Atomic Layer Deposition of Nanocrystalline-As-Deposited (GeTe)_x(Sb₂Te₃)_1−x Films for Endurable Phase Change Memory

This paper introduces a new atomic layer deposition process for highly conformal, nanocrystalline-as-deposited GeTe−Sb₂Te₃ pseudobinary film growth at a deposition temperature of 130 °C. The process utilizes Ge(II)-amidoguanidinate (Ge^IIN(CH₃)₂[(NⁱPr)₂CN(CH₃)₂]), Te(Si(CH₃)₃)₂, and Sb(OC₂H₅)₃ with an NH₃ coreagent. The alternative GeTe and Sb₂Te₃ subcycles produced various film compositions, all consistent with the GeTe−Sb₂Te₃ tie lines, owing to the stoichiometric reactions between the precursors without involvement of undesirable side reactions. The density of the nanocrystalline Ge₂Sb₂Te₅ (GST225) films was 6.2 g·cm⁻³, similar to the density of the bulk crystalline material. The crystallization behaviors indicated that the distribution of the constituent elements of the GST225 films was highly uniform at the atomic level, as opposed to the case of the low-temperature (100 °C)-deposited films. The cubic to hexagonal transition at 350 °C upon postannealing produced (0001) hexagonal planes highly aligned along the substrate. The demonstration of the phase change memory device achieved high cycling endurance (>10⁷). Considering that further scaling and optimization of the cell design can improve the electrical performance, the nanocrystalline GST films introduced herein can provide potential utilities in the large-capacity three-dimensional vertical-type phase change memory.