Amorphous SiGe Research Articles

Regulating the crystal orientation of vapor-transport-deposited GeSe thin films by a post-annealing treatment

Recently, GeSe has emerged as a highly promising photovoltaic absorber material due to its excellent optoelectronic properties, nontoxicity, and high stability. Although many advantages make GeSe well suited for thin-film solar cells, the power conversion efficiency of the GeSe thin-film solar cell is still much below the theoretical maximum efficiency. One of the challenges lies in controlling the crystal orientation of GeSe to enhance solar cell performance. The two-step preparation of GeSe thin films has not yet been reported to grow along the [111] orientation. In this work, we study the effect of a post-annealing treatment on the GeSe thin films and the performance of the solar cells. It was found that amorphous GeSe films can be converted into polycrystalline films with different orientations by changing the post-annealing temperature. [111]-oriented and [100]-oriented GeSe thin films were successfully prepared on the same substrate by optimizing the annealing conditions. With the structure of Au/GeSe/CdS/ITO cell devices, PCEs of 0.14% and 0.16% were ultimately achieved.

Synthesis and Characterization of Type II Ge-Si Clathrate Films for Optoelectronic Applications.

Type II inorganic clathrates consist of cage-like structures with open frameworks, and they are considered promising materials due to their unique properties. However, the difficulty of synthesizing phase-pure and continuous films has hindered their application in practical devices. In this report, we demonstrate the synthesis of type II SiGe clathrate films through the thermal decomposition of a Na-deposited amorphous SiGe film on a sapphire substrate in a high vacuum. The as-prepared films of type II SiGe clathrates showed uniform growth and were evaluated for their structural and optical properties. Morphological studies conducted using a scanning electron microscope showed the presence of cracks on the film surface.

Structure, bonding and electronic characteristics of amorphous Se.

Ovonic threshold switching (OTS) selectors can effectively improve the storage density and suppress the leakage current of advanced phase-change memory devices. As a prototypical OTS material, amorphous GeSe is widely investigated. But the attention paid to amorphous Se (i.e., the functional constituent in amorphous GeSe) has been very limited up to now. Here we have explored the structure, bonding and electronic characteristics of amorphous Se using ab initio molecular dynamics simulations. The results reveal that the Se atoms in amorphous Se tend to form 2-coordinated configurations, and they connect with each other to form long chains. The fraction of the vibrational density of state located in the high frequency range is relatively large, and the formation energy of the Se-Se bond is as large as 4.44 eV, hinting that the Se-Se bonds in chains possess high stability. In addition, the mid-gap state related to the OTS behavior is also found in amorphous Se despite the small proportion. Our findings enrich the knowledge of amorphous Se, which aids the applications of Se-based OTS selectors.

Heat transport evaluation of amorphous Si, SiGe, and Ge thin films using 3ω method and minimum heat conduction model

Heat transport evaluation of amorphous Si, SiGe, and Ge thin films using 3ω method and minimum heat conduction model

Unveiling the structure and electronic characteristics of amorphous GeS for high performance threshold switching

Ovonic threshold switching selectors are widely studied owing to the essential application in high density phase-change memory. Amorphous GeS is proposed as a potential candidate for the excellent performance. However, the knowledge of amorphous GeS is still insufficient up to date. Here, we have studied the structure and electronic characteristics of GeS in the amorphization process, by using ab initio molecular dynamics simulations. The results indicate that the amorphous GeS is mainly made up of Ge–S bonds. The Ge- and S-centered clusters are dominantly in the form of octahedral structures in liquid GeS. During the amorphization process, most of Ge-centered clusters become highly coordinated octahedrons while a small number of Ge-centered clusters change to tetrahedrons, and the S-centered clusters deviate from the octahedral structure gradually. In addition, the large bandgap and the relatively small mid-gap states in amorphous GeS lead to a high switching voltage.

Tailoring Mid‐Gap States of Chalcogenide Glass by Pressure‐Induced Hypervalent Bonding Towards the Design of Electrical Switching Materials

AbstractPhase change memory (PCM) and ovonic threshold switching (OTS) materials using chalcogenide glass are essential elements in advanced 3D memory chips. The mid‐gap states, induced by the disorder and defects in the glass, are the physical mechanisms of the electrical switching behavior, while the origin of these trap states is still under debate and the medium‐range clusters that break the global octet rule, such as over‐coordinated atoms, are known to be responsible in various glass. Here, it is discovered that a large fraction of over‐coordinated clusters fails to generate mid‐gap states, which are probably caused by hypervalent bonding, a multi‐centered covalent bond participated by delocalized lone‐pair electrons. This is confirmed by the pressure‐driven simulations of amorphous GeSe models, in which it is found that octahedral motifs and hypervalent bonds prevent the over‐coordinated medium‐range clusters from providing excessive electrons. In practical applications, compatible dopants can be used to change the number of hypervalent bonds, thus controlling the number of mid‐gap states and consequently the performance of PCM and OTS materials. These results reveal the origin of mid‐gap states in chalcogenide glasses, enabling extensive control in the development of pioneering electrical switching materials.

