The paper reviews materials science aspects of chalcogenide-based phase change thin films relevant for non-volatile memory applications.

Two-dimensional van-der-Waals-bonded chalcogenide heterostructures have recently received a lot of attention due to promising applications in the fields of photonics, plasmonics and data storage. Of particular interest is the interfacial switching process inherent in these structures, which is assumed to occur locally at the van-der-Waals interfaces and thus represents an intracrystalline transition. However, detailed experimental studies on the underlying mechanism are still lacking. In this work, epitaxially grown thin films consisting of van-der-Waals-bonded Ge–Sb–Te and GeTe/Sb₂Te₃ based heterostructures are employed as a model system to investigate structural changes induced by a single ns-laser pulse. A combined approach using X-ray diffraction and advanced transmission electron microscopy is applied to study phase transitions within the Ge–Sb–Te-based thin films in detail. The results reveal ultrafast transitions from 2D-bonded layered structures to 3D-bonded structures via a transient molten phase. Moreover, the interface between the 2D- and 3D-bonded structures is well defined by a single van-der-Waals gap, suggesting that the transition can be controlled very precisely in its spatial extent by an appropriate choice of the laser fluence. Overall, the results of this work offer a new perspective on the switching mechanism in Ge–Sb–Te-based materials and demonstrate the potential of van-der-Waals-bonded Ge–Sb–Te compounds to be applied for novel phase-change memory concepts.

Classification of the optical reflectivity contrasts of single-phase, epitaxial Ge₂Sb₂Te₅ thin films with respect to the vacancy arrangements.

Ge-Sb-Te-based phase change memory alloys have recently attracted a lot of attention due to their promising applications in the fields of photonics, non-volatile data storage, and neuromorphic computing. Of particular interest is the understanding of the structural changes and underlying mechanisms induced by short optical pulses. This work reports on structural changes induced by single nanosecond UV laser pulses in amorphous and epitaxial Ge₂Sb₂Te₅ (GST) thin films. The phase changes within the thin films are studied by a combined approach using X-ray diffraction and transmission electron microscopy. The results reveal different phase transitions such as crystalline-to-amorphous phase changes, interface assisted crystallization of the cubic GST phase and structural transformations within crystalline phases. In particular, it is found that crystalline interfaces serve as crystallization templates for epitaxial formation of metastable cubic GST phase upon phase transitions. By varying the laser fluence, GST thin films consisting of multiple phases and different amorphous to crystalline volume ratios can be achieved in this approach, offering a possibility of multilevel data storage and realization of memory devices with very low resistance drift. In addition, this work demonstrates amorphization and crystallization of GST thin films by using only one UV laser with one single pulse duration and one wavelength. Overall, the presented results offer new perspectives on switching pathways in Ge-Sb-Te-based materials and show the potential of epitaxial Ge-Sb-Te thin films for applications in advanced phase change memory concepts.

Controlling strain in hetero-epitaxial material systems can be used to optimize properties and functionalities for specific applications. For this purpose, a lattice mismatch between substrate and epitaxial layer is typically exploited. Beyond that, vicinal substrate surfaces can be used to enhance strain-induced effects. In both cases, a profound understanding of the impact of strain on the growth mode and the resulting structure and morphology of the epitaxial layer is of great importance in order to achieve the desired modifications. In this study, the impact of vicinal substrate surfaces on the phase, structure as well as on surface pattern formation in epitaxial phase-change material thin films is investigated. It is found that in the case of the prototypical phase-change material $\mathrm{G}{\mathrm{e}}_2\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_5$, the change from nominally flat Si(111) substrates to substrates with a miscut of 6° and terrace widths lying between 2 and 5 nm enhances compressive strain and a transition from 2D island nucleation to step-flow growth. This results in a different $\mathrm{G}{\mathrm{e}}_2\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_5$ phase evolution (change from metastable to stable phase) and surface nanostructuring by regularly arranged terraces that resemble the surface topography of the substrates. This work therefore provides deeper insights into different regimes of growth of epitaxial $\mathrm{G}{\mathrm{e}}_2\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_5$ thin films and paves the way for strain-induced modification possibilities such as band-gap tuning, optimization of the switching energy or the figure of merit.