Direct PEALD Deposition of a HfO₂ Gate Dielectric without the Passivation for TFTs on Rigid and Flexible Substrates

Although hafnium oxide has been widely studied for silicon CMOS and thin-film transistors (TFTs), it has typically been implemented either with a passivating layer or with nanolaminates layered with other dielectric materials to improve the performance of these TFTs. Eliminating postdeposition passivation steps simplifies the fabrication process, potentially reducing manufacturing costs, enhancing throughput, and increasing compatibility with soft substrates. In this work, we have investigated the quality of HfO2 film deposited at four sets of substrate temperatures on silicon via plasma-enhanced atomic layer deposition (PEALD) without any postdeposition annealing. The materials characterization of these as-deposited films was investigated via grazing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), confirming the best quality of film at a substrate temperature of 200 °C. Meanwhile, the resulting 50 nm thick HfO2 gate dielectric exhibited a ∼382 nF/cm2 zero-bias capacitance at 1 kHz and an estimated high dielectric constant value of 22 as extracted from the MOSCAP (Si/HfO2/Al) device. Furthermore, the same method was employed to deposit HfO2 films on glass and polyimide substrates along with Si, followed by the solution-processed In2O3 film as a semiconductor and Al deposition atop as a source/drain electrode to fabricate TFTs on all three substrates. The figure of merit (FOM) parameters including threshold voltage (Vth), subthreshold slope (SS), saturation mobility (μsat), and trap density (Neff) were extracted in each case, and their reliability was examined on multiple devices in each case. Preliminary results demonstrate the potential of the as-deposited HfO2 with a limited thermal budget to achieve acceptable device performance in flexible TFTs, highlighting a promising pathway for the streamlined fabrication of next-generation flexible electronics.