Nonlinear optical response of polycrystalline GeSe films

GeSe exhibits in-plane optical anisotropy similar to black phosphorus and other two-dimensional materials, so it is believed to have remarkable nonlinear optical (NLO) properties. In this work, amorphous GeSe films were prepared by vacuum thermal evaporation. After annealing at 450 °C, GeSe films exhibited a polycrystalline phase. The NLO properties of these GeSe films were investigated by the Z-scan technique. The results indicated that GeSe films exhibited typical reverse saturable absorption (RSA) and self-focusing phenomenon. It was also found that the nonlinear optical parameters and laser-induced damage threshold of annealed recrystallized GeSe films were significantly improved. The nonlinear absorption and refractive coefficients were β = (2.90 ± 0.06)×10−7 m/W and n2= (1.06 ± 0.07)×10−13 m2/W at the incident laser intensity of 0.4 GW/cm2, respectively. The polycrystalline GeSe film showed remarkable RSA-induced optical limiting properties, which exhibited an optical limiting starting threshold of 0.018 GW/cm2 and an optical limiting threshold of 0.98 GW/cm2. These results strongly indicate that GeSe films have potential applications in solid-state optical limiting devices and laser protection.

Graphite Nanosheets as a Platform for the Nanosize GeSe2 Anode with Improved Electrode Performance

GeSe2 anode materials possess energy densities higher than those of the commercial graphite anode for lithium-ion batteries (LIBs). However, the limited cycling life and poor inherent conductivity present significant difficulties for their use in various fields. LIBs’ lifetime and capacity density depend on their electrode materials, where structural stability and a slow diffusion control mechanism are crucial factors. In this study, amorphous nanosize GeSe2 uniformly anchored on graphite sheets (GeSe2/G) was prepared using ball milling. For comparison, the cycle performances of pure GeSe2 and GeSe2 with the same ball milling conditions (M-GeSe2) were tested. The results showed that ball milling enabled uniform anchoring of GeSe2 nanoparticles of ∼200 nm on graphite nanosheets without aggregation, where the introduction of graphite not only served as electron transfer layers to effectively improve the electrical conductivity but also assisted in reducing the volume growth and comminution of GeSe2 during Li+ insertion/extraction procedures. When served as anode materials for LIBs, the GeSe2/G nanocomposites showed high Li+ storage properties, i.e., a high reversible capacity of 812.8 mAh g–1 at 0.1 A g–1. After 50 cycles, GeSe2/G still reached 650 mAh g–1, which was much higher than M-GeSe2 (392.3mAh g–1) and GeSe2 (183 mAh g–1). Even at 1 A g–1, the GeSe2/G anode exhibited a high reversible capacity of 409.6 mAh g–1 and remarkable cycling stability, with a high capacity retention of 63% after 300 cycles.

Transmittance oscillation in amorphous semiconductors: Revisited

Transmittance self-oscillation discovered in amorphous GeSe2 films has attracted considerable interest, while its underlying mechanism remains controversial. We suggest that the oscillation is governed by at least two kinds of changes; one occurs in absorption for thin ([Formula: see text]m) films and the other in refractive-index for thick ([Formula: see text][Formula: see text]mm) plates. We also detail a model, which assumes recoverable micro-crystallization. An idea for increasing the oscillation frequency is proposed for possible applications.

Synthesis of Large-Area GeS Thin Films with the Assistance of Pre-deposited Amorphous Nanostructured GeS Films: Implications for Electronic and Optoelectronic Applications

Synthesis of Large-Area GeS Thin Films with the Assistance of Pre-deposited Amorphous Nanostructured GeS Films: Implications for Electronic and Optoelectronic Applications

Nanostructured Si–Ge thermoelectric material for 1200 V/W highly sensitive infrared thermopile sensor device

In this paper, we report the characteristics of a thermopile infrared sensor in which a nanostructured Si–Ge thermoelectric material is used. Although the thermopile infrared sensor is capable of being operated without power consumption, a challenge has been improving its sensitivity. With the aim of improving its sensitivity, we have realized a nanostructured Si–Ge thermoelectric material with low thermal conductivity (0.8 W/m K or less) by forming nanocrystals through heat treatment of amorphous Si–Ge. The thermopile infrared sensor composed of this material exhibited a high sensitivity of 1200 V/W at a pressure of 1.0 × 10−1 Pa or less.

