A combinatorial study of SiGeAsTe thin films for application as an Ovonic threshold switch selector

Abstract

In this paper, we investigate the thermal stability of a wide range of ternary and quaternary (Si)GeAsTe alloy thin films. These type of materials are reported to show Ovonic threshold switching, which means they conduct the current above a specific threshold voltage and are not conducting at lower voltages, making them ideal as a selector element for crosspoint memory devices. For threshold switching to occur in these chalcogenides, the amorphous state of the material is crucial, and hence the material may not crystallize below 400 degrees C to be compatible with temperatures used in device process flows. A combinatorial deposition technique is used to create a thin film library of 36 different compositions, and in situ X-ray diffraction and X-ray fluorescence spectroscopy is used to investigate the thermal stability of the films. We show that Si doping of GeAsTe improves the thermal stability by increasing the crystallization temperature and small amounts of Si (i.e.5at%) decreases the tendency to lose material by sublimation. Also capping the films with a W cap avoids material loss if the capping layer does not show cracking under annealing. An optimal chalcogenide composition, being As50Te20Ge20Si10 combined with a W electrode is identified and is integrated in 65nm mushroom type devices for electrical characterization. The material shows threshold switching and excellent endurance with over 10^(8) cycles.