Bi Droplets Research Articles

High verticality vapor-liquid-solid growth of GaAs0.99Bi0.01 nanowires using Ga-Bi assisted catalytic droplets.

GaAsBi nanowires (NWs) are promising for optoelectronic applications in the near- and mid-infrared wavelengths due to the optical properties of the Bi-containing compound and the nanowire structure benefits. In general, synthesizing the GaAsBi NWs results in uncontrollable metamorphic structures and spontaneous Bi-containing droplets. Here, we explore the potential of using the droplets as catalysts to form GaAsBi nanowires (hence, the vapor-liquid-solid growth mechanism) on GaAs (111) substrates by molecular beam epitaxy. The GaAsBi NWs experience a two-step growth: Bi droplet deposition and GaAsBi nanowire growth. The optimal droplet deposition temperature (250 °C) is defined based on the droplet morphologies. The gradation of growth temperatures of GaAsBi NWs to 250 °C, 300 °C, and 350 °C results in high-aspect-ratio NWs, tilted NWs, and low-aspect-ratio NWs, respectively. Structural investigation shows that the optimal (low-aspect-ratio) NW has the composition of GaAs0.99Bi0.01 with the catalytic droplet of Ga0.99Bi0.01 decorated on its tip. Detailed structural analyses show that the Bi content progressively increases from the NW stem to the wire-substrate interface. The satisfying GaAsBi NW morphology does not warrant the expected superior optical results. Photoluminescence study suggests that the NW has a strong carrier thermalization from the NW stem to the wire-substrate interface influenced by the graded NW growth temperature profile.

Degradation mechanisms of annealed GaAsPBi films grown by molecular beam epitaxy

GaAsPBi is a novel semiconductor that can broaden the bandgap range of III−V compounds, but its growth on GaAs(001) by molecular beam epitaxy is complicated by the presence of Bi. To avoid the nucleation of Bi droplets, the growth temperature must be low (<400°C), resulting in GaAsPBi films with poor photoluminescent yields. It is shown here that while post-growth annealing of GaAsPBi improves its optical quality, the GaAsPBi surface and the underlying GaAsPBi/GaAs interface undergo structural and chemical changes that limit the usefulness of annealing. Using synchrotron radiation-based spectromicroscopy, morphological and chemical changes on the surface are followed in situ, allowing the temperature limit to be estimated at ∼510°C. Beyond this, the GaAsPBi film degrades, first by the nucleation of interfacial dislocations at ∼520°C, then by the desorption of surface oxides at ∼580°C, and by congruent sublimation of the film itself at ∼670°C. The damages are spatially correlated and inhomogeneous, with areas above the dislocations being the preferential damage sites. These in situ observed events provide critical insights to the mechanisms and chronology of surface and interfacial degradation during high-temperature processing of GaAsPBi/GaAs structure, with implications for the process and device designs of heteroepitaxial III−V films prevalence in optoelectronics.

Multiple ultrasounds assisted phase separation and monotectic solidification of liquid ternary Al81.5Cu14.7Bi3.8 immiscible alloy

Multiple power ultrasounds were employed to investigate the phase transition process of ternary Al81.5Cu14.7Bi3.8 immiscible alloy by various exerting modes. As the ultrasonic sources increased, the liquid phase separation pattern transformed from (Bi)-rich layered macrosegregation into the uniform distribution of secondary (Bi) droplets. Meanwhile, the primary (Al) phase evolved from coarse dendrites into plenty of small spherical grains and also tended to be uniformly dispersed. The subsequently formed ternary (Al) + (Al2Cu) + (Bi) monotectic structure, featured by the alternative (Al) and (Al2Cu) lamellar structure with fine (Bi) grains distributed, was coarsened first and then refined. Numerical simulations showed that the transient cavitation and the acoustic streaming strength were significantly enhanced by increasing ultrasonic beams, with the fourfold ultrasounds producing the most prominent effects on the phase separation process. The intensive and enlarged cavitation areas greatly accelerated the nucleation of both the secondary liquid phase and primary solid phase, which refined the growing (Bi) droplets and (Al) dendrites. The strength and morphology of acoustic streaming were the key factors in offsetting Stokes motion and carrying the growing grains to various regions, resulting in a uniform microstructure. Furthermore, increasing ultrasonic sources improved the friction and wear properties of the solidified alloy, which indicated that the Al81.5Cu14.7Bi3.8 immiscible alloy may become an excellent wear-resistant material owing to the uniform monotectic structure fabricated by the fourfold ultrasounds.

