Amorphous Ge-Se-S chalcogenide alloys via post plasma sulfurization of atomic layer deposition GeSe for ovonic threshold switch applications

For the future scaling of 3D cross-point (X-point) memory, it is necessary to implement atomic layer deposition (ALD) of chalcogenides for ovonic threshold switch (OTS) applications. We investigated ternary Ge-Se-S amorphous chalcogenide alloys based on ALD process, motivated by the expectation of low off-current and stable OTS behavior by partially replacing S in binary Ge-Se. Ge-Se-S alloys were synthesized by post-deposition sulfurization of ALD GeSe ₂ thin films. Especially, we investigated the growth characteristics and film properties of ALD GeSe ₂ using HGeCl ₃ precursors with Se(SiMe ₃) ₂ together with density-functional theory (DFT) calculations. By changing the temperature and the low-temperature plasma sulfurization time, the compositions of 10-nm-thick Ge-Se-S thin films were controlled along the GeSe ₂-Ge ₂ S pseudo-binary line in ternary phase diagram. It was confirmed that the Ge ₅Se ₃S ₂ alloys maintained an amorphous phase and excellent step coverage, similar to ALD GeSe ₂ . Finally, we compared the OTS electrical characteristics of 10-nm-thick ALD GeSe ₂ with Ge ₅Se ₃S ₂ amorphous chalcogenide thin films in a mushroom-type device with a 50-nm bottom electrode. The novel Ge ₅Se ₃S ₂ had a slightly larger threshold voltage (V _th) drift than GeSe ₂ but exhibited the advantages of a higher threshold field, lower off-current, and smaller V _th fluctuation up to 10 ⁶ cycles.