Evidence for Pseudocapacitance and Faradaic Charge Transfer in High-Mobility Thin-Film Transistors with Solution-Processed Oxide Dielectrics

Abstract

In developing low-power electronics, low-voltage transistors have been intensively investigated. One of the most important findings is that some high-k oxide gate dielectrics can lead to remarkable enhancement of apparent mobility in thin-film transistors (TFTs), which is not clearly understood. Here, we investigate InO x TFTs with solution-processed AlO x dielectrics. At very low frequencies (<1 Hz), the AlO x films feature strong voltage-dependent capacitance. Also, cyclic voltammograms show clear features of surface-controlled Faradaic charge transfer. The two independent experiments both point to the formation of pseudocapacitance, which is similar to the mechanism behind some supercapacitors. A physical model including charge transfer is established to describe ion distribution. The charge transfer is probably related to residual hydrogens, as revealed by secondary-ion mass spectroscopy. The results provide direct evidence of the formation of pseudocapacitance in TFTs with high apparent mobilities and advance the understanding of mechanisms, measurements, and applications of such TFTs for low-power electronics.M etal-oxide thin-film transistors (TFTs) have attracted significant attention for applications in the area of active-matrix devices such as high-resolution displays with organic light-emitting diodes. 1−4 The advantages include largearea uniformity, high mobility, and compatibility in a flexible electronic device. To improve the performance of the metaloxide TFTs, high-k dielectrics have been used as gate insulator in metal-oxide TFTs such as Y 2 O 3 , Al 2 O 3 , ZrO 2 , and HfO 2 . 5−8 It is exciting to note that there are considerable reports that the solution-processed high-k gate dielectric induces remarkable enhancement of mobility, 9,10 with several to 10 times as compared with that in the cases with thermal silicon oxide gate. However, no consensus yet exists on how to explain this remarkable enhancement.Lee et al. attributed the mobility enhancement to the increased capacitance and carrier density using a high-k gate dielectric. 11 However, for some TFTs with high-k gate dielectrics, the mobility was not enhanced. 6,12 Zeumault et al. proposed that donor-like traps in ZrO 2 are aligned at shallow energies with respect to the transport band of ZnO and provide more carriers in the ZnO channel by charge migration at the insulator/semiconductor interface, resulting in high mobility. 13 Heo et al. attributed the mobility enhancement to electric double layer (EDL) formation at the solutionprocessed AlO x /InO x interface. 14 In most cases, large capacitance induced by EDL can lead to low operating voltage. However, numerical simulations have revealed that, once the

Engineering Bilayer AlOx /YAlOx Dielectric Stacks for Hysteresis-Free Switching in Solution-Processed Metal-Oxide Thin-Film Transistors

“…Still, this would yield only one order of magnitude difference in the ON current by itself. In addition to the electric double-layer capacitance, faradaic charge exchanges at the quasi-static frequency levels (Liang et al, 2020b), reflects as several orders of magnitude increase in the capacitance of the MOS capacitor. This can explain the difference in the ON current of the devices observed here, as well.…”

“…(Futscher et al, 2020). Residuals in the AlO x dielectrics, such as H + and OH − ions, were reported partly responsible for the enhanced quasi-static frequency polarization in the dielectric (Daunis et al, 2018;Bolat et al, 2020;Liang et al, 2020b). Secondary ion mass spectroscopy (SIMS) measurements were performed on the bilayer dielectrics spin-coated on Si wafers to investigate the hypothesis in detail.…”

“…Such behavior was closely related to the presence of the water within the dielectric (Daunis et al, 2018). In addition to the electron migration hypothesis, Liang et al (Liang et al, 2020b) underlined another phenomenon existing in solution-processed AlO x dielectrics. It was observed that at the quasi-static frequencies, in addition to the readily existing high electric double-layer (EDL) capacitance, a bias dependent capacitance was present due to faradaic charge transfer at the interfaces of the dielectric with semiconductor and metal layers, which increased the gate capacitance by almost two orders of magnitude (the socalled pseudo capacitance effect).…”

Spray‐Pyrolyzed High‐k Zirconium‐Aluminum‐Oxide Dielectric for High Performance Metal‐Oxide Thin‐Film Transistors for Low Power Displays

“…The effective reduction of C ′ in Ar/O 2 plasma‐treated ZAO GIs ascribes the suppression of oxygen‐related defects in the surface and/or bulk of ZAO. [ 48a,49,50 ] Table 1 summarizes all the dielectric properties of ZAO GIs. The areal capacitance and dielectric constant of pristine, 1 and 2 cycles Ar/O 2 plasma‐treated ZAO GIs are 248, 228, and 220 nF cm −2 , and 13.51, 12.35, and 11.96, respectively, at the frequency of 100 Hz.…”

Inkjet‐Printed Ternary Oxide Dielectric and Doped Interface Layer for Metal‐Oxide Thin‐Film Transistors with Low Voltage Operation

“…In accordance with our observations, previous reports suggested that low‐temperature solution‐processed oxide gate dielectric materials can suffer from increased capacitance at low frequencies due to hydrogen content remaining in the films, [ 52 ] for instance, from incomplete sol–gel densification (MOHM), [ 26 ] leading to slow formation/reorientation of dipole moments and/or proton migration upon application of positive gate voltage. [ 53 ] Therefore, an annealing temperature of 500 °C was selected for further studies of the material. The results highlight the importance of measuring the low‐frequency capacitance after plasma treatment for solution‐processed dielectrics, to avoid mobility overestimation in the case that a plasma treatment is performed before the semiconductor film is processed.…”