Heterogeneous nucleation of embedded droplets in the Zn-15Bi immiscible alloy studied by nanocalorimetry

In this study, micro-sized Bi droplets embedded in the Zn matrix were prepared by melt-spinning technique, and their heterogeneous nucleation behaviors were investigated by nanocalorimetry at cooling rates ranging from 500 to 10,000 K/s. Upon rapid heating, a melt layer between the embedded Bi and Zn matrix formed during eutectic melting, which was then followed by the melting of the residual solid Bi phase. On cooling, both eutectic reaction and solidification of Bi droplets occurred with different nucleation kinetics. At a higher cooling rate, the undercooling for the eutectic reaction increased and then reached a maximum value while the undercooling for the solidification of Bi was nearly independent on the cooling rate. With the classical nucleation theory (CNT), the underlying nucleation mechanism for eutectic reaction and solidification of Bi droplets was addressed, revealing that the evolution of active nucleation sites with the cooling rate in different interfaces was responsible for this discrepancy.

Understanding solidification and wear behaviour of Al-3Cu-Bi alloys

Considering that Bi is able to decrease friction between contacting surfaces, wear damage is expected to decrease as well. Bi-containing Al alloys have emerged as a choice for creating self-lubricating parts. The current work compares the effects of two distinct Bi alloy contents on the microstructure, Bi morphology/spacing, hardness, and wear resistance of Al-3.0wt.%Cu-Bi alloys. The novelty is in determining the microstructure constituting the Al-Cu based alloys with different Bi additions. Also, functional relationships between the wear resistance and the Bi droplets are derived. Increasing the Bi content from 2.0 to 3.2 wt.% changed the size and spacing of Bi particles as well as decreased hardness and increased the wear resistance. Even though the Al-3.0wt%Cu-3.2wt%Bi alloy has a lower hardness, the greater Bi fraction seems to be the key factor enhancing wear resistance. The bigger Bi areas spreading over the Al-rich phase under wear allowed higher wear resistance.

Bi incorporation and segregation in the MBE-grown GaAs-(Ga,Al)As-Ga(As,Bi) core\u2013shell nanowires

Incorporation of Bi into GaAs-(Ga,Al)As-Ga(As,Bi) core–shell nanowires grown by molecular beam epitaxy is studied with transmission electron microscopy. Nanowires are grown on GaAs(111)B substrates with Au-droplet assisted mode. Bi-doped shells are grown at low temperature (300 °C) with a close to stoichiometric Ga/As flux ratio. At low Bi fluxes, the Ga(As,Bi) shells are smooth, with Bi completely incorporated into the shells. Higher Bi fluxes (Bi/As flux ratio ~ 4%) led to partial segregation of Bi as droplets on the nanowires sidewalls, preferentially located at the nanowire segments with wurtzite structure. We demonstrate that such Bi droplets on the sidewalls act as catalysts for the growth of branches perpendicular to the GaAs trunks. Due to the tunability between zinc-blende and wurtzite polytypes by changing the nanowire growth conditions, this effect enables fabrication of branched nanowire architectures with branches generated from selected (wurtzite) nanowire segments.

