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Since the 1980s, the generation and detection of spin currents has relied on ferromagnets. Their switching today relies on spin-orbit torque from heavy metals. Nevertheless, spin injection in semiconductors has rather low efficiency. Ferroelectric Rashba semiconductors (FERSC) [1,2] may constitute a new paradigm for semiconductor spintronics, thanks to the combination of semiconductivity, large spin-orbit interaction, and the non-volatility provided by ferroelectricity. Here we report the room-temperature ferroelectric switching of spin-to-charge conversion in epitaxial GeTe films. We first show that ferroelectricity in GeTe can be reversed by electrical gating despite its high carrier density. Then, we reveal a spin-to-charge conversion as effective as in Pt, but whose sign switches with the ferroelectric polarization. These results open a route towards devices combining spin-logic and memory integrated into a silicon-compatible material.

Tellurium switch operates memories through crystalline-liquid-crystalline phase changes

We report the self-assembly of core–shell GeTe/Sb2Te3 nanowires (NWs) on Si (100), and SiO2/Si substrates by metalorganic chemical vapour deposition, coupled to the vapour–liquid–solid mechanism, catalyzed by Au nanoparticles. Scanning electron microscopy, X-ray diffraction, micro-Raman mapping, high-resolution transmission electron microscopy, and electron energy loss spectroscopy were employed to investigate the morphology, structure, and composition of the obtained core and core–shell NWs. A single crystalline GeTe core and a polycrystalline Sb2Te3 shell formed the NWs, having core and core–shell diameters in the range of 50–130 nm and an average length up to 7 µm.

We present a detailed investigation of the mechanisms at play for the incorporation of In and Mg on the GaN(0001) surface during plasma-assisted molecular beam epitaxy (PAMBE). First, we have studied the kinetics of In desorption in the presence of Mg either without or with N supply from the plasma cell by quadrupole mass spectrometry (QMS) in the line of sight. Second, we have explored the effect of Mg doping at a different time along the cycle of (InN/GaN) supply repeated 10 times to form (In,Ga)N/GaN superlattices (SLs). By the complementary ex-situ investigation of these SLs by X-ray diffraction (XRD) and secondary ion mass spectrometry (SIMS), we found that in monolayer-thick (In,Ga)N layer, the In content was maximized when Mg was not supplied simultaneously to In, but it drastically decreased otherwise. In contrast, the Mg concentration strongly increased in the (In,Ga)N monolayers compared to the GaN barriers. We attribute this finding to the surfactant effect of In for Mg, which decreases the binding energy of Mg in GaN in presence of N.

The disordering process in crystalline GeSb2Te4 films has been studied by means of ion irradiation with 150 keV Ar+ ions. The effect of the interfaces and the role of the crystal microstructure has been investigated. The disordering path observed in a randomly oriented polycrystalline material with trigonal structure involves the transition to the disordered rocksalt structure (at fluence 7x1013 cm-2) and then to the amorphous phase (at 1.5x1014 cm-2). In GeSb2Te4 epitaxially grown on Si(111) the formation of the disordered rocksalt phase occurs at much higher fluence (3x1014 cm-2) and it is preceded by the conversion of the stable phase into the ordered rocksalt structure (at 5x1013 cm-2), with the formation of ordered vacancy layers, associated to a local variation of the stoichiometry. Even by increasing the fluence up to 3.5x1014 cm-2, the films remains mainly crystalline. The observed behaviour has been attributed to the effect of the interfaces and suggests the possibility to promote switching between the crystalline phases by interface engineering.

