In this article, we present the structural investigation by Raman spectroscopy of GeSbSeN ovonic threshold switching (OTS) material once integrated in selector devices featuring a top electrode based on a transparent and conductive indium tin oxide layer. The devices are characterized by standard electrical protocols, and the structural evolution of the material is investigated after several switching operations. The results are correlated with the spectra obtained from blanket samples annealed at increasing temperature and are supported by XRD and TEM analyses. We establish a link between the evolution of the material structure with the annealing process and the device behavior along cycling, bringing important advancement in the understanding of the switching mechanism and of the origin of the failure in OTS devices.

The material engineering of GeSbTe alloys has led to the significant improvements of thermal stability, necessary to ensure the data retention of the Phase-Change Memory device (PCM) over extended temperature range. However, despite the proven benefits of Ge enrichment of GeSbTe alloys, the effect of Ge content on the structure and its evolution as a function of annealing temperature remains unclear. In this paper, we present the structural analyses of as-deposited and annealed Ge-rich GeSbTe, considering the Ge enrichment (15–55 at.%) of reference Ge2Sb2Te5 alloy. Based on the combination of Raman spectroscopy and X-ray diffraction techniques, we describe the progressive reorganization of the main structural features common to all investigated compositions. Therefore, we present a model describing the overall crystallization mechansim in Ge-rich GeSbTe. We highlight several competing phenomena in the system at increasing temperature, such as Ge diffusion and segregation as well as nucleation and growth of GeSbTe and Ge phases, which depends on Ge content. Our results contribute to the understanding of the crystallization mechanisms in Ge-rich GeSbTe alloys, unveiling the primary structural reorganization and crystallization of the GeSbTe phase followed by the crystallization of the Ge phase.

In this paper, we present an innovative Multilayer SeAsGeSi-based Ovonic Threshold Switching (OTS) Selector targeting high reliability for Crossbar arrays. We compare our Multilayer (ML) OTS with SeAsGeSi-based bulk alloy (SAGS). We demonstrate the high thermal stability of the ML stack against the Back-End-of-Line (BEOL) thermal budget as well as the reduction of the device-to-device variability and reliable switching operations up to 300°C. We study by Raman and FTIR spectroscopy the integrity of the ML OTS material after an annealing of 3 hours at 400° C. SeAsGeSi Multilayer OTS delays crystallization mechanism along cycling. We finally report the successful co-integration of our ML with Phase-Change Memory technology.

We investigate the recrystallization of thick phosphorus-implanted GeSn layers using 308 nm Ultraviolet Nanosecond Laser Annealing (UV-NLA). We identify the optimal annealing conditions leading to the reconstruction of Ge0.92Sn0.08 crystal amorphized by dopant implantation. The fully recrystallized GeSn layers present specific structures with localized tin and strain variations. Above the non-amorphized and unmelted Ge0.92Sn0.08 seed layer, a first highly tensile strained GeSn sublayer is formed, with a tin gradient from 2.5% up to 10.5%. Closer to the surface, a second sublayer consists of tin-enriched vertical structures in a Ge0.93Sn0.07 matrix. Laser annealing enables us to reverse the strain of the GeSn layer. The initial GeSn presents a compressive strain of −0.10%, while the recrystallized Ge0.93Sn0.07 matrix is tensile strained at 0.39%. UV-NLA presents the advantages of (i) local annealing that recrystallizes amorphized GeSn layers after implantation without excessive tin segregation and (ii) reversing the strain of epitaxial GeSn layers from compressive to tensile. Our results open up promising perspectives for the integration of GeSn mid-IR photonic devices.

Nitrogen doping in chalcogenide materials represents a promising way for the improvement of material properties. Indeed, N doping in GeSbTe phase-change alloys have demonstrated to greatly enhance thermal stability of their amorphous phase, necessary to ensure the data retention of the final phase-change memory device. Although it is suggested that the N doping in such alloys leads to the preferential formation of Ge-N bonds, further questions concerning the bonding, in particular, Sb-N and Te-N, and the structural arrangement remain unclear. In this paper, we present a study of as-deposited elemental Ge, Sb, and Te systems and their nitrides (i.e., GeN, SbN, and TeN alloys), using a large range of N content from 0 up to about 50 at. %. The as-deposited alloys are investigated by Fourier transform infrared and Raman spectroscopy. We identify the active vibrational modes associated with the formation of Ge-N, Sb-N, and Te-N bonds, highlighting the impact of N incorporation on the structure of these elemental systems. We further qualitatively compare the GeN, SbN, and TeN experimental spectra with the “ab initio” simulations of the related ideal nitride structures. Finally, the analysis of elemental nitride layers is extended to N-doped GeSbTe alloys, providing deeper understanding of nitrogen bonding in such ternary systems, employed in memory technology.