Bismuth incorporation and its influence on surface morphology of InAs (1 1 0)

The low solid solubility of Bi in III-V semiconductors complicates achieving high Bi incorporation in those films. Crystalline orientation is often overlooked as a parameter that can be used to influence Bi incorporation. This study investigates the differences in Bi incorporation in superlattice structures grown on InAs(001) versus InAs(110). X-ray diffraction reveal distinct differences in Bi incorporation between the two surfaces as a function of whether or not Bi droplets formed on the surface during growth. Atomic force microscopy shows that the surface morphology also changes as a function of the amount of Bi deposited. Finally we discuss possible mechanisms explaining the changes in Bi incorporation and surface morphology.

Atomic tuning effect of TiB2 particles on the liquid phase separation behavior of an Al-Bi immiscible alloy

Microparticles are usually applied to obtain homogenous microstructure in immiscible alloys, but their role on the atomic-scale structural evolution during the liquid-liquid separation process has not been fully understood. Here we employed in situ high-energy X-ray diffraction (HE-XRD) combined with Reverse Monte Carlo calculations (RMC) to investigate the melt structure of an Al-60 wt.%Bi immiscible alloy with and without TiB2 particles. The results show that liquid separation can be mainly characterized by the changes in atomic spacing of Bi, and the first shell layer of Bi droplets is in crystal order in the Al-Bi melt. Addition of TiB2 particles changes the first shell layer of Bi droplets to a disorder structure, and consequently reduces the temperature for liquid separation. These findings break the conventional cognition that microparticles can promote nucleation of minor droplets, and provide a new perspective of the inhibiting effect of microparticles in immiscible alloys.

Investigation on the Solidification Behavior of Bi Droplets Embedded in Cu Matrix by Nanocalorimetry

Background: The solidification of metals is usually initiated by heterogeneous nucleation, which, however, is less controllable due to the unexpected impurities. In this study, the nucleation behavior of micro- and nano-sized Bi droplets embedded in the Cu matrix, where well-identified catalyst interfaces were introduced, was studied. Objective: This paper demonstrated the nucleation behavior of the nano-sized Bi droplets embedded in the Cu matrix to determine the nucleation sites and contact angle. Methods: Cu-20Bi (wt. %) melt-spun ribbons consisting of numerous micro- and nano-sized Bi droplets embedded in the Cu matrix were prepared. Based on the structure characterization by transmission electron microscopy (TEM), the nucleation behavior of these embedded Bi droplets was demonstrated by nanocalorimetry. Results: The orientation relationship between the nano-sized Bi droplets and the Cu matrix was determined. The undercooling of the nano-sized Bi droplets with the cooling rate was more stable than that of the micro-sized Bi droplets, and a nucleation contact angle of 72o was obtained. Conclusion: The undercooling increases with the reduction of droplet size. The nucleation of the Bi droplets is significantly affected by their interfacial structure with the Cu matrix. Compared with hexagonal Zn, the Cu matrix with face-centered cubic structure has a less catalytic effect on the nucleation of Bi.

Thermodynamics and Reduction of Bi2O3 in Waste Tire-Derived Atmosphere

The present study aimed to investigate reduction behavior of Bi2O3 in the gaseous atmosphere generated by the pyrolysis of waste tires. Thermodynamic analysis carried out in the Bi-O-C-H-S-Ar system predicted that Bi2O3 could be reduced to Bi in the temperature range 700–900 K. Bi2O3 reduction experiments were carried out as a function of reactant mass ratio ( $$ m_{\text{tire}} /m_{{{\text{Bi}}_{ 2} {\text{O}}_{ 3} }} $$ = 0–10) and temperature (600–900 K). The extent of tire pyrolysis increased as the temperature was raised to 900 K. The degree of Bi2O3 reduction increased with increasing tire pyrolysis, while the waste pyrolysis process controlled Bi2O3 reduction. The oxide reduction was incomplete, when only waste tires were used. Adding a minor amount of waste high-density polyethylene to the tires sufficed for full Bi2O3 reduction at 900 K. The reduction reactions and a model for the morphological evolution are presented. Large Bi shots formed owing to the coalescence of the liquid Bi droplets at the reduction temperature.