The crystallization behavior of amorphous nano regions (20–100 nm in diameter) embedded in a textured epitaxial Ge2Sb2Te5 (GST) 25 nm thick film grown on a Si (1 1 1) substrate has been investigated in situ by transmission electron microscopy (TEM) analysis. The amorphous regions were obtained by irradiation with 30 keV Ge+ at a fluence of 1.5 × 1014 ions cm−2 of masked samples. The adopted configuration simulates the GST structure of a device in the RESET state, it is then of relevance for understanding their data retention characteristics. The in situ TEM analysis indicates that the amorphous to crystal transition for 20 nm dots is characterized by a growth velocity of 3.6 pm s −1 at 75 °C, probably related to the external partially damaged area. At 90 °C annealing crystallization is completed for 20 nm dot. In the case of 50 and 100 nm diameter amorphous dots a growth velocity of about 2.6 pm s −1 was observed at 90 °C and of 170 pm s −1 at 110 °C. The transition is governed only by rearrangement (nucleation is absent) of the atoms located at the boundary of the amorphous dot with the surrounding crystalline regions. In some cases a preferential growth along surfaces normal to the [1 1 1] and to the [1 2 1] directions has been found. The crystallographic characterization of the regrowth crystal indicates a good matching with the zone axis of the surrounding material although the seed, a Si (1 1 1) wafer, at the bottom interface is damaged by the implant.

In this study, a generalized guideline is identified to predict the interaction between two-dimensional (2D) layered materials and substrate surfaces. Additionally, the van der Waals (vdW) heterostructures commensurability, the phase formation and the strain relaxation are identified during interface growth. To achieve such a general overview, the case of Ge-Sb-Te (GST) alloys on InAs(111) is studied. In this system, low-lattice mismatch conditions are fulfilled to avoid relaxation due to formation of misfit dislocations and allow to correctly identify vdW epitaxy. At the same time, the substrate can be efficiently prepared into self- and un-passivated surfaces to clarify the role of the surface interaction. Furthermore, the GST epilayer exhibits two different highly ordered 2D structures and a three-dimensional disordered structure, allowing to directly infer the nature of the epitaxy. This study opens the way for the design and mastering of vdW epitaxial growth of 2D heterostructures as well as hybrid 2D and non-layered materials.

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline GexSb2Te3+x with almost Ge1Sb2Te4 composition and improved degree of ordering of the vacancy layers is thus obtained by epitaxial growth of layers on 3–4° stepped Si substrates. These results highlight that it is possible to build and control strain in van der Waals systems, therefore opening up new prospects for the functionalization of epilayers by directly employing vicinal substrates.

We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions.

Phase change memory (PCM) device physics comprehension represents an important chapter of future development of the PCM-based architectures and their placement into the storage class memory (SCM) segment of the memory hierarchy. Here, a reduction of SET and RESET currents by more than 60% with respect to conventional GeTe-Sb2Te3 (GST) alloys is demonstrated by using phase change memory cells containing (GeTe-Sb2Te3)/Sb2Te3 super-lattices (SL). Further, it is demonstrated that our SL PCM devices have similar characteristics in terms of the memory transition as conventional memory cells based on GST, even though showing reduced power consumption, indicative of an efficiency augmented SET-to-RESET transition. The reduced power consumption may be attributed to an increased thermal resistance of the SL with respect to the bulk GST alloy. This demonstrates that it is possible to engineer PCM with enhanced performance by employing SL structures, enlarging the possibility of employing SL as SCM players.

In recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb2Te3/GeTe heterostructures leveraging on the peculiar behavior of Sb2Te3, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb2+xTe3 films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb2Te3 and Sb4Te3 are prepared by molecular beam epitaxy. A combination of X-ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb-2 and Sb-4 slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb2+xTe3/GeTe SLs by controlling at the nanoscale the 2D character of the Sb2+xTe3 alloys is envisioned.

GeTe-Sb2Te3 superlattices have attracted major interest in the field of phase-change memories due to their improved properties compared with their mixed counterparts. However, their crystal structure and resistance-switching mechanism are currently not clearly understood. In this work epitaxial GeTe-Sb2Te3 superlattices have been grown with different techniques and were thoroughly investigated to unravel the structure of their crystalline state with particular focus on atomic stacking and van-der-Waals bonding. It is found that, due to the bonding anisotropy of GeTe and Sb2Te3, the materials intermix to form van-der-Waals heterostructures of Sb2Te3 and stable GeSbTe. Moreover, it is found through annealing experiments that intermixing is stronger for higher temperatures. The resulting ground state structure contradicts the dominant ab-initio results in the literature, requiring revisions of the proposed switching mechanisms. Overall, these findings shed light on the bonding nature of GeTe-Sb2Te3 superlattices and open a way to the understanding of their functionality.