Device‐to‐Materials Pathway for Electron Traps Detection in Amorphous GeSe‐Based Selectors

AbstractThe choice of the ideal material employed in selector devices is a tough task both from the theoretical and experimental side, especially due to the lack of a synergistic approach between techniques able to correlate specific material properties with device characteristics. Using a material‐to‐device multiscale technique, a reliable protocol for an efficient characterization of the active traps in amorphous GeSe chalcogenide is proposed. The resulting trap maps trace back the specific features of materials responsible for the measured findings, and connect them to an atomistic description of the sample. The metrological approach can be straightforwardly extended to other materials and devices, which is very beneficial for an efficient material‐device codesign and the optimization of novel technologies.

In-situ Neutron Reflectometry to Determine Ge Self-Diffusivities and Activation Energy of Diffusion in Amorphous Ge0.8Si0.2

Amorphous Ge-Si solid solutions are an interesting class of materials from the fundamental as well as the technological point of view. Self-diffusion of the constituents is an important process because of the inherent metastability. While self-diffusion was already examined in crystalline GexSi1-x (0 < x <1) this is not the case for the amorphous counterparts. This work reports on Ge self-diffusivities obtained from insitu neutron reflectometry measurements during isothermal annealing of ion-beam sputter-deposited amorphous Ge0.8Si0.2 films. The diffusivities are modified peculiarly fast with annealing time by a maximum factor of two due to structural relaxation. The diffusivities in the relaxed state are lower (higher) than in amorphous germanium (silicon). They follow the Arrhenius law and show an activation energy of (2.06 ± 0.1) eV, which equals that of amorphous germanium, but differs from that of amorphous silicon. Thus, it is concluded that the diffusion mechanism of Ge in amorphous Ge0.8Si0.2 and Ge are similar, despite of the presence of dispersed 20 at.% of Si.

Selective amorphization of SiGe in Si/SiGe nanostructures via high energy Si+ implant

The selective amorphization of SiGe in Si/SiGe nanostructures via a 1 MeV Si+ implant was investigated, resulting in single-crystal Si nanowires (NWs) and quantum dots (QDs) encapsulated in amorphous SiGe fins and pillars, respectively. The Si NWs and QDs are formed during high-temperature dry oxidation of single-crystal Si/SiGe heterostructure fins and pillars, during which Ge diffuses along the nanostructure sidewalls and encapsulates the Si layers. The fins and pillars were then subjected to a 3 × 1015 ions/cm2 1 MeV Si+ implant, resulting in the amorphization of SiGe, while leaving the encapsulated Si crystalline for larger, 65-nm wide NWs and QDs. Interestingly, the 26-nm diameter Si QDs amorphize, while the 28-nm wide NWs remain crystalline during the same high energy ion implant. This result suggests that the Si/SiGe pillars have a lower threshold for Si-induced amorphization compared to their Si/SiGe fin counterparts. However, Monte Carlo simulations of ion implantation into the Si/SiGe nanostructures reveal similar predicted levels of displacements per cm3. Molecular dynamics simulations suggest that the total stress magnitude in Si QDs encapsulated in crystalline SiGe is higher than the total stress magnitude in Si NWs, which may lead to greater crystalline instability in the QDs during ion implant. The potential lower amorphization threshold of QDs compared to NWs is of special importance to applications that require robust QD devices in a variety of radiation environments.

Isotropy‐Induced Stress Relaxation and Strong‐Tolerance for High‐Rate and Long‐Duration Sodium Storage by Amorphous Structure Engineering

AbstractHigh‐performance conversion‐reaction‐based electrodes have received enormous attention due to their high theoretical capacities, yet the poor structural stability and large stress accumulation induced from volume change are identified as major limiting factors for further utilization. Although viable strategies are exerted to acquire durable storage capacity, the controllable technology of inner stress is still required to explore. Herein, the amorphous structured GeS2 with 2D porous sheet morphology is designed for sodium‐ion batteries (SIBs). The differences in reaction mechanism and structure reversibility between amorphous and crystalline states are systematically investigated and unraveled. Notably, the amorphous matrix can ensure evenly distributed intermediate interface and alleviated aggregation of active materials, promoting the complete conversion reaction with decreased hysteresis. Furthermore, it provides more isotropic Na‐ion diffusion channels and active sites, effectively boosting the reaction kinetics. Meanwhile, it can fully relieve the volume expansion and restrain the inner stress of GeS2, resulting in excellent structural tolerance. As expected, it expresses prominent rate capability and cyclic life (512.8 mA h g−1 at a superhigh rate of 10 A g−1 after 1000 cycles) for SIBs. Therefore, amorphization engineering is a reliable strategy to obtain the robust framework and unimpeded diffusion channels for developing high‐performance conversion‐reaction‐based electrode materials.