Fabrication of Al-Bi frozen emulsion alloys due to high-intense ultrasound irradiation

The aim of this study is to develop an ultrasound-assisted process for fabrication of frozen emulsion alloys, which are a type of miscibility gap alloys (MGAs). Al-Bi system was used to produce 2-kg ingots of Al-Bi frozen emulsion alloys in the following two steps: (1) emulsification treatment of Al-Bi melt by introducing high-intense ultrasound vibrations into it in a temperature range of Al-Bi binodal decomposition, and (2) casting of the melt into a book mold. The ingot samples were analyzed with SEM/EDX for the alloy microstructure and bismuth particle distribution. Besides, cold model experiments, thermodynamic and CFD numerical simulations were performed to elucidate the underlying mechanisms of emulsification including droplet fragmentation and acoustic streaming.The results reveal that the alloys with Bi concentration of 4%~9.4% can be fabricated by the proposed technique. Within this range, 90% of Bi particles have a diameter under 10 µm, and the majority of them are incorporated into aluminum grains. Especially, a uniform microstructure is obtained when concentration of Bi is less than 6%. Emulsification was found to occur at the interface between two liquids when cavitation bubbles collapse in the interface vicinity followed by high-velocity micro-jetting which is responsible for the emulsion droplet formation. Another positive effect of ultrasound is acoustic streaming which is responsible for the effective agitation of molten bath preventing the sedimentation of Bi droplets during the ultrasound treatment and casting operation.

Dynamic behaviors of minor droplets and the role of bubbles in phase-separating Al[sbnd]Bi immiscible alloy

Minor droplets growth and solute segregation have their peculiarities in phase-separating immiscible alloys and can take complex pathways during solidification. The dynamic behaviors of liquid phase separation and surface enrichment in solidifying Al-10 wt% Bi immiscible alloys were revealed by synchrotron X-ray imaging technique. The large-sized Bi droplets would precipitate from Al-matrix melt directly and almost all of the droplets possess different existing states. The distribution of Bi droplets, resulting from the heterogeneous nucleation and repeatedly nucleation, varies with the growth time during cooling. And the growth dynamics of Bi droplets is revealed to be inconsistent with the Gaussian model. We also found that hydrogen bubbles can prevent the occurrence of the liquid phase separation process.

The effect of III:V ratio on compositional and microstructural properties of GaAs1-xBix (0 0 1)

Compound semiconductors alloyed with Bi have many interesting properties, but the low solubility of Bi in these materials leads to effects such as Bi segregation and the formation of surface droplets. To study these effects, a series of GaAsBi films were grown as a function of growth conditions, namely the Bi:As and As:Ga flux ratios. Four surface morphologies emerged—surfaces that were free of droplets, contained Bi droplets, Ga droplets, or biphasic Ga/Bi droplets—as a function of flux ratio. Further analysis with X- ray diffraction, Transmission Electron Microscopy, and Atomic Probe Tomography revealed specific microstructures corresponding to the types of droplets present. Finally, we propose a model to explain each microstructure that takes into consideration factors including Ga adatom diffusion, bulk Bi diffusion, Bi droplet mobility, and film growth rate.

Combined growth of α-Al and Bi in a Al-Bi-Cu monotectic alloy analyzed by in situ X-ray radiography

