Amorphous Germanium Research Articles

Optical and microstructural studies of femtosecond laser treated amorphous germanium thin coatings

Optical and microstructural studies of femtosecond laser treated amorphous germanium thin coatings

Structural and electronic properties of amorphous silicon and germanium monolayers and nanotubes: A DFT investigation

A recent breakthrough has been achieved by synthesizing monolayer amorphous carbon (MAC). Here, we used ab initio (DFT) molecular dynamics simulations to study silicon and germanium MAC analogs. Typical unit cells contain more than 600 atoms. We also considered their corresponding nanotube structures. The cohesion energy values for MASi and MAGe are approximately 3.0 eV/atom lower than the energy ordering of silicene and germanene, respectively. Their electronic behavior varies from metallic to small band gap semiconductors. Since silicene, germanene, and MAC have already been experimentally realized, the corresponding MAC-like versions we propose are within our present synthetic capabilities.

Optical Properties of Graphene Nanoplatelets on Amorphous Germanium Substrates.

In this work, the integration of graphene nanoplatelets (GNPs) with amorphous germanium (Ge) substrates is explored. The optical properties were characterized using Variable-Angle Spectroscopic Ellipsometry (VASE). The findings of this study reveal a strong interaction between GNPs and amorphous germanium, indicated by a significant optical absorption. This interaction suggests a change in the electronic structure of the GNPs, implying that amorphous germanium could enhance their effectiveness in devices such as optical sensors, photodetectors, and solar cells. Herein, the use of amorphous germanium as a substrate for GNPs, which notably increases their refractive index and extinction coefficient, is introduced for the first time. By exploring this unique material combination, this study provides new insights into the interaction between GNPs and amorphous substrates, paving the way for the develop of high-performance, scalable optoelectronic devices with enhanced efficiency.

Strain mapping in amorphous germanium thin films with scanning reflectance anisotropy microscopy

Strain imaging is a critical aspect in the design and characterization of opto-electronics, microelectronics, flexible electronics, and on-chip photonics. However, strain mapping techniques are often material specific and strain measurements in amorphous materials remain a challenge. Here, we demonstrate strain mapping and optical characterization of an amorphous semiconductor using scanning reflectance anisotropy microscopy. Using reflection anisotropy spectroscopy and finite element simulations on evaporated amorphous germanium films, we showcase the strain sensitivity of the ellipsometric parameters. We demonstrate nondestructive mapping for simple and complex strain states in amorphous systems. The sub-degree phase and amplitude sensitivity of the microscope is able to determine strain states on the order of 10−3.

Low-frequency electrodynamics in the mixed state of superconducting NbN and a-MoGe films using two-coil mutual inductance technique

We investigate the low-frequency electrodynamics in the vortex state of two type-II superconducting films, namely, a moderate-to-strongly pinned Niobium Nitride (NbN) and a very weakly pinned amorphous Molybdenum Germanium (a-MoGe) film. We employ a two-coil mutual inductance technique to extract the complex penetration depth, λ~ . The sample response is studied through the temperature variation of λ~ in the mixed state, where we employ a model developed by Coffey and Clem (the CC model) to extract different vortex lattice (VL) parameters, such as the restoring pinning force constant (Labusch parameter), VL drag coefficient and pinning potential barrier. We observe that a consistent description of the inductive and dissipative part of the response is only possible when we take the viscous drag on the vortices to be several orders of magnitude larger than the viscous drag estimated from the Bardeen–Stephen model.

Broadband Quantum Efficiency Enhancement of Al0.3InAsSb p–i–n Photodiodes with All‐Dielectric Amorphous Germanium Metasurfaces

Transparent amorphous germanium (a‐Ge) has emerged as a promising material for engineering nanostructures and metasurfaces, offering significant potential for enhancing the performance of photonic devices in the short‐wavelength infrared (SWIR) spectrum. Herein, the successful application of a‐Ge metasurfaces with a truncated pyramid profile to enhance the external quantum efficiency (EQE) of a digital alloy Al0.3InAsSb p–i–n photodiode across a broad‐wavelength range in the SWIR is presented. The experimental findings demonstrate a broadband enhancement in EQE. Two metasurface samples are designed to emphasize different‐wavelength ranges. Notably, 51% improvement in EQE at 1550 nm and 125% enhancement at 2000 nm is achieved. Finite‐difference time domain simulations show that the observed EQE improvement originates from the reduction of reflection and electromagnetic field enhancement. This study underscores the promising role of a‐Ge metasurfaces in advancing the capabilities of SWIR photodetectors. It lays the groundwork for further exploration in optoelectronic device enhancements.

