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Reflectance anisotropy spectroscopy (RAS) has been recently applied to molecular beam epitaxy (MBE) of GaAsBi alloys. The presence of the voluminous Bi atoms induces strain in the crystal lattice, modifying the substrate symmetry of the centrosymmetric GaAs(001) and then producing clear signatures in the anisotropy spectra of the GaAsBi layers. In particular, the amplitude of the characteristic structure measured below 2.5 eV is shown to be directly related to the Bi concentration, while the sign has a meaningful correlation to the strain conditions present in the sample. In this article, the application of RAS is extended to "faulted" GaAsBi samples, i.e., samples that after growth result not satisfactory for research because of problems or errors risen during the complex deposition process (wrong growth temperature, excess or deficiency of Bi flux, formation of dislocations, etc.). Herein, it is demonstrated that also in these cases RAS offers a useful characterization of the sample, possibly (if RAS runs during the deposition) singling out the occurrence of faults eventuality, and thus validating its potential applicability to an all‐optical real‐time monitoring of the deposition process. [ABSTRACT FROM AUTHOR]

The possibility to engineer (GeTe)m(Sb2Te3)n phase‐change materials to co‐host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. Here an experimental and theoretical study of epitaxial (GeTe)m(Sb2Te3)n with GeTe‐rich composition is presented. These layered films feature a tunable distribution of (GeTe)m(Sb2Te3)1 blocks of different sizes. Breakthrough evidence of ferroelectric displacement in thick (GeTe)m(Sb2Te3)1 lamellae is provided. The density functional theory calculations suggest the formation of a tilted (GeTe)m slab sandwiched in GeTe‐rich blocks. That is, the net ferroelectric polarization is confined almost in‐plane, representing an unprecedented case between 2D and bulk ferroelectric materials. The ferroelectric behavior is confirmed by piezoresponse force microscopy and electroresistive measurements. The resilience of the quasi van der Waals character of the films, regardless of their composition, is also demonstrated. Hence, the material developed hereby gathers in a unique 2D platform the phase‐change and ferroelectric switching properties, paving the way for the conception of innovative device architectures. [ABSTRACT FROM AUTHOR]

In this study, we present a significant advance in the growth of epitaxial GeTe-Sb2Te3 alloys on Si(111)-(√3 × √3)R30°-Sb reconstructed surface by means of Molecular Beam Epitaxy. By employing X-ray diffraction and Raman spectroscopy, we show how phase, composition, and vacancy ordering can be tailored by tuning the growth parameters such as Ge and Te elemental fluxes as well as the substrate temperature. The effect of each parameter on the GeSbTe composition and phase is also discussed. A very surprising finding is that GeSbTe alloys are more ordered when grown at lower substrate temperatures. In addition, it was possible to fabricate ordered GeSbTe even for substrate temperatures as low as 120 °C. In situ high-energy electron diffraction is employed to monitor the crystallinity and surface roughness of GeSbTe films at different stages of growth. Thus, we identify the deposition parameter ranges whereby high structural quality GeSbTe with flat surfaces can be obtained. [ABSTRACT FROM AUTHOR]

The morphological, structural, and electrical properties of as-grown and annealed Ge2Sb2Te5 (GST) layers, deposited by RF-sputtering on flexible polyimide, were studied by means of optical microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and electrical characterization. The X-ray diffraction annealing experiments showed the structural transformation of GST layers from the as-grown amorphous state into their crystalline cubic and trigonal phases. The onset of crystallization of the GST films was inferred at about 140 °C. The vibrational properties of the crystalline GST layers were investigated via Raman spectroscopy with mode assignment in agreement with previous works on GST films grown on rigid substrates. The electrical characterization revealed a good homogeneity of the amorphous and crystalline trigonal GST with an electrical resistance contrast of 8 × 106. [ABSTRACT FROM AUTHOR]

The realization of textured films of 2-dimensionally (2D) bonded materials on amorphous substrates is important for the integration of this material class with silicon based technology. Here, we demonstrate the successful growth by molecular beam epitaxy of textured Sb2Te3 films and GeTe/Sb2Te3 superlattices on two types of amorphous substrates: carbon and SiO2. X-ray diffraction measurements reveal that the out-of-plane alignment of grains in the layers has a mosaic spread with a full width half maximum of 2.8°. We show that a good texture on SiO2 is only obtained for an appropriate surface preparation, which can be performed by ex situ exposure to Ar+ ions or by in situ exposure to an electron beam. X-ray photoelectron spectroscopy reveals that this surface preparation procedure results in reduced oxygen content. Finally, it is observed that film delamination can occur when a capping layer is deposited on top of a superlattice with a good texture. This is attributed to the stress in the capping layer and can be prevented by using optimized deposition conditions of the capping layer. The obtained results are also relevant to the growth of other 2D materials on amorphous substrates. [ABSTRACT FROM AUTHOR]

A combination of X‐ray diffraction and Raman scattering is employed to investigate (GeTe)m(Sb2Te3)n alloys crystallized by thermal annealing from the amorphous (a‐) phase. The films are deposited by molecular beam epitaxy on Si(111) substrates. First, a series of a‐GeSbTe (GST) films of different composition is deposited and studied by Raman spectroscopy to identify the Ge‐rich features of the alloys. Second, the crystallization properties of Ge10Sb2Te13 are studied upon different annealing conditions. The aim of this work is to develop a procedure to avoid segregation of Ge and GeTe at the GST crystallization onset (Tx). This is here achieved by means of an incubation step at temperature lower than Tx. The crystallization onset Tx increased to 270 °C, which is about 160 °C higher compared to a reference GeTe sample, while the alloy always crystallizes in the stable Ge1Sb2Te4 composition. The increase of Tx is observed for all annealing conditions, regardless the amount of crystalline Ge segregated. For the optimized annealing treatment, the presence of Ge nanocrystals along with crystalline GeSbTe is unveiled by Raman measurements, paving the way for the control of the microstructure and electrical/thermal properties of Ge‐rich alloys. The crystallization of amorphous Ge10Sb2Te13 deposited by molecular beam epitaxy on Si(111) substrate is investigated upon different annealing conditions. The alloy crystallizes in the Ge1Sb2Te4 composition, along with segregated Ge. An optimized two‐step annealing process is demonstrated to lower the segregation of Ge crystals, paving the way to engineer the microstructure of Ge‐rich alloys and their properties. [ABSTRACT FROM AUTHOR]

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 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. The precise tuning of the properties of epitaxial Sb2+xTe3 in between those of 2D and 3D materials is established. It paves the way to expand the horizon of strain engineering in phase change memory superlattice structures. The interplay between structural and thermoelectric properties in these alloys is presented. [ABSTRACT FROM AUTHOR]