Al-based monotectic alloys show an interesting combination of wear resistance and mechanical strength. While self-lubricating bismuth (Bi) guarantees an adequate wear resistance, the modification with copper (Cu) can increase the ability to support load. Tailoring the micromorphology of the α-Al phase as well as the distribution of the inclusions of Bi embedded in the α-Al matrix is of prime importance in order to improve the properties. In this paper, various solidification experiments are performed with the Al-3.2wt.%Bi-3wt.%Cu monotectic alloy. Real-time radiographs could be acquired during solidification by using the laboratory device SFINX (Solidification Furnace with IN situ X-radiography). The processing conditions could be chosen in such a way that both cellular and dendritic morphologies of the α-Al phase could be clearly seen in a single test. Micromorphological transitions from dendrites to cells are recognized for samples solidified from different cooling rates, i.e. 0.025 K/s and 0.01 K/s. Measurements of the α-Al dendrite velocity and growth parameters of the Bi droplets during solidification have also been extensively performed. A comprehensive model for the cellular-to-dendritic (CDT) transition of Al-based monotectic alloys is outlined. The model considers the interaction between α-Al and Bi droplets during solidification and its impact on the microstructure.

Optical phase change in bismuth through structural distortions induced by laser irradiation

ABSTRACTSemimetal bismuth (Bi) is known to possess a wide range of peculiar properties, owing to its unique electronic band structure. Its electronic band can easily be distorted by structural changes, and thereby undergo transitions between semimetal to either semiconductor or metal states. Utilising a focused laser beam, one can easily introduce structural defects, along with phase changes, oxidation, and morphological modifications. Confocal Raman microscopy indicated that the as-fabricated Bi droplets inhibit the Raman signal from the underlying silicon (Si) substrate. After a laser flash heating step, the intensity of Si optical phonons was strongly enhanced at the positions of Bi droplets, and exceeding the intensity from the bare Si substrate. Thus, such laser irradiating step on the Bi droplets induces an optical phase change. The optical phase change was detected as going from inhibition to strong enhancement of the underlying Si substrate Raman signal. From the observed Bi optical phonon modes (Eg and A1g), alterations in the Raman peaks due to laser exposure indicated that the ordered crystallinity in pristine Bi droplets became deteriorated. The effects of atomic displacements and loss of structural order in Bi droplets impacts its dielectric response. The observed Si Raman signal enhancement is similar to the surface-enhanced Raman scattering effect typically known for noble metals.

Structure tailoring and nucleation behavior of Bi droplets embedded in a Zn matrix

Well-characterized nucleants are important for heterogeneous nucleation because of their catalyst effect. In this study, micro- and nano-sized Bi droplets embedded in the Zn matrix were prepared by tailoring the solidification process of the Zn–xBi (x = 5, 10, 15, 20, 25, 30 wt%) alloys. With the help of nanocalorimetry, the nucleation behavior of the embedded Bi droplets at cooling rates ranging from 1000 K/s to 10,000 K/s was demonstrated based on the classical nucleation theory. Under this condition, the evolution of undercooling with scanning rate suggests a single interface heterogeneous nucleation both in the micro- and nano-sized droplets. The nucleation of the micro-sized droplets is explored by repeated undercooling measurements, producing a contact angle of 37° ± 1.4°. With droplet size down to nanoscale, a transition from rhombohedral to cubic structure is found in the Bi droplet. Based on Cantor's model, the nucleation kinesics of these nano-sized droplets is elaborated, where a contact angle of 63° ± 2.5° is obtained. The discrepancy highlights the dominant role of the interfacial structure in determining the nucleation behavior.

Correlation between unsteady-state solidification thermal parameters and microstructural growth of Zn–8 mass% Al and Zn–8 mass% Al–XBi tribological alloys

In the present study, directionally solidification experiments are performed with Zn–8 mass% Al and Zn–8 mass% Al–X mass% Bi (X = 1.5, 2.3 and 3.0 mass%) alloys using a water-cooled solidification system, which permits a wide range of solidification cooling rates to be investigated in a single experiment. The microstructural phases of the ternary alloys are shown to be quite similar to those of the binary alloy, with equiaxed dendrites immersed in a matrix formed by fibers or lamellae of the Al/Zn eutectoid product, with the difference that the ternary alloys have Bi droplets disseminated into the matrix and dendrite branches. The dendrites, lamellae and fibers evolve from a refined microstructure at regions closer to the water-cooled bottom of the casting, to increasingly coarser microstructures toward the top. Experimental expressions relating the secondary dendritic arm spacing, the spacing between lamellae and the fiber spacing to solidification thermal parameters (growth and cooling rates) are derived. For the alloys having higher Bi content, a bimodal distribution of Bi droplets was shown to occur, with small droplets, consisting of lamellae and fibers, disseminated into the matrix and larger droplets between the dendritic branches.