Experimental and theoretical studies on self-diffusion in amorphous germanium

Experimental and theoretical studies on self-diffusion in amorphous germanium

Kinetic study of GeOx amorphous film disproportionation into a-Ge nanoclusters / GeO2 system using Raman and infrared spectroscopy

In this paper, based on the analysis of Raman spectroscopy data, the kinetics of the disproportionation reaction process (GeOx → (1−x/2)Ge + (x/2)GeO2) of an amorphous GeOx film during furnace annealing were studied. An approximation of the experimental kinetics of the disproportionation reaction to the theoretical Kolmogorov–Johnson–Mel–Avrami dependence has been carried out. By analyzing the temperature dependence of the formation time of amorphous germanium clusters, the activation energy of the formation process was obtained, which amounted to 0.9 ± 0.1 eV. In addition, it was found that the position of the Raman peak from amorphous germanium nanoclusters depends on their size. Thus, the phonon localization model can be applied not only to germanium nanocrystals but also to amorphous germanium clusters in the case of their ultra-small sizes (less than 1.5 nm), which is less than the phonon correlation length in amorphous germanium.

Space-Confined Growth of Ultrathin P-Type GeTe Nanosheets for Broadband Photodetectors.

As p-type phase-change degenerate semiconductors, crystalline and amorphous germanium telluride (GeTe) exhibit metallic and semiconducting properties, respectively. However, the massive structural defects and strong interface scattering in amorphous GeTe films significantly reduce their performance. In this work, two-dimensional (2D) p-type GeTe nanosheets are synthesized via a specially designed space-confined chemical vapor deposition (CVD) method, with the thickness of the GeTe nanosheets reduced to 1.9nm. The space-confined CVD method improves the crystallinity of ultrathin GeTe by lowering the partial pressure of the reactant gas, resulting in GeTe nanosheets with excellent p-type semiconductor properties, such as a satisfactory on/off ratio of 105. Temperature-dependent electrical measurements demonstrate that variable-range hopping and optical-phonon-assisted hopping mechanisms dominate transport behavior at low and high temperatures, respectively. GeTe devices exhibit significantly high responsivity (6589 and 2.2 A W-1 at 633 and 980nm, respectively) and detectivity (1.67 × 1011 and 1.3 × 108 Jones at 633 and 980nm, respectively), making them feasible for broadband photodetectors in the visible to near-infrared range. Furthermore, the fabricated GeTe/WS2 diode exhibits a rectification ratio of 103 at zero gate voltage. These satisfactory p-type semiconductor properties demonstrate that ultrathin GeTe exhibits enormous potential for applications in optoelectronic interconnection circuits.

Enhancement of Mg-induced lateral crystallization of amorphous germanium on an insulating substrate by two-step annealing

Magnesium (Mg)-induced lateral crystallization (Mg-ILC) of amorphous germanium (Ge) on a SiO2 stacked structure was investigated. From Raman mapping images, the critical annealing temperature necessary to induce Mg-ILC of amorphous Ge was estimated to be about 350 °C. Furthermore, the Mg-ILC length was truly narrow (∼2 μm) compared with other metal catalysts after annealing at 350 °C for 1 h. To enhance the Mg-ILC, we have examined a two-step annealing method for Mg-ILC of amorphous Ge on SiO2. The Mg-ILC length is significantly enhanced (∼4.5 times) by using a two-step annealing process, which is due to the enhancement of Mg diffusion into amorphous Ge during first-stage low-temperature annealing.

Ultralow thermal conductivity of amorphous silicon–germanium thin films for alloy and disorder scattering determined by 3ω method and nanoindentation

Abstract The ultralow thermal conductivity (1.3 W/(m∙K)) of amorphous silicon–germanium films for alloy and disorder scattering was investigated using the 3ω method and nanoindentation. The films exhibited the lowest phonon mean free path (MFP) of 0.5 nm compared to that of amorphous silicon (1.1 nm) and germanium (0.9 nm) films, owing to alloy scattering in the silicon–germanium films. Based on Matthiessen’s rule, the phonon MFPs of the amorphous silicon–germanium films contributing to alloy and disorder scattering were calculated to be 1.0 nm for both. Therefore, alloy and disorder scattering contribute equally to the reduction in the phonon MFP.

