PE-ALD of Ge1−xSx amorphous chalcogenide alloys for OTS applications

Abstract

Three-dimensional (3D) cross-point (X-point) technology, including amorphous chalcogenide-based ovonic threshold switching (OTS) selectors, is bringing new changes to the memory hierarchy for high-performance computing systems. To prepare for future 3D X-point memory scaling, we studied the plasma-enhanced atomic layer deposition (PE-ALD) of Ge_1−xS_x amorphous chalcogenide alloy thin films, the selection of which was motivated by their high optical bandgap and wide amorphous forming regions. The PE-ALD Ge_1−xS_x thin films were synthesized using a GeCl₄ precursor and H₂S plasma reactant, and their self-limited growth characteristics were studied in detail as a function of the exposure time of the ALD steps, temperature, and plasma power. The PE-ALD GeS₂ thin film showed an RMS roughness of 0.29 nm and good conformality in the vertical 3D structure. Moreover, the OTS behavior of GeS₂ and Ge₂S₃ mushroom-type devices with a 50 nm bottom electrode contact (BEC) were investigated as well as the trade-off relationship between the threshold voltage (1.9–6.2 V) and the normalized off current (20–250 nA) based on scaling the film thickness down from 30 nm to 5 nm. In particular, the GeS₂ device showed a higher threshold field (∼3.1 MV cm⁻¹) and lower normalized off current characteristics than the Ge₂S₃ device due to the higher trap density (2.1 × 10²¹ cm⁻³), according to the modified Poole–Frenkel (PF) model. The results achieved by this PE-ALD research on this novel binary GeS₂ amorphous chalcogenide for OTS applications will contribute to the development of future 3D cross-point memory scaling.