Molecular beam epitaxy growth of GaSb1-xBix without rotation

GaSb1-xBix thin film was grown on a 2 inch GaSb substrate by molecular beam epitaxy (MBE) without substrate rotation. Bi composition is found to vary from 2.76% to 3.98% across the wafer. The distribution of Bi content is mainly determined by spatial non-uniformity of Sb/Ga flux ratio, while Bi flux has slightly influence. Ostwald ripening process is confirmed to be reason for bigger Bi droplets via Bi surface diffusion. With the increase of Sb/Ga flux ratio, Ostwald ripening process is suppressed. At high Bi flux, excess Ga atoms accumulate on surface and form droplets.

Laser-induced structural disordering and optical phase change in semimetal bismuth observed by Raman microscopy

We observed strong enhancement of the Raman signal from a silicon (Si) substrate hidden beneath nano/micro-droplets of bismuth (Bi) after exposing them to a high-power laser beam (785 nm). Confocal Raman microscopy indicated that the as-fabricated Bi droplets inhibit the Raman signal from the underlying Si substrate. After the laser flash heating step, the intensity of Si optical phonons was strongly enhanced at the positions of Bi droplets, becoming even more intense than that of the bare Si substrate. Analyses of the Bi optical phonon modes, E g and A 1g , confirmed a loss of long-range ordered crystallinity in the laser exposed droplets. Such lattice disordering can increase the available free carrier density in semimetal Bi, as occurs in the case of its amorphous and liquid states. And this can induce an optical phase change, which causes a marked enhancement of Si Raman signals through an effect similar to surface-enhanced Raman scattering (SERS). • Micro-Raman image, showed dramatic intensity enhancement of Si optical phonons hidden beneath Bi droplets after exposure to a 785 nm high-power laser. • Analysis of E g and A 1g optical phonon modes suggests that the Bi lattice is distorted through rapid melting and resolidification caused by short exposure to laser irradiation. • SERS-like Enhancement of the Si Raman signal is explained by a phase change of Bi from a semimetal state to a more metallic state associated with laser-induced lattice disordering.

Epitaxial phases of high Bi content GaSbBi alloys

GaSbBi alloys have recently emerged as attractive materials for mid-infrared optoelectronics owing to strong band gap reduction enabled by Bi incorporation into the GaSb matrix. The fundamental understanding of the epitaxial process required to demonstrate high quality crystals is in an early-developmental phase. From this perspective, we report on the key role played by the Sb/Ga flux ratio in controlling the structural quality and incorporation of high Bi content GaSbBi (up to 14.5%-Bi), revealing three distinct epitaxial phases. The first phase (below stoichiometric Sb/Ga) exhibits Ga-Bi compound droplets, low crystal quality, and reduced Bi content. At the second phase (above stoichiometric Sb/Ga), the crystal exhibits smooth surfaces and excellent crystallinity with efficient Bi incorporation. The last phase corresponds to exceeding a Sb/Ga threshold that leads to reduced Bi incorporation, Bi droplets and degraded crystallinity. This threshold value that defines the optimal growth window is controlled by the temperature as well as the Bi/Ga ratio. Increasing temperature increases the threshold, albeit simultaneously reducing Bi incorporation. Conversely, increasing the Bi/Ga flux ratio increases Bi incorporation, while narrowing down and ultimately closing the window. This study provides a general framework enabling development of high quality GaSbBi heterostructures for emerging mid-infrared optoelectronics.