Atomic layer deposition characteristics of (GeTe2)(1-x)(Sb2Te3)x pseudo-binary thin films for phase change memory application

Abstract

Phase change random access memory appears to be the strongest candidate for next-generation high density non-volatile memory. The fabrication of ultra-high density phase change memory (>> 1 Gb) depends heavily on the thin film growth technique for the phase changing material, most typically containing Ge, Sb, and Te (Ge-Sb-Te). Atomic layer deposition (ALD) at low temperatures is the most preferred growth method for depositing such complex materials over surfaces possessing extreme topology. In this dissertation, [(CH3)3Si]2Te and stable alkoxy-Ge (Ge(OCH3)4 and Ge(OC2H5)4) and alkoxy-Sb (Sb(OC2H5)3) metal-organic precursors were used to deposit various layers with compositions lying on the GeTe2 – Sb2Te3 tie line and [(CH3)3Si]2Te and stable alkoxy-Ge (Ge(OC2H5)4) and alkoxy-Sb (Sb(OC2H5)3), [(CH3)3Si]3Sb metal-organic precursors were used to deposit various layers with compositions lying on the GeTe2 – Sb5Te3 tie line, respectively, at a substrate temperature as low as 70 °C using a thermal ALD process. The adsorption of Ge precursor was proven to be a physisorption type by observing exponential decays of incorporation amount of GeTe2 with increasing purge time, which could be explained well by the isothermal desorption of the physisorbed Ge precursor molecules. The incorporation behaviors of the Ge precursor with different ligands were explained by the adsorption and desorption kinetics based on the Brunauer-Emmett-Teller (BET) isotherm, which explains the behavior of a gas adsorption system where two or more layers of adsorbates are formed on the surface. It was suggested that the tendency to form multiple layers was increase along with the ligand length. Consequently, Ge precursor with the ethoxy ligands forms multiple layers while Ge precursor with the methoxy ligands was not, and the exponential factor of decaying function for Ge(OC2H5)4 precursor was changed to a smaller value due to configuration change from the multilayer to a monolayer. However, the adsorption of Ge precursor was still self-regulated and other precursors showed a chemisorption behavior. The ALD-like film growth behaviors could be well explained by the kinetically limited incorporation of Ge atoms. The saturation behavior as a function of precursor injection time was due to the dynamic balance between the adsorption and desorption of the physisorbed Ge precursor molecules, The ALD window as a function of substrate temperature was observed due to the transition of activation energy of equilibrium coefficient of equilibrium surface coverage. The facile ALD of the pseudo-binary solid solutions with composition (GeTe2)(1-x)(Sb2Te3)x were achieved. This chemistry-specific ALD process was quite robust against process variations, allowing highly conformal, smooth, and reproducible film growth over a contact hole structure with an extreme geometry. The root-mean-square roughness was as low as 0.8 nm and Si, O, and C impurities was not detected by auger electron spectroscopy. The film had a uniform thickness and chemical composition along the depth direction in the contact hole structure with opening diameter ~ 120-150 nm and hole depth of 2500 nm, giving an aspect ratio of ~ 20 This new composition material showed reliable phase change and accompanying resistance switching behavior, which were slightly better than the standard Ge2Sb2Te5 material in the nano-scale. The phase change behavior was confirmed by pulsed voltage application and the average response time was ~350 ns. The resistivity contrast ration was as high as ~ 1x106. The phase change behavior was confirmed also by pumped layer radiation in the miro-scale, a similar crystallization times were observed. The local chemical environment was similar to that of conventional Ge2Sb2Te5 materials.