PEALD deposited aluminum hafnium mixed oxide dielectrics for amorphous-IGZO TFTs

Abstract

(Al–Hf) mixed oxide thin films (HfAlO) are attracted by many researchers because of their outstanding optical and electrical properties compared to hafnium oxide (HfO2). This study prepared HfAlO films at 300 °C using plasma-enhanced atomic layer deposition (PEALD). Changing the cycle ratio of HfO2 and aluminum oxide (Al2O3), the mixing effect on the properties of HfAlO film was studied. The characterization of the HfAlO films was investigated using spectroscopic ellipsometry (SE), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), scanning electron microscopy (SEM), and UV–vis spectroscopy. The results show that the aluminum content and optical bandgap of the HfAlO films increased with the increase of the Al2O3 cycle ratio. The dielectric constant of the HfAlO film with a 20 % Al2O3 cycle ratio (cycles of Al2O3/(cycles of Al2O3 + cycles of HfO2)) reaches ∼ 26, and the breakdown electric field is about 6.5 MV/cm. Pure HfO2 and 20 % Al2O3 cycle ratio deposited HfAlO was used as the gate dielectric layer, and the influence on the performance of amorphous indium gallium zinc oxide (a-IGZO) thin film transistor (TFT) was studied. A-IGZO TFT with a 20 % Al2O3 cycle ratio mixed HfAlO film as the gate dielectric exhibits the best performance, showing a high Ion/Ioff ratio of 1.3 × 109, a saturation mobility of ∼5.8 cm2/Vs, a subthreshold slope of 88 mV per decade and a low threshold voltage of −0.1V.