In the present work, GeTe and Sb2Te3 van der Waals bonded superlattices epitaxially grown by molecular beam epitaxy are investigated. These structures are grown on passivated Si substrates, resulting in one single epitaxial domain and its twinned domain, both sharing the same out-of-plane orientation. Supported by X-ray diffraction and Raman spectroscopy, attention is called to the thermodynamically driven tendency of GeTe and Sb2Te3 to intermix into a Ge–Sb–Te (GST) alloy at the interfaces. A growth model is proposed to explain how these GST structures are formed.

We study the growth of InN films on In 2 O 3 (111) substrates by plasma-assisted molecular beam epitaxy under N excess. InN films deposited directly on In 2 O 3 (111) exhibit a strongly faceted morphology. A nitridation step prior to growth is found to convert the In 2 O 3 (111) surface to InN{0001}. The morphology of InN films deposited on such nitridated In 2 O 3 (111) substrates is characteristic for growth by instable step-flow and is thus drastically different from the three-dimensional growth obtained without nitridation. We show that this change originates from the different polarity of the films: while InN films deposited directly on In 2 O 3 (111) are In-polar, they are N-polar when grown on the nitridated substrate.

Chalcogenide thin films are exciting candidates for electronic applications such as spintronic devices, nonvolatile 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.

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.

A route to realize strain engineering in weakly bonded heterostructures is presented. Such heterostructures, consisting of layered materials with a pronounced bond hierarchy of strong and weak bonds within and across their building blocks respectively, are anticipated to grow decoupled from each other. Hence, they are expected to be unsuitable for strain engineering as utilized for conventional materials which are strongly bonded isotropically. Here, it is shown for the first time that superlattices of layered chalcogenides (Sb2Te3/GeTe) behave neither as fully decoupled two-dimensional (2D) materials nor as covalently bonded three-dimensional (3D) materials. Instead, they form a novel class of 3D solids with an unparalleled atomic arrangement, featuring a distribution of lattice constants, which is tunable. A map to identify further material combinations with similar characteristic is given. It opens the way for the design of a novel class of artificial solids with unexplored properties.

The electric and nonvolatile control of the spin texture in semiconductors would represent a fundamental step toward novel electronic devices combining memory and computing functionalities. Recently, GeTe has been theoretically proposed as the father compound of a new class of materials, namely ferroelectric Rashba semiconductors. They display bulk bands with giant Rashba-like splitting due to the inversion symmetry breaking arising from the ferroelectric polarization, thus allowing for the ferroelectric control of the spin. Here, we provide the experimental demonstration of the correlation between ferroelectricity and spin texture. A surface-engineering strategy is used to set two opposite predefined uniform ferroelectric polarizations, inward and outward, as monitored by piezoresponse force microscopy. Spin and angular resolved photoemission experiments show that these GeTe(111) surfaces display opposite sense of circulation of spin in bulk Rashba bands. Furthermore, we demonstrate the crafting of nonvolatile ferroelectric patterns in GeTe films at the nanoscale by using the conductive tip of an atomic force microscope. Based on the intimate link between ferroelectric polarization and spin in GeTe, ferroelectric patterning paves the way to the investigation of devices with engineered spin configurations.

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline Ge

We present an innovative approach to integrate arrays of isolated, strain-free GaN crystals on patterned Si substrates. First, micrometer-sized pillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5 μm Si substrates are deposited by low-energy plasma-enhanced chemical vapor deposition, forming crystals mostly bounded by {1 1 1}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beam epitaxy is then used for GaN deposition. GaN crystals with slanted {0 0 0 1} facets having a root-mean-square surface roughness of 0.7 nm are obtained for a deposited material thickness of >3 μm. Microphotoluminescence measurements performed at room and cryogenic temperature show no yellow luminescence and a neutral donor-bound A exciton transition at 3.471 eV (10 K) with a full width at half-maximum of 10 meV. Microphotoluminescence and micro-Raman spectra reveal that GaN grown on Si pillars is strain-free. Our results indicate that the shape of GaN crystals can be tuned by the pattern periodicity and that a reduction of threading dislocations is achieved in their top part.