We present the engineering of Ovonic Threshold Switching (OTS) Multilayer (ML) Selector device based on the stacking of N-doped SbSe and Ge layers. By tuning individual layers thicknesses and N content of the ML stack, we demonstrate the possibility to highly improve selector stability during integration Back-End-of-Line (BEOL) and to reduce device-to-device variability. We show how our OTS ML presents fundamental electrical characteristics that are compatible with the ones of standard bulk OTS achieved by co-sputtering technique, but enabling reliable switching operations up to 160° C with lower variability. We study by FTIR and Raman spectroscopy the layers structure revealing the high stability achieved in OTS ML wrt bulk OTS even after 3 hours at 400°C. In TEM/EDX analyses performed on cycled and annealed devices, we highlight the preserved integrity of the amorphous structure in OTS ML wrt bulk. Finally, OTS ML solution allows reliable endurance up to more than 10 9 cycles and improved yield in scaled devices thanks to a higher control of the layer structure and properties.

Ge-rich GeSbTe alloys allowed overcoming temperature limitations of phase-change memory technology. In this paper, we present a thorough investigation of the structural evolution and the crystallization process of these alloys as a function of increasing temperature in annealing. We highlight the progressive rearrangement of the structure toward the demixing of Ge and GeSbTe phases. In particular, we show the stability of Sb–Te units and the development of Ge–Te bonds around these features. We observe the formation of a transient GeSbTe phase, which is driven by crystallization phenomena, leading to a gradual diffusion and expulsion of Ge. Therefore, the system moves toward the complete separation of Ge and Ge2Sb2Te5 stable phases. Furthermore, we investigate the effect of N-doping in Ge-rich GeSbTe, which induces the formation of Ge–N bonds. Such features are demonstrated to be responsible for a delayed structural reorganization to higher temperatures, thus affecting the entire process of crystallization and phase separation in the alloy.

In this paper, we investigate an innovative Ovonic Threshold Switching Selector (OTS) based on Multilayer structure (ML). Thanks to physico-chemical analysis and electrical characterization we show how MLs properties and structure can be tuned thanks to the engineering of each individual layer stoichiometry, thickness and interfaces. Ge/N-doped SbSe-based MLs OTS are analyzed by FTIR and Raman spectroscopy revealing the structural features present in the as-deposited materials and the strong interaction among individual layers at interfaces. We demonstrate the improved variability control of electrical parameters wrt standard OTS achieved by co-sputtering technique, and the high endurance capability of MLs OTS up to more than 2•10 9 cycles with stable nA leakage current. Moreover, we show how Ge-N bonds play a huge role on OTS thermal stability at 400°C and how they can be tuned more easily in ML OTS. These developments pave the way towards a new class of OTS materials and their engineering, ensuring high temperature stability and best tuning of electrical performances.

Crystalline phosphors of Eu3+-doped titania (TiO2:Eu3+) were prepared by hydro and solvothermal synthesis with luminescent ion concentration of 2 mol.%. The structure and shape of the synthesized nanoparticles were characterized using X-ray powder diffraction, transmission electron microscopy, and Raman spectroscopy. Changes in the emission, excitation spectra, and the intensity decay of the photoluminescence for TiO2:Eu(3+)nanoparticles were analyzed their phase composition. The photoluminescence of synthesized TiO2:Eu3+ crystalline phosphors depends on whether the said nanophosphors are formed from organometallic or inorganic precursors under hydro- and solvothermal conditions. Indeed, photoluminescence excitation at wavelengths ranging from 350-550 nm leads to splitting of electron dipole transitions into Stark components according to the symmetry of the Eu3+ surroundings. Also, both nanoparticles with the anatase structure and phosphors predominantly containing rutile showed very short photoluminescence lifetimes.

Recent demonstrations of optically pumped lasers based on GeSn alloys put forward the prospect of efficient laser sources monolithically integrated on a Si photonic platform. For instance, GeSn layers with 12.5% of Sn were reported to lase at 2.5 μm wavelength up to 130 K. In this work, we report a longer emitted wavelength and a significant improvement in lasing temperature. The improvements resulted from the use of higher Sn content GeSn layers of optimized crystalline quality, grown on graded Sn content buffers using reduced pressure CVD. The fabricated GeSn micro-disks with 13% and 16% of Sn showed lasing operation at 2.6 μm and 3.1 μm wavelengths, respectively. For the longest wavelength (i.e., 3.1 μm), lasing behavior was demonstrated up to 180 K, with a threshold of 377 kW/cm2 at 25 K.

The application of high values of strain to Ge considerably improves its light emission properties and can even turn it into a direct band gap semiconductor. Raman spectroscopy is routinely used for strain measurements. Typical Raman-strain relationships that are used for Ge were defined up to ∼1% strain using phonon deformation potential theory. In this work, we have studied this relationship at higher strain levels by calculating and measuring the Raman spectral shift-strain relations in several different strain configurations. Since differences were shown between the usual phonon deformation potential theory and ab-initio calculations, we highlight the need for experimental calibrations. We have then measured the strain in highly strained Ge micro-bridges and micro-crosses using Raman spectroscopy performed in tandem with synchrotron based micro-diffraction. High values of strain are reported, which enable the calibration of the Raman-strain relations up to 1.8% of in plane strain for the (001) biaxial stress, 4.8% strain along ⟨100⟩, and 3.8% strain along ⟨110⟩. For Ge micro-bridges, oriented along ⟨100⟩, the nonlinearity of the Raman shift-strain relation is confirmed. For the ⟨110⟩ orientation, we have shown that an unexpected non-linearity in the Raman-strain relationship has also to be taken into account for high stress induction. This work demonstrates an unprecedented level of strain measurement for the ⟨110⟩ uniaxial stress and gives a better understanding of the Raman-strain relations in Ge.

