Chalcogenide Superlattices Research Articles

Comprehensive study of the ultrafast photoexcited carrier dynamics in Sb2Te3–GeTe superlattices

Abstract Chalcogenide superlattices Sb2Te3-GeTe is a candidate for interfacial phase-change memory (iPCM) data storage devices. By employing terahertz emission spectroscopy and the transient reflectance spectroscopy together, we investigate the ultrafast photoexcited carrier dynamics and current transients in Sb2Te3-GeTe superlattices. Sample orientation and excitation polarization dependences of the THz emission confirm that ultrafast thermo-electric, shift and injection currents contribute to the THz generation in Sb2Te3-GeTe superlattices. By decreasing the thickness and increasing the number of GeTe and Sb2Te3 layer, the interlayer coupling can be enhanced, which significantly reduces the contribution from circular photo-galvanic effect (CPGE). A photo-induced bleaching in the transient reflectance spectroscopy probed in the range of ~1100 nm to ~1400 nm further demonstrates a gapped state resulting from the interlayer coupling. These demonstrates play an important role in the development of iPCM-based high-speed optoelectronic devices.

Internal reverse-biased p–n junctions: A possible origin of the high resistance in chalcogenide superlattice for interfacial phase change memory

Chalcogenide superlattice (CSL) is one of the emerging material technologies for ultralow-power phase change memories. However, the resistance switching mechanism of the CSL-based device is still hotly debated. Early electrical measurements and recent materials characterizations have suggested that the Kooi-phase CSL is very likely to be the as-fabricated low-resistance state. Due to the difficulty in in situ characterization at atomic resolution, the structure of the electrically switched CSL in its high-resistance state is still unknown and mainly investigated by theoretical modelings. So far, there has been no simple model that can unify experimental results obtained from device-level electrical measurements and atomic-level materials characterizations. In this work, we carry out atomistic transport modelings of the CSL-based device and propose a simple mechanism accounting for its high resistance. The modeled high-resistance state is based on the interfacial SbTe bilayer flipped CSL that has previously been mistaken for the low-resistance state. This work advances the understanding of CSL for emerging memory applications.

Comparative study of interfacial strain dependent phonon localization in the beryllium-zinc chalcogenide superlattices

A comprehensive study is reported for the novel (BeX)m/(ZnX)n (X = S, Se and Te) superlattices (SLs) to assess their acoustic and optical phonon characteristics. The use of an elastic continuum model has offered implicit equations for assessing folded acoustic modes (FAMs) and their splitting at the center and at the edge of mini-Brillouin zone. In (BeTe)m/(ZnSe)n SLs grown with Zn–Te interfaces, the Raman spectroscopy measurements revealed well resolved first order doublet and frequency shifts which compared favorably well with the simulated results. For the graded (BeX)10-Δ/(Be0.5Zn0.5X)Δ/(ZnX)10-Δ/(Be0.5Zn0.5X)Δ SLs, we have adopted a modified linear-chain model by meticulously integrating the effects of alloying and interfacial layer thickness Δ to study phonon dispersions (ωj(q→SL)) and Raman intensity profiles. By changing Δ from 1 to 3 monolayers, our study has distinctly revealed (a) a dramatic increase in the Raman scattering intensity of ZnX quasi-confined optical modes (Q-COMs) with slight upward shifts in frequencies, (b) an insignificant change in Raman intensity lines of BeX confined optical modes (COMs) with appreciable downward shifts (up to ∼ - 113 cm−1) in frequencies, and (c) trivial modifications in the FAMs. These atypical effects noticed in the optical modes are attributed to the localization of atomic displacements at the BeX-ZnX interfacial region. The variations of phonon mode frequencies and enhancement of Raman lines for the ZnX Q-COMs can be used as vital tools for establishing interfacial structures in graded SLs of technological importance.

