Abstract
Ge2Sb2Te5 (GST-225) is a chalcogenide material with applications in nonvolatile memories. However, chalcogenide material properties are dependent on the deposition technique. GST-225 thin films were prepared using three deposition methods: magnetron sputtering (MS), pulsed laser deposition (PLD) and a deposition technique that combines MS and PLD, namely MSPLD. In the MSPLD technique, the same bulk target is used for sputtering but also for PLD at the same time. The structural and optical properties of the as-deposited and annealed thin films were characterized by Rutherford backscattering spectrometry, X-ray reflectometry, X-ray diffraction, Raman spectroscopy and spectroscopic ellipsometry. MS has the advantage of easily leading to fully amorphous films and to a single crystalline phase after annealing. MS also produces the highest optical contrast between the as-deposited and annealed films. PLD leads to the best stoichiometric transfer, whereas the annealed MSPLD films have the highest mass density. All the as-deposited films obtained with the three methods have a similar optical bandgap of approximately 0.7 eV, which decreases after annealing, mostly in the case of the MS sample. This study reveals that the properties of GST-225 are significantly influenced by the deposition technique, and the proper method should be selected when targeting a specific application. In particular, for electrical and optical phase change memories, MS is the best suited deposition method.
Highlights
The results can be affected by errors for Sb and Te, with errors up to 10–15%. This is because Sb and Te cannot be energetically separated for the maximum energy used of 4.28 MeV, while for Ge, the error is smaller than 5%
For the films deposited by pulsed laser deposition (PLD) and MS and PLD (MSPLD), a pronounced diffusion effect of the Sb and Te atoms towards the substrate is evidenced
For the film deposited by magnetron sputtering (MS), the diffusion effect is much smaller
Summary
Ge–Sb–Te alloys are intensely studied and used for optical and electrical memory applications due to their fast and reversible crystalline to amorphous phase transition that leads to a remarkable change in reflectivity and resistivity [1,2,3]. GST-225 has enabled the development of phase-change random access memories and can be integrated in nonvolatile memory structures, which have better scaling capabilities when compared to flash memory [4]. This has led to a large number of studies on their structural and optical properties [5,6,7]
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