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 degrees 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 degrees 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.

Phase-change materials based on GeSbTe show unique switchable optoelectronic properties and are an important contender for next-generation non-volatile memories. Moreover, they recently received considerable scientific interest, because it is found that a vacancy ordering process is responsible for both an electronic metal-insulator transition and a structural cubic-to-trigonal transition. GeTe-Sb

Phase change alloys are used for non-volatile random access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here, we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin., Comment: 14 pages, 6 figures, 5 tables

Phase-change materials based on GeSbTe show unique switchable optoelectronic properties and are an important contender for next-generation non-volatile memories. Moreover, they recently received considerable scientific interest, because it is found that a vacancy ordering process is responsible for both an electronic metal-insulator transition and a structural cubic-to-trigonal transition. GeTe Sb2Te3 based superlattices, or specifically their interfaces, provide an interesting platform for the study of GeSbTe alloys. In this work such superlattices have been grown with molecular beam epitaxy and they have been characterized extensively with transmission electron microscopy and X-ray diffraction. It is shown that the van der Waals gaps in these superlattices, which result from vacancy ordering, are mobile and reconfigure through the film using bi-layer defects and Ge diffusion upon annealing. Moreover, it is shown that for an average composition that is close to GeSb2Te4 a large portion of 9-layered van der Waals systems is formed, suggesting that still a substantial amount of random vacancies must be present within the trigonal GeSbTe layers. Overall these results illuminate the structural organization of van der Waals gaps commonly encountered in GeSbTe alloys, which are intimately related to their electronic properties and the metal-insulator transition.

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.

NPG Asia Materials 9(6), e396 (2017). doi:10.1038/am.2017.95, Published by Tokyo

The growth of GeTe thin films on a Si(111)-(√3 × √3)R30°-Sb surface is reported. At growth onset, the rapid formation of fully relaxed crystalline GeTe(0001)-(1 × 1) is observed. During growth, a GeTe(0001)-(√3 × √3)R30° surface reconstruction is also detected. Indeed, density functional theory (DFT) simulations indicate that the reconstructed GeTe(0001)-(√3 × √3)R30° structure is energetically competing with the GeTe(0001)-(1 × 1) reconstruction. The out-of-plane α-GeTe ||Si and in-plane α-GeTe ||Si epitaxial relationships are confirmed by X-ray diffraction (XRD). Suppression of rotational twist and reduction of twinned domains are achieved. The formation of rotational domains in GeTe grown on Si(111)-(7 × 7) is explained by domain matched coincidence lattice formation with the Si(111)-(1 × 1) surface. Atomic force microscopy (AFM) images show the coalescence of well-oriented islands with subnanometer roughness on their top part. van der Pauw measurements are performed to verify the...

The structural evolution of Ge–Sb–Te-based phase-change material during molecular beam epitaxy growth is investigated. The in situ and ex situ synchrotron-based X-ray scattering results reveal the formation and evolution of defects-induced inhomogeneities during growth. The scattered intensity in the vicinity of various Bragg reflections indicates a specific scattering vector-dependent anisotropy in the diffuse scattering regime. The average local static distortion fields of the defects-induced inhomogeneities are estimated to be in between 2.5 and 3.0 nm. The defects-induced diffuse scattering anisotropy, observed for the particular Bragg points, arises from the combination of the elastic and crystalline anisotropies that are caused by the large number of vacancies. The experiments show that the large-size defect clusters create inhomogeneity-driven tetragonal fields inserted into the metastable phase of the Ge–Sb–Te host crystal.

A long-standing question for avant-garde data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved x-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process; meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in PCMs.

Scientific reports 6, 32895 (2016). doi:10.1038/srep32895, Published by Nature Publishing Group, London

Scientific Reports 6: Article number: 23843; published online: 01 April 2016; updated: 16 August 2016 In this Article, Stefania Privitera and Emanuele Rimini are incorrectly listed as being affiliated with ‘Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany’. The correct affiliation is listed below