Epitaxial growth of GeTe/Sb2Te3 superlattices

In the quest for superlattices with engineered interfaces for disruptive applications such as neuromorphic computing, we present a dedicated study on the epitaxial growth of nominal GeTe/Sb2Te3 chalcogenide superlattices (CSL) on the Sb passivated Si(111) − (√3 × √3)R30°− Sb surface. Intermixing at the GeTe and Sb2Te3 interface is assessed by X-Ray diffraction and reflectivity. A new growth procedure with element flux interruptions is proposed to engineer the degree of intermixing and the tailoring of the GeSbTe layers into Sb-rich compositions.

Tunable Electronic Properties of Lateral Monolayer Transition Metal Dichalcogenide Superlattice Nanoribbons.

The structural stability and structural and electronic properties of lateral monolayer transition metal chalcogenide superlattice zigzag and armchair nanoribbons have been studied by employing a first-principles method based on the density functional theory. The main focus is to study the effects of varying the width and periodicity of nanoribbon, varying cationic and anionic elements of superlattice parent compounds, biaxial strain, and nanoribbon edge passivation with different elements. The band gap opens up when the (MoS2)3/(WS2)3 and (MoS2)3/(MoTe2)3 armchair nanoribbons are passivated by H, S and O atoms. The H and O co-passivated (MoS2)3/(WS2)3 armchair nanoribbon exhibits higher energy band gap. The band gap with the edge S vacancy connecting to the W atom is much smaller than the S vacancy connecting to the Mo atom. Small band gaps are obtained for both edge and inside Mo vacancies. There is a clear difference in the band gap states between inside and edge Mo vacancies for symmetric nanoribbon structure, while there is only a slight difference for asymmetric structure. The electronic orbitals of atoms around Mo vacancy play an important role in determining the valence band maximum, conduction band minimum, and impurity level in the band gap.

Local structure of [(GeTe)2/(Sb2Te3)m]n super-lattices by x-ray absorption spectroscopy

Herein, the local structure of [(GeTe)2/(Sb2Te3)m]n chalcogenide super-lattices (SLs), which are at the basis of emerging interfacial Phase-Change Memory (iPCM), is studied by x-ray absoprtion spectroscopy at the Ge-K edge. The quantitative analysis of the first coordination shells reveals that the SLs possess a structure very similar to that of thin film of the canonical Ge2Sb2Te5 (GST225) phase-change alloy. By comparing experimental data with ab initio molecular dynamics simulations of the extended x-ray absorption fine structure spectra, we show that chemical disorder is mandatory in order to reproduce the experimental data in the full spectral range. As a result, we can unambiguously conclude that Ge/Sb intermixing resulting from inter-diffusion of the GeTe and Sb2Te3 layers within SLs is inherent to SLs and is not induced by sample preparation method nor by interaction with the electron beam of electron microscopes used in all the previous studies that were suggesting such a phenomenon. We further evidence that the short Ge-Te distance is the same in GeTe and GST225 films, as well as in SLs. The main difference is the impact of disorder in GST225 and SLs. Intermixing being definitively present in [(GeTe)2/(Sb2Te3)m]n SLs, this parameter must be considered in future models aiming at going further in the understanding and the development of iPCM technology. This seems mandatory in order to allow such technology to emerge in the near future on the non-volatile memory market.

Topologically protected spin diffusion and spin generator using chalcogenide superlattices

Spintronics is expected to be the basis for future ultra-low-energy nanoelectronic devices. To operate such devices at room temperature, amplifiers, batteries, capacitors, as well as spin current sources are required. Here we report a chalcogenide superlattice composed of GeTe and Sb2Te3 layers that have a topologically protected spin diffusion length exceeding 100 μm at room temperature. A spin generator is demonstrated by combining magnetic injectors (TbFeCo) with this superlattice. The spin current was found to increase exponentially with the number of superlattice periods. We used this effect to demonstrate a 15-fold increase in the spin current. In addition, spin rectification is possible by growing the superlattice layers with atomic-level thickness accuracy. The reported chalcogenide superlattice spin generators and rectifiers open new opportunities to design low-energy spintronic integrated circuits and quantum computers.

