Ruining Wang

Interface formation of two- and three-dimensionally bonded materials in the case of GeTe–Sb2Te3 superlattices

GeTe-Sb2Te3 superlattices are nanostructured phase-change materials which are under intense investigation for non-volatile memory applications. They show superior properties compared to their bulk counterparts and significant efforts exist to explain the atomistic nature of their functionality. The present work sheds new light on the interface formation between GeTe and Sb2Te3, contradicting previously proposed models in the literature. For this purpose [GeTe(1 nm)-Sb2Te3(3 nm)]15 superlattices were grown on passivated Si(111) at 230 °C using molecular beam epitaxy and they have been characterized particularly with cross-sectional HAADF scanning transmission electron microscopy. Contrary to the previously proposed models, it is found that the ground state of the film actually consists of van der Waals bonded layers (i.e. a van der Waals heterostructure) of Sb2Te3 and rhombohedral GeSbTe. Moreover, it is shown by annealing the film at 400 °C, which reconfigures the superlattice into bulk rhombohedral GeSbTe, that this van der Waals layer is thermodynamically favored. These results are explained in terms of the bonding dimensionality of GeTe and Sb2Te3 and the strong tendency of these materials to intermix. The findings debate the previously proposed switching mechanisms of superlattice phase-change materials and give new insights in their possible memory application.

Surface Reconstruction-Induced Coincidence Lattice Formation Between Two-Dimensionally Bonded Materials and a Three-Dimensionally Bonded Substrate

Sb2Te3 films are used for studying the epitaxial registry between two-dimensionally bonded (2D) materials and three-dimensional bonded (3D) substrates. In contrast to the growth of 3D materials, it is found that the formation of coincidence lattices between Sb2Te3 and Si(111) depends on the geometry and dangling bonds of the reconstructed substrate surface. Furthermore, we show that the epitaxial registry can be influenced by controlling the Si(111) surface reconstruction and confirm the results for ultrathin films.

Ordered Peierls distortion prevented at growth onset of GeTe ultra-thin films

Using reflection high-energy electron diffraction (RHEED), the growth onset of molecular beam epitaxy (MBE) deposited germanium telluride (GeTe) film on Si(111)-(√3 × √3)R30°-Sb surfaces is investigated, and a larger than expected in-plane lattice spacing is observed during the deposition of the first two molecular layers. High-resolution transmission electron microscopy (HRTEM) confirms that the growth proceeds via closed layers, and that those are stable after growth. The comparison of the experimental Raman spectra with theoretical calculated ones allows assessing the shift of the phonon modes for a quasi-free-standing ultra-thin GeTe layer with larger in-plane lattice spacing. The manifestation of the latter phenomenon is ascribed to the influence of the interface and the confinement of GeTe within the limited volume of material available at growth onset, either preventing the occurrence of Peierls dimerization or their ordered arrangement to occur normally.

Improved structural and electrical properties in native Sb2Te3/GexSb2Te3+x van der Waals superlattices due to intermixing mitigation

Superlattices made of Sb2Te3/GeTe phase change materials have demonstrated outstanding performance with respect to GeSbTe alloys in memory applications. Recently, epitaxial Sb2Te3/GeTe superlattices were found to feature GexSb2Te3+x blocks as a result of intermixing between constituting layers. Here we present the epitaxy and characterization of Sb2Te3/GexSb2Te3+x van der Waals superlattices, where GexSb2Te3+x was intentionally fabricated. X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, and lateral electrical transport data are reported. The intrinsic 2D nature of both sublayers is found to mitigate the intermixing in the structures, significantly improving the interface sharpness and ultimately the superlattice structural and electrical properties.

Laser induced structural transformation in chalcogenide based superlattices

Superlattices made of alternating layers of nominal GeTe and Sb2Te3 have been studied by micro-Raman spectroscopy. A structural irreversible transformation into ordered GeSbTe alloy is induced by high power laser light exposure. The intensity ratio of anti-Stokes and Stokes scattering under laser illumination gives a maximum average temperature in the sample of 177 °C. The latter is lower than the growth temperature and of 400 °C necessary by annealing to transform the structure in a GeSbTe alloy. The absence of this configuration after in situ annealing even up to 300 °C evidences an electronic excitation induced-transition which brings the system into a different and stable crystalline state.

GeTe: a simple compound blessed with a plethora of properties

GeTe is a rather unconventional and complex compound from a fundamental point of view considering its very simple stoichiometry. Within this review, we first discuss the crystal structure of GeTe, focusing our attention on the bonding mechanism and on the distortion of the unit cell. This distortion is responsible for the ferroelectric properties of GeTe that are also considered here. Furthermore, we describe the amorphous phase and review the phase change material properties resorting to examples from several GeTe based alloys. Finally we examine the usage of GeTe for spintronics, a fairly recent application field.

Electrical and optical properties of epitaxial binary and ternary GeTe-Sb 2 Te 3 alloys

Phase change materials such as pseudobinary GeTe-Sb2Te3 (GST) alloys are an essential part of existing and emerging technologies. Here, we investigate the electrical and optical properties of epitaxial phase change materials: α-GeTe, Ge2Sb2Te5 (GST225), and Sb2Te3. Temperature-dependent Hall measurements reveal a reduction of the hole concentration with increasing temperature in Sb2Te3 that is attributed to lattice expansion, resulting in a non-linear increase of the resistivity that is also observed in GST225. Fourier transform infrared spectroscopy at room temperature demonstrates the presence of electronic states within the energy gap for α-GeTe and GST225. We conclude that these electronic states are due to vacancy clusters inside these two materials. The obtained results shed new light on the fundamental properties of phase change materials such as the high dielectric constant and persistent photoconductivity and have the potential to be included in device simulations.

Tailoring the epitaxy of Sb2Te3 and GeTe thin films using surface passivation

Chalcogenide thin films are exciting candidates for electronic applications such as spintronic devices, non-volatile memories and thermoelectric materials. To achieve such applications the understanding of their thin film growth is of paramount importance. In this work the epitaxy of exemplary chalcogenides Sb2Te3 and GeTe on different surfaces of Si(111) with atomically sharp interfaces is presented and compared using plan-view transmission electron microscopy and electron diffraction. It is shown that depending on the monolayer surface termination the resulting films present drastic differences in terms of film morphology and crystallinity. In particular, a profound difference is found between the films grown on H-passivated and Sb-passivated surfaces. In both cases, the out-of-plane texture is strongly c-axis oriented, but the case of Si(111)–H shows the frequent occurrence of random in-plane twist for both films, while for Si(111)–Sb this is strongly suppressed. The role of the substrate-film interface for the epitaxy is discussed and the consequences for the properties of the films are highlighted. In general, the insights of these results shed light on chalcogenide thin film growth for topological insulator, ferroelectric, thermoelectric and phase-change materials research.