Topological data analysis of TEM-based structural features affecting the thermal conductivity of amorphous Ge

Compared to crystalline materials, amorphous materials lack periodicity and exhibit distinct thermal and lattice vibration properties. Hence, analyzing atomic networks in the transmission electron microscopy (TEM) images of amorphous materials is challenging. In this study, we employ topological data analysis (TDA) and principal component analysis (PCA) to extract structural features from the TEM images of amorphous germanium (a-Ge) and analyze its atomic networks. Our findings demonstrate that the thermal conductivity of a-Ge is influenced by larger atomic rings with more vertices, which facilitates the heat transfer through longer atomic chains and results in a higher thermal conductivity. A comparison of the experimental and simulation data confirms the non-random nature of the atomic arrangements in a-Ge. We propose herein an approach that employs the TDA to identify and analyze the atomic networks in amorphous materials, establishing connections to their thermal properties. This study enhances our understanding of amorphous materials and paves a way for tailored material design and engineering to achieve the desired thermal properties.

A-C/GeTe superlattices: Effect of interfacial impedance adaptation modeling on the thermal properties

Recently, nanostructuration has been proposed to improve the performance of phase change memories. This is the case of superlattices composed of amorphous carbon and crystalline germanium telluride, which we have investigated by molecular dynamics. For this, a modified Stillinger–Weber potential is adapted to reproduce their stiffness contrast/impedance ratio. In order to study the effect of the interface interaction, two sets of parameters are used to model the interfaces with different interactions between the two materials using the properties of the softer material or the average properties between the two creating an adaptation of impedance across the layers. The effects of interface roughness and carbon diffusion at grain boundaries are studied. Using equilibrium molecular dynamics as well as the propagation of wave-packets, we show first that without impedance adaptation, the anisotropy is high, and the roughness has a marked impact on the properties. However, the introduction of impedance adaptation destroys those effects on the thermal conductivity. Finally, we show that the periodic texturing of the interface increases the transmission of in-plane transverse phonons.

Ultrafast Infrared Laser Crystallization of Amorphous Ge Films on Glass Substrates.

Amorphous germanium films on nonrefractory glass substrates were annealed by ultrashort near-infrared (1030 nm, 1.4 ps) and mid-infrared (1500 nm, 70 fs) laser pulses. Crystallization of germanium irradiated at a laser energy density (fluence) range from 25 to 400 mJ/cm2 under single-shot and multishot conditions was investigated using Raman spectroscopy. The dependence of the fraction of the crystalline phase on the fluence was obtained for picosecond and femtosecond laser annealing. The regimes of almost complete crystallization of germanium films over the entire thickness were obtained (from the analysis of Raman spectra with excitation of 785 nm laser). The possibility of scanning laser processing is shown, which can be used to create films of micro- and nanocrystalline germanium on flexible substrates.

Temperature dependent aluminum induced crystallization of amorphous germanium thin films

In this work, we present the sequential growth of metal–semiconductor bilayer structure on HF-treated textured glass substrates by thermal evaporation method. The process of aluminum-induced crystallization of amorphous germanium thin films was critically observed under heat treatment. Temperature assisted structural modifications triggered the nucleation phenomenon for controlled grain growth which transformed the nature of Ge-thin film from amorphous-to-polycrystalline. Interestingly, the electrical resistivity also decreased by subsequent annealing which promoted the conduction mechanism. Optical measurements indicated a dwindling in band tail states and related structural defects, leading to better crystallinity.

Cryogenic mechanical loss of amorphous germania and titania-doped germania thin films

The mechanical loss of amorphous thin films of germania (GeO2) and titania-doped germania (Ti:GeO2) deposited by ion-beam sputtering onto silicon double-paddle oscillators was studied from 10 K to 290 K. Undoped germania was found to show a wide cryogenic mechanical loss peak centered at K with , which decreases to as Ti-concentration increases to 44%. In addition to decreasing the height of this low-temperature peak, Ti-doping increases its width, and shifts its position toward lower temperatures. Annealing reduces the room temperature mechanical loss of Ti:GeO2 and increases its cryogenic mechanical loss, which is consistent with trends observed in most amorphous oxides.

Challenges of Electron Correlation Microscopy on Amorphous Silicon and Amorphous Germanium.