Effects of electric and magnetic fields on the resistive switching operation of iPCM

Interfacial phase change memory devices based on chalcogenide superlattices show a remarkable performance improvement over traditional phase change memory devices. Here, we report on the effects of the resistive switching of Ge–Te/Sb–Te superlattices in the presence of an external magnetic field at elevated temperature. In addition to the unique thermal dependence of the switching behavior, a new resistance level was found. This resistance level, once initiated, could be then obtained without a magnetic field. The observed phenomena are associated with the structural reconfiguration of domains at the superlattice interfaces and grain boundaries. It has been proposed that these effects may be caused by the localization of spin-polarized electrons generated by a combination of electric and magnetic fields in the ferroelectric phase of the superlattice.

Role of substrate temperature and tellurium flux on the electrical and optical properties of MBE grown GeTe and Sb2Te3 thin films on GaAs (100)

Van der Waals layered GeTe/Sb2Te3 chalcogenide superlattices have demonstrated outstanding performance for use in dynamic resistive memories in what is known as interfacial phase change memory devices due to their low power requirement and fast switching. These devices are made from the periodic stacking of nanometer thick crystalline layers of chalcogenide phase change materials. The mechanism for this transition is still debated, though it varies from that of traditional phase change melt-quench transition observed in singular layers of GeTe and Sb2Te3. In order to better understand the mechanism and behavior of this transition, a thorough study on each constituent layer and the parameters for growth via molecular beam epitaxy was performed. In this work, the authors show the effect of tellurium overpressure and substrate temperature on the growth of thin film GeTe and Sb2Te3 on (100) GaAs. The authors demonstrate the significant role during growth that tellurium overpressure plays in the transport properties of both GeTe and Sb2Te3, as well as the negligible impact this has on both the structural and optical properties. The highest mobility recorded was 466 cm2/V s with a p-type bulk carrier concentration of 1.5 × 1019 cm−3 in Sb2Te3. For GeTe, the highest achieved was 55 cm2/V s at a p-type bulk carrier concentration of 8.6 × 1020 cm−3. The authors discuss transport properties, orientation, and crystal structure and the parameters needed to achieve high mobility chalcogenide thin films.

Photon energy dependence of Kerr rotation in GeTe/Sb2Te3 chalcogenide superlattices

We report on pump-probe based helicity dependent time-resolved Kerr measurements under infrared excitation of chalcogenide superlattices, consisting of alternately stacked GeTe and Sb2Te3 layers. The Kerr rotation signal consists of the specular inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE), both of which are found to monotonically increase with decreasing photon energy over a sub-eV energy range. Although the dependence of the SIFE can be attributed to the response function of direct third-order nonlinear susceptibility, the magnitude of the SOKE reflects cascading second-order nonlinear susceptibility resulting from electronic transitions between bulk valence/conduction bands and interface-originating Dirac states of the superlattice.

Atomic-scale observation of defects motion in van der Waals layered chalcogenide based materials

Ge-Sb-Te based alloys exhibit unique material properties and are used as active layer for data storage in phase-change memory technology. Interfacial phase-change memory based on chalcogenide superlattices are considered as a promising approach for further engineering of memory devices with lower power consumption. The internal bonding and the nature of layered defects in the superlattices are very similar to those observed in layered van der Waals (vdW) bonded Ge-Sb-Te structures. In this regard, uncovering the dynamic of such defects is crucial for a better comprehension of local structural transformations in those layered compounds. In this work, atomic-scale motion of bilayer defects is directly observed using in situ heating experiments inside an aberration-corrected scanning transmission electron microscope. The observations show lateral movements and self-reconfiguration of bilayer defects at elevated temperatures. These structural changes result in the local reorganization of vdW gaps and in an increase of overall structural order in layered Ge-Sb-Te materials. The present findings reveal the mechanism of structural reconfiguration in vdW layered Ge-Sb-Te materials and is assumed to be general in a wide range of similar layered compounds.

