Structural Transitions in Ge2Sb2Te5 Phase Change Memory Thin Films Induced by Nanosecond UV Optical Pulses.

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.

Direct Measurement of Crystal Growth Velocity in Epitaxial Phase-Change Material Thin Films.

Central to the use of Ge-Sb-Te based phase-change materials for data storage applications is their crystallization capability since it determines memory writing time. Although being intensively studied to identify intrinsic limits and develop strategies to enhance memory performance, the crystallization process in these materials is still not fully explored. Therefore, this study focuses on the determination of crystal growth dynamics in an epitaxial phase-change material thin film model system offering the advantage of high crystalline quality and application-relevant sizing. By introducing a method that combines time-resolved reflectivity measurements with high-resolution scanning transmission electron microscopy, crystal growth velocities upon fast cooling after single ns-laser pulse irradiation of the prototypical phase-change material Ge2Sb2Te5 are determined. As a result, an increase in crystal growth velocity from 0.4 to 1.7 m/s with increasing laser fluence is observed with a maximum rate of 1.7 m/s as the upper detectable limit of the studied material.

Ultrafast interfacial transformation from 2D- to 3D-bonded structures in layered Ge-Sb-Te thin films and heterostructures.

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.