Confined in-fiber solidification and structural control of silicon and silicon−germanium microparticles

Abstract

Crystallization of microdroplets of molten alloys could, in principle, present a number of possible morphological outcomes, depending on the symmetry of the propagating solidification front and its velocity, such as axial or spherically symmetric species segregation. However, because of thermal or constitutional supercooling, resulting droplets often only display dendritic morphologies. Here we report on the crystallization of alloyed droplets of controlled micrometer dimensions comprising silicon and germanium, leading to a number of surprising outcomes. We first produce an array of silicon−germanium particles embedded in silica, through capillary breakup of an alloy-core silica-cladding fiber. Heating and subsequent controlled cooling of individual particles with a two-wavelength laser setup allows us to realize two different morphologies, the first being a silicon−germanium compositionally segregated Janus particle oriented with respect to the illumination axis and the second being a sphere made of dendrites of germanium in silicon. Gigapascal-level compressive stresses are measured within pure silicon solidified in silica as a direct consequence of volume-constrained solidification of a material undergoing anomalous expansion. The ability to generate microspheres with controlled morphology and unusual stresses could pave the way toward advanced integrated in-fiber electronic or optoelectronic devices. multimaterial fibers | microparticles | confined solidification | silicon−germanium spheres | stressed silicon C ontrolling the microstructure or state of stress of microparticles and nanoparticles is often key to attaining the desired properties for a specific application (1-5); however, the ability to do so is strongly limited by the synthesis method. For instance, nonspherically symmetric distributions of inorganic materials are difficult to achieve from bottom-up approaches (4-7). Likewise, controlling the state of stress or strain of semiconductor particles is challenging in unconstrained nucleation-and-growth synthesis methods. However, Janus particles of silicon−germanium (SiGe) could potentially find applications as microswimmers or nanoswimmers owing to asymmetric absorption properties (8), as well as in infrared photodetectors or solar cells for increased infrared absorption (9). Stressed silicon particles, on the other hand, could be used for bandgap tunability in photonic or optoelectronic devices (10-12).In the past few years, thermally drawn multimaterial fibers have emerged as a unique platform for top-down scalable fabrication of microparticles to nanoparticles over a broad range of materials, through controlled in-fiber capillary breakup of the fiber components (13-15). In the case of polymers or chalcogenide glasses, structural control of the particle can be achieved by constructing complex cores at the preform level, which is later broken up in the fiber state to form structured particles (13). However, in the case of traditional semiconductor materials such as silicon and germanium, the same...

Advanced Multimaterial Electronic and Optoelectronic Fibers and Textiles

“…Fabrication of in‐fiber particles has been achieved by softening the prefabricated fibers either by isothermal heating via furnace or by axial thermal‐gradient heating via oxyhydrogen flame or furnace . However, these approaches face some challenges, such as wide and uncontrollable heating profiles, unavoidable satellite particle generation, and a limited selection of functional materials, all of which have limited their study and possible applications.…”

“…The versatility of the laser‐based solidification approach has also been demonstrated for tailoring the microstructure of in‐fiber SiGe spheres obtained via PRI. By precisely controlling the heating and cooling rates, Gumennik et al showed the creation of SiGe compositionally segregated Janus spheres and spheres of dendrites of Ge in Si in silica fibers . Another appealing example is to modify the electronic band structure of in‐fiber semiconductors via the high stress introduced during the laser solidification process.…”

In-Fiber Structured Particles and Filament Arrays from the Perspective of Fluid Instabilities

“…Even further complexity in digital preform design was highlighted for solely or combined radial and azimuthal control on particles [31]. In addition, control on solidification front and recrystallization, microstructure and stress control on SiGe alloy spheres was demonstrated for bandgap modification and in-fiber microelectronics [34].…”

Crystalline GaSb-core optical fibers with room-temperature photoluminescence

“…The literature on Group IV semiconductor-core glass fibers is now significant [1][2][3][4][5], including processing improvements [6][7][8] and device demonstrations [9][10][11][12][13][14][15][16][17]. There are also reports on both SiGe alloy and II-VI core fibers [18][19][20][21][22][23], one including low temperature photoluminescence [21]. However, there is limited information on III-V semiconductor-core fibers [26][27][28] manufactured by either CVD or molten-core techniques.…”