We report optically pumped lasing from GeSn micro-disks with high Sn content active GeSn layers. Buffer layers made by a step-growth show enhanced performances compared to conventional Ge strain relaxed buffers.

GeSn alloys are the subject of intense research activities as these group IV semiconductors present direct bandgap behaviors for high Sn contents. Today, the control of strain becomes an important challenge to improve GeSn devices. Strain micro-measurements are usually performed by Raman spectroscopy. However, different relationships linking the Raman spectral shifts to the built-in strain can be found in the literature. They were deduced from studies on low Sn content GeSn layers (i.e., xSn < 8%) or on GeSiSn layers. In this work, we have calibrated the GeSn Raman relationship for really high Sn content GeSn binaries (6 < xSn < 15%). We have used fully strained GeSn layers and fully relaxed GeSn under-etched microstructures to clearly differentiate the contributions of strain and chemical composition on the Ge-Ge Raman spectral shift. We have shown that the GeSn Raman-strain coefficient for high Sn contents is higher compared with that for pure Ge.

The realization of efficient laser sources compatible with the microelectronics industry is currently one of the main challenges for silicon photonics. As Ge is CMOS compatible, the interest of using tensile strain or n-type doping to improve its light emission properties has significantly increased over the last few years. Theoretically, it has been predicted that the Ge bandgap becomes direct at around 4% strain for uniaxial tensile stress or 2% strain for bi-axial tensile stress. Several methods to induce such extreme levels of strain are currently investigated. The highest value of strain has been reached with Ge micro-bridges fabricated from Ge-On-Insulator (GeOI) substrates in a controllable and reproducible way. In this work we have first of all investigated the material properties of 200-mm GeOI wafers. Very high crystallographic quality is demonstrated at the micron-scale using Raman spectroscopy and synchrotron based Laue micro-diffraction performed at BM32-ESRF. We give then optimized designs of micro-bridge by comparing suspended and landed micro-bridges on different materials. We theoretically show that the thermal management is strongly improved in landed micro-bridges. Finally, we have developed specific processing for landing Ge micro-bridges on Si or SiO2, the photoluminescence measurements performed on landed micro-bridges shows an improvement of the Ge light emission with strain.

Applying sufficient tensile strain to Ge leads to a direct bandgap group IV semiconductor, which emits in the mid-infrared (MIR) wavelength range. However, highly strained-Ge cannot be directly grown on Si because of its large lattice mismatch. In this work, we have developed a process based on Ge micro-bridge strain redistribution intentionally landed to the Si substrate. Traction arms were then partially etched to keep locally strained-Ge micro-blocks. Large tunable uniaxial stresses up to 4.2% strain were demonstrated in Ge, which was bonded on Si. Our approach allows envisioning integrated strained-Ge on Si platform for MIR-integrated optics. Silicon photonics merge optical and electronic components that can be integrated together onto a single microchip.

We present deep insights on the integration and physics of two new strain boosters for FDSOI CMOS. "STRASS" and "BOX creep" techniques (for tensily and compressively stressed channels, respectively) are for the first time integrated in a localized manner on a state-of-the-art 14nm FDSOI route. STRASS enables to achieve +1.6 GPa in SOI active regions (w.r.t. +1.3 GPa for thin BOX sSOI). BOX creep process leads to more than +10% in hole mobility and +6% in Ieff(Ioff) plots. The BOX creep efficiency is investigated with respect to device dimensions: the electrical data evolution matches the proposed mobility model based on 2D simulated stress profiles.

The influence of pattern design and tensile strain on light emission was investigated in Ge layers and suspended membranes. The optical properties were examined by micro-photoluminescence and reflectivity. Tensile strain was extracted from micro-Raman spectroscopy. It has been shown that Fabry–Pérot interference fringes can dominate the photoluminescence spectra. It is crucial to remove them in order to analyze the photoluminescence changes coming from tensile strain; especially if Fabry–Pérot oscillations are in the same energy range compared to the stress-induced spectral shift. This study highlights the fact that this interference must be taken into account in order to examine the strain in suspended Ge layers.

We demonstrate the crossover from indirect- to direct band gap in tensile-strained germanium by temperature-dependent photoluminescence. The samples are strained microbridges that enhance a biaxial strain of 0.16% up to 3.6% uniaxial tensile strain. Cooling the bridges to 20 K increases the uniaxial strain up to a maximum of 5.4%. Temperature-dependent photoluminescence reveals the crossover to a fundamental direct band gap to occur between 4.0% and 4.5%. Our data are in good agreement with new theoretical computations that predict a strong bowing of the band parameters with strain.