Atomic layer deposition of chalcogenides for next-generation phase change memory

Abstract

Atomic layer deposition (ALD) is a thin film growth technique that uses self-limiting, sequential reactions localized at the growing film surface. It guarantees exceptional conformality on high-aspect-ratio structures and controllability over the thickness and composition at the atomic level. The design of the chemical reactions and the synthesis of the corresponding inorganic compounds have been the focus of the growth of various functional films via ALD. This review provides an overview of the recent progress in the ALD of chalcogenides, focusing on Ge–Sb–Te (GST) ternary alloys, the core material for phase-change memory (PCM). First, the concept, materials, and device structure of PCM are briefly introduced, and the benefits of the ALD process in establishing low-power and high-capacity PCM are explained. The following sections discuss precursor options for Ge, Sb, and Te, and reaction mechanisms for the film deposition, with special attention to the challenges in depositing the multi-component films, including the non-ideal chemical interactions between the precursors and the predeposited films. Influences of the growth temperature, co-reactants, and substrate types are also highlighted. Next, the growth of the Ge–Se-based thin film, which is a useful material as the cell selector in a passive array, is discussed. Despite the similar characteristics of Se precursors to those of Te, notable differences were observed in their reaction mechanisms and accompanying film growth behavior. The final section presents an outlook on the ALD of GST alloys and a recent research effort to expand the ALD technique beyond the conventional GST alloys, for example to superlattice-like structures.