Aluminium oxide-capped MoS2 transistors for ultrathin OLED displays

April 24, 2018 // By Julien Happich
In a paper titled "Flexible active-matrix organic light-emitting diode display enabled by MoS2 thin-film transistor" published in Science Advances, a team of South Korean researchers from Yonsei University, and Chung-Ang University propose a novel ultrathin transistor architecture with drastically increased charge mobility, which could be used for transparent flexible OLED displays.

In their paper, the researchers highlight the drawbacks of 2D MoS2 used as an active channel material in thin film transistors, lacking charge mobility due to a large contact barrier between the source/drain (S/D) metal electrode and the MoS2 channel. In such architectures, electron transport is hindered by Coulomb scattering and trap charges at the interface between the gate dielectric and MoS2, lowering charge mobility to under 1 cm2 V−1 s−1 on par with that of a-H Si.

They propose an Al2O3-capped TFT device, whereby a bilayer of Molybdenum disulphide is sandwiched between two layers of high-k dielectric aluminium oxide.

According to the paper, the high-k Al2O3 layer reduces the contact resistance at the metal/MoS2 interface and facilitates considerable n-type doping of the MoS2 layer owing to its oxygen-deficient surface. This in turn reduces the scattering charge impurities. What's more, the bottom Al2O3 layer reduces surface roughness, further improving the device's performance as it effectively decreases the interface-trapped charge density.

Fig. 1: (top) the high-mobility MoS2 TFT using an Al2O3 passivation layer capping the MoS2 channel region. An ultra-thin MoS2-based backplane array drives an AMOLED display (middle), capable of operation when affixed to human skin (bottom).

All these effects combined yield a 28-fold increase in mobility value compared to conventional back-gate structure, report the authors, with a positive threshold voltage of about 5V, a high ON/OFF ratio at circa 108.