Onset Of Amorphization Research Articles

Anisotropic Disorder and Thermal Stability of Silicane.

Atomically thin silicon nanosheets (SiNSs), such as silicane, have potential for next-generation computing paradigms, such as integrated photonics, owing to their efficient photoluminescence emission and complementary-metal-oxide-semiconductor (CMOS) compatibility. To be considered as a viable material for next-generation photonics, the SiNSs must retain their structural and optical properties at operating temperatures. However, the intersheet disorder of SiNSs and their nanoscale structure makes structural characterization difficult. Here, we use synchrotron X-ray diffraction and atomic pair distribution function (PDF) analysis to characterize the anisotropic disorder within SiNSs, demonstrating they exhibit disorder within the intersheet spacing, but have little translational or rotational disorder among adjacent SiNSs. Furthermore, we identify changes in their structural, chemical, and optical properties after being heated in an inert atmosphere up to 475 °C. We characterized changes of the annealed SiNSs using synchrotron-based total X-ray scattering, infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, electron paramagnetic resonance, absorbance, photoluminescence, and excited-state lifetime. We find that the silicon framework is robust, with an onset of amorphization at ∼300 °C, which is well above the required operating temperatures of photonic devices. Above ∼300 °C, we demonstrate that the SiNSs begin to coalesce while keeping their translational alignment to yield amorphous silicon nanosheets. In addition, our DFT results provide information on the structure, energetics, band structures, and vibrational properties of 11 distinct oxygen-containing SiNSs. Overall, these results provide critical information for the implementation of atomically thin silicon nanosheets in next-generation CMOS-compatible integrated photonic devices.

Unconventional high-pressure Raman spectroscopy study of kinetic and peak pressure effects in plagioclase feldspars

We present the results of a high-pressure semi-hydrostatic study of two plagioclase minerals, andesine and albite, using diamond anvil cells (DACs) to characterize in situ variations in Raman spectra under different static pressures. In this work, we also examined the kinetic effects on deformation at both long and short timescales through non-traditional experiments in which the DAC was either dropped or struck with a mallet. We examined the effects of strain rate, quench rate, and pressure duration on the Raman spectra of plagioclase. We observed that amorphization occurred in all the plagioclase samples we analyzed, and that amorphization onset and permanence differ depending on the composition, kinetics, energy input, and peak pressure. In andesine, samples pressurized above a peak pressure of 18 GPa, amorphization is permanent. Below this critical pressure, the phase has ‘memory’, and crystalline andesine reforms on decompression. Our findings suggest the presence of a thermodynamic energy well in andesine around 18 GPa, and we show that any additional energy input while close to 18 GPa results in amorphization becoming permanent. The effect of the energy well may be relevant for longer duration impacts. For such impacts, equilibrium state studies of deformation and phase formation were previously considered to be applicable. These experiments illuminating the presence of memory effects in plagioclase have implications for comparing static compression, in which samples are measured while under compression, with shock compression, for which samples are measured after decompression.

Thermal conductivity of electron-irradiated graphene

We report results of a systematic analysis of thermal transport in electron-irradiated, including irradiation-induced amorphous, graphene sheets based on nonequilibrium molecular-dynamics simulations. We focus on the dependence of the thermal conductivity, k, of the irradiated graphene sheets on the inserted irradiation defect density, c, as well as the extent of defect passivation with hydrogen atoms. While the thermal conductivity of irradiated graphene decreases precipitously from that of pristine graphene, k0, upon introducing a low vacancy concentration, c < 1%, in the graphene lattice, further reduction of the thermal conductivity with the increasing vacancy concentration exhibits a weaker dependence on c until the amorphization threshold. Beyond the onset of amorphization, the dependence of thermal conductivity on the vacancy concentration becomes significantly weaker, and k practically reaches a plateau value. Throughout the range of c and at all hydrogenation levels examined, the correlation k = k0(1 + αc)−1 gives an excellent description of the simulation results. The value of the coefficient α captures the overall strength of the numerous phonon scattering centers in the irradiated graphene sheets, which include monovacancies, vacancy clusters, carbon ring reconstructions, disorder, and a rough nonplanar sheet morphology. Hydrogen passivation increases the value of α, but the effect becomes very minor beyond the amorphization threshold.

Amorphization and nanocrystallization of silicon under shock compression

High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon unveiled remarkable structural changes above a pressure threshold. Two distinct amorphous regions were identified: (a) a bulk amorphous layer close to the surface and (b) amorphous bands initially aligned with {111} slip planes. Further increase of the laser energy leads to the re-crystallization of amorphous silicon into nanocrystals with high concentration of nano-twins. This amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, are in agreement with the experimental results. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed qualitatively by classical nucleation theory.

