Abstract
We investigated the effect of film thickness (geometrical confinement) on the structural evolution of sputtered indium-zinc-tin oxide (IZTO) films as high mobility n-channel semiconducting layers during post-treatment at different annealing temperatures ranging from 350 to 700 °C. Different thicknesses result in IZTO films containing versatile phases, such as amorphous, low-, and high-crystalline structures even after annealing at 700 °C. A 19-nm-thick IZTO film clearly showed a phase transformation from initially amorphous to polycrystalline bixbyite structures, while the ultra-thin film (5 nm) still maintained an amorphous phase. Transistors including amorphous and low crystalline IZTO films fabricated at 350 and 700 °C show reasonable carrier mobility (µFE) and on/off current ratio (ION/OFF) values of 22.4–35.9 cm2 V−1 s−1 and 1.0–4.0 × 108, respectively. However, their device instabilities against positive/negative gate bias stresses (PBS/NBS) are unacceptable, originating from unsaturated bonding and disordered sites in the metal oxide films. In contrast, the 19-nm-thick annealed IZTO films included highly-crystalline, 2D spherulitic crystallites and fewer grain boundaries. These films show the highest µFE value of 39.2 cm2 V−1 s−1 in the transistor as well as an excellent ION/OFF value of 9.7 × 108. Simultaneously, the PBS/NBS stability of the resulting transistor is significantly improved under the same stress condition. This promising superior performance is attributed to the crystallization-induced lattice ordering, as determined by highly-crystalline structures and the associated formation of discrete donor levels (~ 0.31 eV) below the conduction band edge.
Highlights
We investigated the effect of film thickness on the structural evolution of sputtered indium-zinc-tin oxide (IZTO) films as high mobility n-channel semiconducting layers during post-treatment at different annealing temperatures ranging from 350 to 700 °C
N-type IZTO thin films with different thicknesses ranging from 5 to 50 nm were annealed at 350 and 700 °C to investigate the feasibility of using polycrystalline oxide phases in field-effect transistors (FETs) applications
Though the amorphous phase was obtained at TA = 350 °C, annealing at TA = 700 °C resulted in various phases including amorphous, weakly crystalline and strongly crystalline phases depending on the film thickness
Summary
We investigated the effect of film thickness (geometrical confinement) on the structural evolution of sputtered indium-zinc-tin oxide (IZTO) films as high mobility n-channel semiconducting layers during post-treatment at different annealing temperatures ranging from 350 to 700 °C. Transistors including amorphous and low crystalline IZTO films fabricated at 350 and 700 °C show reasonable carrier mobility (μFE) and on/off current ratio (ION/OFF) values of 22.4–35.9 cm[2] V−1 s−1 and 1.0–4.0 × 108, respectively Their device instabilities against positive/negative gate bias stresses (PBS/NBS) are unacceptable, originating from unsaturated bonding and disordered sites in the metal oxide films. Amorphous indium gallium zinc oxide (a-IGZO) has been used as a semiconducting channel material in activematrix thin-film transistors (TFTs) for high-resolution liquid crystal and large organic light-emitting displays (OLEDs) since its discovery by Hosono and co-workers in 2 0041 Its attractive properties, such as high carrier mobility (μFE) of > 10 cm[2] V−1 s−1, superior uniformity, ultra-low leakage current (< 1 pA), low temperature processing and low cost fabrication have facilitated rapid commercialization of pixel switchers and drivers in advanced display devices. In metal oxide semiconductor systems, the GB-driven trap behavior is an interesting basic subject of material science and is important for potential industrial applications such as display, memory, logic and sensor devices, but it has not been systematically investigated
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