Role of grain boundaries in Ge–Sb–Te based chalcogenide superlattices

Interfacial phase change memory devices based on a distinct nanoscale structure called superlattice have been shown to outperform conventional phase-change devices. This improvement has been attributed to the hetero-interfaces, which play an important role for the superior device characteristics. However, the impact of grain boundaries (GBs), usually present in large amounts in a standard sputter-deposited superlattice film, on the device performance has not yet been investigated.Therefore, in the present work, we investigate the structure and composition of superlattice films by high resolution x-ray diffraction (XRD) cross-linked with state-of-the art methods, such as correlative microscopy, i.e. a combination of high-resolution transmission electron microscopy and atom probe tomography to determine the structure and composition of GBs at the nanometer scale. Two types of GBs have been identified: high-angle grain boundaries (HAGBs) present in the upper part of a 340 nm-thick film and low-angle grain boundaries present in the first 40 nm of the bottom part of the film close to the substrate. We demonstrate that the strongest intermixing takes place at HAGBs, where heterogeneous nucleation of Ge2Sb2Te5 can be clearly determined. Yet, the Ge1Sb2Te4 phase could also be detected in the near vicinity of a low-angle grain boundary. Finally, a more realistic view of the intermixing phenomenon in Ge–Sb–Te based chalcogenide superlattices will be proposed. Moreover, we will discuss the implications of the presence of GBs on the bonding states and device performance.

Stoichiometry Determination of Chalcogenide Superlattices by Means of X‐Ray Diffraction and its Limits

In this paper, the potential of stoichiometry determination for chalcogenide superlattices, promising candidates for next‐generation phase‐change memory, via X‐ray diffraction is explored. To this end, a set of epitaxial GeTe/Sb2Te3 superlattice samples with varying layer thicknesses is sputter deposited. Kinematical scattering theory is employed to link the average composition with the diffraction features. The observed lattice constants of the superlattice reference unit cell follow Vegard's law, enabling a straight‐forward and non‐destructive stoichiometry determination.

High Resolution Imaging of Chalcogenide Superlattices for Data Storage Applications: Progress and Prospects

Phase‐change materials (PCMs) based on Ge–Sb–Te alloys are a strong contender for next‐generation memory technology. Recently, PCMs in the form of GeTe–Sb2Te3 superlattices (CSLs) have shown superior performance compared to ordinary PCM memory, which relies on switching between amorphous and crystalline phases. Although detailed atomic structure switching models have been developed with the help of ab‐initio simulations, there is still fierce scientific debate concerning the experimental verification of the actual crystal structures pertaining to the two CSL memory states. One of the strongest techniques to provide this information is (scanning) transmission electron microscopy ((S)TEM). The present article reviews the analyses of CSLs using TEM‐based techniques published during the last seven years since the seminal 2011 Nature Nanotechnology paper of Simpson et al., showing the superior performance of the CSL memory. It is critically reviewed what relevant information can be extracted from the (S)TEM results, also showing the impressive progress that has been achieved in a relatively short time frame. Finally, an outlook is given including several open questions. Although debate on actual switching mechanism in CSL memory is clearly not settled, still there is consensus in this field that CSL research has a bright future.

In situ observations of the reversible vacancy ordering process in van der Waals-bonded Ge-Sb-Te thin films and GeTe-Sb2Te3 superlattices.

Chalcogenide-based thin films are employed in data storage and memory technology whereas van der Waals-bonded layered chalcogenide heterostructures are considered to be a main contender for memory devices with low power consumption. The reduction of switching energy is due to the lowering of entropic losses governed by the restricted motion of atoms in one dimension within the crystalline states. The investigations of switching mechanisms in such superlattices have recently attracted much attention and the proposed models are still under debate. This is partially due to the lack of direct observation of atomic scale processes, which might occur in these chalcogenide systems. This work reports direct, nanoscale observations of the order-disorder processes in van der Waals bonded Ge-Sb-Te thin films and GeTe-Sb2Te3-based superlattices using in situ experiments inside an aberration-corrected transmission electron microscope. The findings reveal a reversible self-assembled reconfiguration of the structural order in these materials. This process is associated with the ordering of randomly distributed vacancies within the studied materials into ordered vacancy layers and with readjustment of the lattice plane distances within the newly formed layered structures, indicating the high flexibility of these layered chalcogenide-based systems. Thus, the ordering process results in the formation of vacancy-bonded building blocks intercalated within van der Waals-bonded units. Moreover, vacancy-bonded building blocks can be reconfigured to the initial structure under the influence of an electron beam, while in situ exposure of the recovered layers to a targeted electron beam leads to the reverse process. Overall, the outcomes provide new insights into local structure and switching mechanism in chalcogenide superlattices.