Multi-technique Approach to Unravel the (Dis)order in Amorphous Materials.

The concept of order in disordered materials is the key to controlling the mechanical, electrical, and chemical properties of amorphous compounds widely exploited in industrial applications and daily life. Rather, it is far from being understood. Here, we propose a multi-technique numerical approach to study the order/disorder of amorphous materials on both the short- and the medium-range scale. We combine the analysis of the disorder level based on chemical and physical features with their geometrical and topological properties, defining a previously unexplored interplay between the different techniques and the different order scales. We applied this scheme to amorphous GeSe and GeSeTe chalcogenides, showing a modulation of the internal disorder as a function of the stoichiometry and composition: Se-rich systems are less ordered than Ge-rich systems at the short- and medium-range length scales. The present approach can be easily applied to more complex systems containing three or more atom types without any a priori knowledge about the system chemical–physical features, giving a deep insight into the understanding of complex systems.

Femtosecond Laser Deposition of Germanium Selenide onto Silicon Platform at Different Substrate Temperatures.

Germanium selenide (GeSe) thin films were fabricated by employing femtosecond pulsed-laser deposition (fs-PLD) on silicon (100) substrates at various substrate temperatures, ranging from 25 °C to 600 °C. The thin films’ surface morphology qualities and optical properties were studied by utilising transmission electron microscopy (TEM) and X-ray diffraction (XRD). The X-ray diffraction result signifies that the thin films deposited on the silicon at a substrate temperature below 400 °C were amorphous Ge-Se. In contrast, those grown at 400 °C and above exhibited crystallised peaks of Ge-Se orthorhombic and tetragonal structures. The deposition growth rate of the thin films was also found to decrease substantially with increasing substrate temperature. These results show that the fs-PLD process has great potential for fabricating good quality Ge-Se thin film. This technique could enable the manufacture of modern optoelectronic devices for applications in optical communication, sensing, and ovonic threshold switching for the high-density crossbar memory array.

How arsenic makes amorphous GeSe a robust chalcogenide glass for advanced memory integration

The 3D integration technology in semiconductor fabrication requires a key component, the ovonic threshold switching (OTS) selector, to suppress the current leakage. The As doped amorphous (a-) GeSe glass is a commercialized OTS material in 3D phase-change memory, but the understanding of such a doping mechanism is still inadequate. Here we systematically explore the effect of As doping on the structural, bonding, and dynamics properties of a-GeAsSe using ab initio molecular dynamics simulations. The results reveal that As atoms form strong bonds with both Ge and Se atoms. The distorted octahedral structures and the 5-fold rings linked by atoms are increased. All of these structural features lead to a more disordered configuration. Moreover, As atoms have notably slowed down the atomic mobility, rendering a-GeAsSe a high stability. Our studies offer insightful understanding of As-doping in OTS materials, paving the way for the design and application of advanced selector devices.

Nanoscale laser flash measurements of diffuson transport in amorphous Ge and Si

The thermal properties of amorphous materials have attracted significant attention due to their technological importance in electronic devices. In addition, the disorder-induced breakdown of the phonon gas model makes vibrational transport in amorphous materials a topic of fundamental interest. In the past few decades, theoretical concepts, such as propagons, diffusons, and locons, have emerged to describe different types of vibrational modes in disordered solids. However, experiments can struggle to accurately determine which types of vibrational states carry the majority of the heat. In this study, we use nanoscale laser flash measurements (front/back time-domain thermoreflectance) to investigate thermal transport mechanisms in amorphous Ge and amorphous Si thin-films. We observe a nearly linear relationship between the amorphous film’s thermal resistance and the film’s thickness. The slope of the film’s thermal resistance vs thickness corresponds to a thickness-independent thermal conductivity of 0.4 and 0.6 W/(m K) for a-Ge and a-Si, respectively. This result reveals that the majority of heat currents in amorphous Si and Ge thin films prepared via RF sputtering at room temperature are carried by diffusons and/or propagons with mean free paths less than a few nanometers.

Raman study of photoinduced changes in Cd-doped amorphous GeSe2 films

For Cd-doped amorphous GeSe2 films Raman spectra reveal a considerable decrease of the amount of edge-sharing GeSe4 tetrahedra and an increasing number of ethane-like Se3Ge–GeSe3 structural units compared to the undoped films. A Raman peak with a frequency close to that of the CdSe LO phonon observed for the Cd-doped films at increasing laser power density (Pexc ≥ 100 kW/cm2) provides evidence for the photoinduced formation of CdSe nanoparticles in the GeSe2:Cd films due to photoenhanced diffusion.