Pressure and shear-induced amorphization of silicon

Here we report that high-power, pulsed, laser-driven shock compression of monocrystalline silicon produces directional amorphization, revealed by high-resolution transmission electron microscopy and confirmed by molecular dynamics simulations. At the lowest energy level experiment, generating a pressure of ∼4 GPa, silicon reacts elastically. At intermediate energy levels (P∼11 and 22 GPa), amorphization is observed both at the surface and directionally, along planes making angles close to the maximum shear. At the highest laser energy level explored here, (Ppeak ∼28 GPa), the recovered sample shows a nanocrystalline microstructure near the surface. This nanocrystalline structure forms by crystallization from the amorphous phase and is thought to be a post-shock phenomenon. Shear-induced lattice defects (stacking faults and twins) on crystallographic slip planes play a crucial role in the onset of amorphization. Molecular dynamics show that silicon behaves elastically until ∼10 GPa and, at slightly higher pressures, partial dislocations and stacking faults are emitted from the surface. Driven by the high-amplitude stress pulse, these defects travel inwards along specific crystallographic orientations and intersect, leading to further defect creation, additional plastic work, and, at higher pressures, amorphous bands in intersecting patterns. The typical high-pressure solid–solid phase transitions of silicon are not observed whereas the high shear stresses are relaxed by localized dislocation motion/interactions and eventually by directional amorphization, which occurs below the critical hydrostatic pressure for melting of silicon in shock compression. It is therefore proposed that the combined effects of hydrostatic and shear stresses lead to directional amorphization.

High-pressure structural and elastic properties of Tl2O3

The structural properties of Thallium (III) oxide (Tl2O3) have been studied both experimentally and theoretically under compression at room temperature. X-ray powder diffraction measurements up to 37.7 GPa have been complemented with ab initio total-energy calculations. The equation of state of Tl2O3 has been determined and compared to related compounds. It has been found experimentally that Tl2O3 remains in its initial cubic bixbyite-type structure up to 22.0 GPa. At this pressure, the onset of amorphization is observed, being the sample fully amorphous at 25.2 GPa. The sample retains the amorphous state after pressure release. To understand the pressure-induced amorphization process, we have studied theoretically the possible high-pressure phases of Tl2O3. Although a phase transition is theoretically predicted at 5.8 GPa to the orthorhombic Rh2O3-II-type structure and at 24.2 GPa to the orthorhombic α-Gd2S3-type structure, neither of these phases were observed experimentally, probably due to the hindrance of the pressure-driven phase transitions at room temperature. The theoretical study of the elastic behavior of the cubic bixbyite-type structure at high-pressure shows that amorphization above 22 GPa at room temperature might be caused by the mechanical instability of the cubic bixbyite-type structure which is theoretically predicted above 23.5 GPa.

Swift heavy ion induced recrystallization in cubic silicon carbide: New insights from designed experiments and MD simulations

3C-SiC single crystals have been initially irradiated in the nuclear energy loss regime with 100keV Fe ions to fluences ranging from 4×1013 to 4×1014cm−2 (i.e. 0.07–0.7dpa). RBS/C measurements indicate that SiC rapidly becomes amorphous (at ∼0.4dpa). Two damaged SiC crystals exhibiting a different defective structure have been subsequently irradiated in the electronic energy loss regime with 870MeV swift heavy (Pb) ions (SHIs) up to a fluence of 4×1013cm−2. Initially fully amorphous SiC layers showed a decrease in size after SHI irradiation with a recrystallization occurring at the amorphous–crystalline interface. On the contrary, partially amorphous crystals for which onset of amorphization just initiated at the damage peak recovered over the entire damage thickness. Variation of amorphous thickness or disorder level has been monitored as a function of Pb ion fluence, which allowed deriving recrystallization kinetics. Data have been fitted with the direct-impact model and recrystallization cross-sections and threshold values for recovery have been determined for both types of initially defective structures. Differences are qualitatively discussed in terms of nature and density of irradiation defects. All experimental trends have been successfully reproduced by molecular dynamics simulations that mimicked thermal spikes induced by SHIs.

The structure and amorphization of germanane

Germanane, a direct band gap, high electron mobility germanium graphane analogue, has attracted considerable interest for optoelectronics. Here, temperature dependent pair distribution function analysis shows, upon annealing, the two-dimensional germanane framework begins to amorphize. The amorphization onset is independent of the existence of halogens or other impurities.