Three-Dimensional Superlattices Based on Unusual Chalcogenide Supertetrahedral In-Sn-S Nanoclusters.

Reported here are two novel metal chalcogenide superlattices built from unusual supertetrahedral TO2-InSnS clusters. With regard to only one previously reported case of a TO2-InS-based 2D-layered structure, such a combination of In-Sn-S components is thought to be reasonable for leading to the first observation of 3D superlattices based on TO2-InSnS clusters. Besides, these title semiconducting materials also display good performance on the electrocatalytic oxygen reduction reaction.

Local magnetization of GeTe/Sb2Te3 superlattice films using a scanning probe microscope

Local magnetization of chalcogenide superlattices (SL) of [(GeTe)2 (Sb2Te3)]n (n=4 and 8) was performed using a magnetic force microscopy (MFM) at room temperature. We show that stripe patterns appeared in MFM phase images after magnetizing of the SL by holding the MFM probe at a short distance to the SL surface at room temperature. The probe-induced stripe pattern remained after storage for several days, and which differs from electric poling. We attributed the SL magnetization to the imbalance in the amount of spin-up and spin-down carriers in helical interface bands under the stray magnetic field.

Interplay between Structural and Thermoelectric Properties in Epitaxial Sb2+xTe3 Alloys

AbstractIn 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 Sb2 and Sb4 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.

Topological Phase Buried in a Chalcogenide Superlattice Monitored by Helicity-Dependent Kerr Measurement.

Chalcogenide superlattices (SLs), formed by the alternate stacking of GeTe and Sb2Te3 layers, also referred to as interfacial phase-change memory (iPCM), are a leading candidate for spin-based memory device applications. Theoretically, the iPCM structure has been predicted to form a three-dimensional topological insulator or Dirac semimetal phase depending on the constituent layer thicknesses. Here, we experimentally investigate the topological insulating nature of chalcogenide SLs using a helicity-dependent time-resolved Kerr measurement. The helicity-dependent Kerr signal is observed to exhibit a four-cycle oscillation with π/2 periodicity, suggesting the existence of a Dirac-like cone in some chalcogenide SLs. Furthermore, we found that increasing the thickness of the GeTe layer dramatically changed the periodicity, indicating a phase transition from a Dirac semimetal into a trivial insulator. Our results demonstrate that thickness-tuned chalcogenide SLs can play an important role in the manipulation of topological states, which may open up new possibilities for spintronic devices based on chalcogenide SLs.

All‐Optical Detection of Periodic Structure of Chalcogenide Superlattice Using Coherent Folded Acoustic Phonons

Chalcogenide superlattices (SL) consist of alternately stacked GeTe and Sb2Te3 layers. The structure can act as a 3D topological insulator depending on the constituent layer thicknesses, making the design of the SL period a central issue for advancing chalcogenide SL as potential candidates for spin devices as well as for optimization of the current generation of phase‐change memory devices. Here we explore the periodic structure of chalcogenide SL by observing coherent folded longitudinal acoustic (FLA) phonons excited by femtosecond laser pulse irradiation. The peak frequency of the FLA modes is observed to change upon variation of the thickness of the GeTe layer, which is well reproduced by means of an elastic continuum model. In addition, a new SL structure is unveiled for a sample consisting of thin GeTe and Sb2Te3 layers, which suggests intermixing of Ge atoms. This all‐optical technique based on observation of coherent FLA modes offers a non‐destructive characterization of superlattice structures at atomic resolution.