(Invited) Atomic-Layer-Deposition of in-Situ Crystallized Ge2Sb2Te5 Alloy and Gete/Sb2Te3 Superlattice, and Their Phase-Change Performances

Abstract

This paper introduces a new atomic layer deposition process for highly conformal, nanocrystalline-as-deposited GeTe-Sb2Te3 pseudo-binary film growth at a deposition temperature of 130 °C. The process utilizes Ge(II)-guanidinate, Te(Si(CH3)3)2, and Sb(OC2H5)3 with the NH3 co-reagent. 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/cm3, 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 post-annealing produced (0001) hexagonal planes highly aligned along the substrate. The demonstration of the phase change memory device achieved high cycling endurance (>107).The same ALD process was further exploited to grow the GeTe/Sb2Te3 superlattice structure, which might be an even more energy-efficient phase change material. It was found that the Sb2Te3 layer could be grown in highly c-axis oriented manner on SiO2 surface, which then can be used as the template layer for the subsequent growth of also highly c-axis oriented GeTe layer. Direct growth of the GeTe layer on SiO2 did not result in the similar optimal results. However, a certain intermixing occurs during the alternating growth of the Sb2Te3 and GeTe layers, of which details are dependent on the thickness ratio of the two layers. Interestingly, the intermixed layers still showed a certain type of layered structure, which appeared to be related to the van der Waals nature of the Sb2Te3 layer. However, such a layered structure could hardly be achieved on metallic surface such as TiN. This might be related with the highly interacting chemical nature of the TiN surface as well as the relatively rough surface topography. Adopting graphene or ultra-thin SiO2 layer on the TiN recovered the highly c-axis aligned growth behavior.