Towards semi-insulating InGaAsP/InP layers by post-growth processing using Fe ion implantation and rapid thermal annealing

In this paper, we report on an effective post-growth processing technique for developing semi-insulating (SI) photonic thin films absorbing in 1.3 µm. For that purpose, we examined a 1 µm thick unintentionally n-doped In0.72Ga0.28As0.61P0.39 epilayer (0.95 eV bandgap) modified by multiple-energy MeV Fe ion implantation. Fe was chosen as a deep-level impurity. The ion beam processing was performed at room temperature, followed by rapid thermal annealing (RTA) at 800 °C for 15 s. We investigated the impact of ion fluence on electrical properties by Hall effect measurements. Channelling Rutherford backscattering spectrometry, x-ray diffraction and photoluminescence measurements were carried out to evaluate crystal quality after each fabrication step. Beyond the onset of amorphization, when the total Fe fluence was more than 4.8 × 1013 cm−2, the implanted InGaAsP layer showed evidence of a poor recrystallization after RTA, and its isolation was impaired. Maximum resistivity values were achieved below the onset of amorphization where annealing reduced ion de-channelling and recovered damage-induced strain. With a total Fe fluence of 1.6 × 1013 cm−2, the electrical resistivity and Hall mobility reached values of 1.4 × 104 Ω cm and 4 × 102 cm2 V−1 s−1. These results add important insights on the optimization of this process for the development of InP-based SI photoconductive films.

Irradiation enhanced paramagnetism on graphene nanoflakes

We have studied the magnetization of vertically aligned graphene nanoflakes irradiated with nitrogen ions of 100 KeV energy and doses in the range 10^11- 10^17 ions/cm2. The non-irradiated graphene nanoflakes show a paramagnetic contribution, which is increased progressively by ion irradiation at low doses up to 10^15 /cm^2. However, further increase on implantation dose reduces the magnetic moment which coincides with the onset of amorphization as verified by both Raman and X-ray photoelectron spectroscopic data. Overall, our results demonstrate the absence of ferromagnetism on either implanted or unimplanted samples from room temperature down to a temperature of 5K

Formation of amorphous sugar in the syrup film – a key factor in modelling of industrial sugar drying

A mathematical model of the steady-state sugar drying operation in a cross-flow rotating drum dryer is proposed. Although the model is based on the classic two-film concept of simultaneous heat and mass transfer, it also allows for the formation and growth of a thin layer of amorphous sugar due to local supersaturation of the sugar syrup. The formation of amorphous sugar is a result of the competition between crystallization and drying kinetics. The onset of amorphization is assumed to occur when the system moves from the metastable zone to the labile zone on the sucrose–water phase diagram. The rate of sucrose amorphization is controlled by the conditions prevailing on the metastability limit. Diffusivity of water through the amorphous sugar, the only adjustable parameter of the proposed model, has been determined by fitting sugar moisture data from an industrial Louvre-type dryer. The layer of amorphous sugar plays a critical role during the falling-rate period of drying. Generally, models ignoring its presence tend to overestimate the moisture removal rate. The presented model predicts a dramatic decline of the evaporation rate once the amorphous sugar begins to form which is observed in the industrial practice. The effect of key operating parameters such as air flow rate, hot air temperature, and crystal size on the dryer performance is also discussed.

Non-hydrostatic compression of zeolite NaA in water medium: connection to anomalous conductivity

High-pressure synchrotron X-ray powder diffraction measurements of a synthetic zeolite NaA were carried out up to 2.5 GPa using pure water as pressure-transmitting (P) medium to provide non-hydrostatic (wet) conditions in a diamond anvil cell. The compressibility of wet zeolite NaA is similar to that measured at hydrostatic compression in water within the P-range 0-0.8 GPa, whereas between 1-2 GPa the zeolite becomes slightly more compressible and progressively amorphizes due to the non-hydrostatic conditions. Rietveld refinement at 0.37 GPa reveals a selective additional filling of the H 2 O sites in α- and β-cage, leading to about 30% increase of the total water content. The over-hydrated state of the zeolite is partially preserved after the pressure release. The over-hydration of zeolite pores, combined with a partial disordering at the onset of amorphization, apparently provides necessary conditions for the P-induced enhancement of water-cationic diffusion and the corresponding increase of ionic conductivity observed in zeolite NaA in non-hydrostatic water medium.

