (Al2O3) layer are electroformed by applying a high bias. The initial stage is reversible and involves trapping of electrons near the oxide/polymer interface. The rate of charge trapping is limited by electron transport through the polymer. Detrapping of charge stored can be accomplished by illuminating with light under short-circuit conditions. The amount of stored charge is determined from the optically induced discharging current transient as a function of applied voltage and oxide thickness. When the charge density exceeds 8 × 1017/m2, an irreversible soft breakdown transition occurs to a non-volatile memory diode.

Improving positive and negative bias illumination stress stability in parylene passivated IGZO transistors

The impact of a parylene top-coating layer on the illumination and bias stress instabilities of indium-gallium-zinc oxide thin-film transistors (TFTs) is presented and discussed. The parylene coating substantially reduces the threshold voltage shift caused by continuous application of a gate bias and light exposure. The operational stability improves by 75%, and the light induced instability is reduced by 35%. The operational stability is quantified by fitting the threshold voltage shift with a stretched exponential model. Storage time as long as 7 months does not cause any measurable degradation on the electrical performance. It is proposed that parylene plays not only the role of an encapsulation layer but also of a defect passivation on the top semiconductor surface. It is also reported that depletion-mode TFTs are less sensitive to light induced instabilities. This is attributed to a defect neutralization process in the presence of free electrons.

Anomalous temperature dependence of the current in a metal-oxide-polymer resistive switching diode

Metal-oxide polymer diodes exhibit non-volatile resistive switching. The current–voltage characteristics have been studied as a function of temperature. The low-conductance state follows a thermally activated behaviour. The high-conductance state shows a multistep-like behaviour and below 300 K an enormous positive temperature coefficient. This anomalous behaviour contradicts the widely held view that switching is due to filaments that are formed reversibly by the diffusion of metal atoms. Instead, these findings together with small-signal impedance measurements indicate that creation and annihilation of filaments is controlled by filling of shallow traps localized in the oxide or at the oxide/polymer interface.

The role of internal structure in the anomalous switching dynamics of metal-oxide/polymer resistive random access memories

The dynamic response of a non-volatile, bistable resistive memory fabricated in the form of Al2O3/polymer diodes has been probed in both the off- and on-state using triangular and step voltage profiles. The results provide insight into the wide spread in switching times reported in the literature and explain an apparently anomalous behaviour of the on-state, namely the disappearance of the negative differential resistance region at high voltage scan rates which is commonly attributed to a “dead time” phenomenon. The off-state response follows closely the predictions based on a classical, two-layer capacitor description of the device. As voltage scan rates increase, the model predicts that the fraction of the applied voltage, Vox , appearing across the oxide decreases. Device responses to step voltages in both the off- and on-state show that switching events are characterized by a delay time. Coupling such delays to the lower values of Vox attained during fast scan rates, the anomalous observation in the on-state that, device currents decrease with increasing voltage scan rate, is readily explained. Assuming that a critical current is required to turn off a conducting channel in the oxide, a tentative model is suggested to explain the shift in the onset of negative differential resistance to lower voltages as the voltage scan rate increases. The findings also suggest that the fundamental limitations on the speed of operation of a bilayer resistive memory are the time- and voltage-dependences of the switch-on mechanism and not the switch-off process.

Influence of Channel Length Scaling on InGaZnO TFTs Characteristics: Unity Current-Gain Cutoff Frequency, Intrinsic Voltage-Gain, and On-Resistance

This paper presents a study concerning the role of channel length scaling on IGZO TFT technology benchmark parameters, which are fabricated at temperatures not exceeding 180 °C. The parameters under investigation are unity current-gain cutoff frequency, intrinsic voltage-gain, and on-resistance of the bottom-gate IGZO TFTs. As the channel length varies from 160 to 3 μm, the measured cutoff frequency increases from 163 kHz to 111.5 MHz, which is a superior value compared to the other competing low-temperature thin-film technologies, such as organic TFTs. On the other hand, for the same transistor dimensions, the measured intrinsic voltage-gain is changing from 165 to 5.3 and the on-resistance is decreasing from 1135.6 to 26.1 kΩ. TFTs with smaller channel length (3 μm) have shown a highly negative turnon voltage and hump in the subthreshold region, which can be attributed to short channel effects. The results obtained here, together with their interpretation based on device physics, provide crucial information for accurate IC design, enabling an adequate selection of device dimensions to maximize the performance of different circuit building blocks assuring the multifunctionality demanded by system-on-panel concepts.