Electron correlation microscopy experiments were conducted on amorphous germanium (a-Ge) and amorphous silicon (a-Si) with the goal to study self-diffusion. For this purpose, a series of tilted dark-field images were acquired during in situ heating of the samples in a transmission electron microscope. These experiments show that the measurements are greatly affected by artefacts. Contamination, crystallization, electron beam-induced sputtering, and macroscopic bending of the samples pose major obstacles to the measurements. Other, more subtle experimental artefacts could occur in addition to these which makes interpretations regarding the structural dynamics nearly impossible. The data were nonetheless evaluated to see if some useful information could be extracted. One such result is that the distribution of the characteristic times τKWW, which were obtained from stretched exponential fits to the intensity autocorrelation data, is spatially heterogeneous. This spatial heterogeneity is assumed to be caused by a potential nonergodicity of the materials, the artefacts or an inhomogeneous amorphous structure. Further data processing shows that the characteristic times τKWW are moreover temperature independent, especially for the a-Ge data. It is concluded that the structural rearrangements over time are primarily electron beam-driven and that diffusive dynamics are too slow to be measured at the chosen, experimentally accessible annealing temperatures.

The role of tin atoms on the crystallization of amorphous germanium films

The thorough investigation of the GeSn system was able to present some new (or intermediary) properties that, if properly applied, can provide interesting outcomes. One of these refer to the crystallization of amorphous Ge as induced by the presence of Sn, also known as metal-induced crystallization. According to it, depending on the relative composition of the GeSn system Ge crystallites can be produced at rather low temperatures in a gradual and very controllable way. This forms the basis of this work that investigated the Sn-induced crystallization of sputter-deposited amorphous Ge films − as a function of the Sn concentration (∼0−30 at.% range) and temperature of thermal annealing (from 25 to 600 °C). In all cases, the atomic structure of the films was investigated by Raman spectroscopy that corroborates the influence of Sn atoms in reducing the crystallization temperature and improving the quality of the Ge crystallites. Accordingly, this work suggests the suitability of the GeSn system in producing cost-effective poly-crystalline Ge films.

Electrical, optical, and magnetic properties of amorphous yttrium iron oxide thin films and consequences for non-local resistance measurements

We present magnetic characterization, charge resistivity, and optical photoluminescence measurements on amorphous yttrium iron oxide thin films (a-Y–Fe–O), with supporting comparisons to amorphous germanium (a-Ge) films. We measured magnetic properties with both SQUID magnetometry and polarized neutron reflectometry. These results not only confirm that a-Y–Fe–O is a disordered magnetic material with strong predominantly antiferromagnetic exchange interactions and a high degree of frustration, but also that it is best understood electrically as a disordered semiconductor. As with amorphous germanium, a-Y–Fe–O obeys expectations for variable-range hopping through localized electron states over a wide range of temperature. We also clarify the consequences of charge transport through such a semiconducting medium for non-local voltage measurements intended to probe spin transport in nominally insulating magnetic materials. We further compare non-local resistance measurements made with “quasi-dc” automated current reversal to ac measurements made with a lock-in amplifier. These show that the “quasi-dc” measurement has an effective ac current excitation with frequency up to approximately 22 Hz, and that this effective ac excitation can cause artifacts in these measurements including incorrect sign of the non-local resistance. This comprehensive investigation of non-local resistance measurements in a-Y–Fe–O shows no evidence of spin transport on micrometer length scales, which is contrary to our original work, and in line with more recent investigations by other groups.

Atomic-scale in situ observation of electron beam and heat induced crystallization of Ge nanoparticles and transformation of Ag@Ge core-shell nanocrystals.

Crystallization of amorphous materials by thermal annealing has been investigated for numerous applications in the fields of nanotechnology, such as thin-film transistors and thermoelectric devices. The phase transition and shape evolution of amorphous germanium (Ge) and Ag@Ge core-shell nanoparticles with average diameters of 10 and 12nm, respectively, were investigated by high-energy electron beam irradiation and in situ heating within a transmission electron microscope. The transition of a single Ge amorphous nanoparticle to the crystalline diamond cubic structure at the atomic scale was clearly demonstrated. Depending on the heating temperature, a hollow Ge structure can be maintained or transformed into a solid Ge nanocrystal through a diffusive process during the amorphous to crystalline phase transition. Selected area diffraction patterns were obtained to confirm the crystallization process. In addition, the thermal stability of Ag@Ge core-shell nanoparticles with an average core of 7.4 and a 2.1nm Ge shell was studied by applying the same beam conditions and temperatures. The results show that at a moderate temperature (e.g., 385°C), the amorphous Ge shell can completely crystallize while maintaining the well-defined core-shell structure, while at a high temperature (e.g., 545°C), the high thermal energy enables a freely diffusive process of both Ag and Ge atoms on the carbon support film and leads to transformation into a phase segregated Ag-Ge Janus nanoparticle with a clear interface between the Ag and Ge domains. This study provides a protocol as well as insight into the thermal stability and strain relief mechanism of complex nanostructures at the single nanoparticle level with atomic resolution.