Surface modifications by swift heavy-ion irradiation of indium phosphide

InP (001) samples were irradiated with 200 MeV Au ions at different fluences. The surface nanotopographical changes due to increasing fluence of swift heavy ions were observed by Atomic Force Microscopy (AFM), where the onset of a large increase in surface roughness for fluences sufficient to cause complete surface amorphization was observed. Transmission Electron Microscopy (TEM) was used to observe bulk-ion tracks that formed in InP, and high resolution TEM (HRTEM) revealed that single-ion tracks might not be amorphous in nature. Surface-ion tracks were observed by AFM in the form of ill-defined pits (hollows) of ~12 nm in diameter (width). In addition, Rutherford backscattering was utilized to follow the formation of disorder to amorphization in the irradiated material. The interpretation of the large increase in surface roughness with the onset of amorphization can be attributed to the plastic phenomena induced by the change of states from crystalline to amorphous by ion irradiation.

Temperature independence of pressure-induced amorphization of the phase-change memory alloy Ge2Sb2Te5

In the temperature range from room temperature to about 150°C, the prototypic phase-change material Ge2Sb2Te5 becomes amorphous upon hydrostatic compression. In the studied temperature range, the onset of amorphization is at about 15GPa and the material completely amorphizes at 25GPa; these values do not depend on temperature. Upon decompression, the amorphous phase is stable at lower temperatures, yet at higher temperatures (145°C), the initial fcc phase is recovered upon decompression. A possible mechanism of pressure-induced amorphization and its implications for phase-change memories are discussed.

Pressure induced structural transformation of pyrochlore Gd 2Zr 2O 7

High pressure x-ray diffraction studies on Gd 2Zr 2O 7 pyrochlore were carried out up to 45 GPa using a diamond anvil cell. A structural transition leading to monoclinic phase is initiated at a very low pressure of ∼3 GPa. Coexistence of both the parent and the daughter phase was seen up to the maximum pressure of 45 GPa. Bulk modulus of the parent phase is found to be 161.5 GPa and the lattice parameters of the high pressure monoclinic phase at 9 GPa are found to be: a=7.4839 Å, b=5.7089 Å, c=12.6498 Å and β = 97.61 . The structural phase transition is found to be reversible when quenched from 10 GPa, but it is irreversible when quenched from 45 GPa. Also, onset of amorphisation is seen above 35 GPa.

Pressure-induced amorphization of cubicZrW2O8studiedin situandex situby synchrotron x-ray diffraction and absorption

The behavior of cubic $\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$ on compression in a DAC to $7.6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ was examined in situ by a combination of synchrotron x-ray diffraction and x-ray absorption spectroscopy (XAS). These data were compared with x-ray absorption measurements on an amorphous sample of $\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$ recovered from $7.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in a multianvil apparatus. The in situ diffraction data show the complete formation of orthorhombic $\mathrm{Zr}{\mathrm{W}}_{2}{\mathrm{O}}_{8}$ at low pressure $(<0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, and amorphization onset at $>2.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ with completion at $<7.6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The corresponding in situ XAS data suggest a continuous evolution of the local tungsten coordination environment on compression after forming the orthorhombic phase, with the average $\mathrm{W}\mathrm{O}$ bond length increasing, indicating an increase in the average coordination number, and the W ${L}_{I}$ pre-edge peak decreasing in magnitude, indicating a movement toward tungsten coordination that is closer to centrosymmetric These observations are inconsistent with a model for the amorphization that simply involves a loss of orientational/positional order among existing coordination polyhedra. The XANES data for the amorphous sample recovered from the multianvil apparatus are unlike any of the XANES seen in the in situ measurements, suggesting that the local structure in the glassy material relaxes on decompression. The XANES for the recovered sample are very similar to those for ammonium paratungstate, a material that contains tungsten in a variety of heavily distorted octahedral environments.

Enhancement of the etch rate of LiNbO3 by prior bombardment with MeV O2+ ions

The implantation of LiNbO3 with 5 MeV O2+ ions has been examined as a means of increasing the rate of subsequent reactive ion etching in CF4/CHF3 plasmas. The etch rate of LiNbO3 implanted at a dose sufficient to amorphize the substrate (1×1015–5×1015 ions cm−2) was ⩽3 times higher than in unimplanted substrates. The measured etch rates in CF4/CHF3 plasmas were essentially independent of the implant dose below 3×1014 ions cm−2 but increased significantly at and above 1×1015 ions cm−2. This transition was correlated with the onset of amorphization. In an Ar plasma, the etch rate of LiNbO3 was unaffected by prior million electron volt implantation and was an order of magnitude lower than in the CF4/CHF3 plasmas. These results have suggested that the damage induced by million electron volt implantation acted to accelerate the process of ion-enhanced chemical etching in LiNbO3 but had no significant influence on the process of physical sputtering in an Ar plasma.