Solution based zinc tin oxide TFTs: the dual role of the organic solvent

Chemical solution deposition is a low cost, scalable and high performance technique to obtain metal oxide thin films. Recently, solution combustion synthesis has been introduced as a chemical route to reduce the processing temperature. This synthesis method takes advantage of the chemistry of the precursors as a source of energy for localized heating. According to the combustion chemistry some organic solvents can have a dual role in the reaction, acting both as solvent and fuel. In this work, we studied the role of 2-methoxyethanol in solution based synthesis of ZTO thin films and its influence on the performance of ZTO TFTs. The thermal behaviour of ZTO precursor solutions confirmed that 2-methoxyethanol acts simultaneously as a solvent and fuel, replacing the fuel function of urea. The electrical characterization of the solution based ZTO TFTs showed a slightly better performance and lower variability under positive gate bias stress when urea was not used as fuel, confirming that the excess fuel contributes negatively to the device operation and stability. Solution based ZTO TFTs demonstrated a low hysteresis (ΔV = −0.3 V) and a saturation mobility of 4–5 cm2 V−1 s−1.

Operational stability of solution based zinc tin oxide/SiO2 thin film transistors under gate bias stress

In this study, we report solution-processed amorphous zinc tin oxide transistors exhibiting high operational stability under positive gate bias stress, translated by a recoverable threshold voltage shift of about 20% of total applied stress voltage. Under vacuum condition, the threshold voltage shift saturates showing that the gate-bias stress is limited by trap exhaustion or balance between trap filling and emptying mechanism. In ambient atmosphere, the threshold voltage shift no longer saturates, stability is degraded and the recovering process is impeded. We suggest that the trapping time during the stress and detrapping time in recovering are affected by oxygen adsorption/desorption processes. The time constants extracted from stretched exponential fitting curves are ≈106 s and 105 s in vacuum and air, respectively.

Bias Stress and Temperature Impact on InGaZnO TFTs and Circuits

This paper focuses on the analysis of InGaZnO thin-film transistors (TFTs) and circuits under the influence of different temperatures and bias stress, shedding light into their robustness when used in real-world applications. For temperature-dependent measurements, a temperature range of 15 to 85 °C was considered. In case of bias stress, both gate and drain bias were applied for 60 min. Though isolated transistors show a variation of drain current as high as 56% and 172% during bias voltage and temperature stress, the employed circuits were able to counteract it. Inverters and two-TFT current mirrors following simple circuit topologies showed a gain variation below 8%, while the improved robustness of a cascode current mirror design is proven by showing a gain variation less than 5%. The demonstration that the proper selection of TFT materials and circuit topologies results in robust operation of oxide electronics under different stress conditions and over a reasonable range of temperatures proves that the technology is suitable for applications such as smart food packaging and wearables.

Boosting Electrical Performance of High-κ Nanomultilayer Dielectrics and Electronic Devices by Combining Solution Combustion Synthesis and UV Irradiation.

In the past decade, solution-based dielectric oxides have been widely studied in electronic applications enabling the use of low-cost processing technologies and device improvement. The most promising are the high-κ dielectrics, like aluminum (AlOx) and hafnium oxide (HfOx), that allow an easier trap filling in the semiconductor and the use of low operation voltage. However, in the case of HfOx, a high temperature usually is needed to induce a uniform and condensed film, which limits its applications in flexible electronics. This paper describes how to obtain HfOx dielectric thin films and the effect of their implementation in multilayer dielectrics (MLD) at low temperatures (150 °C) to apply in thin film transistors (TFTs) using the combination of solution combustion synthesis (SCS) and ultraviolet (UV) treatment. The single layers and multilayers did not show any trace of residual organics and exhibited a small surface roughness (<1.2 nm) and a high breakdown voltage (>2.7 MV·cm-1). The resulting TFTs presented a high performance at a low operation voltage (<3 V), with high saturation mobility (43.9 ± 1.1 cm2·V-1·s-1), a small subthreshold slope (0.066 ± 0.010 V·dec-1), current ratio of 1 × 106 and a good idle shelf life stability after 2 months. To our knowledge, the results achieved surpass the actual state-of-the-art. Finally, we demonstrated a low-voltage diode-connected inverter using MLD/IGZO TFTs working with a maximum gain of 1 at 2 V.