Singly-twinned growth of Si crystals upon chemical modification

Saman Moniri, Xianghui Xiao, and Ashwin J. Shahani

Phys. Rev. Materials 4, 063403 – Published 19 June 2020

Abstract

During crystallization a continuum of patterns could emerge due to the interplay of growth kinetics, material or solution chemistry, and crystallographic defects. The coherent twin boundary is widely known to catalyze growth in pristine crystals including polycrystalline Si. Much remains unknown about the impact of changing the chemical environment of the crystallization process through the deliberate addition of trace metallic species—termed chemical modification. Pristine Si has been reported to grow through the classical model of two parallel twin planes acting in concert to enable steady-state propagation of the solid-liquid interfaces. Here, we achieve a vision on the growth process via in situ synchrotron x-ray microtomography and further corroborated by ex situ crystallographic investigation. We find that steady-state growth is impossible in chemically modified alloys that consist of trace (0.1 wt.%) Sr. This is because the Sr modifier poisons the concave re-entrant grooves, thereby deactivating the advantage of the twin-plane re-entrant edge mechanism and leading to a singly-twinned interface. This study may serve as a proxy to chemically modified crystallization pathways of eutectic Si in Al-Si alloys and, more broadly, as a framework for the crystallization-mediated synthesis of materials.

Received 17 February 2020
Accepted 2 June 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.063403

Physics Subject Headings (PhySH)

Crystal defects Crystal growth Crystallization

X-ray tomography

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Saman Moniri ¹, Xianghui Xiao^2,*, and Ashwin J. Shahani^3,†

¹Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
²X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, USA
³Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA

^*Present address: National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973, USA.
^†Corresponding author: shahani@umich.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 4, Iss. 6 — June 2020

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Schematics of faceted crystal growth with singly- and doubly-twinned interface, according to Wagner [23] Sand Hamilton and Seidensticker [24]. (a),(b) Growth of a crystal with a single twin boundary. (a) The crystal is bounded by {111} habit planes, and a 141° re-entrant corner (type I) appears where the twin intersects the solid-liquid interface. Growth proceeds in 〈112〉 directions (arrows) upon nucleation at the re-entrant groove, compared to the flat {111} habit planes. (b) Growth terminates by the vanishing of the re-entrant grooves and subsequent formation of a triangular mound projecting into the melt. (c)–(e) Growth of a crystal with two parallel twin planes. (c) The doubly-twinned crystal is bounded by {111} habit planes and contains a type-I re-entrant groove. Nucleation readily occurs at the re-entrant groove as opposed to the flat {111} planes, similar to the case of singly-twinned crystal in (a). (d) At the neighboring twin the new layer creates a second re-entrant corner (type II) with an angle of 109.5°. (e) The type-II corner enables the continuous propagation of the crystal in the 〈112〉 direction. Unlike in the singly-twinned interface in (a), the type-I corner does not disappear during growth and no triangular mounds appear upon the termination of growth. Figure reproduced with permission from Ref. [24].
Reuse & Permissions
Figure 2
3D growth behavior of the Si particles from x-ray tomographic reconstructions. (a)–(d) The Si particles are visualized at 912, 907, 884, 879 °C (22.5, 27.5, 69, 89 min after imposing a cooling rate of 1 °C/min to the melt at 934 °C), respectively. The transparent gray wall represents the oxide skin that contained the molten specimen. The Cu-enriched liquid is rendered transparent for clarity. Si particle color reflects crystallization time, with early times in red and late times in blue. (e) A closer examination of the particle boxed in (a) suggests that Si undergoes a morphological instability during growth to attain, eventually, a convex geometry with a triangular mound projecting into the melt [cf. Fig. 1]. Shown are the Si particle at six time steps: 912, 907, 890, 884, 879, 873 °C (22.5, 27.5, 44, 69, 89, and 120 min from start of cooling).
Reuse & Permissions
Figure 3
Evolution of Si morphology during growth. Concavity was calculated from the μXRT results as the length of the normal vector projected from the convex hull to the crystal surface [see arrow in inset (ii)]. The analysis is limited to the single Si particle shown in Fig. 2, and the early times in growth are omitted due to the small size of particle at time step labeled $i$ ). The concavity initially increases and subsequently decreases as the crystal attains a more convex geometry. The latter suggests a disappearance of the concave re-entrant groove. Inset: Si particle (red) and its corresponding convex hull (light blue) at the three indicated time-steps (denoted $i, i i$ , and $i i i$ ) during growth showing that the crystal is fully convex as growth terminates, thus “filling” its convex hull.
Reuse & Permissions
Figure 4
The impact of chemical modification on the twinning behavior of fully solidified, primary Si particles. (a), (b) EBSD orientation maps of two representative unmodified Si particles show the existence of two parallel twin planes running across the length of the Si particles and intersecting the solid-liquid interface at the edges of the particles. (c), (d) Orientation maps of two representative chemically modified Si particles display singly-twinned interface. Similar to the unmodified case, the twin plane intersects the solid-liquid interface at the edge of the particle. In all maps, the mean crystallographic orientation of each grain of Si is colored according to the standard stereographic triangle on top-left of (a); the surrounding eutectic matrix is depicted black.
Reuse & Permissions
Figure 5
Nucleation potency of impurity clusters on the Si crystal surfaces. Ratio of heterogeneous to homogeneous nucleation barriers as a function of wetting strength (contact angle θ) is plotted for a re-entrant groove, flat facet, and ridge (crystal step of angle η), computed via CNT [36]. For all contact angles, nucleation along the concave re-entrant groove is found to be more favorable than on the flat facet plane or along the convex ridge. Inset: Spherical droplet (light brown) nucleating with contact angle θ relative to the twin plane (dashed line) and step angle η.
Reuse & Permissions

Physical Review Materials