Boron-implantation-induced crystalline-to-amorphous transition in nickel: An experimental assessment of the generalized Lindemann melting criterion

The generalized Lindemann melting hypothesis has recently been used to develop a unified thermodynamic criterion for melting applicable to both heat-induced melting and disorder-induced crystalline-to-amorphous (c-a) transformation. The hypothesis stipulates that the sum $〈{\ensuremath{\mu}}^{2}{〉}_{\mathrm{Total}}$ of the static and dynamic root-mean-square (rms) atomic displacements is a constant fraction of the nearest-neighbor distance along the melting curve of a solid. To test this hypothesis, energy-filtered selected area electron-diffraction intensity measurements were used to determine the generalized Lindemann parameter $\ensuremath{\delta}=\sqrt{〈{\ensuremath{\mu}}^{2}{〉}_{\mathrm{Total}}}{/d}_{\mathrm{nn}},$ in which ${d}_{\mathrm{nn}}$ represents the nearest-neighbor distance, as a function of boron concentration during implantation of $50\ensuremath{-}{\mathrm{keV}\mathrm{}\mathrm{B}}^{+}$ into polycrystalline Ni at 77 K. The onset of amorphization was found to occur close to 10 at. % boron, which is in good agreement with the value predicted by ${T}_{o}$ curve calculated using the generalized Lindemann hypothesis. Moreover, the critical value of the generalized Lindemann parameter for amorphization, ${\ensuremath{\delta}}_{\mathrm{Critical}}=0.115\ifmmode\pm\else\textpm\fi{}0.01,$ is within experimental error, identical to that for Ni just below its thermodynamic melting temperature of $T=1728\mathrm{K},$ hence providing a direct confirmation for the generalized Lindemann melting hypothesis.

Early Stages of Solid-State Amorphization Reaction During Mechanical Alloying of a Multicomponent Zr-Powder Mixture

The synthesis of amorphous alloys by non-equilibrium techniques such as rapid quenching or ball milling has become an active subject during the past decade. Recently, a new group of Zr-based multicomponent metallic alloys with large glass forming ability have been found and are characterized by an extended supercooled liquid region and a glass transition before crystallization. Due to their low critical cooling rates, these alloys also offer the possibility to produce bulk metallic glasses by mechanical alloying and subsequent warm consolidation of the obtained amorphous powders above the glass transition temperature. Although amorphous phase formation by ball milling of elemental or intermetallic powders was studied extensively on several binary alloys systems such as Zr combined with late transition metals, the driving forces and the mechanisms of amorphization are still the subject of many investigations and discussions. In this regard, in order to allow for a meaningful comparison of the thermal stability and compositional homogeneity of mechanical alloyed amorphous powders with that of metallic glasses of the same average composition prepared by rapid liquid quenching, it is essential to develop a clear understanding of the microstructural evolution and phase formation during ball milling. This issue is addressed in the present workmore » by showing detailed structural investigations on the early stages of phase formation during mechanical alloying of an elemental Zr{sub 60}Al{sub 10}Ni{sub 9}Cu{sub 18}Co{sub 3} powder mixture. The results reveal that formation of strongly supersaturated solid solutions and fast grain refinement are the prior steps before the onset of amorphization.« less

The production and stability of implantation-induced vacancy excesses in silicon

The spatial separation of implantation-induced Frenkel pairs can yield vacancy and interstitial excesses at depths, respectively, less than and greater than the projected range ( R p). For the present study, the production and stability of such excesses have been characterised with Rutherford backscattering spectrometry (RBS), double-crystal X-ray diffraction (DCXRD) and transmission electron microscopy (TEM) following high-energy, high-temperature implantation of Si substrates. Vacancy and interstitial excesses were manifested as tensile and compressive strains, respectively. At a given temperature, the compressive strain at depths greater than R p increased with dose until the onset of amorphisation. Similarly, the tensile strain at depths less than R p increased with dose until the yield strength of Si was exceeded. Thereafter, strain relaxation through dislocation formation was observed. At a given dose, the tensile strain decreased as the implant temperature increased due to increased dynamic annealing. Similarly, the maximum strain attainable prior to the onset of dislocation formation decreased as the implant temperature increased consistent with increased defect mobility and decreased yield strength.