Field-Effect Mobility Enhancement of a-InGaZnO Thin Film Transistors through Metal Capping Layer Oxidation Induced Carrier Boosting

Amorphous oxide semiconductor (AOS), such as amorphous In-Ga-Zn-O (a-IGZO), has gained attention as channel materials for thin-film transistors (TFTs) based on attractive benefits, including low-temperature thin-film processing, low leakage current characteristics, and excellent optical properties due to its wide band gap. However, with relatively low carrier mobility in the range of 10-40 cm2/Vs, improving the mobility becomes one of the most important research concerns of AOS TFTs.<br/>Various methods to enhance the mobility of AOS have been suggested thus far, including changes in the composition of metal cation and proportion ratio of IGZO to form strong s-orbital overlap to enhance percolation conduction. Recently, there has been active research focused on enhancing the conduction of the channel by adopting a capping layer on AOS TFT channel surfaces. Performance-enhancing strategies were reported using oxidation of Ca/Al capping layers and low-temperature crystallization of various AOS using Mo, Ti and Al. Several papers have reported on using Ti and Ta capping layers for AOS TFTs to enhance the crystallinity of the channel through low-temperature thermal annealing.<br/>Our focus was on increasing the oxygen vacancies within the IGZO channel, thereby improving its conductivity and therefore its field-effect mobility μFE. The XPS results confirmed that Ta capping with PDA at 200 °C increased the oxygen vacancy of the IGZO film from 21% to 31% by breaking M-O bonds in IGZO and allowing oxygen diffusion to the Ta layer, which in turn acts as a carrier source.<br/>Based on the observed carrier boosting effects, the characteristics of Ta capped a-IGZO TFTs were investigated with increasing Ta coverage area. We observed that with 90% channel coverage (Ta capping) demonstrated a remarkable 875% enhancement in μFE, increasing from 16 cm²/Vs to 140 cm²/Vs, while maintaining near-zero threshold voltage and low off-state leakage current. One problem with increasing the carrier concentration throughout the length is a severe negative shift in the threshold voltage Vth. The extremely high carrier concentration raises the Fermi level near the contact high enough to make the TFT operate in the depletion mode, or not turn off with nominal negative gate voltages. However, if a part of the back channel is left uncapped, the uncapped IGZO region would create a homojunction and act as a potential barrier, effectively maintaining the off-state current, while the capped IGZO region acts as a carrier-boosted region, facilitating higher channel conduction. We formed this IGZO homojunction and allowed the TFT to operate in the enhancement mode with a positive Vth and prevented an increase in the off-state current. As the carrier-boosted region becomes wider, the total carrier concentration increases, leading to an increase in on-current and μFE. The results reported in this paper provide a simple method to achieve high-performance metal oxide semiconductor devices, which are expected to be applicable to various next-generation device applications.