Electrical Performance and Stability Improvements of High-Mobility Indium–Gallium–Tin Oxide Thin-Film Transistors Using an Oxidized Aluminum Capping Layer of Optimal Thickness

Abstract

We examined the effects of aluminum (Al) capping layer thickness on the electrical performance and stability of high-mobility indium–gallium–tin oxide (IGTO) thin-film transistors (TFTs). The Al capping layers with thicknesses (tAls) of 3, 5, and 8 nm were deposited, respectively, on top of the IGTO thin film by electron beam evaporation, and the IGTO TFTs without and with Al capping layers were subjected to thermal annealing at 200 °C for 1 h in ambient air. Among the IGTO TFTs without and with Al capping layers, the TFT with a 3 nm thick Al capping layer exhibited excellent electrical performance (field-effect mobility: 26.4 cm2/V s, subthreshold swing: 0.20 V/dec, and threshold voltage: −1.7 V) and higher electrical stability under positive and negative bias illumination stresses than other TFTs. To elucidate the physical mechanism responsible for the observed phenomenon, we compared the O1s spectra of the IGTO thin films without and with Al capping layers using X-ray photoelectron spectroscopy analyses. From the characterization results, it was observed that the weakly bonded oxygen-related components decreased from 25.0 to 10.0%, whereas the oxygen-deficient portion was maintained at 24.4% after the formation of the 3 nm thick Al capping layer. In contrast, a significant increase in the oxygen-deficient portion was observed after the formation of the Al capping layers having tAl values greater than 3 nm. These results imply that the thicker Al capping layer has a stronger gathering power for the oxygen species, and that 3 nm is the optimum thickness of the Al capping layer, which can selectively remove the weakly bonded oxygen species acting as subgap tail states within the IGTO. The results of this study thus demonstrate that the formation of an Al capping layer with the optimal thickness is a practical and useful method to enhance the electrical performance and stability of high-mobility IGTO TFTs.

Complex amorphous oxides: property prediction from high throughput DFT and AI for new material search

“…Finally, in the 2020s, the reported materials become more and more complex with quaternary oxides becoming more common than before. 64–71 A few exceptions excluded, most reported mobilities are now all below 50 cm 2 V −1 s −1 . One would almost be tempted to start believing that electron mobility in complex oxides is a time dependent property.…”

Readily Accessible Metallic Micro‐Island Arrays for High‐Performance Metal Oxide Thin‐Film Transistors

“…This is attributed to saturation of the I on and enhancement of the I off , reducing the transconductance and thus the mobility of the Al‐PL IGZO TFTs when the coverage is >50%. [ 42,48 ] Here, µs at of the best Al‐MIA IGZO device is >400% and >200% that of the pristine IGZO and best Al‐PL IGZO devices, respectively. Both the subthreshold slope ( SS ) and the threshold voltage ( V T ) of IGZO TFTs with Al‐MIA slightly increases from 0.16 to 0.20 V decade −1 and −7.38 to −9.90 V, respectively, as the Al‐MIA coverage increases from 23% to 51%; however, those of the Al‐PL devices dramatically increase from 0.16 to 0.77 V decade −1 and +3.3 to −20.0 V, respectively, as the coverage increases from 20% to 60% (Figure S10, Supporting Information).…”

“…The slightly degraded SS value and negatively shifted V T of both the Al‐MIA and Al‐PL IGZO TFTs, as the Al coverage increases, can be rationalized by the increased carrier density. [ 48,54,55 ] Furthermore, since the SS change correlates with the total trap density ( N T ) within the device channel region according to the equation Δ SS = qkTN T t ch ln(10)/ C ox , [ 56 ] where q is the electron charge, k is Boltzmann's constant, T is the temperature, and t ch is the channel thickness, the N T of these devices can be extracted. It is found that N T of the Al‐MIA TFTs (2.71 × 10 16 eV −1 cm −3 ) is far lower than that of the Al‐PL TFTs (3.64 × 10 17 eV −1 cm −3 ).…”

“…[ 45–47 ] However, this approach suffers considerable issues since the enhanced field‐effect mobility is accompanied by an increased off‐current and more complex device manufacture. [ 42,48 ] Furthermore, the development of unique (sub‐)micrometer patterning fabrication methods for heterojunctions and other TFT architectures remain challenging. [ 37 ]…”

An Empirical Modeling of Gate Voltage-Dependent Behaviors of Amorphous Oxide Semiconductor Thin-Film Transistors including Consideration of Contact Resistance and Disorder Effects at Room Temperature

“…This additionally alludes to a poor quality of metal contacts at the source and drain [1,2,7]. It is known that these non-ideal properties have a significant influence on the electrical performances of the AOS TFTs [2,3,8]; for example, the field-effect mobility of AOS TFTs is inversely proportional to the density of the localized tail states, while poor contact can lead to a higher contact resistance, and thus a lower mobility [9]. In addition, the contact resistance is typically extracted with a transmission line method (TLM), yielding a constant value without a gate-bias dependency [10].…”